Chloroplasts, pigments and the light-dependent stage

Last time

Before we start: last time, in The link reaction, Krebs cycle and oxidative phosphorylation, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Capturing sunlight

Nearly all the energy in living things started as sunlight. A leaf catches it with pigments packed into green organelles, then turns it into chemical energy. The first stage of that conversion, the light dependent stage, makes A T P and reduced N A D P. Both are then spent building sugar, and both come from water and light.

The chloroplast

A chloroplast has a double membrane, the envelope. Inside is the stroma, a fluid with enzymes for the Calvin cycle, starch grains, small circular D N A and 70S ribosomes. Flattened sacs called thylakoids are stacked into grana. The thylakoid membranes contain the pigments, electron carriers and A T P synthase. Stacking gives a huge surface area for the light dependent reactions. Lamellae join the grana together.

A past-paper example

October November 2023, Paper 41, question 1. A chloroplast diagram shows grana. Explain how the structure of the granum relates to its function. A granum is a stack of thylakoids, which gives a large surface area. The membranes hold chlorophyll and other pigments in photosystems, electron carriers and A T P synthase. The light dependent stage happens here, and the stroma is the site of the Calvin cycle.

Photosynthetic pigments

Chlorophyll a is the main pigment. It absorbs mainly red and blue violet light and reflects green. Chlorophyll b and carotenoids are accessory pigments. Accessory pigments, such as carotene and xanthophyll, absorb other wavelengths and pass the energy to chlorophyll a. This widens the range of light used. Carotenoids also protect the chloroplast by absorbing excess light energy. An absorption spectrum plots how much light each pigment absorbs at each wavelength. An action spectrum plots the rate of photosynthesis against wavelength, and matches the absorption spectra.

Chromatography

A leaf extract is spotted on a line near the bottom of chromatography paper. The paper stands in a solvent, which rises and carries the pigments. Pigments move different distances, because they differ in solubility and how strongly they stick to the paper. Carotene runs furthest. Chlorophyll b stays nearest the start. The Rf value is the distance moved by the pigment divided by the distance moved by the solvent front. A spot at four point two centimetres with the front at eight point four gives zero point five.

A past-paper example

May June 2026, Paper 41, question 4. A student separated leaf pigments using paper chromatography. Outline how the pigments were identified. Measure the distance moved by each spot and the solvent front, and calculate the Rf value. Compare the Rf values with known values for each pigment. Colour helps too: carotene is orange, and chlorophyll a is blue green.