Photosynthesis

A photosystem is an array of chlorophyll and accessory pigments held by proteins in a membrane: the thylakoids of chloroplasts, or the membranes of cyanobacteria. Pigments absorb light and pass the energy inwards to a reaction centre chlorophyll, which emits an excited electron. There are two kinds, photosystem II and photosystem I.

Different pigments absorb a wider range of wavelengths; many molecules catch more photons; their precise spacing funnels energy to the reaction centre; and the reaction centre passes the excited electron to an acceptor. A single pigment molecule alone would only re-emit the energy as heat or light.

Photosystem II replaces its lost electrons by splitting water: 2H₂O → 4H⁺ + 4e⁻ + O₂. The protons and electrons are used; oxygen is waste. Oxygen from early photosynthesizers built up about 2.4 billion years ago, oxidized dissolved iron into banded iron deposits, formed the ozone layer and made aerobic respiration possible.

Excited electrons pass along a chain of carriers that pump protons from the stroma into the thylakoid space. Protons flow back through ATP synthase, making ATP. In non-cyclic photophosphorylation electrons come from photosystem II (water) and end on NADP; in cyclic photophosphorylation electrons from photosystem I return to the chain, making only ATP.

Photosystem I emits excited electrons, two of which reduce one NADP, which also takes a proton from the stroma. Use the pairs “NADP and reduced NADP” or “NADP⁺ and NADPH” consistently.

Photolysis happens on the inner side of the thylakoid membrane, releasing protons into the thylakoid space. Proton pumping is across the membrane; ATP and reduced NADP are made on the stroma side, where the Calvin cycle uses them.

In the stroma, rubisco joins CO₂ to RuBP (5C), giving two molecules of glycerate 3-phosphate. Rubisco is the most abundant enzyme on Earth; chloroplasts need a lot of it because it works slowly and poorly at low CO₂ concentrations.

Glycerate 3-phosphate is reduced to triose phosphate: reduced NADP supplies the hydrogen and ATP supplies the energy.

Five triose phosphates are converted into three RuBP, using ATP, so the cycle can continue. For each glucose made, five-sixths of all triose phosphate must return to RuBP.

All the carbon in a plant is fixed in the Calvin cycle. Sugars, starch and cellulose, lipids, and (with nitrogen from nitrate or ammonium) amino acids and nucleotides are made by pathways that start from triose phosphate or other cycle intermediates.

Without light, no ATP or reduced NADP are made, so the Calvin cycle stops within seconds. Without CO₂, ATP and reduced NADP are not used, NADP runs out, electrons back up along the chain and photosystem II stops working.