Leaf structure
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
Overview of Leaf Structure
Leaves are the main organs of photosynthesis. Most leaves have a large surface area and are thin - these are both adaptations for photosynthesis.
- Large surface area - maximises the light absorbed
- Thin - gives carbon dioxide a short diffusion distance to reach all cells, and lets light penetrate to the lower layers
Structures of a Leaf
A cross-section of a dicotyledonous leaf shows the following structures, from top to bottom:
- Cuticle - a waxy, transparent layer covering the upper epidermis; reduces water loss and lets light through
- Upper epidermis - a single layer of transparent cells with no chloroplasts, covered by the cuticle; allows light through to the mesophyll below
- Palisade mesophyll - a layer of column-shaped cells packed tightly just below the upper epidermis, containing many chloroplasts; the main site of photosynthesis
- Spongy mesophyll - a layer of irregularly shaped cells with large air spaces between them, allowing gases to diffuse to and from the cells
- Lower epidermis - the outer layer on the underside of the leaf, containing most of the stomata
- Stomata (singular: stoma) - tiny pores in the epidermis, mainly on the lower surface, that allow gas exchange
- Guard cells - a pair of cells surrounding each stoma, containing chloroplasts, that control the opening and closing of the stoma
- Vascular bundles (veins) - contain xylem vessels, which carry water and mineral ions into the leaf, and phloem, which carries sugars made by photosynthesis away from the leaf
Each layer of a leaf's cross-section has a structure suited to its role in photosynthesis
Guard Cells and Stomata
Guard cells control the size of the stomatal pore by changing shape:
- In light: guard cells photosynthesise, producing sugar; water enters by osmosis, and the cells become turgid - the stoma opens, allowing CO₂ in for photosynthesis
- In darkness, or when water is scarce: guard cells lose water and become flaccid - the stoma closes, reducing water loss
Turgid guard cells open the stoma; flaccid guard cells close it
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