Organisms and their Environment
SEAB syllabus topic 11: energy flow, food chains and webs, pyramids, the carbon cycle, global warming, pollution and conservation, with the why, the traps and the exam checks.
Energy enters a food chain from the Sun, flows along it in one direction and leaves as heat, so the Sun must keep supplying energy.
Carbon is cycled: producers take carbon dioxide from the air, and respiration, decomposition and combustion return it.
Human activities such as deforestation, burning fossil fuels and pollution damage ecosystems, and conservation helps to maintain biodiversity.
Organisms and their environment links four ideas: who eats whom and how energy is lost, how carbon moves between the air and living things, how human activities disturb these flows, and how conservation protects the balance of an ecosystem.
Energy flow: one way only
describe the non-cyclical nature of energy flow
Energy flows through an ecosystem in one direction, from the Sun to the producers, then to the consumers and decomposers, and it leaves the living things as heat, so it is not recycled.
- Energy flow
- Energy flow: the transfer of energy from one organism to the next along a food chain, starting with the Sun and the producers
- Non-cyclical
- Non-cyclical: moving in one direction only and never returning to the start; the flow of energy in an ecosystem is non-cyclical, but the movement of carbon is cyclical
- Chemical energy
- Chemical energy: the energy stored in carbon compounds such as glucose, starch and fat, which passes from organism to organism when food is eaten
- Heat
- Heat: the form in which energy leaves the living things, mainly from respiration; it escapes to the surroundings and producers cannot use it to make food
How energy moves along a food chain
- 1Producers, such as plants and algae, absorb light energy from the Sun, and use it in photosynthesis to make carbon compounds such as glucose, so the light energy is converted into chemical energy.
- 2The chemical energy is passed on, in the carbon compounds, to the consumers that eat the producers, and then to the next consumers along the food chain.
- 3At each stage, some of the energy is used for growth and repair, which builds body tissue that stores energy for the next organism in the chain.
- 4At each stage, most of the rest of the energy is released as heat in respiration, or stays in waste and dead remains.
- 5Decomposers feed on the waste and dead remains, and release the last of this energy as heat when they respire.
- 6The heat escapes to the surroundings and cannot be used again by the producers, so the flow of energy is non-cyclical and the Sun must continually supply energy.
Energy is not recycled, but carbon is
| Feature | Energy | Carbon and other nutrients |
|---|---|---|
| Direction | Energy moves along the food chain in one direction and is lost as heat. | Carbon moves in a cycle between the air, the living things and the soil or water, so the same carbon atoms are used again and again. |
| Source | Energy comes into the ecosystem from the Sun, and must be supplied continually. | Carbon is already in the ecosystem, and is recycled by producers, consumers and decomposers. |
| Can producers reuse it? | Producers cannot use heat to make food, so energy that has been lost as heat cannot be reused. | Producers can reuse carbon, because the carbon dioxide released by respiration is taken in again by photosynthesis. |
For SEAB answers, write that the flow of energy is non-cyclical: energy is lost as heat, it is not recycled, and so the Sun must continually supply energy. supports K325-11(a)
Depth: why and how
- Why must the Sun keep supplying energy? Energy leaves each trophic level as heat and cannot be reused by the producers, so without new light energy the producers would stop making food and the food chain would run out of energy.
- Is energy destroyed when it is lost as heat? No, it is transferred to the surroundings as heat, where the living things cannot use it, so it is lost from the food chain and not destroyed.
- Why is carbon cycled but energy is not? Carbon atoms can be released as carbon dioxide and taken in by producers again, but heat spreads into the surroundings and cannot be converted back into chemical energy by producers.
- Cross-link to Nutrition and Transport in Flowering Plants: photosynthesis converts light energy into the chemical energy in glucose, which is where the flow of energy in a food chain starts.
- Cross-link to Respiration: the energy released in respiration is used for life processes and is eventually lost as heat.
Common mistakes
Energy is recycled in an ecosystem, like carbon.
The flow of energy is non-cyclical: energy passes along the food chain, leaves as heat and is not recycled.
Energy is used up and destroyed in a food chain.
Energy is not destroyed; it is transferred along the food chain or lost to the surroundings as heat.
Producers make energy.
Producers convert light energy into chemical energy, which is stored in the carbon compounds they make.
Energy flows from the consumers back to the Sun.
