Science

Leaves and photosynthesis

Leaves are adapted to perform their function. For example, they have a large surface area and many chloroplasts to absorb sunlight.

Functions of the leaf

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.

The leaf

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.

Leaf, showing the veins, midrib and blade.

 

Adaption of leafPurpose
ThinShort distance for carbon dioxide to diffuse into the leaf
Contains chlorophyll and other pigmentsTo 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 stomaAllow carbon dioxide to diffuse into the leaf
Guard cellsTo open and close the stomata depending on the conditions

Inside the leaf

You should be able to name and locate these parts of a leaf:

  • Cuticle
  • Upper and lower epidermis
  • Palisade and spongy mesophyll layers
  • Stomata and guard cells
  • Vascular bundle

The diagram shows a labelled cross-section through a leaf.

Shows the waxy cuticle on top of the upper epidermis.Under this is the palisade mesophyll layer and spongy mesophyll layer, which has air spaces in it. At the bottom, is the lower epidermis and wax cuticle. Gases are exchanged through the stoma. On each side of the stoma there is a guard cell with chloroplasts.

Inside the leaf - Higher tier

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 leafPurpose
Transparent epidermisAllows more light to reach the palisade cells
Palisade cell layer at top of the leaf and containing most of the chloroplastsTo absorb more light
Spongy layer with air spacesAllows carbon dioxide to diffuse from the stomata to the photosynthesising cells, and to provide a very large internal surface area to volume ratio

Photosynthetic pigments - Higher tier

Plants use more than one photosynthetic pigment to absorb light. This maximises the use of energy from the Sun. These pigments include:

  • Chlorophyll a
  • Chlorophyll b
  • Xanthophyll
  • Carotene

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.

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