PhotosynthesisOxford AQA IGCSE Biology: Revision notes

Photosynthesis is the process by which green plants and algae use light energy to make food. Light is absorbed by a green substance called chlorophyll, which is found in chloroplasts in some plant cells and in algae.

Word equation: carbon dioxide + water → glucose + oxygen (in the presence of light)

Balanced symbol equation: $\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}$

Light is used to convert carbon dioxide (from the air) and water (from the soil) into glucose. Oxygen is released as a by-product. In leaves, the palisade mesophyll is the tissue that carries out most photosynthesis.

In flowering plants, carbon dioxide enters the leaves by diffusion through the stomata. Most of the water and mineral ions are absorbed by the roots.

Adaptations for gas exchange and absorption:

• Root hairs increase the surface area of the roots for absorbing water and mineral ions
• The flattened shape of leaves increases the surface area to volume ratio
• Internal air spaces in the spongy mesophyll also increase the surface area and help gases diffuse

Stomata let carbon dioxide in and let the oxygen made in photosynthesis out. They also help to control the rate of water loss. If a plant loses water faster than its roots can replace it, the stomata can close to prevent wilting.

The rate of photosynthesis may be limited by:

• low temperature
• shortage of carbon dioxide
• shortage of light

These factors interact, and any one of them may be the limiting factor at a given time. The limiting factor is the one in shortest supply, so increasing any other factor will not raise the rate.

Why each one limits the rate:

• Light: less light energy is absorbed by chlorophyll, so less can be used to make glucose
• Carbon dioxide: there is less of this reactant available
• Temperature: at low temperatures the enzymes that control photosynthesis work slowly, so the reaction is slow

On a graph, the rate rises as the limiting factor increases, then levels off when a different factor becomes limiting.

Growers can raise the rate of photosynthesis, and so crop growth, by enhancing the conditions inside a greenhouse:

• Temperature: heaters keep the greenhouse warm
• Carbon dioxide concentration: carbon dioxide can be added, for example by burning a fuel or using a generator
• Light intensity: artificial lighting extends the hours of light and brightens dull days

Each change has a cost. The economics matter: the extra glucose produced, and so the extra crop, must be worth more than the cost of heating, gas and electricity. There is no point raising a factor that is not the limiting one, because the rate will not increase and the money is wasted.

The glucose produced may be used as a source of chemical energy, or converted into larger molecules for storage and use later.

To produce proteins, plants also need nitrate ions, which are absorbed from the soil. Nitrates provide the nitrogen that glucose does not contain.

Starch is insoluble, so it does not affect the water balance of cells, which makes it a good storage compound.

Required practical: investigate how variables affect the rate of photosynthesis.

A common method uses a water plant such as pondweed:

• Place a piece of pondweed in water in a beaker, with a lamp at a measured distance
• Count the bubbles of oxygen released in a set time, or collect and measure the volume of gas
• Change one variable at a time, then repeat and calculate a mean

Variables you can change:

• Light intensity: move the lamp closer or further away
• Carbon dioxide concentration: add different amounts of sodium hydrogencarbonate to the water
• Temperature: use a water bath at different temperatures

Control variables: keep the other factors constant, and use the same piece of pondweed. A heat shield or water bath stops the lamp from changing the temperature.