IB Biology HLEnergy & Matter in EcosystemsPaper 1 & 2~10 min read
Energy Flow in Ecosystems
Energy does not travel through a food chain as a glowing beam. It travels as molecules – sugars, fats and proteins that get eaten, digested and rebuilt. Once you picture it that way, every arrow in a food chain makes sense.
📚 What you need to know
Photosynthesis converts light energy into chemical energy, stored in carbon compounds such as glucose, lipids and amino acids.
That chemical energy passes to a primary consumer when the plant is eaten, digested and absorbed.
Absorbed carbon compounds are either respired or built into new tissue, which is how energy is stored and passed on again.
When an organism dies, the energy in its carbon compounds passes to detritivores and saprotrophs.
A food chain shows feeding relationships; the arrows show the transfer of energy and biomass from one trophic level to the next by feeding.
A food web shows how several food chains in one ecosystem connect, because most species eat more than one thing.
Where the chemical energy comes from
Photosynthesis is the process of converting light energy into chemical energy. Plants use light to power reactions that build organic molecules, also called carbon compounds:
Glucose, which is either respired straight away or stored in plant cells as starch
Lipids
Amino acids, which are joined into proteins
The key idea
Energy is not stored on its own. It is stored inside the bonds of carbon compounds, so wherever those molecules go, the energy goes too.
Passing it on by being eaten
Here is the chain of events, and it is worth learning as a chain because that is how “explain” questions are marked:
🧩 From plant to predator
The plant is ingested by a primary consumer.
The consumer digests the plant tissue and absorbs the carbon compounds, which still contain the stored chemical energy.
Those compounds are either used to fuel respiration, or used to build up animal tissue.
Building tissue means the chemical energy is now stored in the consumer’s own body.
When that consumer is eaten, its carbon compounds and their energy pass to the secondary consumer, and so on up the chain.
And if nothing eats it? When an organism dies, the chemical energy stored in the carbon compounds of its tissues passes to detritivores and saprotrophs instead.
Two things leave every trophic level: heat, which is gone for good, and dead material, which the decomposers deal with.
Food chains
A food chain shows the feeding relationships between organisms in an ecosystem.
What the arrow means
The arrow shows the transfer of energy (as chemical energy in carbon compounds) and biomass, from one trophic level to the next, by the process of feeding.
So the arrow always points from the thing being eaten towards the thing eating it. Grass → rabbit means “energy moves from the grass into the rabbit”. Students who read the arrow as “eats” draw every chain backwards.
If a question asks what the arrows represent, do not just write “eats” or “is eaten by”. Say transfer of energy and biomass by feeding. That exact phrase is what gets the mark.
Food webs
A food web shows how several food chains within one ecosystem are connected.
In a real ecosystem, most species have more than one food source, and are food for more than one consumer.
That makes a web a more honest picture of the ecosystem than any single chain.
Oak leaves feed two different herbivores, and the wood mouse feeds two different predators. That branching is the whole point of a web.
Reading a web in the exam. To pull a food chain out of a web, put your finger on a producer and follow the arrows without ever going backwards. Every chain you can trace is a valid answer, so pick the one the question asks about.
Worked examples
WE 1
Explain how energy reaches a secondary consumer
Explain how energy from sunlight ends up stored in the tissues of a secondary consumer. (4 marks)
Point 1: trapping it
Producers absorb light energy and convert it to chemical energy in carbon compounds during photosynthesis.
Point 2: first transfer
A primary consumer ingests the plant, digests the tissue and absorbs those carbon compounds.
Point 3: storing it
Some absorbed compounds are used to build animal tissue, so the energy is now stored in the primary consumer’s body.
Point 4: second transfer
When the primary consumer is ingested by the secondary consumer, those carbon compounds and their stored energy pass on again.
Light → carbon compounds → eaten → new tissue → eaten againsay “carbon compounds” at least once – vague answers about “energy passing along” lose marks
WE 2
State what an arrow represents
In the food web above, state what the arrow between the aphid and the ladybird represents. (2 marks)
Point 1: what moves
The transfer of energy and biomass, stored as chemical energy in carbon compounds.
Point 2: how and which way
It moves by feeding, from the aphid (the trophic level being eaten) to the ladybird.
Energy and biomass, transferred by feeding, in the direction of the arrowtwo marks means two things: what is transferred, and by what process
WE 3
Use the food web
Using the food web above, construct one food chain containing four organisms, and suggest one effect of a disease killing all the wood mice. (3 marks)
Part 1: the chain
Oak leaves → aphid → ladybird → blue tit
Part 2: the effect
Foxes and owls lose a food source, so their numbers would fall, or foxes would feed more heavily on rabbits instead.
Part 3: why the web matters
Because foxes have another food source in this web, the effect on foxes is smaller than the effect on owls, which only feed on wood mice here.
Species with more food sources are less affected when one disappearsstart every chain with a producer, and never draw an arrow backwards
💡 Exam tips
Always start a food chain with a producer.
Learn the arrow definition: energy and biomass, transferred by feeding, to the next trophic level.
Use the word carbon compounds when describing what carries the energy.
Remember the two fates of an absorbed carbon compound: respired, or built into tissue.
Dead organisms are not a dead end – name detritivores and saprotrophs as the destination.
In “what if this species disappears” questions, look for alternative arrows before you predict a crash.
⚠ Common mistakes
Drawing arrows the wrong way. The arrow follows the energy, so it points at the consumer.
Saying the arrow means “eats”. It means transfer of energy and biomass by feeding.
Writing that plants “make energy”. They convert light energy into chemical energy; they do not create it.
Starting a food chain with the Sun. Sunlight is not an organism, so most mark schemes want the producer first unless the question shows otherwise.
Treating a food web as one long chain. A web is many chains crossing over, which is why one species can appear in several of them.
Forgetting decomposers. Energy in an uneaten organism still goes somewhere.
Up next: Obtaining Carbon Compounds in Ecosystems – where organisms actually get their carbon compounds from, and how autotrophs, heterotrophs and decomposers each solve that problem differently.
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