The Lone Ranger
01-07-2007, 06:04 AM
An Introduction to Human Anatomy and Physiology
Chapter Three: Energy Flow in the Life of a Cell
Introduction: Chemical reactions either release or absorb energy as they proceed. Among the fundamental characteristics of living organisms is their ability to guide and direct the various chemical reactions within their cells. These chemical reactions generate and utilize the energy that organisms need in order to maintain themselves.
The ultimate source of energy for living organisms is almost always sunlight. Plants, algae and some bacteria are known as autotrophs (“self-feeders”) because they can use molecules such as chlorophyll and rhodopsin to absorb solar energy. The captured solar energy is then used to build the monosaccharide glucose from carbon dioxide and water. This is photosynthesis. In photosynthesis, solar energy is used to assemble a single molecule of glucose from six carbon dioxide molecules and six water molecules; six molecules of diatomic oxygen are released as a byproduct.
Photosynthesis: 6CO2 + 6H2O + sunlight energy → C6H12O6 + 6O2
http://www.freethought-forum.com/forum/gallery/files/5/0/photosynthesis.jpg
Plants use glucose to store energy for later use. Animals and other heterotrophs (“other-feeders”) acquire glucose either by eating autotrophs or by eating other heterotrophs. So ultimately, all heterotrophs are dependent on autotrophs for their food, either directly or indirectly.
Organisms decompose glucose in the process called cellular (aerobic) respiration. This releases the stored solar energy that was originally used to manufacture the glucose during photosynthesis. The released energy is used to drive the various chemical reactions that living organisms depend upon for growth, maintenance and repair of their bodies.
Chemically, aerobic respiration is precisely the opposite of photosynthesis. Aerobic respiration is accomplished by adding six oxygen molecules to a glucose molecule. This oxidation of glucose causes it to decompose into six carbon dioxide molecules and six water molecules. Because there is less energy stored in the chemical bonds of the carbon dioxide and water than was stored in the bonds of the glucose, the “excess” energy is released and can be used to do work. In other words, the breaking of a glucose molecule’s chemical bonds releases the energy that was originally used to manufacture it.
[B]Aerobic Respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Organisms use the molecule adenosine triphosphate (ATP) to transfer energy from one part of a cell to another, so ATP is known as an energy carrier molecule. The energy released during aerobic respiration is used to add a phosphate group to a molecule of adenosine diphosphate, making ATP. (ADP, of course, has two phosphate groups, and ATP has three.) The ATP can then be transported to wherever it is needed by the cell, and when the third phosphate group is split off, the energy released is used to power chemical reactions.
[B]ATP Synthesis: ADP + Phosphate Group (Pi) + energy → ATP
http://www.freethought-forum.com/images/anatomy3/atp.jpg
A molecule of adenosine triphosphate consists of a molecule of
the sugar ribose bound to three phosphate groups on one side,
and the nitrogenous base adenine on the other side.
http://www.freethought-forum.com/forum/gallery/files/5/0/adp_atp_original.jpg
In the cells of a living organism, energy is used to add a third phosphate group to a molecule of
adenosine diphosphate (ADP), forming a molecule of adenosine triphosphate (ATP). The ATP can
then be transported to wherever the cell needs energy. When the third phosphate group is
split off the ATP to regenerate ADP, energy is released that can be used to do work.
[B]The Nature of Energy:Energy, as noted earlier, is the capacity to do mechanical work – that is, to move matter. Organisms need an energy supply because they need energy to build and maintain body structures. On a more basic level, organisms need energy to power the chemical reactions in which these body structures are built and maintained.
Kinds of Energy:
Kinetic energy is the energy of movement. Anything that is moving has kinetic energy. For instance, light (electromagnetic energy) is moving photons. Heat (thermal energy) is the random movement of atoms, ions, and molecules. Electricity is moving electrons.
Potential energy is energy that is stored in some way. Chemical bonds are a form of potential energy, because when they’re broken, the energy that was used to make them is released. The electrical energy stored in batteries is potential energy, because it can be converted to electricity when a circuit is completed. When work is done to lift an object against the pull of gravity, the energy is stored as positional energy in the object itself. If the object is then released, it falls and its potential energy is converted to kinetic energy.
http://www.freethought-forum.com/forum/gallery/files/5/0/potential_and_kinetic_energy_original.jpg
[b]The Laws of Thermodynamics and You: Thermodynamics is the study of energy – specifically, it is the study of how energy moves and how it causes movement in matter. The laws of thermodynamics describe the behavior of energy.
