Limiting Factors and Agricultural Practices Revision Notes | A Level AQA Biology

1. Why Limiting Factors Matter for Food Production

Farmers and horticulturalists need plants to photosynthesise as fast as possible — faster photosynthesis means more biomass, more yield, and more profit. Yet plants rarely operate at their maximum rate. Something is always holding them back.

That 'something' is a limiting factor: the one environmental variable that, if increased, would raise the rate of photosynthesis. Understanding which factor is limiting — and by how much — is the foundation of modern controlled-environment agriculture.

This lesson covers both the biology of limiting factors and the agricultural practices designed to overcome them. Exam questions frequently ask you to evaluate data from these practices, so you need to be comfortable reading graphs and constructing evidence-based judgements.

2. Environmental Factors That Limit Photosynthesis

Three key environmental factors limit the rate of photosynthesis: light intensity, carbon dioxide ($CO_2$) concentration, and temperature. A fourth factor — water availability — can also limit the rate, though its effect is often indirect. At any given moment, only one factor is the primary limiting factor; raising the others will have no effect until that bottleneck is removed.

2a. Light Intensity

Light drives the light-dependent reactions in the thylakoid membranes. Photons excite electrons in chlorophyll, generating ATP and reduced NADP (NADPH), and splitting water molecules in photolysis to release oxygen. Without sufficient light, these reactions slow — and so does everything downstream.

As light intensity increases from zero, the rate of photosynthesis rises proportionally — this is the linear region of the light-response curve. Eventually, the rate plateaus. At that plateau, light is no longer the limiting factor; either $CO_2$ concentration or temperature has taken over as the bottleneck.

Notice that both curves start at the same gradient — at low light intensities, light is limiting regardless of $CO_2$ level. Once the low-$CO_2$ curve flattens, increasing light has no further effect because $CO_2$ has become the new bottleneck. The high-$CO_2$ curve continues rising to a higher plateau, confirming that removing the $CO_2$ limitation allows light to drive a greater rate.

2b. Carbon Dioxide Concentration

$CO_2$ is the substrate for carbon fixation in the Calvin cycle. The enzyme RuBisCO catalyses the combination of $CO_2$ with ribulose bisphosphate (RuBP) to produce glycerate-3-phosphate (GP). When $CO_2$ concentration is low, RuBisCO operates below its maximum rate, GP production falls, and the regeneration of RuBP slows — reducing the overall rate of the Calvin cycle.

Atmospheric $CO_2$ is approximately 0.04% (400 ppm). Experiments consistently show that raising $CO_2$ concentration above this level — provided light and temperature are not limiting — increases the rate of photosynthesis significantly. This is the principle behind $CO_2$ enrichment in commercial glasshouses.

2c. Temperature

Temperature affects the enzyme-catalysed reactions of photosynthesis — particularly those in the Calvin cycle. As temperature rises from around 0°C, enzyme activity increases and the rate of photosynthesis rises. Between roughly 25°C and 35°C (depending on species), the rate reaches an optimum.

Above the optimum, enzymes including RuBisCO begin to denature: the tertiary structure is disrupted, the active site changes shape, and the enzyme can no longer bind its substrate effectively. The rate of photosynthesis falls sharply. This is why temperature control in glasshouses is critical — too cold slows enzyme activity; too hot causes irreversible damage.

At low temperatures, even with ample light and $CO_2$, the light-independent reactions become the bottleneck because enzyme-catalysed steps are too slow to use the ATP and NADPH produced by the light-dependent reactions. This is an important distinction: light intensity and $CO_2$ concentration affect different stages of photosynthesis, but temperature affects the enzyme-driven stages most directly.

2d. Water Availability

Water is a direct substrate for the light-dependent reactions: photolysis of water provides the electrons that replace those lost from excited chlorophyll, and releases oxygen as a by-product. Severe water deficit therefore directly reduces the rate of the light-dependent reactions.

Water stress also has an indirect effect. When water is scarce, guard cells lose turgor and stomata close, reducing $CO_2$ entry into the leaf. This means water shortage can cause $CO_2$ concentration to become the secondary limiting factor. In exam answers, it is worth distinguishing these two mechanisms — the direct effect on photolysis and the indirect effect via stomatal closure.

3. Agricultural Practices to Overcome Limiting Factors

Now that you know which factors limit photosynthesis, the next question is: what do farmers actually do about it? Modern agriculture uses a range of controlled-environment techniques to push each limiting factor closer to its optimum, maximising crop yield. Exam questions will give you data from these practices and ask you to evaluate their effectiveness.

3a. Glasshouses and Polytunnels

Glasshouses allow growers to manipulate light, temperature, $CO_2$, and water simultaneously. Glass transmits visible light while retaining infrared radiation, raising internal temperature above ambient — useful in temperate climates where outdoor temperatures limit enzyme activity for much of the year.

Supplementary artificial lighting (typically high-pressure sodium lamps or LEDs) extends the photoperiod and raises light intensity during winter months or in northern latitudes. LED systems can be tuned to emit predominantly red and blue wavelengths, which are most efficiently absorbed by chlorophyll a and b, improving the efficiency of energy conversion.

