Krebs Cycle And Electron Transport example essay topic

Mitochondria Mitochondria are responsible for energy production. They are also the responsible location for which respiration takes place. Mitochondria contain enzymes that help convert food material into adenosine triphosphate (ATP), which can be used directly by the cell as an energy source. Mitochondria tend to be concentrated near cellular structures that require large inputs of energy, such as the flagellum.

The role of the mitochondria is very important in respiration. In the presence of oxygen, pyruvate or fatty acids, can be further oxidized in the mitochondria. Each mitochondrion is enclosed by two membranes separated by an intermembrane space. The intermembrane space extends into the folds of the inner membrane called cristae which dramatically increase the surface area of the inner membrane. Cristae extend into a dense material called the matrix, an area which contains RNA, DNA, proteins, ribosomes and range of solutes. This is similar to the contents of the chloroplast stroma and like the chloroplast, the mitochondrion is a semi-autonomous organelles containing the machinery for the production of some of its own proteins.

The main function of the mitochondrion is the oxidation of the pyruvate derived from glycolysis and related processes to produce the ATP required to perform cellular work. (Campbell 182-9) Pyruvate, or fatty acids from the breakdown of triglycerides or phospholipids, pass easily through pores in the outer mitochondrial membrane made up of a channel protein called por in. The inner membrane is a more significant barrier and specific transport proteins exist to carry pyruvate and fatty acids into the matrix. Once inside the matrix, pyruvate and fatty acids are converted to the two carbon compound acetyl coenzyme A (acetyl CoA). For pyruvate this involves a decarboxylation step which removes one of the three carbons of pyruvate as carbon dioxide. The energy released by the oxidation of pyruvate at this stage is used to reduce NAD to NADH.

(185) The C 2 acetyl CoA is then taken into a sequence of reactions known as Krebs cycle which completes the oxidation of carbon and regenerates an acceptor to keep the cycle going. The oxidation of the carbon is accompanied by the reduction of electron acceptors and the production of some ATP by substrate phosphorylation. The C 2 acetyl CoA is coupled to oxaloacetate, a C 4 acceptor in the cycle. The product is citrate a C 6 compound. This first product, citrate, is the reason the cycle is sometimes called the citric acid or ti carboxylic acid cycle, referring it after the scientist whose lab most advanced our understanding of it, Sir Hans Krebs. (Compton's 160) Two of the early reactions of the cycle are decarboxylation which shorten citrate to succinate a C 4 compound.

The CO 2 lost does not actually derive from acetyl CoA, during that cycle, but two carbons are lost which are the equivalent of the two introduced by acetyl CoA. The decarboxylation steps are again accompanied by the reduction of NAD to NADH. The formation of succinate also sees the formation of an ATP molecule by substrate phosphorylation. (Brit 1041) The last part of the cycle converts C 4 succinate back to C 4 oxaloacetate. In the process another reaction generates NADH while another reduces the electron acceptor FAD (Flavin Adenine Dinucleotide) to FADH. The final stage of respiration in the mitochondria involves the transfer of energy from the reduced compounds NADH and FADH to the potential energy store represented by ATP.

The process is oxidative phosphorylation and it is driven by a chemi osmotic system analogous to that seen in chloroplasts. (Moore 88-9) The inner membrane contains an electron transport chain that can receive electrons from reduced electron carriers. The energy lost as electrons flow between the components of the electron transport chain is coupled to the pumping of protons from the matrix to the intermembrane space. The matrix is alkalinized while the intermembrane space is acidified. The electrons are ultimately combined with molecular oxygen and protons to produce water.

Respiration is aerobic when oxygen is the terminal electron acceptor. (Brit 1042) The energy that was contained in the pyruvate molecule has at this point been converted to ATP by substrate phosphorylation in glycolysis and Krebs cycle and to a free energy gradient of protons across the inner membrane known as the proton motive force (PMF). The gradient of protons will tend to diffuse to equilibrium but charged substances like protons do not easily cross membranes. Proton complexes in the inner membrane provide a channel for the protons to return to the matrix. Those protein complexes function as an ATPase, an enzyme that synthesizes ATP, because the energy liberated as the protons work to diffuse back to the matrix is used to push the equilibrium between ADP+Pi and ATP strongly toward ATP.

(Campbell 182) The electron transport chain has three sites along it that pump protons from the matrix. NADH donates its electrons to the chain at a point where the energy input is sufficient to drive all three proton pumping sites. FADH is less energetic than NADH and its electrons are donated at a point that drives two proton pumping sites. It is also possible for the NADH produced in glycolysis to enter the mitochondrial matrix and donate electrons to the electron transport chain. Depending on the system, NADH from glycolysis may be able to drive two or three proton pumping sites. For eukaryotes, only two pumping sites are driven; for prokaryotes, three.

(184-5) The importance of mitochondria is unremarkably, a key element in the process of respiration. Between the three distinct sections of respiration, glycolysis, Krebs Cycle, and Electron Transport, the mitochondrion is the site of which most of it takes place, either inside of the mitochondrion or outside it.