The big idea: Every living thing needs two things to build its body and stay alive: a source of energy and a source of carbon (to make organic molecules).
An organism's mode of nutrition is just the answer to those two questions: where does its energy come from, and where does its carbon come from?
The first big split is between autotrophs, which make their own organic carbon, and heterotrophs, which take organic carbon from other organisms.
| Mode | Energy source | Carbon source | Plain meaning |
|---|---|---|---|
| Autotroph | Light (photo-) or inorganic chemicals (chemo-) | Carbon dioxide (CO₂) — an inorganic source | Makes its own food / organic carbon |
| Heterotroph | Chemical energy in organic molecules it eats | Organic carbon from other organisms | Takes food / organic carbon from others |
| Mixotroph | Both — can use light AND organic molecules | Both — CO₂ and organic carbon from others | Switches between making and taking food |
- Mode of nutrition
- The way an organism obtains its energy and its carbon (the building blocks for organic molecules).
- Autotroph
- An organism that makes its own organic carbon compounds from an inorganic carbon source (carbon dioxide). 'Auto' = self, 'troph' = feeding — self-feeding.
- Heterotroph
- An organism that obtains its organic carbon by taking in organic molecules made by other organisms. 'Hetero' = other — feeding on others.
- Mixotroph
- An organism that can use both modes — it can make its own food like an autotroph AND take in organic food like a heterotroph (e.g. Euglena).
Read the word: The names tell you the answer:
auto = self → an autotroph feeds itself (makes its own carbon from CO₂).
hetero = other → a heterotroph feeds on others (takes carbon from food).
mixo = mixed → a mixotroph does both.
Both autotrophs and heterotrophs come in more than one kind. The differences come down to the SAME two questions — energy and carbon — asked more precisely.
For autotrophs the carbon source is always CO₂, so we split them by their energy source. For heterotrophs the carbon always comes from other organisms, so we split them by how and where they get and digest that food.
Two kinds of autotroph: All autotrophs build organic carbon from CO₂, but they power that work in two ways:
Photoautotrophs use light energy (photosynthesis) — plants, algae, cyanobacteria.
Chemoautotrophs use energy from oxidising simple inorganic chemicals (such as hydrogen sulfide) — some bacteria, including those around deep-sea vents where there is no light.
| Type of autotroph | Where it gets its energy | Example |
|---|---|---|
| Photoautotroph | Light (sunlight) — captured in photosynthesis | Plants, algae, cyanobacteria |
| Chemoautotroph | Energy released by oxidising simple inorganic chemicals (e.g. H₂S, NH₃) | Some bacteria around deep-sea vents |
Three kinds of heterotroph: Heterotrophs all take organic carbon from other organisms, but how they do it differs:
Holozoic — ingests food and digests it INTERNALLY, inside the body (most animals).
Saprotroph — feeds on dead matter, digesting it EXTERNALLY by secreting enzymes onto it and absorbing the products (many fungi and bacteria — the decomposers).
Parasite — feeds on a living host and harms it (a tapeworm, a head louse).
| Heterotroph type | Where the food comes from | How food is digested | Example |
|---|---|---|---|
| Holozoic | Other organisms, taken in whole or in pieces | INTERNALLY — ingested, then digested inside the body | Most animals (e.g. a frog eating an insect) |
| Saprotroph | Dead organic matter (decaying remains, waste) | EXTERNALLY — releases enzymes onto the food, then absorbs the products | Many fungi and bacteria (decomposers) |
| Parasite | A living host, which is harmed | Takes nutrients from the host (in or on its body) | Tapeworm, head louse, many disease microbes |
Saprotroph
- Feeds on dead organic matter
- Digestion is external (enzymes secreted onto the food)
- Absorbs the digested products
- Does not harm a living host — a decomposer
Holozoic
- Ingests food (often living or freshly dead)
- Digestion is internal (inside a gut / body)
- Egests undigested remains
- Typical of most animals
Mixotrophs — the in-betweeners: A mixotroph is not stuck with one mode. It can photosynthesise like an autotroph and take in organic food like a heterotroph, switching depending on conditions.
A classic example is Euglena: in the light it photosynthesises; in the dark it feeds on organic matter. This is why some protists are hard to slot neatly into one mode.
Don't forget oxygen: An organism's nutrition also depends on whether it needs oxygen to release energy:
an obligate aerobe must have oxygen; an obligate anaerobe is poisoned by it; a facultative anaerobe uses oxygen when present but can manage without it.
Oxygen requirement is part of an organism's niche — its full role in the ecosystem.
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How this is tested: On Paper 1A a single-mark question usually describes an organism's feeding and asks you to name the mode of nutrition — a fungus that externally digests dead wood is a saprotroph; an organism that uses both sunlight and other organisms is a mixotroph.
A data-style Paper 1 row gives an energy source and a carbon source and asks which mode they describe — you classify it from the two sources together (light + CO₂ = photoautotroph; oxidising chemicals + CO₂ = chemoautotroph).
On Paper 2 a 1-mark Define can ask for autotroph (or heterotroph) — give the energy and carbon source in the definition.
IB-style question — classify a mode from a data row
A microbe living deep underground obtains its energy by oxidising hydrogen sulfide and obtains its carbon from carbon dioxide. State its mode of nutrition and justify your answer. [2]
How to score both marks
- Check the carbon source first. Its carbon comes from carbon dioxide — an inorganic source — so the microbe makes its own organic carbon. That makes it an autotroph.
- Now check the energy source to pick the sub-type. Its energy comes from oxidising an inorganic chemical (hydrogen sulfide), not from light, so it is a chemoautotroph. (Mark 1: chemoautotroph / autotroph. Mark 2: carbon from CO₂ AND energy from oxidising inorganic chemicals.)
Final answer
Chemoautotroph — it fixes carbon from CO₂ (so it is an autotroph) and gets its energy by oxidising an inorganic chemical rather than from light.
✓ Why this scores full marks: The answer uses both clues. The carbon source (CO₂) proves it is an autotroph; the energy source (oxidising a chemical, not light) proves it is the chemo kind.
Classifying nutrition always means reading the two sources together, not just one.
| If energy comes from… | …and carbon comes from… | the mode of nutrition is… |
|---|---|---|
| Light | Carbon dioxide | Photoautotroph |
| Oxidising inorganic chemicals | Carbon dioxide | Chemoautotroph |
| Eating other organisms | Organic molecules in that food | Heterotroph |
| Both light and food | Both CO₂ and organic food | Mixotroph |