Cellular Respiration

Respiration occurs in two forms; aerobic (using oxygen) and anaerobic (without oxygen). The purpose of respiration is to generate ATP which is then used as a form of energy for numerous processes such as muscle contraction and protein synthesis.

            Respiration can be broken down into four stages; glycolysis, the link reaction, the Krebs cycle and the electron transport chain.

            Glycolysis is the first stage of respiration and is anaerobic (does not use oxygen). It occurs in the cytoplasm, as this is where the enzymes for the glycotic pathway are. The first stage is the phosphorylation of glucose (C6H12O6) where two phosphates (from the hydrolysis of two ATP molecules) are added to a glucose molecule to form phosphorylated glucose. The phosphorylated glucose (6 carbon) is then split into two 3-carbon groups called triose phosphate. (So far no ATP has been generated and in fact 2 ATP molecules have been used up). The final stage of glycolysis is the conversion of two triose phosphate molecules to two pyruvate molecules (also 3-carbon) by the removal of the phosphate groups. Triose phosphate is oxidised by NAD to form reduced NAD. Enzyme controlled reactions also take place where inorganic phosphate molecules are combined with ADP to form ATP. For each pyruvate molecule formed (of which there are two) there are two ATP molecules formed (as well as 1 reduced NAD). Overall there is a gain of 2ATP molecules as two are used at the start in phosphorylation. The pyruvate molecules are then actively transported into the matrix of the mitochondria where the link reaction and Krebs cycle take place.

- Mitochondria (mitochondrion – singular) – mitochondria are cell organelles that are associated with cell respiration. This is where all AEROBIC respiration takes place. As aerobic respiration generates much more ATP than anaerobic respiration (glycolysis), cells that contain many mitochondria generally have a metabolic function etc and require large amounts of ATP. They therefore need more mitochondria to provide enough energy (in the form of ATP) to meet their metabolic demands.

            The link reaction involves the formation of a compound called acetylcoenzyme A. Pyruvate is oxidised by the removal of hydrogen and forms reduced NAD. A carbon atom (as well as oxygen) is also removed from the pyruvate to produce a 2-carbon molecule called an acetyl group. The removal of carbon and oxygen from the pyruvate directly forms CO2 (it is not formed by the combination of carbon with molecular oxygen). The Acetyl group then combines with a coenzyme (coenzyme A) and this forms acetylcoenzyme A. For each pyruvate, one reduced NAD, 1 CO2 molecule and 1 acetylcoenzyme A is formed. Whilst this process produces no ATP directly, it generates acetylcoenzyme A which is used in the Krebs cycle and the electron transport chain to produce large quantities of ATP which are invaluable to the cell.