The First Law of Thermodynamics tells us that energy can be neither created nor destroyed. Energy can be converted from one form to another, such as conversion of electricity to light, but energy cannot be created, nor can it be destroyed. This means that there’s a limited amount of energy available to living systems. Fortunately, there’s no concern about running out anytime soon.
The Second Law of Thermodynamics tells us that when energy is converted from one form to another, some of the usable energy in the system is lost – usually in the form of heat.
Between them, the first and second laws tell us that while the total amount of energy in a closed system is constant, the amount of usable energy in that system decreases over time. Sadly, there is no such thing as 100% efficiency in energy conversion – some energy is always lost when it is converted from one form to another. That’s one of the reasons why a “perpetual motion machine” is physically impossible – no one has every built one, and no one ever will.
Consider your car. It burns gasoline to produce energy. Gasoline consists of various types of hydrocarbon molecules blended together. Hydrocarbons are molecules that contain hydrogen and carbon only. When gasoline is burned in your car’s engine, it is combined with oxygen, and the resulting chemical reactions convert the hydrocarbons into water (H2O) and carbon dioxide (CO2). Since the chemical bonds of the products (H2O) and CO2) have less energy than did the bonds of the reactants (hydrocarbons), this chemical reaction is exothermic and releases energy.
Not all of the energy released when gasoline is burned goes into moving your car. Much of it is wasted as sound, and much of the remainder is released as heat. Only a fraction of the energy released actually goes to moving your car.
The same is true of human beings. When you decompose food molecules for energy, most of the energy released is not used to build body tissues or to provide power for movement. Instead, most of the energy that is released is lost, mostly in the form of heat.
One consequence of the Second Law is that the amount of disorder or entropy of a closed system tends to increase over time. Strictly, the entropy of a system is the amount of energy in the system that cannot be used to perform work. For practical purposes, it can be seen as the amount of disorder or randomness in the system.
We’re all familiar with this phenomenon. Highly-ordered systems spontaneously become more disordered over time unless we import energy into them and use it to maintain them. For instance, your house gets messy over time and you must therefore expend energy to dust, vacuum and tidy it. Similarly, your car breaks down over time, and you must expend energy to maintain and repair it.
The upshot of this is that living organisms, being highly complex and highly organized, need to use energy in order to maintain themselves. Otherwise, their complex and highly-organized molecular arrangements will begin to spontaneously break down, and homeostasis will no longer be possible. We call an organism that is no longer using energy to maintain itself “dead.”
http://www.freethought-forum.com/forum/gallery/files/5/0/entropy_original.jpg
Robert Heinlein came up with the acronym TANSTAAFL, which stands for “There Ain’t No Such Thing As A Free Lunch.” That’s actually a pretty good summary of the significance of the Second Law to living organisms. Living creatures, in order to survive, must consume energy. But the body tissues you build and repair with that energy will not remain in their highly-ordered state, so you must continue to consume energy if you are to survive. As soon as the cycle is broken, life ceases.
Natural processes tend to proceed in such a direction that the entropy of the Universe increases. “Life,” someone once said, “is a continual battle against the Second Law of Thermodynamics.” But in the end, entropy always wins.
So, eat, drink and be merry, for sooner or later, you will die!
[BREAK=Chemical Reactions and Energy]
[B]Chemical Reactions and Energy:Energy is used to make chemical bonds. As chemical bonds are broken, the energy that was used to make them is released. Since chemical reactions involve the making and breaking of chemical bonds, the reactions either release or absorb energy, depending upon whether there’s more energy in the bonds of the reactants or in the products of the reaction.
Exergonic (exothermic) reactions (such as aerobic respiration), as you recall, are those in which the chemical bonds in the products of the reaction store less energy than do the bonds in the reactants. The “excess” energy is released as the chemical reactions take place, and so exergonic reactions release energy.
Endergonic (endothermic) reactions (such as photosynthesis), are those in which the chemical bonds in the products store more energy than do the bonds in the reactants. Such reactions can only progress by absorbing energy from their surroundings.
Your metabolism functions because your body couples reactions. What this means is that exergonic reactions are used to power the endergonic reactions that build body structures.
http://www.freethought-forum.com/forum/gallery/files/5/0/coupled.jpg
In coupled reactions, an exergonic reaction
provides the energy for an endergonic reaction.
