Cambridge O Level5090

Energy flow

Biology 5090 Chapter Notes

What this chapter covers

Energy flowNutrient cyclesEcosystems and biodiversityEffects of humans on ecosystemsConservation
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In this chapter

  1. 1The Sun: The Ultimate Energy Source
  2. 2Food Chains and Trophic Levels
  3. 3Energy Loss in Food Chains
  4. 4Pyramids of Biomass and Energy
  5. 5Efficiency in Human Food Production

1. The Sun: The Ultimate Energy Source

Nearly all life on Earth is powered by the Sun. The process that captures this energy is called photosynthesis. Producers, such as plants and algae, contain a pigment called chlorophyll which absorbs light energy. They use this energy to convert simple inorganic molecules (carbon dioxide and water) into complex organic molecules (glucose), storing the energy in the chemical bonds of the glucose. This chemical energy is the foundation of almost every food chain, making producers the essential first step in the flow of energy through an ecosystem.

Photosynthesis Word Equation: Carbon dioxide + Water --(Light Energy)--> Glucose + Oxygen

Photosynthesis Chemical Equation: 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 reward students who explicitly link light energy from the sun to chemical energy in producers via the process of photosynthesis.

Fun fact

While most ecosystems are powered by sunlight, some deep-sea vent communities rely on chemosynthesis, where bacteria use chemical reactions, not light, to produce food.

Worked example 13 marks

In a single day, a field of wheat receives 2,000,000 kJ of light energy from the sun. The wheat plants convert 25,000 kJ of this into chemical energy in glucose. Calculate the percentage efficiency of this energy conversion.

  1. 1

    Step 1: Identify the formula for percentage efficiency. Efficiency (%) = (Useful energy output / Total energy input) × 100.

  2. 2

    Step 2: Substitute the given values into the formula. Useful energy output = 25,000 kJ. Total energy input = 2,000,000 kJ.

  3. 3

    Step 3: Calculate the efficiency. Efficiency = (25,000 kJ / 2,000,000 kJ) × 100.

  4. 4

    Step 4: Final calculation. Efficiency = 0.0125 × 100 = 1.25%.

Recap

  • The Sun is the principal source of energy for most biological systems.
  • Producers, like plants, convert light energy into chemical energy through photosynthesis.
  • This chemical energy is stored in organic molecules like glucose.
  • Only a small fraction, typically 1-2%, of the Sun's energy reaching the Earth is captured by producers.

Quick check

  1. What is the name of the process by which producers convert light energy into chemical energy?1 mark
  2. What is the role of a producer in a food chain?1 mark

2. Food Chains and Trophic Levels

A food chain illustrates how energy is transferred from one living organism to another. Each step in the food chain is called a trophic level. The chain always starts with a producer (Trophic Level 1). The organisms that eat producers are called primary consumers (herbivores, Trophic Level 2). Organisms that eat primary consumers are secondary consumers (carnivores or omnivores, Trophic Level 3), and so on. The arrows in a food chain show the direction of energy flow, from the organism being eaten to the one that eats it. A food web is a more realistic model, showing how multiple food chains are interconnected in an ecosystem.

Key term

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

Examiner insight

Use specific terminology: 'primary consumer' is better than 'animal that eats plants'. Marks are awarded for precision.

Common pitfall

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

Worked example 14 marks

Consider the following food chain: Algae → Tadpole → Perch → Heron. Identify the producer, primary consumer, secondary consumer, and tertiary consumer.

  1. 1

    Step 1: The producer is the first organism in the chain that makes its own food. Here, it is the Algae.

  2. 2

    Step 2: The primary consumer is the herbivore that eats the producer. Here, it is the Tadpole.

  3. 3

    Step 3: The secondary consumer eats the primary consumer. Here, it is the Perch.

  4. 4

    Step 4: The tertiary consumer eats the secondary consumer. Here, it is the Heron.

Recap

  • A food chain shows the linear transfer of energy between organisms.
  • Each position in a food chain is a trophic level.
  • Arrows in a food chain indicate the direction of energy flow.
  • Trophic Level 1 is the producer, Trophic Level 2 is the primary consumer, and so on.
  • A food web is a complex network of interconnected food chains.

