ch 7+8
Did you know that..
Aerobic respiration yields up to 16 times as much ATP per glucose molecule as does fermentation - up to 32 molecules of ATP for respiration, compared with 2 molecules of ATP produced by substrate-level phosphorylation in fermentation.
Where does anaerobic respiration take place?
Anaerobic respiration, takes place in certain prokaryotic organisms (yeast?) that live in environments without oxygen. These organisms have an electron transport chain but do not use oxygen as a final electron acceptor at the end of the chain. Oxygen performs this function very well because it is extremely electronegative, but other, less electronegative substances can also serve as final electron acceptors. Some "sulfate-reducing" marine bacteria for instance, use the sulfate ion at the end of their respiratory chain. Operation of the chain builds up a proton-motive force used to produce ATP, but hydrogen sulfide is produced as by product rather than water. The rotten-egg odor you may have smelled while walking through a salt marsh or a mudflat signals the presence of sulfate-reducing bacteria.
How is fermentation an extension of glycolysis?
As an alternative to respiratory oxidation of organic nutrients, fermentation is an extension of glycolysis that allows continuous generation of ATP by the substrate-level phosphorylation of glycolysis. For this to occur, there must be a sufficient supply of NAD+ to accept electrons during the oxidation step of glycolysis. Without some mechanism to recycle NAD+ from NADH, glycolysis would soon deplete the cell's pool of NAD+ by reducing it all to NADH and would shut itself down for a lack of an oxidizing agent. Under aerobic conditions, NAD+ is recycled from NADH by the transfer of electrons to the electron transport chain. An anaerobic alternative is to transfer electrons from NADH to pyruvate, the end product of glycolysis.
How do fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen?
Because most of the ATP generated by cellular respiration is due to the work of oxidative phosphorylation, our estimate of ATP yield from aerobic respiration is contingent on an adequate supply of oxygen to the cell. Without electronegative oxygen to pull electrons down the transport chain, oxidative phosphorylation eventually ceases. However, there are two general mechanisms by which certain cells can oxidize organic fuel and generate ATP without the use of oxygen: Anaerobic respiration and fermentation. The distinction between these two is that an electron transport chain is used in anaerobic respiration but not in fermentation. (The electron transport chain is also called the respiratory chain because of its role in both types of cellular respiration.)
What were the first organisms to produce O2?
Cyanobacteria produced oxygen as a by-product of photosynthesis. Therefore, early prokaryotes may have generated ATP exclusively from glycolysis. The fact that glycolysis is today the most widespread metabolic pathway among Earth's organisms suggests that it evolved very early in the history of life. The cytosolic location of glycolysis also implies great antiquity; the pathway does not require any of the membrane bounded organelles of the eukaryotic cell, which evolved approximately 1 billion years after the prokaryotic cell (how the fk do we know this sht?!) Glycolysis is a metabolic heirloom from early cells that continues to function in fermentation and as the first stage in the breakdown of organic molecules by respiration.
What happens during lactic acid fermentation?
During lactic acid fermentation, pyruvate is reduced directly to NADH to form lactate as an end product with no release of CO2. Lactate is an ionized form of lactic acid. Lactic acid fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and yogurt.
What are the three alternative cellular pathways for producing ATP by harvesting the chemical energy of food?
Fermentation, anaerobic respiration, and aerobic respiration. All three use glycolysis to oxidize glucose and other organic fuels to pyruvate, with a net production of 2 ATP by substrate-level phosphorylation. And in all three pathways, NAD+ is the oxidizing agent that accepts electrons from food during glycolysis.
How does pyruvate act as a key juncture in catabolism? [Figure 9.18]
Glycolysis is common to fermentation and cellular respiration. The end product of glycolysis, pyruvate, represents a fork in the catabolic pathways of glucose oxidation. In a facultative anaerobe or a muscle cell, which are capable of both aerobic and cellular respiration and fermentation, pyruvate is committed to one of those two pathways, usually depending on whether or not oxygen is present.
What occurs in alcohol fermentation?
Here, pyruvate is converted to ethanol (ethyl alcohol) in two steps. The first step releases carbon dioxide from the pyruvate, which is converted to the two-carbon compound acetaldehyde. In the second step, acetaldehyde is reduced by NADH to ethanol. (Acetyl gained an electron). This regenerates the supply of NAD+ needed for the continuation of glycolysis. Many bacteria carry out alcohol fermentation under anaerobic conditions. Yeast (a fungus) also carries out alcohol fermentation. For thousands of years, humans have used yeast in brewing, winemaking, and baking. The CO2 bubbles generated by baker's yeast during alcohol fermentation allow bread to rise.
A glucose-fed yeast cell is moved from an aerobic environment to an anaerobic environment. How would its rate of glucose consumption change if ATP were generated at the same rate?
