Holozoic and saprotrophic nutrition, and archaeaSpec B4.2.4, B4.2.6, B4.2.7
In short
All animals are heterotrophic, and holozoic nutrition means food is ingested, digested internally, absorbed and assimilated. Saprotrophic fungi and bacteria secrete enzymes onto dead organic matter, digest it externally and absorb the products, so they are decomposers. Archaea, one of the three domains of life, are very varied in how they obtain energy for ATP production.
Heterotrophs obtain carbon compounds from other organisms. All animals are heterotrophic, and their mode of nutrition is holozoic nutrition.
- Ingestion: food is taken into the gut.
- Digestion: large molecules are broken down by enzymes inside the body (internal digestion).
- Absorption: small digested molecules pass across the gut wall into body cells and the blood.
- Assimilation: absorbed molecules are taken up by body cells and become part of the organism's own cells and tissues.
Saprotrophic nutrition
Some fungi and bacteria are saprotrophs. They secrete digestive enzymes onto dead organic matter, digest it outside their cells (extracellular digestion), then absorb the products. Fungi and bacteria with this heterotrophic mode of nutrition are called decomposers: they release mineral nutrients from dead matter for reuse.
| Feature | Holozoic | Saprotrophic |
|---|---|---|
| Organisms | Animals | Some fungi and bacteria |
| Food source | Other organisms, living or dead, ingested | Dead organic matter |
| Where digestion happens | Internally, in the gut | Externally, outside the cells |
| Ingestion | Yes | No; products are absorbed directly |
Diversity of nutrition in archaea
Archaea are one of the three domains of life, alongside bacteria and eukaryotes. They are very varied in how they obtain energy for ATP production. Different species use light, the oxidation of inorganic chemicals or the oxidation of carbon compounds as their energy source. Archaea that use light do not photosynthesise with chlorophyll: they absorb light with a different pigment, which pumps protons across the membrane, and the proton gradient drives ATP synthase. Many live in extreme habitats, but archaea are also common in ordinary soils and oceans.
You do not need to name archaea species. Do know the three possible energy sources, since a question may ask you to outline their metabolic diversity. No archaea are known to carry out chlorophyll-based photosynthesis.
Written and checked against the IB Biology HL specification · Updated October 2026