|6.1 Life Depends on Photosynthesis|
1. Photosynthesis is the light-driven reaction of water and carbon dioxide, which produces oxygen, water, and carbohydrate, in the presence of light. The two stages are the light reactions and the carbon reactions.
2. Plants, algae, and some microorganisms photosynthesize. They are autotrophs.
3. All life ultimately depends on photosynthesis.
6.2 How Do Cellular Structures Capture and Use Light Energy?
4. Photons move in waves. The longer the wavelength, the less energy per photon. Visible light occurs in a spectrum of colors representing different wavelengths. Red and blue light are most effective for photosynthesis.
5. Only absorbed light can affect life, and pigment molecules absorb light. Chlorophyll a is the primary photosynthetic pigment in plants. Accessory pigments absorb wavelengths of light that chlorophyll a cannot absorb, extending the range of wavelengths useful for photosynthesis.
6. A chloroplast consists of a gelatinous matrix called the stroma that contains stacks of thylakoid membranes called grana. Pigments aggregate in the grana and absorb light. Antenna complexes funnel the energy to reactive chlorophylls that aggregate with proteins to form reaction centers.
6.3 The Light Reactions Begin Photosynthesis
7. A photosystem consists of an antenna complex and a reaction center. In plants, photosystem II captures light energy and sends electrons from reactive chlorophyll a through an electron transport chain. The energy drives the active transport of protons into the thylakoid space, where they accumulate and then diffuse out through channels in ATP synthase. This movement powers phosphorylation of ADP to ATP. The coupling of the proton gradient and ATP formation is called chemiosmotic phosphorylation.
8. Photosystem II passes excited electrons through an electron transport chain to photosystem I, replacing them with electrons from water. The energy from electrons in photosystem I is used to reduce NADP+ to NADPH.
9. In plants, overall the light reactions oxidize water, produce ATP, and reduce NADP+ to NADPH. The two linked photosystems maximize energy extraction.
6.4 The Carbon Reactions “Fix” Carbon
10. The carbon reactions use ATP and NADPH to fix carbon into organic compounds.
11. In the Calvin cycle, rubisco catalyzes the reaction of CO2 with ribulose bisphosphate (RuBP) to yield two molecules of PGA. ATP and NADPH power the conversion of PGA to PGAL, the immediate carbohydrate product (photosynthate) of photosynthesis. This is C3 photosynthesis.
6.5 How Efficient Is Photosynthesis?
12. Photon energy produces very little carbohydrate. Photorespiration contributes to this inefficiency by fixing oxygen instead of carbon from CO2. Photorespiration wastes CO2 and energy.
13. C4 photosynthesis resists photorespiration by separating the light and carbon reactions into separate cells. An additional pathway recycles CO2 to the Calvin cycle and enables these plants to continue photosynthesis even when little atmospheric CO2 enters their cells.
14. In CAM photosynthesis, certain plants that live in hot, dry habitats open their stomata and take in CO2 at night and store it as malic acid in vacuoles. During the day, they split off CO2 and fix it in chloroplasts in the same cells storing the malic acid.