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The rate of photosynthesis is influenced by several environmental factors. Understanding these factors, and how they interact as limiting factors, is essential for the Edexcel A-Level Biology (9BI0) specification. This lesson covers light intensity, CO₂ concentration, temperature, and the practical techniques used to measure photosynthetic rate.
A limiting factor is the factor that is present at the lowest or least favourable level, and therefore directly controls the rate of a process. When a factor is limiting, increasing it will increase the rate of photosynthesis. Once that factor is no longer limiting, another factor becomes the limiting factor.
This principle was first described by F.F. Blackman (Blackman's Law of Limiting Factors):
"When a process is governed by more than one factor, the rate of the process is limited by the factor nearest its minimum value."
When plotting rate of photosynthesis against a single variable:
Light provides the energy for the light-dependent reactions. As light intensity increases:
| Light intensity | Effect on photosynthesis rate |
|---|---|
| Very low | Rate is very slow; compensation point may not be reached |
| Increasing | Rate increases proportionally (light is limiting) |
| High | Rate plateaus (CO₂ or temperature becomes limiting) |
| Very high | Rate may decrease slightly due to stomatal closure (to prevent water loss) |
In practical experiments, light intensity is often varied by changing the distance between a lamp and the plant. Light intensity is inversely proportional to the square of the distance:
Light intensity ∝ 1/d²
This means doubling the distance reduces light intensity to one-quarter.
Exam Tip: When plotting results, plot rate of photosynthesis against 1/d² (not against distance). This gives a clearer linear relationship during the limiting phase.
CO₂ is the substrate for the Calvin cycle (fixed by RuBisCO). Atmospheric CO₂ is approximately 0.04% (400 ppm).
| CO₂ concentration | Effect |
|---|---|
| Very low | Rate is very slow; little carbon fixation |
| Increasing | Rate increases (CO₂ is limiting) |
| ~0.1% | Rate plateaus in most plants |
| Very high | No further increase; RuBisCO is saturated or another factor limits |
In greenhouse horticulture, CO₂ enrichment is used to increase crop yields. Growers may raise CO₂ levels to 0.1% (1000 ppm) using:
This is only economically viable if light and temperature are also optimised.
Temperature affects the rate of enzyme-controlled reactions in both the light-dependent and light-independent stages.
| Temperature | Effect |
|---|---|
| Below optimum | Increasing temperature increases rate (more kinetic energy, more enzyme-substrate collisions) |
| Optimum (~25–30°C for most plants) | Maximum rate of photosynthesis |
| Above optimum | Rate decreases sharply — enzymes (especially RuBisCO) begin to denature |
| Very high (>40°C) | Enzymes are denatured; rate drops to zero |
The Q₁₀ principle applies:
Q₁₀ = (rate at (T + 10°C)) / (rate at T°C)
For enzymatic reactions, Q₁₀ is typically about 2 (the rate doubles for every 10°C rise) up to the optimum.
Exam Tip: Temperature is the only factor that can cause a decline in rate if it exceeds the optimum. Light and CO₂ cause a plateau but not a decline under normal conditions.
While water is a substrate for photolysis, it rarely directly limits photosynthesis because:
In reality, photosynthesis is affected by multiple factors simultaneously. Graphs showing the interaction typically display families of curves:
| Condition | Limiting factor |
|---|---|
| Low light, any CO₂, any temp | Light |
| High light, low CO₂, moderate temp | CO₂ |
| High light, high CO₂, low temp | Temperature |
| High light, high CO₂, optimum temp | Maximum rate (no single factor limiting) |
The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration.
| Plant type | Compensation point |
|---|---|
| Sun plants | Higher compensation point; adapted to high light |
| Shade plants | Lower compensation point; can achieve net photosynthesis in dim light |
Improvements:
Exam Tip: Always state how you would control variables in a photosynthesis practical: temperature (heat shield/water bath), CO₂ concentration (NaHCO₃), wavelength of light (colour filters), and how you ensured the plant had time to acclimatise.
