The Light-Dependent Reaction — AQA A Level Biology 7402 Revision Notes & Topical Questions

Photosynthesis converts light energy into chemical energy in organic molecules. It happens in two linked stages in the chloroplast. The light-dependent reaction takes place in the thylakoid membranes (grana). It needs light and water and produces ATP, reduced NADP and oxygen. The products ATP and reduced NADP are used in the light-independent reaction in the stroma.

The thylakoid membranes have a large surface area and contain photosystems (chlorophyll and other pigments in protein complexes), electron carriers and the enzyme ATP synthase.

When light is absorbed by chlorophyll in photosystem II (PSII), the energy excites two electrons in a chlorophyll molecule and they are lost from the molecule.

This energy is used to pump protons (H⁺) from the stroma into the thylakoid space (the lumen). The electrons then reach photosystem I (PSI), where they are re-excited by light, and are finally accepted by NADP.

The chlorophyll of PSII has lost electrons and needs replacements. They come from photolysis, the splitting of water by light:

2H₂O → 4H⁺ + 4e^- + O₂

Pumping protons into the thylakoid space produces a high proton concentration inside compared with the stroma, an electrochemical (proton) gradient.

At PSI, the electrons from the chain, together with protons, are accepted by the coenzyme NADP (via the enzyme NADP reductase), forming reduced NADP:

NADP + 2e^- + 2H⁺ → reduced NADP (NADPH)

Cyclic photophosphorylation makes extra ATP when the cell has enough reduced NADP.

The same ideas of electron transport and chemiosmosis occur in respiration (oxidative phosphorylation) on the inner mitochondrial membrane. Plants need ATP and reduced NADP to be made continually, which is why the light-dependent stage is affected by light intensity and wavelength.

Typical questions ask you to explain what happens when a variable is changed.

Remember the direction of each step: electrons move from chlorophyll to the carriers, to NADP; protons move from stroma to thylakoid space (pumping, which needs energy) and back to the stroma (through ATP synthase). The thylakoid space becomes acidic, with a lower pH than the stroma, which can be used as evidence for the proton gradient.

Compare with respiration

• in chloroplasts, the energy comes from light and the final electron acceptor is NADP, while in mitochondria the energy comes from the oxidation of reduced coenzymes and the final acceptor is oxygen.
• Both depend on membranes, electron carriers, a proton gradient and ATP synthase.

Be ready to label a diagram of a chloroplast: outer and inner membranes, stroma, grana (stacks of thylakoids), lamellae joining the grana, starch grains and lipid droplets, and a loop of DNA and 70S ribosomes in the stroma.