IGCSE Biology / Plants and food
How a leaf is adapted revision notes
Edexcel International GCSE Biology (4BI1), chapter 10: Plants and food, specification point 2.21, 2.42B. Free lesson.
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A leaf is adapted for photosynthesis by catching light, allowing carbon dioxide to reach its cells and moving water to where it is needed. Each visible feature and internal layer helps solve one or more of these problems.
How the outside of a leaf helps
The lamina is the flat blade of the leaf. Its broad shape catches light, while its thinness means carbon dioxide has only a short distance to diffuse to the inner cells. The veins and petiole help the leaf get what it needs and stay in a good position for light. A broad lamina has a large surface area, so it absorbs more light. The leaf is thin, so carbon dioxide can reach its inner cells quickly. Veins contain xylem, which brings water in, and phloem, which carries sugars away. They also support the leaf. The petiole, or stalk, holds the leaf out into the light and away from…
What each layer does in cross-section
A cross-section shows how the inside of a leaf is arranged in layers. Light can pass through the upper layers to reach the cells where most photosynthesis happens. The waxy cuticle is transparent, so light passes through, and waterproof, so water is not lost from the upper surface. The upper epidermis is one cell thick and transparent. It has no chloroplasts, so light passes through it. The palisade mesophyll is just below the upper epidermis. Its tall cells are packed with chloroplasts and lie near the top, where light is strongest. Most photosynthesis happens here. The **spongy…
How adaptations solve the main problems
When explaining an adaptation, link the feature to what it helps the leaf do. The key problems are getting light, allowing carbon dioxide to reach cells and supplying water. For light: the broad lamina absorbs more light, and palisade cells are near the top where light is strongest. For carbon dioxide: a thin leaf and the air spaces in the spongy mesophyll shorten the route for diffusion, helping carbon dioxide reach every cell. For gas exchange and water: stomata allow gases to move in and out. They are on the underside, where less water is lost than from the sunlit upper surface. For water…
The key features to remember
The adaptations work together: the leaf catches light, brings carbon dioxide to its cells and receives water through its veins. Broad for light, thin for gas diffusion. Palisade cells near the top contain many chloroplasts and do most photosynthesis. Air spaces in the spongy mesophyll let carbon dioxide reach the cells. Stomata are underneath, where less water is lost.