Modes of nutrition & autotrophs vs heterotrophs
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Question
Define an autotroph.
Answer
An organism that **makes its own organic molecules** from inorganic substances (e.g. CO₂), using an external energy source. 'Auto' = self.
Question
Define a heterotroph.
Answer
An organism that **cannot make its own organic molecules** and must take in ready-made organic food from other organisms. 'Hetero' = other.
Question
What carbon source do all autotrophs use?
Answer
**Inorganic carbon dioxide (CO₂)** — they fix it into organic molecules.
Question
How do photoautotrophs get their energy?
Answer
From **light** (photosynthesis). Examples: plants, algae, cyanobacteria.
Question
How do chemoautotrophs get their energy?
Answer
By **oxidising simple inorganic substances** (chemosynthesis). Examples: deep-sea vent bacteria.
Question
How do photoautotrophs and chemoautotrophs differ?
Answer
Only in their **energy source** (light vs oxidising inorganic substances); both fix **CO₂** for carbon.
Question
What is holozoic nutrition?
Answer
Heterotrophic nutrition where food is **ingested** and **digested internally** — the way animals feed.
Question
What is a saprotroph?
Answer
A heterotroph that feeds on **dead or decaying** matter by releasing enzymes onto it and absorbing the products — **external digestion** (many fungi/bacteria).
Question
What is a mixotroph?
Answer
An organism that uses **both** autotrophic and heterotrophic nutrition — it can make its own food and feed on others (e.g. Euglena).
Question
Which organisms are the producers in an ecosystem?
Answer
**Autotrophs** — they produce organic molecules that feed the heterotrophs.
Question
What two things define a mode of nutrition?
Answer
The **energy source** and the **carbon source** of the organism.
Question
Where does a saprotroph digest its food?
Answer
**Outside** its body (external digestion) — then it absorbs the digested products.
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Topic 3.9 hub
Transfers of energy and matter
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