The different ways in which organisms use ATP
All living organisms need energy in order to stay alive. Initially this energy comes from the sun. Plants use the sun’s energy in process called photosynthesis to produce complex organic molecules such as glucose, from water and carbon dioxide. A cell cannot get its energy directly from glucose. For this reason glucose is broken down in a process known as respiration; this causes glucose to release its energy which is then used to produce adenosine triphosphate (ATP). ATP is now the immediate source of energy for the cell and allows it to carry out processes which are essential to life. ATP is found in both plant and animal cells.
Adenosine triphosphate is composed of the nucleotide base adenine, combined with a ribose sugar and three phosphate groups. The bonds between the phosphate groups are unstable and therefore have a low activation energy, meaning that they can easily be broken. When these bonds are broken significant amounts of energy are released. Usually, it is only the third phosphate that is removed. For this reason ATP is synthesized from ADP (adenosine diphosphate) and an inorganic phosphate molecule (the enzyme ATP synthase often catalyses the reaction). The reaction uses energy from an energy-releasing reaction, e.g. the breakdown of glucose in respiration. Energy is then stored as chemical energy within the final phosphate bond. This process is known as phosphorylation – adding phosphate to a molecule. So it is said that ADP is phosphorylated to become ATP. ATP then diffuses to the part of cell where the energy is needed. Here it is broken back down into ADP and an inorganic phosphate molecule, by a process known as hydrolysis (the splitting of a molecule using water). An enzyme named ATPase often catalyses this reaction. The chemical energy contained within the phosphate bond is released as a result and is then used in the cell.
Phosphorylation of ADP can occur in three different ways. The first...