Every ecosystem runs on borrowed sunlight. The energy arrives, gets passed along from one living thing to the next, and then leaks away as heat — it never comes back. Matter behaves completely differently: the same atoms get used again and again forever. Once you can keep those two ideas apart, most of this topic falls into place.
📘 What you need to know
Ecosystems are open systems — both energy and matter cross the boundary.
Energy flows through an ecosystem in one direction. Matter cycles round inside it.
First law of thermodynamics: energy cannot be made or destroyed, only changed from one form to another.
Second law: every energy transfer is inefficient — some energy is always degraded into heat.
Sunlight is the main energy input; producers turn it into chemical energy by photosynthesis.
Because so much is lost at each step, food chains are short — rarely more than five levels.
Energy in, heat out
Picture a woodland. Light lands on the leaves all day long. That light is the only energy going in, and once producers capture it, everything else in the wood is living off what those leaves managed to catch.
Matter is a different story. The nitrogen in a fox’s muscle was once in a rabbit, before that in a blade of grass, and before that in the soil. When the fox dies, decomposers hand those same atoms back to the soil and the whole thing starts again. Nothing new is delivered — it just goes round.
The red arrows never turn round. The teal loop does. That single difference is the whole reason ecosystems need a constant supply of sunlight but not a constant supply of atoms.
If a question asks you to compare energy and matter, the safe pair of words is flow for energy and cycle for matter. Examiners look for exactly that contrast.
The first law: nothing goes missing
First law of thermodynamics
Energy cannot be created or destroyed — only transformed from one form into another
This is also called the principle of conservation of energy. Applied to a food chain it means the energy going into a system must equal the energy coming out. Nothing vanishes.
So follow it round: light energy hits a leaf, becomes chemical energy in glucose, becomes chemical energy in a caterpillar’s body, becomes movement and heat when a bird eats the caterpillar. Different forms, same total.
Light energy from the Sun enters the food chain.
Producers convert it into chemical energy stored as biomass.
That chemical energy is passed along as one organism eats another.
In the end, all of it leaves as heat.
Careful with the word “lost”. When we say energy is lost at each trophic level, we do not mean it disappeared — that would break the first law. We mean it left the food chain as heat and is no longer useful to living things.
The second law: every transfer leaks
Second law of thermodynamics
Energy transfers are never 100% efficient — some energy is always degraded to heat
Think of energy as coming in two grades. Sunlight is concentrated and ordered. Heat spread through the air is dispersed and disordered. Every time energy moves, a bit of the good grade turns into the useless grade, and there is no way back.
The leaking starts immediately. Even the very first step is poor:
Some light bounces straight off the leaf (reflection).
Some passes straight through it (transmission).
Photosynthesis itself only captures a small slice of what is absorbed.
After that, respiration in every organism burns off more as waste heat.
In ecosystems the single biggest leak is cellular respiration. Organisms have to stay alive, and staying alive costs energy that never reaches the animal above them in the chain.
Values are in kJ per square metre per year. The 10% figure is a rough teaching average, not a fixed law — real ecosystems vary a lot.
🧩 How to answer “why are food chains short?”
State the law. Energy transfers between trophic levels are inefficient (second law).
Give the reason. Energy is lost as heat from respiration, plus uneaten and undigested material.
Follow the consequence. Only about 10% reaches the next level, so the amount shrinks fast.
Finish the chain of logic. After four or five levels there is not enough energy left to support another population.
WORKED EXAMPLE
A grassland traps 12 000 kJ m−2 yr−1 in producer biomass. Assuming 10% is passed on at each transfer, how much energy reaches the tertiary consumers?
Step 1: Count the transfersProducer → primary → secondary → tertiary is three transfersStep 2: Take 10% three times12 000 × 0.1 = 1 200 kJ (primary)1 200 × 0.1 = 120 kJ (secondary)120 × 0.1 = 12 kJ (tertiary)12 kJ m−² yr−¹Always say how many transfers there are before you start multiplying
WORKED EXAMPLE
Producers in a pond store 8 400 kJ m−2 yr−1. The primary consumers store 1 050 kJ m−2 yr−1. What percentage of the energy was lost in this transfer?
Step 1: Work out what was passed on(1 050 ÷ 8 400) × 100 = 12.5%Step 2: The rest was lost100 − 12.5 = 87.5%87.5% lostRead the question carefully – passed on and lost are not the same number
💡 Exam tip
Use flow for energy and cycle for matter. Swapping those words costs marks.
Quote the law by name. Writing “second law of thermodynamics” is worth more than “energy gets wasted”.
Say where the energy went, not just that it went. “Lost as heat during respiration” is a full answer.
Treat 10% as a rough guide. If a question gives you real data, use their numbers, not 10%.
Ecosystems are open systems — be ready to justify that with an example of matter crossing the boundary, like rain or migrating birds.
⚠ Common mix-up
“Energy is destroyed at each level.” No — that breaks the first law. It is transformed into heat and leaves the chain.
Saying matter flows through and energy cycles. This is the right idea written backwards. Slow down and check.
Thinking heat can be reused. Once energy is heat spread through the surroundings, no organism can turn it back into biomass.
Confusing the two laws. First law = nothing is created or destroyed. Second law = transfers are inefficient.
Forgetting decomposers. They are the reason matter can cycle at all, even though they usually sit outside the drawn food chain.
Up next: Photosynthesis as an Energy Input — the one reaction that opens the door and lets sunlight into the living world.
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