Photosynthesis and Limiting Factors
Photosynthesis is the fundamental process by which plants convert light energy into chemical energy, creating their own food. The rate of this vital reaction is not constant; it is controlled by the most scarce environmental resource, known as the limiting factor.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Photosynthesis is an endothermic reaction, meaning it absorbs energy from the surroundings, specifically in the form of light.
- The reactants are carbon dioxide (from the air) and water (from the soil). The products are glucose (a sugar used for energy and growth) and oxygen (released as a waste product).
- A limiting factor is a variable that restricts the rate of a reaction. For photosynthesis, the main limiting factors are light intensity, carbon dioxide concentration, and temperature.
- Increasing the limiting factor will increase the rate of photosynthesis, but only up to a point where another factor becomes limiting.
- Temperature affects the enzymes that catalyse photosynthesis. The rate increases with temperature up to an optimum point, after which the enzymes denature and the rate rapidly decreases.
- At night, light intensity is the limiting factor. On a bright, cold day, temperature might be the limiting factor. In a sealed greenhouse, carbon dioxide concentration often becomes the limiting factor.
Formulae
Carbon Dioxide + Water --(light)→ Glucose + Oxygen To describe the overall process of photosynthesis in words.
6CO2 + 6H2O --(light)→ C6H12O6 + 6O2 To show the balanced chemical equation for photosynthesis, representing the atoms involved.
Definitions
- Photosynthesis
- A chemical process in plant cells that converts light energy, water, and carbon dioxide into chemical energy in the form of glucose, releasing oxygen as a by-product.
- Endothermic Reaction
- A chemical reaction that absorbs energy from its surroundings, typically as heat or light. The products have more energy than the reactants.
- Limiting Factor
- An environmental factor that is in shortest supply and therefore restricts a process, such as photosynthesis, from increasing its rate.
Worked example
A graph shows the rate of photosynthesis versus light intensity for a plant at a constant temperature of 25°C and a CO2 concentration of 0.04%. The rate increases linearly from 0 to 10 units of light, after which the rate plateaus at 50 arbitrary units (a.u.). What is the most likely limiting factor along the linear section (0-10 light units) and along the plateau section (>10 light units)?
- 1
Identify the variable on the x-axis:
Light Intensity.
- 2
Analyse the first section (0-10 light units):
The graph shows that as light intensity increases, the rate of photosynthesis increases directly.
This means that providing more light results in a faster reaction.
- 3
Conclude the limiting factor for the first section:
Because the rate is directly dependent on the light intensity, light intensity is the limiting factor in this range.
- 4
Analyse the second section (>10 light units):
The graph shows that increasing the light intensity beyond 10 units has no effect on the rate; it remains constant at 50 a.u.
- 5
Conclude the limiting factor for the second section:
Since adding more light does not increase the rate, something else must be holding the process back.
Given the stated conditions, this will be either the CO2 concentration (0.04%) or the temperature (25°C).
Answer: Linear section: Light intensity. Plateau section: CO2 concentration or temperature.
Common mistakes
- ×Misreading graphs of limiting factors, such as mixing up the axes or ignoring the labels that state the constant conditions (e.g., a specific temperature or CO2 level).
- ×Forgetting that multiple factors can be limiting. When one factor's curve plateaus, it means another factor has become the primary constraint on the reaction rate.
- ×Incorrectly assuming that temperature has a simple positive correlation with the rate. Remember that beyond the optimum, high temperatures cause enzymes to denature, leading to a sharp drop in the rate.
- ×Mistaking which factor is limiting on different parts of the curve. If the rate is increasing as the x-axis variable increases, that variable is the limiting factor.
No-calculator tips
- ✓When interpreting a limiting factor graph, trace the curve with your finger. If moving right (increasing the x-axis variable) also makes you move up, that variable is limiting. If moving right doesn't make you move up, something else is limiting.
- ✓For questions asking to compare rates, visually check the vertical positions on the graph. A point twice as high represents a rate twice as fast.
- ✓If a question provides two curves on one graph (e.g., at two different temperatures), focus on where they are different. The gap between them shows the effect of changing that specific condition (temperature).