Identify and describe What is a Limiting Factor?, Effect of Carbon Dioxide Concentration and Effect of Light Intensity in the context of rate of photosynthesis.
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GCSE Biology Revision
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GCSE Biology revision
Rate of photosynthesis
Photosynthesis
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Explain the relationships between What is a Limiting Factor?, Effect of Carbon Dioxide Concentration and Effect of Light Intensity and how they contribute to rate of photosynthesis.
Apply and analyse knowledge of What is a Limiting Factor?, Effect of Carbon Dioxide Concentration and Effect of Light Intensity to interpret unfamiliar information about rate of photosynthesis.
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What is a Limiting Factor?
- A limiting factor is a condition that, when in short supply, slows down the rate of photosynthesis.
- The three main limiting factors for photosynthesis are light intensity, carbon dioxide (CO₂) concentration, and temperature.
- Each limiting factor can individually or in combination affect how quickly a plant photosynthesises.
Effect of Carbon Dioxide Concentration
- As CO₂ concentration increases, the rate of photosynthesis increases proportionally.
- At high CO₂ concentrations, the rate levels off and plateaus because the enzymes required for photosynthesis become saturated.
- When the graph plateaus, CO₂ is no longer the limiting factor and another factor must be restricting the rate of reaction.
Effect of Light Intensity
- As light intensity increases, the rate of photosynthesis increases because more light energy is trapped by the chloroplasts.
- At high light intensities, the rate plateaus because the maximum amount of light that can be absorbed has been reached.
- Beyond the plateau, another factor such as chloroplast number or CO₂ concentration becomes the new limiting factor.
Effect of Temperature
- As temperature increases, the rate of photosynthesis increases because enzymes gain more kinetic energy.
- There is an optimum temperature at which the rate of photosynthesis is at its highest.
- Above the optimum temperature, the rate sharply declines because the enzymes denature — their protein structure breaks down as weak hydrogen bonds are disrupted.
- Unlike CO₂ and light intensity graphs which plateau, the temperature graph shows a steep decline after the optimum, making it distinctly different.
Effect of Chlorophyll Concentration
- Chlorophyll is another limiting factor for photosynthesis, as it is required to absorb light energy.
- As the amount of chlorophyll increases, more light can be trapped by the chloroplasts, increasing the rate of photosynthesis.
- Once a sufficient amount of chlorophyll is present, it is no longer the limiting factor and another variable will limit the rate.
Interpreting Graphs with Multiple Variables
- When comparing two temperatures on a light intensity graph, a higher temperature (e.g.
- 30°C vs 20°C) produces a higher rate of photosynthesis at any given light intensity.
- When CO₂ concentration is increased (e.g. from 0.05% to 0.5%), the plateau on a light intensity graph is reached at a higher rate, showing CO₂ was previously limiting.
- On multi-variable graphs, the independent variable is always plotted on the x-axis and the dependent variable (rate of reaction) on the y-axis.
The Canadian Pondweed Experiment
- In the Canadian pondweed experiment, a light source is placed at a measured distance from a beaker containing pondweed, which produces oxygen bubbles as it photosynthesises.
- The rate of oxygen production is proportional to the rate of photosynthesis, as oxygen is a product of the photosynthesis reaction.
- Oxygen produced moves a water droplet along a capillary tube, and the distance the droplet travels is used to measure the rate of photosynthesis.
- Light intensity can be varied by moving the lamp closer to or further from the plant, whilst controlling all other variables to ensure a fair test.
- Temperature can be investigated by placing the boiling tube in water baths set to different temperatures (e.g.
- 10°C, 20°C, 30°C).
- CO₂ concentration can be varied by dissolving different amounts of sodium hydrogen carbonate in the water, as more sodium hydrogen carbonate produces a higher CO₂ concentration.
Experimental Design: Key Principles
- The independent variable (e.g. light intensity, temperature, or CO₂ concentration) must be deliberately varied whilst all other variables are controlled to ensure a fair test.
- Each experiment should be repeated at least three times at each condition to improve the reliability and reproducibility of results.
- A mean (average) should be calculated from the repeated results before plotting a graph.
- When plotting graphs, axes must be labelled with units, and data should occupy at least half the available grid space.
The Inverse Square Law
- Light intensity is inversely proportional to the square of the distance from the light source, expressed as: Light intensity is proportional to (1) / (d²) This means that as the distance between the light source and the plant increases, the light intensity decreases.
- For example, if the distance is 2 cm, the relative light intensity is (1) / (2²) = (1) / (4) = 0.25.
Controlling Conditions in Greenhouses
- Farmers use greenhouses to control limiting factors and maximise crop yields by optimising the conditions for photosynthesis.
- Temperature is regulated using the glass exterior to trap heat, blinds to reduce excess heat, and heaters to provide warmth during winter months.
- CO₂ concentration can be increased by burning paraffin heaters inside the greenhouse, which releases carbon dioxide as a by-product.
- Artificial lighting can be used to supplement or replace natural light, ensuring plants receive sufficient light intensity for photosynthesis.
- Diseases that reduce chlorophyll production can limit photosynthesis, so monitoring plant health is also important for maximising yields.