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GCSE Biology · GCSEChapter 17 · Food chains, energy and the cycles
GCSE · Chapter 17

Food chains, energy and the cycles

By the end of this chapter you will be able to

  • Draw a food chain and label the trophic levels and the roles of organisms
  • Explain energy losses, and why food chains are short
  • Use quadrats to estimate abundance, and calculate a mean density
  • Describe the carbon cycle and the water cycle, and what decomposition needs
  • [H] Calculate an efficiency of energy transfer, and name the steps of the nitrogen cycle

You should already know

  • Photosynthesis and respiration (Chapters 9 and 10)
  • Organisms and habitats (KS3 Biology)
  • Percentages and means (Chapter 1)

Where this leads

  • Biodiversity and the effect of humans (Chapter 18)
  • Carbon and greenhouse gases in climate change (Chapter 18)
  • Microorganisms in food (Chapter 14)

GCSE Biology. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe food chains and webs, and the roles of producers, consumers and decomposers, Draw and interpret pyramids of numbers and biomass, and explain energy losses along a food chain, Investigate the abundance and distribution of organisms, using quadrats and a transect, Describe the carbon and water cycles, and the factors that affect decomposition, [H] Calculate the efficiency of energy transfer, and describe the nitrogen cycle.

Exercise P  Do you remember?

1
FWhich process do plants use to make glucose from carbon dioxide and water? Write the word.
2
FWhich process releases energy from glucose in every living cell? Write the word.
3
FWhat is 1000 divided by 10? Write the number.
4
FName the organisms that break down dead material. Write the word.
5
FWhat is the mean of 2, 4 and 6? Write the number.
17.1

Food chains and webs

GCSE
Describe food chains and food webs, Name the roles of producers, consumers and decomposers
Video lesson: Food chains and websComing soon
Context. Ali watches a bird catch a worm in the garden. “Where does the worm get its energy?” he asks. Every food chain starts with a plant, which makes its own food from light.

A food chain shows the flow of energy from one organism to the next, with arrows pointing in the direction of the energy flow (from the organism eaten to the organism that eats it). A producer makes its own food by photosynthesis, such as grass. A primary consumer eats producers, such as a rabbit. A secondary consumer eats primary consumers, such as a fox. A food web shows many food chains linked together. Decomposers, such as bacteria and fungi, break down dead material and return nutrients to the soil.

grassproducerrabbitprimary consumerfoxsecondary consumerenergy flows along the arrows, from the producer to the consumersthe arrows show the direction of energy flow (eaten by)
A food chain: grass, rabbit and fox with arrows showing the flow of energy

Key ideas

  • A food chain starts with a producer.
  • Arrows point from the organism that is eaten to the organism that eats it.
  • Consumers are primary (plant-eaters) or secondary (meat-eaters).
  • Decomposers break down dead material and return nutrients to the soil.
Worked example 17.1.1
Name the organism that makes its own food in a food chain. Write the word.

Method. Recall the first organism.

  1. Recall. A producer makes its own food by photosynthesis.
    makes its own food
  2. Name it. So the organism is a producer.
    producer
  3. Write the answer.
    producer

Check. Grass is a producer. ✓

Worked example 17.1.2
In the food chain grass to rabbit to fox, which organism is a primary consumer? Write the word.

Method. Find the organism that eats producers.

  1. Find. The rabbit eats the grass, which is a producer.
    eats the grass
  2. Name it. So the rabbit is the primary consumer.
    rabbit
  3. Write the answer.
    rabbit

Check. The fox is a secondary consumer. ✓

Worked example 17.1.3
Name the group of organisms that break down dead material. Write the word.

Method. Recall the group.

  1. Recall. Organisms that break down dead material are decomposers.
    break down dead material
  2. Name it. So the group is decomposers.
    decomposers
  3. Write the answer.
    decomposers

Check. Fungi are decomposers. ✓

Watch out

Watch out: the arrows in a food chain show the direction of energy flow, from the organism eaten to the organism that eats it. Do not draw them the other way.

Exam tip

Exam tip. Name the trophic level of each organism when you draw a food chain: producer, primary consumer, secondary consumer.

Try it (with your teacher). With your teacher, use a picture of a garden or a pond. Draw one food chain of four organisms, label each one with its role, and add one decomposer.

