IB Biology HLEnergy & Matter in EcosystemsPaper 1 & 2~11 min read
Obtaining Carbon Compounds in Ecosystems
Every organism needs carbon compounds, and there are only two ways to get them: build your own from simple inorganic substances, or eat something that already did. That single split explains autotrophs, heterotrophs and everything in between.
📚 What you need to know
Decomposition breaks down dead organisms and waste, releasing the inorganic nutrients locked in their tissues so producers can use them again.
Detritivores break tissues apart; saprotrophs release enzymes that digest organic molecules outside their bodies.
Autotrophs synthesise their own organic molecules from simple inorganic substances. They are the producers.
Photoautotrophs use light energy; chemoautotrophs use energy from the oxidation of inorganic compounds.
Both routes work by oxidation reactions that release electrons used to make ATP.
Heterotrophs gain carbon compounds by ingesting the tissues of other organisms, then digesting and reassembling them.
Both groups respire: the oxidation of carbon compounds releases the energy that powers the functions of life, plus heat.
Decomposers and carbon compounds
When inorganic nutrients enter a food chain they are converted into carbon compounds such as carbohydrates and proteins, and become locked up inside the tissues of living plants and animals.
That matters because the supply of inorganic nutrients is finite. If nothing released them again, producers would eventually run out and the ecosystem would stop working.
DefinitionDecomposition is the breaking down of the bodies of dead organisms and the waste products of living organisms. It is what makes the cycling of nutrients possible.
Decomposition works on three kinds of material: bodies of dead organisms, dead parts of living organisms such as a fallen branch, and animal faeces.
Decomposer
What it does
Example
Detritivore
Starts the process by breaking tissues apart, usually by ingesting them
Earthworms, woodlice
Saprotroph
Releases enzymes onto the tissue, digesting organic molecules externally and releasing inorganic nutrients
Fungi, many bacteria
Saprotrophs absorb some of those nutrients for themselves. Whatever is left stays in the soil and becomes available to other organisms, especially producers. That is the loop that keeps the whole ecosystem supplied.
The green arrow is the part students leave out. Without decomposers the nutrients stay locked inside dead bodies for good.
Autotrophs
Definition
An autotroph synthesises its own organic molecules from simple inorganic substances in its environment.
Because they make their own organic molecules without relying on any other organism, autotrophs are known as producers. Most green plants are autotrophs, along with algae such as seaweeds and photosynthetic bacteria such as cyanobacteria.
Photoautotrophs use light energy to fix carbon dioxide from the air into organic molecules such as carbohydrates.
Chemoautotrophs use energy from the oxidation of inorganic compounds instead of light.
The organic molecules produced can be built up into macromolecules such as proteins and lipids. Reactions that build bigger molecules from smaller ones are described as anabolic.
Where the electrons come from
Here is the bit that ties the two types together. The energy transferred to ATP during ATP synthesis comes from oxidation reactions, and oxidation is the loss of electrons (remember OIL: oxidation is loss). The donated electrons are used to produce ATP. Different autotrophs simply harvest those electrons in different ways.
Iron(II) losing an electron to become iron(III) is oxidation, exactly like the water being split on the left. The electron is the point in both cases.
A neat way to remember it: photo means light, chemo means chemical, and troph means feeding. So a chemoautotroph feeds itself using chemicals. Break the word up and you never have to memorise the definition.
Heterotrophs
Heterotrophic organisms gain their carbon compounds by ingesting the tissues of other organisms: eating plants, killing and consuming animals, consuming dead bodies, or consuming biological waste.
They ingest biological material, break it down by digestion, then build the molecules back up into their own proteins and nucleic acids.
Digestion can happen inside the body (you, a fox) or outside it (a fungus releasing enzymes onto bread).
The new carbon compounds are then assimilated into the body, where they become available to the next trophic level.
Types of heterotroph: consumers, detritivores and saprotrophs.
Watch the wording. “Digestion happens outside the body” sounds odd until you picture mould on bread. The fungus is a heterotroph doing exactly what you do, just with the stomach turned inside out.
