Cambridge IGCSE0970

Energy flow

Biology 0970 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 Source of Energy
  2. 2Food Chains and Trophic Levels
  3. 3Why Energy Transfer is Inefficient
  4. 4Representing Energy Flow: Ecological Pyramids
  5. 5Food Webs and Human Impact

1. The Sun: The Ultimate Source of Energy

All life in most ecosystems depends on a continuous supply of energy from the sun. Producers, such as green plants and algae, are the only organisms that can capture this light energy and convert it into chemical energy stored in organic molecules (like glucose) through photosynthesis. This chemical energy is the foundation for all food chains. However, this energy capture is not very efficient. Only about 1% of the light energy reaching a plant is actually converted into chemical energy. The rest is reflected, passes through the leaf, or is the wrong wavelength for photosynthesis.

6CO₂ + 6H₂O --(light energy)--> C₆H₁₂O₆ + 6O₂

Key term

Producer: An organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis.

Examiner insight

Examiners expect you to state clearly that the initial source of energy for most ecosystems is sunlight.

Fun fact

The total amount of energy from the sun that hits the Earth in one hour is more than the entire world consumes in a year.

Worked example 13 marks

A plant leaf receives 5000 kJ of light energy. Scientists calculate that the chemical energy stored in the new carbohydrates made by the leaf is 50 kJ. Calculate the efficiency of this energy transfer. Show your working.

  1. 1

    Efficiency is calculated as (useful energy output / total energy input) x 100%.

  2. 2

    Useful energy output = 50 kJ (chemical energy).

  3. 3

    Total energy input = 5000 kJ (light energy).

  4. 4

    Efficiency = (50 / 5000) x 100%

  5. 5

    Efficiency = 0.01 x 100% = 1%.

Recap

  • The sun is the ultimate source of energy for most ecosystems.
  • Producers convert light energy into chemical energy via photosynthesis.
  • This energy conversion is inefficient; only about 1% of light energy is captured.
  • Producers form the first trophic level in a food chain.

Quick check

  1. What is the name of the process that converts light energy to chemical energy in producers?1 mark
  2. Apart from being reflected, give one other reason why most of the sun's energy is not used by plants.1 mark

2. Food Chains and Trophic Levels

A food chain shows how energy is transferred from one living organism to another. Each stage in a food chain is called a trophic level. The flow of energy is always from a lower trophic level to a higher one. It starts with a producer (Trophic Level 1), which is eaten by a primary consumer (a herbivore, Trophic Level 2). The primary consumer is then eaten by a secondary consumer (a carnivore, Trophic Level 3), and so on. Decomposers, like bacteria and fungi, break down dead organic matter from all trophic levels, returning nutrients to the soil but releasing the energy as heat.

Key term

Trophic Level: The position an organism occupies in a food chain.

Examiner insight

When drawing food chains, always use arrows that point in the direction of energy flow (from the organism being eaten to the organism that eats it).

Common pitfall

Drawing arrows in a food chain pointing the wrong way, from the consumer to the producer. The arrow shows the direction of energy flow.

Worked example 13 marks

A food chain in a grassland is: Grass -> Grasshopper -> Frog -> Snake.(a) Name the producer.(b) Name the tertiary consumer.(c) What is the trophic level of the frog?

  1. 1

    (a) The producer is the organism that makes its own food. In this food chain, it is the Grass.

  2. 2

    (b) The primary consumer is the grasshopper (eats grass). The secondary consumer is the frog (eats grasshopper). The tertiary consumer is the snake (eats frog).

  3. 3

    (c) The trophic levels are: Grass (Trophic Level 1), Grasshopper (Trophic Level 2), Frog (Trophic Level 3), Snake (Trophic Level 4). Therefore, the frog is at Trophic Level 3.

Recap

  • A food chain shows the flow of energy between organisms.
  • Arrows in a food chain point in the direction of energy transfer.
  • Each step in a food chain is a trophic level.
  • Producers are at Trophic Level 1.
  • Primary consumers are herbivores at Trophic Level 2.
  • Secondary consumers are carnivores or omnivores at Trophic Level 3.

Quick check

  1. An owl eats a mouse that ate some seeds. What trophic level is the owl at?1 mark

3. Why Energy Transfer is Inefficient

The transfer of energy between trophic levels is very inefficient. As a general rule, only about 10% of the energy from one level is incorporated into the biomass of the next level. The remaining 90% is 'lost' from the food chain. This energy isn't destroyed; it's converted into other forms. The main reasons for this loss are:

  1. Life Processes (Respiration): Organisms use a large amount of the energy they consume for their own metabolic processes like movement, keeping warm, and growth. This energy is released as heat during respiration and lost to the surroundings.
  2. Incomplete Consumption: Not all parts of an organism are eaten (e.g., roots, bones, fur).
  3. Indigestible Waste (Egestion): Not all parts that are eaten can be digested. This undigested material is passed out as faeces.
  4. Excretion: Waste products of metabolism (like urea in urine) contain chemical energy and are removed from the body.

