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

This guide teaches the first part of sub-topic C4.2, Transfers of energy and matter, from the IB Biology guide, first assessment 2025. It covers statements C4.2.1 to C4.2.14. All of it is for SL and HL, and the guide marks none of these statements as higher level only. Statements C4.2.15 onward (production and the carbon cycle) are taught elsewhere. For condensed recall use the revision notes; to test yourself use the practice questions. Course hub: IB DP Biology. Printable list: checklist. Not sure where your gaps are? Try the free 10-minute diagnostics.

What this unit covers

Statement	What you must be able to do	Level
C4.2.1	Explain open systems; know that in closed systems only energy crosses	SL and HL
C4.2.2	Explain why sunlight sustains most ecosystems; give exceptions	SL and HL
C4.2.3	Describe how chemical energy passes along a food chain	SL and HL
C4.2.4	Construct food chains and food webs with correct arrows	SL and HL
C4.2.5	State that decomposers get energy from faeces and dead organisms	SL and HL
C4.2.6	Define autotrophs and explain why they need an external energy source	SL and HL
C4.2.7	Contrast photoautotrophs with chemoautotrophs, using iron-oxidizing bacteria	SL and HL
C4.2.8	Define heterotrophs and describe digestion followed by assimilation	SL and HL
C4.2.9	State that both autotrophs and heterotrophs release energy by cell respiration	SL and HL
C4.2.10	Classify organisms into trophic levels, allowing for varied diets	SL and HL
C4.2.11	Construct energy pyramids from research data	SL and HL
C4.2.12	Explain the large energy losses between trophic levels	SL and HL
C4.2.13	Explain heat loss from respiration in autotrophs and heterotrophs	SL and HL
C4.2.14	Explain why energy losses limit the number of trophic levels	SL and HL

C4.2.1 Open systems and closed systems

An open system exchanges both energy and matter with its surroundings. A closed system exchanges energy only.

An ecosystem is open. Energy enters as sunlight and leaves as heat. Matter enters as carbon dioxide, water, mineral ions and the bodies of animals that migrate in, and leaves as gases, dissolved ions, harvested crops and migrating animals. In a forest stream, leaves fall in and water carries dissolved nutrients out.

A sealed glass jar containing plants, soil and small animals models a closed system. Light and heat cross the glass, but no gas or water does. It can run for a long time only because the matter inside is recycled while light keeps supplying energy. The whole Earth behaves in a similar way: energy arrives and leaves, but the matter available to living things is essentially fixed. Matter can be recycled; energy cannot, because chemical energy in organisms ends up as heat that no organism can turn back into food.

C4.2.2 Sunlight and its exceptions

Most ecosystems depend on light, which photosynthetic organisms convert into chemical energy in carbon compounds. Every consumer in those ecosystems ultimately depends on that energy.

Exceptions are ecosystems without light. Examples are caves and the ocean below the depth that light reaches. There, energy comes from chemoautotrophic bacteria that oxidize inorganic substances, or from dead organic matter that sinks or washes in from lit areas. Hydrothermal vent communities are a standard example of the first kind.

Nature of science: laws and generalizations. A law describes a pattern seen in nature and can be used to predict. Unlike a theory, it does not explain the pattern. A useful generalization:

• rests on many observations from different places;
• states a pattern clearly enough to be tested;
• allows predictions in new situations;
• admits exceptions and limits.

“Energy transferred between trophic levels is much less than the energy entering them” is a good example. The statement “about one tenth is passed on” is only a rule of thumb. Measured values vary widely between ecosystems, so treat it as an approximate pattern, not an exact law.

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

Chemical energy is stored in the bonds of carbon compounds such as carbohydrates, lipids and proteins. When a consumer eats an organism, it takes in those compounds and uses them as a source of energy and building materials, so the chemical energy passes to the consumer. A food chain shows this as a line of feeding relationships.

Rules for diagrams.

• Arrows point from the organism that is eaten to the organism that eats it. They show which way energy and biomass are passed on, never “who eats whom” in reverse.
• A food web joins several food chains where organisms share foods. It is a more realistic model of a community.

Example food chain from a freshwater pond: diatoms → water flea → stickleback → heron. Each arrow is one transfer of energy and biomass.

Decomposers. Dead organisms, dead parts (fallen leaves, shed skin, bark) and faeces all contain carbon compounds. Decomposers, mainly fungi and bacteria, feed on this organic matter and get their energy from it. Detritus feeders ingest the same material directly. Decomposers and detritus feeders are not usually drawn in food chains, but they transform energy: they take it from dead material and release most of it as heat.

C4.2.6 to C4.2.9 Modes of nutrition

Autotrophs make their own carbon compounds from simple inorganic substances such as carbon dioxide and water. This needs an external energy source for two jobs: carbon fixation (turning carbon dioxide into organic compounds) and the anabolic reactions that build macromolecules such as starch, proteins and nucleic acids.

• Photoautotrophs use light. Examples are plants, algae and cyanobacteria.
• Chemoautotrophs use energy released by oxidation reactions of inorganic substances. Oxidation releases energy, so it can drive synthesis. Iron-oxidizing bacteria are the example named in the guide. They oxidize iron(II) ions to iron(III) ions and use the energy released to fix carbon dioxide.

Heterotrophs obtain carbon compounds from other organisms. They do not simply keep what they eat. Large molecules (proteins and nucleic acids, for example) must be broken down first, either outside the body (fungi secrete enzymes onto their food) or inside it (a gut). The small products are absorbed and then assimilated: the organism builds its own carbohydrates, proteins and nucleic acids from them.

