B12: Photosynthesis
The process of photosynthesis, factors affecting rate, uses of glucose
Photosynthesis
📋 Key Concepts
Photosynthesis: The process by which plants make glucose using light energy, carbon dioxide and water. Oxygen is produced as a by-product.
Key Terms
- photosynthesis
- chlorophyll
- chloroplasts
- limiting factors
- light intensity
- CO₂ concentration
- inverse square law
- glucose
- starch
- cellulose
- lipids
- nitrates
📝 The Process of Photosynthesis
Photosynthesis takes place in the chloroplasts of plant cells. Chlorophyll (the green pigment) absorbs light energy, which is used to convert carbon dioxide and water into glucose and oxygen.
Word Equation:
carbon dioxide + water → glucose + oxygen
Symbol Equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
carbon dioxide + water → glucose + oxygen
Symbol Equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Key points about the equation:
- Light energy is required (absorbed by chlorophyll)
- It is an endothermic reaction (takes in energy from the surroundings)
- Happens in chloroplasts, which contain chlorophyll
- Oxygen is a by-product (not the main product)
Example 1
A student writes: "Plants make oxygen by photosynthesis." Improve this statement.
Solution:
Oxygen is a by-product. The main product is glucose. A better statement is: "Plants use light energy to convert carbon dioxide and water into glucose. Oxygen is produced as a by-product."
📝 Limiting Factors
Limiting factor: The factor that is at the least favourable level and therefore limits the rate of photosynthesis. At any given moment, whichever factor is in shortest supply is the limiting factor.
| Factor | Effect on Rate | How It Works |
|---|---|---|
| Light intensity | Increasing light increases rate (up to a plateau) | More light = more energy absorbed by chlorophyll. Eventually another factor becomes limiting. |
| CO₂ concentration | Increasing CO₂ increases rate (up to a plateau) | More CO₂ = more raw material for the reaction. Eventually another factor becomes limiting. |
| Temperature | Rate increases to an optimum, then decreases | Higher temperature = more kinetic energy = faster enzyme action. Above ~45°C, enzymes denature and rate falls sharply. |
Example 2
A plant is grown at high light intensity but low CO₂. What is the limiting factor and how could you increase the rate of photosynthesis?
Solution:
CO₂ concentration is the limiting factor because it is in shortest supply.
To increase the rate, increase the CO₂ concentration (e.g. by growing in a greenhouse with enriched CO₂ air).
Example 3
At 20°C, increasing light intensity increases the rate of photosynthesis. At 35°C, increasing light intensity has no effect. Explain why.
Solution:
At 20°C, light is the limiting factor, so increasing light increases the rate.
At 35°C, temperature is now the limiting factor (enzymes are working at their maximum but not yet denatured). Increasing light does not help because the enzymes cannot work any faster at this temperature.
📝 Inverse Square Law (Higher)
Inverse square law: When light is the limiting factor, doubling the distance between the lamp and the plant does NOT halve the light intensity. The light intensity is inversely proportional to the square of the distance.
Light intensity ∝ 1 / d²
If distance is doubled (2d), light intensity becomes 1/(2d)² = 1/4d² = ¼ of the original
If distance is doubled (2d), light intensity becomes 1/(2d)² = 1/4d² = ¼ of the original
Example 4
A lamp is 10 cm from some pondweed. The lamp is moved to 20 cm away. What happens to the light intensity?
Solution:
Distance has doubled from 10 cm to 20 cm.
Light intensity = 1/d², so new intensity = 1/20² = 1/400
Original intensity = 1/10² = 1/100
New intensity = (1/400) ÷ (1/100) = 100/400 = ¼ of original
The light intensity is reduced to one quarter.
Example 5
A lamp is 5 cm from a plant. It is moved to 15 cm away. Calculate the factor by which light intensity changes.
Solution:
Distance has tripled from 5 cm to 15 cm.
New intensity = 1/15² = 1/225
Original intensity = 1/5² = 1/25
Factor = (1/225) ÷ (1/25) = 25/225 = 1/9
Light intensity is reduced to one ninth of the original.
