Organisms differ in where the carbon for their organic molecules comes from. Here, organic carbon means carbon incorporated into carbon-based biomolecules, whereas inorganic carbon includes simple carbon species such as carbon dioxide ($CO_2$).

An autotroph builds organic carbon compounds from inorganic carbon. Autotrophy therefore requires both a carbon source and an energy source.

• Photoautotrophs use light energy to drive carbon fixation. Plants, algae and many bacteria use this strategy.
• Chemoautotrophs obtain energy from chemical reactions involving inorganic substances and use that energy to fix inorganic carbon. Some bacteria and archaea use this strategy in environments without light.

A heterotroph obtains carbon from pre-existing organic molecules synthesized by other organisms. Animals, fungi and many microorganisms are heterotrophs.

These terms describe carbon acquisition, not an organism's complete metabolism. Autotrophs still require energy-releasing metabolic reactions, and heterotrophs can conserve energy through many different pathways.

The ecological importance is that autotrophs introduce newly fixed organic carbon into biological communities, while heterotrophs obtain that carbon from organisms, waste or detritus.

A compact distinction is

$$\boxed{\text{autotroph: inorganic carbon} \rightarrow \text{organic carbon}}$$

$$\boxed{\text{heterotroph: pre-existing organic carbon} \rightarrow \text{biomass and metabolism}.}$$

This distinction provides the carbon-flow foundation for producers, consumers and decomposers in ecosystems.