Exercise 17A  Food chains and webs

1
FName the organism that makes its own food by photosynthesis. Write the word.
2
FIn a food chain, which way do the arrows point? Write the words.
3
FName the group of organisms that break down dead material in the soil. Write the word.
4
FIn the food chain grass, rabbit, fox, what is the fox? Write the words.
5
FName the first organism in every food chain. Write the word.
6
FWhat is a food web? Write the words.
7
FName the role of bacteria in the soil. Write the word.
8
FIn a food chain, is the energy in the rabbit more or less than in the grass? Write the word.
9
PSDraw the food chain grass, rabbit, fox. Write the arrows, using the words eaten by. Write the chain with arrows.
10
PSIn a food web, a rabbit eats grass and clover. Name the number of producers in this part of the web. Write the number.
11
PSExplain why a food chain usually has only four or five organisms. Write one sentence. Accept any sentence that says energy is lost at each level, so little energy reaches the top.
12
RChallenge: explain why a food web is more realistic than a single food chain, in two sentences. Accept any answer that says organisms often eat more than one type of food, so the links form a web rather than a line.
17.2

Energy transfer and pyramids

GCSE
Describe energy transfer and its losses, Draw and interpret pyramids of numbers
Video lesson: Energy transfer and pyramidsComing soon
Context. A farmer grows grass for cows. “Why do I need so much grass to feed a few cows?” she asks. Most of the energy in the grass is lost before it can become meat or milk.

Energy passes from one trophic level to the next, but most of it is lost. The energy is lost in respiration (released as heat), in waste (faeces and urine), and in parts that are not eaten. Only a small amount is stored as growth in the next level.

A pyramid of numbers shows the number of organisms at each trophic level. Numbers usually fall at higher levels, but the pyramid can be inverted in some cases, such as a tree with many insects.

eaten: 100 unitsstored: 10 unitslost as heatlost in waste and excretionmost energy is lost at each level, so food chains are short
Energy transfer: most energy is lost as heat and in waste at each level
plantsinsectsbirdspyramid of numbers: the number of organisms at each level
A pyramid of numbers with plants, insects and birds
LevelEnergy (kJ per year, invented)
grass10000
rabbit1000
fox100
Energy at three trophic levels (invented data)

Key ideas

  • Energy is lost at each level, through respiration, heat, waste and parts not eaten.
  • So food chains are short, usually four or five levels.
  • A pyramid of numbers shows the number of organisms at each level.
  • Numbers usually fall at higher trophic levels.
Worked example 17.2.1
Name two ways that energy is lost at each trophic level. Write the words.

Method. Recall the losses.

  1. Recall. Energy is lost through respiration (as heat) and in waste.
    as heat
  2. Name two. So two losses are respiration and waste.
    respiration and waste
  3. Write the answer.
    respiration (as heat) and waste

Check. Parts that are not eaten are also lost. ✓

Worked example 17.2.2
In the energy table, what is the energy at the rabbit level? Write the number, in kJ per year.

Method. Read the table.

  1. Find the row. Find the row labelled rabbit in the energy table.
    rabbit row
  2. Read. The rabbit level has 1000 kJ per year.
    1000
  3. Write the answer.
    1000

Check. The grass has 10000 kJ. ✓

Worked example 17.2.3
In the pyramid of numbers, which level has the most organisms? Write the word.

Method. Find the widest level.

  1. Look. The base of the pyramid is the widest.
    widest
  2. Name it. So plants have the most organisms.
    plants
  3. Write the answer.
    plants

Check. Plants are the producers at the base. ✓

Watch out

Watch out: energy is lost, not destroyed. It is transferred to the surroundings as heat and in waste, and it is not recycled along the chain.

Exam tip

Exam tip. When you explain why food chains are short, say that energy is lost at each level, so little reaches the top.

Try it (with your teacher). With your teacher, draw a pyramid of numbers for a local habitat, using counts from one quadrat. Explain the shape of your pyramid.

