Daily Newsletter
October 5, 2012
Fermentation & Scientific Article
What happens if the mitochondria runs out of oxygen?
Remember, oxygen is the final electron acceptor. Once the electrons have powered the H+ pumps in the inner mitochondrial membrane, they are at low energy. Electrons don't disappear, they have to be given to another atom/molecule. With the mitochondira, Complex IV passes the electrons (and some hydrogen) to Oxygen, thus forming water.
If not replenished, oxygen concentrations in the mitochondria will start to drop. For humans, this will happen when we either can not get enough oxygen through breathing or when our cells are metabolically very active (e.g., a work-out). If we don't have oxygen, then we don't have a final electron acceptor.
When there is no electron carrier, then Complex IV can not get rid of the electrons it holds. It can not accept electrons from Cytochrom C. If Cytochrom C can not reduce something, then it can not accept electrons from Complex III. This means Ubiquinone will not be able to reduce Complex III, and so can't accept electrons from Complex I. Ultimately, this means that complex I can not oxidize NADH + H+.
Is that really a big deal? YES. Remember talking about Flavin and how it can act up, creating free radicals? One trigger for creating free radicals is an imbalance in the expected ratio of NAD+ and NADH + H+. When you get too much NADH + H+ Complex I starts forming free radicals. (NOTE: this is only one outcome...the point is we need to regenerate a quality of NAD+)
When eukaryotic cells start running out of NAD+, Pyruvate Dehydrogenase becomes inactive (think regulation). This means that the cell has to do something to the building concentration of pyruvate (remember...we don't want to build up product...product should always become a new substrate). Pyruvate is moves to a different reaction or pathway.
At this point remember that the trigger for the shift is a loss of NAD+ and a build up of NADH + H+.
In animal cells, protist, and many fungal cells, the pyruvate will be oxidized into Lactate. In some fungi, the pyruvate enters a small pathway that oxidizes and decarboxylates the molecule, resulting in Ethanol (alcohol fermentation).
So, what is the result of moving to fermentation. In fermentation, we have to oxidize pyruvate. Electron carriers are needed for redox reactions, so in the oxidation of pyruvate NADH + H+→NAD+. This regenerates NAD+. Since we have NAD+, we can also carry out the substrate level phosphorylations that occur in Glycolysis.
You will recall that the substrate level phosphorylations of glycolysis are not strong energy harvests. More energy is harvested in redox reactions than in substrate level phosphorylation. So, why does it matter that we have some minimal ATP production?
Your cells have critical enzymes that have to keep working for the cell to survive. In an anaerobic state (no oxygen), you don't have enough ATP to carry out all life functions, but there is enough thanks to fermentation to keep critical enzymes running. The goal is then to bring more oxygen into the cells.
Is there a time limit? YES, but depends. Some fungi can survive in fermentation indefinitely (but they may not be able to survive they're by-product, e.g. alcohol). For other cells, yes, there is a limit. Lactate in water is Lactic Acid, and it does alter pH. More importantly, there are some cells that have more critical function than others. Neurons in the Brain and Heart muscle cells are both very sensitive to low oxygen. Neither can stay in a fermentative state. If these cells do not get oxygen, they will quickly start to die. Other cells can stay ferementative for longer, such as skeletal muscle cells; they will eventually stop working, but they won't die. Once oxygen is restored, the cells will quickly begin to shift back to respiration (aerobic, using the respiratory chain).
What happens to the Lactate? That depends on the organism. Cells will purge themselves of lactate if they can, and will convert some back to pyruvate. There are some cells that are fond of lactate. Heart Muscles actually prefer lactate to glucose, and liver cells can convert lactate back to glucose.
Lactate is not wasted.
At this point it needs to be noted that bacteria have a very different relationship with fermentation. There are some bacteria that live exclusively as fermenters, and others that happily switch between respiration and fermentation. Also, bacteria have many more ways of altering pyruvate than the production of Lactate or Ethanol. Some of these, such as the formation of acetate, have become industrially important.
Daily Challenge
In your own words, explain why a eukaryotic cell would shift between respiration and fermentation. Why is it imporant for cells to have the ability to switch?Link to Forum
Scientific Article
Read the following article and in the forum answer these questions: Authors intent, Hypothesis, define 3 new terms or background, define 1 technique or procedure, explain 1 figure or table, state the author's conclusion.Hojung Nam, Tom M Conrad, Nathan E Lewis, The role of cellular objectives and selective pressures in metabolic pathway evolution, Current Opinion in Biotechnology, Volume 22, Issue 4, August 2011, Pages 595-600, ISSN 0958-1669, 10.1016/j.copbio.2011.03.006.
(http://www.sciencedirect.com/science/article/pii/S0958166911000450)
Remember: Look for the article through the GSU library website (The link above will not take you to the full article unless you are logged into the GSU library system).
Link to Forum
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