Plants respire continuously, even while photosynthesising. Therefore:
Net photosynthesis = Gross photosynthesis − Respiration
When measuring O₂ production or CO₂ uptake, you are measuring net photosynthesis. To find gross photosynthesis, you must also measure the rate of respiration (e.g. by measuring O₂ uptake or CO₂ output in the dark).
| Measurement | In light | In dark |
|---|---|---|
| O₂ output | Net photosynthesis | −(Respiration) |
| CO₂ uptake | Net photosynthesis | −(Respiration) |
| Term | Definition |
|---|---|
| Limiting factor | The factor in shortest supply that directly limits the rate of a process |
| Compensation point | The light intensity at which the rate of photosynthesis equals the rate of respiration |
| Gross photosynthesis | The total rate of photosynthesis before accounting for respiration |
| Net photosynthesis | The observed rate of photosynthesis after subtracting respiration |
| Photosynthometer | An apparatus used to measure the volume of O₂ produced by an aquatic plant |
This material sits in Edexcel 9BI0 Topic 5 (On the Wild Side — Photosynthesis, Energy and Ecosystems) and concerns the rate of photosynthesis as a function of light intensity, CO2 concentration and temperature, with the Blackman law of limiting factors as the organising principle. Synoptic links run backwards to the light-dependent reactions (lesson 1: light intensity sets photon flux supplying PSII and therefore ATP/NADPH supply); to the Calvin cycle (lesson 2: CO2 sets RuBisCO substrate supply, temperature sets RuBisCO catalytic rate and the carboxylase:oxygenase ratio); to Topic 7 (Exchange and Transport) for stomatal aperture as a CO2-vs-water trade-off; and forwards to C4 and CAM photosynthesis as adaptations to extreme limiting-factor environments. Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
A scientist measured the rate of photosynthesis (in arbitrary units of O2 evolution per minute) of a Cabomba shoot at three CO2 concentrations (0.04%, 0.10%, 0.40%) across a range of light intensities (0–2000 µmol photons m−2 s−1). Temperature was held at 25 °C. The three curves all rise linearly from the origin, coincide up to ~400 µmol photons m−2 s−1, and then plateau at progressively higher rates as CO2 rises.
(a) Explain why the three curves coincide at low light intensity. (3)
(b) Explain why the curves plateau at progressively higher rates as CO2 concentration rises. (3)
(c) Predict, with reasoning, what would happen to the highest plateau (at 0.40% CO2) if temperature were raised from 25 °C to 35 °C. (2)
Solution with mark scheme:
(a) M1 (AO2) — At low light intensity, light is the limiting factor: the rate is controlled by the supply of photons reaching the photosystems and so by the supply of ATP and NADPH from the light-dependent reactions.
A1 (AO2) — Because CO2 supply is not limiting in this region, varying CO2 has no effect — the Calvin cycle is starved of ATP/NADPH long before it would be starved of CO2.
A1 (AO1) — Hence all three curves trace the same straight line in the light-limited region; they only diverge once light intensity is high enough for CO2 to become the next limiting factor.
(b) M1 (AO2) — At high light intensity, the supply of ATP and NADPH from the light reactions is no longer limiting; instead, the rate is set by the rate of CO2 fixation by RuBisCO in the Calvin cycle.
M1 (AO1) — Raising CO2 concentration increases substrate availability for RuBisCO, raising the carboxylation rate and so the rate of photosynthesis.
A1 (AO2) — Raising CO2 also suppresses the oxygenase reaction (photorespiration), because CO2 outcompetes O2 at RuBisCO's active site — so the effective CO2-fixation rate rises faster than CO2 supply alone would predict.
(c) M1 (AO3) — Below RuBisCO's optimum, raising temperature would normally increase the rate (Q10 of enzyme-controlled reactions ≈ 2). At 35 °C, however, two opposing effects compete: (i) RuBisCO catalytic rate rises, but (ii) CO2 solubility falls more steeply than O2 solubility, raising RuBisCO's oxygenase activity and increasing photorespiration.
