Leaf Structure

The top layers of a leaf are heavily specialized to ensure as much light energy as possible reaches the primary site of photosynthesis. The waxy cuticle is a thin, waterproof lipid layer covering the top of the leaf. It is completely transparent so that maximum sunlight can pass through, while simultaneously reducing water loss via evaporation and preventing rain from blocking gas exchange.

Directly beneath this is the upper epidermis, a single layer of thin, transparent cells. Crucially, this layer completely lacks chloroplasts, ensuring no incoming light is blocked or absorbed before it reaches the layers below.

Beneath the epidermis lies the palisade mesophyll, the tissue responsible for approximately 80% of the leaf's photosynthesis. These cells are tall, narrow, and cylindrical, meaning they can be densely packed together vertically at the top of the leaf. They contain the highest concentration of chloroplasts of any leaf tissue so that light absorption is maximised.

Facilitating Gas Exchange

Below the densely packed palisade layer is the spongy mesophyll, a layer of irregularly shaped, loosely packed cells. This structural arrangement creates a vast network of interconnected internal air spaces. This geometry significantly increases the internal surface area to volume ratio, providing a massive surface area for efficient gas exchange.

For photosynthesis to occur, atmospheric carbon dioxide ($CO_2$) must enter the leaf through the lower surface. It moves through the internal air spaces and must first dissolve in a moist film of water coating the spongy mesophyll cells. Only then can it cross the thin cell walls via diffusion and enter the chloroplasts.

Controlling Water Loss

The lower epidermis is located on the shaded, cooler underside of the leaf, which naturally minimizes water loss. It contains microscopic pores called stomata (singular: stoma), which regulate the intake of carbon dioxide and the release of oxygen. However, when stomata are open for gas exchange, water vapour inevitably escapes through a process called transpiration.

To control this delicate trade-off, each stoma is flanked by a pair of kidney-shaped guard cells. These cells have a uniquely uneven structure: a thick, inflexible inner wall facing the pore, and a thin, elastic outer wall.

When the plant has plenty of water, water moves into the guard cells by osmosis, making them firm or turgid. The thin outer walls stretch much more than the thick inner walls, causing the cells to curve outward and pull the pore open. For Higher Tier students, note that this process is triggered by the active transport of potassium ions ($K^+$) into the guard cells, which lowers their water potential and draws water in.

When water is scarce or it is dark, water exits the guard cells via osmosis. They become limp or flaccid, straightening out to close the stoma and conserve the plant's water supply.