Chloroplast structure and function

Energy Flow, Ecosystems and the Environment · Photosynthesis · note 4 of 6

Spec 5.5

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Chloroplast structure and functionSpec 5.5

In short

A chloroplast is surrounded by a double membrane (envelope) and contains stroma, thylakoids and grana. The thylakoid membranes hold pigments, electron carriers and ATP synthase for the light-dependent reactions, and the stroma holds the enzymes for the Calvin cycle. This arrangement lets the two stages occur close together.

Chloroplasts are found mainly in palisade mesophyll cells. They are biconvex discs about 2 to 10 µm long. Each is surrounded by a chloroplast envelope of two phospholipid membranes, which controls the entry and exit of substances such as carbon dioxide, ATP and sugars.

Chloroplast structure related to function
Structure	Description	Role in photosynthesis
Thylakoid membranes	Flattened fluid-filled sacs, with a thylakoid space inside	Contain chlorophyll and other pigments in photosystems, electron carriers and ATP synthase. Site of the light-dependent reactions. The small space inside allows a proton gradient to build up quickly
Grana (singular granum)	Stacks of thylakoids	Give a very large surface area for pigments, so more light is absorbed. Stacks are arranged to capture light efficiently
Intergranal lamellae	Thylakoid membranes joining one granum to the next	Link grana, and keep them in position
Stroma	Colourless fluid-filled matrix surrounding the grana	Contains the enzymes (including rubisco) for the light-independent reactions, starch grains, lipid droplets, and the chloroplast’s own DNA and ribosomes
Starch grains	Insoluble stores in the stroma	Temporary store of the sugars made in the Calvin cycle
Chloroplast DNA and ribosomes	A small circular DNA molecule and 70S ribosomes	Allow chloroplasts to make some of their own proteins, for example some rubisco subunits

Chloroplast section labelled with outer and inner membrane, stroma, granum, thylakoid space, intergranal lamella, starch grain, circular DNA, 70S ribosomes and lipid droplet, with a magnified thylakoid membrane showing PSII, electron carriers, PSI and ATP synthase. — A chloroplast, with a magnified view of the **thylakoid membrane** where the light-dependent reactions happen.

The two stages of photosynthesis are in different places but close together. The ATP and reduced NADP made on the thylakoid membranes are used straight away in the stroma next door, so little energy is lost in transport.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between cyclic and non-cyclic photophosphorylation?

Non-cyclic photophosphorylation uses photosystems I and II, takes electrons from water, and produces ATP, reduced NADP and oxygen. Cyclic uses only photosystem I and recycles its electrons, so it makes ATP only: no reduced NADP and no oxygen are produced.

What happens in the Calvin cycle?

Carbon dioxide combines with RuBP, catalysed by rubisco, to form two molecules of GP. GP is reduced to GALP using ATP and reduced NADP. Most GALP regenerates RuBP, and the rest forms glucose and other organic molecules. It takes place in the stroma of the chloroplast.

Why does photosynthesis need light if the Calvin cycle is light-independent?

The Calvin cycle does not use light directly, but it needs ATP and reduced NADP, which are made in the light-dependent reactions. Without light, these run out, GP cannot be reduced, and the cycle stops. So light is needed indirectly.

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