[B]Factors that Affect the Rates of Chemical Reactions: In order for covalent bonds to form, atoms or molecules must be brought sufficiently close together to overcome the repulsion of the negatively-charged electrons surrounding them, (remember that like charges repel), so that they can merge their orbitals. Because heat causes molecules to move faster, most chemical reactions therefore occur at faster rates when temperatures are warmer. This is because faster-moving molecules are more likely to “bump into” each other, and are more likely to do so with sufficient kinetic energy to overcome the repulsive forces generated by their electrons.
The amount of energy that reactants must have before they will chemically react is known as the activation energy of a chemical reaction. A spontaneous reaction is one in which enough energy is available in the normal environment for the reaction to proceed. For example, oxygen in the atmosphere spontaneously combines with iron to form rust – you don’t have to heat the iron or add a catalyst in order for this to happen. This is why iron objects must be protected from atmospheric oxygen. Of course, other factors being equal, iron will rust faster in a warmer environment than in a cold environment.
An example of a chemical reaction that is not spontaneous is the oxidation of wood. That is, wood does not spontaneously burst into flames at room temperature; it must be heated to a considerably higher temperature before the molecules of the wood have enough energy that they’ll begin combining with oxygen.
Certain substances can lower the activation energy of chemical reactions, and so greatly increase the rates at which those reactions occur. These substances, if they are not consumed themselves in the reactions, are known as catalysts. You’ll notice that catalysts don’t actually cause chemical reactions to occur; they simply make it easier for them to occur, by reducing the amount of energy that must be supplied before the reaction will proceed. So, while catalysts speed up the rates at which chemical reactions occur, they will not cause a reaction to occur if that reaction is energetically unfavorable.
http://www.freethought-forum.com/forum/gallery/files/5/0/catalyst.jpg
[B]Enzymes: Enzymes, as you recall, are organic molecules – proteins, generally – that function as catalysts. Without enzymes, the chemical reactions that support life would occur at only a small fraction of the rate necessary to keep us alive. Enzymes tend to be very specific as to which chemical reactions they facilitate, and so by changing the amounts and activities of specific enzymes, a cell can regulate its metabolism very precisely.
Enzyme Structure: Generally speaking, enzymes are thought to work by bringing reactants sufficiently close together that they can react with each other. The molecules in question (known as the substrate molecules) temporarily bind with the enzyme at what is known as the enzyme’s active site. Since the substrate molecules change shape when they react, they will no long fit into the enzyme’s active site. So after the chemical reaction occurs, the products are released. The enzyme is then free to bind more substrate and the reaction can occur again.
Because the function of an enzyme is utterly dependent on its shape, anything that causes the enzyme molecule to change its shape will either alter or destroy its ability to function as a catalyst. As you may recall, even a slight change in temperature, pH or salinity can cause a protein to change shape or denature. This is one of the most important reasons why it’s so crucial for living organisms to maintain blood pH, body temperature, electrolyte balance, and other factors within very narrow ranges. Even small departures from the normal range of values can impact enzyme function with fatal results.
Enzyme Function: One hypothesis regarding how enzymes function is known as the “lock-and-key” model. According to this hypothesis, a particular enzyme is shaped in such a way that the substrate molecule(s) fit into the active site(s) exactly, much as a key fits exactly into a particular lock. Substrate molecules are attracted to the enzyme’s active sites, and are thus brought close together. When the substrates bond to each other, this changes their overall shape, and so they no longer fit into the enzyme’s active site and are released.
http://www.freethought-forum.com/images/anatomy3/lockandkey.jpg
The “Lock-and-Key” Model of Enzyme Function
http://www.freethought-forum.com/forum/gallery/files/5/0/enzymes_original.jpg
This enzyme catalyzes the hydrolysis of sucrose into glucose and fructose.
Enzyme Cofactors: Some enzymes consist simply of a protein molecule that functions as a catalyst, while others must be bound to other substances before they can function as catalysts. The other substances to which some enzymes must be bound before they can function are known as cofactors.
A cofactor may be inorganic or organic. Some metals, for instance, function as enzyme cofactors, and so are necessary parts of your diet. Organic cofactors are known as coenzymes. Many vitamins, for example, function as coenzymes, and so are essential parts of the diet. A well-known example of a coenzyme is Coenzyme A, which plays a crucial role in regulating the chemical reactions of aerobic respiration.
http://www.freethought-forum.com/forum/gallery/files/5/0/coa.jpg
Coenzyme A, an important regulator of aerobic respiration.