Exercise 17B  Energy transfer and pyramids

1
FName one way energy is lost at each trophic level. Write the words.
2
FName the waste material that carries energy away from the body. Write the words. Accept faeces or urine.
3
FIn the energy table, what is the energy at the fox level? Write the number, in kJ per year.
4
FIn a pyramid of numbers, which level has the fewest organisms in this example? Write the word.
5
FWhich level in a pyramid of numbers has the most organisms: plants or birds? Write the word.
6
FIn the pyramid of numbers, how many insects are there? Write the number.
7
PSCalculate how many times more plants than birds there are in the pyramid of numbers. Write the number.
8
PSExplain why a farmer needs a large area of grass to feed a few cows. Write one sentence. Accept any sentence that says energy is lost at each level, so much more grass energy is needed for the cows.
9
RChallenge: explain why a pyramid of numbers can be inverted (wider at the top) in a food chain with one large tree, in two sentences. Accept any answer that says one tree supports many insects, and the producers are few in number even though they are large.
17.3

Required practical: quadrats and a transect

GCSE
Estimate the abundance of an organism using quadrats, Describe the distribution of organisms along a transect, Calculate a mean density
Video lesson: Required practical: quadrats and a transectComing soon
Context. Hana wants to know how many daisies grow on the school field. She cannot count them all, so she places a square quadrat at random points and counts the daisies inside each one. “Then I can estimate the total,” she says.

A quadrat is a square frame, often 1 m by 1 m, placed at random points across an area. Counting the organisms inside each quadrat gives an estimate of their abundance (density). The mean number per quadrat, multiplied by the area, gives an estimate for the whole area.

A transect is a line across a habitat, such as from a wet area to a dry one. Quadrats are placed at regular intervals along the line, to show how the organisms are distributed as the conditions change.

Safety. Work outdoors with a teacher, on a safe route, and wash hands after handling soil or plants.

QuadratDaisies counted (1 m2)
14
26
35
43
57
64
75
86
92
108
Daisies counted in 10 quadrats of 1 m2 (invented data)

Key ideas

  • A quadrat is a square frame placed at random points to sample an area.
  • Mean number per quadrat = total counted / number of quadrats.
  • Estimate for the whole area = mean per m2 x total area.
  • A transect shows how distribution changes along a line.
Worked example 17.3.1
Find the mean number of daisies per quadrat in the table. Write the number.

Method. Add the counts, then divide by 10.

  1. Add. The total of the 10 counts is 50.
    50
  2. Divide. The mean is 50 / 10 = 5.
    50 / 10 = 5
  3. Write the answer.
    5

Check. 5 x 10 = 50. ✓

Worked example 17.3.2
The area of the field is 200 m2, and the mean is 5 daisies per m2. Estimate the total number of daisies. Write the number.

Method. Multiply the mean density by the area.

  1. Multiply. Estimate = 5 x 200.
    5 x 200
  2. Calculate. The estimate is 1000 daisies.
    1000
  3. Write the answer.
    1000

Check. 1000 / 200 = 5. ✓

Worked example 17.3.3
Name the piece of equipment used to sample a line across a habitat. Write the word.

Method. Recall the line.

  1. Recall. A line across a habitat, with quadrats at intervals, is a transect.
    a line across a habitat
  2. Name it. So the equipment or method is a transect.
    transect
  3. Write the answer.
    transect

Check. A transect shows distribution along a gradient. ✓

Watch out

Watch out: the quadrats must be placed at random for a random sample. Choosing the places yourself makes the estimate biased.

Exam tip

Exam tip. Give the mean from all the quadrats, not from one or two. More quadrats give a more reliable estimate.

Try it (with your teacher). With your teacher, use a 1 m quadrat and count one type of plant in five random places on the school field. Calculate the mean and estimate the total number for the area.

Exercise 17C  Required practical: quadrats and a transect

1
FWhat is a quadrat? Write the words.
2
FWhat is a transect? Write the words.
3
FIn the quadrat table, what is the count for quadrat 4? Write the number.
4
FIn the quadrat table, what is the largest count? Write the number.
5
FCalculate the total number of daisies counted in all 10 quadrats. Write the number.
6
PSCalculate the mean number of daisies per quadrat. Write the number.
7
PSEstimate the total number of daisies in a 200 m2 field with a mean density of 5 per m2. Write the number.
8
PSWhy should the quadrats be placed at random points? Write one sentence. Accept any sentence that says it avoids bias in the choice of places, so the sample is representative.
9
PSWhy is a larger number of quadrats more reliable? Write one sentence. Accept any sentence that says the mean is based on more data, so it is less affected by chance.
10
PSA transect runs from a wet area to a dry area. Name one organism you would expect to be more common at the wet end. Write the word. Accept any one: rushes, or water-loving plants.
11
RChallenge: explain why a transect is useful for showing a change in distribution, in two sentences. Accept any answer that says quadrats at regular intervals show how the numbers change along a gradient of conditions, such as moisture.
17.4

Carbon and water cycles, and decomposition

GCSE
Describe the carbon cycle, Describe the water cycle, Explain how decomposition is affected by temperature and moisture
Video lesson: Carbon and water cycles, and decompositionComing soon
Context. A pile of leaves in a damp corner of the garden rots quickly. The same leaves in a dry, cold corner stay almost the same for months. “Why does the damp, warm pile break down so much faster?” asks Ruth.

