Limiting factors on photosynthesis (Edexcel GCSE Biology): Revision Notes

Limiting factors on photosynthesis

Limiting Factors in Photosynthesis:

• A limiting factor is an environmental condition that restricts the rate of photosynthesis when it is in short supply, even if other factors are in abundance. Examples include light intensity, carbon dioxide concentration, and temperature.
• Graphical Representation:
  • A graph showing the rate of photosynthesis against one factor, such as light intensity, will show a line that initially increases but then levels off when another factor becomes limiting.
  • A graph with two or three lines may represent experiments with different levels of a second or third factor, helping to identify which factor is limiting at different points.

For example, if light intensity is the limiting factor, increasing it will raise the rate of photosynthesis until another factor, such as temperature, becomes limiting. This is often depicted as the curve flattening out on a graph.

Applications in Agriculture:

• Greenhouse Management: Farmers can use knowledge of limiting factors to optimise conditions in greenhouses, such as increasing light intensity, CO₂ levels, and maintaining optimal temperatures. This boosts the rate of photosynthesis, leading to faster plant growth and higher yields, ultimately increasing profits.

Inverse Square Law:

• Inverse Proportion: Light intensity is inversely proportional to the square of the distance from the light source, described by the inverse square law.
• This means that as the distance between the light source and the plant doubles, the light intensity falls to a quarter of its original value.

$$\text{Light Intensity} \propto \frac{1}{\text{Distance}^2}$$

Light

Transfers energy needed for photosynthesis & is directionally proportional to the rate up to a certain point. Beyond that point, it will either be CO2 or temp that affects it

Investigating light

• Moving a light metre closer/ further away from plant
• Plot the rate of photosynthesis against 'Distance from the plant' The distance from the lamp & light intensity are inversely proportional (inverse square law)

$$\text{Light Intensity} \propto \frac{1}{\text{Distance}^2}$$

Carbon Dioxide

Levels of carbon dioxide in the atmosphere are rising because of greenhouse gas emissions. They are currently at around 0.04%. This concentration is still very low in terms of being the optimum for photosynthesis.

Carbon dioxide concentration is therefore an important limiting factor for photosynthesis.

One of the raw materials needed for photosynthesis Same as Light where the rate and CO2 is directionally proportional up to a point

Temperature

• When it is too cold - The enzymes needed for photosynthesis work to slowly
• When it is too hot - The enzymes denature at around 45 Degrees

The optimum temperature for most plants is 15°C to around 40°C.

Temperature affects the rate of photosynthesis in crop plants and affects where certain crops can be grown.

Water

In the UK, water is often the main limiting factor for photosynthesis.

You won't see graphs for its effects, as water is important in many other areas of a plant's life, and not just photosynthesis. Most important is its role as a solvent for all the chemical reactions in cells.

Chlorophyll

Plants that grow in shaded areas increase the chlorophyll content in their leaves to absorb enough light energy for photosynthesis.

The compensation point, where the rate of photosynthesis equals the rate of respiration, differs for plants in shaded environments compared to those in brighter conditions. Shade-adapted plants have a lower compensation point, meaning their photosynthesis rate surpasses their respiration rate at lower light intensities than plants adapted to sunnier environments.