Factors Affecting the Rate of Photosynthesis

To survive, all plants must carry out photosynthesis. Four main factors affect the rate of photosynthesis:

  • The amount of chlorophyll
  • Temperature
  • Light intensity
  • Carbon dioxide concentration

Chlorophyll

Chlorophyll is a green pigment in plants located in the chloroplasts.

The image presents an illustration of a green plant on the left with multiple leaves labelled "GREEN PLANTS". A magnified view of one of the leaves is depicted on the right, revealing the microscopic "LEAF CELLS". Within these cells are prominent green structures labelled as "Chloroplast". The overall image visually connects the broader plant structure to its cellular components, emphasising the role of chloroplasts in green plants.

Chlorophyll absorbs light energy, which is needed for photosynthesis. So, plants that have less chlorophyll will not be able to carry out as much photosynthesis. Plants that grow in darker areas synthesise more chlorophyll, so they can absorb more light.

Plant diseases can affect the amount of chlorophyll an individual plant has or can produce. This can affect the rate at which the plant photosynthesises.

Temperature

Temperature affects the kinetic energy of particles. So, in plants, temperature affects the speed at which enzymes and their substrates (carbon dioxide and water) move within a plant. When the temperature increases, the number of successful collisions increases, so the rate of photosynthesis increases.

Photosynthesis is controlled by enzymes, so the rate doesn’t increase indefinitely. At low temperatures, the rate of photosynthesis is limited by the number of collisions between enzymes and substrate molecules. However, at higher temperatures, the enzymes begin to denature. This is when the active site changes shape so much that it is no longer complementary to the substrate. So, the rate of reaction decreases.

The image displays a bell-shaped curve plotted on a graph. The x-axis is labelled "Temperature (°C)" and represents varying temperatures. The y-axis is labelled "Rate of Photosynthesis" indicating the efficiency of photosynthesis at different temperatures. Three distinct points on the curve are marked with green dots and labelled "A", "B", and "C". Point "A" is on the ascending slope, point "B" is at the peak of the curve indicating the optimum temperature for photosynthesis, and point "C" is on the descending slope, illustrating a decrease in photosynthesis rate at higher temperatures.

AThe rate of photosynthesis increases as the number of collisions between enzymes and substrate increases
BThis is the optimum temperature
CThe enzymes begin to denature, so the rate of photosynthesis decreases

Light Intensity

Plants use light energy for photosynthesis, which makes it essential. Increasing the light intensity will increase the rate of photosynthesis. Eventually, the rate of photosynthesis will stop increasing because another factor (limiting factor) such as carbon dioxide or temperature is in short supply.

At a certain point, increasing the light intensity past the optimum does not affect the rate of photosynthesis.

The image depicts a graph showing the relationship between "Light intensity" on the x-axis and the "Rate of Photosynthesis" on the y-axis. The curve rises steeply from point "A", suggesting an increase in photosynthesis rate with light intensity. As the curve approaches point "B", the rate of increase slows, and by point "C", the curve levels off, indicating that the rate of photosynthesis plateaus despite further increases in light intensity. Points "A", "B", and "C" are highlighted with green dots along the curve.

AAt low light intensities, the increase in the rate of photosynthesis is linear
BAt this point, another factor becomes limiting
CThe graph levels off and the rate becomes constant

For example, as you increase the light intensity, plants can absorb more light energy for photosynthesis. This causes the rate of photosynthesis to increase.

However, it may reach a point when the temperature becomes a limiting factor. So, increasing the light intensity will no longer increase the rate of photosynthesis. At this point, the rate of photosynthesis will not increase until the temperature increases.

  • In this case, increasing the temperature will allow the rate of photosynthesis to plateau at a higher level with increased light intensity.

Carbon Dioxide Concentration

Carbon dioxide is required for photosynthesis. Therefore, increasing the concentration of carbon dioxide will increase the rate of photosynthesis. However, this only holds true up to a certain point, after which another factor (a limiting factor) is in short supply.

The image displays a graph illustrating the relationship between "Carbon dioxide concentration" on the x-axis and the "Rate of Photosynthesis" on the y-axis. Beginning from point "A", the curve ascends sharply, indicating a rise in photosynthesis rate with increasing carbon dioxide concentration. The ascent slows near point "B" and plateaus by point "C", signifying that the rate of photosynthesis stabilises regardless of further increases in carbon dioxide concentration. Points "A", "B", and "C" are marked with green dots on the curve.

AAt lower concentrations, the increase in the rate of photosynthesis is linear
BAt this point, another factor becomes limiting
CThe graph levels off and the rate becomes constant

Increasing the concentration of carbon dioxide above the optimum will not affect the rate of photosynthesis.