Biology - Energy in Food Chains (AQA Syllabus A)

In GCSE Science, students will learn about the transfer of energy and biomass I food chains. This is the first of three quizzes on that topic and it looks specifically at the transfer of chemical energy as biomass and how each stage of the food chain sees the transfer of less energy from one organism to another.

Apart from the extremophiles (some of whom get their energy from chemicals and heat coming from deep-sea hydrothermal vents), animals are solar powered! How come? The answer is food chains. Food chains are concerned with the transfer of chemical energy as biomass to the top predator. They always begin with plants and end with a carnivore and they can be combined into food webs. A food web describes the feeding relationships within a community of plants and animals.

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A food chain can be represented as a pyramid of biomass (mass of living material) by drawing a pyramid style of histogram. Each trophic (feeding) level is drawn to the same scale. The lowest level of the food chain normally has the highest biomass whilst the top predator has the lowest biomass. The reason for this is that energy and materials are lost at each stage of the food chain. Let's consider a simple 3 step food chain to see how this works.

The start of a food chain are the plants living in the community. They absorb the Sun's energy and use it for photosynthesis. During photosynthesis, some of the food (glucose) they produce is used to release energy in the process of respiration and some is used to form the biomass of the plant.

Herbivores eat the plants. Some of the energy from the plants they eat is used immediately for respiration and therefore lost from the food chain. Some is stored in the body or used by the body for growth, repair or both, this keeps the biomass in the food chain. The herbivores excrete waste material which lessens the amount of biomass in the system.

Next in the food chain is a predator. This hunts and eats the herbivores. As it hunts, a lot of the energy from its previous successful kill is used up and lost from the food chain. More is lost as the predator excretes its wastes. Only a proportion of the energy from eating the prey remains in the predator.

OK, let's see what you know about energy and food chains now...

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  1. For every 200kJ a gazelle gets from eating, 125kJ are excreted and 65kJ are used in respiration so how much of the original energy would be passed on to a lion that caught and ate it?
    Most of the biomass that animals eat is either not digested, or used to provide the energy needed for staying alive. In this case, 190 kJ of the original 200kJ are lost meaning 10kJ remain in the gazelle which is 5%.
  2. Students investigated a food chain in a garden. The students found 650 aphids feeding on one rose plant. Five ladybirds were feeding on the aphids. Which of the following statements about the biomass of the ladybirds is true?
    The higher up the food chain you get, the lower the biomass - so the biomass of the ladybirds must be lower than the biomass of the aphids and the rose
  3. During photosynthesis, which of the following is the energy transfer?
    The Sun's energy is stored as chemicals in the plant so when we refer to the energy in a food chain we are talking about chemical energy. Chemical energy is transferred up the food chain as biomass
  4. What is the source of energy for most food chains?
    The sun is the ultimate source of energy for most life on Earth. Plants use it for photosynthesis and this gets passed on up the food chain. There are some food chains in the deepest parts of the ocean where the energy comes from biochemical reactions instead of sunlight
  5. In a food chain what is meant by the word producer?
    Plants are called producers because they produce their own food which is the energy resource for the entire food chain
  6. Students measured the biomass of a community living in leaf litter under the trees in the school conservation area. They found woodlice feeding on the leaves, ground beetles feeding on the woodlice and centipedes feeding on the beetles. Each student weighed the leaves in their sample and the other organisms and put their results together with the rest of the class. Which of the following could represent the class results in order of the food chain starting from the leaves?
    This question tests whether or not you recognise the pattern that the biomass is less at each level
  7. How much of the Sun's energy produces biomass in plants?
    Plants only use a very small amount of the sunlight landing on them to create and store biomass
  8. Other than light, what else is required for photosynthesis?
    Chlorophyll is contained in the plant's chloroplasts. Carbon dioxide enters the plant's leaves through the stomata. Water is transported from the roots to the leaves by the xylem tubes
  9. In a food chain, what does the arrow represent?
    Each level of the food chain is called a trophic level. The arrows show how energy is passed from one trophic level to the next
  10. In one year, the Sun supplies 4 x 106kJ to a particular habitat, 15kJ reaches the top predator. What percentage of the Sun's energy is transferred to the predator? Try working it out rather than trying to guess.
    Divide the amount of energy reaching the predator by the total energy from the Sun and then multiply by 100 for the percentage

Author: Kev Woodward

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