Limiting Factors in Photosynthesis

The rate of photosynthesis is influenced by several environmental factors. At any given time, the factor that is furthest from its optimum is the one that limits the rate of photosynthesis. This factor is called the limiting factor. Understanding limiting factors is essential for interpreting experimental data and for optimising conditions in agriculture and horticulture (e.g., in greenhouses).

Key Definition: A limiting factor is the factor that is present at the lowest or least favourable level, and which therefore restricts the rate of a process. Increasing the limiting factor will increase the rate of the process, up to a point.

This concept was first described by F.F. Blackman (1905), who proposed the law of limiting factors: "When a process depends on more than one essential condition being favourable, its rate is limited by the factor that is nearest its minimum value."

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Light Intensity

Effect on Photosynthesis

• At low light intensities, increasing light intensity increases the rate of photosynthesis proportionally (the graph is a straight line). Light is the limiting factor.
• At moderate light intensities, the rate of increase begins to slow (the curve begins to plateau). Another factor (usually CO₂ concentration or temperature) is becoming limiting.
• At high light intensities, the rate reaches a plateau — increasing light intensity further has no effect. Light is no longer the limiting factor.

Mechanism

• Light energy is needed for the light-dependent reactions: photoionisation of chlorophyll, photolysis of water, and driving the electron transport chain.
• More light → more ATP and NADPH produced → Calvin cycle can proceed faster.
• At very high light intensities, all photosystems are saturated (all P680 and P700 reaction centres are being continuously excited), and the rate of the Calvin cycle is limited by the supply of CO₂ or the activity of RuBisCO.

Inverse Square Law

• Light intensity is inversely proportional to the square of the distance from the light source:
  • Light intensity ∝ 1/d²
• This relationship is important in practical investigations where a lamp is moved to vary light intensity.

Exam Tip: When describing the effect of light intensity on the rate of photosynthesis, use precise language: "As light intensity increases, the rate of photosynthesis increases because more light energy is available for the light-dependent reactions, until another factor becomes limiting."

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Carbon Dioxide Concentration

Effect on Photosynthesis

• At low CO₂ concentrations, CO₂ is the limiting factor. Increasing [CO₂] increases the rate of photosynthesis as more CO₂ is available for carbon fixation by RuBisCO.
• At high CO₂ concentrations, the rate plateaus — RuBisCO is working at its maximum rate (Vmax), and another factor (light or temperature) becomes limiting.

Mechanism

• CO₂ is the substrate for the Calvin cycle (carbon fixation step).
• More CO₂ → more RuBP is carboxylated → more GP is produced → more GALP is made → more glucose is synthesised.
• Atmospheric CO₂ concentration is approximately 0.04% (400 ppm), which is relatively low and often limits photosynthesis in natural conditions.

Agricultural Application

• In commercial greenhouses, CO₂ is sometimes artificially elevated to 0.1% (1000 ppm) or higher to increase crop yields.
• CO₂ enrichment must be balanced against costs (the CO₂ must be supplied, and heating/lighting may also be needed).

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Temperature

Effect on Photosynthesis

• At low temperatures, increasing temperature increases the rate of photosynthesis because the kinetic energy of molecules increases, leading to more frequent enzyme-substrate collisions. Temperature is the limiting factor.
• At the optimum temperature (typically around 25-35°C for most plants), the rate is maximal. All enzymes involved in the Calvin cycle (especially RuBisCO) are functioning at their optimal rate.
• Above the optimum, the rate of photosynthesis decreases rapidly as enzymes begin to denature — hydrogen bonds and other weak interactions holding the tertiary structure of the enzyme break, distorting the active site so that substrates can no longer bind effectively.

Mechanism

• Temperature primarily affects the light-independent reactions (Calvin cycle) because these are enzyme-catalysed.
• The light-dependent reactions are less affected by temperature because they rely on photochemical processes (light absorption) rather than enzyme catalysis (though some steps, such as ATP synthase activity, are temperature-dependent).