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3.5.3 Energy and ecosystems

AQA A-level Biology 3.5.3 practice on Energy and ecosystems, with free MCQs, clues and worked explanations drawn from the Energy transfers in and between organisms section of specification 7402.

Specification route
3.5.3
Question bank
16 questions
Course stage
Year 13 / A-level only

Sample questions

QUESTION 1 · 3.5.3 · LEVEL 2

How can the chemical energy store in dry biomass be accurately estimated?

  • By completely burning a sample in a bomb calorimeter and measuring the temperature rise of a known volume of water.
  • By using a respirometer to measure the oxygen uptake of the dried tissue over time.
  • By dissolving the biomass in acid and titrating it against a strong alkali.
  • By extracting the ATP using cell fractionation and measuring its concentration.
Show clue

This involves converting the chemical energy into heat energy.

Show answer and explanation

Answer: By completely burning a sample in a bomb calorimeter and measuring the temperature rise of a known volume of water.

The chemical energy store in dry mass can be estimated using calorimetry. Bomb calorimetry involves burning the sample in pure oxygen and using the heat released to raise the temperature of water, calculating energy using $Q = mc\Delta T$.

QUESTION 2 · 3.5.3 · LEVEL 1

Which mathematical equation correctly relates gross primary production ($GPP$), net primary production ($NPP$), and respiratory losses ($R$) in plants?

  • $NPP = GPP - R$
  • $GPP = NPP - R$
  • $NPP = GPP + R$
  • $R = NPP - GPP$
Show clue

Net is what is left over after the plant's own maintenance costs are deducted from the total.

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Answer: $NPP = GPP - R$

Net primary production ($NPP$) is the chemical energy store remaining after respiratory losses ($R$) have been deducted from gross primary production ($GPP$). Thus, $NPP = GPP - R$.

QUESTION 3 · 3.5.3 · LEVEL 1

To calculate the net production of consumers ($N$), the formula $N = I - (F + R)$ is used. What does the '$F$' represent in this equation?

  • The chemical energy lost in faeces and urine.
  • The chemical energy used for foraging and feeding.
  • The chemical energy stored in the consumer's fat reserves.
  • The chemical energy passed onto the next trophic level as food.
Show clue

This represents the energy that was ingested but could not be absorbed or used by the body.

Show answer and explanation

Answer: The chemical energy lost in faeces and urine.

In the equation $N = I - (F + R)$, $I$ represents the chemical energy store in ingested food, $F$ represents the chemical energy lost to the environment in faeces and urine, and $R$ represents respiratory losses.

QUESTION 4 · 3.5.3 · LEVEL 2

Why is the transfer of energy between trophic levels in an ecosystem highly inefficient (typically only $10\%$)?

  • Energy is lost as heat from respiration, in excretory products, and through parts of the organism not being consumed or digested.
  • The majority of energy is destroyed when it passes through the cell membranes of the consumer.
  • Consumers actively pump excess energy back into the soil to support the producer population.
  • Most of the chemical energy is immediately converted into light energy and lost to space.
Show clue

Think about what an animal does with its food: moving, keeping warm, and producing waste.

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Answer: Energy is lost as heat from respiration, in excretory products, and through parts of the organism not being consumed or digested.

Energy transfer is inefficient due to respiratory losses (heat from movement/metabolism), energy lost in faeces/urine (undigested parts), and parts of the organism not being eaten (like bones).

QUESTION 5 · 3.5.3 · LEVEL 2

How do farming practices like keeping livestock in confined, heated barns increase the net production of the animals?

  • They reduce energy lost through movement and maintaining body temperature, decreasing respiratory losses ($R$).
  • They increase the gross primary production of the animals by providing more light.
  • They completely stop the animals from producing faeces ($F$), maximising assimilation.
  • They force the animals to switch to anaerobic respiration, which is highly energy-efficient.
Show clue

If less energy is spent on staying warm and running around, more energy is available for growth.

Show answer and explanation

Answer: They reduce energy lost through movement and maintaining body temperature, decreasing respiratory losses ($R$).

Farming practices increase efficiency by restricting movement and keeping environments warm, which significantly reduces respiratory losses ($R$), thereby increasing net production ($N = I - (F + R)$).

QUESTION 6 · 3.5.3 · LEVEL 2

When measuring the biomass of an ecosystem, why is dry mass used rather than fresh mass?

  • The water content of living tissue varies significantly, so dry mass provides a more reliable and comparable measure of the actual organic material.
  • Fresh mass includes the mass of oxygen gas trapped in the cells, which artificially inflates the reading.
  • Dry mass calculations automatically exclude the inorganic ions which are not part of the biomass.
  • Water actively undergoes hydrolysis during measurement, destroying the chemical energy store.
Show clue

Think about why a plant immediately after rainfall might weigh more than the same plant after a week of drought.

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Answer: The water content of living tissue varies significantly, so dry mass provides a more reliable and comparable measure of the actual organic material.

Biomass can be measured in terms of mass of carbon or dry mass of tissue per given area. Fresh mass is unreliable because the water content of organisms fluctuates dramatically based on environmental conditions.

QUESTION 7 · 3.5.3 · LEVEL 2

Which units are most appropriate for recording the gross primary production ($GPP$) of a terrestrial ecosystem?

  • $kJ \ m^{-2} \ yr^{-1}$
  • $kJ \ m^{-3} \ day^{-1}$
  • $kg \ m^{-2} \ yr^{-1}$
  • $J \ cm^{-2} \ sec^{-1}$
Show clue

Production is a rate of energy transfer over an area, measured over a full seasonal cycle.

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Answer: $kJ \ m^{-2} \ yr^{-1}$

Primary production is measured as energy per area per given time (e.g., $kJ \ m^{-2} \ yr^{-1}$). A year is used to account for the seasonal impact on photosynthesis.

QUESTION 8 · 3.5.3 · LEVEL 1

What is the biological definition of Gross Primary Production ($GPP$)?

  • The chemical energy store in plant biomass, in a given area or volume, created by photosynthesis.
  • The chemical energy store remaining after respiratory losses have been deducted.
  • The total amount of light energy from the sun that hits the surface of a plant leaf.
  • The chemical energy passed from producers to primary consumers.
Show clue

Gross means the total amount before any 'taxes' or losses are taken away.

Show answer and explanation

Answer: The chemical energy store in plant biomass, in a given area or volume, created by photosynthesis.

Gross primary production ($GPP$) is the chemical energy store in plant biomass, in a given area or volume. It represents the total energy converted by photosynthesis before respiration ($R$) is accounted for.

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