Leaf structure

Leaf structure is built for photosynthesis. Seen in cross-section, a leaf has a waxy cuticle on top, a thin upper epidermis, a layer of tall palisade mesophyll cells packed with chloroplasts, a spongy mesophyll layer full of air spaces, veins of xylem and phloem, and a lower epidermis with tiny pores called stomata, each between two guard cells.

Leaf structure diagram

A leaf is a plant organ, made of several tissues working together. From top to bottom:

Each part of the leaf in detail

Waxy cuticle and upper epidermis

The top surface faces the sun, so it loses the most water. The waxy cuticle cuts down evaporation. The upper epidermis is transparent, so light passes straight through to the palisade layer, where it is needed.

Palisade mesophyll

Palisade cells are long and thin, standing upright like a fence (palisade means a fence of stakes). Lots of them fit side by side near the top of the leaf, where the light is brightest, and each contains many chloroplasts to absorb it. This is where most of the leaf's photosynthesis happens.

Spongy mesophyll

The spongy cells are rounder and loosely packed, leaving large air spaces. These connect to the stomata, so carbon dioxide can diffuse from the air to every cell, and oxygen and water vapour can diffuse out. The air spaces also give a large internal surface area for gas exchange.

Veins: xylem and phloem

Veins run through the spongy layer. Xylem vessels bring water (for photosynthesis and to keep the cells firm) and mineral ions from the roots. Phloem carries the sugars made in photosynthesis to the rest of the plant. The xylem and phloem guide explains how they work.

Stomata and guard cells

Stomata (one is a stoma) are pores, mostly in the lower epidermis. Each is surrounded by two guard cells. When the guard cells take in water and become turgid they curve apart and the stoma opens; when they lose water they become flaccid and the stoma closes. Closing the stomata saves water, but it also stops carbon dioxide getting in.

Most stomata are on the underside of the leaf. Out of direct sunlight it stays cooler, which cuts the amount of water lost through the open pores.

Check your understanding.

How leaf structure is adapted for photosynthesis

• Broad and flat: a large surface area to absorb light.
• Thin: a short distance for carbon dioxide to diffuse to the cells.
• Transparent upper epidermis: light reaches the palisade layer.
• Palisade cells near the top, full of chloroplasts: as much light as possible is absorbed.
• Air spaces in the spongy mesophyll: gases diffuse easily, with a large surface area for exchange.
• Stomata: carbon dioxide enters and oxygen leaves.
• Veins: water arrives and sugars are taken away.

Practise the photosynthesis equation that all these adaptations support in this question from the Cell Biology course.

How to draw a leaf structure diagram

1. Draw two long parallel lines for the upper epidermis, and two more near the bottom for the lower epidermis.
2. Fill the space under the upper epidermis with a row of tall, narrow, tightly packed rectangles: the palisade layer.
3. Below that, draw rounded, loosely spaced cells with gaps between them: the spongy layer.
4. Add a vein in the spongy layer, and leave a gap in the lower epidermis with a guard cell on each side for a stoma.
5. Label the cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, air space, vein (xylem and phloem), lower epidermis, stoma and guard cells. Use a ruler for label lines.

Worked example: counting stomata

A student paints nail varnish on the lower surface of a leaf, peels it off and looks at the print under a microscope. In a circular field of view 0.4 mm across, they count 18 stomata. Estimate the number of stomata per mm².

The radius of the field of view is 0.2 mm, so its area is π × 0.2² = 0.126 mm². The density is 18 ÷ 0.126 ≈ 143 stomata per mm². Counting several fields of view and taking the mean gives a more reliable estimate. If the upper surface of the same leaf gave 3 stomata in the same field of view, its density would be about 24 per mm², showing that most stomata are on the lower surface.

Common mistakes

• Saying the epidermis photosynthesises the most. It is the palisade mesophyll.
• Mixing up palisade and spongy. Palisade cells are tall and tightly packed; spongy cells are round with air spaces.
• Saying stomata are mostly on the top surface. Most are on the lower surface.
• Saying guard cells open when they lose water. They open when they gain water and become turgid.
• Leaving out what each part does. Exam questions usually ask you to link a structure to its function.

Practise with a short timed paper

Plant tissues, photosynthesis and transpiration come up in GCSE Biology. The daily GCSE Biology Higher paper is a short timed paper of 8 questions, with a new one every day.

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