Energy Flow in Ecosystems

Sunlight lands on a leaf. Weeks later, some of that same energy is powering the muscles of a hawk. Food chains and food webs are simply maps of that journey — and the arrows mean something very specific.

Where the chemical energy comes from

Photosynthesis is the doorway. Producers absorb light energy and use it to build carbon compounds from carbon dioxide and water.

The glucose that comes out is more than food. Plants convert it into a whole range of carbon compounds:

• Starch — the storage form of glucose in plant cells.
• Cellulose — the tough material that makes up cell walls.
• Lipids — energy-dense storage, especially in seeds.
• Amino acids and proteins — built by adding nitrogen taken up from the soil.

Every one of those molecules holds chemical energy in its bonds. That is the energy the rest of the ecosystem lives on.

What happens when something gets eaten

Follow a single mouthful. A rabbit bites off a blade of grass and swallows it.

1. The rabbit digests the plant tissue, breaking large molecules into small soluble ones.
2. It absorbs those small molecules through its gut wall into the blood.
3. Some are respired straight away, releasing energy to power the rabbit’s life processes.
4. The rest are built up into rabbit tissue — muscle, fat, skin. The chemical energy is now stored in the rabbit.

Only that last portion is available to whatever eats the rabbit next. That is the whole reason food chains get shorter and shorter as you go up, which we will look at properly on the Energy Losses page.

Food chains and what the arrows mean

A food chain shows one feeding pathway through an ecosystem.

This is worth labouring because it is the single most common slip in the whole topic. The arrow does not mean “eats”. It means “is eaten by”, because it is tracking where the energy goes.

Food webs

Real ecosystems are messier than a single chain. Most animals eat several things and are eaten by several things. A food web draws all those chains together.

Reading a web is a common Paper 1 question. Two skills come up again and again:

• Pull out a food chain. Start at a producer and follow arrows upwards, writing each organism in order.
• Predict a knock-on effect. If one species is removed, follow the arrows outwards. Anything that ate it has less food; anything it ate is now less heavily eaten.

What happens to the energy in dead bodies

Not every organism gets eaten. Plenty die of old age, disease or cold. The chemical energy locked in their tissues does not vanish — it passes to the decomposers.

Detritivores usually go first, tearing tissue into smaller pieces and hugely increasing the surface area. Saprotrophs then finish the job, releasing inorganic nutrients back into the soil.

Why energy flow is one-way

At every single step — producer, consumer, decomposer — organisms respire, and respiration releases heat. Heat spreads out into the surroundings and no organism can capture it and put it back into a carbon compound.

So energy enters as light, gets passed along a few times, and dribbles away as heat the whole way. It never comes back round. Matter, as you saw on the last page, does come back round. Keep the two straight.