What you'll learn
This topic covers how plants produce glucose through photosynthesis, the chemical equation representing this process, and how environmental factors limit the rate at which it occurs. Understanding photosynthesis and its limiting factors is essential for Edexcel GCSE Biology, appearing frequently in Paper 1 and requiring you to interpret graphs, analyse experimental data, and explain how farmers optimise growing conditions.
Key terms and definitions
Photosynthesis — the process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen.
Chlorophyll — the green pigment found in chloroplasts that absorbs light energy for photosynthesis.
Endothermic reaction — a reaction that requires energy from the surroundings; photosynthesis absorbs light energy.
Limiting factor — a variable that, when in short supply, restricts the rate of photosynthesis even if other conditions are favourable.
Light intensity — the brightness of light reaching a plant, inversely proportional to the square of the distance from the light source.
Stomata — pores on the underside of leaves through which carbon dioxide diffuses into the leaf and oxygen diffuses out.
Glucose — the simple sugar produced during photosynthesis, used for respiration or converted into other substances.
Rate of photosynthesis — the speed at which photosynthesis occurs, typically measured by the volume of oxygen produced per unit time.
Core concepts
The photosynthesis equation
The word equation for photosynthesis:
carbon dioxide + water → glucose + oxygen
The balanced symbol equation you must learn for Edexcel GCSE Biology:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Photosynthesis is an endothermic reaction because it requires light energy from the environment. This energy is absorbed by chlorophyll molecules located in the chloroplasts of plant cells. The light energy is transferred to chemical energy stored in the glucose molecules produced.
Where photosynthesis occurs
Photosynthesis takes place primarily in the palisade mesophyll cells near the upper surface of leaves. These cells contain high concentrations of chloroplasts to maximise light absorption. The leaf structure is adapted for efficient photosynthesis:
- Large surface area to capture maximum light
- Thin structure so diffusion distances for gases are short
- Stomata allow carbon dioxide to diffuse in from the air
- Air spaces in the spongy mesophyll enable gases to reach all cells
- Network of xylem vessels transports water from roots
- Phloem vessels transport glucose away to other parts of the plant
Uses of glucose produced
Plants do not simply store all the glucose they produce. The glucose has multiple uses:
Respiration — glucose is broken down to release energy for cellular processes, growth, and active transport of mineral ions.
Starch synthesis — excess glucose is converted into starch for storage in roots, stems, and leaves. Starch is insoluble, so it doesn't affect water movement by osmosis and can be stored in large quantities.
Cellulose production — glucose is converted into cellulose to strengthen cell walls, providing structural support.
Amino acid synthesis — glucose combines with nitrate ions (absorbed from soil) to form amino acids, which are then assembled into proteins.
Fat and oil production — glucose is converted into lipids for storage, particularly in seeds.
Limiting factors of photosynthesis
A limiting factor is any variable that prevents the rate of photosynthesis from increasing, even when other factors are optimal. The three main limiting factors tested in Edexcel GCSE Biology are light intensity, carbon dioxide concentration, and temperature.
Light intensity
At low light levels, photosynthesis occurs slowly because insufficient light energy is available for chlorophyll to absorb. As light intensity increases, the rate of photosynthesis increases proportionally because more energy is available for the reaction. Eventually, light is no longer the limiting factor — the graph plateaus because either carbon dioxide concentration or temperature becomes limiting instead.
The inverse square law describes the relationship between distance and light intensity:
Light intensity ∝ 1/distance²
If you double the distance from a light source, light intensity decreases to one-quarter of the original value. This relationship appears in required practicals where students investigate light intensity effects on photosynthesis rate.
Carbon dioxide concentration
Carbon dioxide is a raw material for photosynthesis. At atmospheric levels (approximately 0.04%), carbon dioxide often limits photosynthesis rate. Increasing CO₂ concentration causes the rate to increase because more substrate molecules are available for the reaction. Beyond approximately 0.4% concentration, CO₂ is no longer limiting and the graph plateaus as light intensity or temperature becomes the limiting factor.