📝 Uses of Glucose
Glucose produced by photosynthesis is used by the plant in several ways. Plants cannot store glucose (it is soluble and would affect osmosis), so they convert it to other substances.
| Use of Glucose | Details |
|---|---|
| Respiration | Glucose is broken down to release energy for cellular processes (transferred to all cells) |
| Starch (storage) | Glucose is converted to starch (insoluble) for storage in roots, stems and leaves. Starch does not affect water potential. |
| Cellulose (cell walls) | Glucose is converted to cellulose for making strong cell walls, especially in fast-growing plants |
| Lipids (fats/oils) | Glucose is converted to lipids for energy storage, particularly in seeds |
| Amino acids (with nitrates) | Glucose is combined with nitrate ions (absorbed from soil) to make amino acids, which are built into proteins |
Example 6
A plant is growing in soil that is deficient in nitrates. Explain why the plant may show poor growth even though it is photosynthesising normally.
Solution:
Without nitrates, the plant cannot convert glucose into amino acids.
Without amino acids, the plant cannot make proteins needed for growth (e.g. enzymes, structural proteins).
Therefore growth is limited despite sufficient glucose from photosynthesis.
Example 7
Explain why plants convert glucose to starch for storage rather than storing glucose itself.
Solution:
Glucose is soluble in water. If stored as glucose, it would dissolve and affect the water potential of the cell, drawing water in by osmosis and potentially causing the cell to burst.
Starch is insoluble, so it does not affect the water potential and can be stored safely in large amounts without affecting osmosis.
📝 Photosynthesis Practical
Required practical: Investigate the effect of light intensity on the rate of photosynthesis using pondweed (Cabomba or Elodea).
Method 1 — Counting bubbles:
- Cut a piece of pondweed and place it upside down in a beaker of water with sodium hydrogencarbonate (provides CO₂)
- Place a lamp at a measured distance from the pondweed
- Count the number of oxygen bubbles produced in a set time (e.g. 1 minute)
- Repeat at different distances from the lamp
- Calculate rate = number of bubbles per minute
Method 2 — Oxygen sensor (more accurate):
- Set up the pondweed in a sealed flask of water with sodium hydrogencarbonate
- Use an oxygen sensor connected to a data logger to measure the concentration of oxygen over time
- This is more accurate because bubble size varies and some bubbles may be missed when counting
Variables:
- Independent variable: distance of lamp from pondweed (or light intensity)
- Dependent variable: number of bubbles per minute or oxygen concentration
- Control variables: temperature (use a water bath), CO₂ concentration (same amount of NaHCO₃), same length of pondweed, same species of plant
Example 8
In a photosynthesis experiment, a student counts bubbles at distances of 5 cm, 10 cm, 15 cm and 20 cm from a lamp. The results are 120, 32, 14 and 8 bubbles per minute. Explain the pattern.
Solution:
As distance increases, light intensity decreases (inverse square law: light intensity ∝ 1/d²).
At 5 cm: high light intensity → fast photosynthesis → 120 bubbles/min
At 10 cm: light intensity is ¼ of 5 cm value → 32 bubbles/min (roughly ¼ of 120)
At 15 cm and 20 cm: light intensity is very low, so light is still the limiting factor and rate continues to drop.
The pattern approximately follows the inverse square law.
❓ Practice Questions
Q1: Write the word equation and symbol equation for photosynthesis.
Q2: Name the three limiting factors for photosynthesis and explain how each affects the rate.
Q3: Higher A lamp is moved from 10 cm to 30 cm away from a plant. By what factor does the light intensity change?
Q4: State three ways plants use the glucose produced by photosynthesis.
Q5: Explain why a plant converts glucose to starch for storage rather than storing glucose directly.
Q6: Describe how you would investigate the effect of light intensity on the rate of photosynthesis using pondweed, including the variables you would control.
✅ Answers
- Word equation: carbon dioxide + water → glucose + oxygen. Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. (Light energy is required; absorbed by chlorophyll.)
- The three limiting factors are: (1) Light intensity — more light provides more energy for the reaction, increasing rate until another factor limits; (2) CO₂ concentration — more CO₂ provides more raw material, increasing rate until another factor limits; (3) Temperature — higher temperature increases enzyme activity and rate, but above ~45°C enzymes denature and rate drops sharply.
