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Key concept
Leaves are adapted for maximum photosynthesis: they are broad and thin (large surface area, short diffusion distances), have transparent upper epidermis (light reaches palisade cells), packed palisade mesophyll (many chloroplasts near the top), air spaces in spongy mesophyll (CO₂ diffusion), and stomata for gas exchange.
The leaf is the organ of photosynthesis. Every tissue within it has a specific role in maximising the rate of photosynthesis. Examiners frequently ask you to label a leaf cross-section and explain how each structure is adapted for its function.
Leaf Tissues and Their Functions
Tissue
Location
Function / Adaptation
Upper epidermis
Top surface
Transparent: allows light to pass through to the palisade layer. Covered by a waxy cuticle to reduce water loss.
Palisade mesophyll
Just below upper epidermis
Main site of photosynthesis. Cells are tall and columnar, packed tightly with many chloroplasts. Near the top of the leaf where light intensity is highest.
Spongy mesophyll
Below palisade layer
Loosely packed cells with large air spaces between them. Air spaces allow CO₂ to diffuse from stomata to photosynthesising cells and O₂ to diffuse out. Some chloroplasts present.
Lower epidermis
Bottom surface
Contains stomata (pores) for gas exchange. Guard cells control the opening and closing of stomata.
Guard cells
Around each stoma
Kidney-shaped cells that can change shape. When turgid → stoma opens (for gas exchange). When flaccid → stoma closes (to reduce water loss).
Xylem
In leaf veins
Transports water and mineral ions from roots to the leaf.
Phloem
In leaf veins
Transports sucrose (made from glucose) from the leaf to other parts of the plant (translocation).
Why Leaves Are Thin and Broad
Broad: large surface area to absorb maximum light energy.
Thin: short diffusion distance for gases (CO₂ in, O₂ out). Ensures light can reach all photosynthesising cells.
Network of veins: provides structural support and delivers water to every cell.
Stomata and Gas Exchange
Stomata (singular: stoma) are small pores on the underside of most leaves. They allow:
CO₂ to diffuse in for photosynthesis.
O₂ to diffuse out (waste product of photosynthesis).
Water vapour to diffuse out (transpiration).
Guard cells regulate stomatal opening:
In light: guard cells photosynthesise → become turgid → stoma opens. This allows CO₂ in for photosynthesis.
In darkness or when the plant is losing too much water: guard cells lose water → become flaccid → stoma closes. This reduces water loss.
Most stomata are on the underside of the leaf: this reduces water loss because the lower surface receives less direct sunlight and heat.
Try a focused question (4 marks)
Explain how two features of the palisade mesophyll layer are adaptations for photosynthesis. (4 marks)
Show the answer and marking guidance
Palisade cells contain many chloroplasts: more chlorophyll to absorb more light energy.
Palisade cells are positioned near the top of the leaf: they receive the most light before it is absorbed by other layers.
Palisade cells are tall and tightly packed: maximising the number of chloroplasts per unit area of leaf.
The cells are arranged in a regular, columnar pattern: light can pass through to deeper cells.
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 stomata with stoma: stomata is plural, stoma is singular.
Saying guard cells "open and close": guard cells change shape; the stoma opens or closes as a result.
Forgetting that stomata are mainly on the lower epidermis, not the upper.