Greenhouses are often enriched with additional carbon dioxide (using paraffin heaters or CO₂ generators) to increase crop yields by removing this limiting factor.
Temperature
Temperature affects photosynthesis because enzymes control the reactions involved. As temperature increases from cold conditions, enzyme and substrate molecules have more kinetic energy, leading to more frequent successful collisions. The rate of photosynthesis approximately doubles for every 10°C increase in temperature (within the optimal range).
At temperatures above 40-45°C, enzymes involved in photosynthesis begin to denature. The active sites change shape permanently, preventing enzyme-substrate complexes from forming. The rate of photosynthesis rapidly decreases to zero.
The optimum temperature for most plants is around 25-35°C, though this varies by species and their natural habitat.
Interpreting limiting factor graphs
Exam questions frequently present graphs showing how photosynthesis rate changes with varying conditions. To identify the limiting factor at any point:
- On a flat plateau, the x-axis variable is not limiting — something else is restricting the rate
- On an upward slope, the x-axis variable is limiting — increasing it causes the rate to increase
- At low light intensity, rate increases linearly with light — light is limiting
- At high light intensity with a plateau, either CO₂ or temperature is limiting
- Lines at different temperatures/CO₂ levels help identify which factor limits at the plateau
Required practical: investigating photosynthesis
The Edexcel specification requires investigation of how light intensity affects photosynthesis rate using aquatic plants like Canadian pondweed (Elodea). The method involves:
- Place pondweed in a test tube of water with sodium hydrogencarbonate solution (provides CO₂)
- Position a light source at a measured distance from the plant
- Allow the plant to acclimatise for five minutes
- Count the number of oxygen bubbles produced per minute, or collect gas in an inverted measuring cylinder
- Repeat at different distances from the light source
- Calculate light intensity using 1/d²
Control variables must be maintained: temperature (using a water bath or heat shield), carbon dioxide concentration (same volume of sodium hydrogencarbonate solution), same piece of pondweed, time period for counting bubbles.
Variables:
- Independent variable: distance from light source (or light intensity)
- Dependent variable: rate of photosynthesis (measured as oxygen produced per minute)
Commercial applications
Understanding limiting factors allows farmers and horticulturalists to optimise growing conditions in greenhouses:
Temperature control — heaters maintain optimal temperature overnight and in winter; ventilation prevents overheating in summer.
Artificial lighting — supplementary LED lights extend growing hours and increase light intensity, particularly in winter months.
CO₂ enrichment — paraffin heaters release CO₂ as a by-product while also heating; dedicated CO₂ generators can raise concentration to 0.1%.
Cost-benefit analysis — farmers must balance the increased costs of providing optimal conditions against increased crop yield and earlier harvests. The additional expense is only worthwhile if the profit from increased production exceeds the running costs.
Worked examples
Example 1: A student investigated how light intensity affects the rate of photosynthesis in pondweed. They placed a lamp at different distances from the plant and counted oxygen bubbles produced per minute.
| Distance (cm) | Bubbles per minute |
|---|---|
| 10 | 40 |
| 20 | 10 |
| 30 | 4 |
| 40 | 3 |
(a) Explain why fewer bubbles were produced when the lamp was further away. [2 marks]
Answer: Light intensity decreases as distance increases [1 mark]. Less light energy is available for photosynthesis, so the rate decreases and less oxygen is produced [1 mark].
(b) Calculate the light intensity at 20 cm compared to 10 cm using the inverse square law. [2 marks]
Answer: Light intensity ∝ 1/d², so at 10 cm: 1/10² = 1/100, at 20 cm: 1/20² = 1/400 [1 mark]. Light intensity at 20 cm is ¼ (or 0.25 or 25%) of the intensity at 10 cm [1 mark].
Example 2: The graph below shows how increasing carbon dioxide concentration affects photosynthesis rate at three different light intensities.
