Mitochondria are organelles found in all cells in the human body, with the exception of mature red blood cells. They are able to migrate through the cytoplasm of a cell, and it is believed that they are able to reproduce themselves, since mitochondria contain their own primitive form of DNA. They all may vary in size and shape, but all have the same basic structure. Each is surrounded by an outer membrane that contains the transport protein, porin, which forms wide channels through the lipid bilayer. In contrast, the inner membrane of the mitochondria is impermeable to ions and other small molecules, except where a path is provided by transport proteins. Thus, the contents within the inner membrane are highly specialized.
The inner membrane has many folds called cristae, which extend into the matrix, or central area, of the organelle. The cristae and matrix create different compartments within mitochondria, and each have different roles in the organelle's function of cellular respiration. Therefore, mitochondria serve as the sites for the production of cellular energy using the Krebs cycle.
A few things must happen in the cytoplasm of a cell before the Krebs cycle can begin to work. This little process is the aerobic respiration of glucose (glycolysis) into two molecules of pyruvic acid.
Glucose + 2NAD + 2ADP +2Phosphate groups --> 2 pyruvic acid + 2NADH + 2ATP
The glycolysis pathway begins when 2 ATP molecules are phosphorylated and the energy derived is used to activate the breakdown of the glucose molecule. The phosphate groups are added to the new intermediate molecules present in the pathway. At this point two molecules of NAD (Nicotinamide adenine dinucleotide) are reduced by two pairs of hydrogen atoms. Each pair of hydrogen atoms donates two electrons to each NAD molecule, thus reducing it. Each reduced NAD binds one hydrogen proton and leaves one hydrogen ion. So, 2 NADH molecules and 2 hydrogen ions are created. Finally, at the end of the reaction as energy is released, the oxidation of an aldehyde group to a carboxylic acid group in the intermediate molecules produces enough energy to convert 4 molecules of ADP to 4 ATP; what is left are 2 molecules of pyruvic acid. Since 2 ATP molecules were used to activate the reaction, the net gain of the reaction is 2 molecules of ATP, 2 molecules of NADH and 2 molecules of pyruvic acid.
The molecules of pyruvic acid then enter the mitochondrion from the cytoplasm and the Krebs cycle is initiated.