Photosynthesis

• Chloroplast structure: grana of thylakoids for the light-dependent stage, stroma for the light-independent stage (Calvin cycle).
• Pigments (chlorophyll a and b, carotene, xanthophyll) absorb light; absorption and action spectra; separation by chromatography using Rf values.
• Cyclic photophosphorylation (PSI only, ATP) and non-cyclic photophosphorylation (PSI and PSII, photolysis, ATP and reduced NADP).
• ATP is made by chemiosmosis across the thylakoid membrane.
• Calvin cycle: rubisco fixes CO₂ with RuBP to give GP; GP is reduced to TP; RuBP is regenerated.
• Light intensity, CO₂ concentration and temperature are limiting factors; investigated with redox indicators and whole plants.

Two stages linked by ATP and reduced NADP

In the light-dependent stage, light energy is absorbed and used to make ATP and reduced NADP, and water is split to release oxygen. In the light-independent stage (the Calvin cycle), the energy in ATP and the hydrogen in reduced NADP are used to convert CO₂ into complex organic molecules. ADP, Pᵢ and NADP return to the thylakoids to be used again.

Chloroplast structure and function

Within a chloroplast, the thylakoids (thylakoid membranes and thylakoid spaces), which occur in stacks called grana, are the site of the light-dependent stage. The stroma is the site of the light-independent stage.

Chloroplast pigments

Pigments are held in the thylakoid membranes in photosystems. In each, many pigment molecules form a light-harvesting complex (antenna) that absorbs light and passes the energy on to a pair of chlorophyll a molecules at the reaction centre. PSI has a reaction centre absorbing best at 700 nm (P700); PSII at 680 nm (P680).