Aerobic refers to organisms that require oxygen to exist; for example, most living organisms need oxygen to stay alive. As humans, our cells get our energy by using oxygen to fuel our metabolism. Anaerobic refers to organisms that need little or no oxygen to exist; it often refers to bacteria, such as those found in the human small intestines.

ATP (adenosine triphosphate) is the universal energy currency of a cell for both plants and animals. Its secret lies in its structure: ATP contains three negatively charged phosphate groups. When the bond between the outermost two phosphate groups is broken, ATP becomes ADP (adenosine diphosphate). This reaction releases 7.3 kcal/mole of ATP, which is a great deal of energy by cell standards.

All cells need the ATP in order to survive. For example, in humans, ATP is used for a large range of biological actions, with each cell in the body estimated to use between one to two billion ATPs per minute, from muscle contractions to providing the energy needed to move the “tail” of a sperm cell in order to reach the female’s egg cell. In plants, ATP is not only used in photosynthesis, but in the plant’s root hair cells, which need ATP to absorb the essential mineral ions from the soil in order to grow.