Cambridge IGCSE0610

Energy flow

Biology 0610 Chapter Notes

What this chapter covers

Energy flowFood chains and food websNutrient cyclesPopulations
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In this chapter

  1. 1The Sun: The Ultimate Energy Source
  2. 2Food Chains and Trophic Levels
  3. 3The Inefficiency of Energy Transfer
  4. 4Food Webs and Ecosystem Stability
  5. 5Pyramids of Numbers and Biomass
  6. 6Pyramids of Energy: The Best Picture

1. The Sun: The Ultimate Energy Source

Nearly all life on Earth is powered by the Sun. Organisms called producers, such as plants and algae, are the foundation of every food chain. They are autotrophs, meaning they make their own food. Using a process called photosynthesis, they capture light energy from the sun and convert it into chemical energy. This chemical energy is stored in the bonds of organic molecules, like glucose, which provides the fuel for the entire ecosystem. Without this constant input of solar energy, most ecosystems would collapse.

Photosynthesis: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Key term

Producer: An organism that produces its own food, usually through photosynthesis, forming the base of a food chain.

Examiner insight

Marks are often awarded for explicitly stating that light energy is converted into chemical energy stored in organic molecules.

Fun fact

Less than 1% of the sun's energy that reaches Earth's surface is actually captured by plants and used for photosynthesis.

Worked example 13 marks

Explain the role of a producer in an ecosystem and state the energy conversion that takes place.

  1. 1

    Step 1: Identify the role of a producer. Producers, like plants, form the first trophic level and are the foundation of the ecosystem's energy.

  2. 2

    Step 2: State how they obtain energy. They synthesise their own organic food molecules (like glucose) from simple inorganic substances.

  3. 3

    Step 3: Name the process and the energy conversion. They do this via photosynthesis, which converts light energy from the sun into chemical energy stored in the food molecules.

Recap

  • The Sun is the ultimate source of energy for most ecosystems.
  • Producers are organisms that make their own food, typically via photosynthesis.
  • Photosynthesis converts light energy into chemical energy stored in organic molecules.
  • This chemical energy fuels all other life in the ecosystem.
  • Producers form the first trophic level in any food chain.

Quick check

  1. What is the term for an organism that makes its own food?1 mark
  2. What form of energy is light converted into during photosynthesis?1 mark

2. Food Chains and Trophic Levels

A food chain is a simple model that shows how energy flows through an ecosystem. It illustrates a sequence of organisms where each one is eaten by the next. The arrows in a food chain represent the direction of energy transfer. Each step in a food chain is called a trophic level. The levels are: Producer (Trophic Level 1), Primary Consumer (Trophic Level 2), Secondary Consumer (Trophic Level 3), and so on. Primary consumers are herbivores that eat producers. Secondary consumers are carnivores or omnivores that eat primary consumers. Decomposers, like bacteria and fungi, break down dead organisms from all trophic levels, returning nutrients to the soil.

Key term

Trophic Level: The position an organism occupies in a food chain, food web, or ecological pyramid.

Common pitfall

Confusing the direction of the arrows in a food chain; they show the direction of energy flow, from the organism being eaten to the organism that eats it.

Worked example 13 marks

A food chain consists of grass, a grasshopper, a frog, and a snake.a) Construct the food chain using arrows.b) Identify the secondary consumer.

  1. 1

    Step 1 (a): Write down the organisms in the correct order of feeding. Grass is the producer, eaten by the grasshopper, which is eaten by the frog, which is eaten by the snake.

  2. 2

    Step 2 (a): Draw arrows between the organisms to show the direction of energy flow. The arrow points from the organism being eaten to the organism that eats it. So: Grass → Grasshopper → Frog → Snake.

  3. 3

    Step 3 (b): Identify the trophic levels. Grass is the producer (Trophic Level 1). The grasshopper is the primary consumer (Trophic Level 2). The frog eats the primary consumer, so it is the secondary consumer (Trophic Level 3).

