3.8.6Additional lessons
Energy Losses — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 96 of 142
Part 1 of 5Routes of Energy Loss
Energy transfer between trophic levels is inefficient because most of the energy is lost to the surroundings.
Uneaten parts
- Uneaten parts Roots, bones, shells and other parts that the consumer does not eat retain their chemical energy, which passes to decomposers.
Egestion
- Egestion Food that cannot be digested, such as cellulose or fur, leaves as faeces and carries its energy out of the food chain.
Respiration
- Respiration Cell respiration releases energy as heat, and this heat is radiated to the environment at every level.
Excretion
- Excretion Metabolic wastes like urea in urine contain chemical energy that the animal cannot use.
Uneaten organisms
- Uneaten organisms Some individuals die without being eaten, so their energy goes to detritivores and saprotrophs instead.
Part 2 of 5Role of Decomposers
1
Detritivores break up dead plant and animal material, increasing the surface area available for enzyme action.
2
Saprotrophs secrete enzymes that digest the remains, and they absorb the soluble products.
3
Their cell respiration transfers part of the stored energy to the environment as heat.
4
As a result, energy that is not passed along the food chain is eventually all lost as heat.
5
Nutrients, unlike energy, are returned to the soil and used again by producers.
Part 3 of 5Consequences for Ecosystems
Consequences for Ecosystems
Short food chains
- Short food chains Food chains rarely exceed four or five levels because too little energy remains to support more consumers.
Falling biomass
- Falling biomass Less energy means fewer or smaller organisms at higher levels, so biomass tends to decrease up the chain.
Diet size
- Diet size A predator must eat a large mass of prey to obtain enough energy because most of that prey's energy is lost.
Open energy flow
- Open energy flow Energy flows in one direction and is not recycled, which is why ecosystems need a constant input of light.
| Loss route | Where the energy goes |
|---|---|
| Heat from respiration | Radiated to the environment |
| Faeces | Decomposers |
| Urea | Environment and decomposers |
| Uneaten parts | Decomposers |
Part 4 of 5Worked Example
Question
- A rabbit eats grass containing 2 500 kJ.
- It egests 600 kJ in faeces and loses 1 300 kJ as heat through respiration (ignore excretion).
- Calculate the energy added to its biomass and the percentage of ingested energy this represents.
- Energy absorbed = 2 500 − 600 = 1 900 kJ.
- Energy remaining after respiration = 1 900 − 1 300 = 600 kJ.
- Percentage of ingested energy = 600 ÷ 2 500 × 100 = 24%.
Answer 600 kJ is added to the rabbit's biomass, which is 24% of the energy it ingested.
Part 5 of 5Exam Checklist & Tips
3.8.6 – Key Points for the Exam
- Egestion The removal of undigested material from the gut as faeces.
- Excretion The removal of metabolic waste products from the body.
- Heat loss Energy released as thermal energy during respiration and radiated to the surroundings.
- Energy transfer efficiency The proportion of energy at one trophic level that ends up in the tissues of the next.
Common Mistakes
- Saying energy is destroyed; it is converted to heat and other forms in line with the conservation of energy.
- Thinking egestion and excretion are the same; egestion is undigested food, excretion is metabolic waste.
- Forgetting respiration as the biggest loss route.
- Claiming nutrients are lost from ecosystems at the same rate as energy; they are recycled.
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