1. An autotroph (such as a plant) is an organism that "feeds itself" by synthesizing its own organic molecules. Heterotrophs (such as animals) must consume their food.
2. Digestion breaks down organic macromolecules into their component amino acids, fatty acids, and monosaccharides. Catabolism goes farther to break the bonds occurring in these smaller molecules. The breaking of the bonds during catabolism generates more ATP and therefore provides more energy than digestion.
3. Substrate-level phosphorylation is the endergonic generation of ATP from ADP and Pi by coupling the reaction with a highly exergonic reaction. It occurs as one of the first steps in glycolysis in eukaryotic cells.
4. Glycolysis occurs in the cytoplasm of the cell. Pyruvate is produced by the process. Four ATPs are produced, although the net production of ATP is two because two ATPs are used to mobilize glucose.
5. Two additional ATPs are produced during glycolysis when oxygen is present. There are only small amounts of NAD+ in each cell, and when these are all converted to NADH, glycolysis stops, no matter how much glucose is available.
6. The two mechanisms that can convert NADH back to NAD+ are oxidative respiration and fermentation. There are many different nonoxygen electron acceptors used in fermentation, such as organic acids and alcohol. There is no additional energy production, but the processes recycle the NADH to NAD+ so that the reaction can continue producing its small amount of ATP per glucose molecule. Oxidative respiration is aerobic; that is, it requires oxygen. Fermentation is anaerobic, and does not require oxygen.
7. Oxidation of pyruvate, the Krebs Cycle, and the electron transport chain all occur in the mitochondria. At the end of the electron transport chain, 36 molecules of ATP should be produced, theoretically. In actuality, closer to 30 molecules of ATP are generated due to "leaky" mitochondrial membranes and use of ATP to power other processes at the same time.
8. The electron transport chain employs proton pumps, which, during the course of electron transfer, concentrate protons outside of the mitochondrial matrix. When the protons diffuse back into the matrix, they move through channels that drive the coupling of ADP to Pi, creating ATP in a process called chemiosmosis. Without oxygen, oxidative respiration cannot occur, and if it doesn't, neither can electron transport.
9. In carbohydrate catabolism, acetyl CoA is produced from the final product of glycolysis, pyruvate. The pyruvate is cleaved into CO2 and acetyl-CoA. Fatty acids from fat molecules are broken down within the mitochondrial matrix to two-carbon acetyl groups, which then bind to coenzyme A, forming acetyl-CoA in a process called _-oxidation. In both cases then, the newly-formed acetyl-CoA enters into the Krebs Cycle.
10. Catabolism of fatty acids yields about 20% more ATP than the catabolism of glucose. Fat contains more energy, almost twice as much, on a per-weight basis than glucose.