Photosynthesis

Photosynthetic reaction

Photosynthesis is an endothermic reaction — energy is transferred from the environment to the chloroplasts by light. Chlorophyll in the chloroplasts absorbs the light energy.

Word equation: carbon dioxide + water → glucose + oxygen

Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Carbon dioxide enters the leaf by diffusion through the stomata, and water is absorbed by the roots and carried up in the xylem. The oxygen produced is released into the air or used in respiration.

Rate of photosynthesis

Four factors affect how fast photosynthesis happens:

• light intensity,
• carbon dioxide concentration,
• temperature,
• the amount of chlorophyll.

At any moment, the factor in shortest supply holds the rate back. This is the limiting factor. On a graph, the rate rises as one factor increases and then levels off — on the flat part, something else has become limiting, so increasing the original factor further has no effect.

Temperature behaves differently: photosynthesis is controlled by enzymes, so the rate rises up to an optimum and then falls quickly as the enzymes denature.

Light intensity follows the inverse square law: light intensity is proportional to 1 ÷ distance². Doubling the distance between a lamp and a plant reduces the light intensity to a quarter.

Growers use this knowledge commercially. In greenhouses they can add heaters, lighting and extra carbon dioxide to keep conditions near ideal, but they must weigh the cost of doing so against the value of the extra crop.

Required practical: rate of photosynthesis

A common method uses pondweed in a test tube of water, with a lamp placed at measured distances:

• Leave the pondweed to adjust for a few minutes at each distance.
• Count the bubbles of oxygen released in a set time, or collect the gas in a measuring cylinder or gas syringe and measure its volume.
• Repeat at several distances and calculate a mean for each.

To make it a fair test, keep the temperature constant (the lamp gives off heat, so use an LED or a beaker of water as a heat shield), keep carbon dioxide constant by adding sodium hydrogencarbonate to the water, and use the same piece of pondweed throughout.

Measuring the volume of gas is more accurate than counting bubbles, because bubbles vary in size.

Uses of glucose from photosynthesis

The glucose a plant makes is used:

• for respiration,
• converted into insoluble starch for storage,
• to make fats and oils for storage, especially in seeds,
• to make cellulose, which strengthens cell walls,
• to make amino acids for building proteins — combined with nitrate ions absorbed from the soil.

Starch is used for storage rather than glucose because it is insoluble, so it does not affect the movement of water into and out of cells by osmosis.