- Define and explain Limiting Factors in your own words
- Use key terms such as limiting factor accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Limiting Factors: investigate how light, carbon dioxide and temperature limit the rate of photosynthesis.
Investigate how light, carbon dioxide and temperature limit the rate of photosynthesis.
Key ideas
Limiting factors
The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature. On a graph of rate against light intensity, the rate rises steeply at first, then levels off when another factor, such as carbon dioxide, becomes limiting. Greenhouse growers exploit this by adding extra light, carbon dioxide and warmth — but only raising the limiting factor helps.
Photosynthesis
Photosynthesis takes place in the chloroplasts of green cells: carbon dioxide + water → glucose + oxygen, using light energy trapped by chlorophyll. Leaves are adapted with a large surface area, thin structure for a short diffusion distance, and stomata that let gases in and out. Glucose is used for respiration, converted to starch for storage, or built into cellulose for cell walls.
Key term — limiting factor: The factor in shortest supply that restricts the rate of a process; raising it increases the rate until another factor takes over.
On a graph of photosynthesis rate against light intensity, the curve levels off at high light. Explain why.
Light is no longer the limiting factor; something else, such as carbon dioxide concentration or temperature, is now in shortest supply, so extra light cannot raise the rate further.
Answer: Light is no longer the limiting factor; something else, such as carbon dioxide concentration or temperature, is now in shortest supply, so extra light cannot raise the rate further.
- Writing the photosynthesis equation backwards Photosynthesis builds glucose from carbon dioxide and water; respiration breaks glucose down. Mixing them up reverses the direction of energy flow.
- Saying plants only respire at night Plants respire all the time to release energy. At night photosynthesis stops but respiration continues.
Practice
Carbon dioxide + water → glucose + oxygen (using light energy).
Glucose + oxygen → carbon dioxide + water (energy is released).
Any two of: large surface area to catch light; thin for a short diffusion distance; chlorophyll in chloroplasts to trap light energy; stomata for gas exchange; veins to transport water and glucose.
In human muscle, anaerobic respiration converts glucose to lactic acid. In yeast, it converts glucose to ethanol and carbon dioxide.
Quick check
Which of these best defines "limiting factor"?
Grass at one trophic level contains 20 000 kJ of energy, and the rabbits feeding on it contain 2 400 kJ. Calculate the percentage efficiency of energy transfer.
- Limiting Factors: investigate how light, carbon dioxide and temperature limit the rate of photosynthesis.
- Limiting factors: The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature.
- chlorophyll: The green pigment in chloroplasts that absorbs light energy for photosynthesis.
- Watch out for: writing the photosynthesis equation backwards