B8

Photosynthesis can be carried out only by green plants and algae.

• Chlorphyll in the chloroplasts absorbs the Sun's light.
• The equation for photosynthesis is:
  • Carbon dioxide + water ------------> glucose + oxygen
•  Photosynthesis is an endothermic reaction. Energy is transferred from the environment when the light is absorbed by the chlorophyll.
• The proess for photosynthesis is:
  • Carbon dioxide is taken by the leaves, and water is taken up by the roots.
  • The chlorophyll traps the light energy needed for photosynthesis.
  • This energy is used to convert the carbon dioxide and water into glucose (a sugar)
• Oxygen is released as a by-product of photosynthesis
• Some of the glucose is converted into insoluble starch for storage

Leaves are well adapted for photosynthesis:

• Leaves are broad and flat so they have a large surface area, and thin so the diffusion path is short.
• The guard cells open the stomata so that gases can diffuse in and out of the air spaces inside the leaves.
• The photosynthetic cells in the leaves contain chlorophyll.
• The veins contain xylem, which brings water to the leaves, and phloem takes glucose away.

• A lack of light would slow down the rate of photosynthesis, because light transfers the energy for the process. Even on sunny days, light may be limited to plants that are shaded by trees.
• If it is cold, then enzymes do not work as effectively and this will slow down the rate of photosynthesis.
• If there is too little carbon dioxide, then the rate of photosynthesis will slow down. Carbon dioxide may be limited in an enclosed space, such as in a greenhouse on a sunny day where there is plenty of light available but the plants use up the carbon dioxide.
• Anything that stops the rate of photosynthesis increasing above a certain level is a limiting factor.

Some plants have patches of white among the green parts of the leaves. A variegated leaf will have less photosynthetic activity than one that is completely green.

The use of soluble glucose:

• The glucose produced by photosynthesis may be:
  • Converted into insoluble starch for storage in organs such as a potato tuber
  • Used for respiration
  • Converted into fats and oils for storage
  • Used to produce cellulose, which strengthens cell walls
  • Used to produce proteins

• Glucose is stored as insoluble starch so that it does not affect the water balance of the plant. High concentration of dissolved glucose would affect the way the water moves by osmosis.
• Plant and algal cells also need a supply of mineral ions, such as nitrate ions, in order to produce protein for glucose. Plants absorb nitrate ions from the soil. Algae absorb nitrate ions from the water they live in.
• Carnivorous plants such as the venus flytrap are adapted to live in nutrient-poor soil.
• The venus flytrap obtains most of its nutrients from the animals, such as insects, that it catches and digests. The plant obtains nitrates by digesting animal protein.