Photosynthesis: The Light-Dependent Reactions

Photosynthesis converts light energy into chemical energy, and it happens in two connected stages. The light-dependent reactions come first, taking place on the thylakoid membranes inside the chloroplast, and they produce the ATP and reduced NADP that the light-independent reactions — the Calvin cycle — then use to fix carbon dioxide into sugar. To understand the whole process you have to be clear about what the light-dependent stage supplies and where.

Embedded in the thylakoid membrane are two protein complexes called photosystems, labelled Photosystem II and Photosystem I for historical reasons even though PSII acts first. Each photosystem contains chlorophyll and other pigments arranged so that light energy is funnelled toward a special pair of chlorophyll molecules at the reaction centre. When a photon is absorbed, an electron in that reaction centre is raised to a higher energy level — it is said to be photoactivated — and is passed to an electron acceptor.

The electron lost from Photosystem II has to be replaced, and this is where water comes in. An enzyme complex splits water molecules in a reaction called photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂. The electrons replace those lost from PSII, the protons accumulate inside the thylakoid space, and the oxygen is released as a waste product — which is, incidentally, the source of essentially all the oxygen in the atmosphere.

The photoactivated electrons from PSII pass along a chain of carrier proteins in the membrane, the electron transport chain. As they move from carrier to carrier they release energy, and that energy is used to pump additional protons from the stroma into the thylakoid space. The result is a steep concentration gradient of protons — many more inside the thylakoid than outside. This stored gradient is a form of potential energy, exactly like water held behind a dam.

The electrons eventually arrive at Photosystem I, where a second absorption of light re-energises them. From PSI they are passed to NADP along with a proton to form reduced NADP (NADPH), a molecule that carries chemical reducing power to the Calvin cycle. Because electrons flow in one direction from water through PSII and PSI to NADP, this route is called non-cyclic photophosphorylation.

Meanwhile the proton gradient is put to work. Protons can only cross back into the stroma through a membrane enzyme called ATP synthase. As they flow down their concentration gradient through this enzyme, the movement drives the synthesis of ATP from ADP and inorganic phosphate. This coupling of proton flow to ATP production is called chemiosmosis, and it is the same fundamental mechanism used in respiration in the mitochondria — a striking example of how evolution reuses a good idea.

So the light-dependent reactions deliver three things: oxygen, which is released; ATP, made by chemiosmosis; and reduced NADP, made at Photosystem I. The ATP and reduced NADP then diffuse the short distance to the stroma, where the Calvin cycle uses them to reduce carbon dioxide into triose phosphate and ultimately glucose. Without the light-dependent …