Leaf structure

Biology · Plant nutrition. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A leaf is built for photosynthesis. It is broad, giving a large surface area to absorb light and take in carbon dioxide. It is thin, so gases have only a short distance to diffuse and light reaches all the cells.

From top to bottom the layers are: waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis and cuticle. Most chloroplasts are in the palisade cells near the top, where the light is brightest.

Gases enter and leave through pores called stomata, mostly in the lower epidermis. Each stoma lies between two guard cells, which open and close it. Veins (vascular bundles) bring water and take sugars away.

Rules to remember

Cuticle: waxy and waterproof, reduces water loss; transparent, so light passes through.
Upper epidermis: transparent cells without chloroplasts, letting light through to the palisade layer.
Palisade mesophyll: tall, tightly packed cells with many chloroplasts; the main site of photosynthesis.
Spongy mesophyll: loosely packed cells with large air spaces, so gases diffuse quickly to and from the cells.
Guard cells turgid: stoma open. Guard cells flaccid: stoma closed.
Xylem brings water and mineral ions to the leaf; phloem carries sucrose and amino acids away.
Most stomata are on the lower surface, which is cooler and shaded, so less water is lost.

Common mistake

Mixing up the two mesophyll layers. Palisade cells are the tall, closely packed ones near the top with the most chloroplasts. Spongy cells are the irregular ones lower down with air spaces between them.

Worked example

The stomata of a leaf are blocked with a layer of grease on both surfaces. The leaf is left in bright light. Why does it make very little starch?

Carbon dioxide enters a leaf by diffusion through the stomata.
With the stomata blocked, carbon dioxide cannot reach the air spaces and mesophyll cells.
Carbon dioxide is a raw material for photosynthesis, so very little glucose is made.
With little glucose, little starch can be formed.

Answer: Carbon dioxide cannot diffuse in through the blocked stomata, so photosynthesis almost stops.

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Nutrition in Plants

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