We have to obtain it from other organisms. And this includes humans, while this include plants. Now, the energy source can be broken down into two types. We have phototrophs, which only use light as the energy source, and for example, plants use light energy to synthesize our carbohydrates, while chemotrophs are those that actually obtain our energy source by breaking down or oxidizing either organic or inorganic matter. And finally, we can also categorize based on electron source. So we have organotropes, which basically means we obtain our electrons from organic materials such as sugar, as proteins and lipids, while inorganic, while our organisms that obtain our electrons from inorganic starting materials.