Leaves: Plants' food factories
Adrian Whyte and Joanna Johnson, Contributor

External features of the leaf and its importance to photosynthesis.

THE MAIN function of leaves is to make food for plants. The process is known as photosynthesis. The process requires energy from light, (generally from the sun) water and carbon dioxide. The food made is glucose.

LEAF STRUCTURE

In biology, always remember that 'form follows function'. This means that for every structure that you should learn during your course it is not enough that you are able to recognise the structure and label its parts, but you should also be able to identify any characteristics that allow it to carry out its job efficiently. Sometimes each part may have a specific job to do, therefore, it is also important that you are able to identify the characteristics that allow each part to do its job.

(See diagram at top left)

Most leaves are quite thin, this allows for the quick diffusion of gases and easy passage of sunlight into the cells. It is, therefore, quite amazing to see the four distinctive layers that comprise it when you see it under the microscope.

(See diagram at bottom left)

* Cuticle: a waxy layer that protects the leaf from environmental strains.

* Epidermis: The top and bottom surfaces of the leaf are covered with a single layer of cells called the epidermis. (The bottom layer is called the lower epidermis and the top layer is called the upper epidermis). It is therefore thin and transparent, making it easy for light to penetrate it and reach the leaf's interior where most of the photosynthesis takes place.

The internal structure of the leaf

* Palisade mesophyll: Row of tightly packed columnar cells filled with a lot of chloroplasts. These are the cells that make the leaves look green. It should not be a surprise that it is here the bulk of photosynthesis occurs.

* Spongy mesophyll: Roughly spherical cells that are not tightly packed. Thus there are a lot of air spaces between the cells. This is important to facilitate the diffusion of carbon dioxide gas that is needed for the process of photosynthesis to occur.

* Vascular bundle: In the mesophyll area there is a bundle of specialised cells, consisting of phloem vessels and xylem vessels. As usual, there is a perfectly good reason for the vascular bundle to be in the particular area. The xylem is responsible for bring the needed water to the cells to carry out the process of photosynthesis. So it being so close allows for quick and easy osmosis of the water to the mesophyll cells the phloem is responsible for taking the food made by the mesophyll cells to other parts of the plant that need it.

* Stomata: These are tiny 'holes' found in the lower epidermis of the leaf. They are very important to the process of photosynthesis. The very important carbon dioxide gas diffuses into the leaf through these 'holes'. It is also important that these 'holes' are on the underside of the leaf. Water is also able to diffuse out of the plant through the stomata. If they were on the top side of the leaf the direct exposure to the sun would cause the plant to become dehydrated.

With all these features we can see that the leaf is well suited for its job as the plant's food factory.

* Adrian Whyte and Joanna Johnson teach Biology at Ardenne High School.