3.9.3Additional lessons
Obtaining Carbon Compounds in Ecosystems — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 21 of 101
Part 1 of 5Autotrophs
Definition
- Autotrophs make their own organic carbon compounds from inorganic carbon dioxide, so they are the producers of an ecosystem.
Photoautotrophs
- Plants, algae and cyanobacteria use light energy to fix carbon dioxide during photosynthesis.
Chemoautotrophs
- Some bacteria oxidise inorganic substances such as hydrogen sulfide and use the energy to fix carbon dioxide.
Role
- They form the base of food chains, because all other organisms depend on the organic compounds they build.
Part 2 of 5Heterotrophs
Heterotrophs
Definition
- Heterotrophs cannot make their own food and must obtain carbon compounds by consuming other organisms or their remains.
Consumers
- Herbivores, carnivores and omnivores ingest living or recently killed food and digest it inside the body.
Detritivores
- These animals, such as earthworms, feed on dead organic matter and break it into smaller pieces.
Saprotrophs
- Fungi and bacteria secrete digestive enzymes onto dead matter and absorb the soluble products, a process of external digestion.
Part 3 of 5Mixotrophs and Classification
1
Look at whether the organism builds organic compounds from carbon dioxide, which makes it an autotroph.
2
If it must take in ready-made organic compounds, it is a heterotroph.
3
Decide how the food is obtained: ingestion followed by internal digestion for consumers and detritivores, or secretion of enzymes for saprotrophs.
4
Some organisms such as Euglena are mixotrophs and can do both, photosynthesising in light and feeding on organic matter in darkness.
| Group | Source of carbon | Example |
|---|---|---|
| Photoautotroph | Carbon dioxide, using light | Grass, algae |
| Chemoautotroph | Carbon dioxide, using chemical energy | Sulfur bacteria |
| Consumer | Living organisms | Zebra, lion |
| Detritivore | Dead organic matter, ingested | Earthworm |
| Saprotroph | Dead organic matter, absorbed | Mushroom, bread mould |
Part 4 of 5Why Carbon Compounds Matter
Structure
- Carbon compounds such as proteins, lipids and cellulose are used to build cells and tissues, so every organism needs a carbon source.
Energy
- Glucose and fats store chemical energy that is released in respiration and used to make ATP.
Carbon fixation
- Autotrophs convert inorganic carbon dioxide into organic compounds,
- and this step brings carbon from the atmosphere into living things.
Transfer
- Heterotrophs pass the carbon on by feeding and return it as carbon dioxide when they respire, which links nutrition to the carbon cycle.
Part 5 of 5Exam Checklist & Tips
3.9.3 – Key Points for the Exam
- Autotroph An organism that makes organic compounds from inorganic carbon.
- Heterotroph An organism that obtains carbon from other organisms.
- Detritivore An organism that ingests dead organic matter.
- Saprotroph An organism that digests dead matter externally and absorbs the products.
- Mixotroph An organism using both autotrophic and heterotrophic nutrition.
Common Mistakes
- Saying detritivores and saprotrophs feed in the same way;
- one ingests and digests internally, the other secretes enzymes.
- Assuming all autotrophs use light, forgetting chemoautotrophs.
- Calling fungi consumers in the sense of eating, although they absorb nutrients.
- Thinking plants are never heterotrophic, although some parasitic plants are.
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