The Lone Ranger
01-07-2007, 06:10 AM
An Introduction to Human Anatomy and Physiology
Chapter Four: Cell Structure and Function
Introduction: In 1665, the English scientist Robert Hooke observed thin slices of cork under a microscope and noted that the wood was made of tiny, box-like structures that he named “cells.” Subsequent observations by Hooke and other scientists showed that not just plants, but also fungi and animals were made up of cells. In fact, all living creatures appeared to be made of cells.
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Hooke’s drawing of “cells” in a piece of cork.
In 1839, German biologists Matthias Schleiden and Theodor Schwann formulated the Cell Theory of Life, which has four claims. First, according to the cell theory, all living organisms consist of one or more cells. (Since a virus is not cellular, according to the cell theory, viruses are not alive.) Second, all cells arise through division of preexisting cells. Third, all vital functions of an organism occur within its cells. Finally, cells contain the hereditary information necessary both for regulating cellular function and for reproduction.
Basic Cellular Structure: All cells have at least three components: a plasma membrane, genetic material, and cytoplasm.
The plasma membrane (or cell membrane) regulates the flow of materials into and out of the cell, allows interactions between cells, and protects the interior of the cell from the external environment.
The genetic material provides the “recipe” for making the enzymes and structural proteins that regulate cellular functions and make up much of the internal structure of the cell. In all living organisms, genetic material consists of DNA, and RNA is used to “translate” information stored in DNA into proteins. (Many viruses use RNA as their primary genetic material.) In eukaryotic cells, the DNA is contained within a structure called the nucleus of the cell.
The cytoplasm of the cell is the substance between the plasma membrane and the nucleus. The cytoplasm of a eukaryotic cell consists of various organelles embedded within cytosol. The organelles are smaller subunits within a cell that perform specific functions. The cytosol consists of a watery solution of proteins, carbohydrates, lipids, and electrolytes.
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A generalized animal cell. Cells contain genetic material
and cytoplasm, and are enclosed by a plasma membrane.
Physical Constraints on Cell Size: The relationship between an object’s surface area and its volume determines how large a cell can grow. Because surface area is a square function, if you double an object’s size, its surface area increases by a factor of 22 or four times. Volume, however, is a cubic function, so if you double an object’s size, its volume increases by 23 or eight times. This means that as objects grow larger, their volumes increase at a much faster rate than do their surface areas.
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If its proportions remain the same, as an object grows larger,
its volume increases much faster than does its surface area.
What does this have to do with cells? Well, it’s important to remember that everything that comes into a cell – oxygen, food, water, etc. – must come in across the plasma membrane. Similarly everything that is expelled from a cell – poisonous metabolic waste products, for example – must leave across the plasma membrane. This means that the size of the plasma membrane sets a limit to how quickly a cell can absorb and excrete materials.
So, the size of a cell’s plasma membrane determines how quickly it can absorb necessary substances like oxygen and nutrients, and how quickly it can expel dangerous substances like CO2. The problem is that the size of the cell’s plasma membrane is a function of the cell’s surface area.
On the other hand, the amount of material inside the cell that requires oxygen and nutrients – and that generates dangerous waste products like CO2 – is a function of the cell’s volume.
So as a cell increases in size, it quickly reaches a point where it simply isn’t possible for it to grow any larger and survive. If it were to grow any larger, the cell’s relatively small surface area relative to its relatively large volume would mean that it couldn’t bring in oxygen and nutrients (and expel poisonous metabolic wastes) fast-enough to keep itself alive.
So if an organism is to grow large, it must be made of many small cells, each of which has sufficient surface area relative to its volume to keep itself supplied with nutrients and oxygen, and to avoid poisoning by its own metabolic wastes. Movies such as The Blob that feature enormous single-celled organisms wandering about the countryside and devouring unwary teenagers are pure fantasy. No single-celled organism could get even close to that size. In fact, the largest single-celled organisms are just barely visible to the naked eye – if you have good eyesight, that is.
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Not that big a threat, actually.
One of the advantages of being multicellular is that body cells can be specialized to perform different functions. This allows multicellular organisms to be much more efficient than if each cell in the body were identical and performed identical functions.
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An organism can increase its surface area while keeping its volume
constant by dividing into lots of smaller subunits (i.e. cells).
[B]Prokaryotic and Eukaryotic Cells: Prokaryotes are organisms with prokaryotic cells. (Quelle surprise, eh?) Almost certainly, the earliest living organisms were prokaryotes. You’d probably find it less than surprising to learn that eukaryotes are organisms with eukaryotic cells.
Prokaryotic cells are small and relatively simple cells with no distinct nuclei or other organelles. Bacteria and Archaea are the only surviving prokaryotes. There are no known prokaryotes that are truly multicellular, perhaps because their cells are insufficiently complex to be capable of coming together to form a multicellular organism.
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A prokaryotic cell. Note that there are no complex organelles, and
that the genetic material is more or less dispersed throughout the
cytoplasm, rather than being confined in a nuclues.
Eukaryotic cells are generally much larger than are prokaryotic cells, and are much more complex. A eukaryotic cell has a nucleus containing its genetic material, and other organelles within its cytoplasm. These organelles apparently make eukaryotic cells much more efficient than are prokaryotic cells, and eukaryotic cells can grow to much larger sizes than can prokaryotic cells.
Also unlike prokaryotic cells, eukaryotic cells can also cluster together to form multicellular organisms that are far larger than any single cell could possibly become. Protists, fungi, plants, and animals are all eukaryotic organisms. Many protists and fungi are multicellular, and all plants and animals are multicellular by definition. (An amoeba is not a “single-celled animal,” as you sometimes hear it called; it’s a protist. Unless you’re talking about a zygote, a “single-celled animal” is a contradiction in terms.)
[B]The Organelles of Eukaryotic Cells: Humans are eukaryotes. Specifically, we’re animals. So, let’s take a look at the makeup of a typical animal cell. It’s worth keeping in mind that since different cells are specialized to perform different tasks, many will look quite different from a “typical” cell. For example, a skeletal muscle cell looks very different from a neuron. Still, there are a great many more similarities than differences.
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A Typical Animal Cell
[B]The Nucleus: Control Center of the Cell: The nucleus of the cell contains its genetic material. Here is where the DNA is stored that contains the information the cell uses to make its proteins. Since these proteins make up much of the internal structure of the cell and (in the form of enzymes) control its metabolism, the nucleus is often referred to as the “control center” of the cell. A cell cannot survive for very long without its nucleus. (Red blood cells or erythrocytes lose their nuclei as they mature; as you would expect, a mature erythrocyte does not live for very long.)
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The nucleus of a eukaryotic cell.
The nucleus is surrounded by a nuclear envelope (or nuclear membrane) that is virtually identical in makeup to the plasma membrane that surrounds the cell itself. In fact, the nuclear envelope and the plasma membrane are actually continuous with each other, but we’ll get to that in a bit.
Just as the plasma membrane determines what gets in and out of a cell, the nuclear envelope determines what passes into and out of the nucleus of the cell. One distinction between the nuclear envelope and the plasma membrane is that the nuclear envelope has numerous relatively large holes in it known as nuclear pores. These pores allow relatively large molecules such as proteins and RNA to cross the nuclear envelope.
Under a microscope, the interior of the nucleus usually looks somewhat granular. The grainy substance is called chromatin, and it consists of unraveled DNA molecules and their associated protein molecules. Sections of DNA molecules can be “unzipped” and “transcribed” to make molecules of messenger-RNA, which then transport the information necessary for protein synthesis into the cytoplasm of the cell, where protein synthesis actually occurs. Because DNA never leaves the nucleus of the cell, the danger that it will be damaged during the production of proteins is greatly lessened.
Cells reproduce by dividing in two. When a cell is ready to reproduce, the chromatin condenses into structures known as chromosomes. So if you can see chromosomes in the nucleus of the cell, it is either in the process of reproducing or it is preparing for reproduction.
You may be able to see a darker structure inside the nucleus called the nucleolus (“little nucleus”). The nucleolus is made mostly of RNA and protein, and it’s where structures called ribosomes are assembled. After they’re assembled in the nucleolus, ribosomes are transported out of the nucleus and into the cytoplasm of the cell, where they’re important in protein synthesis.
[B]The Endomembrane System of the Cell: All eukaryotic cells have an elaborate system of membranes surrounding the cell and extending throughout the cytoplasm. This endomembrane system consists of the plasma membrane, plus several internal organelles, including the endoplasmic reticulum, the nuclear envelope, the Golgi complex, and smaller structures such as lysosomes and vesicles. As you’ve probably realized, this means that the endomembrane system connects every part of the cell.
These membranes are phospholipid bilayers that have cholesterol and various proteins embedded in them. Because the membranes of different organelles are made of the same substance, they can readily fuse. This allows different organelles within a cell to easily exchange their contents.
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The endomembrane system of a typical animal cell.
The Plasma Membrane: The plasma membrane surrounds the cell and separates it from the external environment. The membrane is selectively permeable, meaning that it allows some substances to cross but not others. In this way, the plasma membrane controls what substances can get into and out of the cell.
The Endoplasmic Reticulum: The endoplasmic reticulum is a series of membranous channels that extends throughout the cytoplasm of the cell. The ER is continuous with both the plasma membrane and the nuclear envelope, and so allows communication between all parts of the cell and the external environment.
Rough endoplasmic reticulum has small organelles called ribosomes embedded in its membranes. The ribosomes are the sites of protein synthesis, and so rough ER manufactures proteins and then transports them to other portions of the cell, or transports them to the plasma membrane where they can be exported from the cell.
Smooth endoplasmic reticulum lacks ribosomes and so does not manufacture proteins. Instead, it synthesizes lipids and some other organic molecules, which can then be transported elsewhere in the cell or to the plasma membrane for export.
Since ER can synthesize all the components of cellular membranes, it can synthesize itself, as well as the membranous portions of other cellular organelles.