Energy flows from the Sun to the producers and then to the consumers, and the heat that is lost goes to the surroundings.
Energy comes from the Sun, passes along the food chain in one direction, and leaves as heat, so it is non-cyclical and the Sun must keep supplying it.
Exam check
Q1 Which statement describes the flow of energy in an ecosystem?
- A It is recycled between the organisms and the air
- B It moves from the consumers back to the producers
- C It moves in one direction and is lost as heat
- D It is made by the consumers and passed to the producers
Show answer
MCQ answer key C (It moves in one direction and is lost as heat): energy flows from the Sun along the food chain in one direction and leaves as heat, so it is non-cyclical.
Q2 What is the original source of energy for most food chains?
- A The Sun
- B The producers
- C The decomposers
- D The soil
Show answer
MCQ answer key A (The Sun): light energy from the Sun is absorbed by the producers and converted into chemical energy, so the Sun is the original source.
Q3 Why must the Sun continually supply energy to an ecosystem?
- A Carbon dioxide runs out if the Sun stops
- B Decomposers stop working without light
- C Consumers store energy for only a few hours
- D Heat that is lost from the food chain cannot be reused by the producers
Show answer
MCQ answer key D (Heat that is lost from the food chain cannot be reused by the producers): energy is lost as heat at each stage and producers cannot use heat to make food, so new energy is needed all the time.
Structured Energy enters a woodland food chain and leaves it.
- Name the source of the energy that enters the food chain. [1]
- Name the process by which the producers convert this energy into chemical energy. [1]
- State two ways in which energy leaves the organisms in the food chain. [2]
- Explain why the flow of energy is described as non-cyclical. [2]
Total: 6 marks
Show mark scheme
(a) 1 mark
- Mark scheme: the Sun (light energy)
(b) 1 mark
- Mark scheme: photosynthesis
(c) 2 marks
- Mark scheme: heat released in respiration
- Mark scheme: energy in faeces, urine and other waste
- Mark scheme: energy in uneaten parts and dead remains
- Mark scheme: any two of these
(d) 2 marks
- Mark scheme: energy is lost as heat to the surroundings
- Mark scheme: producers cannot reuse it, so the Sun must continually supply energy
Producers, consumers and decomposers
describe the roles of producers, consumers and decomposers in food chains and food webs
Producers make their own food by photosynthesis, consumers feed on other organisms and decomposers feed on dead organisms and waste, and together they pass energy along food chains and food webs.
- Population
- Population: all the organisms of one species that live together in the same habitat supports K325-11(b)
- Habitat
- Habitat: the place where an organism lives, which provides its food, water and shelter supports K325-11(b)
- Community
- Community: all the populations of different species that live and interact with one another in the same habitat supports K325-11(b)
- Ecosystem
- Ecosystem: a community of living organisms together with the non-living physical environment, such as light, temperature, water and soil, with which they interact supports K325-11(b)
In a mangrove swamp, all the mudskippers form one population; the mudskippers, crabs, mangrove trees and all the other species together form the community; and the community together with the water, mud and sunlight forms the ecosystem. supports K325-11(b)
Food chains and trophic levels
- Food chain
- Food chain: a series of organisms in which each one is the food of the next, showing how energy and nutrients pass from a producer to the consumers
- Trophic level
- Trophic level: the feeding position of an organism in a food chain
- Producer
- Producer: an organism that makes its own food from simple inorganic substances by photosynthesis, so it forms the first trophic level
- Consumer
- Consumer: an organism that gets its energy by feeding on other organisms
- Decomposer
- Decomposer: an organism, such as a bacterium or a fungus, that feeds on dead organisms and waste and breaks them down
- Herbivore
- Herbivore: a consumer that eats only plants
- Carnivore
- Carnivore: a consumer that eats only other animals
- Omnivore
- Omnivore: a consumer that eats both plants and animals
In a food chain, the arrow points from the organism that is eaten to the organism that eats it, so the arrow shows the direction in which energy flows.
| Trophic level | Feeding position | In the food chain: sunflower, snail, frog, fox |
|---|---|---|
| 1st | The producer is at the first trophic level, and it makes its own food. | The sunflower is the producer. |
| 2nd | A primary consumer is at the second trophic level, and it is a herbivore. | The snail is the primary consumer, because it eats the sunflower. |
| 3rd | A secondary consumer is at the third trophic level, and it eats primary consumers. | The frog is the secondary consumer, because it eats the snail. |
| 4th | A tertiary consumer is at the fourth trophic level, and it eats secondary consumers. | The fox is the tertiary consumer, because it eats the frog. |
What decomposers do
- Decomposers feed on dead organisms, faeces and other waste from the organisms at all the trophic levels.
