The Structure of a Leaf

Layers of a dicotyledonous leaf

• Cuticle, a waxy, waterproof layer on the upper and lower epidermis that reduces water loss.
• Upper epidermis, a single transparent layer of cells with no chloroplasts, allowing light through to the mesophyll below.
• Palisade mesophyll, tall, closely packed cells with many chloroplasts, positioned near the upper surface to absorb the most light.
• Spongy mesophyll, loosely packed, irregularly shaped cells with air spaces between them, allowing gases to circulate.
• Vascular bundle (leaf vein), contains xylem, which brings water and minerals in, and phloem, which carries away the glucose made by photosynthesis.
• Lower epidermis, contains most of the stomata, each guarded by a pair of guard cells that control its opening.

How structure matches function

A leaf is thin and broad, giving it a large surface area to absorb light and exchange gases efficiently. The palisade layer being near the upper surface, where light is strongest, and packed with chloroplasts, maximises the rate of photosynthesis, while the air spaces in the spongy mesophyll let carbon dioxide and oxygen diffuse to and from the cells easily.

Why palisade cells are positioned near the top

Light intensity decreases as it passes through leaf tissue, so placing the chloroplast-rich palisade cells closest to the upper surface means they receive the most light, allowing them to carry out the most photosynthesis.

How it is examined

A labelled diagram of a leaf cross-section is common, asking you to name a layer, describe an adaptation, or explain how a particular structure, such as the arrangement of palisade cells or the position of stomata, suits the leaf's function in photosynthesis, gas exchange or transpiration.

How guard cells control stomatal opening

Each stoma is bordered by a pair of bean-shaped guard cells, the only epidermal cells that contain chloroplasts and can therefore photosynthesise. In light, photosynthesis and the active transport of ions into the guard cells lower their water potential, so water enters by osmosis and the guard cells become turgid; because their inner walls, facing the pore, are thicker and less elastic than their outer walls, this turgidity makes them bow outward and open the pore.

In darkness, or when the leaf is short of water, the guard cells lose turgor and become flaccid, straightening and closing the pore, which reduces water loss by transpiration at the cost of also limiting the entry of carbon dioxide for photosynthesis.

Worked exam-style question

Question. Diagram Z shows a transverse section of a leaf with four layers labelled J, K, L and M, where J lies directly below the upper epidermis and contains tall, closely packed cells full of chloroplasts, and M lies just above the lower epidermis and contains loosely arranged cells with large air spaces. (a) Name layer J and layer M.

(b) Explain why layer J is positioned directly below the upper epidermis rather than lower in the leaf. (c) State one function of the air spaces in layer M.

(d) The lower epidermis contains far more stomata than the upper epidermis. Suggest one advantage of this arrangement to the plant.

Model answer. (a) J is the palisade mesophyll; M is the spongy mesophyll. (b) Light intensity decreases as it passes through leaf tissue, so positioning the chloroplast-rich palisade layer closest to the upper surface, where light is strongest, allows it to absorb the maximum light for photosynthesis.

(c) The air spaces allow carbon dioxide and oxygen to diffuse to and from the mesophyll cells easily. (d) The lower surface faces away from direct sunlight and is cooler, so having more stomata there reduces the rate of water loss by transpiration compared with placing them mainly on the hotter, more exposed upper surface.

Practice question

Try this. A student notices that most land plants have far more stomata on the lower epidermis of their leaves than on the upper epidermis. Suggest why this arrangement helps the plant conserve water.

Exam tip

Key terms

These Chapter 17 terms name the structures examined in this topic:

• Cuticle, the waxy, waterproof layer that reduces water loss from the leaf surface.
• Guard cell, one of a pair of cells bordering a stoma that controls its opening and closing.
• Palisade mesophyll, the tall, chloroplast-rich layer beneath the upper epidermis where most photosynthesis occurs.
• Stoma, a pore in the epidermis, mainly the lower epidermis, through which gases and water vapour pass.

Source:SRC-DSKP-EN

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