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Edexcel GCSE Biology · 1BI0
Edexcel 1BI0 · 6.11B Check the specification (PDF) (opens in a new tab)
Photosynthesis uses light energy to react carbon dioxide and water, producing glucose and oxygen. A leaf therefore needs to capture light, receive water and take in carbon dioxide. It also needs a way for oxygen to leave. Its broad, thin shape and its internal layers help meet these needs together.
A broad leaf presents a large surface area to incoming light. The upper epidermis is transparent, so light can pass through it to the palisade mesophyll beneath. Palisade cells contain many chloroplasts, especially useful near the upper surface where they receive most light. Chlorophyll in the chloroplasts absorbs light energy for photosynthesis. Because the leaf is thin, light can reach cells deeper inside it too.
Carbon dioxide enters through small pores called stomata, found mainly in the lower epidermis of many leaves. When a stoma is open, carbon dioxide diffuses from the air into the leaf, while oxygen produced by photosynthesis can diffuse out. Diffusion is the net movement of particles from a higher to a lower concentration.
The spongy mesophyll has loosely arranged cells and connected air spaces. These let gases move between the stomata and photosynthesising cells. The leaf's thinness keeps this diffusion distance short. Guard cells beside each stoma change shape to open or close it: opening allows gas exchange but also lets water vapour escape. This is why a leaf's gas-exchange structure also affects water loss.
Veins form a network through the leaf. Their xylem supplies water needed for photosynthesis, while their phloem carries sugars made from the glucose produced to other parts of the plant, chiefly as sucrose. The veins also help support the broad leaf blade. A waxy cuticle over the epidermis reduces water loss while allowing light to pass through.
Leaf layers work together: the upper layers capture light, air spaces and stomata allow gas exchange, and veins supply water and carry sugars away.
Broad leaf → large area for light absorption. Thin leaf → light reaches cells and gases have a short diffusion path. Transparent upper epidermis → light reaches palisade mesophyll; its many chloroplasts absorb light.
Stomata, mainly in the lower epidermis, allow carbon dioxide in and oxygen out by diffusion. Guard cells open and close them. Spongy mesophyll air spaces help gases move through the leaf. Xylem delivers water; phloem carries sugars away. The cuticle reduces water loss.
Link each leaf feature to what it does: for example, a thin leaf gives gases a short diffusion distance.
Do not confuse stomata, which are pores, with guard cells, which control whether the pores are open.
In a labelled leaf cross-section, identify where light enters, where gases diffuse and where water is delivered.
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Last updated
Photosynthesis
The process in which plants use light energy to react carbon dioxide with water, producing glucose and oxygen.
Diffusion
The movement of particles from an area of higher concentration to an area of lower concentration.
Palisade mesophyll
A layer of closely packed leaf cells near the upper surface, containing many chloroplasts.
Spongy mesophyll
A layer of loosely packed leaf cells with air spaces that allow gases to move through the leaf.
Stomata
Small pores in a leaf surface through which gases pass; each pore is a stoma.
Guard cells
Cells beside a stoma that change shape to open or close the pore.
Xylem
Transport tissue that carries water and dissolved mineral ions to leaves.
Phloem
Transport tissue that carries dissolved sugars, mainly sucrose, away from leaves to other parts of a plant.
Put your knowledge into practice — try past paper questions for Biology
Photosynthesis
The process in which plants use light energy to react carbon dioxide with water, producing glucose and oxygen.
Diffusion
The movement of particles from an area of higher concentration to an area of lower concentration.
Palisade mesophyll
A layer of closely packed leaf cells near the upper surface, containing many chloroplasts.
Spongy mesophyll
A layer of loosely packed leaf cells with air spaces that allow gases to move through the leaf.
Stomata
Small pores in a leaf surface through which gases pass; each pore is a stoma.
Guard cells
Cells beside a stoma that change shape to open or close the pore.
Xylem
Transport tissue that carries water and dissolved mineral ions to leaves.
Phloem
Transport tissue that carries dissolved sugars, mainly sucrose, away from leaves to other parts of a plant.