- Decomposers break down the dead matter and release mineral ions, such as nitrates, into the soil, where producers absorb them and use them again.
- Decomposers release carbon dioxide into the air when they respire, which producers use in photosynthesis.
- Without decomposers, dead matter would pile up and the nutrients in it would stay locked away, so the producers would run short of mineral ions.
- Detritivores, such as earthworms, feed on dead matter and break it into small pieces, which gives the decomposers a larger surface area to work on. supports K325-11(b)
Food webs
- Food web
- Food web: a network of interlinked food chains in a community, which shows the many feeding relationships between the organisms
Reading a food web
| Question about the food web | Answer and reason |
|---|---|
| Which organism is the producer? | The grass is the only producer, because it is the only organism with no arrow pointing to it. |
| Which organisms are primary consumers? | The mouse and the grasshopper are the primary consumers, because they eat the grass. |
| How many food chains are there? | There are four food chains: grass, mouse, snake, hawk; grass, mouse, hawk; grass, mouse, fox; and grass, grasshopper, frog, snake, hawk. |
| Which is the longest food chain? | The longest food chain is grass, grasshopper, frog, snake, hawk, which has five organisms and so five trophic levels. |
| Which organisms eat the mouse? | The snake, the hawk and the fox all eat the mouse. |
| Which organisms are top consumers? | The hawk and the fox are the top consumers, because no arrow leads away from them to another organism. |
| At which trophic levels is the hawk? | The hawk is a secondary consumer in the chain grass, mouse, hawk; a tertiary consumer in the chain grass, mouse, snake, hawk; and a quaternary consumer in the chain grass, grasshopper, frog, snake, hawk. |
An organism in a food web can be at more than one trophic level, because it can be part of different food chains, so always say which food chain you are describing. supports K325-11(b)
How to read a food web in an exam
- 1Find the producer, which has no arrow pointing to it.
- 2Follow the arrows from the producer to trace each food chain, and remember that each arrow points from the food to the feeder.
- 3Count the organisms in a chain to find the trophic level of each one.
- 4Find the top consumers, which have no arrow leading away from them.
- 5To predict a change, follow the arrows both ways: what the organism eats, and what eats it.
Depth: what happens if a link changes?
- What would probably happen to the fox population if all the mice died from a disease? The fox has no other food in this web, so its population would probably fall, and the snake and the hawk would depend more on the frogs and the snakes, so the frog and snake populations would probably fall.
- What would probably happen to the frog and grasshopper populations if the snakes were removed? Fewer frogs would be eaten, so the frog population would probably rise, and the larger number of frogs would eat more grasshoppers, so the grasshopper population would probably fall.
- Why is a food web more realistic than a single food chain? Most organisms eat more than one kind of food and are eaten by more than one kind of consumer, so a food web shows all these links, and a consumer can switch to another food if one becomes scarce.
Depth: why and how
Common mistakes
The arrow points from the eater to the food.
The arrow points from the organism that is eaten to the organism that eats it, in the direction of energy flow.
A population is all the living things in a habitat.
A population is the organisms of one species only, and all the populations in a habitat form the community.
A habitat and an ecosystem are the same thing.
A habitat is the place where an organism lives, but an ecosystem includes all the living things and the physical environment.
Decomposers are not important because they are not in the food chain.
Decomposers break down dead matter and return mineral ions and carbon dioxide to the environment, so the producers can use them again.
The hawk in the food web is always a tertiary consumer.
An organism in a food web can be at more than one trophic level, depending on the food chain that is followed.
Producers make food, consumers eat other organisms and decomposers break down dead matter; a food chain shows one feeding path, and a food web joins the food chains together.
Exam check
Q1 Which term describes all the populations of different species living and interacting in a habitat?
- A ecosystem
- B community
- C population
- D habitat
Show answer
MCQ answer key B (community): a community is all the populations of different species that live and interact in the same habitat; an ecosystem also includes the physical environment.
Q2 In a food web, grass is eaten by a mouse and a grasshopper, and the mouse is eaten by a fox. Which food chain is correct?
