Most organisms on Earth obtain energy derived from a chemical process known as photosynthesis. Photosynthesis occurs in most plants and some bacteria. During photosynthesis, these plants and bacteria will combine carbon dioxide (CO₂) and water (H₂O) and using energy from light, convert these ingredients into oxygen (O₂) and sugar (C₆H₁₂O₆). The balanced chemical equation looks like this:

6(CO₂) + 6(H₂O) + light -----> C₆H₁₂O₆ + 6(H₂O)

This sugar is then food for the organism. Even organisms, such as humans, which don't undergo photosynthesis, indirectly get our food from this chemical process. When you eat vegetables, the stored energy you are eating exists because those plants underwent photosynthesis. Even when you eat meat, say beef, that cow ate grass or hay which in turn got their energy through photosynthesis.

However, there are a few organisms that do not get their food from photosynthesis. When light isn't available as a source of energy, some bacteria have evolved to harness energy from carbon-containing molecules in a process known as chemosynthesis. This process has only been found to occur in bacteria which live near the ocean floor, where there is no sunlight. During this process, bacteria convert carbon dioxide (CO₂) or methane (CH₄) into more complex organic matter, allowing them to obtain energy without sunlight. Although, it should be noted that the number of organisms obtaining their food from chemosynthesis is much smaller than through photosynthesis.

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