Carbon moves between the air, living things and the soil in the carbon cycle. Photosynthesis takes carbon dioxide from the air, feeding passes carbon along food chains, and respiration releases carbon dioxide back. Decomposition by microorganisms releases carbon dioxide from dead material, and burning fuel (combustion) releases carbon dioxide too.

The water cycle moves water from the sea and land to the air by evaporation and transpiration, and it falls back as precipitation. Decomposition is faster when it is warm and moist, because the decomposers are more active.

carbon dioxide in airplantsanimalsdecomposersphotosynthesisfeedingrespirationcombustion, decomposition
The carbon cycle: photosynthesis, feeding, respiration, decomposition and combustion
ProcessWhat happens to carbon
photosynthesiscarbon dioxide is taken into plants
respirationcarbon dioxide is released by living things
feedingcarbon passes along food chains
decompositionmicroorganisms release carbon dioxide from dead material
combustionburning fuel releases carbon dioxide
What happens to carbon at each step
Temperature (C)Mass of leaves lost in 4 weeks (%)
54
1512
2524
Mass of leaves lost in 4 weeks at three temperatures (invented data)

Key ideas

  • Photosynthesis removes carbon dioxide from the air; respiration and decomposition return it.
  • Combustion of fuel also releases carbon dioxide.
  • The water cycle: evaporation, transpiration, condensation and precipitation.
  • Decomposition is faster when it is warm and moist.
Worked example 17.4.1
Name the process that takes carbon dioxide from the air into plants. Write the word.

Method. Recall the process.

  1. Recall. Plants take in carbon dioxide during photosynthesis.
    during photosynthesis
  2. Name it. So the process is photosynthesis.
    photosynthesis
  3. Write the answer.
    photosynthesis

Check. Respiration releases carbon dioxide. ✓

Worked example 17.4.2
In the decomposition table, how much mass is lost at 25 C? Write the number, as a percentage.

Method. Read the table.

  1. Find the row. Find the row for 25 C in the decomposition table.
    25 C row
  2. Read. At 25 C, 24% of the leaf mass is lost.
    24
  3. Write the answer.
    24%

Check. It is double the loss at 15 C. ✓

Worked example 17.4.3
Name the process that moves water into the air from plants. Write the word.

Method. Recall the process.

  1. Recall. Water vapour leaves the leaves by transpiration.
    leaves the leaves
  2. Name it. So the process is transpiration.
    transpiration
  3. Write the answer.
    transpiration

Check. Evaporation from the sea and land is also part of it. ✓

Watch out

Watch out: decomposition releases carbon dioxide, so it is part of the carbon cycle, and it is not the same as combustion. Both release carbon dioxide, but decomposition is done by microorganisms.

Exam tip

Exam tip. When you explain decomposition, name the factor, and say why it speeds up the decomposers: for example, warmth increases their enzyme activity.

Try it (with your teacher). With your teacher, put equal amounts of damp leaves in two bags, one warm and one cold. Weigh each after two weeks and compare the masses. Explain the result.

Exercise 17D  Carbon and water cycles, and decomposition

1
FName the process by which plants take carbon dioxide from the air. Write the word.
2
FName the process that releases carbon dioxide from living things. Write the word.
3
FName the process by which burning fuel releases carbon dioxide. Write the word.
4
FName the process by which microorganisms release carbon dioxide from dead material. Write the word.
5
FIn the water cycle, what is the name of the process by which water vapour leaves plants? Write the word.
6
FIn the decomposition table, what is the mass lost at 5 C? Write the number, as a percentage.
7
FWhich temperature in the table gives the most decomposition? Write the number, in degrees C.
8
PSWhy does decomposition speed up in warm conditions? Write one sentence. Accept any sentence that says warmth increases the activity of the decomposers and their enzymes.
9
PSWhy does decomposition need moisture? Write one sentence. Accept any sentence that says the decomposers need water to live and carry out their processes.
10
PSExplain how the carbon cycle is kept in balance, in one sentence. Accept any sentence that says carbon dioxide taken in by photosynthesis is returned by respiration and decomposition.
11
RChallenge: explain why burning fossil fuels changes the carbon cycle, in two sentences. Accept any answer that says it releases carbon dioxide that was stored for millions of years, adding carbon to the air faster than it can be removed.
17.5

Efficiency of energy transfer and the nitrogen cycle [H]

GCSE
[H] Calculate the efficiency of energy transfer, [H] Describe the nitrogen cycle
Video lesson: Efficiency of energy transfer and the nitrogen cycle [H]Coming soon
Context. A farmer compares how much energy a cow gets from grass with how much it gets back as meat. “Which part of the energy is lost?” she asks. Efficiency is a way to compare the energy that passes on with the energy that was available.