- A grass, mouse, fox
- B fox, mouse, grass
- C mouse, grass, fox
- D grass, fox, mouse
Show answer
MCQ answer key A (grass, mouse, fox): the arrows point from the food to the feeder, so the chain runs from the producer, the grass, to the mouse and then to the fox.
Q3 In the food chain sunflower, snail, frog, fox, which row describes the frog?
- A second trophic level, primary consumer
- B third trophic level, tertiary consumer
- C third trophic level, secondary consumer
- D fourth trophic level, secondary consumer
Show answer
MCQ answer key C (third trophic level, secondary consumer): the frog eats the snail, which is the primary consumer, so the frog is a secondary consumer at the third trophic level.
Structured In a food web, the grass is eaten by the mouse and the grasshopper. The grasshopper is eaten by the frog, the frog is eaten by the snake, and the mouse is eaten by the snake, the hawk and the fox. The snake is eaten by the hawk.
- Name the producer in the food web. [1]
- Write down one food chain of four organisms from the food web. [1]
- Name one organism that is a tertiary consumer in one of the food chains. [1]
- Predict and explain what would happen to the fox population if all the mice died. [2]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: grass
(b) 1 mark
- Mark scheme: grass, mouse, snake, hawk, or grass, grasshopper, frog, snake, with the arrows pointing from the food to the feeder
(c) 1 mark
- Mark scheme: the snake, in the chain grass, grasshopper, frog, snake, or the hawk, in the chain grass, mouse, snake, hawk
(d) 2 marks
- Mark scheme: the fox population would fall
- Mark scheme: because the fox has no other food in the web
Energy losses and efficiency
explain how energy losses occur along food chains, and discuss the efficiency of energy transfer between trophic levels
Only about 10% of the energy at one trophic level is passed on to the next, because most of it is lost as heat, in waste or in parts that are not eaten, so the transfer of energy is not very efficient.
- Efficiency of energy transfer
- Efficiency of energy transfer: the percentage of the energy at one trophic level that is passed on to the next trophic level
- Excreta
- Excreta: waste products of the body's chemical reactions that are removed from the body, such as urea and carbon dioxide supports K325-11(c)
- Egesta
- Egesta: food that has been swallowed but not digested, which leaves the body as faeces supports K325-11(c)
How energy is lost at each trophic level
Respiration
Organisms release energy from their food in respiration, and much of this energy is lost to the surroundings as heat, including the energy used for movement and other activities.
Excreta and egesta
Some of the chemical energy stays in the faeces and other waste, which the consumer does not use, so it is not passed on to the next consumer.
Uneaten parts
Some parts of an organism are not eaten, such as bones, roots and fur, so the energy in them is not passed on to the next consumer.
Decomposers (not a separate loss)
Decomposers feed on the waste and uneaten parts, and release the energy in them as heat; this is energy that had already left the food chain in waste and uneaten parts, so it is not a separate way of losing energy.
- Birds and mammals use a large part of their energy to keep their body temperature steady, so they lose more energy as heat than many other animals. supports K325-11(c)
RUE: the three ways energy is lost along a food chain are Respiration, Uneaten parts and Excreta and egesta.
Decode key
- R = Respiration releases the energy as heat
- U = Uneaten parts, such as bones, are not passed on
- E = Excreta and egesta still contain energy that is not passed on
How much energy is passed on?
| Trophic level | Energy passed on, using the 10% rule (kJ) |
|---|---|
| Producers | If the producers contain 10 000 kJ of energy, this is the energy available to the food chain. |
| Primary consumers | About 10% of 10 000 kJ is passed on, so the primary consumers contain about 1000 kJ. |
| Secondary consumers | About 10% of 1000 kJ is passed on, so the secondary consumers contain about 100 kJ. |
| Tertiary consumers | About 10% of 100 kJ is passed on, so the tertiary consumers contain about 10 kJ. |
The value of about 10% is an average, and the real value varies between food chains, so in an exam use the figures given in the question. supports K325-11(c)
Calculating the efficiency of energy transfer
- Efficiency of energy transfer = (energy in the higher trophic level ÷ energy in the lower trophic level) × 100%.
- Worked example: a trophic level contains 2000 kJ and the next trophic level contains 180 kJ, so the efficiency is 180 ÷ 2000 × 100% = 9%.
