Life Needs a Carbon Flow

All organisms are made of carbohydrates, lipids and proteins, which are needed for growth, reproduction and movement. These molecules all contain carbon, making them organic molecules.

The Law of Conservation of Mass states that the quantity of mass in a closed system must remain constant over time. This means that mass can neither be created nor destroyed. It can only be rearranged/converted into new forms, with no net loss or gain.

In other words, living things cannot create from nothing the carbohydrates, lipids and proteins they need to live, grow and reproduce — they must continually obtain a source of carbon (and other elements) from which to synthesise these. These carbon (and other) atoms must enter the organism in a form the organism can make use of, where, through biochemical pathways they ultimately end up in other carbon molecules the organism then uses for life or expels as waste.

An organism will die when a carbon source is no longer available.

Carbon Sources

Some organisms use the carbon in an inorganic carbon source (carbon dioxide, or CO₂) to make their carbohydrates, lipids and proteins. These are the autotrophs.

Other organisms use the carbon in organic carbon sources (carbohydrates, lipids and proteins) to make their own particular carbohydrates, lipids and proteins. These are the heterotrophs.

Autotroph

From the Ancient Greek combining form αὐτο-, auto-, from αὐτός, autós, ‘self’; and τροφή, trophḗ, ‘nourishment’: ‘nourishment from self’.

An autotroph is an organism that uses inorganic carbon (CO₂) as its carbon source. It does not need a (once-)living (organic) carbon source (such as carbohydrates, lipids and proteins) to make its own carbohydrates, fats and proteins.

Autotrophs are also known as primary producers, as they produce the food source (eg grass, leaves) at the beginning of all food chains and webs.

Heterotroph

From the Ancient Greek ἕτερος, héteros, ‘other, another, different’; and τροφή, trophḗ, ‘nourishment’: ‘nourishment from other’.

A heterotroph is an organism that uses organic compounds as its carbon source. It does need a (once-)living (organic) carbon source (such as carbohydrates, fats and proteins) to make its own carbohydrates, fats and proteins.

Heterotrophs are also known as primary, secondary, tertiary and quarternary consumers as they are consumers of autotrophs (making them primary consumers) and/or other heterotrophs (making them secondary, tertiary or quarternary consumers).