B18: Photosynthesis
Photosynthesis equations, limiting factors, inverse square law, and the required practical on pondweed and light intensity for GCSE Biology.
Key Definitions
- Photosynthesis – the process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen.
- Limiting factor – a factor that, when in shortest supply, limits the rate of a process such as photosynthesis.
- Inverse square law – the rule that light intensity is inversely proportional to the square of the distance from the light source (Higher only).
- Chlorophyll – the green pigment in chloroplasts that absorbs light energy for photosynthesis.
Photosynthesis Equations
Word Equation
carbon dioxide + water → glucose + oxygen
Symbol Equation
6CO2 + 6H2O → C6H12O6 + 6O2
Photosynthesis takes place in the chloroplasts of plant cells. Chlorophyll absorbs light energy, which is used to convert carbon dioxide and water into glucose. Oxygen is released as a by-product.
Conditions Needed for Photosynthesis
- Light energy (absorbed by chlorophyll)
- Carbon dioxide (from the air via stomata)
- Water (absorbed by roots from soil)
- A suitable temperature (enzymes involved)
Limiting Factors
The rate of photosynthesis is affected by several factors. At any given time, the factor that is in the shortest supply will be the limiting factor – it determines the overall rate.
1. Light Intensity
As light intensity increases, the rate of photosynthesis increases steadily (light is limiting). Eventually the rate plateaus – another factor (usually CO2 or temperature) becomes limiting.
2. Carbon Dioxide Concentration
As CO2 concentration increases, the rate of photosynthesis increases (CO2 is limiting). At higher concentrations the rate levels off – another factor becomes limiting.
3. Temperature
As temperature increases, the rate of photosynthesis increases because enzymes work faster. However, above the optimum temperature (around 25–30 °C for most plants), enzymes begin to denature and the rate drops sharply.
| Limiting Factor | Effect of Increasing Factor | When Rate Plateaus |
|---|---|---|
| Light intensity | Rate increases proportionally | Another factor (CO2 or temperature) becomes limiting |
| CO2 concentration | Rate increases proportionally | Another factor (light or temperature) becomes limiting |
| Temperature | Rate increases up to optimum | Enzymes denature above optimum; rate falls sharply |
Worked Example 1 – Identifying the Limiting Factor
Question: A plant is growing at 15 °C with high light intensity and high CO2. The rate of photosynthesis is low. What is the limiting factor and why?
Solution: The temperature (15 °C) is the limiting factor. Even though light and CO2 are in plentiful supply, the enzymes involved in photosynthesis work slowly at low temperatures. Increasing the temperature would increase the rate until the optimum is reached.
Worked Example 2 – Explaining a Limiting Factor Graph
Question: A graph shows the rate of photosynthesis increasing with light intensity, then levelling off. Explain both sections.
Solution: Initially, light intensity is the limiting factor, so increasing it increases the rate. At the plateau, light is no longer limiting – another factor such as CO2 concentration or temperature has become the limiting factor. Further increases in light have no effect because the rate is now limited by this other factor.
Worked Example 3 – Temperature and Denaturation
Question: Explain why the rate of photosynthesis decreases above 35 °C.
Solution: At temperatures above 35 °C, the enzymes involved in photosynthesis begin to denature. Denaturation changes the shape of the enzyme's active site, so it can no longer bind to its substrate. This means the reactions slow down and the rate of photosynthesis decreases.
Inverse Square Law (Higher Only)
Inverse Square Law
Light intensity ∝ 1 / d²
Where d = distance from the light source.
If the distance is doubled, the light intensity is reduced to 1/4 of the original value.
Worked Example 4 – Inverse Square Law Calculation (Higher)
Question: A pondweed is 10 cm from a lamp. The lamp is moved to 20 cm away. What is the new light intensity as a fraction of the original?
Solution:
Original distance: d1 = 10 cm → Light intensity ∝ 1/10² = 1/100
New distance: d2 = 20 cm → Light intensity ∝ 1/20² = 1/400
New / Original = (1/400) / (1/100) = 100/400 = 1/4
The light intensity at 20 cm is one quarter of the intensity at 10 cm.
