Respiration and Glycolysis
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Respiration can be defined as the oxidation of the end products of glycolysis with the storage of the energy in the form of ATP. Cellular respiration occurs when oxygen is available, and the products are carbon dioxide and water. There are three main pathways in the cellular respiration process. These are: pyruvate oxidation, the citric acid cycle, and the respiratory chain.
Pyruvate oxidation in eukaryotic cells occurs inside the mitochondrion in the inner membrane, and in prokaryotes on the inner face of the plasma membrane. This step is the crucial link between the steps of glycolysis and cellular respiration. In this step, pyruvate is oxidized into acetate. Pyruvate from the end of the glycolysis cycle diffuses into the mitochondria, where it gets oxidized. The three-carbon pyruvate loses two of its hydrogen atoms and also a carboxyl grouping. A two-carbon acetyl group, free energy, and carbon dioxide are made. Coenzyme A links to the acetyl group, and captures the free energy that is there. A little of the energy that was made gets saved when NAD+ is reduced to NADH+H+. Some of the rest of the remaining energy is stored temporarily when the acetyl group combines with CoA. Pyruvate dehydrogenase complex catalyzes the reaction. This catalyzing agent alone contains 72 polypeptide chains. Acetyl coenzyme A is the product of this cycle, and moves into the citric acid cycle to continue the process.
The citric acid cycle receives the acetyl CoA, and begins its system. This system occurs inside the mitochondrion matrix in eukaryotes and in the cytoplasm in prokaryotes. The inputs that start the CAC are water, acetyl, and oxidized electron carriers. For every acetyl group the cycle goes over, there are usually two carbons in the form of carbon dioxide removed, and four pairs of hydrogen atoms are used to reduce carrier molecules. The two-carbon acetyl group combines with the four-carbon oxalacetate and in turn form a six-carbon citrate. The energy that was stored from before in the CoA drove that reaction. Here the coenzyme A goes away to be recycled, as it was just a carrier molecule for the acetyl group. In the next reaction, the citrate from before gets reorganized, and it becomes isocitrate. This then gets converted into alpha-
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ketoglutarate when one carbon dioxide and two hydrogens are taken away. This part of the reaction series makes a large drop in the free energy that is around. The energy that does get released gets stored in NADH+H+. The next part of the reaction chain is when the five-carbon alpha-k molecule gets oxidized into a four-carbon molecule called succinate. Here again, carbon dioxide is released, some energy is preserved with the combination of CoA and succinate, and some of the oxidation energy is stored in NADH+H+ again. The energy that is in the succinyl CoA is then removed and used to make GTP from GDP and Pi. This is an example of phosphorylation on the substrate level. Next, ADP is used to make ATP by using GTP. More free energy gets released when the succinyl CoA gets oxidized to yield fumarate. During this, two more hydrogens are moved to an enzyme that has the carrier FAD. One more NAD+ reduction occurs and makes oxaloacetate from malate. Water is added, which makes an OH- group, and the hydrogen from the group gets taken off in the next step to make NAD+ reduce to NADH+H+. Water is used here, and in turn provides lots f energy because of its abundance. The finishing product that we have is oxaloacetate, however, this process has
to be repeated again. The final product gets to combine with another CoA and go around the whole circle again. This happens two times per glucose that goes through glycolysis. The CAC reactions together are one of the most efficient energy gatherers of any of the systems along the process of respiration. The end outputs of the CAC are: 2 carbon dioxide molecules, ATP, 2 NADH and an FADH2 molecule. From this phase, the respiratory chain takes over.
The actions of the respiratory chain occur for prokaryotes on