Because of this continuous loss of energy at each level, food chains are rarely longer than four or five trophic levels. There simply isn't enough energy left to support organisms at higher levels.

Efficiency of transfer (%) = (Energy in higher trophic level / Energy in lower trophic level) × 100

Key term

Respiration: The chemical reactions in cells that break down nutrient molecules to release energy for metabolism.

Examiner insight

Marks are often awarded for explaining *how* energy is lost. Be specific: mention heat loss from respiration, inedible parts, and waste products like faeces.

Common pitfall

Stating that energy is 'lost' or 'used up' without explaining that it is converted to heat during respiration and dissipated into the environment.

Worked example 14 marks

The producers in a food chain contain 25,000 kJ/m²/year of energy. The primary consumers incorporate 2,500 kJ/m²/year. The secondary consumers incorporate 250 kJ/m²/year.(a) Calculate the percentage of energy transferred from the producers to the primary consumers.(b) Give two reasons why the transfer of energy is not 100% efficient.

  1. 1

    (a) Use the formula: Efficiency = (Energy at higher level / Energy at lower level) x 100.

  2. 2

    Efficiency = (2,500 / 25,000) x 100.

  3. 3

    Efficiency = 0.1 x 100 = 10%.

  4. 4

    (b) Reason 1: Energy is lost as heat to the surroundings during respiration by the primary consumers.

  5. 5

    (b) Reason 2: Not all of the producer is eaten by the primary consumer, or some parts are indigestible and are egested as faeces.

Recap

  • Only about 10% of energy is transferred from one trophic level to the next.
  • The other 90% is lost from the food chain.
  • Energy is lost as heat during respiration.
  • Energy is also lost through inedible parts and undigested waste.
  • This inefficiency limits the length of food chains.

Quick check

  1. If a herbivore eats 500 kJ of energy in grass, what is the approximate amount of energy that will be available to a carnivore that eats the herbivore?1 mark
  2. State the main life process that causes energy to be lost as heat from a food chain.1 mark

4. Representing Energy Flow: Ecological Pyramids

Ecological pyramids are diagrams used to represent the relative amounts of energy or matter at each trophic level in a food chain. The producers are always at the base. There are three main types:

  • Pyramid of Numbers: Shows the total number of individual organisms at each trophic level. These can sometimes be 'inverted' (not pyramid-shaped), for example, when one large producer (like an oak tree) supports thousands of consumers (like caterpillars).
  • Pyramid of Biomass: Shows the total dry mass (biomass) of all organisms at each trophic level. Biomass is a better indicator of the energy available than numbers. These are usually pyramid-shaped, but can be inverted in some aquatic ecosystems where producers (phytoplankton) have a very short lifespan and reproduce quickly.
  • Pyramid of Energy: This is the most accurate representation of energy flow. It shows the total amount of energy that flows through each trophic level over a period of time (e.g., in kJ/m²/year). Pyramids of energy are *always* pyramid-shaped because a large amount of energy is lost at each successive trophic level.

Biomass = Number of individuals × Mass of one individual

Key term

Pyramid of Energy: A diagram that shows the rate of energy flow through the trophic levels of an ecosystem, which is always pyramid-shaped due to energy loss at each level.

Examiner insight

When asked to explain why a pyramid of energy is always pyramid-shaped, you must refer to the significant loss of energy at each trophic level, primarily as heat from respiration.

Common pitfall

Forgetting that pyramids of numbers and biomass can be inverted, while pyramids of energy are never inverted.

Worked example 14 marks

(a) Explain why a pyramid of numbers for the food chain 'Oak tree -> Caterpillars -> Blue tits' would not be a true pyramid shape.(b) Which type of ecological pyramid is always upright and why?

  1. 1

    (a) The pyramid would be inverted. There is only one producer (the oak tree), but it supports a very large number of primary consumers (caterpillars). The number of secondary consumers (blue tits) would be smaller than the number of caterpillars.

  2. 2

    This results in a narrow base (1 tree), a very wide middle layer (many caterpillars), and a narrower top layer (fewer birds).

  3. 3

    (b) A pyramid of energy is always upright (pyramid-shaped).