Worked Example 5 – Inverse Square Law with Three Distances (Higher)
Question: A student places pondweed at distances of 5 cm, 15 cm, and 30 cm from a lamp. Calculate the relative light intensity at each distance.
Solution:
At 5 cm: Light intensity ∝ 1/5² = 1/25 = 0.04
At 15 cm: Light intensity ∝ 1/15² = 1/225 = 0.0044
At 30 cm: Light intensity ∝ 1/30² = 1/900 = 0.0011
The light intensity at 15 cm is roughly 1/9 of that at 5 cm (distance tripled, intensity = 1/3² = 1/9). At 30 cm it is roughly 1/36 of that at 5 cm (distance ×6, intensity = 1/6² = 1/36).
Uses of Glucose from Photosynthesis
| Use of Glucose | Details |
|---|---|
| Respiration | Glucose is broken down to release energy for cellular processes |
| Starch | Glucose is converted to starch for storage (insoluble, does not affect water potential) |
| Cellulose | Glucose is converted to cellulose to make strong cell walls |
| Lipids (fats and oils) | Glucose is converted to lipids for energy storage, especially in seeds |
| Amino acids | Glucose is combined with nitrates from soil to make amino acids for protein synthesis |
Why Plants Store Glucose as Starch
Glucose is soluble in water, which means it would draw water into cells by osmosis and affect the water potential. Starch is insoluble, so it can be stored in large quantities without affecting the water balance of the cell.
Worked Example 6 – Uses of Glucose
Question: A plant has produced excess glucose through photosynthesis. Describe three ways the plant uses this glucose.
Solution:
- Some glucose is used for respiration, releasing energy for processes such as active transport of mineral ions from the soil.
- Some glucose is converted to starch and stored in the leaves or roots as an energy reserve for when photosynthesis cannot occur (e.g. at night or in winter).
- Some glucose is combined with nitrates absorbed from the soil to form amino acids, which are then used to synthesise proteins for growth and repair.
Required Practical: Pondweed and Light Intensity
Investigating the Effect of Light Intensity on the Rate of Photosynthesis
- Method: Place a piece of pondweed in a beaker of water at a fixed temperature. Add sodium hydrogencarbonate solution to supply 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), or use a gas syringe to measure the volume of oxygen.
- Variables: Independent = distance of lamp (light intensity). Dependent = number of O2 bubbles per minute (or volume of gas collected). Control = temperature, CO2 concentration, same piece of pondweed.
- Improving accuracy: Use a gas syringe to collect and measure the volume of oxygen, rather than counting bubbles (bubbles may vary in size). Repeat and calculate a mean.
- Using inverse square law (Higher): Plot rate vs 1/d² to get a straight line through the origin, confirming the inverse square relationship.
Practice Questions
Foundation
Q1. Write the word equation for photosynthesis.
Show Answer
Carbon dioxide + water → glucose + oxygen
Light energy is absorbed by chlorophyll and is needed for the reaction.
Foundation
Q2. Name three limiting factors that affect the rate of photosynthesis.
Show Answer
Light intensity, carbon dioxide concentration, and temperature.
Higher
Q3. A lamp is moved from 10 cm to 30 cm away from pondweed. Calculate the relative change in light intensity.
Show Answer
Original: Light intensity ∝ 1/10² = 1/100
New: Light intensity ∝ 1/30² = 1/900
Relative change = (1/900) / (1/100) = 100/900 = 1/9
The light intensity is now 1/9 (one ninth) of the original intensity.
Foundation
Q4. Explain why a plant converts glucose to starch for storage rather than storing glucose directly.
Show Answer
Glucose is soluble in water. If stored in large amounts, it would draw water into the cell by osmosis, which could damage the cell. Starch is insoluble, so it can be stored in large quantities without affecting the water potential of the cell.
Higher
Q5. In the pondweed required practical, explain why using a gas syringe is more accurate than counting bubbles, and state the purpose of adding sodium hydrogencarbonate to the water.
Show Answer
A gas syringe directly measures the volume of oxygen produced. Counting bubbles is less accurate because bubbles can vary in size, so the number of bubbles does not reliably represent the volume of gas. Sodium hydrogencarbonate is added to supply dissolved carbon dioxide to the pondweed, ensuring CO2 is not a limiting factor.
