3.9.2Additional lessons
Energy Flow in Ecosystems — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 20 of 101
Part 1 of 5Capturing Energy
Sunlight
- The Sun supplies almost all the energy for ecosystems, although only a small fraction of the light reaching producers is used.
Photosynthesis
- Chlorophyll absorbs light and the energy is stored as chemical energy in glucose and other organic compounds.
Chemosynthesis
- In a few ecosystems, such as deep-sea vents, bacteria obtain energy from inorganic chemicals instead of light.
Transfer by feeding
- Energy stored in the organic compounds of producers passes to consumers when they are eaten.
Part 2 of 5Energy Transformations
Energy conversions are never completely efficient.
1
Light energy is converted by producers into chemical energy in organic molecules.
2
Consumers eat these molecules and release the energy in cell respiration, where it is transferred to ATP.
3
Cells use ATP for growth, movement, active transport and other life processes.
4
At every transformation some energy is lost as heat, which is a less useful form and cannot be used by living things to make organic compounds.
5
Heat is lost to the environment and eventually radiated into space.
Part 3 of 5Why Energy Does Not Cycle
Why Energy Does Not Cycle
One-way flow
- Energy moves from the Sun to producers, to consumers and out to the environment as heat.
Second law
- Each transformation increases disorder, so energy becomes more dispersed and less available.
Need for input
- Life on Earth depends on a constant supply of light, because energy lost as heat cannot be reused.
Decomposers
- Decomposers also respire, so the energy in dead matter is released as heat in the same way.
| Feature | Energy | Nutrients |
|---|---|---|
| Direction | One-way | Cyclic |
| Source | Sun | Soil, water, air |
| Final fate | Lost as heat | Reused by producers |
Part 4 of 5Following the Energy in a Chain
Producer to herbivore
- Only the energy stored in the plant material that is actually eaten can pass to the herbivore;
- roots and tough tissue may be left.
Herbivore to carnivore
- The carnivore receives only the energy in the herbivore's body tissues, not the energy the herbivore used for movement.
Egestion
- Material that is eaten but not absorbed leaves as faeces and still contains chemical energy that decomposers can use.
Heat loss
- Warm-blooded animals lose more heat to keep body temperature constant, so less of their intake becomes new tissue.
Overall pattern
- The amount of usable energy falls at each step, which is why the diagrams that show it narrow towards the top.
Part 5 of 5Exam Checklist & Tips
3.9.2 – Key Points for the Exam
- Energy flow The movement of energy through an ecosystem in one direction.
- Producer An organism that makes organic compounds from inorganic sources.
- Cell respiration Release of energy from organic compounds to make ATP.
- Chemosynthesis Production of organic compounds using energy from chemical reactions.
- Heat Energy lost from the food chain that living things cannot reuse.
Common Mistakes
- Writing that energy is recycled in an ecosystem.
- Saying energy is destroyed, when it is converted to heat.
- Giving the Sun as the energy source for every ecosystem, ignoring chemosynthetic ones.
- Forgetting that respiration in decomposers also releases heat.
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