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Chapter 7: Cellular Respiration, Fermentation, and Secondary Metabolism

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Learning Objectives
Chapter Summary
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Learning Objectives

Describe the distinct metabolic pathways used by cells to harvest the energy stored in glucose under aerobic conditions.
Know the specific locations of these pathways in a generalized eukaryotic cell.
Understand the chemical relationship between the glucose molecules used by cells as fuel and the carbon dioxide generated by the same cells as waste.
Trace the path of high-energy electrons from glucose to molecular oxygen in aerobic respiration.
Describe glycolysis in general terms, including the molecules that exist at its start and its end, as well as its net versus total ATP production.
Explain how the electron transport chain uses the high energy electrons harvested originally from glucose to provide the direct source of energy used by ATP synthase to make ATP.
Explain why the NADH produced in glycolysis and the NADH and FADH₂ produced in the Krebs cycle differ from one other in the amount of energy they provide for the production ATP by oxidative phosphorylation.
Understand how proteins and fats are metabolized.
Compare the number of ATPs produced in the degradation of carbohydrates, proteins, and fats.
Compare the overall amount of ATP produced by the complete metabolic breakdown of one molecule of glucose under aerobic conditions with the ATP produced from the breakdown of one glucose molecule under anaerobic conditions.
Describe how the NADH molecules produced during glycolysis are oxidized back to NAD+ under aerobic and anaerobic conditions and explain why this oxidation is important to glucose metabolism and ATP production.
Describe fermentation in plant and animal cells and explain the importance of this process in terms of energy harvest and ATP production.
Compare and contrast the various secondary metabolites described in the chapter.

Chapter Summary

Biological endergonic reactions do not occur spontaneously and are generally coupled with reactions that split energy-carrying molecules like ATP. ATP is not a long-term energy storage molecule, it is made only when needed. It is an extremely valuable molecule because it is used to do most of the work in a cell and is used to drive endergonic reactions. Cells generate ATP through two different processes, substrate level phosphorylation and chemiosmosis. The substrate level phosphorylation produces ATP from ADP and phosphate by association with an exergonic reaction and is the more ancient process. Chemiosmosis occurs when protons pumped out through specific transmembrane channels re-enter through other channels coupled to ATP synthesis. Most biological ATP is produced in this manner.

Glycolysis occurs in the cytoplasm of a cell and is catalyzed by enzymes not associated with any membranes or organelles. Glucose is converted to two glyceraldehyde-3-phosphate (G3P) molecules in a reaction that costs two ATP molecules. G3P is then converted to pyruvate and produces four ATPs via substrate level phosphorylation, a process that occurs with or without oxygen. In addition, a pair of electrons and one proton are removed from G3P reducing the coenzyme NAD⁺ to NADH. The net energy yield at this point is two ATPs per glucose. Glycolysis continues as long as there is a fresh supply of glucose and there is sufficient NAD⁺. It is advantageous for a cell to do something with its NADH other than allowing it to build up because its supply of NAD⁺ is generally limited. NADH returns to NAD⁺ through aerobic respiration.

The process of aerobic respiration includes the oxidation of pyruvate to acetyl-CoA and the Krebs cycle. Pyruvate is oxidized to a two-carbon molecule, acetyl-CoA, one NAD⁺ is reduced to NADH and one molecule of CO₂is given off. This reaction occurs within the mitochondria of eukaryotes or on special membranes in a few bacteria. The Krebs cycle is a complex set of reactions in which a four-carbon molecule is added to the acetyl-CoA from pyruvate oxidation. During the cycle, two molecules of CO₂are given off and three NADH, one FADH₂, and one ATP are produced. These quantities are, of course, for a single molecule of pyruvate. The degradation of a whole molecule of glucose produces twice the quantity of each substance.

Oxidative respiration in itself produces no more ATP than glycolysis, but it becomes highly efficient only when it is coupled to the fourth stage, the chemiosmotic generation of ATP via an electron transport chain. This process occurs on the inner mitochondrial membrane, requires oxygen as a final electron acceptor, and therefore occurs only in aerobic organisms. In theory, each NADH from the oxidative respiration (a total of eight per glucose) activates three pumps and produces three ATPs (a total of 24). Each FADH₂ from the Krebs cycle (two per glucose) activates two pumps and generates two ATPs (a total of four). The cell uses one ATP to get the NADH from glycolysis (a total of two) into the mitochondrion, thus the net value of each is only two ATPs (a total of four). Overall, glycolysis plus complete oxidative respiration produces 32 ATPs via chemiosmosis and four ATPs by substrate level phosphorylation. In actuality, the mitochondrial membrane is leaky, and only 2.5 ATPs are produced per NADH and 1.5 per FADH₂. Thus, on average, closer to 30 ATP are produced by chemiosmosis in the electron transport chain.

Proteins and fats are also metabolized. Proteins provide the same efficiency as glucose as constituent amino acids are converted to participants in the Krebs cycle. Fats are metabolized via beta-oxidation during which two carbon chunks are converted to acetyl-CoA, NADH, and FADH₂molecules. A six carbon fatty acid molecule produces 36 actual ATP compared to 30 actual from a six carbon sugar. The NADH produced in glycolysis also returns to NAD⁺ through various anaerobic fermentations. A carbohydrate serves as the final electron acceptor in most fermentations. Products of familiar eukaryotic fermentations include ethyl alcohol and carbon dioxide by yeast and lactic acid by overworked muscle cells.