Aerobic respiration is the major process used by oxygen- requiring organisms to generate energy. During respiration, glucose is metabolized to generate chemical energy in the form of ATP:C6H12O6 + 6O2 6CO2 + 6H2O + 36ATPThe biochemical machinery necessary for cellular respiration is found in the mitochondria, small organelles scattered throughout the cytoplasm of most eukaryotic cells. The number of mitochondria per cell varies by tissue type and cell function.Mitochondria are unusual in that they have their own genetic systems that are entirely separate from the cell's genetic material. However, mitochondrial replication is still dependent upon the cell's nuclear DNA to encode essential proteins required for replication. Despite this fact, mitochondria seem to replicate randomly, out of phase with both the cell cycle and other mitochondria.The nature of the mitochondrial genome and protein synthesizing machinery has led many researchers to postulate that mitochondria may have arisen as the result of the ingestion of a bacterium by a primitive cell millions of years ago. It is postulated that the two may have entered into a symbiotic relationship and eventually became dependent on each another; the cell sustained the bacterium, while the bacterium provided energy for the cell. Gradually, the two evolved into the present-day eukaryotic cell, with the mitochondrion retaining some of its own DNA. This is known as the endosymbiotic hypothesis. Because mitochondrial DNA is inherited in a non-Mendelian fashion (mitochondria are inherited from the maternal parent, who supplies most of the cytoplasm to the fertilized egg), it has been used to look at evolutionary relationships among different organisms.Four different human cell cultures erythrocytes, epidermal cells, skeletal muscle cells, and intestinal cells were grown in a medium containing radioactive adenine. After 10 days, the mitochondria were isolated via centrifugation, and their level of radioactivity was measured using a liquid scintillation counter. Which of the following cells would be expected to have the greatest number of counts per minute of radioactive decay?
Answer(s): C
According to the question stem, four different human cell cultures were grown in a medium containing radioactive adenine. So, the first thing that you should be thinking about is DNA replication; while these cells are replicating they're going to incorporate this radioactive adenine into all of their DNA. This includes chromosomal DNA, as well as mitochondrial DNA. Mitochondria replicate independently of their cells. Since all autosomal human cells have the same amount of DNA in their nuclei, the only difference in radioactivity will be the amount that was incorporated into the mitochondrial DNA. And this is why the cells' mitochondria were isolated via centrifugation, and then the radiation from each sample was measure using a scintillation counter. The cells with the greatest number of mitochondria will have the highest radioactive count when their mitochondria are separated. So, you need to determine which of the four cell types would have the greatest number of mitochondriA. This number is dependent on the energy needs of the tissue. Given the choices erythrocytes, epidermal cells, skeletal muscle cells, and intestinal cells you should know that the correct answer is choice C, skeletal muscle cells. Muscle cells need a lot of energy in order to contract. ATP is required every time a molecule of myosin binds to actin in the sarcomeres. In general, muscle cells have a higher content of mitochondria than do any other type of autosomal cell. Choice A, erythrocytes, do not even contain any mitochondriA. Choice B, epidermal cells, do not have any special energy requirements. The same thing applies to intestinal cells, choice D.
Aerobic respiration is the major process used by oxygen- requiring organisms to generate energy. During respiration, glucose is metabolized to generate chemical energy in the form of ATP:C6H12O6 + 6O2 6CO2 + 6H2O + 36ATPThe biochemical machinery necessary for cellular respiration is found in the mitochondria, small organelles scattered throughout the cytoplasm of most eukaryotic cells. The number of mitochondria per cell varies by tissue type and cell function.Mitochondria are unusual in that they have their own genetic systems that are entirely separate from the cell's genetic material. However, mitochondrial replication is still dependent upon the cell's nuclear DNA to encode essential proteins required for replication. Despite this fact, mitochondria seem to replicate randomly, out of phase with both the cell cycle and other mitochondria.The nature of the mitochondrial genome and protein synthesizing machinery has led many researchers to postulate that mitochondria may have arisen as the result of the ingestion of a bacterium by a primitive cell millions of years ago. It is postulated that the two may have entered into a symbiotic relationship and eventually became dependent on each another; the cell sustained the bacterium, while the bacterium provided energy for the cell. Gradually, the two evolved into the present-day eukaryotic cell, with the mitochondrion retaining some of its own DNA. This is known as the endosymbiotic hypothesis. Because mitochondrial DNA is inherited in a non-Mendelian fashion (mitochondria are inherited from the maternal parent, who supplies most of the cytoplasm to the fertilized egg), it has been used to look at evolutionary relationships among different organisms.Which of the following pieces of evidence would NOT support the hypothesis that mitochondria were once independent bacteria that eventually formed a symbiotic relationship with eukaryotic cells?
