Life Needs an Energy Flow

All living things need energy to constantly fuel the biochemical pathways that enable them to grow, reproduce and move.

The Law of Conservation of Energy states that energy can neither be created nor destroyed. It can only be transformed or transferred from one form to another.

In other words, living things cannot make their own energy, but must acquire it somehow. They must continuously input energy into their systems so as to drive those systems. An organism will die without an energy source forever entering it.

Energy Sources

For some organisms, their energy source comes directly from the sun, via light photons. These are the phototrophs.
Other organisms must break the chemical bonds in molecules to release — and use — the energy stored within. These are the chemotrophs.

This energy, once released from photons or chemical bonds, is then stored in the organism as potential energy in adenosine triphosphate (ATP), carbohydrates, lipids (a more accurate word than ‘fats’), and proteins until needed for living, growth and reproduction..

Phototroph 

From the Ancient Greek combining form φωτω-, phōtō-, from φῶς, phôs, ‘light’; and τροφή, trophḗ, ‘nourishment’: ‘nourishment from light’.

Phototrophs convert photon energy into the chemical potential energy of ATP, carbohydrates, lipids and proteins. Plants, archaea and many bacteria are phototrophs.

It’s worth nothing that not all phototrophs are photosynthesisers. Photosynthesisers are the sub-group of phototrophs known as photoautotrophs, which convert inorganic carbon (typically carbon dioxide, CO₂) into their own organic carbon molecules (carbohydrates, which are then used to make lipids and proteins). The other sub-group, the photoheterotrophs, convert organic carbon (carbohydrates, lipids and proteins) into their own organic carbon (carbohydrates, lipids and proteins).

Chemotroph 

From the combining word chemo-, ‘chemical’; and τροφή, trophḗ, ‘nourishment’: ‘nourishment from chemicals’.

Chemotrophs release energy by breaking the bonds of chemical compounds, which is converted into the chemical potential energy of ATP, carbohydrates, lipids and proteins. Animals, fungi, archaea and many bacteria are chemotrophs.