A1 (AO3) — The plateau is therefore likely to rise modestly or stay flat, not double — and at 40 °C+ the rate would fall as RuBisCO's oxygenase activity dominates and as stomata close to limit water loss, choking off CO2 supply. (Total: 8 marks; M3 A5.)
Question (6 marks): A horticulturalist running a commercial tomato glasshouse measures crop yield against weekly CO2 enrichment cost. At ambient CO2 (~400 ppm), yield is 50 kg m−2 year−1. Enrichment to 1000 ppm raises yield to 70 kg m−2 year−1. Further enrichment to 1500 ppm raises yield only to 72 kg m−2 year−1 but increases enrichment cost by 50%.
Use limiting-factor reasoning to explain the diminishing returns of CO2 enrichment, and identify what factor(s) the grower should optimise next to lift the yield ceiling further.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating the Blackman law: rate is controlled by the factor in shortest supply |
| 2 | AO2.1 | Identifying that at 400 ppm, CO2 is the limiting factor for tomato photosynthesis |
| 3 | AO2.2 | Explaining that raising CO2 to 1000 ppm relieves CO2 limitation, transferring control to a different factor |
| 4 | AO3.1 | Recognising that the small additional gain from 1000 → 1500 ppm shows CO2 is no longer limiting above ~1000 ppm |
| 5 | AO3.2 | Predicting that light intensity (in winter) and/or temperature become the next limiting factor — the curve has hit a new plateau |
| 6 | AO3.3 | Concluding that the grower should next invest in supplementary lighting (LED grow-lamps) and/or temperature control (heating in winter, ventilation in summer) rather than further CO2 |
Total: 6 marks (AO1 = 1, AO2 = 2, AO3 = 3). This question structure mirrors Edexcel's preference for applying limiting-factor logic to commercial horticulture data.
Lesson 1 (light-dependent reactions) — light intensity sets photon supply. Light intensity in photosynthesis is photon flux density (mol photons m−2 s−1, 400–700 nm), not lumens or watts. Each absorbed photon excites one electron at PSII or PSI; doubling photon flux at sub-saturating light doubles photolysis rate and therefore ATP/NADPH production — the linear rise in rate.
Lesson 2 (Calvin cycle) — CO2 supply and RuBisCO catalysis. CO2 concentration sets RuBisCO substrate supply; temperature sets RuBisCO's catalytic rate and the carboxylase:oxygenase ratio. Above ~30 °C, CO2 solubility falls more steeply than O2 solubility, intensifying photorespiration — a second reason high temperature lowers rate, separate from denaturation.
Topic 7 (Exchange and Transport) — stomatal aperture trade-off. Stomata admit CO2 but lose water by transpiration. Under heat or drought stress, abscisic acid (ABA) triggers closure, choking CO2 supply. The high-temperature rate fall has two environmental causes — rising oxygenase activity (intrinsic) and stomatal closure (ABA-mediated). A* candidates separate these.
Commercial horticulture — greenhouse CO2 enrichment. Ambient CO2 (~400 ppm) is below RuBisCO saturation for C3 crops; growers raise CO2 to ~1000 ppm — lifting the rate ceiling provided light and temperature are also optimised. Diminishing returns above ~1000 ppm reflect transfer of limitation to another factor.
Lesson 1 — absorption vs action spectrum. The action spectrum tracks the absorption spectrum because absorbed photons drive photochemistry. Green light is poorly absorbed (hence leaves look green) and contributes least to photosynthesis.
Topic 5 next lessons — C4 and CAM as limiting-factor adaptations. C4 plants (maize, sugarcane) concentrate CO2 around RuBisCO via PEP carboxylase, suppressing photorespiration and lifting the high-temperature ceiling. CAM plants (cacti, pineapple) fix CO2 at night with stomata open, releasing it by day with stomata closed — an arid-survival adaptation.
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