Cellular Respiration
I. Oxidation and reduction
A. Electrons in the bonds of molecules contain energy.
B. Electrons can be transferred from one molecule to another and the energy captured by doing so. This transfer is called oxidation and reduction.
C. Oxidation - a loss of electrons; reduction - a gain of electrons.
D. Energy is released when electrons move toward electronegative atoms (such as oxygen); energy must be added to pull electrons away from electronegative atoms. Why do electrons move?
E. Oxidation is always accompanied by a net release of energy, producing products that are more stable than the reactants.
F. In cells, this energy is captured by ADP to form ATP; the resulting phosphate makes ATP quite unstable and to regain stability it must lose the phosphate (i.e., the energy).
G. H+ and electrons (and their energy) are removed from glucose and accepted by NAD+ which becomes NADH (a reduction); NADH is the carrier of electrons.
II. Cellular Respiration - C6H12O6 + 6O2 ➝ 6CO2 + 6H2O
A. Glycolysis (Fig. 9.8)
1. Purpose is to split glucose into two molecules of pyruvate (a 3C compound). The cell also gets ATP and NADH from the process.
2. Occurs in the cytosol, outside the mitochondrion.
3. Note in step 5 that the energy released from the oxidation was used to attach a phosphate to PGAL. The resulting 2 phosphate compound immediately gives up one of the phosphates to ADP and two ATP are created. The cell has capitalized on the fact that oxidation reactions release energy.
4. Pyruvic acid (or pyruvate) is the starting material for the next step (the Krebs cycle) and it enters the mitochondrion where that occurs.
5. Notice that glycolysis does not produce any CO2 and that it can occur under anaerobic conditions. If O2 is present, the NADH can be used to produce ATP through the ETC and the pyruvate can be oxidized in the Krebs cycle to produce even more ATP.
B. Krebs Cycle (Fig 9.11) - occurs in the mitochondrial matrix
1. After glycolysis, most of the energy in glucose is still there, very little was removed by glycolysis; remember the energy yield.
2. In the presence of O2 pyruvate enters the mitochondrion to be used in the Krebs cycle.
3. The purpose of the Krebs cycle is to produce NADH - the electron carrier.
4. Notice that carbon atoms enter the cycle in reduced form as pyruvate and exit in oxidized form as CO2 - a highly oxidized, therefore low energy, product. Where did the energy go?
C. Electron Transport Chain (Fig. 9.13)
1. Glycolysis and Krebs cycle produce little actual ATP but they do produce NADH. NADH provides the link between the first two parts of cellular respiration and the third - the ETC.
2. The metabolic machinery for the ETC is found on the inner mitochondrial membrane. The folds of this membrane provide a large surface area for this purpose.
3. High energy electrons carried by NADH are delivered to the ETC.
4. The electrons are passed to stronger and stronger electron acceptors and, each time, energy is released and used to make ATP (note: the ETC does not actually make any ATP).
5. FADH2, the other electron carrier also delivers electrons to the ETC but does not drop them off at the first acceptor in the chain. As a result, they produce less ATP.
6. The final electron acceptor is oxygen which combines with H+ to form water. Note that water is a low energy product which shows that most of the energy in the original fuel has been extracted.
7. Chemiosmosis (Fig. 9.14)
a. On the inner mitochondrial membrane is a protein called an ATP synthase.
b. As the electron acceptors of the ETC pass electrons, they also move H+ ions from the mitochondrial matrix to the intermembrane space.
c. This build up of protons in the intermembrane space is called the proton motive force (PMF).
d. Protons are charged and cannot cross the membrane. They move down their concentration gradient through the ATP synthases, making ATP in the process.
8. This method of producing ATP is called oxidative phosphorylation because the energy is derived from the redox reactions that transfer electrons from food to oxygen. The process forming ATP during glycolysis and Krebs cycle is called substrate level phosphorylation because an enzyme transfers a phosphate from a substrate to ADP.
9. The overall net energy yield from cellular respiration is 36 ATP/glucose molecule. This is only an estimate because the PMF is sometimes used to do other work in the mitochondrion as well.
III. Control of cellular respiration (Fig. 9.20)
A. If a cell is working hard so that it’s ATP supply begins to drop, respiration speeds up. When there is plenty of ATP to meet demands, respiration slows down and some of the intermediates are diverted to other pathways.
B. Phosphofructokinase (PFK) is the enzyme that catalyzes the step which adds the second phosphate to glucose.
C. PFK has both inhibitor and activator sites. It is inhibited by ATP and activated by ADP.
D. It is also regulated by the [citrate]. As [citrate] increases, the enzyme is slowed, helping to coordinate the rates of glycolysis in the cytosol with Krebs in the mitochondria.
IV. Variety of fuels (Fig. 9.19)
A. Many different carbohydrates, lipids, and proteins can be used as fuel for cellular respiration.
B. Proteins are digested to amino acids and the amino group from each amino acid is removed. In humans, this nitrogenous waste is excreted as urea in urine.
C. Lipids can also be used after the fatty acids are removed from glycerol.
D. Remember also that cells can make many of the molecules they need by using intermediates in the pathways of cellular respiration.
V. Anaerobic Respiration (fermentation) (Fig. 9.16)
A. In the absence of sufficient oxygen, cells can extract energy from fuel using fermentation.
B. The net energy gain in fermentation is 2 ATP; cf. 36 in aerobic respiration
C. If no O2 is present, the ETC stops, so pyruvate acts as the electron acceptor to oxidize NADH to NAD+; NAD+ is then available to oxidize more glucose. In the absence of NAD+, even glycolysis would stop.
D. There are many types of fermentation, distinguished by the end product. Two common ones are
1. Alcohol fermentation
a. Pyruvate is converted to ethyl alcohol by the removal of a C in the form of CO2, followed by the reduction of the resulting 2C compound to ethanol.
b. Because the pyruvate does not enter the Krebs cycle, there is still a lot of energy which is not removed from the fuel. This is evident in yeast fermentation where the end product is alcohol - a high energy fuel. Why is wine never more than ~12-14% alcohol?
2. Lactic acid fermentation
a. Pyruvate is reduced to lactic acid (or lactate). In human muscle cells this occurs during oxygen shortages which often accompany strenuous exercise. The lactic acid causes muscle pain and cramping. This process is also used to make cheese and yogurt.