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. We can categorize organisms by how they obtain the energy source, and we can also categorize organisms by how they obtain their electron source. So let's begin with carbon. So, basically, humans are examples of heterotrophs.