Chloroplast Structure

Chloroplast Structure and Function

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Chloroplasts are organelles found in green plant cells. They contain chlorophyll, which absorbs light energy for photosynthesis. Their internal structure includes thylakoid membranes (stacked into grana) where light-dependent reactions occur, and stroma where carbon fixation takes place.

Chloroplasts are where photosynthesis happens. For the Supplement tier, you need to understand the internal structure of chloroplasts and how this structure relates to their function. This connects to the photosynthesis equation and leaf structure topics.

Chloroplasts are lens-shaped organelles about 5–10 μm long, surrounded by a double membrane (envelope). Inside, they contain:

Thylakoid membranes: flattened membrane sacs where chlorophyll is embedded. The light-dependent reactions of photosynthesis occur here: light energy is absorbed and used to split water molecules.
Grana (singular: granum): stacks of thylakoids. The stacking increases the surface area for light absorption.
Stroma: the fluid-filled matrix surrounding the grana. The light-independent reactions (Calvin cycle) occur here: CO₂ is fixed into glucose using the energy captured in the light-dependent stage.
Starch grains: glucose produced by photosynthesis may be temporarily stored as starch granules within the chloroplast.
Own DNA and ribosomes: chloroplasts have their own small circular DNA and 70S ribosomes, similar to bacteria (evidence for the endosymbiotic theory).

Chlorophyll and Light Absorption

Chlorophyll is the green pigment that absorbs light energy. It absorbs red and blue light most effectively and reflects green light: which is why plants appear green.

There are several forms of chlorophyll:

Chlorophyll a: the primary pigment, absorbs red and blue-violet light.
Chlorophyll b: an accessory pigment, absorbs different wavelengths and passes energy to chlorophyll a.
Carotenoids: orange and yellow accessory pigments that absorb blue-green light. Visible in autumn when chlorophyll breaks down.

For IGCSE, the key point is: chlorophyll absorbs light energy and converts it to chemical energy. Without chlorophyll, photosynthesis cannot occur.

Linking Structure to Function

Double membrane: separates the internal environment from the cytoplasm, allowing different conditions inside.
Stacked thylakoids (grana): large surface area maximises the amount of chlorophyll and light energy that can be absorbed.
Fluid stroma: contains enzymes needed for the light-independent reactions (carbon fixation).
Many chloroplasts per cell: palisade cells can contain up to 50 chloroplasts, maximising the cell's photosynthetic capacity.

Chloroplasts are most abundant in palisade mesophyll cells near the top of the leaf where light intensity is highest. Root cells and cells deep within the plant have few or no chloroplasts.

Try a focused question (2 marks)

Explain how the structure of grana in chloroplasts is related to the function of photosynthesis. (2 marks)

Show the answer and marking guidance

Grana are stacks of thylakoid membranes, providing a large surface area.
More chlorophyll molecules can be embedded in the membranes, increasing the amount of light energy absorbed for photosynthesis.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Confusing chloroplasts with mitochondria: chloroplasts are for photosynthesis; mitochondria are for respiration.
Saying chlorophyll absorbs green light: it reflects green light; it absorbs red and blue.
Forgetting that chloroplasts have their own DNA: this is a key structural feature.

Chloroplast Structure and Function