The Golgi Complex: The Golgi complex (or Golgi bodies or Golgi apparatus) are often described as looking like stacked pancakes when viewed under a sufficiently powerful microscope. The Golgi bodies are closely associated with the endoplasmic reticulum, and in fact, are formed by it. If the endoplasmic reticulum is the “factory” of a cell, where most cellular components are manufactured, the Golgi complex is the “finishing plant.” In the Golgi complex, molecules synthesized in the endoplasmic reticulum are chemically altered into their final form and then packaged into small membranous structures called vesicles. The vesicles then transport the finished molecules through the cytoplasm to other parts of the cell, or to the plasma membrane and out of the cell. (To continue the analogy, that would make the vesicles the “delivery trucks” of the cell.)
Lysosomes: Lysosomes are relatively small membranous organelles in the cell that contain digestive enzymes. When food is brought into a cell inside vesicles, lysosomes can fuse with the vesicles and release their digestive enzymes into the vesicles to digest the food. Once it has been broken down into its component molecules by the lysozome’s digestive enzymes, the food is then be absorbed into the cytoplasm of the cell.
White blood cells (leukocytes) are cells that are specialized for engulfing and destroying bacteria and other foreign cells. As you might expect, most leukocytes contain large numbers of lysosomes, allowing them to quickly and efficiently digest and destroy viruses, bacteria, and other invaders.
If the lysosomes of a cell rupture, the digestive enzymes they release will digest and destroy the cell itself. This process is called autolysis, and it normally occurs when cells are damaged or soon after they die. Some degenerative disorders, such as Alzheimer’s disease and Bovine Spongiform Encephalopathy (“Mad Cow Disease”), may be the result of foreign agents triggering autolysis and causing destruction of body tissues.
Vacuoles: Vacuoles are relatively large membranous sacs which cells use for (relatively) long-term storage of materials, especially liquids. Vacuoles can be formed by endoplasmic reticulum or can be formed from the plasma membrane. If you’ve ever seen an amoeba engulf a smaller organism by flowing around it, the sac that encased the amoeba’s unfortunate victim and was formed from the amoeba’s plasma membrane was a vacuole. Lysosomes can fuse with vacuoles and release digestive enzymes into them, allowing digestion of the substances contained in the vacuoles.
Vesicles: Vesicles are smaller membranous structures than are vacuoles. As mentioned earlier, they’re used to contain and transport substances within the cell. There’s really no clear distinction between vesicles and vacuoles except that vesicles are smaller.
Vesicles can be formed by endoplasmic reticulum or by Golgi bodies. Since the membrane surrounding a vesicle is the same substance that makes up the plasma membrane, if a vesicle should come into contact with the plasma membrane, the vesicle will be incorporated into the plasma membrane, and so its contents will be emptied to the outside of the cell. In this way, vesicles can be used to transport substances out of the cell.
Similarly, relatively small objects can be brought into a cell by vesicle formation. If the plasma membrane folds inward and pinches off, it will form a vesicle, and so substances can be transported into the cell from the outside. Once it forms, the vesicle can then transport the substance it contains to any other part of the cell, or it can fuse with lysosomes so that the substance it contains is digested.
[B]Mitochondria: Though they are indeed membranous, the mitochondria are not formed by or associated with the endoplasmic reticulum, Golgi bodies, or other membranous organelles. As such, mitochondria are not considered part of the endomembrane system of a cell.
A mitochondrion looks somewhat “sausage-shaped” under a microscope, and it has an elaborately folded inner membrane. The mitochondria are where aerobic (cellular) respiration occurs. That is, the mitochondria are where carbohydrate molecules are broken down for the energy that powers the cell’s metabolism.
Interestingly, mitochondria have their own DNA, and they reproduce independently of the rest of the cell. Analysis of mitochondrial DNA shows that it’s actually much more similar to the DNA of certain bacteria than it is to the DNA in the nucleus of the cell within which it resides.
Given the similarity of mitochondrial DNA to bacterial DNA, it is thought that mitochondria are the descendants of bacterial cells that – at some time in the distant past – either invaded larger cells or were engulfed by them. In either case, these cells weren’t destroyed in the process. Instead, the bacterial cells remained within the larger cells and the arrangement turned out to be mutually beneficial. The bacterial cells gained a (relatively) safe and stable haven, and the larger cells benefited from the energy the bacterial cells produced when they broke down carbohydrates.
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A mitochondrion within a cell.
[B]Ribosomes: Ribosomes are tiny organelles composed of RNA (specifically, ribosomal-RNA) and protein. Messenger-RNA produced in the nucleus by copying information from DNA travels into the cytoplasm of the cell and to ribosomes, where the information it contains is used to manufacture proteins. Ribosomes are found embedded within the membranes of “rough” endoplasmic reticulum and mitochondria, but they are also scattered throughout the cytoplasm of the cell.
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A ribosome, under extremely high magnification. Each
ribosome consists of two subunits called, somewhat
unimaginatively, the “large subunit” and the “small subunit.”
[B]The Cytoskeleton: The cytoskeleton is a three-dimensional network of protein fibers that extends throughout the cell. These protein fibers help to support the cell and they give the cell its shape. The cytoskeleton helps to hold the various organelles in place, and it plays an important role in movement of cells. Three principle types of proteins make up the cytoskeleton: microfilaments, intermediate fibers, and microtubules.
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The Cytoskeleton
Microfilaments are thin fibers that are made mostly of the protein actin. Microfilament fibers can change length when supplied with energy by ATP, and so these fibers allow cells to change shape, and they also guide the movements of organelles within the cell. Interactions of actin and the protein myosin are important in contraction of muscle cells.
Intermediate fibers (intermediate filaments) are somewhat larger in diameter than are microfilaments, and are made of several different proteins. Intermediate fibers form a 3-D network throughout the cell that forms much of the framework of the cell and holds the organelles in position.
Microtubules are about 24 nanometers in diameter, somewhat thicker than microfilaments or intermediate fibers. Microtubules are hollow and composed largely of the protein tubulin. Microtubules help to position, anchor, and move organelles. They are also important in cellular reproduction and in the movement of cells. Microtubules are important components of both cilia and flagella, and of centrioles.
[B]Propulsive Organelles: Cilia and flagella are extensions of the plasma membrane that are supported by microtubules. Both cilia and flagella can be moved back-and-forth, and either move the cell itself or move fluid past the cell.
Cilia are relatively short and numerous structures. The cells of many microorganisms are ciliated, and these organisms swim by beating their cilia. Paramecium is a well-known example of a ciliated swimmer. In humans, cells lining the respiratory tract and the female reproductive tract are ciliated. The ciliated cells in the respiratory tract help to keep mucus moving, and thereby prevent it from blocking respiratory passages. Ciliated cells in the female reproductive tract create currents that draw ova down the reproductive tract and to the uterus.
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Ciliated cells lining the human respiratory tract.
Flagella are nearly identical to cilia, but they’re much longer and less-numerous. A typical flagellated cell has only a single flagellum, though some have more. The best-known of flagellated cells is surely the spermatozoan. A sperm cell uses its flagellum to swim up the female reproductive tract in search of an ovum.
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A spermatozoan, one of the few flagellated cells in the human body.
Centrioles: Centrioles are structures that, like cilia and flagella, are composed mostly of microtubules. In fact, specialized centrioles called basal bodies manufacture cilia and flagella.
Centrioles are also important in cellular reproduction. A cell typically has two centrioles located near the nucleus and oriented at right angles to each other. Just before a cell is ready to reproduce, it duplicates its centrioles. During cellular reproduction, the centrioles produce spindle fibers that organize the movement of the cell’s chromosomes.
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A pair of centrioles. Each centriole is made up of nine microtubule triplets.
[B]Structure and Function of the Plasma Membrane: Every cell interacts with its external environment through its plasma membrane. The membrane makes it possible for a cell to defend itself, to exchange materials with its environment, and to communicate with other cells. In order to understand how cells can perform these functions, we must take a closer look at the structure of the plasma membrane.
[B]The Fluid Mosaic Model of Membrane Structure: A cellular membrane is a fluid made of a phospholipid bilayer with inclusions of cholesterol (a steroid) and protein molecules. Because cellular membranes are so fluid, they can easily change shape.
A phospholipid molecule, as you recall, has a polar “head” and two nonpolar “tails.” Because the heads are attracted to water and the tails are not, phospholipids spontaneously form a bilayer in water, with the heads pointing out towards the water and the tails pointing inward, away from the water. On its own, a phospholipid bilayer has roughly the consistency of a soap bubble. Cholesterol molecules embedded in the bilayer strengthen and stiffen it, making it much less likely to rupture.
Because of their charged nature, most ionic and polar substances cannot easily cross cellular membranes, since they typically lack the energy to force their way through the nonpolar interior portion of the membrane. This is one of the reasons that cellular membranes are so selective about what can cross them.
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The fluid mosaic model of a cellular membrane.
Membrane Proteins: Broadly speaking, there are three different kinds of proteins embedded within cellular membranes. These are transport proteins, receptor proteins, and recognition proteins.
Transport Proteins: Transport proteins regulate the movement of molecules into and out of cells.
Channel proteins are transport proteins that take the form of hollow tubes penetrating cellular membranes. When present, they effectively open holes in membranes, and so allow substances that would not normally be able to cross the plasma membrane to enter or exit cells. Channel proteins do not cause substances to move against the concentration gradient. Some channel proteins, called “gate proteins” or “gated channels,” can change shape, and so the channels they create can be opened or closed at need to regulate the flow of substances into and out of the cell.
Carrier proteins can form temporary bonds with molecules and so can pull substances across cellular membranes. By taking energy from ATP molecules, carrier proteins can even transport substances against the concentration gradient.
Receptor Proteins: Receptor proteins can bind to hormone molecules or other chemical messengers, and cause changes inside the cell. For example, hormones released into the blood by one organ can attach to binding sites on receptor proteins in the plasma membranes of the cells of the “target organ.” When hormones bind to receptor proteins, the proteins change shape. This triggers changes in the interior of the target cells. In this way, hormones can cause changes in cells without ever entering them
Recognition Proteins: Most recognition proteins consist of carbohydrates bound to proteins to form glycoproteins. The exact arrangement of glycoproteins in the membranes of your body cells is determined by your unique genetic makeup (assuming you aren’t an identical twin). Because the glycoproteins in your cellular membranes are distinctive, your immune system has a means of recognizing any “foreign” cells that might be present – bacterial cells, for instance. Some of your white blood cells (leukocytes) systematically “test” the cells they encounter, and attempt to destroy any that have the wrong recognition proteins in their membranes.
