In order for a living organism to do work endergonic reactions must be linked with exergonic reactions. The energy that the body requires is provided by respiration where the aerobic oxidation of a glucose molecule provides 2870 kJ of energy. Respiration is not just one big reaction but is in-fact a series of small steps with each reactions being regulated by enzymes. The enzymes help to lower the activation energy of glucose so that aerobic respiration can take place relatively easily. Theoretically, the energy released from respiration could be harnessed in the cell so that it can be used to do work directly. However, a much more flexible system actually occurs where exergonic reactions in all organisms are linked to the production of an intermediary molecule, ATP (adenosine triphosphate).
The structure of ATP consists of the purine base, adenine, a 5C ribose and 3 phosphate groups.
When ATP is hydrolysed, a phosphate Pi is lost and ADP is formed. This reaction releases 30.5 kJ of energy and is reversible, it is the interconversion of ATP and ADP that is all important in providing energy for the cell.
ATP + H2O ↔ ADP + H3PO4 30.5 kJ
ATP is an instant source of energy within the cell, it is mobile and transports chemical potential energy to endergonic reactions anywhere in the cell. When energy is needed it is simply a matter of hydrolysing the ATP. It is a universal energy carrier that is found in all cells and acts as the cell’s energy currency; trading it for ADP and vice versa. When hydrolysis of ATP releases more energy than required for a particular endergonic reaction the excess is released as heat, maintaining the body’s temperature. With its reversible character ATP acts as an intermediate between respiration and energy requiring processes with phosphates being constantly removed and replaced.
In aerobic respiration, most of the ATP that is produced comes from the energy that is released when hydrogen ions and electrons pass down a respiratory chain of carriers in a series of redox reactions – this process is called oxidative phosphorylation.
There is no definitive answer to how ATP is generated without going through the respiration pathway. However, there is a theory developed by Peter Mitchell – the Chemiosmosis theory. It states that ATP is generated using electrical potential energy. This energy is stored as a difference in the concentration in hydrogen ions across the phospholipid membrane of the mitochondria/choloroplast. The hydrogen ions are actively transported through the inner membrane/thylakoid membrane into the inter-membranal space/stroma. It is then allowed to flow down the gradient into the organelle via carrier proteins in the organelle membrane. Part of this protein acts as an enzyme which synthesises ATP and is called ATP synthase. The transfer of three H+ ions through the protein channel with ADP and Pi in the organelle allows ATP to be produced.
Active transport is crucial to cells in order to maintain the ionic content of cells which directly affects the metabolism of the cell. All cells show differences in concentrations of ions, especially sodium and potassium, with respect to the cell’s surroundings. Most cells have sodium and potassium pumps to regulate ion concentration, this requires energy as carrier proteins are moving ions against the concentration gradient. The energy which maintains operation of these proteins is provided by ATP.