Biology - Leaves and Photosynthesis

Photosynthesis occurs mainly in the leaves of plants. Leaves are especially adapted for efficiency. For example, a leaf:

• Contains a pigment chlorophyll (which absorbed light) in millions of chloroplasts, plus other pigments to absorb light from different parts of the spectrum.
• Is broad and flat to provide a huge surface area to absorb sunlight.
• Has a network of vascular bundles for support, and to transport water to the cells and remove the products of photosynthesis, i.e. glucose.
• Has a thin structure so the gases (carbon dioxide and oxygen) only have a short distance to travel to and from the cells.
• Has stomata (tiny pores) on the underside of the leaf to allow the exchange of gases; these are open and closed by guard cells.

During photosynthesis carbon dioxide diffuses in through the stomata (leaf pores) and oxygen diffuses out of the stomata. Water is absorbed through the roots.

A lead has four distinct layers: the upper epidermis the palisade layer, the spongy mesophyll and the lower epidermis. 

In a typical leaf:

• The upper epidermis is transported to allow sunlight through to the layer below.
• The cell in the palisade layer are near the top of the leaf and are packed with chloroplasts so they can be absorb the maximum amount of light.
• The spongy mesophyll contains lots of air spaces connected to the optimum exchange of gases.

This internal structure provides a very large surface area to volume ratio for effecient gaseous exchange.

Plant cells contain chloroplasts and are long so they can absorb lots of light. Chloroplasts are not found in all plant cells, for example, root cells don't have chloroplasts as they obviously don't receive any light.

Leaves contain chlorophyll and other pigments which absorb different wave lengths of light.

Chlorophyll is a mixture of pigments including chlorphyll a, chlorophyll b, xanthophylls and carotene. This chart shows that when lights of different colours are shone on chlorophyll a and b, they absorb different ranges of colours, but both tend to absorb colours int the red and violet ends of the spectrum. When lights of different colours are shone on a plant and the rate of photosynthesis is measured, the maximum rates are obtained in the red and violet ends too. The greener colours are reflected, which is why plants tend to be green.