Which of the following factors does NOT affect photosynthesis?

Analyze the question: We need the factor that does NOT directly affect the rate of photosynthesis.

Concept: Photosynthesis is summarised as:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light and chlorophyll)

The main factors that affect its rate are:

• Carbon (iv) oxide concentration: it is a raw material, so more CO₂ increases the rate until another factor becomes limiting.
• Light intensity: it supplies the energy for the light reactions.
• Temperature: it affects the enzymes that control the dark (light-independent) reactions.
• Water availability: water is a raw material, and a shortage causes stomata to close, which reduces CO₂ intake.
• Chlorophyll content: it traps light energy.

Going through the options:

• A. CO₂ is a raw material, so it affects photosynthesis.
• B. Light intensity supplies energy, so it affects photosynthesis.
• C. Relative humidity mainly affects the rate of transpiration. Its effect on photosynthesis is only indirect and minor, so it is not counted as a main factor.
• D. Soil water is the source of water for photosynthesis, so it affects it.
• E. Temperature affects enzyme activity, so it affects photosynthesis.

Answer: The factor that does not (directly) affect photosynthesis is C. relative humidity.

Note: Humidity can have a small indirect effect, because it influences stomatal opening. In exam questions of this type, though, it is the accepted "odd one out", since the other four are the classic limiting factors or raw materials.

Exam tip: Remember the "CLTW" factors for photosynthesis: CO₂, Light, Temperature, Water (plus chlorophyll). Humidity is linked with transpiration, not photosynthesis.