Factors affecting the rate of photosynthesis

What you'll learn

This revision guide covers how environmental and internal factors control the rate at which plants photosynthesise. You'll learn to interpret graphs showing limiting factors, apply the inverse square law to light intensity experiments, and explain how greenhouses optimise conditions for commercial crop production. These concepts regularly appear in AQA GCSE Biology exam questions worth 4-6 marks.

Key terms and definitions

Photosynthesis — the endothermic reaction by which plants use light energy to convert carbon dioxide and water into glucose and oxygen (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂)

Limiting factor — an environmental condition that, when in short supply, restricts the rate of photosynthesis from increasing even if other factors are at optimal levels

Inverse square law — the principle that light intensity is inversely proportional to the square of the distance from the light source (light intensity ∝ 1/distance²)

Rate of photosynthesis — the speed at which a plant produces glucose and oxygen, typically measured by the volume of oxygen released per unit time or the rate of carbon dioxide uptake

Chlorophyll — the green pigment in chloroplasts that absorbs light energy for photosynthesis

Greenhouse — a controlled environment structure that allows growers to optimise temperature, light and CO₂ levels to maximise photosynthesis and crop yield

Compensation point — the light intensity at which the rate of photosynthesis equals the rate of respiration in a plant

Stomata — pores on the underside of leaves through which carbon dioxide enters and oxygen exits the leaf

Core concepts

The four main factors affecting photosynthesis

Four environmental or internal factors can limit the rate of photosynthesis:

Light intensity — photosynthesis requires light energy to drive the reaction. Without sufficient light, the rate of photosynthesis remains low regardless of other conditions.

Carbon dioxide concentration — CO₂ is a reactant in photosynthesis. The atmosphere contains approximately 0.04% carbon dioxide, which is often insufficient for maximum photosynthetic rate.

Temperature — photosynthesis involves enzyme-controlled reactions. Temperature affects the kinetic energy of molecules and enzyme activity.

Chlorophyll concentration — the amount of chlorophyll in leaves determines how much light energy can be absorbed. Disease, mineral deficiencies or variegated leaves reduce chlorophyll content.

These factors interact constantly in real plants, but examination questions typically ask you to analyse how changing one factor affects the rate whilst others remain constant.

How limiting factors work

At any given moment, only one factor limits the rate of photosynthesis — this is the limiting factor. Even if you increase other factors to optimal levels, the rate cannot increase until you address the limiting factor.

Key principles:

• When a factor is limiting, increasing its level increases the rate of photosynthesis
• When a factor is not limiting, changing its level has no effect on the rate
• As you increase the limiting factor, eventually a different factor becomes limiting
• You can identify the limiting factor from a graph by seeing which factor, when increased, causes the rate to increase

Typical graph patterns:

When light intensity is limiting (low light levels), increasing light intensity increases the rate proportionally. The graph shows a steep positive correlation. Once light intensity is sufficient, the line plateaus — another factor (usually CO₂ or temperature) becomes limiting.

When temperature is limiting, increasing temperature increases the rate because enzymes work faster with more kinetic energy. However, above approximately 45°C, enzymes denature and the rate drops sharply to zero.

When CO₂ concentration is limiting, increasing CO₂ concentration increases the rate until another factor becomes limiting, shown by the graph plateauing.

Light intensity and the inverse square law

Light intensity decreases rapidly as you move away from a light source. This relationship follows the inverse square law:

$$\text{Light intensity} \propto \frac{1}{\text{distance}^2}$$

Or expressed as an equation:

$$\text{Light intensity} = \frac{k}{\text{distance}^2}$$

where k is a constant.

Practical implications:

• If you double the distance from a lamp to a plant, light intensity decreases to ¼ (not ½)
• If you halve the distance, light intensity increases by 4 times
• If you treble the distance, light intensity decreases to 1/9

Using 1/d² in experiments:

Because light intensity is difficult to measure directly in school laboratories, AQA exams often ask you to use 1/d² as a proxy measurement. When the distance is 10 cm, 1/d² = 1/100 = 0.01. When the distance is 20 cm, 1/d² = 1/400 = 0.0025.

You can plot 1/d² on the x-axis against rate of photosynthesis on the y-axis. This produces a linear relationship when light is the limiting factor, making analysis simpler than plotting distance directly.

Temperature as a limiting factor

Temperature affects photosynthesis because the reactions are controlled by enzymes, particularly those in the Calvin cycle and light-dependent reactions within chloroplasts.