[break=Regulation of Enzyme Activity]
Regulation of Enzyme Activity: One way that living cells can regulate the activity of enzymes is by controlling how much of a given enzyme is synthesized. You may recall that enzymes are proteins, and that the “instructions” for making proteins are encoded in the DNA of a cell’s nucleus. So, by regulating how many copies of a given enzyme are made, the cell can regulate the rate at which the chemical reactions it mediates proceed.
Cells can also temporarily activate or inhibit the function of enzymes that are present, and so alter the rates at which chemical reactions take place. Typically, this occurs when the products of the chemical reactions mediated by the enzyme(s) in question function to inhibit the enzyme’s activity. This is known as feedback inhibition, because the products of the enzyme influence the enzyme’s function. (This will hopefully sound familiar, as it’s an example of negative feedback.)
Allosteric regulation occurs when the products of a chemical reaction bind to some portion of the enzyme other than the active site, and in so doing, change the shape of the enzyme. By changing the enzyme’s shape, they alter its function.
Imagine that enzyme “A” catalyzes a reaction that produces product “B.” If “B” binds to the enzyme and changes its shape, the enzyme will either start to catalyze a different reaction and so produce a different product (product “C”), or it will simply cease to function. So, the more of product “B” is present in the cell, the less of it will be produced.
Competitive regulation is similar to allosteric regulation, except that the product(s) of the reaction bind to the enzyme’s active site, and so stop it from functioning. As a result, the more of the product is present, the less substrate can bind to the enzyme and be converted to product, and so the less product is produced.
These forms of inhibition allow cells to very precisely regulate the amounts of product formed by enzyme-mediated chemical reactions.
[break=Naming Enzymes]
Naming Enzymes: Generally speaking, enzymes are named according to what they do, and the last part of the enzyme’s name consists of the suffix -ase. For example, the enzyme that catalyzes the breakdown of the alcohol ethanol is known as alcohol dehydrogenase.
Amylases are enzymes that catalyze the breakdown of complex carbohydrates. The next time you eat some crackers, you may notice that if you hold them in your mouth for some time, they begin to taste sweet. That’s because the amylase in your saliva breaks the starch in the crackers down into the sugar maltose. Proteases are enzymes that accelerate the breakdown of proteins into their component amino acids. Lipases catalyze the breakdown of fats and other lipids. Nucleases catalyze the breakdown of nucleic acids. And so forth.
[break=Coupled Reactions and Energy-Carrier Molecules]
[B]Coupled Reactions and Energy-Carrier Molecules: Many of the chemical reactions in organisms’ metabolisms are endergonic. How then, do cells manage to get those reactions going, since these reactions absorb energy as they progress? Cells do so by employing coupled reactions, in which exergonic reactions provide the energy necessary to power endergonic reactions. In this way, metabolic pathways often involve many chemical reactions that are closely interlinked, and the products of one reaction become the reactants in the next reaction. The synthesis of particularly large organic molecules, for example, might easily involve dozens, hundreds, or even thousands of closely-interlinked chemical reactions.
As you recall, the principle molecule used by living organisms as an energy carrier, is ATP. Ultimately, breakdown of glucose is used to provide the energy for powering the various endergonic reactions in a cell’s metabolism. ATP transports energy from where it is produced to where it is needed, in order for necessary chemical reactions to proceed.
Another way in which energy is transported within cells is by electron carriers. Some metabolic reactions produce high-energy electrons, which could damage cellular components if left free. Electron acceptors are molecules that can accept these electrons, and so prevent them from doing damage. However, it would be a shame to waste all that energy, wouldn’t it? So, metabolic pathways have evolved in which high-energy electrons are passed from one molecule to another, and at each step in the progression, some of the electrons’ energy is removed and used to make ATP. Electron carrier molecules can thus be used to transport energy from one part of a cell to another, in much the same way that ATP does.
Removal of energy from high-energy electrons to generate ATP is a key component of aerobic respiration. Ultimately, the “spent” electrons are captured by hydrogen and oxygen atoms to make water, which is one of the reasons why oxygen is necessary for aerobic respiration.
http://www.freethought-forum.com/forum/gallery/files/5/0/nad_original.jpg
NAD+ (nicotinamide adenine dinudeotide) is a coenzyme that functions as an electron-carrier.
It can bind to and transport hydrogen ions and electrons. When NAD+ is reduced through gain
of electrons, it forms NADH. Oxidation of NADH produces NAD+ and releases free electrons.