Recap

  • A food chain shows the linear transfer of energy between organisms.
  • Arrows in a food chain show the direction of energy flow.
  • Each feeding level is called a trophic level.
  • Producers are at the first trophic level, followed by consumers at subsequent levels.
  • Decomposers break down dead organic matter from all trophic levels.

Quick check

  1. What is the name for an animal that only eats plants?1 mark

3. The Inefficiency of Energy Transfer

Energy transfer between trophic levels is very inefficient. As a general rule, only about 10% of the energy from one trophic level is incorporated into the biomass of the next level. The remaining 90% is 'lost' from the food chain. This loss occurs for several reasons:

  1. Not all parts of an organism are eaten (e.g., bones, roots, fur).
  2. Not all material that is eaten can be digested and absorbed; some is egested as waste (faeces).
  3. A large amount of energy is used by the organism for its own life processes, such as respiration, movement, and keeping warm. This energy is ultimately transferred to the environment as heat.

This significant loss of energy at each step is why food chains are rarely longer than four or five trophic levels. There is simply not enough energy left to support organisms at higher levels.

Energy Transfer Efficiency (%) = (Energy available at next level / Energy available at previous level) × 100

Key term

Energy Transfer: The movement of energy from one trophic level to the next in a food chain or food web.

Examiner insight

High-scoring answers quantify the energy loss, often citing the '90% lost' or '10% transferred' rule, and give specific, distinct reasons for the loss such as respiration and indigestible parts.

Fun fact

This inefficiency is why a blue whale, the largest animal on Earth, feeds on tiny krill (primary consumers). By feeding low on the food chain, it accesses a much larger pool of energy.

Worked example 13 marks

In a food chain, the producers contain 25,000 kJ/m²/year of energy. The primary consumers contain 2,500 kJ/m²/year. Calculate the percentage efficiency of energy transfer between the producer and the primary consumer.

  1. 1

    Step 1: Write down the formula for energy transfer efficiency. Efficiency (%) = (Energy at higher level / Energy at lower level) × 100.

  2. 2

    Step 2: Substitute the given values into the formula. Efficiency (%) = (2,500 kJ/m²/year / 25,000 kJ/m²/year) × 100.

  3. 3

    Step 3: Calculate the result. Efficiency = 0.1 × 100 = 10%.

  4. 4

    Step 4: State the final answer with units. The efficiency of energy transfer is 10%.

Recap

  • Only about 10% of energy is transferred from one trophic level to the next.
  • Energy is lost as heat during respiration.
  • Energy is lost because not all parts of an organism are consumed or digested.
  • This energy loss limits the length of food chains.
  • The 'lost' energy is not destroyed, but converted to other forms like heat.

Quick check

  1. State two reasons why a large proportion of energy is not transferred between trophic levels.2 marks

4. Food Webs and Ecosystem Stability

In reality, ecosystems are not simple food chains. They are complex networks of interconnected food chains called food webs. A food web is a more realistic representation of feeding relationships because most organisms eat more than one type of food, and are in turn eaten by more than one type of predator. This complexity provides stability to the ecosystem. If one food source for a particular consumer becomes scarce, the consumer can switch to another food source. This resilience helps to prevent populations from crashing and maintains the overall balance of the ecosystem. In contrast, an organism in a simple food chain is highly vulnerable; if its only food source disappears, it will likely die out.

Key term

Food Web: A complex of interrelated food chains in an ecological community.

Examiner insight

Examiners look for the understanding that increased biodiversity and more complex food webs lead to greater ecosystem stability and resilience because of alternative food pathways.

Fun fact

The reintroduction of wolves to Yellowstone National Park in 1995 created a 'trophic cascade', changing not just animal populations but also the physical geography of the park by altering river banks.

