Energy flow, energy loss



The Sun is the principal source of energy input to biological systems. The Earth receives 2 main types of energy from the Sun: light (solar) and heat. Photosynthetic plants and some bacteria can trap light energy and convert it into chemical energy.


Non-cyclical nature of energy flow


Heterotrophic organisms obtain their energy by eating plants or animals that have eaten plants. So all organisms, directly or indirectly, get their energy from the Sun. The energy is passed from one organism to another in a food chain but, unlike water and elements such as carbon and nitrogen, energy does not return in a cycle. Energy give out by organisms is lost to the environment.



Energy is lost at each level in the food chain, as in the examples below.


  • Energy lost through the process of respiration (as heat)
  • Energy used up for movement (to search for food, find a mate, escape from predators…).
  • Warm-blood animals (birds and mammals) maintain a standard blood temperature – they lose heat to the environment.
  • Warm-blood animals lose heat energy in faeces and urine.
  • Some of the material in the organism being eaten is not used by the consumer, for example a locust does not eat the roots of maize, and some of the parts eaten are not digestible.


Even plants do not make use of all the light energy available to them. This is because some light:


  • is reflected off shiny leaves
  • is the wrong wavelength for chlorophyll to trap
  • passes through the leaves without passing through any chloroplasts
  • does not fall on the leaves.


On average, about 90% of the energy is lost at each level in a food chain. This means that in long food chains, very little of the energy entering the chain through the producer is available to the top carnivore. So there tend to be small numbers of top carnivores. The food chain below shows how energy reduces through the chain. It is based on maize obtaining 100 units of energy.



locust à




100 units


10 units

1 unit


0.1 unit


On shorter food chains, less energy is lost.

Try this

Figure below shows the flow of energy through a complete food chain:


  1. a) Which form of the Sun’s energy is trapped by the producer? [1 mark]


b) Into which energy form is the Sun’s energy converted when it is trapped by the producer? [1 mark]


  1. a)The first consumer has received 6000 units of energy. How many units of energy (X on the figure) have been passed to the second consumer? [1 mark]

b) How many units of energy (Y on the figure) are lost from the third consumer to the decomposers. [1 marks]

  1. a) Suggest why the proportion of the energy intake which a producer loses to the environment (20%) is smaller than that lost to the environment by a first consumer (30%). [2 marks]

b) Many countries have difficulty in producing enough food for their population. How might it help to overcome this problem if humans were always fed as first consumers, rather than second or third consumers? [3 marks]




  1. a) Light (or solar) energy


b) Chemical energy


  1. a) 1200 units


b) 48 units


  1. a) The consumer may be warm-blooded, so some energy is lost as heat. Consumers usually move around to find food, a mate, or escape from predators, which uses up energy, but producers do not move.


b) Feeding as a first consumer involves eating plants. Less energy is lost to the environment when feeding at this level, so food production is more efficient in terms of energy conservation.


Food chain


Food chain is a chart showing the flow of energy (food) from one organism to the next beginning with a producer.






    • o mahogany tree à caterpillar à song bird à hawk
    • o maize à locust à lizard à snake





    • A food chain usually starts with a photosynthetic plant, which gains its energy from the Sun.
    • The arrows used to link each organism to the next represent the direction of energy flow. They always points towards the ‘eater’, and away from the plant.
    • The feeding level is known as the trophic level.
    • Plant are producers (they make/produce food for other organisms).
    • Animals that eat plants are primary consumers (a consumer is an ‘eater’). They are also called carnivores.

Examiner’s tips


    • Make sure you can write a food chain involving 3 consumers, with the arrows in the correct direction.
    • Don’t include the Sun (it is not an organism).
    • Always start with the producer on the left of diagram.
    • Practice labeling each trophic level in your food chain under the organisms (producer, primary consumer, etc.).
    • Don’t waste time drawing plants and animals: this will not get you any extra marks.


Common misconceptions


Marks are often lost when students write out food chains and webs because they draw the arrows the wrong way round or put the chain back-to-front (or both). The following example was seen in a recent paper:

jackal à sheep à grass

This student is suggesting that grass eats sheep and sheep eat jackals!

Food web

Food web is a network of interconnected food chains showing the energy flow through part of an ecosystem.




These are a more accurate way of showing feeding relationships than food chains, because most animals have more than one food source. For example, in the food webs in figure below, the leopard feeds on baboons and impala.




The leopard can be placed at 2 different trophic levels:


    • secondary consumer (feeding on imlala)
    • quaternary or fourth level consumer (feeding on baboons).

Another example of food web.




Food webs are easily unbalanced, especially if one population of organisms in the web dies or disappears. This may happen for a number of reasons, including:


    • over–predation or hunting
    • disease
    • pollution
    • use of pesticides
    • lack of food (or other resources)
    • emigration.


