Chapter Summary

1. Enzymes catalyze reactions of metabolism. Many of these include oxidation-reduction reactions. Oxidation is loss of electrons; reduction is gain of electrons.

2. Photosynthesis is an anabolic process that combines carbon dioxide and water in the presence of light with the aid of chlorophyll; oxygen is a by-product. All life depends on photosynthesis, which takes place in chloroplasts.

3. Carbon dioxide constitutes 0.036% of the atmosphere, but the percentage has been rising in recent years. Increased carbon dioxide levels have potential to elevate global temperatures through the "greenhouse effect."

4. Chlorophyll b and carotenoids are antenna pigments that direct light energy to chlorophyll a. Photosynthetic units containing chlorophylls and accessory pigments absorb units of light energy, become excited, and pass this energy to acceptors during the light-dependent reactions of photosynthesis.

5. During the light-dependent reactions of photosynthesis, which occur in thylakoid membranes of chloroplasts, water molecules are split, and oxygen gas is released. Hydrogen ions and electrons are released from water and transferred to produce NADPH and ATP.

6. The two types of photosynthetic units present in most chloroplasts are photosystems I and II. Each photosystem has a reaction-center molecule of chlorophyll a that boosts an electron to a higher energy level as it reacts to light energy.

7. Photosystem I boosts electron excitation energy to a level that, when it encounters photosystem II, has the potential to reduce NADP to NADPH through non-cyclic electron flow. Photosystem II, by itself, can cycle electrons for generation of ATP. Electron transport while the photosystems are operating and proton movement across thylakoid membranes are both involved in ATP production.

8. The light-independent reactions occur through a series of reactions known as the Calvin cycle, which takes place in the stroma of chloroplasts. In the first step, carbon dioxide is combined with RuBP through catalytic action of the enzyme rubisco to form two molecules of the 3-carbon compound, GA3P. The ATP and NADPH from the light-dependent reactions furnish energy to eventually convert GA3P to 6-carbon carbohydrates. This cycle also regenerates RuBP to enable continued carbon fixation.

9. In the light-independent reactions of C₄ plants, 4-carbon oxaloacetic acid is initially produced instead of 3-carbon PGA. In the leaf mesophyll of C₄ plants, there are large chloroplasts, which contain rubisco in the bundle sheaths, and small chloroplasts, which contain higher concentrations of PEP carboxylase that facilitate the conversion of carbon dioxide to carbohydrate at much lower concentrations than is possible in C₃ plants.

10. CAM photosynthesis occurs in succulent plants whose stomata are closed and admit little CO₂ during the day. Regular photosynthesis occurs as the 4-carbon compounds that accumulate at night are converted back to carbon dioxide during the day.

11. Respiration is a catabolic process that takes place in the cytoplasm and mitochondria of cells. The energy is released, with the aid of enzymes, from simple sugar and organic acid molecules.

12. In aerobic respiration, stored energy release requires oxygen; CO₂ and water are by-products of the process.

13. Anaerobic respiration and fermentation do not require oxygen gas, and much less energy is released. The remaining energy is in the ethyl alcohol, lactic acid, or other such substances produced. Some released energy is stored in ATP molecules. Temperature, available water, and environmental oxygen affect respiration rates.

14. Glycolysis, which occurs in the cytoplasm, requires no molecular oxygen; two phosphates are added to a 6-carbon sugar molecule, and the prepared molecule is split into two 3-carbon sugars (GA3P). Some hydrogen, energy, and water are removed from the GA3P, producing pyruvic acid. There is a net gain of two ATP molecules. Hydrogen ions and electrons released during glycolysis are picked up by NAD, which becomes NADH.

15. In aerobic respiration, which occurs in the mitochondria, pyruvic acid loses some CO₂, is restructured, and becomes acetyl CoA. Energy, CO₂, and hydrogen are removed from the acetyl CoA in the citric acid cycle, which involves enzyme-catalyzed reactions of a series of organic acids.

16. NADH passes the hydrogen gained during glycolysis and the citric acid cycle along an electron transport system; small increments of energy are released and partially stored in ATP molecules, and the hydrogen is combined with oxygen gas, forming water in the final step of aerobic respiration.

17. Hydrogen removed from glucose during glycolysis is combined with an inorganic ion in anaerobic respiration. The hydrogen is combined with the pyruvic acid or one of its derivatives in fermentation. Both processes occur in the absence of oxygen gas, with only about 7% of the total energy in the glucose molecule being released, for a net gain of two ATP molecules.

18. Two molecules of NADH and two ATP molecules are gained during glycolysis when two 3-carbon pyruvic acid molecules are produced from a single glucose molecule. Another molecule of NADH is produced when the pyruvic acid molecule is restructured and becomes acetyl CoA prior to entry into the citric acid cycle.

19. In the citric acid cycle, acetyl CoA combines with 4-carbon oxaloacetic acid, producing first a 6-carbon compound, next a 5-carbon compound, and then several 4-carbon compounds. The last 4-carbon compound is oxaloacetic acid. Two CO₂ molecules are also released during this process.

20. Some hydrogen removed during the citric acid cycle is picked up by FAD and NAD; one molecule of ATP, three molecules of NADH, and one molecule of FADH₂ are produced during one complete cycle. Energy associated with electrons and/or with hydrogen picked up by NAD and FAD is gradually released as the electrons are passed along the electron transport system; some of this energy is transferred to ATP molecules during oxidative phosphorylation.

21. Energy used in ATP synthesis during oxidative phosphorylation is believed to be derived from a gradient of protons formed across the inner membrane of a mitochondrion, while electrons are moving in the electron transport system by chemiosmosis.

22. Altogether, 38 ATP molecules are produced during the complete aerobic respiration of one glucose molecule; two are used to prime the process, so there is a net gain of 36 ATP molecules.

23. In addition to photosynthesis and respiration, other metabolic pathways are required for growth, development, reproduction, and survival. Essential products of additional pathways include nucleotides, proteins, chlorophylls, and fatty acids. Secondary metabolites include alkaloids, phenolics, and terpenoids.

24. Conversion of sugar produced by photosynthesis to fats, proteins, complex carbohydrates, and other substances is termed assimilation. Digestion takes place within plant cells with the aid of enzymes. During digestion, large insoluble molecules are broken down by hydrolysis to smaller soluble forms that can be transported to other parts of the plant.