Cellular Respiration and the Role of Mitochondria

Cellular Respiration and the Role of Mitochondria

Cellular respiration is the process of oxidising food molecules, such as glucose, to carbon dioxide and water and releasing the covalent bond energy in the form of ATP for use by all the energy-consuming activities of the cell. Mitochondria are membrane-enclosed organelles distributed through the cytosol of most eukaryotic cells. They are where cellular aerobic respiration occurs; indeed cells without mitochondria cannot respire aerobically.

Cellular respiration consists of two broad phases, initially, glycolysis (the breakdown of glucose to pyruvic acid) Occurs, this is followed by the oxidation of pyruvic acid to carbon dioxide and water. In eukaryotes, glycolysis occurs in the cytosol (The fluid in which cell organelles are suspended). The remaining processes take place in the mitochondria.

The first stage, glycolysis is the anaerobic catabolism of glucose, it occurs in almost all cells. The process uses glucose and co-enzyme NAD (Nicotinamide Adenine Dinucleotide), and yields 2 molecules of Pyruvic acid, as below

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C6H12O6 + 2NAD+ -> 2C3H4O3 + 2NADH + 2H+

The free energy stored in 2 molecules of pyruvic acid is somewhat less than that in the original glucose molecule.

Some of this difference is captured in 2 molecules of ATP.

The Krebs Cycle then decarboxylates the pyruvic acid resulting in a 2-carbon fragment of acetate. This 2-carbon fragment is coupled to a molecule of oxaloacetic acid , this results in a molecule of citric acid (the process is also known as the citric acid cycle) undergoes a series of enzymatic steps. Each of the 3 carbon atoms present in the pyruvate that entered the mitochondrion leaves as a molecule of carbon dioxide (CO2). A pair of electrons (2e-) are removed and transferred to NAD+ reducing it to NADH + H+ A pair of electrons is removed from succinic acid and reduces FAD to FADH2. The electrons of NADH and FADH2 are transferred to the respiratory chain. The final step regenerates a molecule of oxaloacetic acid and the cycle is ready to turn again.

The four complexes of integral membrane proteins that make up the respiratory chain accomplish the stepwise transfer of electrons from NADH (and FADH2) to oxygen atoms to form (with the aid of protons) water molecules (H2O). The energy released is harnessing by the pumping of protons (H+) from the matrix to the intermembrane space. Protons are then pumped at 3 (of the four) complexes, NADH dehydrogenase,

cytochrome c reductase and cytochrome c oxidase. An average of 3 protons is pumped out at each complex as each pair of electrons passes through it. Thus some 9 protons are pumped for each pair of electrons contributed to the respiratory chain by NADH; (6 for each pair contributed by FADH2). The gradient of protons formed across the inner membrane by this process of active transport acts as energy storage. The protons can flow back down this gradient, re-entering the matrix, only through another complex of integral proteins in the inner membrane, the ATP synthase complex.

The energy released as electrons pass down the gradient from NADH to oxygen is harnessed by the NADH dehydrogenase, Cytochrome c reductase and Cytochrome c oxidase complexes to pump protons (H+) against their concentration gradient from the matrix of the mitochondrion into the intermembrane space (this is an example of active transport). As their concentration increases there (which is the same as saying that the pH decreases), a strong diffusion gradient is set up. The only exit for these protons is through the ATP synthase complex. As in chloroplasts, the energy released as these electrons flow down their gradient is harnessed to the synthesis of ATP. The process is called chemiosmosis and is an example of facilitated diffusion.

The energy stored in the proton gradient is used for a number of other mitochondrial functions such as the active transport of a variety of essential molecules and ions through the mitochondrial membranes. NADH is also used as reducing agent for many cellular reactions. So the actual yield of ATP as mitochondria respire varies with conditions. It is generally regarded as seldom reaching 30

Mitochondria are vital to the proper function of almost all cells (the only exceptions being a few species of yeast's) because of the role they play in cellular respiration, they do however, unusually have their own DNA in the form of plasmids within the inner membrane. It is believed that mitochondria are the evolutionary descendants of a prokaryote that established an endosymbiotic relationship with the ancestors of eukaryotic cells early in the history of life on earth.