Limiting Factors in Photosynthesis

Photosynthesis Overview

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy in organic molecules. The overall equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The process occurs in two main stages within the chloroplast:

1. Light-dependent reactions — occur on the thylakoid membranes; light energy is absorbed by photosynthetic pigments (chlorophyll a, chlorophyll b, carotenoids) and used to produce ATP and reduced NADP (NADPH). Water is split by photolysis, releasing O₂.

2. Light-independent reactions (Calvin cycle) — occur in the stroma; CO₂ is fixed by RuBisCO into glycerate-3-phosphate (GP), which is reduced to triose phosphate (TP) using ATP and NADPH. TP is used to regenerate RuBP and to synthesise glucose and other organic molecules.

The Concept of Limiting Factors

The principle of limiting factors (Blackman, 1905) states that at any given time, the rate of a physiological process is limited by the factor that is nearest to its minimum value. Increasing this limiting factor will increase the rate until another factor becomes limiting.

For photosynthesis, the three main limiting factors are:

• Light intensity
• Carbon dioxide concentration
• Temperature

Light Intensity

Light provides the energy for the light-dependent reactions. As light intensity increases from zero:

• The rate of photosynthesis increases linearly at first — light is the limiting factor
• At a certain point, the rate plateaus — another factor (CO₂ concentration or temperature) has become limiting
• Further increases in light intensity have no effect on the rate
• At very high light intensities, photoinhibition can occur — damage to photosystems reduces the rate

The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration. Below this point, the plant is a net consumer of O₂; above it, the plant is a net producer.

Interpreting graphs: A graph of rate vs. light intensity at constant CO₂ and temperature shows a curve that rises steeply, then levels off. Increasing CO₂ or temperature shifts the plateau to a higher rate.

Carbon Dioxide Concentration

CO₂ is the substrate for the Calvin cycle — it is fixed by RuBisCO to form GP. Atmospheric CO₂ concentration is approximately 0.04% (400 ppm), which is well below the optimum for photosynthesis.

• Increasing CO₂ concentration increases the rate of the Calvin cycle, producing more GP and TP
• The rate increases until another factor (light intensity or RuBisCO activity/amount) becomes limiting, and the curve plateaus
• In glasshouses, CO₂ enrichment (to ~0.1%) is used commercially to increase crop yields

At very high CO₂ concentrations, the rate still plateaus because:

• All RuBisCO active sites are saturated
• The light-dependent reactions cannot supply ATP and NADPH fast enough to keep up with carbon fixation

Temperature

Temperature affects the rate of enzyme-catalysed reactions in both the light-dependent and light-independent stages.

• As temperature rises from low values, the rate increases — molecules have more kinetic energy, increasing the frequency of enzyme-substrate collisions (approximately doubling for every 10°C rise: Q₁₀ ≈ 2)
• The rate reaches an optimum (typically around 25-30°C for temperate plants, higher for tropical species)
• Above the optimum, the rate decreases sharply — enzymes (including RuBisCO) begin to denature as hydrogen bonds and other weak interactions maintaining the tertiary structure are disrupted
• At very high temperatures, enzymes are permanently denatured and the rate drops to zero

Temperature also affects:

• The fluidity of thylakoid membranes — extreme temperatures disrupt the arrangement of photosystems
• The solubility of CO₂ — CO₂ is less soluble in water at higher temperatures, which can reduce the concentration available to RuBisCO
• Photorespiration — at high temperatures, RuBisCO increasingly fixes O₂ instead of CO₂ (oxygenase activity), reducing the efficiency of the Calvin cycle. This is a significant problem for C3 plants.

Interaction of Limiting Factors

In practice, the three factors interact. To determine which factor is limiting at any given moment, consider which factor, if increased, would increase the rate:

Example scenario:

• On a bright, warm summer day with normal atmospheric CO₂: CO₂ is the limiting factor — light and temperature are sufficient, but CO₂ is at only 0.04%
• On a cold, bright morning: temperature is the limiting factor — enzymes are working slowly despite adequate light and CO₂
• At dawn on a warm day: light intensity is the limiting factor — temperature and CO₂ are adequate but light is low

Multi-factor Graphs

AQA commonly presents graphs showing rate of photosynthesis against one factor at different levels of a second factor. For example:

• Rate vs. light intensity at two different CO₂ concentrations: both curves rise initially, but the higher CO₂ curve plateaus at a higher rate. At low light intensity, the curves overlap (light is limiting for both). At high light intensity, only the lower CO₂ curve has plateaued (CO₂ is now limiting), while the higher CO₂ curve may still be rising or plateaus higher.

Application: Glasshouses

Commercial growers manipulate limiting factors to maximise photosynthesis and crop yield:

Factor	Method of control
Light	Supplementary artificial lighting extends the photoperiod and increases intensity
CO₂	CO₂ generators (propane burners) or cylinders raise concentration to ~1000 ppm
Temperature	Heating in winter; ventilation/shading in summer; thermostatically controlled
Water	Irrigation systems maintain optimal water availability

The cost of providing these conditions must be balanced against the increase in yield — the optimum economic level is not the same as the biological optimum.

Measuring Photosynthesis

Common methods include:

• Volume of O₂ evolved — using an aquatic plant (e.g. Elodea/Cabomba) and counting bubbles or collecting gas in a syringe
• Uptake of CO₂ — using a CO₂ sensor or indicator solution
• Increase in dry mass over time (destructive; long-term)
• Changes in pH of surrounding solution (as CO₂ is removed, pH rises)

When measuring the effect of light intensity, the inverse square law applies: light intensity ∝ 1/d², where d is the distance between the light source and the plant.

Exam Tips

• Always identify which factor is limiting by asking which one, if increased, would raise the rate — AQA mark schemes require this reasoning
• On multi-factor graphs, describe what is happening in EACH section of the curve (rising = factor X is limiting; plateau = factor Y is now limiting)
• The compensation point is commonly tested — at this point, net gas exchange is zero (CO₂ released by respiration = CO₂ fixed by photosynthesis)
• When discussing glasshouses, always mention the economic consideration — it is not worth providing unlimited CO₂ if the cost exceeds the value of extra yield
• Remember that gross photosynthesis = net photosynthesis + respiration