By the end of this lesson
- Describe how substances move along a food chain.
- Describe how energy is transferred along a food chain.
- Distinguish the cycling of matter from the flow of energy.
BIG QUESTION
Does a fox contain atoms that were once in grass?
Matter moves along food chains and returns to the environment; energy flows through and is eventually lost as heat.
Substances pass on when organisms are eaten
Producers build carbohydrateA food molecule made of carbon, hydrogen and oxygen, such as glucose, starch and glycogen, used mainly as an energy source., proteins and lipids from carbon dioxide, water and mineral ions. When a consumer eats the producer, it digests and absorbs these molecules and uses them to build its own tissues.
The carbon, nitrogen and other elements in a fox may therefore have been in grass, then in a rabbit. When organisms die, decomposers return these elements to the environment, so matter is recycled.
Energy flows in one direction
Light energy is transferred to chemical energyEnergy stored in the bonds of organic molecules, such as glucose, which is released by reactions such as respiration. by photosynthesis. When an organism is eaten, some of the chemical energy stored in its biomass passes to the consumer.
At every level, respiration releases energy, which is lost to the surroundings, ultimately as heat. Energy is not recycled; ecosystems need a continuous input of light energy.
Not everything is passed on
Some material is not eaten (roots, bones), some is not digested and is egested, and some is excreted, for example as urea. These substances and their energy pass to decomposers instead.
This explains why less biomass and energy is available at each higher trophic level.
Matter and energy in a food chain
Use “cycle” for matter and “flow” for energy.
| Feature | Substances (matter) | Energy |
|---|---|---|
| Enters the chain | CO₂, water and mineral ions taken up by producers. | Light absorbed by chlorophyll. |
| Passed on | Organic molecules eaten, digested and assimilated. | Chemical energy stored in biomass. |
| Leaves each level | Egestion, excretion, death and respiration (CO₂). | Heat from respiration, energy in waste and uneaten parts. |
| Fate | Recycled by decomposers. | Not recycled; finally lost as heat. |
Language that earns precision
- assimilation
- Incorporating absorbed food molecules into the body’s own tissues.
- chemical energy
- Energy stored in the bonds of organic molecules.
- recycling
- Return of elements to the environment so that they can be used again.
Common misconceptions
Claim: Energy is recycled like carbon.
Correction: Energy flows through ecosystems and is eventually lost as heat; only matter is recycled.
Claim: Plants get their food from the soil.
Correction: Plants make organic molecules by photosynthesis; the soil supplies water and mineral ions.
Claim: Energy is “used up”.
Correction: Energy is transferred to the surroundings, mainly as heat, not destroyed.
Match the answer to the command word
Name the process at each step: photosynthesis, feeding, digestion, absorption, assimilation, respiration.
Say “chemical energy stored in biomass”, not “energy in food is used up”.
Recall, apply, then justify
Describe how carbon in grass can become part of the body of a fox.
Hint
Follow the carbon through the rabbit.
Suggested marking points
- A rabbit eats the grass and digests the carbon compounds.
- The products are absorbed and used to build rabbit tissue.
- The fox eats the rabbit and assimilates the carbon compounds into its own tissues.
Explain why energy, unlike carbon, is not recycled in an ecosystem.
Hint
Where does the energy end up?
Suggested marking points
- Energy released by respiration is lost to the surroundings as heat.
- Heat cannot be used by producers, so new light energy must enter.
A seed company claims its plants “take their food from the soil”. Evaluate this claim using your knowledge of food chains.
Planning hint
What does soil provide, and what does photosynthesis provide?
What a strong design includes
- Plants make organic molecules (glucose) by photosynthesis.
- The carbon comes from carbon dioxide in the air.
- The energy comes from light, not soil.
- Soil supplies water and mineral ions such as nitrates and magnesium.
- Minerals are needed to make proteins and chlorophyll.
- So the claim is incorrect for the main source of biomass, though soil nutrients are essential.