IB DP Biology: Energy flow: food chains, trophic levels and energy pyramids (DP Biology) – Revision Notes

These notes condense the first part of sub-topic C4.2, Transfers of energy and matter, from the IB Biology guide, first assessment 2025 (statements C4.2.1 to C4.2.14). Everything here is for SL and HL; none of it is marked higher level only. For full explanations and worked examples, read the study guide. Then test yourself with the practice questions. Course hub: IB DP Biology. Printable list: checklist. To find weak spots, use the free 10-minute diagnostics.

Key definitions

Term	Meaning
Open system	Exchanges both energy and matter with its surroundings (an ecosystem)
Closed system	Exchanges energy only (a sealed jar ecosystem; Earth, approximately, for matter)
Autotroph	Makes carbon compounds from simple inorganic substances using an external energy source
Photoautotroph	Autotroph that uses light
Chemoautotroph	Autotroph that uses energy from oxidation reactions of inorganic substances
Heterotroph	Makes the carbon compounds it needs from compounds obtained from other organisms
Decomposer	Gets energy from carbon compounds in faeces and dead organisms
Trophic level	Position in a food chain: producer, primary, secondary, tertiary consumer
Energy pyramid	Bars for each trophic level, scaled to energy per unit area per unit time

C4.2.1 and C4.2.2: systems and energy sources

• Ecosystems are open. Energy in: sunlight. Energy out: heat. Matter crosses the boundary in both directions (gases, water, ions, migrating animals, harvested crops).
• In a closed system only energy can cross.
• Most ecosystems rely on sunlight captured by photosynthesis.
• Exceptions: caves and ocean depths below light penetration. Energy comes from chemoautotrophs or from organic matter that arrives from elsewhere.
• Matter is recycled in ecosystems. Energy is not, because chemical energy ends up as heat.
• Nature of science: a law describes a pattern and can predict; it does not explain. A useful generalization rests on many observations, is testable, predicts, and admits exceptions. “About one tenth passes on” is a rule of thumb, not an exact value.

C4.2.3 to C4.2.5: food chains, webs and decomposers

Method: drawing a food chain or web

1. List the organisms and what each eats.
2. Start with a producer.
3. Draw each arrow from the food to the feeder.
4. Join chains into a web where organisms share a food source.
5. Check that every arrow points in the direction of energy and biomass transfer.

• Chemical energy passes to a consumer when it eats the previous organism.
• Decomposers get energy from carbon compounds in faeces, dead parts (such as leaf litter) and whole dead organisms.
• Decomposers and detritus feeders are not usually drawn in chains, but they take energy from dead matter and release much of it as heat.

C4.2.6 to C4.2.9: nutrition and respiration

	Photoautotroph	Chemoautotroph	Heterotroph
Carbon source	Carbon dioxide	Carbon dioxide	Compounds from other organisms
External energy	Light	Oxidation of inorganic substances	Chemical energy in food
Example	Plants, algae, cyanobacteria	Iron-oxidizing bacteria	Animals, fungi

• Autotrophs need external energy for two jobs: fixing carbon and powering the anabolic synthesis of macromolecules.
• Oxidation releases energy, which is why it is useful to living things.
• Heterotrophs digest large molecules (externally or internally), absorb the products and assimilate them into their own compounds.
• All autotrophs and heterotrophs respire. Cell respiration oxidizes carbon compounds to release energy. Photoheterotrophs are not required.

C4.2.10: trophic levels

• Producer, primary consumer, secondary consumer, tertiary consumer.
• Many organisms eat more than one kind of food, so they sit at different levels in different chains. An omnivore eating plants and herbivores is a primary and a secondary consumer.
• Always say which chain you are using when you classify an organism.

C4.2.11: energy pyramids

• One bar per level, producers at the bottom, bars to one scale.
• Units: energy per unit area per unit time, for example kJ m⁻² year⁻¹.
• Data come from research on specific ecosystems.

Method: percentage transfer

1. Take the energy in the higher level.
2. Divide by the energy in the level below.
3. Multiply by 100.

Worked reminder. A coastal lagoon has 36 000 kJ m⁻² year⁻¹ in producers. A transfer of 8% gives 36 000 × 0.08 = 2 880 in primary consumers. A second transfer of 8% gives 2 880 × 0.08 = 230.4 kJ m⁻² year⁻¹ in secondary consumers. The energy not passed from producers to primary consumers is 36 000 − 2 880 = 33 120 kJ m⁻² year⁻¹.