- Distance triples from 10 cm to 30 cm. Light intensity ∝ 1/d². New intensity = 1/30² = 1/900. Original = 1/10² = 1/100. Factor = 100/900 = 1/9. Light intensity is reduced to one ninth.
- Any three from: respiration (to release energy), converted to starch for storage, converted to cellulose for cell walls, converted to lipids for storage in seeds, combined with nitrates to make amino acids/proteins.
- Glucose is soluble and would dissolve in the cell sap, lowering the water potential and causing excess water to enter by osmosis (potentially bursting the cell). Starch is insoluble, so it does not affect water potential and can be stored safely in large quantities.
- Cut pondweed and place it in a beaker of water with sodium hydrogencarbonate. Place a lamp at a measured distance. Count oxygen bubbles per minute. Repeat at different distances. Control variables: temperature (use water bath), CO₂ concentration (same amount of NaHCO₃), same length/species of pondweed. Independent variable: distance of lamp. Dependent variable: bubble count per minute.
🎯 Exam Tips
- Make sure you can write both the word and symbol equation from memory — these are frequently tested
- When asked about limiting factors, always identify WHICH factor is limiting and explain WHY
- Higher tier: learn the inverse square law (light intensity ∝ 1/d²) — if distance triples, intensity drops to 1/9th
- Remember: photosynthesis is ENDOTHERMIC (takes in energy), respiration is EXOTHERMIC (releases energy)
- When describing the pondweed practical, always mention control variables to get full marks
- Counting bubbles is less accurate than using an oxygen sensor — be ready to explain why (variable bubble size, missed bubbles)
- Don't forget that nitrates are needed to make amino acids from glucose — plants get nitrates from the soil, NOT from photosynthesis
🔬 Required Practical
Effect of Light Intensity on Photosynthesis Rate Using Pondweed
Cut a piece of pondweed (Cabomba or Elodea) and place it upside down in a beaker of water with sodium hydrogencarbonate (provides CO₂). Place a lamp at a measured distance and count the oxygen bubbles produced per minute. Repeat at different distances. For greater accuracy, use an oxygen sensor connected to a data logger instead of counting bubbles. Control variables: temperature (use a water bath), CO₂ concentration (same NaHCO₃), same length and species of pondweed. Independent variable: distance of lamp. Dependent variable: number of bubbles per minute or oxygen concentration.
🔢 Maths Skills
Mathematical Skills
Inverse square law: Light intensity is inversely proportional to the square of the distance: light intensity ∝ 1/d². If the distance doubles, the light intensity falls to ¼. If the distance triples, the light intensity falls to 1/9. Calculate using: new intensity = original intensity × (original distance / new distance)².
⚠️ Common Misconceptions
Watch Out!
1. Plants only respire at night. Wrong: plants respire only in the dark. Correct: plants respire continuously, day and night — respiration happens in all living cells all the time.
2. More CO₂ always means more photosynthesis. Wrong: increasing CO₂ will always increase the rate. Correct: other factors (light intensity, temperature) can become limiting, so increasing CO₂ beyond a certain point has no further effect.
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how light intensity affects the rate of photosynthesis.
As light intensity increases, the rate of photosynthesis increases because more light energy is absorbed by chlorophyll, providing more energy for the reaction. The rate increases proportionally at low light intensities where light is the limiting factor. However, at higher light intensities the rate plateaus because another factor (such as CO₂ concentration or temperature) becomes the limiting factor. The relationship between light intensity and distance follows the inverse square law: light intensity ∝ 1/d², so doubling the distance reduces the light intensity to one quarter, causing a significant drop in the rate of photosynthesis.
Mark scheme: 1 mark — light increases rate; 1 mark — more energy absorbed by chlorophyll; 1 mark — proportional increase at low intensity; 1 mark — plateau when another factor limits; 1 mark — inverse square law; 1 mark — correct use of 1/d² relationship.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student measured the volume of oxygen gas collected from pondweed at different light intensities. At light intensities of 5, 10, 20 and 40 arbitrary units, the oxygen collected in 5 minutes was 2 cm³, 4 cm³, 7 cm³ and 7.5 cm³ respectively. Explain the pattern in the results and suggest why the increase in oxygen volume becomes smaller at higher light intensities. Identify the limiting factor at 40 arbitrary units of light and suggest how you could test your answer.
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