[Graph shows three curves, each rising then plateauing, with higher light intensities reaching higher plateaus]
Explain the shape of the line for high light intensity. [3 marks]
Answer: At low CO₂ concentration, carbon dioxide is the limiting factor, so increasing CO₂ increases the rate [1 mark]. The rate plateaus because CO₂ is no longer limiting [1 mark]. Either light intensity or temperature becomes the limiting factor instead [1 mark].
Example 3: A farmer wants to increase crop yield by enriching the greenhouse atmosphere with CO₂. The CO₂ generator costs £1,200 and running costs are £800 per year. The increased crop yield will generate £1,500 additional profit per year.
Calculate whether this is worthwhile over a five-year period. [3 marks]
Answer: Total costs over 5 years = £1,200 + (5 × £800) = £5,200 [1 mark]. Total additional profit over 5 years = 5 × £1,500 = £7,500 [1 mark]. Net profit = £7,500 - £5,200 = £2,300, so it is worthwhile [1 mark].
Common mistakes and how to avoid them
Mistake: Writing that plants photosynthesise during the day and respire at night. Correction: Plants respire continuously, 24 hours per day, to release energy from glucose. Photosynthesis only occurs when light is available.
Mistake: Confusing the limiting factor — stating temperature is limiting when the graph shows rate increasing with light intensity. Correction: The limiting factor is always the one that's in short supply. If increasing light causes rate to increase, light is limiting at that point. Read the axes carefully.
Mistake: Writing the photosynthesis equation incorrectly, with products and reactants reversed or missing the light energy requirement. Correction: Learn the equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The arrow should show light energy above it. Carbon dioxide and water are reactants (left side); glucose and oxygen are products (right side).
Mistake: Stating that light intensity doubles when you halve the distance. Correction: Light intensity is inversely proportional to the square of distance. Halving distance increases light intensity by a factor of four (2² = 4), not two.
Mistake: Describing enzymes as "dying" or "killing" at high temperatures. Correction: Enzymes are proteins, not living organisms. At high temperatures, they denature — the active site permanently changes shape, preventing substrate binding.
Mistake: Only stating that stomata "let gases in and out" without specifying which gases or their direction. Correction: Be precise: carbon dioxide diffuses in through stomata for photosynthesis; oxygen (the waste product) diffuses out through stomata.
Exam technique for "Photosynthesis: the process and limiting factors"
"Explain" questions require you to give reasons why something happens, not just describe what happens. For a 3-mark explain question on limiting factors, identify the limiting factor [1], state how it affects the rate [1], and explain why using scientific principles [1]. Chain your reasoning logically.
Graph interpretation questions frequently ask you to identify limiting factors at specific points or explain the shape of curves. Always refer to specific sections of the graph (e.g., "between 0 and 200 units on the x-axis" or "at the plateau"). State which factor is limiting and which is not limiting if the curve is flat.
Calculation questions involving the inverse square law require you to show your working clearly. Write the formula (light intensity ∝ 1/d²), substitute values, and compare ratios. Marks are awarded for method even if your final answer is incorrect.
Required practical questions may ask about control variables, safety precautions, or how to improve accuracy. For photosynthesis practicals, common control variables are temperature, CO₂ concentration, and the same plant. To improve accuracy: repeat readings and calculate a mean, use a more precise measuring instrument (e.g., gas syringe instead of counting bubbles), or increase the time period for measurements.
Quick revision summary
Photosynthesis converts carbon dioxide and water into glucose and oxygen using light energy absorbed by chlorophyll: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. It's an endothermic reaction occurring mainly in palisade mesophyll cells. The three main limiting factors are light intensity, CO₂ concentration, and temperature. Light intensity follows the inverse square law. At low levels, each factor limits the rate; beyond optimal levels, another factor becomes limiting. Greenhouses control these factors to maximise crop yield, though farmers must consider whether increased costs are offset by additional profit.