Chapter Three: Energy Flow in the Life of a Cell
Introduction: Chemical reactions either release or absorb energy as they proceed. Among the fundamental characteristics of living organisms is their ability to guide and direct the various chemical reactions within their cells. These chemical reactions generate and utilize the energy that organisms need in order to maintain themselves.
The ultimate source of energy for living organisms is almost always sunlight. Plants, algae and some bacteria are known as autotrophs (“self-feeders”) because they can use molecules such as chlorophyll and rhodopsin to absorb solar energy. The captured solar energy is then used to build the monosaccharide glucose from carbon dioxide and water. This is photosynthesis. In photosynthesis, solar energy is used to assemble a single molecule of glucose from six carbon dioxide molecules and six water molecules; six molecules of diatomic oxygen are released as a byproduct.
Photosynthesis: 6CO2 + 6H2O + sunlight energy → C6H12O6 + 6O2
http://www.freethought-forum.com/forum/gallery/files/5/0/photosynthesis.jpg
Plants use glucose to store energy for later use. Animals and other heterotrophs (“other-feeders”) acquire glucose either by eating autotrophs or by eating other heterotrophs. So ultimately, all heterotrophs are dependent on autotrophs for their food, either directly or indirectly.
Organisms decompose glucose in the process called cellular (aerobic) respiration. This releases the stored solar energy that was originally used to manufacture the glucose during photosynthesis. The released energy is used to drive the various chemical reactions that living organisms depend upon for growth, maintenance and repair of their bodies.
Chemically, aerobic respiration is precisely the opposite of photosynthesis. Aerobic respiration is accomplished by adding six oxygen molecules to a glucose molecule. This oxidation of glucose causes it to decompose into six carbon dioxide molecules and six water molecules. Because there is less energy stored in the chemical bonds of the carbon dioxide and water than was stored in the bonds of the glucose, the “excess” energy is released and can be used to do work. In other words, the breaking of a glucose molecule’s chemical bonds releases the energy that was originally used to manufacture it.
[B]Aerobic Respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Organisms use the molecule adenosine triphosphate (ATP) to transfer energy from one part of a cell to another, so ATP is known as an energy carrier molecule. The energy released during aerobic respiration is used to add a phosphate group to a molecule of adenosine diphosphate, making ATP. (ADP, of course, has two phosphate groups, and ATP has three.) The ATP can then be transported to wherever it is needed by the cell, and when the third phosphate group is split off, the energy released is used to power chemical reactions.
[B]ATP Synthesis: ADP + Phosphate Group (Pi) + energy → ATP
http://www.freethought-forum.com/images/anatomy3/atp.jpg
A molecule of adenosine triphosphate consists of a molecule of
the sugar ribose bound to three phosphate groups on one side,
and the nitrogenous base adenine on the other side.
http://www.freethought-forum.com/forum/gallery/files/5/0/adp_atp_original.jpg
In the cells of a living organism, energy is used to add a third phosphate group to a molecule of
adenosine diphosphate (ADP), forming a molecule of adenosine triphosphate (ATP). The ATP can
then be transported to wherever the cell needs energy. When the third phosphate group is
split off the ATP to regenerate ADP, energy is released that can be used to do work.
[B]The Nature of Energy:Energy, as noted earlier, is the capacity to do mechanical work – that is, to move matter. Organisms need an energy supply because they need energy to build and maintain body structures. On a more basic level, organisms need energy to power the chemical reactions in which these body structures are built and maintained.
Kinds of Energy:
Kinetic energy is the energy of movement. Anything that is moving has kinetic energy. For instance, light (electromagnetic energy) is moving photons. Heat (thermal energy) is the random movement of atoms, ions, and molecules. Electricity is moving electrons.
Potential energy is energy that is stored in some way. Chemical bonds are a form of potential energy, because when they’re broken, the energy that was used to make them is released. The electrical energy stored in batteries is potential energy, because it can be converted to electricity when a circuit is completed. When work is done to lift an object against the pull of gravity, the energy is stored as positional energy in the object itself. If the object is then released, it falls and its potential energy is converted to kinetic energy.
http://www.freethought-forum.com/forum/gallery/files/5/0/potential_and_kinetic_energy_original.jpg
[b]The Laws of Thermodynamics and You: Thermodynamics is the study of energy – specifically, it is the study of how energy moves and how it causes movement in matter. The laws of thermodynamics describe the behavior of energy.
