Biochemistry
Biochemistry can be viewed in the context of metabolism, which nicely covers a lot of biochemistry concepts. Human metabolism has many fuels, but primary among them are carbohydrates, or sugars.
The breakdown of carbohydrates results in energy, which is necessary for life. Based on evolution, humans and animals have evolved different ways to break down carbohydrates, create them, store them, and use them as building blocks.
Lipids (fats) can also be used to store energy and be broken down to release energy.