Since cancer is often caused by genetic mutations, cancerous cells often fail to produce the proper recognition proteins. As a result, the immune system is even capable of detecting and destroying cancer cells in many instances. This is despite the fact that the cancerous cells have the same basic genetic makeup as the rest of your body cells.
[B]Transport of Substances Across Cellular Membranes: When you consider the movement of substances across cellular membranes, there are three things to keep in mind. First of all, molecules are always in motion, even if they’re simply vibrating in place. Second, in a fluid such as water (which makes up well over 90% of a typical cell), the molecules are free to move and are not constrained to vibrate in place. Finally, movement of molecules from areas of high concentration to areas of low concentration is spontaneous.
Let’s consider that last point for a moment. Remember that the molecules in a fluid are moving randomly – in other words, in all directions. If you concentrate the molecules, they will tend to spread out again until they fill the available space uniformly.
Maybe it’s easier to understand why this occurs if we consider only two dimensions for now, instead of three. Imagine I have a container that is divided into two halves by a partition. Now imagine that I put a 10% sugar solution into the right half of the container. (This means that 10% of the solution is sugar, and 90% is water. If you prefer, you could think of it as a 90% water solution.) Now imagine that I put a 50% sugar solution (i.e. a 50% water solution) into the left half of the container. What will happen if I remove the partition that separates the solutions?
Since the solution in the left half of the container contains five times the sugar as does the solution in the right half of the container, this means that for every sugar molecule that happens to be moving from the right half of the container to the left half, there will be five sugar molecules (on average) moving from left to right. So, sugar molecules will tend to migrate from the left half of the container, where they’re more concentrated, to the right half of the container, where they’re less concentrated. This will continue until the sugar concentrations in both halves of the container are the same.
When the concentration of sugar molecules in both sides of the container is equal, the solution is said to be in equilibrium. At that point, for every sugar molecule moving from left to right, there will be one moving from right to left, and so there will be no net movement of sugar molecules, even though the individual molecules will still be moving about quite energetically.
(Incidentally, the water molecules would be moving too. The water was initially more concentrated in the right side of the container, so there would be a net movement of water from right to left, until the concentrations were equal.)
This spontaneous movement of molecules in solution from high concentration to low concentration until equilibrium is reached is called diffusion.
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Diffusion across a cellular membrane.
(Click “refresh” to see the animation.)
Just for fun, what would happen if the partition were selectively permeable, and allowed water to cross but not sugar? Well, if the left half of the container holds a 50% sugar solution, that means it holds a 50% water solution. And if the right half of the container holds a 10% sugar solution, that means it holds a 90% water solution. Let’s assume that I carefully poured equal volumes of solution into each half of the container, so that the levels were exactly equal.
Since water is more concentrated on the right side of the partition, there will be a net flow of water to the left side of the container. But since the sugar cannot cross the partition, there will be no corresponding flow of sugar to the right.
So, the level of solution in the left side of the container will rise and the level of solution in the right side of the container will fall as water moves to the left. In effect, the solution in the left side of the container will absorb water from the solution in the right side of the container. This will continue until the two solutions reach the same concentration or until gravity halts the rise of solution in the left side of the container.
Osmosis is the diffusion of water across a selectively-permeable membrane when the solute dissolved in it cannot cross the membrane. As you can see, osmosis is just a variation of diffusion, since the water molecules are diffusing from where they are more concentrated to where they are less concentrated.
Osmosis is typically described as diffusion of water from low solute concentration to high solute concentration, so it might sound a little confusing at first. But remember that if a solution’s solute concentration is low, that means its water concentration is high, and if its solute concentration is high, then its water concentration is low. Just remember that the water always diffuses from where it is more concentrated to where it is less concentrated, and osmosis should make sense.
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Osmosis is the diffusion of water across a selectively-permeable
membrane, from low solute concentration (high water concentration)
to high solute concentration (low water concentration).
[b]Passive Transport of Substances Across Cellular Membranes: When substances are transported across cellular membranes passively, the cells do not expend any energy to move the substances in question. The three kinds of passive transport are diffusion, facilitated diffusion, and osmosis.
Diffusion:
Diffusion occurs when water, O2, CO2 and other such small molecules spontaneously cross cellular membranes as they move from areas of high concentration to areas of low concentration.
Most polar molecules cannot cross cellular membranes, because they cannot traverse the nonpolar central region of the membrane. Water, however, can readily cross cellular membranes despite its polar nature. This is because water molecules are so small that they can slip right between the relatively enormous phospholipid molecules that make up the membrane.
As they go about their normal activities, cells consume oxygen and produce carbon dioxide. As a result, concentration of oxygen in the blood is generally higher than it is inside cells. For that reason, oxygen diffuses from the blood and into cells. By contrast, since cells produce carbon dioxide, CO2 concentration in cells is generally higher than it is in the blood. As a result, CO2 diffuses out of cells and into the blood.
Facilitated Diffusion:
Facilitated diffusion occurs when carrier and channel proteins allow substances to cross cellular membranes from high concentration to low concentration, even though the substances in question cannot normally pass through the hydrophobic interior portion of the cellular membrane.
Osmosis:
Osmosis is the spontaneous movement of water across selectively permeable membranes. It’s really just another form of diffusion, but if the substance(s) dissolved in the water cannot cross the membrane, then water will move across the membrane until concentrations on both sides of the membrane are equal.
[b]Solutions, Tonicity, and Osmosis: Solutions, as you know, consist of solutes dissolved into fluid solvents. Since water is the solvent in almost all biological fluids, if the solute(s) won’t cross cellular membranes and concentrations on the opposite sides of a cellular membrane are different, then osmosis will occur. The less water there is in a given solution (in other words, the more concentrated is the solute), the higher is the solution’s osmotic pressure and so the faster is the flow of water across the membrane.
A hypertonic solution is a solution that has a greater solute concentration than does a living cell. Cells that find themselves in a hypertonic solution lose water, because water diffuses from where it’s more concentrated (inside the cells) to where it’s less concentrated (outside the cells). If the cells lose too much water, they’ll be killed.
This is one reason why it’s not a good idea to drink seawater. The salt content of your body tissues is 0.85%, whereas the salt content of seawater is generally about 3.5%, so seawater is a hypertonic solution, so far as your body tissues are concerned, and it will tend to draw water out of them. (Contrary to what a lot of people think, your body tissues do not have the same salt concentration as does seawater.) The kidneys can excrete the excess salt, but they cannot make urine that has a greater salt concentration than seawater has, so they must dilute the salt you absorb with water in order to excrete it. In other words, drinking enough seawater will ultimately cause you to lose water from body tissues.
An isotonic solution is one that has the same solute concentration as a living cell. A cell placed into an isotonic solution will neither gain nor lose water. You may have heard mention of Ringer’s lactate on medical shows from time to time. Ringer’s lactate is an isotonic solution that’s often given intravenously to people who’ve suffered serious blood loss. Since it’s isotonic to body tissues, it won’t cause them to either absorb too much water or lose water. So if someone has lost a lot of blood, giving them Ringer’s lactate is a good way to quickly restore fluid volume and so keep the blood pressure up. (If your blood pressure becomes too low because of fluid loss, all sorts of problems can occur. For instance, your kidneys will stop functioning.)
Normal saline is a 0.9% salt solution, so it’s isotonic to body tissues. Burn victims are often wrapped in cloth that has been soaked in normal saline, because the normal saline won’t draw water out of damaged and exposed tissues, nor will it cause them to become overhydrated, as pure water would.
A hypotonic solution is one that has a lower solute concentration than does a living cell (and thus a higher water concentration). A living cell placed into a hypotonic solution will absorb water, perhaps until it bursts. (Taking a saltwater fish out of its aquarium and tossing it into a tank of fresh water will generally kill it, because the cells of the poor fish’s gills will absorb so much water so quickly that they may rupture.)
Strange as it may seem, it’s entirely possible to drink too much water, though it’s a condition that’s usually seen only in athletes such as marathon runners. The condition that results is called “water intoxication” or hyponatremia. Drinking too much water, especially when you’re exercising heavily and excreting lots of salt in your sweat, can cause your cells to absorb so much water that the body’s electrolyte balance is affected. The salt content of the blood falls to a point that nerve, heart, and muscle cells can no longer function properly, and death can result.
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Tonicity and cells. In a hypotonic solution, a cell gains water. An animal
cell may gain so much water that it undergoes lysis (that is, it bursts).
A plant cell will not undergo lysis, because its rigid cell wall prevents that.
In an isotonic solution, a cell gains and loses water at the same rate. In
a hypertonic solution, a cell loses water.
[b]Active Transport of Substances Across Cellular Membranes: When substances are transported across cellular membranes actively, the cells must expend energy in the process. Usually, it’s because the cells are transporting substances against the concentration gradient. Active transport involves either the use of membrane proteins to transport substances, or movement of the cellular membrane itself. Glucose is a good example of a molecule that is actively transported into cells against the concentration gradient.
Active Transport:
It may be a little confusing, but when carrier proteins temporarily bind to molecules and physically transport them across cellular membranes against the concentration gradient, the process is called “active transport.” In other words, “active transport” is a kind of active transport.
Endocytosis and Exocytosis:
Endocytosis and exocytosis are the other form of active transport. Endocytosis and exocytosis are basically the same process, but run in different directions. Both involve movement of the cellular membranes themselves, and movement of vesicles or vacuoles within the cytoplasm.
“Endo” means “inside,” and “cyto” means “cell.” So endocytosis means to bring something inside of a cell. Specifically, endocytosis occurs when cells engulf substances with their plasma membranes and bring them inside the cytoplasm in vacuoles or vesicles. For instance, when an amoeba engulfs a victim, it is performing endocytosis. Substances brought into a cell through endocytosis are enclosed within a vacuole or vesicle, which can travel through the cytoplasm. Lysosomes may fuse with the vesicle and release digestive enzymes into it.
If the substance engulfed by the cell is relatively large, and especially if it’s solid, endocytosis is referred to as phagocytosis (“cell eating”). For example, some white blood cells can engulf and destroy cells nearly as large as themselves.
If the substance engulfed by the cell is relatively small, and especially if it’s a liquid, endocytosis is referred to as pinocytosis (“cell drinking”).
