Leaf structure

The leaf is the plant's main organ for photosynthesis, gas exchange and transpiration. Its layered structure is adapted for all three.

Parts and functions

How structure suits function

The broad, thin blade gives a large surface area for light and a short distance for gases. The palisade layer near the top catches the most light, the spongy layer allows diffusion, and stomata on the lower surface reduce water loss.

• The broad, flat blade gives a large surface area for capturing light and absorbing carbon dioxide.
• The blade is thin, so gases have only a short distance to diffuse between the air spaces and the mesophyll cells.
• Palisade cells are long and column-shaped, tightly packed with chloroplasts just below the upper surface where they receive the most light.
• Interconnecting air spaces in the spongy mesophyll link to the stomata, letting carbon dioxide diffuse in to the cells and oxygen diffuse out.
• A branching network of veins carries water to every part of the blade and removes the glucose made, and the guard cells open and close the stomata so the leaf can exchange gases while limiting water loss.

Related processes

The leaf carries out three linked processes. Photosynthesis takes place mainly in the palisade mesophyll, where chloroplasts use light energy to build glucose from carbon dioxide and water.

Gaseous exchange supplies that carbon dioxide and removes the oxygen made: gases move through the stomata and diffuse through the moist air spaces of the spongy mesophyll. Transpiration is the loss of water vapour through the same stomata; it is a side effect of keeping the stomata open for gas exchange, but it also pulls a stream of water up from the roots through the xylem in the veins.

Common labelling errors

Worked exam-style question

Question. The diagram shows a section through a leaf. (a) Name the tissue that is packed with chloroplasts and is the main site of photosynthesis.

(b) Explain two ways the leaf is adapted to absorb light efficiently. (c) Explain how carbon dioxide in the air outside reaches a palisade cell.

(d) State one way the leaf reduces water loss from its surface.

Model answer. (a) The palisade mesophyll. (b) The blade is broad and flat, giving a large surface area to capture light, and the palisade cells are packed with chloroplasts near the upper surface where the light is strongest, while the upper epidermis and cuticle are transparent so light passes straight through to them.

(c) Carbon dioxide diffuses in through the stomata on the lower surface, moves through the air spaces of the spongy mesophyll, and then diffuses down its concentration gradient into the palisade cell, dissolving in the moist cell surface on the way. (d) The waxy cuticle reduces evaporation from the surface (or: most stomata are on the lower surface, and the guard cells can close them to limit water loss).

Source:SRC-DSKP-EN

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