Quick check

  1. In the food chain Grass → Vole → Owl, which organism is at the second trophic level?1 mark

3. Energy Loss in Food Chains

The transfer of energy between trophic levels is extremely inefficient. On average, only about 10% of the energy stored in one trophic level is converted into biomass at 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.
  2. Much of the energy consumed is used by the organism for its own life processes, such as respiration, movement, and maintaining body temperature. This energy is ultimately lost as heat to the surroundings.
  3. Energy is lost in waste products like faeces and urine.
  4. Some parts of the organism may be indigestible.

This significant energy loss 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.

Efficiency of energy transfer (%) = (Energy available at higher trophic level / Energy available at lower trophic level) × 100

Key term

Decomposer: An organism, such as a bacterium or fungus, that gets its energy by breaking down dead or waste organic material.

Common pitfall

Simply stating that '90% of energy is lost' is not a full answer. You must be able to explain how it is lost, mentioning respiration (as heat), waste, and incomplete consumption.

Worked example 13 marks

A producer level in an ecosystem contains 25,000 kJ/m²/year of energy. Assuming a 10% energy transfer efficiency at each step, calculate the energy available to the tertiary consumers.

  1. 1

    Step 1: Energy at Producer level (Trophic Level 1) = 25,000 kJ/m²/year.

  2. 2

    Step 2: Calculate energy at Primary Consumer level (Trophic Level 2). Energy = 10% of 25,000 = 0.10 × 25,000 = 2,500 kJ/m²/year.

  3. 3

    Step 3: Calculate energy at Secondary Consumer level (Trophic Level 3). Energy = 10% of 2,500 = 0.10 × 2,500 = 250 kJ/m²/year.

  4. 4

    Step 4: Calculate energy at Tertiary Consumer level (Trophic Level 4). Energy = 10% of 250 = 0.10 × 250 = 25 kJ/m²/year.

Recap

  • Only about 10% of energy is transferred from one trophic level to the next.
  • Most energy is lost as heat during respiration.
  • Energy is also lost through waste, movement, and uneaten parts.
  • This energy loss limits the length of food chains.

Quick check

  1. List two ways in which energy is lost between trophic levels.2 marks

4. Pyramids of Biomass and Energy

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

  • A Pyramid of Numbers shows the total number of individual organisms at each level. It can sometimes be 'inverted' (e.g., one large oak tree supporting thousands of caterpillars).
  • A Pyramid of Biomass shows the total dry mass (biomass) of organisms at each level. This is a more reliable representation as it accounts for the size of organisms. These pyramids are almost always upright because the total mass of organisms decreases at each successive trophic level.
  • A Pyramid of Energy is the most accurate representation of energy flow. It shows the total amount of energy at each trophic level over a period of time (e.g., kJ/m²/year). These pyramids are always upright because energy is always lost at each transfer, so the energy at a lower level is always greater than the energy at the level above it.

Key term

Biomass: The total dry mass of all organisms at a particular trophic level or in a specific area.

Examiner insight

Marks are often awarded for explaining why a pyramid of biomass is a more accurate representation of the relationships between organisms than a pyramid of numbers.

Common pitfall

Drawing a pyramid of biomass with bars of equal width or failing to label each trophic level correctly with both the organism and its biomass value.

Worked example 13 marks

The biomass of organisms in a food chain was measured: Grass (10,000 kg), Grasshoppers (1,000 kg), Frogs (100 kg), Snakes (10 kg). Construct and label a pyramid of biomass for this food chain.

  1. 1

    Step 1: Draw the base of the pyramid for the producer. Label it 'Grass' and write '10,000 kg'. The width of the bar should be the largest.

  2. 2

    Step 2: Draw the next level up for the primary consumer. Label it 'Grasshoppers' and write '1,000 kg'. The bar should be proportionally smaller than the base.

  3. 3

    Step 3: Draw the third level for the secondary consumer. Label it 'Frogs' and write '100 kg'. This bar should be smaller still.

  4. 4

    Step 4: Draw the top level for the tertiary consumer. Label it 'Snakes' and write '10 kg'. This should be the smallest bar at the apex of the pyramid.

