The light-dependent reactionAQA A-Level Biology: Revision notes

The light-dependent reaction takes place in the thylakoid membranes of the chloroplasts. Light energy absorbed by chlorophyll is conserved in two products, ATP and reduced NADP, which are used in the light-independent reaction. Oxygen is released as a waste product.

When chlorophyll absorbs light, electrons in it are raised to a higher energy level and are released. This is photoionisation of chlorophyll. The chlorophyll is left positively charged, so it needs replacement electrons.

The released electrons pass along an electron transfer chain of carriers in the thylakoid membrane, losing energy at each step. This energy is used to move protons across the membrane into the thylakoid space, building up an electrochemical gradient.

Protons then diffuse back down the gradient through ATP synthase, which catalyses ADP + $P_i$ → ATP. This is the chemiosmotic theory. At the end of the chain, electrons are accepted by NADP, which also takes up a proton to form reduced NADP.

Photolysis is the splitting of water using light energy:

$2H_2O \rightarrow 4H^+ + 4e^- + O_2$

The electrons replace those lost by chlorophyll, the protons contribute to the proton gradient and reduce NADP, and oxygen is released as a by-product.

Chloroplasts are extracted in ice-cold, isotonic buffer to prevent damage by enzymes and osmosis. DCPIP, a blue dye, is added; it accepts electrons in place of NADP and becomes colourless. The rate of colour loss (measured with a colorimeter) shows the rate of dehydrogenase activity.

Controls: a tube in the dark, and a tube of boiled chloroplasts, show that the colour change depends on light and on functioning chloroplasts. The effect of a named factor, such as light intensity, can be investigated.