On page 197, Shubin discusses mitochondrial diseases. What problems does Shubin discuss that are associated with mitochondria? What is the function of mitochondria? Explain the process of cellular respiration. Discuss the relationship between mitochondria and bacteria. Explain endosymbiosis.
(Bobby Muttilainen, rmuttilainen@gmail.com)
The list of problems that arise from malfunctioning mitochondria is an extremely long one, which Shubin points out is because "if there is a problem in the chemical reactions in which oxygen is consumed, energy production can be impaired" (197). Any problems with the functioning of mitochondria, depending on where and how severe the malfunction is, can lead to a huge assortment of problems, from weakness to death. The specific disease that Shubin discusses is called cardioencehalomyopathy, which "results from a genetic change that interrupts the normal metabolic function of mitochondria" (198). While studying this disease, the scientists changed the genes of a microbe known as Paracoccus denitrificans in the same way as the DNA of the mitochondria in the patient had been altered. They produced this exact change and were able to simulate parts of the mitochondrial disease in the bacteria, which supports the theory of endosymbiosis.
ReplyDeleteThe theory of endosymbiosis suggests that "mitochondria and plastids were formerly small prokaryotes that began living within larger cells" (Campbell 516). It proposes that the free-living microbes probably entered the host cell as undigested prey or internal parasites, a process which scientists have actually observed. Thus, our mitochondria arose from ancient bacteria. This relationship between mitochondria and bacteria helps scientists study the diseases of mitochondria because they "can do all kinds of experiments with bacteria that are not possible with human cells" (Shubin 197).
These prokaryotes that became mitochondria due to the selective advantages of having a mutually beneficial symbiosis serve very important purposes in our cells. Mitochondria serve as the location of cellular respiration in cells. Cellular respiration is the "metabolic process that generates ATP by extracting energy from sugars, fats, and other fuels with the help of oxygen" (Campbell 109). The process contains three steps, which include glycolysis, the citric acid cycle and oxidative phosphorylation along the electron transport chain. Without mitochondria, the cells would have no energy and would not be able to function.
Clever title, Bobby.
Hannah Kay (hgkay@aol.com)
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ReplyDeleteHannah’s explanation of the problems related to mitochondrial disease is pretty accurate; any oxygen-related process in the body relies heavily on the mitochondria. Another mitochondrial disease is Leigh’s disease, in which mitochondrial DNA in the brain stem is mutated, creating dysfunctional mitochondria (http://www.ninds.nih.gov/disorders/leighsdisease/leighsdisease.htm). These mutations result in a chronic lack of energy which then leads to motor function and central nervous system problems, and eventually death. In essence, dysfunctional mitochondria mean loss of energy throughout the body, which will almost always result in death.
ReplyDeleteMitochondria are the “powerhouses” of the cell, turning sugar into ATP (adenosine triphosphate, an energy-laden molecule) or metabolizing toxins and regulating cell function (197). They produce this energy through one of the most important metabolic processes, cellular respiration, which Hannah touched on in her response. The first step, glycolysis, in which glucose is broken into two smaller molecules called pyruvate by first spending a small amount of energy (2 ATP) to use phosphofructokinase to put a phosphate group onto the sugar before breaking it down (Campbell 168). After glucose is originally broken down, twice as much ATP (4 ATP) is gained through substrate-level phosphorylation (Campbell 167), where a phosphate group is transferred to ADP (adenosine diphosphate, a molecule with less energy than ATP) by a kinase, creating more ATP. Additionally, NAD+ (an electron carrier) is reduced to store an electron for energy later. Before the second step, the Krebs (or citric acid) cycle, pyruvate must be converted into a usable form through the bridge reaction, where pyruvate is actively transported into the mitochondrion and, after the reduction of NAD+, is combined with coenzyme A to create acetyl CoA. Acetyl CoA is what initiates the Krebs cycle, by adding two carbons to oxeloacetate to create citrate. This molecule is repeatedly oxidized to reduce NAD+, create ATP and reduce FAD (another electron carrier). After several rounds of oxidation, the molecule is again oxeloacetate, restarting the cycle (Campbell 171). The final step of cellular respiration is oxidative phosphorylation; previously, all ATP created was made through substrate-level phosphorylation, yielding only 4 ATP molecules (Campbell 172). The electron transport chain is the first part of oxidative phosphorylation, where electrons are transferred from NADH and FADH2 to membrane proteins to create hydrogen ions (H+); the gradient created by the high [H+] in the space results in their return through a transmembrane protein called ATP synthase in the second part of oxidative phosphorylation, chemiosmosis; the ions flow through the protein, powering it to attach free phosphate groups to ADP, creating high amounts of ATP. Overall, each glucose molecule leads to 36-38 molecules of ATP, a vital source of energy.
Hannah also described endosymbiosis pretty well; the general definition of endosymbiosis is the relationship between an organism and another organism (the endosymbiont) that lives inside its body or a cell. Thus, endosymbiotic theory centers on the idea that some organelles, namely mitochondria and chloroplasts, originated as bacterial endosymbionts that were at one point engulfed in a cell which eventually developed in to the cells we have now. The evidence for endosymbiotic theory in relation to mitochondria is rather bountiful, as mitochondria and bacteria have multiple similarities: they both have a similar cellular and genetic structure (197), cannot be formed in a cell, but can only come from previous mitochondria or bacteria, are surrounded by two membranes, and both have a single circular DNA molecule (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endosymbiosis.html).
Eugene Bulkin (doubleaw002@gmail.com)