Worked example 14 marks

In a marine food web, sea otters eat sea urchins. Sea urchins eat kelp. If the sea otter population is drastically reduced by hunting, predict what would happen to the populations of sea urchins and kelp.

  1. 1

    Step 1: Identify the relationship between otters and urchins. Sea otters are predators of sea urchins.

  2. 2

    Step 2: Predict the effect on the sea urchin population. With fewer predators (otters), the sea urchin population would likely increase.

  3. 3

    Step 3: Identify the relationship between urchins and kelp. Sea urchins are herbivores that eat kelp.

  4. 4

    Step 4: Predict the effect on the kelp population. With an increased population of sea urchins eating it, the kelp population would likely decrease significantly.

Recap

  • A food web consists of multiple interconnected food chains.
  • Food webs are more realistic models of ecosystems than food chains.
  • Greater complexity in a food web leads to greater ecosystem stability.
  • In a complex food web, consumers have alternative food sources.
  • The removal of one species can have knock-on effects throughout the food web.

Quick check

  1. Explain in one sentence why a food web is more stable than a food chain.1 mark

5. Pyramids of Numbers and Biomass

Ecological pyramids are diagrams that represent the relative amounts of organisms, biomass, or energy at each trophic level.

Pyramid of Numbers: This shows the total number of individual organisms at each trophic level. The base is the producers, and successive levels are stacked on top. While simple to create, it can be misleading. For example, one large oak tree (producer) can support thousands of caterpillars (primary consumers), which would create an 'inverted' pyramid where the base is smaller than the level above it.

Pyramid of Biomass: This pyramid represents the total dry mass (biomass) of all organisms at each trophic level. Biomass is a better indicator of the energy available than numbers. To construct it, you would find the mass of organisms at each level in a given area (e.g., g/m²). Pyramids of biomass are almost always pyramid-shaped, as the biomass decreases at each successive trophic level. This solves the problem of inverted pyramids of numbers.

Biomass = Number of individuals × Dry mass of one individual

Key term

Biomass: The total dry mass of all living organisms in a given area, often at a specific trophic level.

Common pitfall

Forgetting that biomass refers to 'dry mass'. Water content can vary greatly between organisms, so scientists dry them out in an oven to get a standardised, comparable measure.

Worked example 15 marks

An oak tree supports 5,000 caterpillars, which are eaten by 50 birds.a) Sketch the likely shape of the pyramid of numbers.b) Sketch the likely shape of the pyramid of biomass.c) Explain the difference in shape.

  1. 1

    Step 1 (a): For the pyramid of numbers, the base (producer) is 1 tree. The next level is 5,000 caterpillars, and the top level is 50 birds. This creates a narrow base, a very wide middle, and a smaller top. It is an inverted pyramid.

  2. 2

    Step 2 (b): For the pyramid of biomass, the single oak tree has a very large mass. The total mass of the 5,000 caterpillars will be much less than the tree. The total mass of the 50 birds will be less than the caterpillars. This creates a standard, upright pyramid shape.

  3. 3

    Step 3 (c): The pyramid of numbers is inverted because it just counts individuals, and one large producer can support many smaller consumers. The pyramid of biomass is upright because it represents the total mass, and the mass of organisms always decreases up the food chain due to energy loss.

Recap

  • Pyramids of numbers show the count of individual organisms at each trophic level.
  • Pyramids of numbers can be inverted if the producer is very large (e.g., a tree).
  • Pyramids of biomass show the total dry mass of organisms at each trophic level.
  • Biomass is measured in units like g/m² or kg/ha.
  • Pyramids of biomass are a more reliable representation of energy flow than pyramids of numbers.