Respiration in both groups. Autotrophs and heterotrophs both oxidize carbon compounds in cell respiration to release energy as ATP. A plant respires day and night but photosynthesizes only in the light. You are not required to know about photoheterotrophs.

Feature	Photoautotroph	Chemoautotroph	Heterotroph
Carbon source	Carbon dioxide	Carbon dioxide	Other organisms
Energy for synthesis	Light	Oxidation of inorganic substances	Chemical energy in food
Example	Grass	Iron-oxidizing bacteria	Rabbit, fungus

C4.2.10 Trophic levels

A trophic level is the position an organism occupies in a food chain. Use these terms:

• producer: first level, an autotroph;
• primary consumer: eats producers (herbivore);
• secondary consumer: eats primary consumers;
• tertiary consumer: eats secondary consumers.

Organisms with varied diets occupy different levels in different chains. A fox that eats berries is a primary consumer. When it eats a vole that ate seeds, it is a secondary consumer. When it eats a shrew that ate beetles, it is a tertiary consumer. Always classify an organism in the chain you are given, and say if it feeds at more than one level.

C4.2.11 Energy pyramids

An energy pyramid is a stack of horizontal bars, one per trophic level, with producers at the bottom. The length of each bar is drawn to the same scale and is proportional to the energy available at that level. The units are energy per unit area per unit time, such as kJ m⁻² year⁻¹. Using area and time lets you compare ecosystems fairly.

Worked example: constructing a pyramid. Energy stored in biomass on an upland heather moor was measured over one year.

Trophic level	Energy stored (kJ m⁻² year⁻¹)
Heather and other producers	24 600
Primary consumers	2 214
Secondary consumers	177
Tertiary consumers	14.2

Choose a scale. With 1 character = 1 000 kJ, the bars are about 25, 2, 0.2 and 0.01 characters long, so the top bars are almost invisible. Label each bar with its level and value, and put the unit on the figure.

Percentage transferred from one level to the next = (energy in the higher level ÷ energy in the lower level) × 100.

• Producers to primary consumers: 2 214 ÷ 24 600 × 100 = 9.0%
• Primary to secondary: 177 ÷ 2 214 × 100 = 8.0%
• Secondary to tertiary: 14.2 ÷ 177 × 100 = 8.0%

Energy not passed on from producers to primary consumers is 24 600 − 2 214 = 22 386 kJ m⁻² year⁻¹. Only 14.2 ÷ 24 600 × 100 = 0.058% of the producers’ stored energy reaches the tertiary consumers.

C4.2.12 and C4.2.13 Why so much energy is lost

1. Parts not eaten. Roots, bones, shells and fur may be left behind. This material goes to decomposers.
2. Parts eaten but not digested. Cellulose and other materials leave in faeces. The energy in faeces stays in the ecosystem and goes to decomposers.
3. Waste products. Excretion removes energy-containing compounds such as urea in urine.
4. Cell respiration. Organisms oxidize carbon compounds to release ATP. This is the largest single loss in most food chains.
5. Death without being eaten. The energy of these organisms passes to decomposers.

Heat. Energy transfers are never 100% efficient. Some of the energy in glucose is captured in ATP and the rest is released as heat. When cells use ATP for movement, active transport or synthesis, heat is released again. This happens in autotrophs as well as heterotrophs. The heat escapes to the surroundings and cannot be used by another organism to make carbon compounds. Warm-blooded animals lose more this way because they use respiration to keep their body temperature high, so a smaller share of their intake goes into growth.

Worked example: an energy budget. Field voles ate grass containing 640 kJ m⁻² year⁻¹ of energy. Of this, 150 kJ was lost in faeces, 20 kJ in urine, and 376 kJ was released in respiration (all as heat eventually). The rest was added to vole biomass.

• Energy added to biomass = 640 − 150 − 20 − 376 = 94 kJ m⁻² year⁻¹
• Share of intake added to biomass = 94 ÷ 640 × 100 = 14.7%
• Share lost as heat through respiration = 376 ÷ 640 × 100 = 58.8%

The 150 kJ in faeces is not lost from the ecosystem straight away: decomposers feed on it.

C4.2.14 Limits to the number of trophic levels

Because so little energy moves up each step, the top level of a chain has very little to share. At each stage there are fewer organisms or smaller organisms, so total biomass falls. The energy content per gram of biomass does not fall: a gram of dry grass and a gram of dry caterpillar hold similar amounts of energy. The fall in total energy is due to the smaller amount of biomass.

Worked example. Suppose producers capture 20 000 kJ m⁻² year⁻¹ and each transfer passes on 10%. The energy available is 20 000, 2 000, 200, 20 and 2 kJ m⁻² year⁻¹ at levels one to five, and 0.2 at level six. A predator population needs a minimum energy supply. If that minimum is more than 0.2 kJ m⁻² year⁻¹, a sixth level cannot be supported. This is why most food chains have only four or five trophic levels.

Common errors

• Drawing arrows from consumer to food. The arrow shows energy moving toward the consumer.
• Saying energy is “used up”. It is transformed into heat and ATP-driven work, and the total is conserved.
• Saying energy is recycled. Matter is recycled. Energy flows through and leaves as heat.
• Calling faeces energy “lost from the ecosystem”. It passes to decomposers first.
• Forgetting that autotrophs respire, so they also lose energy as heat.
• Treating the one-tenth rule of thumb as exact.

Official syllabus

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