Glossary: Leaf Structure and Function

This chapter's terms in context

Chapter 17 covers four terms describing the structure of a leaf and how that structure suits its main functions of photosynthesis, gas exchange and transpiration: cuticle, palisade mesophyll, stoma and guard cell. The cuticle is a waxy, waterproof layer secreted onto the outer surface of the upper (and, more thinly, the lower) epidermis of a leaf, and its main function is to reduce uncontrolled water loss from the leaf surface by evaporation; because the cuticle is transparent, it also allows light to pass through to the photosynthetic tissue beneath without significantly blocking it. A frequent exam point is explaining why the cuticle is typically thicker on the upper surface of a leaf, which normally receives more direct sunlight and would otherwise lose more water by evaporation, than on the lower surface.

The palisade mesophyll is a layer of tightly packed, elongated, column-shaped cells found just beneath the upper epidermis, and it is adapted to be the leaf's main site of photosynthesis: its cells contain a very large number of chloroplasts concentrated near the top of the cell, close to the leaf surface receiving the most light, and their elongated shape allows many cells to be packed close together to maximise the number of chloroplasts per unit area exposed to incoming light. Comparing the palisade mesophyll with the more loosely arranged, irregularly shaped spongy mesophyll beneath it, which contains air spaces that allow gases to circulate freely to and from the stomata, is a standard structure-function comparison built from this chapter.

A stoma (plural: stomata) is a small pore, usually on the lower epidermis of a leaf, through which gases (carbon dioxide, oxygen and water vapour) are exchanged between the leaf's internal tissues and the surrounding air; stomata are also the main route through which transpiration occurs, since water vapour escapes through them. Each stoma is bordered by a pair of guard cells, bean-shaped cells that are the only epidermal cells to contain chloroplasts, and it is these guard cells, not any external muscle, that open and close the stomatal pore by changing shape as their internal water content (turgidity) changes: when guard cells become turgid, their unevenly thickened cell walls cause them to bow outward and open the pore, and when they lose turgidity, they straighten and close it. A common exam error is describing the stoma itself as actively opening or closing, when it is in fact a passive gap whose size is controlled entirely by the guard cells surrounding it.

Together, these four structures explain why a leaf's cross-section is never uniform: the cuticle controls water loss at the surface, the palisade mesophyll maximises light capture just beneath it, and the stoma and guard cells regulate gas exchange and water loss lower down, mostly on the shaded underside of the leaf, a division of labour examiners frequently test by asking a candidate to relate a named leaf structure to more than one of the leaf's overall functions at once, rather than to only one in isolation.

Paper 3 questions in this chapter often ask a candidate to compare stomatal density on the upper and lower epidermis of different leaves, usually using nail varnish or clear adhesive tape impressions viewed under a microscope, and full marks typically require stating both the counting method and a correctly reasoned explanation for why most land plants have far more stomata on the lower epidermis, reducing water loss from the surface exposed to direct sunlight.

A related question type asks a candidate to explain why a wilted plant recovers when watered even though its stomata were fully open throughout, testing whether the candidate understands that wilting reflects a temporary water deficit in the mesophyll cells rather than any permanent damage to the leaf's structure, and that restoring water restores turgor pressure throughout the leaf, including in the guard cells themselves. Because guard cells are the only epidermal cells containing chloroplasts, a candidate can also be asked to justify this feature specifically, linking it to the guard cell's own need to photosynthesise and generate the solutes that draw water in by osmosis to make it turgid.