Quick check

  1. State one disadvantage of using a pyramid of numbers to represent an ecosystem.1 mark

6. Pyramids of Energy: The Best Picture

The pyramid of energy is the most accurate and fundamental way to represent the flow of energy through an ecosystem. Unlike pyramids of numbers or biomass, which are snapshots in time, a pyramid of energy shows the rate at which energy is transferred from one trophic level to the next over a period, typically a year. The units are energy per unit area per unit time, such as kilojoules per square metre per year (kJ/m²/year). A pyramid of energy is ALWAYS upright and can never be inverted. This is because of the second law of thermodynamics and the massive energy loss at each trophic level. As energy flows from one level to the next, a significant portion is lost as heat during respiration. Therefore, the energy available at each successive trophic level is always less than the level below it.

Units for Pyramid of Energy: kJ m⁻² year⁻¹

Key term

Pyramid of Energy: A diagram that shows the rate of energy flow through the trophic levels of an ecosystem, which is always upright.

Examiner insight

Students who can clearly explain why a pyramid of energy can never be inverted, linking it to energy loss via respiration (heat), demonstrate a deep understanding of the topic.

Fun fact

It takes about 10,000 kg of phytoplankton (producers) to produce just 1 kg of a top predator like a tuna (tertiary/quaternary consumer).

Worked example 15 marks

A pyramid of energy for a grassland shows the producers have 1,200,000 kJ/m²/year. The tertiary consumers have 120 kJ/m²/year.a) Assuming 10% efficiency at each step, what is the energy value for the primary consumers?b) Explain why the pyramid of energy is never inverted.

  1. 1

    Step 1 (a): The primary consumers are one level above the producers. Calculate 10% of the producer's energy. 10% of 1,200,000 is (1,200,000 × 0.10).

  2. 2

    Step 2 (a): The energy available to primary consumers is 120,000 kJ/m²/year.

  3. 3

    Step 3 (b): Explain the principle of energy loss. At every transfer between trophic levels, a large amount of energy is lost from the food chain.

  4. 4

    Step 4 (b): State the main reason for this loss. The majority of this energy is converted to heat during metabolic processes like respiration and is lost to the environment.

  5. 5

    Step 5 (b): Conclude why this leads to an upright pyramid. Because energy is always lost, each successive trophic level must have less energy than the one before it, ensuring the pyramid shape.

Recap

  • Pyramids of energy show the rate of energy flow through trophic levels.
  • They are the most accurate type of ecological pyramid.
  • Units are typically energy per area per time (e.g., kJ/m²/year).
  • A pyramid of energy can never be inverted.
  • The upright shape reflects the continuous loss of energy at each trophic level.

Quick check

  1. State the typical units for a pyramid of energy.1 mark
  2. Why is a pyramid of energy a better model than a pyramid of biomass?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the terms 'producer' and 'primary consumer', giving an example of each.4 marks
  2. Draw a food chain with four organisms from a marine ecosystem and label the trophic level of each organism.4 marks
  3. Explain why the transfer of energy between trophic levels is described as inefficient.4 marks
  4. A producer in an ecosystem captures 2,000,000 kJ of energy. Assuming a 10% energy transfer efficiency at each trophic level, calculate the amount of energy available to the secondary consumers. Show your working.3 marks
  5. Describe the difference between a pyramid of numbers and a pyramid of biomass. Explain one situation where a pyramid of numbers might not be a standard pyramid shape.5 marks
  6. Explain why an ecosystem with a complex food web is likely to be more stable than one with simple food chains.4 marks
  7. Consider the food web: Grass -> Rabbit -> Fox; Grass -> Grasshopper -> Lizard -> Fox. A disease suddenly wipes out most of the rabbit population. Predict and explain the likely short-term effects on the populations of (a) grass and (b) lizards.4 marks
  8. Explain why a pyramid of energy is considered the most accurate representation of an ecosystem's structure and why it is never inverted.5 marks
  9. State the principal source of energy for most ecosystems on Earth and name the biological process responsible for capturing it.2 marks
  10. It is often said that it is more energy-efficient for the human population to eat plants than to eat meat. Using your knowledge of energy flow and trophic levels, explain this statement.5 marks

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