The leaf is a thin structure designed to absorb sunlight. To work efficiently it is thin yet has a large surface area exposed to the light. There are numerous air spaces between the cells of the spongy mesophyll which allow for rapid diffusion. The small diffusion pathway from the atmosphere to the palisade mesophyll cells is another design feature. The upper surface has a thick waxy cuticle which makes the surface waterproof, reducing water loss, and being shiny it makes dust build up less likely. Light is absorbed by the chloroplasts of the palisade and spongy mesophyll cells. The greatest numbers of chloroplasts occur in the palisade cells. The arrangement of the palisade cells reduces the number of cell walls the light must pass through before being likely to strike a chloroplast and be absorbed. The spongy mesophyll cells are covered by a thin layer of water which is evaporated away to cool the leaf down (long term sun-bathing gets very hot even for a plant!). As water evaporates from the surface of the spongy mesophyll cells, the cells become more concentrated (their water potential falls) and they will absorb water by osmosis from the cells next to them, these cells will then absorb water by osmosis from the nearby xylem vessels and thus pull water up the xylem. The evaporating water enters the interstitial air spaces in the spongy mesophyll and then passes out of the leaves through the stomata.

The leaf is supported by a network of veins which contain the xylem and the phloem. The xylem carries water to the leaf, while the phloem cells are designed to transport the products made by the leaf to other areas of the plant where they are used for storage, respiration or growth. On the lower surface of the leaf there are numerous small pores called stomata (singular stoma). Each pore has a pair of guard cells making up its edges. These cells can alter their shape under osmotic pressure changes and thus can open or close the stomatal aperture. This allows the plant to control to some extent the rate of water loss from the leaf.

 

The guard cells have a thick inelastic wall on the side next to the pore and a thin wall elsewhere. When such a cell absorbs water by osmosis the thin walls will stretch while the thick walls will remain their original size. Therefore as guard cells absorb water by osmosis they bend and a pore becomes visible between them. The diagram opposite illustrates this idea.

The only controversy about this process is what makes the guard cells absorb water and swell! There are two theories, the starch glucose mechanism proposes that as the level of CO₂ in the guard cells drops due to photosynthesis in the leaf, a reversible phosphorylase enzyme converts starch into glucose-phosphate. This decreases the water potential of the cell because glucose is soluble while starch is not. The guard cells therefore absorb water from adjacent cells which do not have such a high glucose content. This theory is supported by the fact that in some plants chloroplasts are found in the guard cells.

 The alternative theory is that light, or some internal mechanism alters the activity of potassium pumps in the cell membrane so that potassium ions increase in concentration within the cell. This causes the osmotic uptake of water. The evidence in support of this theory includes:-

1. We know such pumps are common in cell membranes;
2. The use of ATP inhibitors tends to prevent guard cell opening;
3. In many plants, the guard cells open and close on a diurnal rhythm even if the plant is kept in continual darkness!

It is likely that both theories are correct to some extent in the various plant species known to occur.