Well, basically, they need energy to power the different types of processes that take place within that organism. They need carbon to create the biomolecules used by those organisms, and they need electrons to basically undergo oxidation reduction reactions that are used to create different types of molecules. For example, if we examine the human body, humans need energy to basically power different types of processes. 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.