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How Cells Harvest Energy

9.1 Cells harvest the energy in chemical bonds.
Using Chemical Energy to Drive Metabolism
• Autotrophs produce their own chemical energy, while heterotrophs live on the energy autotrophs produce. (p. 160)
• The energy of a chemical bond is contained in the potential energy of the electrons that make up the bond. (p. 160)
• Cells use some of the energy gained by catabolizing food to drive ATP production. (p. 160)
• ATP stores energy by linking charged phosphate groups near one another. (p. 160)
• Cells use ATP to facilitate movement and to drive endergonic reactions. (p. 160)
• The vast majority of ATP produced in the cell is made by ATP synthase. (p. 161)

9.2 Cellular respiration oxidizes food molecules.
An Overview of Glucose Catabolism
• Cells can make ATP from the catabolism of organic molecules two ways: substrate-level phosphorylation and aerobic respiration. (p. 162)
• In many organisms, cells harvest energy from glucose molecules in a sequence of four pathways: glycolysis, pyruvate oxidation, the Krebs cycle, and the electron transport chain. (pp. 162—163)
• Anaerobic respiration donates harvested electrons to inorganic compounds other than oxygen. (p. 163)
Stage One: Glycolysis
• Glycolysis generates ATP by shuffling the bonds in glucose molecules. Two molecules of NAD+ are reduced to NADH. (p. 164)
• NAD+ must be regenerated for glycolysis to continue. (pp. 166—167)
Stage Two: The Oxidation of Pyruvate
• Pyruvate is decarboxylated within the mitochondrion, yielding acetyl-CoA, NADH, and CO2. (p. 168)
Stage Three: The Krebs Cycle
• The Krebs cycle is a series of nine reactions that oxidize acetyl-CoA in the matrix of a mitochondrion. (p. 169)
• The Krebs cycle yields two molecules of ATP per molecule of glucose. (p. 170)
Harvesting Energy by Extracting Electrons
• Glucose catabolism involves a series of oxidation-reduction reactions that release energy by repositioning electrons closer to oxygen atoms. (pp. 172—173)
• Energy is harvested in gradual steps, using NAD+ as an electron carrier. (p. 172)
Stage Four: The Electron Transport Chain
• The electron transport chain is a series of membrane-associated proteins. Electrons delivered by NADH and FADH2 are passed along the protein chain, and are used to pump protons out of the mitochondrial matrix via the electron transport chain. The return of protons into the matrix through ATP synthase generates ATP. (p. 175)
Summarizing the Yield of Aerobic Respiration
• The theoretical yield of aerobic respiration is 36 molecules of ATP, while the actual yield is around 30 molecules of ATP. (p. 176)
• Aerobic respiration harvests about 32% of the energy available in glucose. (p. 176)
Regulating Aerobic Respiration
• Relative levels of ADP and ATP control the catabolic pathway at the committing reactions of glycolysis and the Krebs cycle. (p. 177)

9.3 Catabolism of proteins and fats can yield considerable energy.
Cellular Respiration of Protein
• Proteins are first broken down into their individual amino acids, and then the amino group is removed from each amino acid by deamination. (p. 178)
• Glycolysis and the Krebs cycle then extract high-energy electrons from the molecules and use them in producing ATP. (p. 178)
Cellular Respiration of Fat
• Fats are metabolized for energy by b oxidation. (p. 179)

9.4 Cells can metabolize food without oxygen.
• Fermentation occurs in the absence of oxygen as electrons from the glycolytic breakdown of glucose are donated to an organic molecule, regenerating NAD+ from NADH. (p. 181)

9.5 The stages of cellular respiration evolved over time.
The Evolution of Metabolism
• The six major innovations of metabolism are degradation, glycolysis, anaerobic photosynthesis, oxygen-forming photosynthesis, nitrogen fixation, and aerobic respiration. (p. 182)

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