[H] Higher tier only. This section is higher-tier content. It covers the efficiency calculation and the nitrogen cycle.

[H] The efficiency of energy transfer = (energy in the next level / energy in this level) x 100%. With the grass and rabbit values in this chapter: 1000 / 10000 x 100 = 10%. So only about a tenth of the energy passes on.

[H] The nitrogen cycle moves nitrogen through the environment. Some bacteria fix nitrogen gas into nitrates. Plants absorb nitrates and make proteins. Animals eat plants and then excrete waste containing nitrogen. Decomposers return ammonium to the soil, and other bacteria turn it back into nitrates or nitrogen gas.

LevelEnergy (kJ per year, invented)
grass10000
rabbit1000
fox100
Energy at three levels (invented data) for an efficiency calculation

Key ideas

  • [H] Efficiency = energy in the next level / energy in this level x 100%.
  • [H] Most energy passes on at about 10% at each level in this data.
  • [H] Some bacteria fix nitrogen gas into nitrates for plants.
  • [H] Decomposers return nitrogen to the soil as ammonium.
Worked example 17.5.1
[H] Calculate the efficiency of energy transfer from grass (10000 kJ) to rabbit (1000 kJ). Write the number, as a percentage.

Method. Divide the energy passed on by the energy available, then multiply by 100.

  1. Divide. 1000 / 10000 = 0.1.
    1000 / 10000 = 0.1
  2. Multiply. Multiply by 100 to give 10%.
    10%
  3. Write the answer.
    10%

Check. 10% of 10000 is 1000. ✓

Worked example 17.5.2
[H] Name the process by which bacteria change nitrogen gas into nitrates. Write the word.

Method. Recall the process.

  1. Recall. Bacteria convert nitrogen gas into a form plants can use, which is fixing.
    fix nitrogen
  2. Name it. So the process is nitrogen fixation.
    nitrogen fixation
  3. Write the answer.
    nitrogen fixation

Check. Plants absorb nitrates from the soil. ✓

Worked example 17.5.3
[H] Name the group of bacteria that return ammonium to the soil as nitrates. Write the word.

Method. Recall the group.

  1. Recall. Nitrifying bacteria convert ammonium into nitrates in the soil.
    nitrates
  2. Name it. So the group is nitrifying bacteria.
    nitrifying bacteria
  3. Write the answer.
    nitrifying bacteria

Check. Plants use the nitrates. ✓

Watch out

Watch out: efficiency is always calculated from the energy in the level being transferred from, not the one it goes to. A rabbit does not add energy to the grass.

Exam tip

Exam tip. [H] Write the formula, then substitute the values, then divide, then multiply by 100 for a percentage.

Try it (with your teacher). With your teacher, sketch the nitrogen cycle as a loop, with arrows for nitrogen fixation, uptake by plants, feeding, excretion and decomposition.

Exercise 17E  [H] Efficiency of energy transfer and the nitrogen cycle

1
F[H] Write the formula for efficiency of energy transfer. Write the words.
2
PS[H] Calculate the efficiency when 200 kJ passes from 2000 kJ. Write the number, as a percentage.
3
PS[H] Calculate the efficiency when 500 kJ passes from 5000 kJ. Write the number, as a percentage.
4
PS[H] Calculate the efficiency when 100 kJ passes from 1000 kJ, which is the fox level in this chapter. Write the number, as a percentage.
5
F[H] Name the bacteria that fix nitrogen gas into a form that plants can use. Write the words.
6
F[H] Which substance do plants absorb from the soil for proteins? Write the word.
7
F[H] Name the process that returns nitrogen from dead material to the soil. Write the word.
8
PS[H] Explain why only about 10% of energy passes to the next level, in one sentence. Accept any sentence that says most energy is lost in respiration, heat and waste at each step.
9
R[H] Challenge: explain why a food chain of five levels would have very little energy at the top, using the efficiency of 10%. Write two sentences. Accept any answer that says each level passes on about 10%, so after several steps only a small fraction reaches the top.