Recap

  • Pyramids of numbers can be inverted if the producer is a large organism.
  • Pyramids of biomass represent the total dry mass at each trophic level and are usually upright.
  • Pyramids of energy show energy flow and are always upright, as energy is lost at each level.
  • Pyramids of energy provide the most accurate picture of an ecosystem's energy dynamics.

Quick check

  1. Why is a pyramid of energy always pyramid-shaped?1 mark
  2. Why is biomass measured as 'dry mass'?1 mark

5. Efficiency in Human Food Production

The principles of energy loss in food chains have significant implications for human food production. When humans eat plants (producers), they are acting as primary consumers and tapping into the food chain at an efficient point. When humans eat meat from herbivores (e.g., cows, sheep), they are acting as secondary consumers. Because approximately 90% of energy is lost between the producer (e.g., grass) and the primary consumer (the cow), eating meat is a much less energy-efficient way of obtaining food. A given area of land can produce far more energy and support a larger population if used to grow crops for direct human consumption, rather than to grow crops to feed livestock. Intensive farming methods aim to increase efficiency by reducing energy losses from livestock. This is done by restricting movement and keeping animals in warm indoor environments, so less energy is used for movement and maintaining body temperature, and more is converted into biomass (meat).

Key term

Intensive Farming: A type of agriculture that aims to maximize yield from a minimal area of land, often by using high inputs of energy and technology.

Examiner insight

In questions about food security, linking the inefficiency of meat production to the potential for feeding more people with plant-based diets is a high-level skill that attracts marks.

Fun fact

It can take over 7 kg of grain to produce just 1 kg of beef. That same grain could provide many more calories if consumed directly by humans.

Worked example 14 marks

Explain, using the concept of energy transfer, why a vegetarian diet is considered more energy-efficient than a diet that includes meat.

  1. 1

    Step 1: State the principle of energy loss. Energy is lost at each trophic level in a food chain, with only about 10% being passed on.

  2. 2

    Step 2: Relate this to a vegetarian diet. When humans eat plants, they are primary consumers (Trophic Level 2). They are accessing the energy from the producers directly.

  3. 3

    Step 3: Relate this to a meat-eating diet. When humans eat herbivores like cattle, they are secondary consumers (Trophic Level 3). The cattle (primary consumers) have already lost 90% of the energy they got from the plants.

  4. 4

    Step 4: Conclude by comparing the two. By eating at a lower trophic level, a vegetarian diet avoids the major energy loss that occurs between producers and primary consumers, making it far more efficient in terms of the energy gained from a given amount of plant matter.

Recap

  • Eating plants (producers) is more energy-efficient than eating animals (consumers).
  • Significant energy is lost when crops are fed to livestock instead of being eaten by humans.
  • A given area of land can support more people on a plant-based diet.
  • Intensive farming increases meat production efficiency by reducing energy 'wasted' by livestock on movement and warmth.

Quick check

  1. State one reason why feeding on plants is more energy-efficient for humans than feeding on herbivores.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.2 marks
  2. Explain why most food chains have no more than five trophic levels.3 marks
  3. A field of clover contains 800,000 kJ of energy. If the energy transfer efficiency between trophic levels is 10%, calculate the energy that would be available to the secondary consumers that feed on rabbits eating the clover. Show your working.3 marks
  4. Draw and label a pyramid of biomass for the following food chain: Oak tree → Aphids → Ladybirds → Blue tits. Explain why a pyramid of numbers would be misleading for this chain.4 marks
  5. Describe the pathway of energy from the Sun to a muscle contraction in a human who has eaten a piece of salmon (a carnivore).4 marks
  6. A caterpillar eats a leaf. Explain two reasons why not all of the chemical energy in the leaf is converted into new biomass in the caterpillar.2 marks
  7. 'It is more sustainable for the growing human population to eat plant-based foods than meat.' Use your knowledge of energy flow in ecosystems to evaluate this statement.5 marks
  8. Distinguish between a food chain and a food web.2 marks
  9. A scientist measures the biomass in an ecosystem. They find 2,500 kg of producers, 300 kg of primary consumers, and 35 kg of secondary consumers. Calculate the percentage efficiency of energy transfer between the primary consumers and the secondary consumers.3 marks
  10. Explain how two specific practices in the intensive farming of livestock are designed to maximize the efficiency of energy transfer to the consumer.4 marks

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