The limiting factors of photosynthesis (light intensity, carbon dioxide concentration, temperature and chlorophyll), the inverse square law for light, and the economics of controlling these factors in greenhouses.

What this dot point is asking

AQA wants you to explain the four limiting factors of photosynthesis, interpret rate graphs, use the inverse square law for light intensity (higher tier), and explain how greenhouse growers control these factors to increase yield economically. It links directly to the required practical on light intensity and photosynthesis.

Limiting factors

The four factors AQA wants are:

• Light intensity: light supplies the energy for the reaction, so more light gives a faster rate, until another factor becomes limiting.
• Carbon dioxide concentration: carbon dioxide is a raw material, so more of it increases the rate, again up to a limit. Air is only about 0.04 percent carbon dioxide, so it is often the limiting factor outdoors on a bright day.
• Temperature: the reactions are controlled by enzymes, so the rate rises with temperature up to an optimum; too hot (above roughly 40 to 45 degrees Celsius) and the enzymes denature, so the rate falls sharply.
• Chlorophyll: the amount of chlorophyll affects how much light can be absorbed. It can be reduced by disease (such as tobacco mosaic virus) or by a lack of magnesium ions, which the plant needs to make chlorophyll.

Interpreting rate graphs

On a rate-against-light-intensity graph the line rises steeply at first (light is limiting), then levels off (plateaus) when a different factor (carbon dioxide or temperature) becomes limiting. If you then repeat the experiment at a higher carbon dioxide concentration or higher temperature, the line plateaus at a higher rate, which proves that the second factor was the one limiting the first plateau. AQA often asks you to identify, from such a graph, which factor is limiting at a labelled point: where the line is still rising the named axis variable is limiting; where it is flat, something else is.

The inverse square law

For light intensity (higher tier), light obeys the inverse square law: intensity is proportional to $\dfrac{1}{d^2}$, where $d$ is the distance from the light source.

Photosynthesis in greenhouses

Commercial growers control the limiting factors to increase yield:

• Adding artificial light so plants photosynthesise for longer and through the night.
• Burning fuel such as paraffin to add carbon dioxide to the air and provide some heat at the same time.
• Using heaters to raise the temperature towards the enzymes' optimum.

This all costs money, so the grower balances the extra cost against the increased yield and profit. The aim is to make every factor non-limiting so the plants photosynthesise as fast as possible.

Required practical

You can investigate the effect of light intensity on the rate of photosynthesis using pondweed such as Cabomba or Elodea. Count the bubbles of oxygen given off in a set time, or measure the volume of gas collected, at several distances from a lamp. Control temperature (the lamp heats the water, so use a heat shield or beaker of water as a filter) and carbon dioxide (add sodium hydrogencarbonate). Plot rate against light intensity (or against $\dfrac{1}{d^2}$ at higher tier).

Try this

Q1. Explain why the rate of photosynthesis levels off as light intensity increases. [2 marks]

• Cue. Light is no longer limiting; another factor (carbon dioxide or temperature) has become the limiting factor.

Q2. A lamp is moved from $10\ cm$ to $30\ cm$ from a plant. By what factor does the light intensity change? [2 marks]

• Cue. Distance trebles, so intensity falls to $1/3^2 = 1/9$ of the original.