<P> A dominant role for the mitochondria is the production of ATP, as reflected by the large number of proteins in the inner membrane for this task . This is done by oxidizing the major products of glucose: pyruvate, and NADH, which are produced in the cytosol . This type of cellular respiration known as aerobic respiration, is dependent on the presence of oxygen . When oxygen is limited, the glycolytic products will be metabolized by anaerobic fermentation, a process that is independent of the mitochondria . The production of ATP from glucose has an approximately 13 - times higher yield during aerobic respiration compared to fermentation . Plant mitochondria can also produce a limited amount of ATP without oxygen by using the alternate substrate nitrite . ATP crosses out through the inner membrane with the help of a specific protein, and across the outer membrane via porins . ADP returns via the same route . </P> <P> Pyruvate molecules produced by glycolysis are actively transported across the inner mitochondrial membrane, and into the matrix where they can either be oxidized and combined with coenzyme A to form CO, acetyl - CoA, and NADH, or they can be carboxylated (by pyruvate carboxylase) to form oxaloacetate . This latter reaction" fills up" the amount of oxaloacetate in the citric acid cycle, and is therefore an anaplerotic reaction, increasing the cycle's capacity to metabolize acetyl - CoA when the tissue's energy needs (e.g. in muscle) are suddenly increased by activity . </P> <P> In the citric acid cycle, all the intermediates (e.g. citrate, iso - citrate, alpha - ketoglutarate, succinate, fumarate, malate and oxaloacetate) are regenerated during each turn of the cycle . Adding more of any of these intermediates to the mitochondrion therefore means that the additional amount is retained within the cycle, increasing all the other intermediates as one is converted into the other . Hence, the addition of any one of them to the cycle has an anaplerotic effect, and its removal has a cataplerotic effect . These anaplerotic and cataplerotic reactions will, during the course of the cycle, increase or decrease the amount of oxaloacetate available to combine with acetyl - CoA to form citric acid . This in turn increases or decreases the rate of ATP production by the mitochondrion, and thus the availability of ATP to the cell . </P> <P> Acetyl - CoA, on the other hand, derived from pyruvate oxidation, or from the beta - oxidation of fatty acids, is the only fuel to enter the citric acid cycle . With each turn of the cycle one molecule of acetyl - CoA is consumed for every molecule of oxaloacetate present in the mitochondrial matrix, and is never regenerated . It is the oxidation of the acetate portion of acetyl - CoA that produces CO and water, with the energy thus released captured in the form of ATP . </P>

Role of mitochondria and chloroplast in energy transduction