How Cells Release Energy
7.1 How Does Cellular Respiration Differ From Breathing?
1. The respiratory system provides the oxygen that aerobic cellular respiration requires as a final electron acceptor.
2. Cellular (aerobic) respiration is a common biochemical pathway that extracts energy from the bonds of nutrient molecules in the presence of oxygen.
7.2 How Do Cells Release Energy?
3. The overall reaction for cellular respiration is:
C6H12O6 + 6O2 Æ 6CO2 + 6H2O + 30ATP
4. All cells begin energy release from nutrients with glycolysis and then may follow any of several pathways.
5. Cellular (aerobic) respiration harvests energy gradually. Acetyl CoA formation, the Krebs cycle, and an electron transport chain follow glycolysis. Glycolysis takes place in the cytoplasm; acetyl CoA formation, the Krebs cycle, and the electron transport chain occur in mitochondria.
6. The two membranes of a mitochondrion enclose the innermost matrix, and the intermembrane compartment is between the membranes.
7. ATP synthesis occurs by substrate-level phosphorylation (phosphate transfer between organic compounds) or by chemiosmotic phosphorylation (passage of electrons along carrier molecules through oxidation-reduction reactions, setting up a proton gradient that powers phosphorylation of ADP to ATP).
7.3 How Does Glycolysis Break Down Glucose to Pyruvic Acid?
8. In the first half of glycolysis, glucose is broken down into two molecules of the three-carbon compound PGAL.
9. In the second half of glycolysis, the PGALs are oxidized as NAD+s are reduced to NADHs, contribute phosphate groups to form two ATPs, and react and are rearranged to form two molecules of pyruvic acid.
7.4 After Glycolysis - In the Presence of Oxygen
10. In the mitochondria, pyruvic acid is broken down into acetyl CoA in a coupled reaction that also reduces NAD+ to NADH.
11. Acetyl CoA enters the Krebs cycle, a series of oxidation-reduction reactions that produces ATP, NADH, FADH2, and CO2. Substrate-level phosphorylation produces ATP in the Krebs cycle.
12. Energy-rich electrons from NADH and FADH2 fuel an electron transport chain. Electrons move through a series of carriers that release energy at each step. The terminal electron acceptor, oxygen, is reduced to form water.
13. Electron transport energy establishes a proton gradient that pumps protons from the mitochondrial matrix into the intermembrane compartment. As protons diffuse back into the matrix through channels in ATP synthase, their energy drives phosphorylation of ADP to ATP.
14. Negative feedback coordinates the rates of glycolysis, acetyl CoA formation, and the Krebs cycle.
15. Amino acids enter the energy pathways as pyruvic acid, acetyl CoA, or an intermediate of the Krebs cycle. Fatty acids and glycerol enter as acetyl CoA.
7.5 After Glycolysis-In the Absence of Oxygen
16. In the absence of oxygen, alcoholic, lactic acid, or other fermentation pathways may run. Fermentation does not produce ATP but oxidizes NADH to NAD+, which is recycled to glycolysis. Alcoholic fermentation reduces pyruvic acid to ethanol and loses carbon dioxide. Lactic acid fermentation reduces pyruvic acid to lactic acid.
7.6 Possible Origins of the Energy Pathways
17. The energy pathways are interrelated, with common intermediates and some reactions that mirror the reactions of the others.
18. Glycolysis may be the oldest energy pathway because it is more prevalent—the other pathways are more specialized.
19. Cellular respiration and photosynthesis may have arisen when larger cells engulfed prokaryotes that were forerunners to mitochondria and chloroplasts.