CCEA AS Topics‎ > ‎

Structure and function of a mesophytic leaf

Like insects, plants must meet the opposing demands of water retention and gas exchange. The site of photosynthesis in plants, as well as the gas exchange site, is the leaf. This is what a section through a leaf looks like:



The large surface area of most leaves maximises photosynthesis, while the tightly packed palisade mesophyll cells contain chlorophyll to carry out photosynthesis to meet the plant's energy needs.

The mesophyll cells (more specifically, the spongy mesophyll) are surrounded by quite a lot of empty space for air to mingle around, providing plenty of surface area for gas exchange by diffusion. They are tightly packed and perform photosynthesis.

Air with its carbon dioxide (necessary for photosynthesis) enters the leaf through the stomata. Stomata are holes on the leaf surface, made by the guard cells. They can open and close depending on environmental factors such as humidity, temperature and wind. This controls the amount of water loss. Oxygen, the byproduct of photosynthesis, also leaves the leaf through the stomata.

The waxy cuticle is waterproof and protects the leaf from water loss, alongside the upper epidermis. The lower epidermis and its respective waxy cuticle serve the same role.

The spongy mesophyll contains irregularly shaped cells that are separated by air pockets. They facilitate movement of air that contains key gases for photosynthesis such as carbon dioxide.

The vascular bundle allows transport of water and nutrients to parts of the plant away from the leaf, since the leaf is the source of glucose for example (made through photosynthesis). A closer look reveals specialised structures such as the xylem and the phloem. They carry water and nutrients respectively.

Roots, as you may have seen in real life, are hairy. All those tiny and not so tiny root hairs buried into the soil greatly increase the surface area of the root. This exposes it to more water molecules which can be taken up. The hairs are nothing like human hairs; they are extensions of the outer layer of the root, made up of cells. This layer is called the epidermis.

Why does water move inside the root? Simple: osmosis. The cell sap (i.e. cell juice) has a lower water potential than the fluid found in the soil, so the water in the soil kindly makes its way into the thirsty awaiting root. Once the water reaches the first cell in its path, the water potential of that cell is increased compared to the cell next to it. Therefore, water moves into the next cell, leaving the current cell. This in turn results in the previous cell taking up water all over again, and so forth, until water makes its way across all cells of the cortex

Reaching the endodermis, water then enters the xylem. The xylem is a tissue of dead cells which contributes to the vascular system of plants by being the transportation medium for water and dissolved mineral ions. The xylem brings them to the leaves and plants' other organs.


Transpiration

Transpiration is water loss through the parts of a plant which are found above soil level i.e. not the roots. As water streams through a plant, transpiration affects the speed of the stream. Increased transpiration will lead to a quicker uptake of water through the roots to maintain the water flow throughout the plant. So what affects transpiration?

1. Light causes stomata to open, resulting in increased water loss (transpiration).

2. Temperature going up also raises the rate of transpiration, as more water molecules evaporate.

3. Humidity. An increase in humidity around the leaves means that there is less space for water molecules from the plant to evaporate into, so transpiration is decreased.

4. Air movement (wind) can displace water molecules from around the stomata, so that more space becomes available for additional water molecules to go into. Transpiration increases.

A leaf's components have adaptations, previously outlined, to counteract all these environmental stresses.

<< Previous topic: Absorption by the epithelium                                                    Next topic: Surface area to volume ratio >>
Comments