Some substances will be brought into a cell through endocytosis only if they first bind to receptor proteins in the cell’s membrane. This is known as receptor-mediated endocytosis.
“Exo” means “outside,” so exocytosis is precisely the opposite of endocytosis. In exocytosis, a vacuole or vesicle containing some substance to be eliminated is transported to the plasma membrane, where it fuses with the membrane. When a vesicle or fuses with the plasma membrane, it is incorporated into it and therefore ceases to exist. This leaves the substance formerly contained within the vacuole or vesicle on the outside of the cell.
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Endocytosis (top) and Exocytosis (bottom)
[B]Cellular Connections and Communications: In multicellular organisms such as humans, cells must be knit together somehow if they are to form a single organism. Imagine what would happen if they weren’t!
(As an aside, the cells of sponges are only loosely interconnected. You can force a living sponge through some terrycloth and break it up into its component cells. Not only can you do this without killing the cells, but they’ll eventually crawl back together and re-form the sponge. It’s a pretty nifty trick to perform in Marine Biology classes, and it forces you to reconsider your notions of what constitutes an “individual” organism.)
In order to coordinate bodily functions, the cells of a multicellular organism must be able to communicate with each other. Specialized connections between cells allow them to exchange chemicals directly, greatly enhancing the ability of adjacent cells to communicate with each other and to coordinate their activities.
Desmosomes: Desmosomes are protein/carbohydrate structures that penetrate the plasma membranes of adjacent cells and hold them together in much the same way that a nail holds together two pieces of wood. Some space remains between the cells, however, and so fluid can circulate around and between cells linked by desmosomes.
If the desmosomes connecting skin cells fail to function properly, layers of skin cells can pull apart, and fluid can accumulate between the separated cell layers. This causes blisters to form under the skin surface.
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Desmosomes, made largely of the protein keratin, penetrate
the membranes of adjacent cells and loosely knit them together.
Tight Junctions: Tight junctions are protein strands that bind cells tightly together and seal the spaces between them. Tight junctions join cells together so tightly that even water has difficulty passing between them. A good place to find tight junctions is in the cells lining the urinary bladder; they join the cells together so tightly that urine cannot leak out of the bladder and into surrounding body tissues.
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Tight junctions knit cells together much more closely than
desmosomes do, so even water cannot pass between them.
Gap Junctions: Gap junctions are protein channels that penetrate the plasma membranes of adjacent cells and allow them to directly exchange substances between them. A substance contained in one cell can quickly be transferred to another cell through gap junctions. If several layers of cells are connected by gap junctions, nutrients and other substances can be quickly and easily transferred some distance through tissues, even substances that cannot easily cross plasma membranes.
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Gap junctions are channels that penetrate cellular membranes,
allowing adjacent cells to easily exchange materials.
[B]Cellular Reproduction and the Cell Cycle: Cells reproduce by duplicating their genetic material and then physically dividing. The cell cycle is the time period from one cellular division to the next.
During cellular division, a single parent cell divides into two daughter cells. As the cell divides, a complete set of genetic information is transferred to each of the daughter cells. In addition, the essential cytoplasmic materials are transferred to each daughter cell, including mitochondria. So long as each daughter cell receives some endoplasmic reticulum and some mitochondria, it can synthesize most of the remaining organelles.
Humans, like virtually all animals, are diploid. This means that normal body cells have two sets of paired chromosomes in their nuclei. Each chromosome is a single DNA molecule. For any particular trait, you have genes on two different chromosomes – one chromosome that you inherited from your mother and one that you inherited from your father. The two chromosomes that contain the genes governing some particular trait are homologous chromosomes.
As it happens, humans have 23 pairs of chromosomes. Of those, 22 pairs are known as autosomes, and are inherited the same way in men and women. The remaining pair are known as the sex chromosomes, because they’re inherited differently in men and women. If you have two “X”-shaped sex chromosomes, you’re a female, and if you have an “X” chromosome plus a smaller “Y” chromosome, you’re a male. (A woman can only give her child an “X” chromosome, but a man can give his child either an “X” or a “Y” chromosome. So it’s the father’s genetic contribution that determines the sex of a child.)
During most of a cell’s life cycle, the DNA in the nucleus is not readily visible. Instead, it is in an unraveled state and is called chromatin. Just before the cell is ready to reproduce, it duplicates all of its DNA. The DNA then condenses to form the chromosomes. In this condensed, highly compacted form, DNA cannot function, so cells don’t spend much time in this state. (At this point, the cell briefly contains not the normal two sets, but four sets of DNA, and is tetraploid.)
The normal process of cellular reproduction is called mitosis. In mitosis, a diploid parent cell duplicates its DNA and is briefly tetraploid. It then divides into two daughter cells, each of which is genetically identical to the original parent cell.
Sex cells (gametes) are produced by a variation of mitosis known as meiosis. Meiosis starts out like mitosis in that a diploid cell duplicates its DNA and is briefly tetraploid. The tetraploid cell divides to produce two daughter cells, and then the daughter cells divide again – but without duplicating their DNA first. The end result is four haploid cells (each has only one set of DNA) – none of which are genetically identical.
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[B]Mitosis involves a single cell division and produces 2 diploid daughter cells
Meiosis involves two cell divisions and produces 4 haploid daughter cells
The Cell Cycle: A cell that is not in the process of reproducing is said to be in interphase. A cell that is in the process of reproducing is said to be in mitosis.
Interphase: Cells spend most of their existence in interphase. This is the stage in a cell’s life cycle during which the cell performs its normal growth and maintenance functions. Near the end of interphase, a cell duplicates its genetic material and prepares for reproduction. Because the DNA is being used for protein synthesis, it is unraveled and takes the form of chromatin during interphase.
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A Cell in [B]Interphase
Interphase has three sub-phases. The G1 (Growth) Phase is that portion of interphase before the cell duplicates its genetic material The S (Synthesis) Phase is that portion of interphase during which the cell duplicates its DNA and its centrioles. The G2 Phase is the portion of interphase during which the cell prepares to enter into mitosis and divide.
Mitosis: Strictly speaking, mitosis is the division of a cell’s nucleus. After mitosis, the cell itself typically divides as well, but this is not always the case. Mitosis has four stages: prophase, metaphase, anaphase, and telophase.
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A Cell Undergoing [B]Mitosis and Cytokinesis
During prophase, the chromatin in the parent cell’s nucleus condenses to form the chromosomes. Each chromosome consists of two identical strands of DNA linked by a structure called a centromere. Each of the identical DNA molecules that makes up a chomsosome is known as a chromatid.
As prophase progresses, the nucleolus and the nuclear envelope disintegrate. The centrioles begin to move toward the opposite ends of the cell (its poles), stretching spindle fibers between them (the spindle fibers are made of microtubules). The chromosomes attach to the spindle fibers on both sides of their centromeres as the centrioles move apart.
At metaphase, the spindle fibers attached to the moving centrioles have pulled the chromosomes into a line along the equator of the cell.
At the beginning of anaphase, the tension from the spindle fibers pulling on both sides of the centromeres causes them to split, and the chromosomes begin moving toward the opposite poles of the cell. (You’ll remember that each chromosome consisted of two identical DNA molecules, so when the centromeres split, this ensures that each daughter cell will wind up with an identical set of DNA.)
During telophase, the chromosomes reach the poles of the cell and begin to unravel back into chromatin. As the chromosomes unravel, the nuclear envelope and nucleolus reappear. The spindle fibers begin to break down. At this point, cytokinesis may begin to occur.
Cytokinesis: Cytokinesis is the physical division of a single parent cell into two daughter cells. It generally begins to occur during telophase or even late in anaphase when the cell begins to pinch together. (This is called furrowing.) The cell continues to pinch together, growing ever narrower in the center until it splits into two daughter cells.
[B]Meiosis: Meiosis is the process by which gametes are formed. Unlike normal body cells, a gamete is haploid and contains only a single set of DNA. The union of a haploid male gamete (spermatozoan) and a haploid female gamete (ovum) at conception forms a diploid zygote that has the potential to grow into an adult human.
Meiosis I: The first part of meiosis is virtually indistinguishable from mitosis. A single tetraploid cell divides into two daughter cells, each of which is diploid. A closer look reveals some important differences, however. For one thing, the two daughter cells produced during Meiosis I are not genetically identical.
During Prophase I of meiosis, spindle fibers attach to only one side of each chromosome’s centromere. This means that the centromeres won’t be split during the first cytokinesis.
The truly unique event during Prophase I is that the homologous chromosomes come together and literally swap parts of themselves with each other, apparently at random. This process is called crossing over and it ensures that the daughter cells produced after the first cytokinesis will not be genetically identical.
Metaphase I in meiosis is very much like metaphase in mitosis.
During Anaphase I in meiosis, the homologous chromosomes are separated, but because spindle fibers are attached to only one side of each centromere, the chromosomes are not broken apart into their separate chromatids like they are in anaphase of mitosis.
Following Telophase I of meiosis, the parent cell undergoes cytokinesis and divides to form two diploid daughter cells. Because of crossing over during Prophase I, however, the daughter cells are not genetically identical.
After the first cytokinesis, the daughter cells do not go into interphase, nor do they duplicate their DNA. Instead, they go right into Meiosis II.
Meiosis II: During Prophase II, spindle fibers attach to both sides of each chromosome’s centromere. This ensures that when cytokinesis occurs, the centromeres will be broken and the chromatids will be separated.
During Anaphase II, the centromeres split, so each of the daughter cells will wind up with a single set of DNA.
During the second cytokinesis, four cells are formed. Thanks to the fact that there was no duplication of DNA between Meiosis I and Meiosis II, each of these four cells is haploid, rather than diploid. And thanks to crossing over during Prophase I, none of the four cells is genetically identical.
In fact, given the tens of thousands of genes in the human genome, and given the fact that the process of crossing over seems to involve (more or less) random swapping of genetic material between homologous chromosomes, it’s a safe bet that none of the tens of thousands of ova an individual human female produces during her lifetime will be genetically identical. Perhaps one or two of the hundreds of billions of spermatozoa a human male produces in his lifetime will happen to be genetically identical. Still, the literally astronomical number of possible gene combinations a single man and a single woman can produce ensures that there is a tremendous amount of potential genetic variability in their children.