Low temperatures (0-25°C):

• Molecules have low kinetic energy
• Few successful collisions between enzymes and substrates
• Rate of photosynthesis is slow
• Increasing temperature increases the rate as the Q₁₀ rule applies (rate approximately doubles for every 10°C rise)

Optimal temperature (25-35°C):

• Enzymes work at their maximum efficiency
• Rate of photosynthesis is at its peak
• Most plants in the UK thrive around 25°C
• Tropical plants may have higher optimal temperatures (30-35°C)

High temperatures (above 40-45°C):

• Enzyme active sites change shape (denaturation)
• Substrates no longer fit the active sites
• Rate of photosynthesis decreases rapidly
• Eventually stops completely as all enzymes denature

A graph of temperature vs rate of photosynthesis typically shows a curve that rises steadily, peaks at the optimum, then drops steeply.

Carbon dioxide concentration as a limiting factor

CO₂ is a raw material for photosynthesis. The atmospheric concentration of 0.04% (400 parts per million) is relatively low, making CO₂ concentration often limiting in bright, warm conditions.

How CO₂ enters the plant:

• CO₂ diffuses into leaves through stomata
• Dissolves in moisture on mesophyll cell walls
• Diffuses into chloroplasts where it is fixed in the Calvin cycle

Effect of increasing CO₂:

• Below 0.04%, CO₂ is severely limiting and rate is very low
• Between 0.04% and 0.4%, increasing CO₂ increases rate proportionally
• Above 0.4%, CO₂ is no longer limiting — light or temperature restricts the rate
• The graph plateaus when another factor becomes limiting

Agricultural applications: Commercial greenhouses in the UK often enrich the air with CO₂ to 0.1% (1000 ppm) to maximise growth of crops like tomatoes, cucumbers and lettuce. This increases the rate of photosynthesis by up to 2.5 times provided light and temperature are also optimal.

Chlorophyll and mineral deficiencies

Chlorophyll concentration affects how much light energy a leaf can absorb. Several factors reduce chlorophyll content:

Mineral deficiencies:

• Magnesium — essential for chlorophyll production; deficiency causes yellowing (chlorosis) between leaf veins
• Nitrogen — needed for amino acids and proteins including chlorophyll; deficiency causes pale yellow-green leaves and stunted growth
• Iron — required for chlorophyll synthesis; deficiency causes young leaves to turn yellow

Disease:

• Tobacco mosaic virus causes a mosaic pattern of discolouration, reducing chlorophyll in affected areas
• Fungal infections can damage chloroplasts

Variegation:

• Some ornamental plants naturally have white or yellow areas lacking chlorophyll
• These areas cannot photosynthesise, reducing the overall rate for the whole leaf

Optimising conditions in greenhouses

Commercial growers use greenhouses to control environmental factors and maximise the rate of photosynthesis. This increases crop yield and allows year-round production in the UK climate.

Temperature control:

• Heaters maintain optimal temperatures in winter (typically 20-25°C)
• Ventilation and shading prevent overheating in summer
• Automated systems monitor and adjust temperature
• Energy costs must be balanced against increased yield

Light supplementation:

• LED or high-pressure sodium lamps extend daylight hours in winter
• Particularly important in northern latitudes like Scotland where winter days are very short
• Lamps are positioned to ensure even light distribution
• Timers control the photoperiod (light duration)

CO₂ enrichment:

• Burning propane or natural gas produces CO₂ and heat simultaneously
• Bottled CO₂ gas can be released in controlled amounts
• Concentrations of 0.1% (1000 ppm) are typical
• Must be timed to coincide with daylight when photosynthesis occurs

Cost-benefit analysis: Growers must calculate whether the increased yield justifies the costs of heating, lighting and CO₂ enrichment. For high-value crops like salad leaves or tomatoes sold in supermarkets like Tesco or Sainsbury's, the investment is usually profitable. For low-value crops, the costs may exceed the benefits.

Worked examples

Example 1: A student investigates how light intensity affects the rate of photosynthesis in pondweed. She places a lamp at different distances from the pondweed and counts oxygen bubbles produced per minute.

(a) Explain why the student uses 1/d² rather than distance on her graph. [2 marks]

Mark scheme answer: Light intensity is inversely proportional to the square of the distance [1 mark]. Using 1/d² produces a linear relationship when light is limiting, making the graph easier to interpret [1 mark].

