The Chloroplast and the Light-Dependent Stage

The chloroplast is built for two different jobs

A chloroplast is surrounded by a double membrane. Inside, a system of flattened sacs called thylakoids is stacked into grana, and the stacks are joined by intergranal lamellae. The thylakoid membranes carry the photosystems, the electron carriers and ATP synthase, so this is where the light-dependent reactions happen; stacking increases the surface area available for them. Surrounding the grana is the stroma, a fluid containing the enzymes of the Calvin cycle, together with circular DNA, $70$S ribosomes, starch grains and lipid droplets. The two reactions are therefore separated in space but linked by two products.

The light-dependent reactions

Light striking chlorophyll in photosystem II raises electrons to a higher energy level and they leave the molecule. They are replaced by electrons from the photolysis of water: $\ce{2H2O -> 4H+ + 4e- + O2}$. This is the only source of the oxygen released by a plant. The excited electrons pass along carriers in the thylakoid membrane, and the energy released pumps hydrogen ions into the thylakoid space, creating a gradient used by ATP synthase to make ATP. This is photophosphorylation, and it is chemiosmosis again. In non-cyclic photophosphorylation the electrons end up at photosystem I and are used with hydrogen ions to reduce NADP. In cyclic photophosphorylation only photosystem I is involved and the electrons return to it; this makes ATP but no reduced NADP and no oxygen.

Pigments

Chlorophyll $a$ and $b$ absorb strongly in the blue and red regions and reflect green, which is why leaves look green. An absorption spectrum shows how much light of each wavelength a pigment absorbs; an action spectrum shows the rate of photosynthesis at each wavelength. The close match between them is evidence that these pigments are the ones driving photosynthesis. Pigments are separated by chromatography and identified by their $R_f$ value, the distance moved by the pigment divided by the distance moved by the solvent front.