- Define and explain Energy in Ecosystems in your own words
- Use key terms such as biomass accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Energy in Ecosystems: calculate the percentage efficiency of energy transfer between trophic levels.
Calculate the percentage efficiency of energy transfer between trophic levels.
Key ideas
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.
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.
Key term — biomass: The total mass of living material in an organism, trophic level or ecosystem, often measured as dry mass.
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.
2 400 ÷ 20 000 = 0.12; 0.12 × 100 = 12% efficiency. The rest is lost through respiration, movement, excretion and uneaten parts.
Answer: 2 400 ÷ 20 000 = 0.12; 0.12 × 100 = 12% efficiency. The rest is lost through respiration, movement, excretion and uneaten parts.
- 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.
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.
Quick check
Which of these best defines "biomass"?
Compare the products of anaerobic respiration in human muscle and in yeast.
- Energy in Ecosystems: calculate the percentage efficiency of energy transfer between trophic levels.
- Photosynthesis: Photosynthesis takes place in the chloroplasts of green cells: carbon dioxide + water → glucose + oxygen, using light energy trapped by chlorophyll.
- chlorophyll: The green pigment in chloroplasts that absorbs light energy for photosynthesis.
- Watch out for: writing the photosynthesis equation backwards