Leaves and photosynthesis
Leaves are adapted to perform their function. For example, they have a large surface area and many chloroplasts to absorb sunlight.
Chloroplastschloroplast: Microscopic structure containing chlorophyl found in green plant cells where photosynthesis takes place. are the tiny structures in plant cells where photosynthesis happens. Chloroplasts contain chlorophyllchlorophyll: The green chemical inside the chloroplasts of plant cells. It enables photosynthesis to take place., a green pigment that absorbs light energy for photosynthesis. However, chloroplasts are not found in all plant cells. For example, they are not found in root hair cellsroot hair cell: Tiny hairs covering the ends of the smallest roots. They give the root a very large surface area through which to absorb water and mineral salts from the soil.. These cells absorb the water needed by the plant for photosynthesis. However, since they are usually underground and in the dark, they are unable to photosynthesise anyway.
Leaves contain many cells with chloroplasts. Leaves are often broad so that they have a large surface area to absorb sunlight. Broader leaves enable more sunlight to be absorbed.
|Adaption of leaf||Purpose|
|Thin||Short distance for carbon dioxide to diffuse into the leaf|
|Contains chlorophyll and other pigments||To absorb light from different parts of the Sun's spectrum|
|Network of vascular bundles (veins)||To support the leaf, and to transport water and carbohydrates|
|Stomatastomata: Tiny holes in the epidermis (skin) of a leaf - usually on the undersides of leaves. They control water loss and gas exchange by openng and closing. Singular is stoma||Allow carbon dioxide to diffuse into the leaf|
|Guard cells||To open and close the stomata depending on the conditions|
You should be able to name and locate these parts of a leaf:
The diagram shows a labelled cross-section through a leaf.
You should be able to explain how the cellular structure of a leaf is adapted for efficient photosynthesis. The table summarises some of these features.
|Adaption of leaf||Purpose|
|Transparent epidermis||Allows more light to reach the palisade cells|
|Palisade cell layer at top of the leaf and containing most of the chloroplasts||To absorb more light|
|Spongy layer with air spaces||Allows carbon dioxide to diffuse from the stomata to the photosynthesising cells, and to provide a very large internal surface area to volume ratio|
Plants use more than one photosynthetic pigment to absorb light. This maximises the use of energy from the Sun. These pigments include:
Chlorophyll a is the main pigment. It absorbs light mainly in the red and blue regions of the spectrum.
Chlorophyll b, xanthophyllxanthophyll: A yellow pigment responsible for the colour seen in autumn leaves. and carotene are 'accessory pigments'. They absorb light from other regions of the spectrum and pass the energy onto chlorophyll a.