The First Law of Thermodynamics tells us that energy can be neither created nor destroyed. Energy can be converted from one form to another, such as conversion of electricity to light, but energy cannot be created, nor can it be destroyed. This means that there’s a limited amount of energy available to living systems. Fortunately, there’s no concern about running out anytime soon.
The Second Law of Thermodynamics tells us that when energy is converted from one form to another, some of the usable energy in the system is lost – usually in the form of heat.
Between them, the first and second laws tell us that while the total amount of energy in a closed system is constant, the amount of usable energy in that system decreases over time. Sadly, there is no such thing as 100% efficiency in energy conversion – some energy is always lost when it is converted from one form to another. That’s one of the reasons why a “perpetual motion machine” is physically impossible – no one has every built one, and no one ever will.
Consider your car. It burns gasoline to produce energy. Gasoline consists of various types of hydrocarbon molecules blended together. Hydrocarbons are molecules that contain hydrogen and carbon only. When gasoline is burned in your car’s engine, it is combined with oxygen, and the resulting chemical reactions convert the hydrocarbons into water (H2O) and carbon dioxide (CO2). Since the chemical bonds of the products (H2O) and CO2) have less energy than did the bonds of the reactants (hydrocarbons), this chemical reaction is exothermic and releases energy.
Not all of the energy released when gasoline is burned goes into moving your car. Much of it is wasted as sound, and much of the remainder is released as heat. Only a fraction of the energy released actually goes to moving your car.
The same is true of human beings. When you decompose food molecules for energy, most of the energy released is not used to build body tissues or to provide power for movement. Instead, most of the energy that is released is lost, mostly in the form of heat.
One consequence of the Second Law is that the amount of disorder or entropy of a closed system tends to increase over time. Strictly, the entropy of a system is the amount of energy in the system that cannot be used to perform work. For practical purposes, it can be seen as the amount of disorder or randomness in the system.
We’re all familiar with this phenomenon. Highly-ordered systems spontaneously become more disordered over time unless we import energy into them and use it to maintain them. For instance, your house gets messy over time and you must therefore expend energy to dust, vacuum and tidy it. Similarly, your car breaks down over time, and you must expend energy to maintain and repair it.
The upshot of this is that living organisms, being highly complex and highly organized, need to use energy in order to maintain themselves. Otherwise, their complex and highly-organized molecular arrangements will begin to spontaneously break down, and homeostasis will no longer be possible. We call an organism that is no longer using energy to maintain itself “dead.”
http://www.freethought-forum.com/forum/gallery/files/5/0/entropy_original.jpg
Robert Heinlein came up with the acronym TANSTAAFL, which stands for “There Ain’t No Such Thing As A Free Lunch.” That’s actually a pretty good summary of the significance of the Second Law to living organisms. Living creatures, in order to survive, must consume energy. But the body tissues you build and repair with that energy will not remain in their highly-ordered state, so you must continue to consume energy if you are to survive. As soon as the cycle is broken, life ceases.
Natural processes tend to proceed in such a direction that the entropy of the Universe increases. “Life,” someone once said, “is a continual battle against the Second Law of Thermodynamics.” But in the end, entropy always wins.
So, eat, drink and be merry, for sooner or later, you will die!
[BREAK=Chemical Reactions and Energy]
[B]Chemical Reactions and Energy:Energy is used to make chemical bonds. As chemical bonds are broken, the energy that was used to make them is released. Since chemical reactions involve the making and breaking of chemical bonds, the reactions either release or absorb energy, depending upon whether there’s more energy in the bonds of the reactants or in the products of the reaction.
Exergonic (exothermic) reactions (such as aerobic respiration), as you recall, are those in which the chemical bonds in the products of the reaction store less energy than do the bonds in the reactants. The “excess” energy is released as the chemical reactions take place, and so exergonic reactions release energy.
Endergonic (endothermic) reactions (such as photosynthesis), are those in which the chemical bonds in the products store more energy than do the bonds in the reactants. Such reactions can only progress by absorbing energy from their surroundings.
Your metabolism functions because your body couples reactions. What this means is that exergonic reactions are used to power the endergonic reactions that build body structures.
http://www.freethought-forum.com/forum/gallery/files/5/0/coupled.jpg
In coupled reactions, an exergonic reaction
provides the energy for an endergonic reaction.