3b. CO2 Enrichment

Commercial glasshouses frequently inject $CO_2$ to raise internal concentrations to around 0.1–0.2% (1000–2000 ppm) — two to five times the atmospheric level. This removes $CO_2$ as a limiting factor, allowing the Calvin cycle to operate at a higher rate provided light and temperature are also adequate.

The economic trade-off is important here. $CO_2$ enrichment increases running costs (fuel for $CO_2$ generators or purchase of compressed $CO_2$). The benefit — increased yield — must outweigh this cost for the practice to be commercially viable. Exam questions often ask you to evaluate this balance using yield data and cost figures.

3c. Evaluating Data from Agricultural Practices

AQA exam questions on this topic almost always present data — a graph, a table of yields, or a cost-benefit summary — and ask you to evaluate. The command word 'evaluate' requires you to weigh evidence on both sides and reach a justified conclusion. Here is a worked approach using a realistic data set.

Worked example: Evaluating Data: CO2 Enrichment and Crop Yield

A grower tests three CO2 concentrations in identical glasshouses at 25°C with constant high light intensity. Tomato yield (kg per m2 per year) is recorded. Evaluate whether CO2 enrichment is an effective agricultural practice based on the data below. CO2 concentration: 0.04% → Yield: 18 kg/m2 CO2 concentration: 0.10% → Yield: 26 kg/m2 CO2 concentration: 0.20% → Yield: 29 kg/m2

1. Yield increases as CO2 concentration increases from 0.04% to 0.20%. — State the direction of the relationship clearly. Examiners award a mark for correctly describing the trend before any interpretation.
2. From 0.04% to 0.10%: increase = 26 - 18 = 8 kg/m2 (a 44% increase). From 0.10% to 0.20%: increase = 29 - 26 = 3 kg/m2 (an 12% increase). — Using actual numbers from the data is essential for evaluation marks. Notice the rate of increase is slowing — this is evidence of diminishing returns.
3. Higher CO2 increases substrate availability for RuBisCO, raising the rate of carbon fixation in the Calvin cycle and increasing GP production, leading to greater glucose synthesis and biomass accumulation. — Link the data trend to the underlying biology. This is what separates a 4-mark answer from a 6-mark answer.
4. The yield increase from 0.10% to 0.20% CO2 is much smaller (3 kg/m2) than from 0.04% to 0.10% (8 kg/m2). This suggests another factor — possibly light intensity or temperature — is becoming limiting at higher CO2 levels. — Recognising diminishing returns and attributing them to a new limiting factor demonstrates higher-order analysis.
5. CO2 enrichment to 0.10% is effective: it produces a substantial yield increase of 44% with a relatively modest doubling of CO2 concentration. Enrichment beyond 0.10% shows diminishing returns and may not be cost-effective unless light and temperature are also optimised. — A good evaluation conclusion is conditional — it states when the practice is effective and when it is not, supported by the data.

Answer: CO2 enrichment is an effective practice up to approximately 0.10% CO2, producing a 44% yield increase. Beyond this, diminishing returns suggest another factor is limiting, making further CO2 enrichment less cost-effective without additional interventions.

4. Interpreting Limiting Factor Graphs

Graph interpretation is a core exam skill for this topic. The shape of a rate-of-photosynthesis curve tells you which factor is limiting at each point — and where the transition between limiting factors occurs.

The key features to identify are: the initial gradient (steeper = factor is strongly limiting), the plateau (where the tested factor is no longer limiting), and the effect of changing a second variable on the plateau height (which reveals the new limiting factor).

Worked example: Reading a Limiting Factor Graph

A graph shows rate of photosynthesis (arbitrary units) against light intensity for a plant at two temperatures: 20°C and 30°C. Both curves have the same initial gradient. The 20°C curve plateaus at a rate of 20 units; the 30°C curve plateaus at a rate of 35 units. CO2 concentration is constant at 0.04% throughout. Explain what the graph shows about limiting factors.

1. Both curves rise at the same rate at low light intensities. — Identical gradients at low light intensity show that light is the limiting factor in this region for both temperatures. Temperature is not limiting here because changing it has no effect on the gradient.
2. The 20°C curve flattens at 20 units. Increasing light intensity beyond this point has no effect. — At the plateau, light is no longer limiting. Either CO2 concentration or temperature (or both) has become the new limiting factor.
3. The 30°C curve plateaus at 35 units — higher than the 20°C plateau. — Raising temperature from 20°C to 30°C increases the plateau rate. This means temperature was a limiting factor at 20°C. At 30°C, enzyme-catalysed reactions in the Calvin cycle proceed faster, allowing a higher maximum rate.
4. At the 30°C plateau, increasing light intensity still has no effect. CO2 concentration (0.04%) is now the most likely limiting factor. — Since temperature has been optimised and light is in excess at the plateau, the fixed CO2 concentration is the factor preventing any further increase in rate.

Answer: At low light intensities, light is the limiting factor for both temperatures. At the plateau, temperature becomes limiting at 20°C (raising it to 30°C increases the maximum rate). At the 30°C plateau, CO2 concentration (0.04%) is the remaining limiting factor.

5. Exam Technique: Evaluating Agricultural Data

Questions on this topic frequently carry 4–6 marks and use the command word 'evaluate'. Many students lose marks by describing data without interpreting it, or by giving one-sided answers. Here is how to structure a full-mark response.