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Biology · 19 Organisms and their environment

19.1 Energy flow

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Before you start

  • Photosynthesis: plants use light energy, carbon dioxide and water to make glucose.
  • Respiration: cells break down glucose to release energy for life processes.
  • That energy can be stored in chemical substances and can be transferred as heat.
  • The idea of a food chain, even informally: grass is eaten by a rabbit, which is eaten by a fox.
Syllabus objectives covered (2)
  • State that the Sun is the principal source of energy input to biological systems
  • Describe the flow of energy through living organisms, including light energy from the Sun and chemical energy in organisms, and its eventual transfer to the environment

Part 1 of 5

Everything living is running on sunlight

  1. Start with the sentence the syllabus actually wants from you: the Sun is the principal source of energy input to biological systems. If a question asks where the energy in an ecosystem comes from, that is the answer, in those words.
  2. Notice the word principal. It means main, not only. Write "the principal source" or "the main source" rather than "the only source" — you are describing where essentially all of the energy in a food chain came from, not making a claim that nothing else exists.
  3. The energy enters at one place and one place only: a producer. In every food chain you will be shown, the first organism is a plant (or an alga) that can capture light. Consumers cannot take energy from sunlight — a rabbit sitting in the sun is warmed by it, but it gains no usable energy from it.
  4. So the whole of the rest of this topic hangs on one doorway. Light arrives, a producer photosynthesises, and from that moment on the energy is inside living things.

Part 2 of 5

Doorway one: light energy becomes chemical energy

  1. A green plant absorbs light energy using chlorophyll and uses it to build glucose from carbon dioxide and water. That is photosynthesis, which you already know as a reaction. Here you need it as an energy statement.
  2. The energy statement is: light energy from the Sun is transferred into chemical energy stored in the plant's organic nutrients. The light has not disappeared — it is now held in the bonds of glucose, and then in the starch, cellulose, proteins and fats the plant makes from it.
  3. This is the only step in the whole ecosystem where new energy comes in. Every later step just moves that same energy around, or lets it escape.
  4. Watch your wording. "The plant gets energy from the Sun" is loose. "The plant absorbs light energy and converts it to chemical energy in glucose during photosynthesis" names the form before, the form after, and the process. That is what an examiner is looking for.

Part 3 of 5

Feeding moves the chemical energy along

  1. When an animal eats a plant, it digests the plant's molecules and absorbs them. Some of that material is built into the animal's own body. The energy has changed owner but not form — it was chemical energy in the plant, and it is chemical energy in the animal.
  2. So say it precisely: energy is transferred from one organism to the next by feeding. An animal's energy comes from its food, not from the Sun directly, even though the Sun is where that energy originally came from. Both halves of that sentence are true and examiners want the distinction.
  3. Not everything gets eaten. Dead plants, dead animals and waste materials still contain chemical energy, and decomposers feed on them. That is how the energy in an uneaten leaf still gets used rather than being locked away.

Part 4 of 5

Doorway two: the energy leaves as heat

  1. Every organism in the chain respires — plants included, all the time, not just animals. Respiration breaks down glucose and releases the stored chemical energy so the organism can use it.
  2. Use it for what? Movement, active transport, cell division, making new proteins, and in birds and mammals, keeping the body warm. Each of those transfers happens inside a living body.
  3. Here is the part the objective is really about. Whenever energy is transferred in a living organism, some of it is transferred to the surroundings as thermal energy — heat. Over time, all of the energy that entered the ecosystem ends up in the environment as heat.
  4. That heat spreads out into the air, soil and water. No organism can capture it and put it back into a molecule. It is out of the biological system for good.
  5. Learn this sentence and use it whenever an answer needs it: energy is eventually transferred to the environment as heat, released during respiration. "The energy is lost" on its own earns nothing — lost how, and to where? Those two words are the mark.

Part 5 of 5

Why it is called a flow and not a cycle

  1. Put the two doorways together. Energy comes in at the producer as light, travels along the chain as chemical energy in food, and leaks out at every organism as heat. It never comes back in.
    Sunlight energychemical energy, passed on by feedinglettuceproducerslugprimary consumerthrushsecondary consumersparrowhawktertiary consumerheatheatheatheatthe surroundings — energy leaves as heat, released in respirationenergy enters once, at the producer, and leaks at every organism
    Every heat arrow is respiration, and none of that energy ever comes back.
  2. A one-way journey with a permanent exit has a consequence: the supply has to keep arriving. If the Sun stopped shining, producers could not photosynthesise, no new chemical energy would enter, and organisms would go on respiring until the stored energy ran out.
  3. Contrast this with the atoms. Carbon and nitrogen atoms are used, released and used again — they go round and round, which is why 19.3 is called nutrient cycles. Energy does not do that. It flows through, once.
  4. Examiners test this contrast directly, so hold both halves: nutrients cycle and are reused; energy flows in from the Sun and out to the environment as heat, and must be constantly resupplied.

