Cellular Respiration: A Brief Overview:
Cellular Respiration: (The Simplified Version)
The first stage, glycolysis, is anaerobic and takes place in the cytoplasm of the cell. The remaining stages require oxygen and take place inside of mitochondria.
The Stages of Cellular Respiration:
Cellular respiration begins in the cytoplasm of a cell, when a glucose molecule is split into two molecules of pyruvate
. (Strictly speaking, pyruvate is a negatively-charged ion, rather than a molecule.) This process is known as glycolysis
, and since it does not require oxygen, it is a form of anaerobic respiration. (Many bacteria are capable of respiring anaerobically only
, and are killed by exposure to oxygen.) The energy released during glycolysis is used by the cell to generate 2 ATP molecules by adding a single phosphate group to each of 2 ADP molecules. (Covalent chemical bonds, as you recall, store energy that is released when those bonds are broken.)
This splitting of glucose during glycolysis also produces high-energy hydrogen ions (H+
). A pyruvate (C3
) ion is negatively-charged because it’s “missing” a hydrogen ion, and so it has an “extra” electron that isn’t paired with a proton. In addition, you may have noticed that two pyruvate ions, even if you take into account the “missing” hydrogen ions, don’t have as many hydrogen atoms as did the original glucose molecule. The uncharged hydrogen atoms that are liberated when glucose is split into pyruvate ultimately combine with oxygen to form water molecules.
Meanwhile, there’s still that H+
to account for. The highly energetic ions would surely do damage if left to themselves. They are gotten rid of by combining them with the coenzyme NAD
inucleotide), to form NADH.
So, glycolysis produces 2 molecules of ATP that store and transport energy, plus two molecules of NADH, which get rid of the energetic and therefore potentially dangerous H+
Of course, the fact that the H+
ions are so energetic means that it is possible to extract energy from them, and the cell does exactly that. Energy stored in the NADH molecules is used to generate an additional 4 molecules of ATP. (Cardiac muscle cells and liver cells are more efficient at this, and can generate not 4 but 6
ATP molecules in this way.)
So, at the end of this first stage in cellular respiration, the cell will typically have generated a total of 6 molecules of ATP from the splitting of a glucose molecule – 2 ATP molecules from glycolysis and 4 more from processing of electrons temporarily stored in molecules of NADH. If the 2 pyruvates produced by the splitting of the original glucose molecule are to be broken down any further, however, oxygen is required. Otherwise, the pyruvate is processed into less-dangerous substances and excreted.
Glycolysis: (The Simplified Version)
A single 6-carbon glucose molecule is ultimately broken down into two 3-carbon pyruvate ions. Note that the net production of ATP molecules is 2, because while 4 ATP molecules are produced by splitting of the glucose into pyruvate, 2 ATP molecules were used to generate the energy necessary to split the glucose molecule in the first place.
Complete breakdown of pyruvate into CO2
O can only take place inside the mitochondria, so after glycolysis has been completed, the pyruvate is normally transported into mitochondria for further breakdown. A few cells in the body lack mitochondria (notably, red blood cells), and so are capable only
of anaerobic respiration. (Perhaps that’s one of the reasons that RBCs don’t live very long.)
A Typical Mitochondrion - Slightly Larger than Life-Size:
A single cell can contain several thousand mitochondria, and cells with especially high energy demands generally have the largest number of mitochondria.
During heavy exercise, skeletal muscle cells can find themselves in an anaerobic (lacking oxygen) condition, because their metabolic rates are so high that they use up all the oxygen that can be delivered to them by the blood. They can survive and continue to function for a time by producing energy through glycolysis. Because the end-products of glycolysis are poisonous to cells, however, there’s a sharp limit to how long skeletal muscles can function anaerobically. (One of the major effects of repeated exercise is that the circulatory system becomes more efficient at delivering oxygen to skeletal muscles, and so they can function for longer periods of time before running out of oxygen.)
The TCA Cycle:
When oxygen is available, the 2 pyruvate ions produced during glycolysis are transported into a mitochondrion. Inside the mitochondrion, the pyruvates are completely broken down in the TCA Cycle
– that is, the Tricarboxylic Acid Cycle
. The TCA Cycle produces 2 more ATP molecules, plus 8 NADH and 2 FADH2
, or flavin adenine dinucleotide
, like NADH is a coenzyme.)
The TCA Cycle is also known as the Citric Acid Cycle
or the Krebs Cycle
. An acquaintance of mine has the last name of Krebs, and she is a quite proficient unicyclist. She refers to her unicycle as “The Krebs Cycle” – biologist humor at its finest.
The TCA Cycle:
The cycle begins when a 2-carbon acetyl-CoA molecule binds to a 4-carbon oxaloacetate molecule to form a 6-carbon molecule of citrate (citric acid). Two of the carbon atoms are stripped off the citrate, one at a time, and combined with oxygen to form CO2, which is excreted. Ultimately, the stripping of 2 carbons from citrate regenerates oxaloacetate, which can then combine with another molecule of acetyl-CoA. (The molecules named in blue but not shown are the enzymes that drive the chemical reactions.)
The Electron Transport System:
The NADH and FADH2
molecules feed the energetic electrons they’ve captured into electron transport chains
, which strip energy from the electrons. Ultimately, the energetic electrons and H+
ions are gotten rid of by combining them with oxygen to form water (H2
O). The electron transport system
produces an additional 28 ATP molecules.
The Advantages of Aerobic Respiration:
Typically, anaerobic respiration can produce only 2 molecules of ATP for each glucose molecule broken down. Additionally, it produces highly toxic waste products. Even under the best of conditions, anaerobic respiration produces only 6 molecules of ATP for each molecule of glucose consumed.
By contrast, aerobic respiration can produce up to 38 molecules of ATP for each molecule of glucose consumed. Not only that, but its waste products – water and carbon dioxide – are much less toxic. So, organisms that can respire aerobically can generate far more energy per glucose molecule than can anaerobic organisms. It’s unsurprising, therefore, that virtually every organism on the planet is capable of aerobic respiration.