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3.5.4 Nutrient cycles

AQA A-level Biology 3.5.4 practice on Nutrient cycles, with free MCQs, clues and worked explanations drawn from the Energy transfers in and between organisms section of specification 7402.

Specification route
3.5.4
Question bank
24 questions
Course stage
Year 13 / A-level only

Sample questions

QUESTION 1 · 3.5.4 · LEVEL 2

What crucial role do saprobionts play in nutrient cycles?

  • They secrete enzymes to digest organic matter externally, breaking down complex molecules and absorbing the nutrients.
  • They perform nitrogen fixation, converting nitrogen gas directly into solid plant proteins.
  • They are apex predators that control the population sizes of primary consumers.
  • They convert carbon dioxide and water into glucose during the light-independent reaction.
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These are decomposers like fungi and certain bacteria that feed on dead/waste material.

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Answer: They secrete enzymes to digest organic matter externally, breaking down complex molecules and absorbing the nutrients.

Saprobionts decompose dead/waste organic material by extracellular digestion. They secrete enzymes onto the matter, breaking down complex molecules into simpler ones, releasing nutrients like ammonia back into the cycle.

QUESTION 2 · 3.5.4 · LEVEL 2

Mycorrhizae are associations between certain types of fungi and the roots of plants. How do these associations benefit the plant?

  • The fungal hyphae vastly increase the surface area for the absorption of water and scarce inorganic ions like phosphate.
  • The fungi actively perform photosynthesis, supplying the roots with excess glucose.
  • The fungi provide a protective physical shield of chitin to stop herbivores eating the roots.
  • The fungi release antibiotics that directly stimulate rapid cell division in the plant meristem.
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The fungi act like massive extensions of the plant's own root hair cells.

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Answer: The fungal hyphae vastly increase the surface area for the absorption of water and scarce inorganic ions like phosphate.

Mycorrhizae are mutualistic associations. The fungal hyphae vastly increase the surface area of the root system, facilitating the uptake of water and scarce inorganic ions, particularly phosphates.

QUESTION 3 · 3.5.4 · LEVEL 2

In the nitrogen cycle, which biological process describes the conversion of nitrogen-containing organic compounds (like proteins and DNA) from dead organisms into ammonia?

  • Ammonification by saprobionts
  • Nitrification by nitrifying bacteria
  • Denitrification by denitrifying bacteria
  • Nitrogen fixation by mutualistic bacteria
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This is the first step of decomposition regarding nitrogen.

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Answer: Ammonification by saprobionts

Ammonification is the process where saprobionts break down organic nitrogen-containing compounds (proteins, urea, nucleic acids) into ammonia, which then forms ammonium ions in the soil.

QUESTION 4 · 3.5.4 · LEVEL 3

What sequence of chemical conversions occurs during nitrification in the soil?

  • Ammonium ions are oxidised to nitrite ions, which are then further oxidised to nitrate ions.
  • Nitrogen gas is reduced to ammonia, which is then oxidised to nitrate ions.
  • Nitrate ions are reduced to nitrite ions, which are then reduced to nitrogen gas.
  • Amino acids are hydrolysed into ammonia, which is then condensed into nitrate ions.
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This is a two-step oxidation process performed by free-living soil bacteria.

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Answer: Ammonium ions are oxidised to nitrite ions, which are then further oxidised to nitrate ions.

Nitrification is a two-stage oxidation reaction carried out by nitrifying bacteria: firstly, oxidation of ammonium ions ($NH_{4}^{+}$) to nitrite ions ($NO_{2}^{-}$), and secondly, oxidation of nitrites to nitrate ions ($NO_{3}^{-}$).

QUESTION 5 · 3.5.4 · LEVEL 2

Denitrifying bacteria convert soil nitrates back into nitrogen gas. Under what environmental conditions do these bacteria typically thrive?

  • Anaerobic conditions, such as waterlogged soils.
  • Highly aerobic conditions, such as freshly ploughed topsoil.
  • Extremely dry conditions, such as sandy deserts.
  • Highly acidic conditions, such as in peat bogs.
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They use nitrates as an alternative to oxygen for respiration.

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Answer: Anaerobic conditions, such as waterlogged soils.

Denitrifying bacteria are anaerobic. They thrive in waterlogged, poorly aerated soils, converting valuable soil nitrates back into atmospheric nitrogen gas.

QUESTION 6 · 3.5.4 · LEVEL 1

Unlike the carbon and nitrogen cycles, the phosphorus cycle lacks which major component?

  • A gaseous phase in the atmosphere.
  • A solid phase stored in sedimentary rocks.
  • A liquid phase dissolved in the oceans.
  • A biological phase incorporated into living cells.
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Phosphorus does not form a stable gas at normal Earth temperatures.

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Answer: A gaseous phase in the atmosphere.

The phosphorus cycle differs from the carbon and nitrogen cycles because it lacks a gaseous phase in the atmosphere. The main reservoir of phosphorus is in mineral form in sedimentary rocks.

QUESTION 7 · 3.5.4 · LEVEL 2

What is the primary environmental danger of using highly soluble artificial fertilisers on agricultural land?

  • The nutrients are easily leached by rain into nearby waterways, causing eutrophication.
  • The artificial nutrients permanently bind to soil particles, making the soil hard and infertile.
  • They release large amounts of ozone-depleting gases into the atmosphere.
  • They selectively kill all natural saprobionts, stopping the nitrogen cycle entirely.
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Soluble means it dissolves in water, and rain washes things down into rivers and lakes.

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Answer: The nutrients are easily leached by rain into nearby waterways, causing eutrophication.

The environmental issues arising from the use of fertilisers include leaching. Because artificial fertilisers are highly soluble, excess nutrients not taken up by crops are washed into waterways, triggering eutrophication.

QUESTION 8 · 3.5.4 · LEVEL 2

During the process of eutrophication, what causes the initial death of submerged aquatic plants?

  • A massive algal bloom on the water surface blocks light, preventing submerged plants from photosynthesising.
  • The high nitrate concentration acts as a direct toxin to the submerged plant cells.
  • Saprobiontic bacteria aggressively attack and digest the living plant tissues.
  • The water temperature rises dramatically due to the rapid respiration of the algae.
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Think about what plants fundamentally need to survive and how a thick green layer on top of a pond affects that.

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Answer: A massive algal bloom on the water surface blocks light, preventing submerged plants from photosynthesising.

Eutrophication begins with an influx of nutrients causing an algal bloom. This dense layer of algae blocks light from reaching lower depths, causing submerged plants to die from a lack of photosynthesis.

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