Oxidation of carbon compounds
Autotrophs make their own carbon compounds; heterotrophs take theirs from other organisms. After that, both groups do the same thing with them.
The chemical energy stored in carbon compounds is released by respiration.
Glucose is the fuel for respiration. Other carbon compounds such as lipids can be converted into glucose first.
Respiration releases that energy by the oxidation of carbon compounds.
Respiration also releases heat as a by-product – which is where all that lost energy in the next few pages goes.
What the energy is spent on: the functions of life
🧠
MR H GREN
Metabolism, Reproduction, Homeostasis, Growth, Response, Excretion, Nutrition. Seven functions, and every one of them is paid for out of respiration.
One of those seven matters more than the others for this topic. During growth, some of the chemical energy in ingested carbon compounds is incorporated into the organism’s tissues. That stored chemical energy is exactly what can be passed to the next trophic level in the food chain.
Remember that cellular respiration happens in autotrophs too. Plants respire day and night. The carbon compounds that fuel respiration are supplied in different ways, but the respiration itself is common to both groups.
Worked examples
WE 1
Distinguish autotrophs and heterotrophs
Distinguish between the mode of nutrition of an autotroph and a heterotroph. (3 marks)
Point 1: autotroph
An autotroph synthesises its own organic molecules from simple inorganic substances, so it is a producer.
Point 2: heterotroph
A heterotroph gains carbon compounds by ingesting the tissues of other organisms, then digesting and reassembling them.
Point 3: the shared bit
Both then respire those carbon compounds to release energy for the functions of life.
Build your own, or take someone else’s“distinguish” wants both sides compared on the same point, not two separate definitions
WE 2
Explain the role of decomposers
Explain why decomposers are essential to a functioning ecosystem. (4 marks)
Point 1: the problem
Inorganic nutrients are converted into carbon compounds and locked up in the tissues of living organisms.
Point 2: why that matters
The supply of inorganic nutrients is finite, so they must be released again when organisms die.
Point 3: how it happens
Detritivores break tissues apart and saprotrophs release enzymes that digest the organic molecules, releasing inorganic nutrients.
Point 4: the result
Those nutrients enter the soil and become available to producers again, so nutrient cycling continues.
Locked up → broken down → released → reused by producersuse the word “finite” – it is often the mark for explaining why recycling is necessary at all
WE 3
Chemoautotrophs at a vent
Explain how bacteria can act as producers in a deep-sea vent ecosystem where no light is available. (3 marks)
Point 1: the energy source
They are chemoautotrophs: they oxidise inorganic chemicals in the environment, for example iron(II) to iron(III).
Point 2: the electrons
Oxidation is the loss of electrons, and the donated electrons are used in the production of ATP.
Point 3: why that makes them producers
That ATP lets them build their own carbon compounds from inorganic substances, so other organisms in the vent can feed on them.
No light needed – the electrons come from a chemical insteadsay “oxidation of inorganic compounds”, not just “they use chemicals”
💡 Exam tips
Learn autotroph and heterotroph as full definitions – they are common one-mark openers.
Detritivore = ingests and breaks apart. Saprotroph = external enzymes. Do not swap them.
Link ATP production to oxidation and electrons in both photo- and chemoautotrophs.
Say both autotrophs and heterotrophs respire, if you are ever asked who respires.
Use “carbon compounds” or “organic molecules”, not “food”, in explanation answers.
For growth questions, link stored chemical energy to the next trophic level.
⚠ Common mistakes
Saying plants do not respire. They do, constantly.
Calling all bacteria decomposers. Some are chemoautotrophic producers, some are pathogens.
Saying photosynthesis “makes energy”. It converts light energy into chemical energy.
Mixing up chemoautotroph and heterotroph. A chemoautotroph still builds its own carbon compounds – it just uses a chemical rather than light as the energy source.
Forgetting that saprotrophs digest externally. This is the detail that separates them from detritivores.
Writing that decomposers “remove” nutrients. They release them.
Up next: Trophic Levels – putting numbers on the positions in a food chain, and why the same animal can sit at three different levels at once.
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