19.1 Energy Flow

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Understand that the Sun is the principal source of energy input to most biological systems
  2. Explain why most forms of life are completely dependent on photosynthesis
  3. Describe the flow of energy through food chains and webs, including energy from light, energy in living organisms, and its eventual transfer to the environment
  4. Construct and interpret simple food chains
  5. Understand the terms: producer, consumer, herbivore, carnivore, and decomposer
  6. Describe food webs as networks of interconnected food chains, and construct and interpret them
  7. Explain why the transfer of energy from one trophic level to another is inefficient
  8. Explain why food chains usually have fewer than five trophic levels
  9. Explain why it is more energy efficient for humans to eat crop plants than to eat livestock fed on crop plants
  10. Construct and interpret pyramids of numbers, biomass, and energy

Main Content

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1. The Sun: The Principal Source of Energy

  • Almost all life on Earth depends on energy from the Sun.
  • The Sun produces energy in the form of light (and heat). This light energy travels through space and reaches the Earth.
  • Plants, algae, and some bacteria are able to capture this light energy and convert it into chemical energy stored in food (glucose).
  • This process is called photosynthesis (explained in detail in the next section).
  • Without the Sun constantly supplying energy, almost no life on Earth could survive.

Think of it this way: The Sun is like a giant battery charger. It keeps charging up life on Earth by providing the energy that plants trap and pass on to everything else.

2. Why Most Life is Completely Dependent on Photosynthesis

  • Photosynthesis is the process by which plants (and algae) use light energy from the Sun to make glucose (a type of sugar) from carbon dioxide and water.

  • The glucose made during photosynthesis contains stored chemical energy. This energy originally came from sunlight.

  • Almost every living thing on Earth either:

    • Carries out photosynthesis itself (plants, algae), OR
    • Eats something that carries out photosynthesis (animals eating plants), OR
    • Eats something that ate a plant (a lion eating a zebra that ate grass)

  • This means that photosynthesis is the entry point for energy into almost every food chain and web on Earth.

  • Without photosynthesis, there would be no glucose, no stored energy in plants, and therefore no energy for any animal to consume. All food chains would collapse.

  • Even animals that eat other animals ultimately trace their energy back to a plant that photosynthesised.

Key point: Photosynthesis is the only process that brings new light energy into the living world. Without it, life as we know it could not exist.

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