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Photosynthesis, food chains and pyramids Cambridge IGCSE Environmental Management revision

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In plain words

All the energy in your lunch started as sunlight on a leaf. This unit follows that energy from the Sun into plants, then from mouth to mouth along a food chain, and explains why so little of it makes it to the top.

9 things to know

  1. Photosynthesis is the process in which plants make glucose from carbon dioxide and water. Sunlight is the source of energy. Chlorophyll, the green substance in leaves, captures the light energy.
  2. The word equation is: carbon dioxide + water → glucose + oxygen.
  3. A food chain shows what eats what, starting with a producer. The arrows show the direction in which energy flows.
  4. A food web is several food chains linked together. It shows that most organisms eat, and are eaten by, more than one species.
  5. A trophic level is a position in a food chain. Producers make their own food. Primary consumers eat producers. Secondary consumers eat primary consumers. Tertiary consumers eat secondary consumers.
  6. Only about 10% of the energy at one trophic level is passed to the next.
  7. The rest is lost during life processes: movement, respiration, digestion and excretion.
  8. A pyramid of numbers shows the number of organisms at each trophic level. It is not always a pyramid shape: one tree can feed thousands of insects.
  9. A pyramid of energy shows the energy at each trophic level. It is always widest at the bottom, because energy is lost at every step.

Worked example

The producers in a field contain 20 000 kJ of energy. Calculate the energy passed to the secondary consumers.

  1. About 10% passes from one level to the next.
  2. Primary consumers receive 10% of 20 000 kJ = 2000 kJ.
  3. Secondary consumers receive 10% of 2000 kJ = 200 kJ.

Tips and tricks

  • The arrow in a food chain means "is eaten by". It points from the food to the feeder, the way the energy goes.
  • Food chains are short, usually four or five links, because so much energy is lost at each step that too little is left to support another level.
6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Photosynthesis, food chains and pyramids: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. What captures light energy in a leaf?
    • glucose
    • oxygen
    • chlorophyll (the answer)
    • carbon dioxide

    Chlorophyll is the green substance in leaves.

  2. Which are the products of photosynthesis?
    • carbon dioxide and water
    • glucose and oxygen (the answer)
    • glucose and carbon dioxide
    • oxygen and water

    Carbon dioxide and water are what the plant takes in.

  3. What is a primary consumer?
    • an organism that makes its own food
    • an animal that eats other animals
    • an organism that breaks down dead material
    • an animal that eats producers (the answer)

    It is the second trophic level.

  4. About how much energy passes from one trophic level to the next?
    • 1%
    • 10% (the answer)
    • 50%
    • 90%

    The other 90% is lost in life processes.

  5. What do the arrows in a food chain show?
    • the direction of energy flow (the answer)
    • which organism is larger
    • where each organism lives
    • the number of organisms

    From the organism eaten to the organism that eats it.

  6. Producers hold 50 000 kJ. How much energy reaches the tertiary consumers?Stretch
    • 5000 kJ
    • 500 kJ
    • 50 kJ (the answer)
    • 5 kJ

    Three steps of 10%: 5000, then 500, then 50 kJ.

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