4.1.2a Rate of photosynthesis

Limiting Factors Control the Rate of Photosynthesis

1. The rate of photosynthesis depends on light intensity, temperature, carbon dioxide concentration and the amount of chlorophyll.
2. At any time, the factor that is in shortest supply is the limiting factor, because it holds the rate back.
3. Water is also needed, but it is not usually a limiting factor because a shortage would damage the plant long before it slowed photosynthesis.

How Light, Temperature, Carbon Dioxide and Chlorophyll Affect the Rate

Light intensity

1. Light provides the energy for photosynthesis, so the brighter the light, the faster the rate.
2. With no light, photosynthesis stops; the rate keeps rising with light until another factor becomes limiting.

Temperature

1. As temperature rises, particles gain more kinetic energy and collide more often, so the rate increases.
2. Photosynthesis is controlled by enzymes, so if the temperature gets too high (around 40 to 50°C) the enzymes are denatured and the rate falls.

Carbon dioxide concentration

1. Carbon dioxide is a raw material for making glucose, so more carbon dioxide gives a faster rate.
2. The air is only about 0.04% carbon dioxide, so it is often the limiting factor; the rate rises with carbon dioxide until another factor limits it.

Amount of chlorophyll

1. Chlorophyll absorbs the light, so the more chlorophyll a plant has, the faster it can photosynthesise.
2. The amount of chlorophyll is reduced by disease (such as tobacco mosaic virus), a lack of magnesium, or the loss of leaves, which all lower the rate.

Reading One-Factor Graphs

1. For light intensity and carbon dioxide, the rate rises steeply at first and then levels off (plateaus).
2. On the rising part, that factor is limiting; on the plateau, something else has become the limiting factor.
3. The temperature graph is a different shape: the rate rises to an optimum and then falls sharply as the enzymes denature.

Required Practical: Light Intensity and Pondweed

This practical investigates how light intensity affects the rate of photosynthesis in an aquatic plant such as pondweed (Elodea or Cabomba).

Measuring and Calculating the Rate

1. The rate is measured from the oxygen given off, either as the number of bubbles per minute or the volume of gas per minute.
2. The rate is calculated as the amount of oxygen divided by the time taken.
3. The results are plotted on a graph, with the factor being changed on the x-axis and the rate on the y-axis.
4. The exact way distance links to light intensity (the inverse square law) is covered in the Higher tier article.

4.1.2b Rate of photosynthesis (Higher tier)

Factors Interact, and Only One Limits at a Time

1. In real conditions, light intensity, temperature and carbon dioxide concentration all change at once.
2. At any moment, the factor in shortest supply is the limiting factor, and it is the one holding the rate back.
3. Early in the morning light and temperature often limit the rate; later, as they rise, carbon dioxide often becomes limiting.
4. On a bright, cold winter day, temperature is usually the limiting factor.

Reading Graphs With Two or Three Factors

1. A graph may show two or three curves taken at different temperatures or carbon dioxide levels.
2. While the curves rise together, light intensity is the limiting factor for all of them.
3. Where the curves level off at different heights, light is no longer limiting; the higher curve has more of another factor, which was limiting the lower curve.
4. For example, a curve at 25°C that levels off higher than one at 15°C shows that temperature was limiting the lower curve.
5. In a three-factor graph, the highest curve has the highest temperature and carbon dioxide, so those factors were limiting the lower curves.

Light Intensity and the Inverse Square Law

1. Light intensity and distance are inversely proportional: as the distance from the lamp increases, the light intensity falls.
2. The link is not linear; light intensity is proportional to one divided by the distance squared.

light intensity ∝ 1 ÷ distance²

1. This means doubling the distance does not halve the intensity; it cuts it to a quarter.

Limiting Factors and Greenhouse Economics

1. Growers use greenhouses to control light, temperature and carbon dioxide and get the fastest possible rate of photosynthesis.
2. Controlling these factors costs money, for example heating, lighting and adding carbon dioxide from burning fuel.
3. The grower must balance the extra cost against the extra income from a bigger, faster crop, so the aim is the most profit, not the highest possible rate.
4. Raising one factor beyond the point where another factor becomes limiting wastes money, because it no longer increases the rate.