Heterotroph

A heterotroph is an organism that cannot synthesize its own food from inorganic raw materials. Unlike autotrophs — which capture energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis) to build organic molecules from carbon dioxide and water — heterotrophs must obtain organic carbon by consuming or absorbing it from other organisms, living or dead.

Animals and fungi are both heterotrophs, but they feed in fundamentally different ways. Animals are ingestive heterotrophs: they take food inside their bodies, then digest it internally using enzymes in a gut or digestive cavity. Fungi are absorptive heterotrophs: they secrete digestive enzymes into the surrounding substrate, break down complex molecules outside their bodies, and absorb the small molecules through their hyphal walls. This distinction — absorptive versus ingestive heterotrophy — is one of the sharpest lines between the fungal and animal kingdoms, despite their close evolutionary relationship (both belong to the clade Opisthokonta, sharing a common ancestor that diverged from plants over a billion years ago).

Heterotrophy is what makes fungi dependent on other organisms — directly or indirectly — for their carbon and energy. Saprotrophic fungi obtain both from dead organic matter. Mycorrhizal fungi obtain carbon from living plant partners (in exchange for mineral nutrients). Parasitic fungi obtain carbon from living hosts at the host’s expense. In every case, the fungus exists in an obligate nutritional relation to other organisms or their remains. It cannot exist in isolation. This dependency is not a weakness but a relational fact: the fungus is constituted through its nutritional relations with the organic world around it.

The distinction between heterotrophy and autotrophy is also why fungi were once misclassified as plants. Both grow in soil, both are sessile, both have branching forms. But plants make their own food from sunlight; fungi eat the dead (and living) products of that photosynthesis. The resemblance is superficial — the metabolic strategies are opposite.

Related terms

• Extracellular Digestion — the specific heterotrophic strategy of fungi
• Saprotroph — heterotrophs that feed on dead organic matter
• Substrate — the material from which heterotrophic fungi obtain nutrition
• Photosynthesis — the autotrophic process that heterotrophs ultimately depend on
• Mycorrhiza — a heterotrophic strategy based on symbiotic carbon exchange
• Fungal Taxonomy — the animal-fungal relationship within Opisthokonta