AQA Biology AS, UNIT 2

Features of a leaf palisade cell to help its function of photosynthesis:

• Long, thin cells which form a continuous layer on the surface of the leaf to absorb sunlight
• Lots of chloroplasts arranged in the best positions to absorb as much sunlight as possible
• A large vacuole which forces the chloroplasts and the cytoplasm to the edge of the cell

The main features of chloroplasts:

• The chloroplast envelope which surrounds the cell and controls what enters and leaves due to its highly selective properties
• The grana (where the first stage of photosynthesis takes place) are stacks of disc-like structures called thylakoids. The thylakoids contain chlorophyll
• The stroma (where the second stage of photosynthesis takes place) is a 'fluid-filled matrix' and also contains other structures, such as starch grains

Chloroplasts are adapted for absorbing sunlight and carrying out photosynthesis in the following ways:

• The granal membranes = large surface area for the attachment of chlorophyll, enzymes and electron carriers
• Chloroplasts contain both ribosomes and DNA so they can make some of the proteins needed for photosynthesis

Cell walls have these features:

• Large number of polysaccharides e.g. cellulose
• Middle lamella = holds adjacent cells together but also marks the boundary between them

FUNCTIONS:

• To give strength to the plant as a whole
• To prevent the cell bursting under the osmotic pressure created by the entry of water
• To allow water to pass along it, therefore contributing to the movement of water through a plant