Chapter Four: Cell Structure and Function
Introduction: In 1665, the English scientist Robert Hooke observed thin slices of cork under a microscope and noted that the wood was made of tiny, box-like structures that he named “cells.” Subsequent observations by Hooke and other scientists showed that not just plants, but also fungi and animals were made up of cells. In fact, all living creatures appeared to be made of cells.
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Hooke’s drawing of “cells” in a piece of cork.
In 1839, German biologists Matthias Schleiden and Theodor Schwann formulated the Cell Theory of Life, which has four claims. First, according to the cell theory, all living organisms consist of one or more cells. (Since a virus is not cellular, according to the cell theory, viruses are not alive.) Second, all cells arise through division of preexisting cells. Third, all vital functions of an organism occur within its cells. Finally, cells contain the hereditary information necessary both for regulating cellular function and for reproduction.
Basic Cellular Structure: All cells have at least three components: a plasma membrane, genetic material, and cytoplasm.
The plasma membrane (or cell membrane) regulates the flow of materials into and out of the cell, allows interactions between cells, and protects the interior of the cell from the external environment.
The genetic material provides the “recipe” for making the enzymes and structural proteins that regulate cellular functions and make up much of the internal structure of the cell. In all living organisms, genetic material consists of DNA, and RNA is used to “translate” information stored in DNA into proteins. (Many viruses use RNA as their primary genetic material.) In eukaryotic cells, the DNA is contained within a structure called the nucleus of the cell.
The cytoplasm of the cell is the substance between the plasma membrane and the nucleus. The cytoplasm of a eukaryotic cell consists of various organelles embedded within cytosol. The organelles are smaller subunits within a cell that perform specific functions. The cytosol consists of a watery solution of proteins, carbohydrates, lipids, and electrolytes.
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A generalized animal cell. Cells contain genetic material
and cytoplasm, and are enclosed by a plasma membrane.
Physical Constraints on Cell Size: The relationship between an object’s surface area and its volume determines how large a cell can grow. Because surface area is a square function, if you double an object’s size, its surface area increases by a factor of 22 or four times. Volume, however, is a cubic function, so if you double an object’s size, its volume increases by 23 or eight times. This means that as objects grow larger, their volumes increase at a much faster rate than do their surface areas.
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If its proportions remain the same, as an object grows larger,
its volume increases much faster than does its surface area.
What does this have to do with cells? Well, it’s important to remember that everything that comes into a cell – oxygen, food, water, etc. – must come in across the plasma membrane. Similarly everything that is expelled from a cell – poisonous metabolic waste products, for example – must leave across the plasma membrane. This means that the size of the plasma membrane sets a limit to how quickly a cell can absorb and excrete materials.
So, the size of a cell’s plasma membrane determines how quickly it can absorb necessary substances like oxygen and nutrients, and how quickly it can expel dangerous substances like CO2. The problem is that the size of the cell’s plasma membrane is a function of the cell’s surface area.
On the other hand, the amount of material inside the cell that requires oxygen and nutrients – and that generates dangerous waste products like CO2 – is a function of the cell’s volume.
So as a cell increases in size, it quickly reaches a point where it simply isn’t possible for it to grow any larger and survive. If it were to grow any larger, the cell’s relatively small surface area relative to its relatively large volume would mean that it couldn’t bring in oxygen and nutrients (and expel poisonous metabolic wastes) fast-enough to keep itself alive.
So if an organism is to grow large, it must be made of many small cells, each of which has sufficient surface area relative to its volume to keep itself supplied with nutrients and oxygen, and to avoid poisoning by its own metabolic wastes. Movies such as The Blob that feature enormous single-celled organisms wandering about the countryside and devouring unwary teenagers are pure fantasy. No single-celled organism could get even close to that size. In fact, the largest single-celled organisms are just barely visible to the naked eye – if you have good eyesight, that is.
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Not that big a threat, actually.
One of the advantages of being multicellular is that body cells can be specialized to perform different functions. This allows multicellular organisms to be much more efficient than if each cell in the body were identical and performed identical functions.
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An organism can increase its surface area while keeping its volume
constant by dividing into lots of smaller subunits (i.e. cells).
[B]Prokaryotic and Eukaryotic Cells: Prokaryotes are organisms with prokaryotic cells. (Quelle surprise, eh?) Almost certainly, the earliest living organisms were prokaryotes. You’d probably find it less than surprising to learn that eukaryotes are organisms with eukaryotic cells.
Prokaryotic cells are small and relatively simple cells with no distinct nuclei or other organelles. Bacteria and Archaea are the only surviving prokaryotes. There are no known prokaryotes that are truly multicellular, perhaps because their cells are insufficiently complex to be capable of coming together to form a multicellular organism.
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A prokaryotic cell. Note that there are no complex organelles, and
that the genetic material is more or less dispersed throughout the
cytoplasm, rather than being confined in a nuclues.
Eukaryotic cells are generally much larger than are prokaryotic cells, and are much more complex. A eukaryotic cell has a nucleus containing its genetic material, and other organelles within its cytoplasm. These organelles apparently make eukaryotic cells much more efficient than are prokaryotic cells, and eukaryotic cells can grow to much larger sizes than can prokaryotic cells.
Also unlike prokaryotic cells, eukaryotic cells can also cluster together to form multicellular organisms that are far larger than any single cell could possibly become. Protists, fungi, plants, and animals are all eukaryotic organisms. Many protists and fungi are multicellular, and all plants and animals are multicellular by definition. (An amoeba is not a “single-celled animal,” as you sometimes hear it called; it’s a protist. Unless you’re talking about a zygote, a “single-celled animal” is a contradiction in terms.)
[B]The Organelles of Eukaryotic Cells: Humans are eukaryotes. Specifically, we’re animals. So, let’s take a look at the makeup of a typical animal cell. It’s worth keeping in mind that since different cells are specialized to perform different tasks, many will look quite different from a “typical” cell. For example, a skeletal muscle cell looks very different from a neuron. Still, there are a great many more similarities than differences.
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A Typical Animal Cell
[B]The Nucleus: Control Center of the Cell: The nucleus of the cell contains its genetic material. Here is where the DNA is stored that contains the information the cell uses to make its proteins. Since these proteins make up much of the internal structure of the cell and (in the form of enzymes) control its metabolism, the nucleus is often referred to as the “control center” of the cell. A cell cannot survive for very long without its nucleus. (Red blood cells or erythrocytes lose their nuclei as they mature; as you would expect, a mature erythrocyte does not live for very long.)
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The nucleus of a eukaryotic cell.
The nucleus is surrounded by a nuclear envelope (or nuclear membrane) that is virtually identical in makeup to the plasma membrane that surrounds the cell itself. In fact, the nuclear envelope and the plasma membrane are actually continuous with each other, but we’ll get to that in a bit.
Just as the plasma membrane determines what gets in and out of a cell, the nuclear envelope determines what passes into and out of the nucleus of the cell. One distinction between the nuclear envelope and the plasma membrane is that the nuclear envelope has numerous relatively large holes in it known as nuclear pores. These pores allow relatively large molecules such as proteins and RNA to cross the nuclear envelope.
Under a microscope, the interior of the nucleus usually looks somewhat granular. The grainy substance is called chromatin, and it consists of unraveled DNA molecules and their associated protein molecules. Sections of DNA molecules can be “unzipped” and “transcribed” to make molecules of messenger-RNA, which then transport the information necessary for protein synthesis into the cytoplasm of the cell, where protein synthesis actually occurs. Because DNA never leaves the nucleus of the cell, the danger that it will be damaged during the production of proteins is greatly lessened.
Cells reproduce by dividing in two. When a cell is ready to reproduce, the chromatin condenses into structures known as chromosomes. So if you can see chromosomes in the nucleus of the cell, it is either in the process of reproducing or it is preparing for reproduction.
You may be able to see a darker structure inside the nucleus called the nucleolus (“little nucleus”). The nucleolus is made mostly of RNA and protein, and it’s where structures called ribosomes are assembled. After they’re assembled in the nucleolus, ribosomes are transported out of the nucleus and into the cytoplasm of the cell, where they’re important in protein synthesis.
[B]The Endomembrane System of the Cell: All eukaryotic cells have an elaborate system of membranes surrounding the cell and extending throughout the cytoplasm. This endomembrane system consists of the plasma membrane, plus several internal organelles, including the endoplasmic reticulum, the nuclear envelope, the Golgi complex, and smaller structures such as lysosomes and vesicles. As you’ve probably realized, this means that the endomembrane system connects every part of the cell.
These membranes are phospholipid bilayers that have cholesterol and various proteins embedded in them. Because the membranes of different organelles are made of the same substance, they can readily fuse. This allows different organelles within a cell to easily exchange their contents.
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The endomembrane system of a typical animal cell.
The Plasma Membrane: The plasma membrane surrounds the cell and separates it from the external environment. The membrane is selectively permeable, meaning that it allows some substances to cross but not others. In this way, the plasma membrane controls what substances can get into and out of the cell.
The Endoplasmic Reticulum: The endoplasmic reticulum is a series of membranous channels that extends throughout the cytoplasm of the cell. The ER is continuous with both the plasma membrane and the nuclear envelope, and so allows communication between all parts of the cell and the external environment.
Rough endoplasmic reticulum has small organelles called ribosomes embedded in its membranes. The ribosomes are the sites of protein synthesis, and so rough ER manufactures proteins and then transports them to other portions of the cell, or transports them to the plasma membrane where they can be exported from the cell.
Smooth endoplasmic reticulum lacks ribosomes and so does not manufacture proteins. Instead, it synthesizes lipids and some other organic molecules, which can then be transported elsewhere in the cell or to the plasma membrane for export.
Since ER can synthesize all the components of cellular membranes, it can synthesize itself, as well as the membranous portions of other cellular organelles.
The Golgi Complex: The Golgi complex (or Golgi bodies or Golgi apparatus) are often described as looking like stacked pancakes when viewed under a sufficiently powerful microscope. The Golgi bodies are closely associated with the endoplasmic reticulum, and in fact, are formed by it. If the endoplasmic reticulum is the “factory” of a cell, where most cellular components are manufactured, the Golgi complex is the “finishing plant.” In the Golgi complex, molecules synthesized in the endoplasmic reticulum are chemically altered into their final form and then packaged into small membranous structures called vesicles. The vesicles then transport the finished molecules through the cytoplasm to other parts of the cell, or to the plasma membrane and out of the cell. (To continue the analogy, that would make the vesicles the “delivery trucks” of the cell.)
