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 Glycolysis Breaks 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.
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.