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Biology - Food chains and energy flow

Grade 4Cambridge (IGCSE)

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Sun is the primary source of energy for almost all life on Earth. Energy is transferred from the sun to producers via photosynthesis.

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Producers (Autotrophs) are organisms, such as green plants and algae, that capture light energy to create glucose. The chemical equation is: 6CO2+6H2O→lightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2.

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Consumers (Heterotrophs) must eat other organisms to obtain energy. They are classified as primary consumers (herbivores), secondary consumers (carnivores/omnivores), and tertiary consumers.

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A Food Chain represents the flow of energy. The arrows in a food chain represent the direction of energy transfer, pointing from the organism being eaten to the consumer.

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Trophic Levels describe the position of an organism in a food chain. Level 1 is always the producer.

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Energy Loss: At each trophic level, energy is lost to the environment as heat (QQ), through movement, and via undigested waste. On average, only 10%10\% of energy is passed to the next level.

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Decomposers (like fungi and bacteria) break down dead organic matter, recycling nutrients back into the soil, though energy is not recycled but flows one way.

📐Formulae

Energy available to next level=Energy at current level×0.10\text{Energy available to next level} = \text{Energy at current level} \times 0.10

Efficiency of Energy Transfer=(Energy at higher levelEnergy at lower level)×100%\text{Efficiency of Energy Transfer} = \left( \frac{\text{Energy at higher level}}{\text{Energy at lower level}} \right) \times 100\%

Total Energy Intake=Energy for growth+Energy for respiration+Energy in waste\text{Total Energy Intake} = \text{Energy for growth} + \text{Energy for respiration} + \text{Energy in waste}

💡Examples

Problem 1:

In a food chain: Grass→Rabbit→Fox\text{Grass} \rightarrow \text{Rabbit} \rightarrow \text{Fox}. If the Grass produces 10,000 kJ10,000\text{ kJ} of energy, how much energy is likely available to the Fox?

Solution:

The Fox receives 100 kJ100\text{ kJ}.

Explanation:

Using the 10%10\% rule: The Rabbit (primary consumer) receives 10,000 kJ×0.10=1,000 kJ10,000\text{ kJ} \times 0.10 = 1,000\text{ kJ}. The Fox (secondary consumer) receives 1,000 kJ×0.10=100 kJ1,000\text{ kJ} \times 0.10 = 100\text{ kJ}.

Problem 2:

Explain why food chains rarely have more than 4 or 5 trophic levels.

Solution:

There is insufficient energy left to support another level.

Explanation:

Because approximately 90%90\% of energy is lost at each level (as heat or waste), the amount of available energy decreases rapidly. By the 4th4^{th} or 5th5^{th} level, the energy remaining is too small to sustain a population of larger predators.

Problem 3:

Calculate the efficiency of energy transfer if a caterpillar consumes 500 J500\text{ J} of energy from a leaf, but only 50 J50\text{ J} is stored as new biomass for the bird that eats it.

Solution:

Efficiency=10%\text{Efficiency} = 10\%

Explanation:

Using the formula Efficiency=(50 J500 J)×100%=0.1×100%=10%\text{Efficiency} = \left( \frac{50\text{ J}}{500\text{ J}} \right) \times 100\% = 0.1 \times 100\% = 10\%.