AQA GCSE Biology Biology Paper 2 (Foundation), November 2021: Question 11

6 marks · Standard Demand difficulty · Extended Answer

Describe how microorganisms in compost soil recycle chemicals from dead plants and how living plants reuse these chemicals.

Practise this question

Question

Figure 19 depicts a cross-section of a compost heap showing alternating bands of material, with thicker layers labeled 'Layers of dead plant material' separated by 'Thin layers of soil'. The question asks students to describe how microorganisms in the soil layers help recycle chemicals in dead plants and how those chemicals are used again by living plants, worth 6 marks.
Question text

11 Decay occurs in a compost heap.

Figure 19 shows a compost heap.

Figure 19

Describe:

• how microorganisms in the layers of soil help to recycle chemicals in the

dead plants

• how the chemicals are used again by living plants.

[6 marks]

Mark scheme

Show the mark scheme Mark scheme table for a 6-mark extended response question. It outlines a two-level mark scheme: Level 2 (4 to 6 marks) requires relevant processes for both microorganisms and plants to be described accurately in detail; Level 1 (1 to 3 marks) requires basic relevant points. Indicative points include microbial digestion, enzymes, respiration producing carbon dioxide, and release of mineral ions, along with plant uptake of carbon dioxide via stomata for photosynthesis and uptake of mineral ions by roots through active transport.

AO /

Question Answers Mark

Spec. Ref.

Level 2: Scientifically relevant facts, events or processes are 4‒6

11 AO1

identified and given in detail to form an accurate account.

4.7.2.1

Level 1: Facts, events or processes are identified and simply 1‒3

4.7.2.2

stated but their relevance is not clear.

4.7.2.3

4.2.2.1

No relevant content 0

4.2.3.1

4.2.3.2

4.4.1.1

Indicative content 4.4.1.3

in microorganisms 4.4.2.1

• digestion or large molecules to small molecules

• enzymes or named example

• respiration

• production of carbon dioxide

• release of mineral ions or named example such as nitrate /

phosphate / magnesium

in plants

• carbon dioxide (from air) taken in by leaves

• by diffusion

• via stomata

• carbon dioxide used in photosynthesis

• making glucose / sugar / starch / cellulose or making other

correctly named example

• (named) ions taken in by roots

• by active transport

• nitrate ions for making amino acids / proteins / DNA / chlorophyll

• phosphate for making DNA

For Level 2 processes in microorganisms and in plants should be

considered

Total 6

How to answer it

Recycling Nutrients: Decay in a Compost Heap

📌 What this question tests

This 6-mark extended writing question assesses your understanding of the carbon cycle and decay (AQA Spec 4.7.2), specifically:

  • Role of Decomposers: How microorganisms (bacteria/fungi) digest organic waste, respire, and release carbon dioxide and mineral ions.
  • Reabsorption & Use by Plants: How green plants take up carbon dioxide (gas exchange via stomata) and minerals (active transport via roots) to build new biological molecules.
  • Quality of Written Communication: Structuring a logical cause-and-effect narrative linking microorganisms directly to plant growth.
Question 11 • 6 Marks Extended Response

Microorganisms, Compost & Plant Reuse

AQA GCSE Biology (Ecology & Material Recycling)

💡 Key Knowledge: The Two Stages

Stage 1: In the Compost (Microorganisms)

  • Microorganisms (bacteria & fungi) secrete extracellular enzymes to digest large, insoluble plant polymers into smaller soluble molecules.
  • They absorb these molecules and use them in aerobic respiration, releasing CO₂ into the atmosphere.
  • They release dissolved mineral ions (e.g. nitrates, phosphates, magnesium) back into the soil.

Stage 2: In Living Plants (Reuse)

  • CO₂ enters leaves through stomata by diffusion for photosynthesis → produces glucose, starch, and cellulose.
  • Nitrate ions are taken up by root hair cells via active transport → used to make amino acids, proteins, and DNA.
  • Magnesium ions are absorbed → used to make chlorophyll.

🧠 Exam Technique: Securing Level 2 (5–6 Marks)

  • Rule of Both Halves: The mark scheme explicitly states that to get into Level 2 (4–6 marks), your answer must cover both microorganisms AND plants. Writing brilliantly about decay alone caps you at Level 1 (3 marks)!
  • Use Paired Pairs: For every chemical released, state how it is taken up and what the plant makes:
    • Carbon released as CO₂ → taken up by stomata → makes glucose/cellulose.
    • Nitrogen released as nitrates → absorbed by roots → makes proteins.
  • Use Precise Transport Terms: Name diffusion for gas exchange at the stomata and active transport for mineral ions in roots.
Mark Scheme Levels Breakdown:
Level Marks Description
Level 2 4 – 6 Scientifically relevant facts, events or processes are identified and given in detail to form an accurate, coherent account covering both microorganisms and plants.
Level 1 1 – 3 Isolated facts, events or processes are stated, but relevance is not fully developed (e.g. only mentions microorganisms, or only mentions plants).

✅ Full-Mark Model Answer (Level 2, 6/6 Marks)

Role of Microorganisms in the Compost Heap:

Microorganisms, such as bacteria and fungi found in the soil layers, decompose dead plant tissues. They secrete digestive enzymes onto the dead material to break down large, insoluble molecules (like proteins and carbohydrates) into small, soluble molecules. The microorganisms absorb these molecules and break them down during aerobic respiration, which releases carbon dioxide (CO₂) into the surrounding air. Decomposers also break down organic matter to release soluble mineral ions (such as nitrates and phosphates) into the soil.

How Living Plants Reuse These Chemicals:

Living plants reuse these released chemicals in two main ways:

  • Carbon recycling: The released carbon dioxide diffuses from the atmosphere into the leaves of growing plants through tiny pores called stomata. Inside the chloroplasts, CO₂ is used in photosynthesis to make glucose. This glucose is then used to synthesize structural cellulose for cell walls or stored as starch.
  • Mineral recycling: Soluble mineral ions dissolved in soil water are absorbed by plant roots (specifically root hair cells) via active transport. Nitrate ions are combined with glucose to synthesize amino acids and proteins for growth, as well as DNA. Magnesium ions are used by the plant to synthesize chlorophyll for trapping light.

❌ Common Student Errors to Avoid

  • The "Plants Eat Soil" Misconception: Writing that plant roots absorb "nutrients/glucose directly from the compost heap". Plants do NOT absorb organic food through roots; they make their own glucose via photosynthesis!
  • CO₂ through Roots: Believing plants take in carbon dioxide through their roots. CO₂ is a gas absorbed via stomata in the leaves.
  • Omitting Microorganism Respiration: Forgetting to mention that microorganisms respire. Microbes do not just "turn" dead plants into CO₂; carbon dioxide is a direct byproduct of microbial respiration.
  • Vague Vocabulary: Writing "plants need nitrates to grow" rather than specifying the molecular purpose: "nitrates are needed to make amino acids / proteins / DNA".
  • Confusing Transport Mechanisms: Stating that minerals enter roots by diffusion. In fertile soil, ion concentration is often higher inside root hair cells than in soil water, requiring active transport against a concentration gradient.

Topics

Biology · B1: Cell Biology · B2: Organisation · B4: Bioenergetics · B7: Ecology

Question and mark scheme from the AQA GCSE Biology examination, Biology Paper 2 (Foundation), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.