Lysosomes: Lysosomes are relatively small membranous organelles in the cell that contain digestive enzymes. When food is brought into a cell inside vesicles, lysosomes can fuse with the vesicles and release their digestive enzymes into the vesicles to digest the food. Once it has been broken down into its component molecules by the lysozome’s digestive enzymes, the food is then be absorbed into the cytoplasm of the cell.
White blood cells (leukocytes) are cells that are specialized for engulfing and destroying bacteria and other foreign cells. As you might expect, most leukocytes contain large numbers of lysosomes, allowing them to quickly and efficiently digest and destroy viruses, bacteria, and other invaders.
If the lysosomes of a cell rupture, the digestive enzymes they release will digest and destroy the cell itself. This process is called autolysis, and it normally occurs when cells are damaged or soon after they die. Some degenerative disorders, such as Alzheimer’s disease and Bovine Spongiform Encephalopathy (“Mad Cow Disease”), may be the result of foreign agents triggering autolysis and causing destruction of body tissues.
Vacuoles: Vacuoles are relatively large membranous sacs which cells use for (relatively) long-term storage of materials, especially liquids. Vacuoles can be formed by endoplasmic reticulum or can be formed from the plasma membrane. If you’ve ever seen an amoeba engulf a smaller organism by flowing around it, the sac that encased the amoeba’s unfortunate victim and was formed from the amoeba’s plasma membrane was a vacuole. Lysosomes can fuse with vacuoles and release digestive enzymes into them, allowing digestion of the substances contained in the vacuoles.
Vesicles: Vesicles are smaller membranous structures than are vacuoles. As mentioned earlier, they’re used to contain and transport substances within the cell. There’s really no clear distinction between vesicles and vacuoles except that vesicles are smaller.
Vesicles can be formed by endoplasmic reticulum or by Golgi bodies. Since the membrane surrounding a vesicle is the same substance that makes up the plasma membrane, if a vesicle should come into contact with the plasma membrane, the vesicle will be incorporated into the plasma membrane, and so its contents will be emptied to the outside of the cell. In this way, vesicles can be used to transport substances out of the cell.
Similarly, relatively small objects can be brought into a cell by vesicle formation. If the plasma membrane folds inward and pinches off, it will form a vesicle, and so substances can be transported into the cell from the outside. Once it forms, the vesicle can then transport the substance it contains to any other part of the cell, or it can fuse with lysosomes so that the substance it contains is digested.
[B]Mitochondria: Though they are indeed membranous, the mitochondria are not formed by or associated with the endoplasmic reticulum, Golgi bodies, or other membranous organelles. As such, mitochondria are not considered part of the endomembrane system of a cell.
A mitochondrion looks somewhat “sausage-shaped” under a microscope, and it has an elaborately folded inner membrane. The mitochondria are where aerobic (cellular) respiration occurs. That is, the mitochondria are where carbohydrate molecules are broken down for the energy that powers the cell’s metabolism.
Interestingly, mitochondria have their own DNA, and they reproduce independently of the rest of the cell. Analysis of mitochondrial DNA shows that it’s actually much more similar to the DNA of certain bacteria than it is to the DNA in the nucleus of the cell within which it resides.
Given the similarity of mitochondrial DNA to bacterial DNA, it is thought that mitochondria are the descendants of bacterial cells that – at some time in the distant past – either invaded larger cells or were engulfed by them. In either case, these cells weren’t destroyed in the process. Instead, the bacterial cells remained within the larger cells and the arrangement turned out to be mutually beneficial. The bacterial cells gained a (relatively) safe and stable haven, and the larger cells benefited from the energy the bacterial cells produced when they broke down carbohydrates.
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A mitochondrion within a cell.
[B]Ribosomes: Ribosomes are tiny organelles composed of RNA (specifically, ribosomal-RNA) and protein. Messenger-RNA produced in the nucleus by copying information from DNA travels into the cytoplasm of the cell and to ribosomes, where the information it contains is used to manufacture proteins. Ribosomes are found embedded within the membranes of “rough” endoplasmic reticulum and mitochondria, but they are also scattered throughout the cytoplasm of the cell.
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A ribosome, under extremely high magnification. Each
ribosome consists of two subunits called, somewhat
unimaginatively, the “large subunit” and the “small subunit.”
[B]The Cytoskeleton: The cytoskeleton is a three-dimensional network of protein fibers that extends throughout the cell. These protein fibers help to support the cell and they give the cell its shape. The cytoskeleton helps to hold the various organelles in place, and it plays an important role in movement of cells. Three principle types of proteins make up the cytoskeleton: microfilaments, intermediate fibers, and microtubules.
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The Cytoskeleton
Microfilaments are thin fibers that are made mostly of the protein actin. Microfilament fibers can change length when supplied with energy by ATP, and so these fibers allow cells to change shape, and they also guide the movements of organelles within the cell. Interactions of actin and the protein myosin are important in contraction of muscle cells.
Intermediate fibers (intermediate filaments) are somewhat larger in diameter than are microfilaments, and are made of several different proteins. Intermediate fibers form a 3-D network throughout the cell that forms much of the framework of the cell and holds the organelles in position.
Microtubules are about 24 nanometers in diameter, somewhat thicker than microfilaments or intermediate fibers. Microtubules are hollow and composed largely of the protein tubulin. Microtubules help to position, anchor, and move organelles. They are also important in cellular reproduction and in the movement of cells. Microtubules are important components of both cilia and flagella, and of centrioles.
[B]Propulsive Organelles: Cilia and flagella are extensions of the plasma membrane that are supported by microtubules. Both cilia and flagella can be moved back-and-forth, and either move the cell itself or move fluid past the cell.
Cilia are relatively short and numerous structures. The cells of many microorganisms are ciliated, and these organisms swim by beating their cilia. Paramecium is a well-known example of a ciliated swimmer. In humans, cells lining the respiratory tract and the female reproductive tract are ciliated. The ciliated cells in the respiratory tract help to keep mucus moving, and thereby prevent it from blocking respiratory passages. Ciliated cells in the female reproductive tract create currents that draw ova down the reproductive tract and to the uterus.
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Ciliated cells lining the human respiratory tract.
Flagella are nearly identical to cilia, but they’re much longer and less-numerous. A typical flagellated cell has only a single flagellum, though some have more. The best-known of flagellated cells is surely the spermatozoan. A sperm cell uses its flagellum to swim up the female reproductive tract in search of an ovum.
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A spermatozoan, one of the few flagellated cells in the human body.
Centrioles: Centrioles are structures that, like cilia and flagella, are composed mostly of microtubules. In fact, specialized centrioles called basal bodies manufacture cilia and flagella.
Centrioles are also important in cellular reproduction. A cell typically has two centrioles located near the nucleus and oriented at right angles to each other. Just before a cell is ready to reproduce, it duplicates its centrioles. During cellular reproduction, the centrioles produce spindle fibers that organize the movement of the cell’s chromosomes.
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A pair of centrioles. Each centriole is made up of nine microtubule triplets.
[B]Structure and Function of the Plasma Membrane: Every cell interacts with its external environment through its plasma membrane. The membrane makes it possible for a cell to defend itself, to exchange materials with its environment, and to communicate with other cells. In order to understand how cells can perform these functions, we must take a closer look at the structure of the plasma membrane.
[B]The Fluid Mosaic Model of Membrane Structure: A cellular membrane is a fluid made of a phospholipid bilayer with inclusions of cholesterol (a steroid) and protein molecules. Because cellular membranes are so fluid, they can easily change shape.
A phospholipid molecule, as you recall, has a polar “head” and two nonpolar “tails.” Because the heads are attracted to water and the tails are not, phospholipids spontaneously form a bilayer in water, with the heads pointing out towards the water and the tails pointing inward, away from the water. On its own, a phospholipid bilayer has roughly the consistency of a soap bubble. Cholesterol molecules embedded in the bilayer strengthen and stiffen it, making it much less likely to rupture.
Because of their charged nature, most ionic and polar substances cannot easily cross cellular membranes, since they typically lack the energy to force their way through the nonpolar interior portion of the membrane. This is one of the reasons that cellular membranes are so selective about what can cross them.
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The fluid mosaic model of a cellular membrane.
Membrane Proteins: Broadly speaking, there are three different kinds of proteins embedded within cellular membranes. These are transport proteins, receptor proteins, and recognition proteins.
Transport Proteins: Transport proteins regulate the movement of molecules into and out of cells.
Channel proteins are transport proteins that take the form of hollow tubes penetrating cellular membranes. When present, they effectively open holes in membranes, and so allow substances that would not normally be able to cross the plasma membrane to enter or exit cells. Channel proteins do not cause substances to move against the concentration gradient. Some channel proteins, called “gate proteins” or “gated channels,” can change shape, and so the channels they create can be opened or closed at need to regulate the flow of substances into and out of the cell.
Carrier proteins can form temporary bonds with molecules and so can pull substances across cellular membranes. By taking energy from ATP molecules, carrier proteins can even transport substances against the concentration gradient.
Receptor Proteins: Receptor proteins can bind to hormone molecules or other chemical messengers, and cause changes inside the cell. For example, hormones released into the blood by one organ can attach to binding sites on receptor proteins in the plasma membranes of the cells of the “target organ.” When hormones bind to receptor proteins, the proteins change shape. This triggers changes in the interior of the target cells. In this way, hormones can cause changes in cells without ever entering them
Recognition Proteins: Most recognition proteins consist of carbohydrates bound to proteins to form glycoproteins. The exact arrangement of glycoproteins in the membranes of your body cells is determined by your unique genetic makeup (assuming you aren’t an identical twin). Because the glycoproteins in your cellular membranes are distinctive, your immune system has a means of recognizing any “foreign” cells that might be present – bacterial cells, for instance. Some of your white blood cells (leukocytes) systematically “test” the cells they encounter, and attempt to destroy any that have the wrong recognition proteins in their membranes.
