Summary

Green plants make their own food by photosynthesis. During this process, carbon dioxide from the atmosphere is taken in by the leaves through stomata and used to make carbohydrates, principally glucose and starch. Photosynthesis takes place only in the green parts of plants, mainly the leaves. Within the leaves, the mesophyll cells have a large number of chloroplasts responsible for CO2 fixation. Within the chloroplast, the membranes are the sites for the light reaction, while the chemosynthetic (biosynthetic) pathway occurs in the stroma.

Photosynthesis has two stages — the light reaction and the carbon fixing reactions:

• In the light reaction, light energy is absorbed by the pigments of the antenna and funnelled to special reaction centre chlorophyll a molecules. There are two photosystems: PS I with a P700 reaction centre, and PS II with a P680 reaction centre that absorbs red light at 680 nm.
• After absorbing light, electrons are excited and transferred through PS II, then PS I, and finally to NADP, forming NADPH. During this a proton gradient is created across the thylakoid membrane; its breakdown, through the F0 (CF0) part of the ATP synthase, releases enough energy to synthesise ATP.
• Splitting of water is associated with PS II, releasing O2, protons, and electrons that are transferred to PS II.

In the carbon fixation (Calvin) cycle, CO2 is added by the enzyme RuBisCO to the 5-carbon compound RuBP, which is converted to two molecules of the 3-carbon PGA. This is then converted to sugar by the Calvin cycle, and RuBP is regenerated. ATP and NADPH made in the light reaction are utilised here. RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants — photorespiration. …