Leaf Structure and Gas Exchange

About this topic

A leaf is adapted for photosynthesis. The waxy cuticle reduces water loss; the upper epidermis is transparent; palisade mesophyll cells packed with chloroplasts capture light; spongy mesophyll has air spaces for gas diffusion; veins contain xylem and phloem; and stomata on the lower epidermis, controlled by guard cells, allow CO₂ in and oxygen and water vapour out.

Stomata close in dry conditions to save water, a trade-off with photosynthesis. The flat shape gives a large surface area.

Guard cells around stomata swell with water to open the pore in daylight, letting in carbon dioxide, and become flaccid to close it, saving water when dry. Most stomata are on the underside of leaves, where it is cooler, reducing water loss. Air spaces in the spongy layer let gases diffuse freely to cells.

Leaves are thin and flat to give a large surface area and short diffusion distance. In a cross-section under a microscope, you can identify the layers in order. Plants in dry places, like cacti, have fewer stomata or open them at night. A common misconception is that plants breathe only carbon dioxide; they take in and give out both gases, depending on whether photosynthesis or respiration dominates at that time.