OCR GCSE Biology A: Material cycling and decomposition in ecosystems (J247) – Revision Notes

Quick recall for statements B4.1a to B4.1d of the OCR GCSE (9-1) Gateway Science Biology A specification (J247), Version 4.0, in topic B4 Community level systems. None of these statements is printed in bold, so they are for both tiers. For full explanations and longer worked examples use the study guide, then test yourself with the practice questions. Course hub: OCR GCSE Biology. Printable checklist. Find weak spots with a free diagnostic.

Key ideas at a glance

Statement	One-line summary
B4.1a	Materials (for example carbon, nitrogen, water) are recycled between abiotic and biotic parts of an ecosystem
B4.1b	Microorganisms decompose dead material and waste, returning carbon dioxide and mineral ions
B4.1c	Carbon and water cycles supply carbon, fresh water and nutrients and help maintain habitats
B4.1d	Decay is fastest when warm (up to an optimum), moist but not waterlogged, and with plenty of oxygen

Definitions

Term	Meaning
Abiotic	Non-living parts of an ecosystem (air, water, soil, rock)
Biotic	Living organisms and their remains
Cycle	Materials pass again and again between abiotic and biotic parts
Decomposer	A microorganism (bacterium or fungus) that breaks down dead material and waste
Decomposition (decay)	Breakdown of dead organisms and waste into simpler substances
Aerobic	With oxygen
Anaerobic	Without oxygen
Composting	Controlled decay of plant waste to make a nutrient-rich material
Rate of decay	Mass lost per unit time

Microorganisms and decomposition

Method in steps:

1. Decomposers secrete enzymes onto the dead material.
2. Enzymes break large insoluble molecules into small soluble ones.
3. Decomposers absorb the products for growth and respiration.
4. Respiration releases carbon dioxide.
5. Unused mineral ions enter the soil for plants to absorb.

Remember what comes back and where: carbon dioxide to the air, mineral ions to the soil. Producers need both.

Earthworms and other small animals only break material into pieces; bacteria and fungi do the chemical breakdown.

The carbon cycle in six words

Photosynthesis in, respiration out, feeding passes it on, decay and combustion return it.

Process	Direction of carbon
Photosynthesis	Carbon dioxide (air or water) into producers
Feeding	From one organism to another
Respiration	From all organisms into carbon dioxide
Decomposition	From dead material into carbon dioxide
Combustion	From wood or fossil fuels into carbon dioxide

Why it matters: every organic molecule contains carbon; the cycle keeps carbon dioxide available for photosynthesis and passes carbon along food chains (flow of nutrients).

The water cycle in order

Evaporation (and transpiration), then condensation, then precipitation, then run-off and drainage, then uptake by roots or drinking. Then it repeats.

Why it matters: fresh water for land organisms; solvent that carries mineral ions into plants; reactant in photosynthesis; keeps rivers, ponds and wetlands as habitats.

Factors affecting the rate of decay

Factor	Rate	Say this
Low temperature	Slow	Enzymes work slowly and microorganisms are barely active
Warm, near the optimum	Fast	Enzymes work faster and microorganisms grow and respire quickly
Too hot	Falls to almost nothing	Enzymes denature and microorganisms die
Dry	Slow	Not enough water for enzyme action and growth
Moist	Fast	Water available and air spaces remain
Waterlogged	Slow	Water fills air spaces, so oxygen is short
Oxygen plentiful (aerobic)	Fast	Aerobic respiration releases more energy
Little oxygen (anaerobic)	Slow	Decay is by anaerobic microorganisms, which can produce methane

Examples: peat bogs (waterlogged, anaerobic, very slow decay); compost heaps turned to add air.

Calculations

Skill	Formula
Mass lost	Start mass - final mass
Percentage loss	Mass lost / start mass x 100
Mean rate of decay	Mass lost / time
Fraction remaining	Final mass / start mass

Worked reminder. A 90 g sample falls to 54 g in 9 days.

Mass lost = 90 - 54 = 36 g
Percentage loss = 36 / 90 x 100 = 40 %
Rate = 36 / 9 = 4.0 g per day
Fraction remaining = 54 / 90 = 3/5

Graphs: the factor you changed goes along the bottom, the measured result goes up the side, both with units, and the scales should spread the points over most of the grid.

Materials that cycle (B4.1a)

Material	Leaves the abiotic store by	Returns by
Carbon	Photosynthesis	Respiration, decomposition, combustion
Water	Uptake by roots, drinking	Transpiration, respiration, evaporation, waste
Nitrogen	Uptake of nitrogen-containing ions by plants	Decomposition of dead material and waste

Only the idea is needed for nitrogen: it is a cycled material, and microorganisms return it to the soil. The specification gives no stages for the nitrogen cycle.