Foundation
Q6. Explain how nitrates are used by plants in connection with glucose produced by photosynthesis.
Show Answer
Glucose produced by photosynthesis is combined with nitrates absorbed from the soil to make amino acids. These amino acids are then joined together to make proteins, which the plant uses for growth and repair. Without nitrates, the plant cannot make proteins even if it has plenty of glucose.
Exam Tips
- Always state the full balanced symbol equation when asked – make sure the numbers balance (6CO2 + 6H2O → C6H12O6 + 6O2).
- When explaining limiting factor graphs, always use the phrase “another factor becomes limiting” for the plateau region.
- For Higher tier: remember the inverse square law formula – if distance triples, intensity drops to 1/9.
- In practical questions, always mention controlling variables and repeating for a mean.
- Don't confuse the uses of glucose – starch is for storage, cellulose is for cell walls.
🔬 Required Practical
RP: Effect of Light Intensity on Photosynthesis Using Pondweed
Aim: Investigate the effect of light intensity on the rate of photosynthesis in pondweed.
Method: Place a piece of pondweed in a beaker of water at a fixed temperature. Add sodium hydrogencarbonate solution to supply 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), or use a gas syringe to measure the volume of oxygen. Repeat at different distances and calculate the mean.
Variables: Independent = distance of lamp (light intensity); Dependent = number of O₂ bubbles per minute or volume of gas; Control = temperature, CO₂ concentration, same piece of pondweed, same lamp.
Improving accuracy: Use a gas syringe rather than counting bubbles (bubbles vary in size). Repeat and calculate a mean. Use a thermostat-controlled water bath to keep temperature constant.
Expected results: As distance decreases (light intensity increases), the rate of photosynthesis increases until another factor becomes limiting. Plotting rate vs 1/d² gives a straight line through the origin (Higher tier).
Safety: Do not touch the hot lamp. Keep water away from electrical connections.
🔢 Maths Skills
Mathematical Skills
Inverse square law: Light intensity is proportional to 1/d², where d is the distance from the light source. If distance doubles, light intensity becomes 1/4. If distance triples, intensity becomes 1/9. Rate calculations: calculate the rate of photosynthesis as bubbles per minute or cm³ of O₂ per minute. Plot graphs of rate vs light intensity and rate vs 1/d².
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Plants only respire at night Correct: Respiration is continuous — it occurs 24 hours a day. During the day, the rate of photosynthesis is usually higher than respiration, so there is a net uptake of CO₂ and net release of O₂. At night, only respiration occurs.
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how light intensity affects photosynthesis rate using inverse square law.
As light intensity increases, the rate of photosynthesis increases because more light energy is available for chlorophyll to absorb, driving the light-dependent reactions. However, at a certain point the rate plateaus because another factor (CO₂ concentration or temperature) becomes the limiting factor. The inverse square law states that light intensity is proportional to 1/d², where d is the distance from the light source. This means that if the distance from the lamp to the pondweed is doubled, the light intensity falls to 1/4 of the original value, and the rate of photosynthesis decreases proportionally (when light is the limiting factor). When plotting rate vs 1/d², a straight line through the origin confirms this relationship.
Mark scheme: 1 mark for rate increases with light intensity; 1 mark for plateau when another factor limits; 1 mark for inverse square law stated (1/d²); 1 mark for explaining doubling distance = 1/4 intensity; 1 mark for linking to photosynthesis rate when light is limiting; 1 mark for QWC
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student measures the volume of oxygen produced by pondweed at different distances from a lamp: 5 cm = 12 cm³/min; 10 cm = 3.2 cm³/min; 15 cm = 1.4 cm³/min; 20 cm = 0.8 cm³/min; 30 cm = 0.35 cm³/min. Temperature and CO₂ were kept constant.
1. Calculate 1/d² for each distance. 2. Plot rate vs 1/d² and describe the relationship. 3. The value at 10 cm is slightly lower than expected. Suggest an explanation for this anomaly and how the student could improve the reliability of their results.
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