- In the same example, the energy lost is 2000 − 180 = 1820 kJ, which is 91% of the energy at the lower trophic level.
Discussing efficiency
- Because only a small proportion of the energy is passed on at each step, there is too little energy left after four or five trophic levels to support more consumers, so food chains usually have only four or five trophic levels.
- A large amount of producers is needed to support a small amount of top consumers, which is why the pyramids of biomass and of energy get narrower towards the top.
- Feeding at a lower trophic level gives the feeder more of the original energy, so a given area of land can feed more people with plant crops than with animals that have been fed on plants. supports K325-11(c)
- Humans can increase the efficiency of energy transfer in farming by keeping animals warm and limiting their movement, which reduces the energy they lose as heat and in activity. supports K325-11(c)
Depth: why and how
- Where does the 90% of energy go that is not passed on? It is lost as heat from respiration, it stays in the excreta and egesta, and it is in the parts that are not eaten, and decomposers finally release it as heat.
- Why can there be more producers than top consumers? Energy is lost at each trophic level, so the producers must capture a large amount of energy to support even a small number of top consumers.
- Why is less energy lost in a shorter food chain? Fewer transfers take place, and energy is lost at each transfer, so more of the original energy reaches the last organism in a shorter food chain.
Common mistakes
All the energy is passed on to the next trophic level.
Only about 10% of the energy is passed on to the next trophic level, and the rest is lost.
Energy is lost because the animals use it up.
Energy is lost as heat from respiration, in excreta and egesta and in uneaten parts, so it is transferred to the surroundings and not destroyed.
Energy that is lost as heat is used again by the producers.
Heat cannot be used by producers to make food, so it is lost from the food chain.
Efficiency is the energy lost divided by the energy at the start.
Efficiency is the energy passed on divided by the energy at the lower trophic level, multiplied by 100%.
Only a small proportion of the energy is passed on at each step, because energy is lost through respiration, excreta and egesta, uneaten parts and decomposers, so food chains are short.
Exam check
Q1 Which is a way in which energy is lost from a food chain?
- A Photosynthesis in the producers
- B Heat released in respiration
- C Absorption of light by chlorophyll
- D Growth of the consumers
Show answer
MCQ answer key B (Heat released in respiration): energy is lost as heat from respiration, and growth stores energy in body tissue that can be passed on.
Q2 A trophic level contains 5000 kJ of energy and the next trophic level contains 400 kJ. What is the efficiency of energy transfer?
- A 12.5%
- B 92%
- C 0.8%
- D 8%
Show answer
MCQ answer key D (8%): efficiency = 400 ÷ 5000 × 100% = 8%.
Q3 Why do food chains usually have only four or five trophic levels?
- A The consumers at the top are too large to be eaten
- B Producers cannot support more than four consumers
- C Too little energy is left after several energy transfers
- D Decomposers eat the top consumers
Show answer
MCQ answer key C (Too little energy is left after several energy transfers): only about 10% of the energy is passed on at each step, so too little energy remains to support more consumers.
Structured In a grassland food chain, the grass contains 20 000 kJ of energy, the grasshoppers contain 2000 kJ and the frogs that eat them contain 150 kJ.
- Calculate the efficiency of the transfer of energy from the grass to the grasshoppers. [1]
- Calculate the efficiency of the transfer of energy from the grasshoppers to the frogs. [2]
- State two ways in which energy is lost between the grasshoppers and the frogs. [2]
- Suggest why a food chain has only a small number of trophic levels. [2]
Total: 7 marks
Show mark scheme
(a) 1 mark
- Mark scheme: 2000 ÷ 20 000 × 100% = 10%
(b) 2 marks
- Mark scheme: 150 ÷ 2000 × 100%
- Mark scheme: = 7.5%
(c) 2 marks
- Mark scheme: heat released in respiration
- Mark scheme: energy in faeces and other waste
- Mark scheme: energy in parts that are not eaten
- Mark scheme: any two of these
(d) 2 marks
- Mark scheme: energy is lost at each trophic level
- Mark scheme: so too little energy is left to support more consumers
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- Pyramids of numbers and biomass
- The carbon cycle and carbon sinks
- Human activities and global warming
- Reducing global warming
- Pollution: sewage, plastic and insecticides
- Conservation and sustainable use
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Notes version 1.0 · Last updated 30 Sept 2026 · Topic 11 of 14