And one particular process is basically the creation of an electrochemical gradient on our nerve cell, and that creates an action potential that propagates and sends electrical signals. Humans basically need a carbon source to create different types of biomolecules, such as our proteins, that are used throughout the human body. And humans need electrons to basically create ATP energy molecules, our adenosine triphosphates that are used by the cell. So we see that all these three things are needed by humans, but are also needed by every single type of living organism that exists on Earth. And because these three things, energy, carbon, electrons are needed by all the organisms, we can categorize organisms by how they actually obtain these three things. So we can categorize organisms by how they obtain their carbon source.