state that light intensity, carbon dioxide concentration and temperature are examples of limiting factors of photosynthesis

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Investigation of Limiting Factors

Investigation of Limiting Factors

Learning Objective

State that light intensity, carbon dioxide concentration and temperature are examples of limiting factors of photosynthesis.

What is a Limiting Factor?

A limiting factor is any variable that, when altered, changes the rate of a biological process. In photosynthesis, the process proceeds only as fast as the most restrictive factor allows.

Key Limiting Factors for Photosynthesis

  • Light intensity – the amount of photons reaching the chloroplasts.

  • Carbon dioxide (CO₂) concentration – the substrate for the Calvin cycle.

  • Temperature – influences enzyme activity and membrane fluidity.

How Each Factor Affects the Rate of Photosynthesis

FactorEffect on PhotosynthesisTypical Experimental Observation
Light intensityIncreases the rate up to a saturation point where all photosystems are excited.Rate rises sharply with increasing light, then plateaus.
CO₂ concentrationProvides substrate for the Calvin cycle; rate rises until Rubisco becomes saturated.Linear increase at low CO₂, leveling off at higher concentrations.
TemperatureAccelerates enzyme-catalysed reactions up to an optimum; beyond that, enzymes denature.Bell‑shaped curve: rate increases to an optimum (≈25‑30 °C for most C₃ plants) then declines.

Typical Experimental Design

  1. Choose a fast‑growing aquatic plant (e.g., Elodea) or a leaf disc.

  2. Place the specimen in a sealed chamber with a known volume of water.

  3. Measure the rate of oxygen evolution (or CO₂ uptake) using a gas‑collection apparatus.

  4. Vary one factor while keeping the other two constant:

    • Light intensity: use neutral‑density filters or adjustable lamps.

    • CO₂ concentration: bubble known concentrations of CO₂‑enriched air.

    • Temperature: use a water bath or thermostatically controlled chamber.

  5. Record the volume of gas produced over a fixed time interval.

  6. Plot the results to identify the point at which the rate no longer increases – the saturation point.

Interpreting the Data

When a graph shows a plateau, the factor being varied has become the limiting factor. The factor that is not varied is said to be in excess. By repeating the experiment for each factor, you can determine which factor is most restrictive under the given conditions.

Key Points to Remember

  • Only one factor can be limiting at any one time; the others are in excess.

  • The optimum temperature is species‑specific; C₄ plants often have higher optima.

  • Light saturation occurs when all photosystems are operating at maximum capacity.

  • CO₂ saturation reflects the maximum rate at which Rubisco can fix carbon.

Suggested diagram: A multi‑panel sketch showing (a) light intensity vs. photosynthetic rate, (b) CO₂ concentration vs. rate, and (c) temperature vs. rate, each illustrating the characteristic saturation or optimum point.

Summary

In photosynthesis, light intensity, carbon dioxide concentration and temperature each have a range over which they increase the rate of the process. Beyond their respective optimum or saturation points, the rate no longer rises, indicating that the factor has become limiting. Understanding these limiting factors is essential for designing experiments and for interpreting how environmental changes affect plant productivity.