The big idea: Every living thing needs two things: a source of energy and a source of carbon (to build its organic molecules).
Organisms are sorted by how they get them.
An autotroph makes its own organic food from simple inorganic raw materials.
A heterotroph cannot — it must take in ready-made organic food from other organisms.
- Autotroph
- An organism that makes its own organic molecules (food) from simple inorganic substances such as carbon dioxide, using an external energy source. 'Auto' = self, 'troph' = feeding — 'self-feeding'.
- Heterotroph
- An organism that cannot make its own organic molecules and so must obtain them by taking in (eating or absorbing) other organisms or organic matter. 'Hetero' = other — 'feeding on others'.
- Mode of nutrition
- The way an organism obtains its energy and its carbon (its organic molecules).
- Producer
- An autotroph in a food chain — it produces organic molecules that feed the rest of the ecosystem.
- Consumer
- A heterotroph that obtains organic molecules by eating other organisms.
| Feature | Autotroph | Heterotroph |
|---|---|---|
| Makes its own food? | Yes — synthesises organic molecules itself | No — must take in ready-made organic molecules |
| Carbon source | Inorganic carbon dioxide (CO₂) | Organic carbon from other organisms |
| Energy source | Light (photo-) or chemical reactions (chemo-) | Chemical energy stored in food |
| Examples | Plants, algae, some bacteria | Animals, fungi, most bacteria |
| Role in a food chain | Producer | Consumer or decomposer |
A quick way to remember: Auto = self → an autotroph feeds itself by making food from CO₂.
Hetero = other → a heterotroph feeds on others, eating ready-made food.
Autotrophs are the producers; heterotrophs are the consumers and decomposers.
To name a mode of nutrition precisely, look at two separate things: where the organism gets its energy, and where it gets its carbon.
Autotrophs all get their carbon from inorganic CO₂, but they split into two groups by their energy source — and that is a favourite exam point.
Two kinds of autotroph: Photoautotrophs use light as their energy source (photosynthesis). Plants, algae and cyanobacteria are photoautotrophs.
Chemoautotrophs use the energy released by oxidising simple inorganic substances (such as hydrogen sulfide or ammonia) — this is called chemosynthesis. Some bacteria, for example those around deep-sea vents, are chemoautotrophs.
Both fix carbon dioxide for carbon — they differ only in their energy source: photo = light, chemo = chemical reactions.
Photoautotroph
- Energy from light (photosynthesis)
- Carbon from inorganic CO₂
- Makes its own organic food
- Examples: plants, algae, cyanobacteria
Chemoautotroph
- Energy from oxidising inorganic substances (chemosynthesis)
- Carbon from inorganic CO₂
- Makes its own organic food
- Examples: deep-sea vent bacteria, some soil bacteria
Heterotrophs feed in different ways: Heterotrophs all take in ready-made organic food, but they do it in different ways:
Holozoic feeders ingest (take in) food and then digest it inside the body — this is how animals feed.
Saprotrophs feed on dead or decaying matter by releasing enzymes onto the food and absorbing the digested products — external digestion (many fungi and bacteria).
Mixotrophs can switch between making their own food (autotrophic) and feeding on others (heterotrophic) — they use both modes (for example Euglena).
| Feeding mode | What it means | Where digestion happens | Example |
|---|---|---|---|
| Holozoic | Ingests (takes in) food, then digests it | INSIDE the body (internal digestion) | An animal eating and digesting its prey |
| Saprotroph | Feeds on dead or decaying organic matter | OUTSIDE the body — releases enzymes onto food, then absorbs it (external digestion) | Many fungi and decomposer bacteria |
| Mixotroph | Switches between making its own food and feeding on others | Both — can be autotrophic and heterotrophic | Euglena and some other protists |
The whole map on two axes: Put it together and every mode is defined by energy source × carbon source:
photoautotroph = light + CO₂; chemoautotroph = chemical reactions + CO₂; heterotroph = food + organic carbon; mixotroph = both.
Reading a table row correctly means checking both columns, not just one.
| Mode of nutrition | Where the ENERGY comes from | Where the CARBON comes from |
|---|---|---|
| Photoautotroph | Light (sunlight) | Inorganic — fixes carbon dioxide (CO₂) |
| Chemoautotroph | Chemical reactions (oxidising inorganic substances) | Inorganic — fixes carbon dioxide (CO₂) |
| Heterotroph | Chemical energy in food (other organisms) | Organic — taken from other organisms |
| Mixotroph | Both light/chemicals AND food, depending on conditions | Both inorganic (CO₂) AND organic (other organisms) |
A memory hook: Photo = light. Chemo = chemicals. Auto = makes its own. Hetero = eats others.
And for heterotrophs: holozoic digests inside (whole food taken in), a saprotroph digests outside (on dead matter), a mixotroph does both.
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How this is tested: On Paper 2 a 1-mark Define question can ask you to define autotroph — give a complete definition (makes its own organic molecules from inorganic substances).
On Paper 1A a multiple-choice question often asks you to compare the energy sources of photoautotrophs and chemoautotrophs, or to identify which organisms get energy by oxidising inorganic substances (chemoautotrophs), or to spot the mixotroph in an energy-flow diagram.
A data-style Paper-1B question gives a table of organisms with an energy source and a carbon source and asks which row is correctly labelled — you must check both columns.
IB-style question — compare photoautotrophs and chemoautotrophs
Both photoautotrophs and chemoautotrophs are autotrophs. Compare their energy sources and their carbon sources. [3]
How to score all three marks
- Energy — photoautotrophs. Photoautotrophs obtain their energy from light (they photosynthesise).
- Energy — chemoautotrophs. Chemoautotrophs obtain their energy from chemical reactions — specifically by oxidising simple inorganic substances (chemosynthesis).
- Carbon — both the same. Both are autotrophs, so both fix inorganic carbon dioxide (CO₂) to build their organic molecules. (Mark 1: photoautotroph uses light. Mark 2: chemoautotroph oxidises inorganic substances. Mark 3: both use CO₂ as their carbon source.)
Final answer
Photoautotrophs get energy from light; chemoautotrophs get energy by oxidising inorganic substances; but both fix inorganic CO₂ for their carbon.
✓ Why this scores full marks: A Compare answer must give both similarities and differences, treating each axis separately.
Here the energy sources differ (light vs oxidising chemicals) but the carbon source is the same (CO₂) — both points are scored.
Writing only 'one uses light' would miss the carbon mark.
| Mode of nutrition | Where the ENERGY comes from | Where the CARBON comes from |
|---|---|---|
| Photoautotroph | Light (sunlight) | Inorganic — fixes carbon dioxide (CO₂) |
| Chemoautotroph | Chemical reactions (oxidising inorganic substances) | Inorganic — fixes carbon dioxide (CO₂) |
| Heterotroph | Chemical energy in food (other organisms) | Organic — taken from other organisms |
| Mixotroph | Both light/chemicals AND food, depending on conditions | Both inorganic (CO₂) AND organic (other organisms) |