For example, in the food web here, if all the baboons were killed by hunters the leopard would have only impala to eat. So the impala population would decrease. The scorpion population may increase because of less predation by baboons, but if there are more scorpions they will eat more locusts, reducing the locust population, and so on.

Try this

Figure below shows a food web:



  1. Select appropriate organisms form the food web to complete each column in the table below. [4 marks]







Organism 1


Organism 2



  1. Ladybirds eat aphids. A very large number of ladybirds arrive in the habita where these organisms live. Predict some of the possible effects this could have on the organisms in the above food web.

[6 marks]






  1. 6 suggestion such as:decrease in aphids because ladybirds eat aphids


    • increase in ivy because there will be fewer aphids feeding
    • decrease in wrens because there are fewer aphids to eat
    • decrease in caterpillars because the wrens now have only caterpillars for food
    • increase in oak trees because there will be fewer aphids feeding
    • increase in hogweed because there will be fewer aphids feeding
    • increase in goldfinches because there is more hogweed to eat.


There are other possible suggestions.

Food pyramids of numbers, biomass and energy



food pyramid shows the relative sizes of different components at the various trophic levels of a food chain. There are three types of ecological pyramid we use: numbersbiomass and energy.


In a food pyramid, each trophic level in a food chain is represented by a horizontal bar, with the width of the bar representing the number of organisms, the amount of biomass or the amount of energy available at that level. The base of the pyramid represents the producer; the second level is the primary consumer; and so non.


  1. Pyramids of numbers


A pyramid of numbers shows the relative number of organisms at each stage of a food chain.


Example 1: clover → snail → thrush → hawk



Clover is a plant and it is the producer in this food chain. Its bar goes at the bottom of the pyramid.


Energy is lost to the surroundings as we go from one level to the next, so there are fewer organisms at each level in this food chain. A lot of clover is needed to support the snail population. A thrush eats lots of snails, and a hawk eats lots of thrushes, so the population of hawks is very small.

Other pyramid shapes


Sometimes the pyramid of numbers doesn't look like a pyramid at all. This could happen if the producer is a large plant such as a tree, or if one of the animals is very small. Remember, though, that whatever the situation, the producer still goes at the bottom of the pyramid.


Here are two examples like this:


Example 2: Oak tree → Insects → Woodpecker


An oak tree is very large so many insects can feed on it.




Example 3: Grass → Rabbit → Flea


Fleas are very small and lots of them can feed on a rabbit.



  1. Pyramids of biomass


Sometimes a pyramid of numbers is not the best way to represent a food chain. In this case a pyramid of biomass (the dry mass of an organism) is a better diagram to use. It shows the total mass of organisms at each stage of a food chain.


In general, all producers have a higher biomass than the primary consumer, so a pyramid will always be produced.


The total energy (and biomass) present at a lower tier of the pyramid, must be greater than the higher tiers in order to support the energy requirements of the subsequent organisms.



Pyramid of numbers and pyramid of biomass

  1. Pyramids of energy


Pyramid of energy shows amount of energy trapped per unit time and area at each stage of a food chain.




A normal-shaped pyramid is always produced because there is a reduced amount of energy at each successive level.


Food chain and energy efficiency

In term of conversations of energy, there is an increased efficiency in supplying green plants as human food and a relative inefficiency in feeding crop plants to animals.




Short food chains are more efficient than long ones in providing energy to the top consumer. Below are two food chains and energy values for each level in them. Both food chains have a human being as the top consumer.

maize → cow → human unit of energy 100 10 1

maize → human

unit of energy 100 10

Ten times more energy is available to the human in the second food chain than in the first. In the second food chain, the human is a herbivore (vegetarian). But eating parts of a cow provide humans with other nutrients, as well as those we gain energy from – it would be very difficult to persuade everyone to become vegetarian for the sake of energy efficiency.




Some farmers try to maximize meat production by reducing movement of their animals (keeping them in pens or cages with a food supply) and keeping them warm in winter. This means less stored energy is wasted by the animals.


Why food chains usually have fewer than 5 trophic levels?


As the energy is passed along the chain, each organism uses some of it. So the further along the chain you go, the less energy there is. The loose of energy along the food chain limits the length of it. There rarely more than 5 links in a chain, because there is not enough energy left to supply the next link. Many food chains only have 3 links.


Nutrient cycles - Carbon and water cycles


Most of the chemicals that make up living tissue contain carbon. When organisms die the carbon

is recycled so that it can be used by future generations.


Four main processes are involved:

photosynthesisrespirationdecomposition combustion.


  1. Carbon cycle



Carbon cycle

    1. Carbon enters the atmosphere as CO2 from respiration and





Credit: BBC Bitesize


    1. CO2 is absorbed by producers to make carbohydrates in




    1. Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as CO2 formed during respiration. The animals and plants eventually die.




    1. The dead organisms are eaten by decomposers and the carbon in their bodies is returned to the atmosphere as CO2. In some