ATP is produced by almost all living things. It stands for adenosine triphosphate and it is the major 'currency' of energy in the body. When the third phosphate bond is created, it immediately is broken and energy is released that can fuel the metabolic chemical reactions required by living organisms, which otherwise would be inefficient. This creates ADP which has one less phosphate attached to the group. ATP has many functions in body such as it is used in muscle contraction, active transport and biosynthetic reactions.
ATP is also used in muscle contraction which is obviously essential for numerous animals including human beings. ATP molecules attach to the myosin heads on actin filaments which causes them to become detached. Then the enzyme ATPase (which is activated by calcium ions) hydrolyses the ATP to ADP. The energy lost from the ATP is used to move the myosin heads back to their original positions. Without this mechanism muscles would remain contracted after stimulation and would therefore not be able to respond to further stimuli. This is exactly what happens in rigor mortis. As there is no food source in the body and no respiration, there is no ATP and therefore the myosin heads remain attached to the actin filaments.
Active transport is especially important in maximising the amount of nutrients we get from food into our cells. An example of this is the absorption of glucose in the small intestine. By diffusion alone (a passive process) only 50% of the glucose in the intestine can diffuse into the epithelial cells and then the blood. This means that unless active transport occurs there is a huge amount of food wastage. Active transport can be defined as the movement of a substance across a membrane from a region where it is in a lower concentration to a region of higher concentration and therefore moving against the concentration gradient.
As this process works against the concentration gradient energy is required and this is supplied in