  4. 4

    (b) This is because there is a large loss of energy at each trophic level. Only about 10% of the energy from one level is passed to the next, so the bar for each successive level must be smaller.

Recap

  • Ecological pyramids model the quantities at each trophic level.
  • Pyramids of numbers show the count of individuals and can be inverted.
  • Pyramids of biomass show the total dry mass and can occasionally be inverted.
  • Pyramids of energy show energy flow over time and are always upright.
  • The shape of the pyramid of energy reflects the inefficiency of energy transfer.

Quick check

  1. What are the units for a pyramid of energy?1 mark
  2. Give an example of a food chain that would produce an inverted pyramid of numbers.1 mark

5. Food Webs and Human Impact

In any ecosystem, organisms usually have multiple food sources and are also eaten by various predators. This creates a complex network of interconnected food chains called a food web. Food webs are more realistic representations of feeding relationships than simple food chains. Ecosystems with high biodiversity and complex food webs tend to be more stable. If one food source becomes unavailable, a consumer can often switch to another, preventing the collapse of the food web. Humans can have a significant impact on energy flow. By eating producers (plants) instead of primary consumers (herbivores), we are consuming at a lower trophic level. This is much more energy-efficient because it cuts out the energy loss that occurs between producers and primary consumers. For example, a field of wheat can provide far more energy to humans directly than if it were fed to cattle which are then eaten by humans.

Key term

Food Web: A system of interlocking and interdependent food chains in an ecosystem.

Examiner insight

Questions about food webs often test your ability to predict the effects of removing or adding a species. Think about both the direct and indirect consequences.

Fun fact

The most complex food webs are found in soil. A single gram of forest soil can contain thousands of different species of bacteria, fungi, insects, and worms, all interacting.

Worked example 14 marks

A simple marine food web is: Phytoplankton -> Zooplankton -> Small Fish -> Tuna. Humans fish for tuna. Explain why it would be more energy-efficient for humans to harvest and eat the small fish instead of the tuna.

  1. 1

    Energy is lost at each trophic level in the food chain.

  2. 2

    Tuna are at a higher trophic level (tertiary or quaternary consumers) than small fish (secondary consumers).

  3. 3

    When small fish are eaten by tuna, about 90% of the energy is lost (e.g., through respiration, movement).

  4. 4

    Therefore, there is significantly less energy available in the total population of tuna compared to the total population of small fish.

  5. 5

    By eating the small fish, humans are consuming at a lower trophic level and obtaining a greater proportion of the original energy from the phytoplankton.

Recap

  • A food web is a network of interconnected food chains.
  • Complex food webs lead to more stable ecosystems.
  • Removing one species from a food web can have widespread effects.
  • It is more energy-efficient for humans to eat producers than consumers.
  • This is because eating at a lower trophic level avoids the energy losses of higher levels.

Quick check

  1. Why is a food web a more accurate model of an ecosystem than a food chain?1 mark
  2. If a disease wiped out the zooplankton population in the ocean, what would be an immediate effect on the phytoplankton?1 mark

End-of-chapter exercise

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

  1. Define the terms (a) producer and (b) primary consumer.2 marks
  2. Draw and label a food chain from a woodland ecosystem that contains four trophic levels.2 marks
  3. Explain why most food chains have no more than five trophic levels.3 marks
  4. In a grassland ecosystem, the energy in the grass is 10,000 kJ/m²/year. The rabbits that eat the grass contain 1,100 kJ/m²/year of energy. The foxes that eat the rabbits contain 100 kJ/m²/year of energy. Calculate the efficiency of energy transfer from the rabbits to the foxes. Give your answer to two significant figures.2 marks
  5. A pyramid of numbers for a food chain is inverted, with one large tree at the base supporting thousands of aphids. Explain how it is possible for the pyramid of biomass for this same food chain to be an upright pyramid.3 marks
  6. Describe two distinct ways in which energy is lost between trophic levels in a food chain.4 marks
  7. A large area of forest is cut down and replaced with fields for growing corn. The corn is then used to feed cattle. Explain, in terms of energy flow, why this is an inefficient way to produce food for a growing human population compared to humans eating the corn directly.5 marks
  8. A marine food web includes phytoplankton, krill, seals, and killer whales. Krill feed on phytoplankton, seals feed on krill, and killer whales feed on seals. If a chemical spill kills most of the krill, predict and explain the likely effects on the populations of (i) phytoplankton, (ii) seals, and (iii) killer whales.6 marks
  9. Explain why a pyramid of energy is a more useful representation of an ecosystem than a pyramid of numbers.4 marks
  10. What is the principal source of energy for almost all ecosystems on Earth?1 mark

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