(b) The student carries out the experiment at 15°C. Suggest how the results might differ at 25°C. Explain your answer. [3 marks]

Mark scheme answer: The number of bubbles per minute would increase at each distance [1 mark]. Temperature affects enzyme activity in photosynthesis [1 mark]. At 25°C, enzymes work faster than at 15°C due to increased kinetic energy / more successful collisions [1 mark].

Example 2: The graph below shows how the rate of photosynthesis changes with CO₂ concentration at two different temperatures.

[Imagine a graph with CO₂ concentration on x-axis, rate on y-axis, showing two curves: one for 25°C that rises then plateaus higher, one for 15°C that rises then plateaus lower]

(a) Identify the limiting factor at point X where CO₂ is 0.01% at 25°C. [1 mark]

Mark scheme answer: CO₂ concentration [1 mark]

(b) Explain why increasing CO₂ concentration above 0.15% does not increase the rate at 25°C. [2 marks]

Mark scheme answer: CO₂ is no longer the limiting factor [1 mark]. Another factor such as light intensity is now limiting the rate [1 mark].

(c) Compare the maximum rates achieved at 15°C and 25°C. Explain the difference. [3 marks]

Mark scheme answer: The maximum rate at 25°C is higher than at 15°C [1 mark]. Photosynthesis involves enzyme-controlled reactions [1 mark]. Enzymes work faster at higher temperatures (within the optimal range) because molecules have more kinetic energy [1 mark].

Example 3: A farmer in Jamaica grows lettuce in a greenhouse. Explain why enriching the air with CO₂ to 0.1% may not be cost-effective if the greenhouse is not also heated and artificially lit. [4 marks]

Mark scheme answer: In bright sunlight and warm temperatures, CO₂ is the limiting factor [1 mark], so enrichment increases photosynthesis and yield [1 mark]. However, without supplementary heating at night or artificial lighting on dull days / during short winter days [1 mark], temperature or light intensity becomes limiting and the increased CO₂ provides no benefit / does not increase the rate [1 mark].

Common mistakes and how to avoid them

• Confusing correlation with causation — Don't say "light makes photosynthesis happen faster." Instead write "increased light intensity increases the rate of photosynthesis when light is the limiting factor."

• Forgetting that only one factor limits at a time — Students often write "light intensity and CO₂ are both limiting." Only one factor is limiting at any moment. Write "CO₂ is now limiting because light intensity is sufficient."

• Misunderstanding the inverse square law — Doubling the distance does NOT halve the light intensity; it reduces it to one quarter. Always square the distance when calculating relative light intensity.

• Saying enzymes "die" at high temperatures — Enzymes are proteins, not living things. The correct term is "denature." Write "enzyme active sites change shape permanently" or "enzymes denature."

• Vague explanations of temperature effects — Don't just write "enzymes work better." Be specific: "Increased temperature increases kinetic energy, causing more frequent successful collisions between enzymes and substrates."

• Ignoring the context in longer questions — If a question mentions greenhouses or commercial growers, discuss cost-benefit analysis and profitability, not just biological principles.

Exam technique for "Factors affecting the rate of photosynthesis"

• Graph interpretation questions — Identify which part of the graph shows the factor being limiting (rising line) vs not limiting (plateau). Use data from the graph in your answer: "Between 0% and 0.1% CO₂, the rate increases from 5 to 20 units."

• "Explain" command words — Provide a reason or mechanism. For 2-3 mark questions, you need to make a statement then justify it. "Increasing temperature increases the rate [statement] because enzymes and substrates have more kinetic energy and collide more frequently [justification]."

• Required practicals — You must know the pondweed experiment measuring oxygen production at different light intensities. Be ready to describe the method, identify variables (independent: distance/light intensity; dependent: number of bubbles; control: temperature, CO₂ concentration, same plant species) and suggest improvements.

• Maths skills — Practice calculating 1/d² values and plotting graphs. You may need to convert units (e.g., mm to cm). Show your working even in calculations as you can gain method marks if your final answer is incorrect.

Quick revision summary

Photosynthesis rate depends on four main factors: light intensity, CO₂ concentration, temperature and chlorophyll. At any moment, only one factor is limiting — the one in shortest supply. Light intensity follows the inverse square law (intensity ∝ 1/d²). Temperature affects enzyme activity: rate increases with temperature until enzymes denature above 40-45°C. Commercial greenhouses optimise all factors to maximise crop yield, though growers must balance increased production against energy and equipment costs.