Factors Affecting the Rate of Photosynthesis

What is the limiting factor principle in photosynthesis?

• The rate of photosynthesis is limited by the factor that is closest to its minimum value. Increasing any other factor will have no effect on the rate until the limiting factor is also increased.

What are the three main environmental factors that limit the rate of photosynthesis?

• Light intensity

• Carbon dioxide (CO₂) concentration

• Temperature

Which stage of photosynthesis is most directly limited by light intensity?

• The Light-Dependent Reaction (LDR), as light energy is the starting input required to excite electrons and split water (photolysis).

Describe the relationship between light intensity and the rate of photosynthesis.

• Initially: The rate is directly proportional to light intensity (light is limiting).

• Eventually: The rate plateaus (becomes constant) when another factor (like CO₂ or temperature) becomes limiting.

Which stage of photosynthesis is most directly limited by CO₂ concentration?

• The Light-Independent Reaction (LIR) / Calvin Cycle, as CO₂ is required for carbon fixation.

What is the role of CO₂ in the LIR, and which enzyme is involved?

• CO₂ combines with Ribulose Bisphosphate (RuBP). This reaction is catalysed by the enzyme RuBisCO.

What happens in the LIR when CO₂ concentration is low?

• The reaction between CO₂ and RuBP is limited. RuBP levels will increase (as it can't be used up), and Glycerate-3-Phosphate (GP) levels will decrease (as the first product can't be made).

Why does temperature affect the rate of photosynthesis?

• Photosynthesis involves many enzyme-catalysed reactions, especially the LIR (which relies heavily on RuBisCO). Temperature affects the kinetic energy of molecules and the integrity of enzyme structures.

What happens to the rate of photosynthesis when the temperature is above the optimum?

• The rate decreases rapidly because the enzymes (like RuBisCO) and other proteins (e.g., in the thylakoid membranes) begin to denature, permanently changing their active site.

What is the effect of water stress (drought) on the rate of photosynthesis?

• Water stress causes the plant to produce Abscisic Acid (ABA), which leads to stomatal closure. This stops the uptake of CO₂, making CO₂ concentration the limiting factor, thus reducing the rate.

What is the main commercial use of controlling limiting factors in a greenhouse?

• To increase the yield (e.g., of tomatoes or flowers) by keeping light intensity, temperature, and CO₂ concentration at their optimum (or highest cost-effective) levels.

What is the "compensation point"?

• The light intensity at which the rate of photosynthesis is equal to the rate of aerobic respiration. At this point, there is no net exchange of CO₂ or O₂ with the environment.

A plant is photosynthesising at 25°C and 0.04% CO₂. When CO₂ is increased to 0.5%, the rate increases significantly. When light intensity is then doubled, the rate does not change. Explain these observations.(4 marks)

• Initial Increase: The rate increased when CO₂ was added because CO₂ was the initial limiting factor (1 mark).

• Saturation: The rate stopped increasing when light was doubled because the high CO₂ allowed the LIR to run faster, and now a new factor (light intensity or temperature) became limiting (1 mark).

• Light/Temp: Since increasing light had no effect, temperature must have become the limiting factor (1 mark).

• Enzymes: Temperature limits the kinetic energy and rate of collision between enzymes (like RuBisCO) and substrates in the LIR (1 mark).

Explain the limiting effect of low CO₂ concentration on the Calvin cycle.(3 marks)

• CO₂ is required for carbon fixation (1 mark).²⁰

• When CO₂ is low, there is not enough substrate to combine with RuBP (5C), even though light products (ATP/NADPH) are available.

• This means GP levels decrease (as it is not being made), and RuBP levels accumulate (as it cannot be used up).

Explain how temperature limits the rate of photosynthesis at temperatures above the optimum.(3 marks)

• High temperatures cause the enzymes (such as RuBisCO) to vibrate excessively (1 mark).

• This vibration breaks the hydrogen bonds and ionic bonds that maintain the enzyme's tertiary structure (1 mark).

• The enzyme's active site changes shape and can no longer bind to the substrate (denaturation), causing a rapid drop in the rate (1 mark).

Explain why increasing light intensity may not always increase the rate of photosynthesis.(4 marks)

• Light energy is only required for the Light-Dependent Reaction (LDR) (1 mark).

• If the LDR is already running at its maximum speed (saturated), increasing light will have no further effect (1 mark).

• The rate is then limited by an input to the LIR (Calvin cycle), such as CO2​ concentration (low CO2​) (1 mark).

• Alternatively, the rate could be limited by temperature (due to enzyme activity) (1 mark).

A plant suffers from water stress. Explain how this limits photosynthesis, even if light and temperature are optimum.(3 marks)

• Water stress causes the release of Abscisic Acid (ABA), which leads to stomatal closure (1 mark).

• Stomatal closure stops the uptake of CO₂ from the atmosphere (1 mark).

• This makes CO₂ concentration the limiting factor for the Calvin cycle, severely reducing the rate of photosynthesis (1 mark).