Reading data on decay

1. Read the axis titles and units first.
2. Find the highest and lowest values and say where they occur.
3. Describe the trend in order: rises, peaks, falls, with figures from the table or graph.
4. Compare two values with a calculation (a ratio, a difference or a percentage change).
5. Explain with biology: enzymes, respiration, water, oxygen.
6. Write a conclusion that only claims what the data show.

Worked reminder. Mass lost by 100 g samples in 10 days: 2 g at 5 °C, 12 g at 15 °C, 30 g at 25 °C, 20 g at 45 °C.

Rate at 15 C = 12 / 10 = 1.2 g per day
Rate at 25 C = 30 / 10 = 3.0 g per day
Ratio = 3.0 / 1.2 = 2.5 times faster
Drop from 25 C to 45 C = 30 - 20 = 10 g, which is 10 / 30 x 100 = 33.3 %

The best temperature must lie somewhere between 15 °C and 45 °C, and the data cannot say exactly where.

Answer frame for a longer explanation

For a question such as “Explain why a gardener turns a compost heap and keeps it damp”:

• Turning mixes air through the heap, so oxygen is available for aerobic respiration.
• Aerobic respiration releases more energy, so the decomposers grow and break material down faster.
• Moisture gives the enzymes and microorganisms the water they need.
• Not too wet, because a soggy heap loses its air spaces and becomes anaerobic and slow.
• Decomposers release mineral ions and carbon dioxide; the finished compost returns mineral ions to soil when it is spread.

Must-know distinctions

• Decomposition releases carbon dioxide; photosynthesis removes it.
• Aerobic means with oxygen; anaerobic means without. The terms are about oxygen, not about air in general.
• Rate of decay (g per day) is not the same as percentage loss (%).
• Evaporation is the change to vapour; transpiration is evaporation from plant leaves; precipitation is the water falling.
• Too little water and too much water both slow decay, for different reasons.

Investigating the best conditions for composting

• Same mass and type of material in each container; same size of pieces.
• Change one factor only; keep the others constant.
• Use mesh bags or measure a property that changes during decay; reweigh at set intervals.
• Repeat and find a mean to deal with variation.
• Name a hazard: microorganisms grow in warm, moist conditions, so wash your hands and keep the containers closed.

Quick self-test

1. Name two materials that cycle through ecosystems.
2. What do decomposers secrete, and why?
3. Which gas do decomposers release into the air?
4. Name the process that removes carbon dioxide from the air.
5. List the stages of the water cycle in order, starting with evaporation.
6. Leaf litter falls from 60 g to 45 g in 5 days. Calculate the mean rate of decay.
7. In question 6, what is the percentage loss of mass?
8. A 360 g heap loses one quarter of its mass. What mass remains?
9. A 150 g sample decays at a mean 12 g per week. What mass remains after 6 weeks?
10. Explain why decay is slow in a waterlogged soil.
11. Give two reasons why decay stops almost completely at 60 °C.

Answers

1. Any two of carbon, nitrogen, water.
2. Enzymes, which break large insoluble molecules into small soluble ones that can be absorbed.
3. Carbon dioxide (from respiration).
4. Photosynthesis.
5. Evaporation (and transpiration), condensation, precipitation, run-off and drainage, uptake by plants and animals.
6. (60 - 45) / 5 = 3.0 g per day.
7. 15 / 60 x 100 = 25 %.
8. 360 x 3/4 = 270 g.
9. Lost 12 x 6 = 72 g, so 150 - 72 = 78 g remains.
10. Water fills the air spaces, so oxygen is scarce; aerobic respiration is limited and decay is slower.
11. The enzymes of the decomposers are denatured, and many of the microorganisms are killed.

Where marks are usually lost

• Writing that decomposers “eat” or “digest inside” the dead material without mentioning enzymes.
• Forgetting to say that mineral ions are returned to the soil as well as carbon dioxide to the air.
• Leaving out respiration when describing how carbon returns to the air.
• Leaving out a stage of the water cycle, or writing “rain” without naming precipitation.
• Saying that rate increases with temperature without mentioning an optimum and denaturing.
• Explaining slow decay in a wet heap as “no water” instead of “no oxygen”.
• Giving a rate without units, or giving a percentage when asked for a rate.
• Calculating from the wrong starting value, or mixing weeks and days.
• Plotting a graph with the axes the wrong way round, or with a scale that squashes the data into a corner.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247), Version 4.0 (August 2026), for first assessment in 2018, published by OCR: statements B4.1a to B4.1d and mathematical learning outcomes BM4.1i to BM4.1v.