Respiration is a process were chemical energy in organic molecules are released by oxidation in the form of ATP. Carbohydrates, fats and proteins can be used. ATP is the energy carrier in a cell.

There is four main steps in respiration, with the first being Glycolysis. Glycolysis is the sequence of reactions which convert Glucose into pyruvate. This occurs in the cytoplasm and it involves the oxidation of glucose to pyruvate. This process does not require oxygen. There are three main processes in Glycolysis. The first is the Phosphorilation of sugar (Glucose). This is the energising step. Then there is lysis during which the phosphorilated sugar is split up into two sugar phosphates. The third is the oxidation of dehydrogenation. Each C3 sugar is converted to Pyruvate, This involves dehydrogenation making a reduced NAD and two ATP.

The second is the link step, where Pyruvate enters mitochondrion through mitochondrial envelope. It is converted to an acetyl group. These are carried by co enzyme A as acetyl co enzyme A or acetyl co A. Acetyl groups have only 2 carbons, whereas pyruvate has three so one is lost by a decarboxylation reaction. Reduced NAD produced in this reaction.

The third step is the Krebs cycle. Acetyl coenzyme A links with 4 carbons starting the cycle forming 6 carbon. Oxidative decarboxylation converts 6C to 5 C to 4C. The cycle regenerates the 4C to 6C by picking up the next acetyl group. There is a big energy difference from 5C to 4C so there is direct production of ATP from ADP + Pi. Dehydrogenation reactions occur forming 3 NADH2 and 1 FADH. These reduced coenzymes are used in the next stage.

The next step is the electron transport chain (ETC). This is a means by which energy from the Krebs cycle, which is now in the form of hydrogen atoms, is converted to ATP. This occurs on the cristae of mitochondria. The hydrogen atoms (carried as 10 reduced NAD and 2 reduced FAD) enter the inner membrane of the mitochondria. Hydrogen can be oxidised to water using oxygen. The respiratory chain is a series of hydrogen and electron carriers ending with oxygen. Hydrogen or electrons are passed from one carrier to the next until they reach oxygen. There are 3 hydrogen carriers, NAD, F, Co. Q. There is also three electron carriers Cyt b, Cyt c and Cyt a. H2 transferred to Co. Q is released as 2H+. 2 electrons are transferred along the chromosomes to Cyt a (oxidase). During the progression of NADH2 to Cyt a there are 3 ATPs produced. During the progression of FADH to cyt a there are 2 ATPs produced. ATP production is only possible ate three stages of the pathway (NAD to FP, Cyt b to Cyt c to Cyt a). The final stage involves cytochrome oxidase which contains copper. This stage can be inhibited by cyanide or carbon monoxide. Cyanide combines with the copper and prevents O2 combining with it so stops the chain.

So altogether there is 38 ATP formed. 34 in the Electron Transport Chain and 4 from the 3 other processes (Links, Krebs and Glycolysis). When Glucose is not available protein or fat is then used. Fatty acids are oxidised in a beta oxidation reaction and this removes the acetyl groups to feed them to the Krebs cycle. The number of Krebs cycles occurring simultaneously is limited only by molecules available and the size of the matrix.