C4.2.12 and C4.2.13: causes of loss

Loss	Where the energy goes
Parts not eaten (bones, roots)	Decomposers
Material eaten but not digested (faeces)	Decomposers
Urine and other excretion	Released to the environment; the energy in it may later reach decomposers
Cell respiration	Heat to surroundings
Organisms that die uneaten	Decomposers

Method: energy budget for a consumer

1. Write the energy ingested.
2. Subtract faeces, urine and energy released in respiration.
3. The remainder is energy added to the consumer’s biomass.
4. Divide by the energy ingested and multiply by 100 for the percentage that becomes biomass.

For example, a consumer ingests 500 kJ, loses 120 kJ in faeces and 15 kJ in urine, and releases 290 kJ in respiration. Biomass gain is 500 − 120 − 15 − 290 = 75 kJ, which is 15% of intake.

• Respiration converts chemical energy to ATP and to heat. ATP use (movement, active transport, synthesis) releases more heat. No transfer is 100% efficient.
• Autotrophs lose heat too, so not all energy captured in photosynthesis becomes biomass.
• Heat cannot be reused to make carbon compounds, so energy flows through and does not cycle.
• Warm-blooded animals lose more energy as heat than cold-blooded animals of similar mass, because they use respiration to maintain body temperature.

C4.2.14: limits to trophic levels

• Energy falls at every transfer, so higher levels have fewer or smaller organisms and less biomass.
• Energy content per gram of biomass stays about the same.
• Food chains rarely have more than four or five trophic levels, because the top level would receive too little energy to support a viable population.

Worked reminder. Start at 36 000 kJ m⁻² year⁻¹ with 8% transfer. The values are 36 000, 2 880, 230.4, 18.4 and 1.5 kJ m⁻² year⁻¹ for levels one to five. At the fifth level only 1.5 kJ m⁻² year⁻¹ remains.

Must-know distinctions

• Open vs closed system: matter and energy vs energy only.
• Photoautotroph vs chemoautotroph: light vs oxidation of inorganic substances.
• Autotroph vs heterotroph: makes carbon compounds from inorganic substances vs obtains them from other organisms.
• Energy vs matter: energy flows one way and is lost as heat; matter is recycled.
• Digestion vs assimilation: breaking down food vs building the organism’s own compounds from the products.

Quick self-test

1. Define an open system.
2. Give one example of an ecosystem that does not depend on sunlight.
3. In the food chain grass → grasshopper → toad → snake, which direction does each arrow point and what does it show?
4. Name the trophic level of the toad.
5. State two reasons why an autotroph needs an external energy source.
6. Which type of autotroph is an iron-oxidizing bacterium?
7. Why do plants lose energy as heat?
8. A pond has 4 500 kJ m⁻² year⁻¹ in producers and 10% of this passes to primary consumers. Calculate the energy in the primary consumers.
9. If 10% passes on again, what energy reaches secondary consumers?
10. Explain why most food chains have four or five trophic levels.
11. Faeces contain energy. State where it goes.
12. State what happens to the energy content per gram of biomass at higher trophic levels.

Answers

1. A system that exchanges both energy and matter with its surroundings.
2. A cave, or the deep ocean below the reach of light (for example a hydrothermal vent community).
3. Each arrow points from the food to the feeder, showing which way energy and biomass move.
4. Secondary consumer, in this chain.
5. Fixing carbon from carbon dioxide, and driving the anabolic building of macromolecules.
6. Chemoautotroph.
7. Respiration converts chemical energy to ATP with some released as heat, and using ATP releases more heat.
8. 4 500 × 0.10 = 450 kJ m⁻² year⁻¹.
9. 450 × 0.10 = 45 kJ m⁻² year⁻¹.
10. Energy is lost at each transfer, mainly as heat from respiration, so too little remains to support a viable population at a sixth level.
11. It stays in the ecosystem and passes to decomposers and detritus feeders.
12. It is not reduced. Total energy falls because there is less biomass.

Where marks are usually lost

• Drawing arrows the wrong way round.
• Writing “energy is recycled” or “energy is used up” instead of “converted to heat and lost from the food chain”.
• Saying only “lost as heat” when the question asks for causes. List several: respiration, faeces, uneaten parts, excretion.
• Quoting a percentage without showing the division.
• Leaving out units, or writing kJ when the data are per area and per year.
• Calling an omnivore a single trophic level without naming the chain.
• Confusing chemoautotrophs with heterotrophs because both use chemical energy. The chemoautotroph builds its own carbon compounds from inorganic substances.
• Giving the one-tenth rule as exact. Say “approximately” and use the supplied data.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Statements C4.2.1 to C4.2.14.