Since cancer is often caused by genetic mutations, cancerous cells often fail to produce the proper recognition proteins. As a result, the immune system is even capable of detecting and destroying cancer cells in many instances. This is despite the fact that the cancerous cells have the same basic genetic makeup as the rest of your body cells.
[B]Transport of Substances Across Cellular Membranes: When you consider the movement of substances across cellular membranes, there are three things to keep in mind. First of all, molecules are always in motion, even if they’re simply vibrating in place. Second, in a fluid such as water (which makes up well over 90% of a typical cell), the molecules are free to move and are not constrained to vibrate in place. Finally, movement of molecules from areas of high concentration to areas of low concentration is spontaneous.
Let’s consider that last point for a moment. Remember that the molecules in a fluid are moving randomly – in other words, in all directions. If you concentrate the molecules, they will tend to spread out again until they fill the available space uniformly.
Maybe it’s easier to understand why this occurs if we consider only two dimensions for now, instead of three. Imagine I have a container that is divided into two halves by a partition. Now imagine that I put a 10% sugar solution into the right half of the container. (This means that 10% of the solution is sugar, and 90% is water. If you prefer, you could think of it as a 90% water solution.) Now imagine that I put a 50% sugar solution (i.e. a 50% water solution) into the left half of the container. What will happen if I remove the partition that separates the solutions?
Since the solution in the left half of the container contains five times the sugar as does the solution in the right half of the container, this means that for every sugar molecule that happens to be moving from the right half of the container to the left half, there will be five sugar molecules (on average) moving from left to right. So, sugar molecules will tend to migrate from the left half of the container, where they’re more concentrated, to the right half of the container, where they’re less concentrated. This will continue until the sugar concentrations in both halves of the container are the same.
When the concentration of sugar molecules in both sides of the container is equal, the solution is said to be in equilibrium. At that point, for every sugar molecule moving from left to right, there will be one moving from right to left, and so there will be no net movement of sugar molecules, even though the individual molecules will still be moving about quite energetically.
(Incidentally, the water molecules would be moving too. The water was initially more concentrated in the right side of the container, so there would be a net movement of water from right to left, until the concentrations were equal.)
This spontaneous movement of molecules in solution from high concentration to low concentration until equilibrium is reached is called diffusion.
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Diffusion across a cellular membrane.
(Click “refresh” to see the animation.)
Just for fun, what would happen if the partition were selectively permeable, and allowed water to cross but not sugar? Well, if the left half of the container holds a 50% sugar solution, that means it holds a 50% water solution. And if the right half of the container holds a 10% sugar solution, that means it holds a 90% water solution. Let’s assume that I carefully poured equal volumes of solution into each half of the container, so that the levels were exactly equal.
Since water is more concentrated on the right side of the partition, there will be a net flow of water to the left side of the container. But since the sugar cannot cross the partition, there will be no corresponding flow of sugar to the right.
So, the level of solution in the left side of the container will rise and the level of solution in the right side of the container will fall as water moves to the left. In effect, the solution in the left side of the container will absorb water from the solution in the right side of the container. This will continue until the two solutions reach the same concentration or until gravity halts the rise of solution in the left side of the container.
Osmosis is the diffusion of water across a selectively-permeable membrane when the solute dissolved in it cannot cross the membrane. As you can see, osmosis is just a variation of diffusion, since the water molecules are diffusing from where they are more concentrated to where they are less concentrated.
Osmosis is typically described as diffusion of water from low solute concentration to high solute concentration, so it might sound a little confusing at first. But remember that if a solution’s solute concentration is low, that means its water concentration is high, and if its solute concentration is high, then its water concentration is low. Just remember that the water always diffuses from where it is more concentrated to where it is less concentrated, and osmosis should make sense.
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Osmosis is the diffusion of water across a selectively-permeable
membrane, from low solute concentration (high water concentration)
to high solute concentration (low water concentration).
[b]Passive Transport of Substances Across Cellular Membranes: When substances are transported across cellular membranes passively, the cells do not expend any energy to move the substances in question. The three kinds of passive transport are diffusion, facilitated diffusion, and osmosis.
Diffusion:
Diffusion occurs when water, O2, CO2 and other such small molecules spontaneously cross cellular membranes as they move from areas of high concentration to areas of low concentration.
Most polar molecules cannot cross cellular membranes, because they cannot traverse the nonpolar central region of the membrane. Water, however, can readily cross cellular membranes despite its polar nature. This is because water molecules are so small that they can slip right between the relatively enormous phospholipid molecules that make up the membrane.
As they go about their normal activities, cells consume oxygen and produce carbon dioxide. As a result, concentration of oxygen in the blood is generally higher than it is inside cells. For that reason, oxygen diffuses from the blood and into cells. By contrast, since cells produce carbon dioxide, CO2 concentration in cells is generally higher than it is in the blood. As a result, CO2 diffuses out of cells and into the blood.
Facilitated Diffusion:
Facilitated diffusion occurs when carrier and channel proteins allow substances to cross cellular membranes from high concentration to low concentration, even though the substances in question cannot normally pass through the hydrophobic interior portion of the cellular membrane.
Osmosis:
Osmosis is the spontaneous movement of water across selectively permeable membranes. It’s really just another form of diffusion, but if the substance(s) dissolved in the water cannot cross the membrane, then water will move across the membrane until concentrations on both sides of the membrane are equal.
[b]Solutions, Tonicity, and Osmosis: Solutions, as you know, consist of solutes dissolved into fluid solvents. Since water is the solvent in almost all biological fluids, if the solute(s) won’t cross cellular membranes and concentrations on the opposite sides of a cellular membrane are different, then osmosis will occur. The less water there is in a given solution (in other words, the more concentrated is the solute), the higher is the solution’s osmotic pressure and so the faster is the flow of water across the membrane.
A hypertonic solution is a solution that has a greater solute concentration than does a living cell. Cells that find themselves in a hypertonic solution lose water, because water diffuses from where it’s more concentrated (inside the cells) to where it’s less concentrated (outside the cells). If the cells lose too much water, they’ll be killed.
This is one reason why it’s not a good idea to drink seawater. The salt content of your body tissues is 0.85%, whereas the salt content of seawater is generally about 3.5%, so seawater is a hypertonic solution, so far as your body tissues are concerned, and it will tend to draw water out of them. (Contrary to what a lot of people think, your body tissues do not have the same salt concentration as does seawater.) The kidneys can excrete the excess salt, but they cannot make urine that has a greater salt concentration than seawater has, so they must dilute the salt you absorb with water in order to excrete it. In other words, drinking enough seawater will ultimately cause you to lose water from body tissues.
An isotonic solution is one that has the same solute concentration as a living cell. A cell placed into an isotonic solution will neither gain nor lose water. You may have heard mention of Ringer’s lactate on medical shows from time to time. Ringer’s lactate is an isotonic solution that’s often given intravenously to people who’ve suffered serious blood loss. Since it’s isotonic to body tissues, it won’t cause them to either absorb too much water or lose water. So if someone has lost a lot of blood, giving them Ringer’s lactate is a good way to quickly restore fluid volume and so keep the blood pressure up. (If your blood pressure becomes too low because of fluid loss, all sorts of problems can occur. For instance, your kidneys will stop functioning.)
Normal saline is a 0.9% salt solution, so it’s isotonic to body tissues. Burn victims are often wrapped in cloth that has been soaked in normal saline, because the normal saline won’t draw water out of damaged and exposed tissues, nor will it cause them to become overhydrated, as pure water would.
A hypotonic solution is one that has a lower solute concentration than does a living cell (and thus a higher water concentration). A living cell placed into a hypotonic solution will absorb water, perhaps until it bursts. (Taking a saltwater fish out of its aquarium and tossing it into a tank of fresh water will generally kill it, because the cells of the poor fish’s gills will absorb so much water so quickly that they may rupture.)
Strange as it may seem, it’s entirely possible to drink too much water, though it’s a condition that’s usually seen only in athletes such as marathon runners. The condition that results is called “water intoxication” or hyponatremia. Drinking too much water, especially when you’re exercising heavily and excreting lots of salt in your sweat, can cause your cells to absorb so much water that the body’s electrolyte balance is affected. The salt content of the blood falls to a point that nerve, heart, and muscle cells can no longer function properly, and death can result.
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Tonicity and cells. In a hypotonic solution, a cell gains water. An animal
cell may gain so much water that it undergoes lysis (that is, it bursts).
A plant cell will not undergo lysis, because its rigid cell wall prevents that.
In an isotonic solution, a cell gains and loses water at the same rate. In
a hypertonic solution, a cell loses water.
[b]Active Transport of Substances Across Cellular Membranes: When substances are transported across cellular membranes actively, the cells must expend energy in the process. Usually, it’s because the cells are transporting substances against the concentration gradient. Active transport involves either the use of membrane proteins to transport substances, or movement of the cellular membrane itself. Glucose is a good example of a molecule that is actively transported into cells against the concentration gradient.
Active Transport:
It may be a little confusing, but when carrier proteins temporarily bind to molecules and physically transport them across cellular membranes against the concentration gradient, the process is called “active transport.” In other words, “active transport” is a kind of active transport.
Endocytosis and Exocytosis:
Endocytosis and exocytosis are the other form of active transport. Endocytosis and exocytosis are basically the same process, but run in different directions. Both involve movement of the cellular membranes themselves, and movement of vesicles or vacuoles within the cytoplasm.
“Endo” means “inside,” and “cyto” means “cell.” So endocytosis means to bring something inside of a cell. Specifically, endocytosis occurs when cells engulf substances with their plasma membranes and bring them inside the cytoplasm in vacuoles or vesicles. For instance, when an amoeba engulfs a victim, it is performing endocytosis. Substances brought into a cell through endocytosis are enclosed within a vacuole or vesicle, which can travel through the cytoplasm. Lysosomes may fuse with the vesicle and release digestive enzymes into it.
If the substance engulfed by the cell is relatively large, and especially if it’s solid, endocytosis is referred to as phagocytosis (“cell eating”). For example, some white blood cells can engulf and destroy cells nearly as large as themselves.
If the substance engulfed by the cell is relatively small, and especially if it’s a liquid, endocytosis is referred to as pinocytosis (“cell drinking”).
