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  1. State that the Sun is the principal source of energy input to biological systems

The Sun is the principal source of energy input to biological systems.

If you’re having trouble understanding why, visualise a food web. Carnivores gain their energy from consuming other animals, herbivores gain their energy from consuming plants, and plants gain their energy from the Sun.

Therefore, the Sun is the ultimate source of energy in all biological systems.


  1. Define the terms:
    • Food chain as showing the transfer of energy from one organism to the next, beginning with a producer
    • Food web as a network of interconnected food chains
    • Producer as an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
    • Consumer as an organism that gets its energy by feeding on other organisms
    • Herbivore as an animal that gets its energy by eating plants
    • Carnivore as an animal that gets its energy by eating other animals
    • Decomposer as an organism that gets its energy from dead or waste organic matter

Definitions are important – learn ‘em.


  1. Define the terms:
    • Ecosystem as a unit containing all of the organisms and their environment, interacting together, in a given area, e.g. a lake
    • Trophic level as the position of an organism in a food chain or food web

Learn these definitions too!


  1. Describe how energy is transferred between trophic levels

Energy is transferred from one trophic level to the next when a consumer eats another organism and gains the energy-rich molecules from its body.


  1. Explain why food chains usually have fewer than five trophic levels

In any self-respecting food chain, the first item is the Sun. Then comes the producer (the first trophic level), then the primary consumer (the second trophic level), the secondary consumer (third trophic level), sometimes there’s a tertiary consumer (fourth trophic level), and even more rarely, there’s a quaternary consumer (the fifth trophic level).

We all know that the Sun has an amazing amount of energy.

The proportion of that energy that arrives at Earth is tiny.

Of all the energy that does reach Earth, slightly less than 34% is reflected back to space by clouds.

The Earth itself reflects another 66% back to space.

Less than one percent of the total energy that reaches Earth is used by plants for photosynthesis.

From here on, around 10% or less is passed to the next trophic level.

The rest is lost as heat, sound, in metabolism, used for movement, in faeces, etc.

So why do food chains usually have less than five trophic levels?

As explained earlier, a lot of energy is lost between trophic levels.

The most energy is available for producers, so there are a large number of them. Less energy is available for primary consumers, so there are fewer primary consumers, and even fewer secondary consumers, etc.

Due to the loss in energy along the food chain, it makes it very difficult for a fifth consumer (the sixth trophic level) to exist.


  1. Construct simple food chains

This is pretty simple. All you have to do is apply what you’ve learned so far about food chains and make one!


grass -> rabbit -> fox

cabbage -> cabbage worms -> spiders -> birds


  1. Interpret food chains and food webs in terms of identifying producers and consumers

As a general rule, producers are the organisms that don’t consume anything else – they create their own organic nutrients. Consumers eat other organisms.


  1. State that consumers may be classed as primary, secondary and tertiary according to their position in a food chain

Consumers may be classed as primary, secondary and tertiary according to their position in a food chain.

Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumes.


  1. Identify the producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers as the trophic levels in food webs and food chains.

Producers are the organisms that don’t consume anything else. Primary consumers consume producers, secondary consumers consume primary consumers, tertiary consumers consume secondary consumers, and quaternary consumers consume tertiary consumers.



Notes submitted by Sarah

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