Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 8

Photosynthesis is the light-driven conversion of water and CO2 to carbohydrate. All life depends on photosynthesis; photosynthesis depends on light.
Photons move in waves. The longer the wavelength, the less energy per photon. Light occurs in a spectrum of colors representing different wavelengths. Red and blue light are most effective for photosynthesis.
Only absorbed light can affect life, and pigment molecules absorb light. Chlorophyll a is the primary photosynthetic pigment and occurs in all photosynthetic organisms except bacteria, which use bacteriochlorophyll. Accessory pigments absorb wavelengths of light that chlorophyll a cannot absorb, extending the range of wavelengths useful for photosynthesis. They may also protect.
Some protists photosynthesize using thylakoid membranes encapsulated in plastids. Photosynthetic bacteria use unencapsulated membranes. Plants and algae use specialized plastids, the chloroplasts.
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, funneling the energy to reactive chlorophylls that aggregate with proteins to form reaction centers.
In reaction centers, light energizes electrons, which move along an electron transport system, releasing energy. The energy pumps protons from the stroma into the thylakoid space. The resulting proton gradient powers ATP synthesis as protons flow back into the stroma.
In plants, the light-dependent reactions oxidize water, produce ATP, and reduce NADP{pos} to NADPH. This happens along two photosystems, each consisting of a photon-harvesting pigment molecule complex and an electron transport system. The two photosystems are linked, and electrons flow from one to the other.
The light-independent reactions use ATP and NADPH to fix carbon into organic compounds that can form nutrients. 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.
Photon energy produces very little carbohydrate. Photorespiration contributes to this inefficiency by fixing oxygen instead of carbon from CO2. This happens when stomata close to conserve water, cutting off the supply of atmospheric CO2. Photorespiration wastes CO2.
C4 plants use C4 photosynthesis to resist photorespiration. An additional pathway recycles CO2 to the Calvin cycle and enables these plants to continue photosynthesis even when little atmospheric CO2 enters their cells.
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.
The energy pathways are interrelated, with common intermediates and some reactions that mirror the reactions of the others. Glycolysis may be the oldest energy pathway because it is more prevalent---the other pathways are more specialized. The pathways may have arisen when larger cells engulfed prokaryotes that were forerunners to mitochondria and chloroplasts.