I believe if the bacteria would have ATP generated 16 times slower if moved into an anaerobic environment. [9.5 - 2] The cell would need to consume glucose at a rate about 16 times the consumption rate in the aerobic environment (2 ATP are generated by fermentation versus up to 32 ATP by cellular respiration.) Nice was right.
Consider the NADH formed during glycolysis. What is the final acceptor for its electrons during fermentation? What is the final acceptor for its electrons during aerobic respiration?
I believe the final acceptor in fermentation is NAD+, or possibly the lactate/ethanol molecules themselves. For aerobic respiration I believe the final electron acceptor is oxygen. [9.5 - 1] The final acceptor for its electrons during fermentation is a derivative of pyruvate, such as acetaldehyde during alcohol fermentation, or pyruvate itself during lactic acid fermentation. The final acceptor for electrons during aerobic respiration is oxygen.
Figure 9.17 - Fermentation. Describe it.
In the absence of oxygen, many cells use fermentation to produce ATP by substrate-level phosphorylation. Pyruvate, the end product of glycolysis, serves as an electron acceptor for oxidizing NADH back to NAD+, which can then be reused in glycolysis.
What does fermentation consist of aside from glycolysis?
It consists of glycolysis plus reactions that regenerate NAD+ by transferring electrons from NADH to pyruvate or derivatives of pyruvate. The NAD+ can then be reused to oxidize sugar by glycolysis, which nets two molecules of ATP by substrate-level phosphorylation.
What is fermentation?
It is a way of harvesting chemical energy without using oxygen or any electron transport chain, in other words without cellular respiration.
What type of cells can carry out aerobic oxidation of pyruvate (taking away electrons), but not fermentation?
Only a few types such as cells of the vertebrate brain cannot carry out fermentation.
How can food be oxidized without cellular respiration?
Remember, oxidation simply refers to the loss of electrons to an electron acceptor, so it does not need to involve oxygen. Glycolysis oxidizes glucose to two molecules of pyruvate. The oxidizing agent of glycolysis is NAD+, and neither oxygen nor any electron transfer chain is involved. Overall glycolysis is exergonic and some of the energy made available is used to produce 2 ATP (net) by substrate-level phosphorylation. If oxygen is present, then additional ATP is made by oxidative phosphorylation when NADH passes electrons removed from glucose to the electron transport chain. But glycolysis generates 2 ATP whether oxygen is present or not - that is, whether conditions are aerobic or anaerobic.
What is the evolutionary significance of Glycolysis?
The role of glycolysis in both fermentation and respiration has an evolutionary basis. Ancient prokaryotes are thought to have used glycolysis to make ATP long before oxygen was present in Earth's atmosphere. The oldest known fossils of bacteria date back 3.5 billion years, but appreciable quantities of oxygen probably did not begin to accumulate in the atmosphere until 2.7 billion years ago (how the **** do we know this?).
What are the types of fermentation, that differ in the end products formed from pyruvate?
The two common types are alcohol fermentation and lactic acid fermentation.
What are obligate anaerobes?
These are organisms that carry out only fermentation or anaerobic respiration. In fact, these organisms cannot survive in the presence of oxygen, some forms of which can actually be toxic (wow imagine that - life that doesn't need oxygen to exist) if protective systems are not present in the cell.
What are facultative anaerobes?
These are organisms, including yeasts and many bacteria, that can make enough ATP to survive using either fermentation or respiration. On the cellular level, our muscle cells behave as facultative anaerobes. In such cells, pyruvate is a fork in the metabolic road that leads to alternative catabolic routes. Under aerobic conditions, pyruvate can be converted to acetyl CoA, and oxidation continues in the citric acid cycle via aerobic respiration. Under anaerobic conditions, lactic acid fermentation occurs: Pyruvate is diverted from the citric acid cycle, serving instead as an electron acceptor to recycle NAD+. To make the same amount of ATP, a facultative anaerobe has to consume sugar at a much faster rate when fermenting than when respiring.
How do human cells make ATP by lactic acid fermentation when oxygen is scarce?
This occurs during the early stages of strenuous exercise, when sugar catabolism for ATP production outpaces the muscle's supply of oxygen from the blood. Under these conditions, the cells switch from aerobic respiration to fermentation. (Holy snipes no fking wonder this is what happens! I wonder how long it takes). The lactate that accumulates was previously thought to cause muscle fatigue and pain, but recent research suggests instead that increased levels of potassium ions may be to blame, while lactate appears to enhance muscle performance. In any case, the excess lactate is gradually carried away by the blood to the liver, where it is converted back to pyruvate by liver cells. Because oxygen is available, this pyruvate can then enter the mitochondria in liver cells and complete cellular respiration.