Chapter summary

SkillMethod
Food chainsProducers, then primary and secondary consumers; arrows show energy flow; decomposers break down dead material.
Energy lossesRespiration (heat), waste and parts not eaten; food chains are short.
PyramidsPyramid of numbers shows the number at each level; numbers usually fall at higher levels.
Quadrats and transectsQuadrats at random estimate abundance; mean x area; a transect shows distribution along a gradient.
Carbon cyclePhotosynthesis, feeding, respiration, decomposition and combustion move carbon.
DecompositionFaster when warm and moist; microorganisms release carbon dioxide.
[H] Efficiency and nitrogenEfficiency = energy passed on / energy available x 100%; the nitrogen cycle uses fixation, nitrifying and decomposer bacteria.

Can you…?

  • I can draw a food chain and label the roles of producers, consumers and decomposers
  • I can explain energy losses and why food chains are short
  • I can use quadrats to estimate abundance and calculate a mean density
  • I can describe the carbon and water cycles, and what decomposition needs
  • [H] I can calculate an efficiency of energy transfer and describe the nitrogen cycle

Key terms

food chain
a sequence showing the flow of energy from one organism to the next
food web
many food chains linked together
producer
an organism that makes its own food by photosynthesis
consumer
an organism that eats other organisms
decomposer
an organism that breaks down dead material
quadrat
a square frame placed at random to sample an area
transect
a line across a habitat along which samples are taken
abundance
how many of an organism there are in an area
transpiration
the loss of water vapour from plants
[H] efficiency
the energy passed on divided by the energy available, as a percentage

Exercise 17Z  Check-up

1
Name the organism that makes its own food in a food chain. Write the word.
2
Name the group of organisms that break down dead material. Write the word.
3
Name the process that releases carbon dioxide from living things. Write the word.
4
Name the process that moves water vapour out of plants. Write the word.
5
Name the energy lost from each level as heat, in respiration. Write the words.
6
In the quadrat table, what is the total count for all 10 quadrats? Write the number.
7
Calculate the mean number of daisies per quadrat from the table. Write the number.
8
In the decomposition table, what is the percentage lost at 15 C? Write the number.
9
Explain in one sentence why food chains are short. Accept any sentence that says energy is lost at each level, so little reaches the top.
10
Name one factor that speeds up decomposition. Accept any one: warmth, or moisture.

Total 27 marks. Allow about 27 minutes.

Exercise 17X  Exam-style questions

1
FName the process that makes glucose in plants. Write the word.[1 mark]
2
FName the organism that eats primary consumers in a food chain, such as the fox. Write the words.[1 mark]
3
FName the process that returns carbon dioxide from dead material. Write the word.[1 mark]
4
FDescribe two ways in which energy is lost along a food chain. Write three points. Accept any two of: respiration (heat); waste (faeces and urine); parts not eaten.[2 marks]
5
FDescribe how to use quadrats to estimate the number of daisies in a field of 400 m2. Write three steps. Accept any three of: place quadrats at random points; count the daisies in each; find the mean and multiply by the area.[3 marks]
6
RExplain why the carbon cycle depends on photosynthesis and respiration. Write three sentences. Accept any answer that says photosynthesis takes carbon dioxide from the air, respiration returns it, and the two balance.[3 marks]
7
RExplain why decomposition is slower in a cold, dry place than in a warm, damp one. Write three sentences. Accept any answer that says decomposers are less active in the cold, and they need moisture to work, so both factors slow the process.[3 marks]
8
PS[H] The energy available at the grass level is 10000 kJ and at the rabbit level is 1000 kJ. Calculate the efficiency of the transfer, and explain why it is so low. Accept 10%, and an answer that says most energy is lost in respiration, heat and waste.[4 marks]
9
R[H] Describe the role of bacteria in the nitrogen cycle, naming two different groups and what each does. Write four sentences. Accept any answer that gives nitrogen-fixing bacteria changing nitrogen gas to nitrates, and decomposers or nitrifying bacteria returning nitrogen to the soil.[4 marks]
10
R[H] Challenge: explain why a food chain with five levels, each passing on 10%, supports much less life at the top than the bottom. Write five sentences. Accept any answer that says each level passes on 10% of the energy, so the energy falls to a tenth at each step, and fewer organisms can be supported at the top.[5 marks]
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