Active transport is the process by which molecules move across a membrane against the concentration gradient. This is a process that requires energy in the form of ATP to drive it, as it is not passive like diffusion. The process of active transport is vitally important, and differing ion concentrations are observed across the membranes of every cell in the body, which is due to active transport. The most commonly observed cell membrane pump is the sodium-potassium pump, which pumps sodium ions out of the cell, and potassium ions into the cell. The transport membrane involved in the sodium-potassium pump has a site for ATP to bind on in the interior of the cell, and the protein catalyses the hydrolysis of ATP into ADP+P, releasing energy as it does so, which drives the active process of the pump. The use of ATP in this way is the single greatest way that it is used, using up 50% of the ATP we use in a day, about 20Kg at rest. ATP is used in maintaining the ionic balance in all cells, and in this method of diffusion the potential difference is increased across the membranes of the cells, aiding the processes needed for the further synthesis of ATP.
Another use of ATP is in the process of protein synthesis, such as in the replication of DNA. In this process ATP is the source of energy used in the polymerisation of nucleotides to form new DNA strands. This is because the nucleotides that serve as substrates for DNA polymerase are similar to ATP in that they have three phosphates, but with an oxyribose sugar group. It is also driven by the energy released by the hydrolysis of the terminal phosphate.
Another chief use of ATP as the means for transferring and using energy in the body is in mechanical processes, such as the contraction of muscles, the beating of cilia and the movement of chromosomes during cellular reproduction. Once again the energy for these processes come from the crucial hydrolysis of ATP into ADP and an inorganic phosphate, which provides the energy that is turned into muscular work. This process can be prolonged by the regeneration of ATP involving creatine phosphate (ADP + PCr -> ATP + Cr) but this can only be done for a few seconds before the reserves of creatine run out, which are replaced by ATP from respiration, and then via the lactate pathway ATP is formed and the muscles incur an oxygen debt. The muscles are actually moved by the transfer of these phosphate groups to contractile proteins.
When ATP releases its terminal phosphate it releases energy that is harnessed by the cell to endergonic processes by transferring the third phosphate group to another molecule, when this transfer of the molecule has been completed the recipient is phosphorylated, and this intermediary is more reactive (unstable) than the original ATP molecule. This effectively transfers the energy that is made available by the ATP to whatever molecule that is phosphorylated, enabling it to carry out any one of the many processes for which ATP is required. It is not only its ability to transfer energy to other molecules that makes ATP so useful, but also the fact that it is small and water-soluble molecule enables it to get to all parts of the body. The fact that there are surprisingly small amounts of ATP in the body is due to the fact that it can be regenerated so quickly, so much so that there is only about 5g of ATP in the body at any one time, and 40kg of it is turned over in a day, and 10 million molecules of ATP are consumed and regenerated per second per cell. This shows how important and how widespread ATP is in the body, as it is needed in all non-passive processes.
Sources
Biology 2 – Cambridge Advanced Sciences
Biology (6th ed) – Campbell ● Reece
http://www.eyesight.org/Research/Research-ATP/research-atp.html
