---
title: "Heterotroph | General Biology I"
description: "Heterotrophs are organisms that get energy by consuming organic matter, a core idea in General Biology I for photosynthesis, food webs, and respiration."
canonical: "https://fiveable.me/college-bio/key-terms/heterotroph"
type: "key-term"
subject: "General Biology I"
unit: "Unit 8"
---

# Heterotroph | General Biology I

## Definition

A heterotroph is an organism that gets energy and carbon by consuming other organisms or organic matter instead of making its own food. In General Biology I, it’s the consumer side of energy flow in ecosystems.

## What It Is

In General Biology I, a heterotroph is any organism that cannot make its own organic food from carbon dioxide and light or inorganic chemicals, so it has to get energy by taking in organic molecules made by other living things. That includes animals, fungi, and many bacteria. The term is less about what an organism looks like and more about how it gets carbon and energy into its cells.

Heterotrophs do not make glucose from scratch the way autotrophs do. Instead, they eat, absorb, or break down organic material and then use cellular respiration or fermentation to release usable energy from those molecules. A deer gets carbohydrates and other nutrients by eating plants. A mushroom gets them by secreting enzymes onto dead matter and absorbing the smaller molecules that result. Many bacteria live in similar ways, either as decomposers or as parasites.

This matters because heterotrophs sit on the consumer side of food webs. They are the organisms that move energy from producers to higher trophic levels. If a plant stores energy from sunlight in glucose, a rabbit can eat the plant, and a fox can eat the rabbit. Energy keeps moving, but it also decreases at each step because living cells use some of it for metabolism, movement, growth, and repair.

A common misconception is that heterotrophs only mean animals. Animals are one big group of heterotrophs, but they are not the whole story. Fungi are heterotrophs too, and so are many protists and bacteria. Another mistake is to think heterotrophs only “consume” by chewing. In biology, consuming can also mean absorbing nutrients after external digestion, which is how fungi work.

In the photosynthesis unit, heterotroph is the partner term to autotroph. Autotrophs make the organic molecules that heterotrophs depend on. That relationship is why photosynthesis is the foundation of most ecosystems and why heterotrophs are never fully independent of producers, even when they are carnivores at the top of a food chain.

## Why It Matters

Heterotroph is one of the fastest ways to connect cell chemistry with ecology in General Biology I. Once you know whether an organism is a heterotroph or autotroph, you can predict where it gets carbon, how it gets energy, and what it depends on in its environment.

The term also helps you trace energy flow. Photosynthesis stores light energy in glucose, and heterotrophs pull that energy back out through respiration or digestion. That is the basic reason food webs have producers at the bottom and consumers above them. Without heterotrophs, ecosystems would not have the same recycling of matter, because dead organic material would not be broken down and returned to the environment as efficiently.

It also connects to nutrient cycling. Heterotrophs eat, digest, excrete, and eventually die, and all of that moves carbon, nitrogen, and other elements through ecosystems. In a lab or class discussion, you may be asked to explain why a fungus on a log, a wolf in a forest, and a bacterium in soil are all part of the same bigger cycle even though they get nutrients in different ways.

You’ll see the term again whenever a question asks you to classify organisms, interpret a food web, or explain how photosynthesis and respiration fit together. If you can identify the heterotrophs in a system, you can usually follow the path of matter and energy more accurately.

## Connections

### Autotroph

Autotrophs are the organisms that make their own organic food, usually by photosynthesis. Heterotrophs depend on them directly or indirectly for energy and carbon. In a food web, autotrophs are the source of new chemical energy, while heterotrophs are the consumers that pass that energy along.

### Consumer

Consumer is the broader ecological label for organisms that get energy by eating or absorbing other organisms. Most heterotrophs are consumers, but the term heterotroph focuses more on metabolism and nutrition. In a food chain, a herbivore, carnivore, or omnivore is both a consumer and a heterotroph.

### Decomposer

Decomposers are a special kind of heterotroph that break down dead organisms and waste. Fungi and many bacteria do this by releasing enzymes outside their body, then absorbing the smaller molecules. That makes decomposers a major part of nutrient cycling, not just energy transfer.

### Calvin cycle

The Calvin cycle is the part of photosynthesis that makes sugar from carbon dioxide. Heterotrophs do not run the Calvin cycle to make their own food, which is why they must depend on organic molecules made by autotrophs. If you confuse the two, check whether the process is building sugar or using sugar.

## On the AP Exam

A quiz question might show a diagram of a food web and ask you to identify which organisms are heterotrophs, or to explain why fungi count even though they do not eat like animals. On lab worksheets, you may sort organisms by trophic level or predict what happens to energy if producers are removed from an ecosystem. In short-answer responses, use the term to trace where an organism gets carbon and energy, not just to name an animal. If you see a question about photosynthesis, remember the heterotroph side is everything that depends on the glucose produced by autotrophs.

## Heterotroph vs Autotroph

These are the most common pair to mix up. An autotroph makes its own organic food from inorganic sources, while a heterotroph gets organic molecules by eating or absorbing them. The easiest way to tell them apart is to ask, does the organism build its own food base, or does it depend on something else already made?

## Key Takeaways

- A heterotroph gets energy and carbon by consuming or absorbing organic matter instead of making its own food.
- Animals, fungi, and many bacteria are heterotrophs, but they do not all get nutrients in the same way.
- Heterotrophs are the consumer side of food webs, so they move energy from producers to higher trophic levels.
- The term connects directly to photosynthesis because autotrophs make the glucose that heterotrophs depend on.
- Heterotrophs also matter in nutrient cycling because they break down, transform, and return matter to ecosystems.

## FAQs

### What is a heterotroph in General Biology I?

A heterotroph is an organism that cannot make its own food from inorganic materials, so it gets energy by consuming or absorbing organic matter. In General Biology I, it usually comes up when you compare consumers with autotrophs and trace energy flow through ecosystems.

### Are fungi heterotrophs?

Yes. Fungi are heterotrophs because they do not photosynthesize or make glucose from carbon dioxide and light. Instead, they release enzymes onto food sources, break complex molecules into smaller ones, and absorb those nutrients.

### How is a heterotroph different from an autotroph?

An autotroph makes its own organic molecules, usually through photosynthesis, while a heterotroph gets those molecules from other organisms. That difference shows up in food webs, where autotrophs form the base and heterotrophs depend on them directly or indirectly.

### Why do heterotrophs matter in photosynthesis topics?

Photosynthesis stores energy in glucose, and heterotrophs use that glucose, either by eating plants or by eating other organisms that ate plants. That connection is what makes photosynthesis the starting point for most food chains and energy transfers.

## Related Study Guides

- [8.1 Overview of Photosynthesis](/college-bio/unit-8/1-overview-photosynthesis/study-guide/PYdZOTe3dPJ0INw9)

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