[B]Factors that Affect the Rates of Chemical Reactions: In order for covalent bonds to form, atoms or molecules must be brought sufficiently close together to overcome the repulsion of the negatively-charged electrons surrounding them, (remember that like charges repel), so that they can merge their orbitals. Because heat causes molecules to move faster, most chemical reactions therefore occur at faster rates when temperatures are warmer. This is because faster-moving molecules are more likely to “bump into” each other, and are more likely to do so with sufficient kinetic energy to overcome the repulsive forces generated by their electrons.
The amount of energy that reactants must have before they will chemically react is known as the activation energy of a chemical reaction. A spontaneous reaction is one in which enough energy is available in the normal environment for the reaction to proceed. For example, oxygen in the atmosphere spontaneously combines with iron to form rust – you don’t have to heat the iron or add a catalyst in order for this to happen. This is why iron objects must be protected from atmospheric oxygen. Of course, other factors being equal, iron will rust faster in a warmer environment than in a cold environment.
An example of a chemical reaction that is not spontaneous is the oxidation of wood. That is, wood does not spontaneously burst into flames at room temperature; it must be heated to a considerably higher temperature before the molecules of the wood have enough energy that they’ll begin combining with oxygen.
Certain substances can lower the activation energy of chemical reactions, and so greatly increase the rates at which those reactions occur. These substances, if they are not consumed themselves in the reactions, are known as catalysts. You’ll notice that catalysts don’t actually cause chemical reactions to occur; they simply make it easier for them to occur, by reducing the amount of energy that must be supplied before the reaction will proceed. So, while catalysts speed up the rates at which chemical reactions occur, they will not cause a reaction to occur if that reaction is energetically unfavorable.
http://www.freethought-forum.com/forum/gallery/files/5/0/catalyst.jpg
[B]Enzymes: Enzymes, as you recall, are organic molecules – proteins, generally – that function as catalysts. Without enzymes, the chemical reactions that support life would occur at only a small fraction of the rate necessary to keep us alive. Enzymes tend to be very specific as to which chemical reactions they facilitate, and so by changing the amounts and activities of specific enzymes, a cell can regulate its metabolism very precisely.
Enzyme Structure: Generally speaking, enzymes are thought to work by bringing reactants sufficiently close together that they can react with each other. The molecules in question (known as the substrate molecules) temporarily bind with the enzyme at what is known as the enzyme’s active site. Since the substrate molecules change shape when they react, they will no long fit into the enzyme’s active site. So after the chemical reaction occurs, the products are released. The enzyme is then free to bind more substrate and the reaction can occur again.
Because the function of an enzyme is utterly dependent on its shape, anything that causes the enzyme molecule to change its shape will either alter or destroy its ability to function as a catalyst. As you may recall, even a slight change in temperature, pH or salinity can cause a protein to change shape or denature. This is one of the most important reasons why it’s so crucial for living organisms to maintain blood pH, body temperature, electrolyte balance, and other factors within very narrow ranges. Even small departures from the normal range of values can impact enzyme function with fatal results.
Enzyme Function: One hypothesis regarding how enzymes function is known as the “lock-and-key” model. According to this hypothesis, a particular enzyme is shaped in such a way that the substrate molecule(s) fit into the active site(s) exactly, much as a key fits exactly into a particular lock. Substrate molecules are attracted to the enzyme’s active sites, and are thus brought close together. When the substrates bond to each other, this changes their overall shape, and so they no longer fit into the enzyme’s active site and are released.
http://www.freethought-forum.com/images/anatomy3/lockandkey.jpg
The “Lock-and-Key” Model of Enzyme Function
http://www.freethought-forum.com/forum/gallery/files/5/0/enzymes_original.jpg
This enzyme catalyzes the hydrolysis of sucrose into glucose and fructose.
Enzyme Cofactors: Some enzymes consist simply of a protein molecule that functions as a catalyst, while others must be bound to other substances before they can function as catalysts. The other substances to which some enzymes must be bound before they can function are known as cofactors.
A cofactor may be inorganic or organic. Some metals, for instance, function as enzyme cofactors, and so are necessary parts of your diet. Organic cofactors are known as coenzymes. Many vitamins, for example, function as coenzymes, and so are essential parts of the diet. A well-known example of a coenzyme is Coenzyme A, which plays a crucial role in regulating the chemical reactions of aerobic respiration.
http://www.freethought-forum.com/forum/gallery/files/5/0/coa.jpg
Coenzyme A, an important regulator of aerobic respiration.
