Factors Affecting the Rate of Photosynthesis

What does a rate of photosynthesis mean?

Photosynthesis transfers light energy to chloroplasts and uses it to make glucose. Its rate tells us how much photosynthesis occurs in a given time. A faster rate means that products such as glucose and oxygen are being made more quickly.

Four important conditions affect this rate: light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll. If one of these is inadequate, it can restrict the process. Such a condition is called a limiting factor.

Think of a production line: supplying more of one raw material increases output only while that material is holding the process back. Once enough is available, adding still more cannot help unless the next restriction is removed. On a photosynthesis graph, this commonly produces a curve that rises and then levels off.

Light intensity

Chlorophyll absorbs light energy. At low light intensity, relatively little energy is available, so photosynthesis is slow. Increasing light intensity allows more light to be absorbed and the rate increases. Over the initial part of an idealised graph, the rate may be directly proportional to light intensity: doubling the light intensity doubles the rate.

Eventually the graph reaches a plateau. Photosynthesis is still occurring, but increasing light intensity no longer makes it faster. Light is no longer in short supply, so some other condition prevents a further increase. A one-factor graph alone does not necessarily identify what that other condition is.

Carbon dioxide concentration

Carbon dioxide is a reactant in photosynthesis. At a low carbon dioxide concentration, relatively few carbon dioxide molecules are available for the reactions, so the rate is low. Increasing the concentration supplies more reactant and increases the rate.

This curve also commonly levels off. At the plateau, adding more carbon dioxide has no further effect because carbon dioxide is no longer the factor restricting the rate. The plateau is therefore a maximum under those particular conditions, not necessarily the greatest rate the plant could ever achieve.

Temperature

Photosynthesis includes enzyme-controlled reactions. At low temperatures, particles have little kinetic energy and successful enzyme–substrate collisions occur relatively infrequently, so the rate is low. As temperature rises, particles move faster and successful collisions become more frequent. The rate therefore increases up to an optimum temperature.

Above the optimum, the pattern changes. High temperature disrupts the bonds maintaining an enzyme's shape. Its active site changes shape, so substrates fit less successfully; the enzyme is denatured and the rate of photosynthesis decreases. Consequently, a temperature graph usually forms a peak rather than merely rising to a plateau.

Heating does not supply the light energy required by photosynthesis. Temperature affects the activity of the enzymes involved, whereas light supplies energy to the process.

Amount of chlorophyll

Chlorophyll is the pigment in chloroplasts that absorbs light. A plant with less chlorophyll absorbs less light energy, so its rate of photosynthesis is lower when the other conditions are unchanged. Increasing the amount of chlorophyll increases the plant's capacity to absorb light and can increase the rate.

However, extra chlorophyll cannot keep increasing the rate indefinitely. Once enough light is being captured for the prevailing conditions, some other requirement prevents any further rise. This can again produce a plateau.

Reading a one-factor graph

First read the axis labels and units. The factor deliberately changed—such as light intensity—is the independent variable and belongs on the x-axis. The rate of photosynthesis is the dependent variable and belongs on the y-axis.

Then divide the graph into regions and describe each one:

1. Rising region: as the factor increases, the rate increases. A straight line through the origin indicates direct proportionality over that region.
2. Plateau: the rate remains constant even though the factor continues to increase. The factor on the x-axis is no longer limiting.
3. Falling region: for temperature, a fall beyond the optimum indicates reduced enzyme activity caused by denaturation.

When extracting information, trace from a value on one axis to the curve and then across to the other axis. Quote both the value and its unit. When translating a graph into words, describe the overall pattern and important regions rather than listing every plotted point.

Plotting an appropriate graph

Light intensity, carbon dioxide concentration, temperature and photosynthesis rate are continuous variables, so they are normally shown using a scatter graph with a suitable line or smooth curve of best fit. Label both axes, include units where given and choose a simple, even scale that uses most of the plotting area. Plot points accurately; do not force a best-fit curve through every point if the data show normal variation.

The shape matters biologically. A light-intensity or carbon-dioxide curve normally rises and then plateaus, whereas a temperature curve rises to an optimum and then falls. These patterns connect the numerical data to the underlying mechanism.

Core recall

The rate of photosynthesis is affected by light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll.