Heterotrophs are organisms that cannot produce their own food. They must obtain their nutrition by consuming external sources of organic matter. The biological process of “making food” refers specifically to the conversion of inorganic materials, like carbon dioxide and water, into organic molecules, such as sugars, within an organism’s body.
Autotrophs: The Organisms That Make Their Own Food
Organisms capable of producing their own food are called autotrophs, meaning “self-feeders.” They form the base of most food chains by converting abiotic energy sources into stored chemical energy within organic compounds.
The primary method autotrophs use is photosynthesis, where they capture light energy. During photosynthesis, green plants, algae, and certain bacteria utilize carbon dioxide and water, along with sunlight, to create glucose (a sugar) and oxygen. Another method is chemosynthesis, employed by some bacteria and archaea, particularly in environments without sunlight, such as deep-sea hydrothermal vents. These chemoautotrophs use energy from the oxidation of inorganic chemical compounds, like hydrogen sulfide, to produce organic molecules.
Heterotrophs: Obtaining Energy by Consuming Others
Heterotrophs must acquire organic carbon from other sources. They rely on consuming organic compounds, such as carbohydrates, fats, and proteins, that were originally produced by autotrophs or are part of other living or once-living organisms.
The acquisition of these organic compounds often involves ingesting or absorbing food from their environment. Once consumed, the complex organic compounds are broken down into simpler forms through digestion, allowing the heterotroph to absorb nutrients and release energy for its metabolic activities. All animals, fungi, and many bacteria and protists are examples of heterotrophs, serving as consumers in food chains.
Diverse Ways Heterotrophs Acquire Nutrients
Heterotrophs display a wide array of strategies for obtaining the organic compounds they need, categorized largely by their dietary sources. Herbivores, for instance, are heterotrophs that consume only plants. Examples include deer and cows, which derive their energy directly from plant matter. Carnivores, conversely, are animals that obtain their nutrients by consuming other animals, such as lions or wolves.
Omnivores have a more varied diet, consuming both plants and animals. Humans and bears are common examples of omnivores, adapting their diets based on available food sources.
Decomposers represent a distinct group of heterotrophs that play an important role in nutrient cycling by breaking down dead organic matter. This category includes saprotrophs, like fungi and many bacteria, which secrete digestive enzymes externally onto dead organisms or waste, then absorb the resulting simpler molecules. Detritivores, such as earthworms, are another type of decomposer that internally ingest and digest dead organic material. Parasites obtain nutrients by living on or inside a living host organism, often causing harm to the host in the process. Ticks and tapeworms exemplify parasitic nutrition.