[break=Regulation of Enzyme Activity]
Regulation of Enzyme Activity: One way that living cells can regulate the activity of enzymes is by controlling how much of a given enzyme is synthesized. You may recall that enzymes are proteins, and that the “instructions” for making proteins are encoded in the DNA of a cell’s nucleus. So, by regulating how many copies of a given enzyme are made, the cell can regulate the rate at which the chemical reactions it mediates proceed.
Cells can also temporarily activate or inhibit the function of enzymes that are present, and so alter the rates at which chemical reactions take place. Typically, this occurs when the products of the chemical reactions mediated by the enzyme(s) in question function to inhibit the enzyme’s activity. This is known as feedback inhibition, because the products of the enzyme influence the enzyme’s function. (This will hopefully sound familiar, as it’s an example of negative feedback.)
Allosteric regulation occurs when the products of a chemical reaction bind to some portion of the enzyme other than the active site, and in so doing, change the shape of the enzyme. By changing the enzyme’s shape, they alter its function.
Imagine that enzyme “A” catalyzes a reaction that produces product “B.” If “B” binds to the enzyme and changes its shape, the enzyme will either start to catalyze a different reaction and so produce a different product (product “C”), or it will simply cease to function. So, the more of product “B” is present in the cell, the less of it will be produced.
Competitive regulation is similar to allosteric regulation, except that the product(s) of the reaction bind to the enzyme’s active site, and so stop it from functioning. As a result, the more of the product is present, the less substrate can bind to the enzyme and be converted to product, and so the less product is produced.
These forms of inhibition allow cells to very precisely regulate the amounts of product formed by enzyme-mediated chemical reactions.
[break=Naming Enzymes]
Naming Enzymes: Generally speaking, enzymes are named according to what they do, and the last part of the enzyme’s name consists of the suffix -ase. For example, the enzyme that catalyzes the breakdown of the alcohol ethanol is known as alcohol dehydrogenase.
Amylases are enzymes that catalyze the breakdown of complex carbohydrates. The next time you eat some crackers, you may notice that if you hold them in your mouth for some time, they begin to taste sweet. That’s because the amylase in your saliva breaks the starch in the crackers down into the sugar maltose. Proteases are enzymes that accelerate the breakdown of proteins into their component amino acids. Lipases catalyze the breakdown of fats and other lipids. Nucleases catalyze the breakdown of nucleic acids. And so forth.
[break=Coupled Reactions and Energy-Carrier Molecules]
[B]Coupled Reactions and Energy-Carrier Molecules: Many of the chemical reactions in organisms’ metabolisms are endergonic. How then, do cells manage to get those reactions going, since these reactions absorb energy as they progress? Cells do so by employing coupled reactions, in which exergonic reactions provide the energy necessary to power endergonic reactions. In this way, metabolic pathways often involve many chemical reactions that are closely interlinked, and the products of one reaction become the reactants in the next reaction. The synthesis of particularly large organic molecules, for example, might easily involve dozens, hundreds, or even thousands of closely-interlinked chemical reactions.
As you recall, the principle molecule used by living organisms as an energy carrier, is ATP. Ultimately, breakdown of glucose is used to provide the energy for powering the various endergonic reactions in a cell’s metabolism. ATP transports energy from where it is produced to where it is needed, in order for necessary chemical reactions to proceed.
Another way in which energy is transported within cells is by electron carriers. Some metabolic reactions produce high-energy electrons, which could damage cellular components if left free. Electron acceptors are molecules that can accept these electrons, and so prevent them from doing damage. However, it would be a shame to waste all that energy, wouldn’t it? So, metabolic pathways have evolved in which high-energy electrons are passed from one molecule to another, and at each step in the progression, some of the electrons’ energy is removed and used to make ATP. Electron carrier molecules can thus be used to transport energy from one part of a cell to another, in much the same way that ATP does.
Removal of energy from high-energy electrons to generate ATP is a key component of aerobic respiration. Ultimately, the “spent” electrons are captured by hydrogen and oxygen atoms to make water, which is one of the reasons why oxygen is necessary for aerobic respiration.
http://www.freethought-forum.com/forum/gallery/files/5/0/nad_original.jpg
NAD+ (nicotinamide adenine dinudeotide) is a coenzyme that functions as an electron-carrier.
It can bind to and transport hydrogen ions and electrons. When NAD+ is reduced through gain
of electrons, it forms NADH. Oxidation of NADH produces NAD+ and releases free electrons.