The first law of thermodynamics states that energy in our universe is never lost or destroyed but is instead converted from one form to another. This concept can be demonstrated by analysing the way in which we derive energy from our environment and how this energy is converted into a form which can be used to power the cellular processes necessary for life. As animals, we consume this energy first through our diet. Chemical energy stored within glucose in food is released and converted to potential energy through a catabolic pathway known as cellular respiration. This potential energy is then transformed to kinetic energy, ready to be used to power the various chemical, mechanical or locomotive functions required by the cell. This essay will describe the processes which instigate this energy release and how it is put to use in the body. It also aims to explain the idea of 'energy currency' in the cell by outlining the structure and formation of the molecule responsible for providing it: ATP or adenosine triphosphate.
ATP is a nucleoside comprised of a central ribose sugar, a purine adenine base and a chain of three phosphate groups. It is an immediate energy source in the cell and is formed during three stages. The first stage begins by harvesting chemical energy from oxidation of a glucose molecule. This process takes place in the cytoplasm and is known as glycolysis. Since the energy within organic molecules is stored within the individual atoms, it can only be released by breaking the bonds which hold the atoms together. This requires an 'energy spend' of two ATP molecules to assist the breakdown of glucose into intermediate substrates called glyceraldehyde-3-phosphates. Further breakdown enables the coenzyme NAD+ to pick up high-energy electrons and hydrogen ions, forming two NADH molecules.
It also releases energy allowing phosphate group to bond with ADP, forming two molecules of ATP in a process called substrate level phosphorylation. Further breakdown to pyruvate generates an additional two molecules of ATP, giving glycolysis an overall energy 'profit' of two ATP. The next stage of cellular respiration also yields ATP by substrate level phosphorylation. This stage, known as the Citric Acid Cycle, completes the oxidation of glucose and takes place in the mitochondria of the cell. Pyruvate diffuses through the cell membrane and undergoes several chemical reactions to form Acetyl Co-A, producing carbon dioxide as a waste product. NADH and FADH2 also carry electrons during this stage as well. Another two molecules of ATP are produced which can be immediately used by the cell for energy.
The majority of ATP produced by our body is formed by the third and final stage of cellular respiration in a process called oxidative phosphorylation. This is known as the electron transfer stage in which NADH and FADH2 give up the electrons they gained from glycolysis and the Citric Acid Cycle, releasing energy. ATP is then generated by an enzyme called ATP synthase which uses a hydrogen ion gradient to capture the energy released from the high-energy electrons. In this way, oxidative phosphorylation yields 34 molecules of ATP for every molecule of glucose. Thus, all the chemical energy harvested from the original glucose molecule is now as ATP in the form of potential energy, ready to be used for cellular work.
It is the molecular arrangement of ATP which then allows the release of this potential energy. The breakdown of ATP to ADP and consequent regeneration is what affords each cell the currency to survive and carry out the cellular work for a particular function. Since all three phosphate groups are negatively charged, the molecule is unstable and readily gives up it's terminal phosphate group through hydrolysis to form ADP (adenosine diphosphate) and an inorganic phosphate molecule. This reaction is exergonic, releasing approx triangle -13 kcal.mol. A reaction is described as exergonic when it releases energy into its surroundings and occurs spontaneously, giving products that have less potential energy than their reactants. An endergonic reaction requires an input of energy from its surrounding and products have more potential energy than reactants. It is the ability of ATP to couple endergonic and exergonic reactions that makes life able to continue. By giving up a phosphate group in the exergonic transformation of ATP to ADP, this allows other reactants to pick them up and gain energy to allow an endergonic reaction to take place. This process is known as energy coupling in the cell.
The notion of ATP as an energy currency is bourne from the idea that ATP must first be 'spent' (in the form of 2 ATP molecules in the glycolysis stage) in order to gain a 'profit' of 36 ATP molecules per oxidised glucose molecule. Without continuous recycling and management of the energy in our cells by ATP, we wouldn't have the energy to make new blood, move around, rid our bodies of poisons or even reproduce. The significance of ATP was highlighted in 1997 when the Nobel Prize for Chemistry was shared between Boyer, Walker & Skou, when they consolidated the structure of ATP and the role of ATP synthase.