Worked examples

Example 1. Describe the flow of energy through the food chain grass → rabbit → fox, from its source to its final destination. (4 marks)

Show the working
  1. A four-mark Describe on energy flow always wants four separate transfers, in order. Plan them before you write: in, converted, passed on, out.
  2. Start at the source, because the first mark is almost always for naming the Sun. Light energy from the Sun reaches the grass.
  3. Name the conversion and the process together. The grass absorbs the light energy and, in photosynthesis, converts it to chemical energy stored in its organic nutrients.
  4. Now move it along, naming the mechanism. Chemical energy is transferred from the grass to the rabbit, and from the rabbit to the fox, by feeding.
  5. Finish at the exit, in full. Each organism respires, releasing the stored chemical energy, and some energy is transferred to the environment as heat at every stage.
  6. Read it back and check you never wrote the bare word "lost". If you did, add "as heat, from respiration" to it.

Answer: Light energy from the Sun is absorbed by the grass and converted, in photosynthesis, to chemical energy in its organic nutrients. This chemical energy is transferred to the rabbit and then to the fox by feeding. Each organism respires, and energy is transferred from each of them to the environment as heat.

Example 2. A student says: "Energy is recycled in an ecosystem in the same way that carbon is." Explain why this statement is wrong. (3 marks)

Show the working
  1. An Explain that begins from a wrong statement wants you to say what actually happens, not just "it is wrong". Deal with the energy first, then the comparison.
  2. State the exit route for energy. At every stage, energy is transferred to the environment as heat released in respiration.
  3. Say why that ends the story for that energy. Heat spreads out into the surroundings and cannot be taken back up by any organism, so it is not available to be passed on again.
  4. Now give the contrast that makes the statement wrong. Carbon atoms are returned to the air and soil and are taken in again by producers, so they are reused; energy is not, which is why the Sun must supply it continuously.

Answer: Energy is transferred to the environment as heat during respiration at every stage, and that heat cannot be recaptured by any organism, so it is not passed on again. Carbon atoms are released and taken up by producers again, so they are recycled; energy is not, and must be constantly resupplied by the Sun.

Example 3. A sealed glass sphere contains soil, water, a small plant and a snail, and is kept on a sunny windowsill. Suggest what would happen inside it if it were moved into a completely dark cupboard. (3 marks)

Show the working
  1. Suggest means apply what you know to a new situation. The situation has changed one thing only — the light input — so start there and follow the energy.
  2. Remove the input. In the dark the plant cannot photosynthesise, so no light energy is converted to chemical energy and no new energy enters the sphere.
  3. Keep the outflow running, because respiration does not stop. Both the plant and the snail continue to respire, and energy continues to be transferred to the surroundings as heat.
  4. State the consequence in energy terms. With energy leaving and none entering, the store of chemical energy in the organisms runs down, so the organisms eventually die.
  5. Notice this is exactly the point about flow rather than cycling — inside a sealed sphere, atoms can be reused, but energy cannot be, so light is the one thing that has to keep coming in.

Answer: In the dark the plant cannot photosynthesise, so no light energy is converted into chemical energy and nothing new enters. Both organisms continue to respire and transfer energy to the surroundings as heat, so the chemical energy stored in them runs out and they die.

Where people go wrong

Mistake: Writing "the energy is lost" as the whole answer.

Instead: Write "energy is transferred to the environment as heat, released during respiration".

Energy is never destroyed, so "lost" describes nothing an examiner can credit. The mark is for naming where it goes (the environment/surroundings), in what form (heat), and from which process (respiration).

Mistake: Saying that animals get their energy from the Sun.

Instead: Animals get their energy from their food; the Sun is where that energy originally entered the food chain, through a producer.

Consumers have no way of converting light energy into chemical energy. Only producers can, and only in photosynthesis, so the Sun's energy reaches an animal second-hand.

Mistake: Thinking plants photosynthesise and animals respire, as though plants do not respire.

Instead: All living organisms respire, all the time, including plants.

If plants did not respire they could not release the energy stored in their glucose to use it. Forgetting this makes the plant look like a place where energy stops leaking out, which it is not.

Mistake: Saying energy is recycled in an ecosystem.

Instead: Nutrients such as carbon and nitrogen are recycled; energy flows through and leaves as heat.

Once energy has been transferred to the surroundings as heat, no organism can take it back up, so it cannot go round again. This is exactly why the Sun's input has to be continuous.

Mistake: Saying that light energy is converted into food.

Instead: Light energy is converted into chemical energy, which is stored in the organic nutrients that make up food.

Food is a material; energy is not a material. Naming the two forms — light energy and chemical energy — is what earns the conversion mark.

