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Cambridge IGCSE Biology · 0610

Chapter 6: Plant nutrition (Part 2)

Limiting factors of photosynthesis

Limiting factor
Something present in the environment in such short supply that it restricts life processes. It is the factor which is least available to the plant.
Light intensity
As light intensity increases, the rate of photosynthesis increases until another factor becomes limiting.
Carbon dioxide concentration
As CO2 concentration increases, the rate increases until another factor becomes limiting.
Temperature
Photosynthesis is controlled by enzymes.
  • At low temperatures: Enzymes have little kinetic energy, resulting in few enzyme-substrate complexes being formed.
  • At high temperatures: Enzymes denature, slowing or stopping the reaction.
  • The optimum temperature is typically around 25°C.
Three graphs of rate of photosynthesis against light intensity, carbon dioxide concentration, and temperature showing plateaus and optimum temperature peak
Diagram 1: Limiting factor graphs. Three graphs show rate of photosynthesis against light intensity, CO2 concentration, and temperature. Each curve plateaus when another factor becomes limiting; the temperature graph peaks at the optimum then falls as enzymes denature.

Exam Traps

  • Do not say rate increases indefinitely with light or CO2 — it levels off when another factor limits.
  • Avoid treating oxygen concentration as a standard limiting factor of photosynthesis in 0610.

Investigations and testing

Starch test
Iodine is used; a blue-black colour indicates starch presence (photosynthesis has occurred), while orange-brown indicates no starch.
Need for chlorophyll
Uses a variegated leaf (green and white patches); starch only forms in the green areas where chlorophyll is present.
Need for light
One leaf is partially covered with aluminium foil; only the exposed areas produce starch.
Need for CO2
One plant is placed with soda lime (absorbs CO2) and another with sodium hydrogencarbonate (produces CO2); only the latter produces starch.

Exam Traps

  • Do not expect starch in white areas of a variegated leaf — chlorophyll is absent there.
  • Avoid confusing soda lime (absorbs CO2) with sodium hydrogencarbonate (provides CO2).

Gas exchange investigation

Hydrogencarbonate indicator
Used to detect CO2 levels (orange = normal, purple = low CO2, yellow = high CO2).
In light
Rate of photosynthesis > rate of respiration; CO2 is removed (turns purple).
In dark
Only respiration occurs; CO2 is released (turns yellow).
Dim light
Rates are equal; no net change in CO2 (remains orange).

Exam Traps

  • Do not say respiration stops in the light — both processes occur, but photosynthesis is faster in bright light.
  • Avoid reversing purple and yellow: purple means CO2 removed, yellow means CO2 released.

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