Some substances will be brought into a cell through endocytosis only if they first bind to receptor proteins in the cell’s membrane. This is known as receptor-mediated endocytosis.
“Exo” means “outside,” so exocytosis is precisely the opposite of endocytosis. In exocytosis, a vacuole or vesicle containing some substance to be eliminated is transported to the plasma membrane, where it fuses with the membrane. When a vesicle or fuses with the plasma membrane, it is incorporated into it and therefore ceases to exist. This leaves the substance formerly contained within the vacuole or vesicle on the outside of the cell.
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Endocytosis (top) and Exocytosis (bottom)
[B]Cellular Connections and Communications: In multicellular organisms such as humans, cells must be knit together somehow if they are to form a single organism. Imagine what would happen if they weren’t!
(As an aside, the cells of sponges are only loosely interconnected. You can force a living sponge through some terrycloth and break it up into its component cells. Not only can you do this without killing the cells, but they’ll eventually crawl back together and re-form the sponge. It’s a pretty nifty trick to perform in Marine Biology classes, and it forces you to reconsider your notions of what constitutes an “individual” organism.)
In order to coordinate bodily functions, the cells of a multicellular organism must be able to communicate with each other. Specialized connections between cells allow them to exchange chemicals directly, greatly enhancing the ability of adjacent cells to communicate with each other and to coordinate their activities.
Desmosomes: Desmosomes are protein/carbohydrate structures that penetrate the plasma membranes of adjacent cells and hold them together in much the same way that a nail holds together two pieces of wood. Some space remains between the cells, however, and so fluid can circulate around and between cells linked by desmosomes.
If the desmosomes connecting skin cells fail to function properly, layers of skin cells can pull apart, and fluid can accumulate between the separated cell layers. This causes blisters to form under the skin surface.
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Desmosomes, made largely of the protein keratin, penetrate
the membranes of adjacent cells and loosely knit them together.
Tight Junctions: Tight junctions are protein strands that bind cells tightly together and seal the spaces between them. Tight junctions join cells together so tightly that even water has difficulty passing between them. A good place to find tight junctions is in the cells lining the urinary bladder; they join the cells together so tightly that urine cannot leak out of the bladder and into surrounding body tissues.
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Tight junctions knit cells together much more closely than
desmosomes do, so even water cannot pass between them.
Gap Junctions: Gap junctions are protein channels that penetrate the plasma membranes of adjacent cells and allow them to directly exchange substances between them. A substance contained in one cell can quickly be transferred to another cell through gap junctions. If several layers of cells are connected by gap junctions, nutrients and other substances can be quickly and easily transferred some distance through tissues, even substances that cannot easily cross plasma membranes.
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Gap junctions are channels that penetrate cellular membranes,
allowing adjacent cells to easily exchange materials.
[B]Cellular Reproduction and the Cell Cycle: Cells reproduce by duplicating their genetic material and then physically dividing. The cell cycle is the time period from one cellular division to the next.
During cellular division, a single parent cell divides into two daughter cells. As the cell divides, a complete set of genetic information is transferred to each of the daughter cells. In addition, the essential cytoplasmic materials are transferred to each daughter cell, including mitochondria. So long as each daughter cell receives some endoplasmic reticulum and some mitochondria, it can synthesize most of the remaining organelles.
Humans, like virtually all animals, are diploid. This means that normal body cells have two sets of paired chromosomes in their nuclei. Each chromosome is a single DNA molecule. For any particular trait, you have genes on two different chromosomes – one chromosome that you inherited from your mother and one that you inherited from your father. The two chromosomes that contain the genes governing some particular trait are homologous chromosomes.
As it happens, humans have 23 pairs of chromosomes. Of those, 22 pairs are known as autosomes, and are inherited the same way in men and women. The remaining pair are known as the sex chromosomes, because they’re inherited differently in men and women. If you have two “X”-shaped sex chromosomes, you’re a female, and if you have an “X” chromosome plus a smaller “Y” chromosome, you’re a male. (A woman can only give her child an “X” chromosome, but a man can give his child either an “X” or a “Y” chromosome. So it’s the father’s genetic contribution that determines the sex of a child.)
During most of a cell’s life cycle, the DNA in the nucleus is not readily visible. Instead, it is in an unraveled state and is called chromatin. Just before the cell is ready to reproduce, it duplicates all of its DNA. The DNA then condenses to form the chromosomes. In this condensed, highly compacted form, DNA cannot function, so cells don’t spend much time in this state. (At this point, the cell briefly contains not the normal two sets, but four sets of DNA, and is tetraploid.)
The normal process of cellular reproduction is called mitosis. In mitosis, a diploid parent cell duplicates its DNA and is briefly tetraploid. It then divides into two daughter cells, each of which is genetically identical to the original parent cell.
Sex cells (gametes) are produced by a variation of mitosis known as meiosis. Meiosis starts out like mitosis in that a diploid cell duplicates its DNA and is briefly tetraploid. The tetraploid cell divides to produce two daughter cells, and then the daughter cells divide again – but without duplicating their DNA first. The end result is four haploid cells (each has only one set of DNA) – none of which are genetically identical.
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[B]Mitosis involves a single cell division and produces 2 diploid daughter cells
Meiosis involves two cell divisions and produces 4 haploid daughter cells
The Cell Cycle: A cell that is not in the process of reproducing is said to be in interphase. A cell that is in the process of reproducing is said to be in mitosis.
Interphase: Cells spend most of their existence in interphase. This is the stage in a cell’s life cycle during which the cell performs its normal growth and maintenance functions. Near the end of interphase, a cell duplicates its genetic material and prepares for reproduction. Because the DNA is being used for protein synthesis, it is unraveled and takes the form of chromatin during interphase.
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A Cell in [B]Interphase
Interphase has three sub-phases. The G1 (Growth) Phase is that portion of interphase before the cell duplicates its genetic material The S (Synthesis) Phase is that portion of interphase during which the cell duplicates its DNA and its centrioles. The G2 Phase is the portion of interphase during which the cell prepares to enter into mitosis and divide.
Mitosis: Strictly speaking, mitosis is the division of a cell’s nucleus. After mitosis, the cell itself typically divides as well, but this is not always the case. Mitosis has four stages: prophase, metaphase, anaphase, and telophase.
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A Cell Undergoing [B]Mitosis and Cytokinesis
During prophase, the chromatin in the parent cell’s nucleus condenses to form the chromosomes. Each chromosome consists of two identical strands of DNA linked by a structure called a centromere. Each of the identical DNA molecules that makes up a chomsosome is known as a chromatid.
As prophase progresses, the nucleolus and the nuclear envelope disintegrate. The centrioles begin to move toward the opposite ends of the cell (its poles), stretching spindle fibers between them (the spindle fibers are made of microtubules). The chromosomes attach to the spindle fibers on both sides of their centromeres as the centrioles move apart.
At metaphase, the spindle fibers attached to the moving centrioles have pulled the chromosomes into a line along the equator of the cell.
At the beginning of anaphase, the tension from the spindle fibers pulling on both sides of the centromeres causes them to split, and the chromosomes begin moving toward the opposite poles of the cell. (You’ll remember that each chromosome consisted of two identical DNA molecules, so when the centromeres split, this ensures that each daughter cell will wind up with an identical set of DNA.)
During telophase, the chromosomes reach the poles of the cell and begin to unravel back into chromatin. As the chromosomes unravel, the nuclear envelope and nucleolus reappear. The spindle fibers begin to break down. At this point, cytokinesis may begin to occur.
Cytokinesis: Cytokinesis is the physical division of a single parent cell into two daughter cells. It generally begins to occur during telophase or even late in anaphase when the cell begins to pinch together. (This is called furrowing.) The cell continues to pinch together, growing ever narrower in the center until it splits into two daughter cells.
[B]Meiosis: Meiosis is the process by which gametes are formed. Unlike normal body cells, a gamete is haploid and contains only a single set of DNA. The union of a haploid male gamete (spermatozoan) and a haploid female gamete (ovum) at conception forms a diploid zygote that has the potential to grow into an adult human.
Meiosis I: The first part of meiosis is virtually indistinguishable from mitosis. A single tetraploid cell divides into two daughter cells, each of which is diploid. A closer look reveals some important differences, however. For one thing, the two daughter cells produced during Meiosis I are not genetically identical.
During Prophase I of meiosis, spindle fibers attach to only one side of each chromosome’s centromere. This means that the centromeres won’t be split during the first cytokinesis.
The truly unique event during Prophase I is that the homologous chromosomes come together and literally swap parts of themselves with each other, apparently at random. This process is called crossing over and it ensures that the daughter cells produced after the first cytokinesis will not be genetically identical.
Metaphase I in meiosis is very much like metaphase in mitosis.
During Anaphase I in meiosis, the homologous chromosomes are separated, but because spindle fibers are attached to only one side of each centromere, the chromosomes are not broken apart into their separate chromatids like they are in anaphase of mitosis.
Following Telophase I of meiosis, the parent cell undergoes cytokinesis and divides to form two diploid daughter cells. Because of crossing over during Prophase I, however, the daughter cells are not genetically identical.
After the first cytokinesis, the daughter cells do not go into interphase, nor do they duplicate their DNA. Instead, they go right into Meiosis II.
Meiosis II: During Prophase II, spindle fibers attach to both sides of each chromosome’s centromere. This ensures that when cytokinesis occurs, the centromeres will be broken and the chromatids will be separated.
During Anaphase II, the centromeres split, so each of the daughter cells will wind up with a single set of DNA.
During the second cytokinesis, four cells are formed. Thanks to the fact that there was no duplication of DNA between Meiosis I and Meiosis II, each of these four cells is haploid, rather than diploid. And thanks to crossing over during Prophase I, none of the four cells is genetically identical.
In fact, given the tens of thousands of genes in the human genome, and given the fact that the process of crossing over seems to involve (more or less) random swapping of genetic material between homologous chromosomes, it’s a safe bet that none of the tens of thousands of ova an individual human female produces during her lifetime will be genetically identical. Perhaps one or two of the hundreds of billions of spermatozoa a human male produces in his lifetime will happen to be genetically identical. Still, the literally astronomical number of possible gene combinations a single man and a single woman can produce ensures that there is a tremendous amount of potential genetic variability in their children.