Check yourself

  1. State the principal source of energy input to biological systems.
    AnswerThe Sun.
  2. At which organism in a food chain does energy enter, and by which process?
    AnswerAt the producer, by photosynthesis — the producer absorbs light energy and converts it to chemical energy in its organic nutrients.
  3. Name the form the energy is in as it passes from a rabbit to a fox, and the process that transfers it.
    AnswerChemical energy, stored in the rabbit's organic molecules, transferred by feeding.
  4. Complete this sentence so that it would earn a mark: "At each stage in a food chain, energy is …"
    Answer"… transferred to the environment as heat, released during respiration." The words heat, environment/surroundings and respiration are all doing work.
  5. Why can the heat released by a respiring animal not be used by the animal that eats it?
    AnswerThe heat has already been transferred to the surroundings and spreads out. No organism can absorb it and store it as chemical energy again, so it has left the biological system.
  6. Explain in one sentence why the Sun's energy must keep arriving, when carbon atoms do not have to keep arriving.
    AnswerEnergy leaves as heat at every stage and cannot be reused, so it must be replaced continuously; carbon atoms are released and taken up by producers again, so the same atoms are used over and over.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A simple grassland food chain is: grass → grasshopper → lizard.

(a) State the principal source of energy input to this food chain. 1 mark No calculator

Mark scheme
  • B1 the Sun / (light energy from) sunlight

(b) Describe how energy from this source becomes chemical energy in the grass. 2 marks No calculator

Mark scheme
  • B1 light energy is absorbed by the grass (by chlorophyll) and used in photosynthesis
  • B1 the light energy is converted to chemical energy stored in organic nutrients / glucose / starch

(c) Explain why the energy that entered this food chain is eventually transferred to the environment. 3 marks No calculator

Mark scheme
  • B1 all the organisms respire / respiration occurs in the grass, grasshopper and lizard
  • B1 respiration releases the stored chemical energy, which is used for processes such as movement, active transport or growth
  • B1 energy is transferred to the surroundings as heat / thermal energy ('energy is lost' alone is insufficient)

(d) Suggest what would happen to the organisms in this food chain if the input of light energy stopped completely. 2 marks No calculator

Mark scheme
  • B1 the grass could not photosynthesise, so no new chemical energy would enter the food chain
  • B1 organisms would continue to respire and transfer energy to the surroundings, so their energy stores would run out and they would die, ORA

Question 2. A student writes in her notes: "Plants get their energy from minerals in the soil, and this energy is then recycled around the ecosystem."

(a) Identify the actual source of a plant's energy input, and the process by which the plant takes it in. 2 marks No calculator

Mark scheme
  • B1 light energy from the Sun
  • B1 photosynthesis

(b) Explain why energy is not recycled within an ecosystem. 3 marks No calculator

Mark scheme
  • B1 at each stage energy is transferred to the environment as heat, released in respiration
  • B1 this heat spreads out into the surroundings and cannot be taken up / converted back into chemical energy by organisms
  • B1 so energy passes through the ecosystem once and must be replaced continuously by light from the Sun

How this is examined

  • Where the energy in a food chain finally goes: A two- or three-mark Explain, often at the end of a question about a food chain or a pyramid: "Explain what happens to the energy at each stage of this food chain." You must name the destination, the form and the process: "energy is transferred to the environment (surroundings) as heat, which is released during respiration". A third mark usually comes from adding that this heat cannot be used by other organisms, so it is not passed on. The bare phrase "energy is lost" scores nothing on its own.
  • The conversion of light energy into chemical energy by a producer: A two-mark Describe near the start of an ecology question: "Describe how the energy in this food chain originally entered it." One mark for "light energy from the Sun is absorbed by the producer/plant (using chlorophyll) in photosynthesis", one for "it is converted to chemical energy stored in organic nutrients such as glucose". Naming both forms of energy is what separates the two marks.
  • Energy flows through an ecosystem but nutrients cycle: A two- or three-mark Explain, often phrased as a challenge: "Explain why energy must be supplied continuously to an ecosystem." "Energy is transferred to the surroundings as heat at every stage" plus "heat cannot be recaptured or reused by organisms" plus "so new energy must enter continuously as light from the Sun". If the question mentions carbon or nutrients, add that atoms are recycled by contrast.
  • The Sun as the principal energy input: A one-mark State, very often the opening part of a longer ecology question. "The Sun" (or "light energy from the Sun"). Say principal or main rather than only, and do not lose the easy mark by naming a producer instead — the plant is where energy enters, the Sun is where it comes from.

Words to know

Energy flow
The one-way passage of energy through an ecosystem: in as light at the producer, along the chain as chemical energy in food, out to the environment as heat.
Chemical energy
Energy stored in the bonds of organic molecules such as glucose, starch, fats and proteins. This is the form energy travels in between organisms.
Photosynthesis
The process in which a producer absorbs light energy and converts it into chemical energy stored in organic nutrients.
Respiration
The process in all living cells that releases the chemical energy in food for the organism to use. Some of the energy released is transferred to the surroundings as heat.
Producer
An organism that makes its own organic nutrients, usually using energy from sunlight in photosynthesis. The point at which energy enters a biological system.
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