Factors Affecting Photosynthesis

The rate of photosynthesis is influenced by several environmental factors. Understanding these factors, and how they interact as limiting factors, is essential for the Edexcel A-Level Biology (9BI0) specification. This lesson covers light intensity, CO₂ concentration, temperature, and the practical techniques used to measure photosynthetic rate.

---

The Concept of Limiting Factors

A limiting factor is the factor that is present at the lowest or least favourable level, and therefore directly controls the rate of a process. When a factor is limiting, increasing it will increase the rate of photosynthesis. Once that factor is no longer limiting, another factor becomes the limiting factor.

This principle was first described by F.F. Blackman (Blackman's Law of Limiting Factors):

"When a process is governed by more than one factor, the rate of the process is limited by the factor nearest its minimum value."

Graphical Representation

When plotting rate of photosynthesis against a single variable:

• The graph initially shows a positive correlation (the variable is limiting).
• The graph then plateaus (another factor has become limiting).
• Increasing the plateau variable further has no effect unless other limiting factors are also increased.

---