Limiting Factors of Photosynthesis

Photosynthesis depends on several environmental conditions. When any one of these conditions is at a suboptimal level, it limits the overall rate — that is, it becomes the limiting factor. OCR specification module 5.2.1(g)–(h) requires you to discuss how light intensity, temperature and carbon dioxide concentration affect the rate of photosynthesis, to interpret graphs of these effects, and to describe experimental methods used to investigate them. Understanding limiting factors is also the key to understanding how glasshouse growers maximise crop yields.

Key Definitions:

• Limiting factor — a factor that, when in short supply, directly limits the rate of a process (such as photosynthesis).
• Compensation point — the light intensity at which the rate of photosynthesis exactly equals the rate of respiration, so net gas exchange is zero.
• Optimum temperature — the temperature at which the rate of a reaction is highest.

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Blackman's Law of Limiting Factors

Blackman (1905) proposed that when a process is affected by more than one factor, the rate is limited by the factor in shortest supply. Increasing any other factor will have no effect until the limiting one is raised. This principle explains the characteristic shape of photosynthesis rate graphs and is a favourite OCR topic.

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The Three Main Limiting Factors

1. Light Intensity

Light provides the energy for the light-dependent stage. As light intensity increases, more photons hit the photosystems, more electrons are excited, and more ATP and reduced NADP are made.

flowchart LR
    L0[Low light] --> R1[Low rate - light limiting]
    L1[Increasing light] --> R2[Rate rises proportionally]
    L2[High light] --> R3[Plateau - other factor limiting]

• At low light, rate is proportional to intensity — light is limiting.
• As light increases, the rate rises until the graph plateaus — now a different factor (CO₂ or temperature) is limiting.
• The point where the graph levels off is called light saturation.
• Very high light intensities can actually damage chlorophyll and reduce rate (photoinhibition) — but at A-Level this is only mentioned briefly.

Compensation point: the light level at which photosynthetic oxygen production exactly balances respiratory oxygen use. Below this, the plant is a net consumer of oxygen; above it, a net producer.

2. Carbon Dioxide Concentration

CO₂ is the substrate for RuBisCO in the Calvin cycle. Atmospheric CO₂ is only about 0.04% (400 ppm), which is actually quite low compared to what plants could use.

• As CO₂ increases, rate rises — CO₂ is limiting.
• At high CO₂, rate plateaus because another factor becomes limiting (typically light or temperature).
• In commercial glasshouses, CO₂ is often artificially raised to 0.1% (1000 ppm) to increase rates.
• Above about 0.15%, stomata may partially close, reducing CO₂ uptake again.

3. Temperature

Temperature affects the enzymes of both the Calvin cycle and the electron transport chain (the light-dependent stage is relatively less temperature-sensitive because it is mostly physical absorption of photons).

• Below the optimum, rate increases with temperature (Q₁₀ ≈ 2 — rate roughly doubles for every 10°C rise) as enzyme-substrate collisions increase.
• At the optimum (typically 25–30°C for temperate plants), rate is maximal.
• Above the optimum, rate decreases sharply as enzymes (especially RuBisCO) begin to denature.
• Very high temperatures also open stomata wider, which can increase water loss and cause stomatal closure — reducing CO₂ uptake.

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Interacting Graphs

OCR often presents combined-factor graphs. A typical example:

flowchart LR
    A[Low CO2 and low temp] -->|Plateau at low rate| B[Raise CO2: plateau rises]
    B -->|Raise temp: plateau rises further| C[Now light-limited at higher level]

• Three lines on a rate-vs-light graph might show: low CO₂ low temperature (lowest plateau), high CO₂ low temperature (middle), high CO₂ high temperature (highest).
• The fact that raising CO₂ or temperature increases the plateau height proves that at high light intensity, the original plateau was limited by CO₂ or temperature, not by light.

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Investigating Photosynthesis: Aquatic Plant Method

OCR students should know at least one classical method for investigating limiting factors. The aquatic plant method (e.g. with Elodea or Cabomba) is commonly used.