3.9.6Additional lessons
Energy Losses — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 24 of 101
Section 1 of 5Where the Energy Goes
Energy transfers between trophic levels are inefficient for several reasons.
Respiration
- Organisms oxidise organic compounds to release energy for ATP, and much of this ends up as heat that cannot be passed on.
Uneaten parts
- Some parts of an organism, such as bones, shells, roots and hair, are not eaten, so their energy does not move to the next level.
Egestion
- Material that is eaten but not digested, such as cellulose in many animals, is lost in faeces.
Excretion
- Energy-containing waste such as urea leaves the body in urine.
Death without being eaten
- Some organisms die of disease or old age and pass their energy to decomposers instead of consumers.
Section 2 of 5Accounting for Energy in a Consumer
1
Energy ingested is the chemical energy in all the food taken in by the animal.
2
Subtracting the energy lost in faeces gives the energy assimilated, which is absorbed into the body.
3
A large part of the assimilated energy is used in respiration for movement, heat and active transport.
4
The remaining energy is stored in new tissue as growth and reproduction, which is the secondary production available to the next level.
Section 3 of 5Consequences
Consequences
Short food chains
- Because energy declines at each step, there is rarely enough left to support a fifth or sixth trophic level.
Feeding at lower levels
- Eating plants directly allows people to use more of the energy captured by producers than eating animals fed on them.
Warm-blooded animals
- Endotherms use a large share of their food to produce body heat, so less is converted into biomass.
Biomass
- Biomass also declines up the trophic levels, since carbon is lost as carbon dioxide in respiration.
| Loss | Where the energy goes |
|---|---|
| Respiration | Heat to the environment |
| Faeces | Decomposers |
| Uneaten parts | Decomposers |
| Urine | Environment and decomposers |
Section 4 of 5Worked Example
Question
- A herbivore ingests 800 kJ of plant material.
- 300 kJ is egested in faeces and 400 kJ of the assimilated energy is used in respiration.
- Calculate the energy assimilated, the energy available for growth and the percentage of ingested energy that becomes new tissue.
- Energy assimilated = ingested − egested = 800 − 300 = 500 kJ.
- Energy for growth = assimilated − respired = 500 − 400 = 100 kJ.
- Percentage of ingested energy = 100 ÷ 800 × 100 = 12.5%.
Answer 500 kJ is assimilated, 100 kJ is available for growth, and 12.5% of the ingested energy becomes new tissue.
Section 5 of 5Exam Checklist & Tips
3.9.6 – Key Points for the Exam
- Assimilation Absorption of digested food into the body.
- Egestion Removal of undigested material from the gut.
- Biomass The dry mass of organic material in organisms.
- Respiration loss Energy released as heat during cell respiration.
- Endotherm An animal that maintains body temperature using metabolic heat.
Common Mistakes
- Saying energy lost as heat disappears; it is dispersed to the environment.
- Confusing egestion with excretion.
- Stating that the energy lost in faeces is wasted, when it supports decomposers.
- Believing that exactly ten percent is always passed on.
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