Factors affecting the rate of photosynthesis

The rate of photosynthesis is how quickly a plant carries out photosynthesis. It depends on conditions including light intensity, carbon dioxide concentration and temperature. A factor that restricts the rate under the current conditions is called a limiting factor.

Light intensity and carbon dioxide concentration

Photosynthesis requires light and carbon dioxide. If light intensity is limiting, increasing it raises the rate of photosynthesis. If carbon dioxide concentration is limiting, increasing it raises the rate.

The rate does not keep rising indefinitely as either factor increases. Eventually, another factor becomes limiting, so increasing the first factor further has little or no effect. For example, increasing light intensity may initially increase the rate, but the rate may then level off because carbon dioxide is limiting. Increasing the carbon dioxide concentration could allow the rate to rise again.

Temperature and photosynthesis

Photosynthesis involves enzyme-controlled reactions. As temperature rises, these reactions generally become faster, so the rate of photosynthesis increases towards an optimum temperature.

Above the optimum, the rate falls. High temperatures can cause enzymes to lose their working shape, so they can no longer carry out their reactions effectively.

Investigating a factor affecting photosynthesis

To investigate one factor, change that factor and measure the rate in the same way at each value. Keep other relevant conditions controlled so that they do not cause the observed change.

For example, to investigate light intensity, a water plant can be exposed to different light intensities while its temperature and carbon dioxide supply are kept constant. The volume of oxygen produced in a fixed time can be used as an indicator of the photosynthetic rate. Comparing the measurements shows how the rate changes as light intensity changes.

Chlorophyll and photosynthesis rate

Plants use light energy to carry out photosynthesis. Chlorophyll absorbs some of this light, making its energy available for the process.

Under otherwise comparable conditions, a smaller amount of chlorophyll means that less light can be absorbed. Less energy is then available for photosynthesis, so its rate decreases.

When more chlorophyll increases the rate

Increasing the amount of chlorophyll can increase the rate of photosynthesis if the amount of light absorbed is limiting the process. More chlorophyll can absorb more light, providing more energy for photosynthesis.

More chlorophyll does not necessarily increase the rate. If another factor is limiting photosynthesis, absorbing more light will not remove that limit, so the rate may not increase.

Measuring photosynthesis rate

Photosynthesis rate can be estimated by measuring how much a quantity changes over a known time interval. For example, photosynthesis releases oxygen and takes up carbon dioxide, so the amount of oxygen released or carbon dioxide taken up can indicate the rate.

Measuring oxygen from an aquatic plant

An aquatic plant releases oxygen as bubbles. The oxygen can be collected over a measured time and its volume recorded. Alternatively, the bubbles can be counted over the same interval. Bubble count is less precise because bubbles can differ in size: equal numbers of bubbles do not necessarily contain equal volumes of oxygen.

Calculating and comparing rates

Calculate the mean rate by dividing the measured change by the time taken:

For example, if an aquatic plant releases $$12\ \text{cm}^3$$ of oxygen in $$4$$ minutes, its mean rate of oxygen release is:

The unit reflects both what was measured and the time unit: here it is cubic centimetres of oxygen per minute. A bubble count would instead give a rate in bubbles per minute.

To compare rates, use a consistent measurement method and keep conditions such as temperature and light intensity controlled. If one condition is being investigated, change that condition while keeping the others the same.

Photosynthesis rate graphs

A photosynthesis rate graph shows how the rate changes as one factor, such as light intensity, changes. The factor being varied is plotted on the horizontal axis; the rate of photosynthesis is plotted on the vertical axis. Other factors should be kept constant so that the effect of the chosen factor can be examined.

Plotting a photosynthesis rate graph

Label each axis with the quantity and its unit. For example, the horizontal axis could show light intensity in lux, and the vertical axis could show the volume of oxygen produced in $$\mathrm{cm^3/min}$$.

Choose a consistent scale for each axis that covers all the data and uses the available space appropriately. For each pair of measurements, find the factor value on the horizontal scale and the corresponding rate on the vertical scale. Plot a point where the two positions meet.

Draw an appropriate line or curve to show the pattern. It should follow the overall trend of the points; it does not have to pass through every point if the measurements vary.

For example, suppose the data include a light intensity of $$200\ \mathrm{lux}$$ and a rate of $$4\ \mathrm{cm^3/min}$$. Plot the point at $$200$$ on the horizontal scale and $$4$$ on the vertical scale. To read that point from the graph, reverse the process: read its horizontal and vertical positions using the axis scales.

Interpreting the pattern

Read the graph from left to right to see what happens to the rate as the factor increases:

• An upward line or curve shows that the rate rises.
• A plateau, or level section, shows that further increases in the plotted factor do not increase the rate.
• A downward line or curve shows that the rate falls.

A plateau may indicate that another factor is limiting photosynthesis. Increasing the factor on the horizontal axis then has no further effect because the other factor restricts the rate.