Answer(s): B
This question requires a little bit of reasoning on your part. In the passage you're told that many scientists believe that mitochondria were once independent unicellular entities, possibly prokaryotic in origin, which formed a symbiotic relationship with eukaryotic cells that was mutually beneficial to both parties. This theory is known as the endosymbiotic hypothesis. If mitochondria are believed to have been prokaryotic in origin, then any findings that reveal similarities between mitochondria and bacteria would support the hypothesis. So let's take a look at the answer choices and find the one that does NOT support the hypothesis, since that's what the question asks you to do. Choice A says that mitochondrial DNA is circular and not enclosed by a nuclear membrane, which is, in fact, true. Well, that sounds like the way things are set up in bacteriA. Bacteria have a single circular chromosome located in a region of the cell known as the nucleoid, which is not bound by a membrane. So choice A is incorrect because it SUPPORTS the endosymbiotic hypothesis. Choice B states that mitochondrial ribosome more closely resemble those found in eukaryotes than those found in prokaryotes. This does NOT support the hypothesis any similarity to eukaryotic cells doesn't support it. So choice B appears to be the correct answer, but let's continue on anyway. Choice C says that there are many present-day bacteria that have symbiotic relationships with eukaryotic cells. This DOES support the concept that the mitochondrial ancestor could have lived within a eukaryotic cell in a mutualistic relationship. So, choice C is wrong because it supports the hypothesis. Choice D says that mitochondrial DNA codes for its own ribosomal RNA. This provides evidence that at some point, mitochondria existed as free entities, capable of directing their own protein synthesis and cell division, and all the cellular activities associated with independently living organisms.So, choice D is also incorrect. By the way, the information contained in choice B is NOT true; mitochondrial ribosomes actually DO resemble prokaryotic ribosomes, thereby providing further support for the endosymbiotic hypothesis. However, if it were true, it would not support the hypothesis, which is why choice B is the correct answer.
The mechanism for the acid-catalyzed esterification of a carboxylic acid, carried out with R'OH, is shown below. The tagged alcohol R'18OH is used to study the reaction mechanism. The resulting ester is separated from the reaction mixture; the water from the reaction mixture is then distilled off completely and collected as a separate fraction. Assuming that only the forward reaction occurs, which of the following statements is correct?
Answer(s): A
This question concerns the acid-catalyzed esterification of a carboxylic acid. In this reaction, the carbonyl carbon of the carboxylic acid reacts with the alcohol oxygen to form an ester linkage. Therefore, if the alcohol has a tagged oxygen, the ester itself will be tagged when the reaction is complete. Thus Choice A is correct.Choices B, and C are incorrect because the water fraction will not contain labeled oxygen, since the oxygen of the water comes from the hydroxyl group on the carboxylic acid, which was never labeled. The answer CAN be determined from knowledge of the mechanism, so choice D is incorrect as well.
The mechanism for the acid-catalyzed esterification of a carboxylic acid, carried out with R'OH, is shown below. The tagged alcohol R'18OH is used to study the reaction mechanism. The resulting ester is separated from the reaction mixture; the water from the reaction mixture is then distilled off completely and collected as a separate fraction. The rate of the reaction is negligible without the acid catalyst. The catalyst:
This question asks why the esterification of a carboxylic acid with an alcohol should be carried out under acid catalysis. To answer this, it's important to understand the mechanism presented in the passage. In the first step, the proton of the acid catalyst attacks the oxygen of the carboxylic acid carbonyl group. This produces a positively charged carbon, which is very susceptible to nucleophilic attack by the alcohol. Thus the reaction occurs between the carbonyl carbon, which is electrophilic and the alcohol oxygen, which is nucleophilic.Choice A agrees with this mechanism and is therefore the correct response.
The mechanism for the acid-catalyzed esterification of a carboxylic acid, carried out with R'OH, is shown below. The tagged alcohol R'18OH is used to study the reaction mechanism. The resulting ester is separated from the reaction mixture; the water from the reaction mixture is then distilled off completely and collected as a separate fraction. Esterification may also occur between parts of the same molecule. Which of the following compounds would most easily undergo internal esterification to form a cyclic ester?
This question asks which compound would most easily undergo internal esterification to form a cyclic ester. In this case, the stability of ring structures must be considered. The least amount of ring strain occurs in structures that can form six-membered rings, like cyclohexane. If choice A were to form a cyclic ester, it would be a four- membered ring with a lot of angle and eclipsing strain. Choice B would be a five-membered ring, which is not ideally stable. Choice C, if it underwent internal esterification, would form a six-membered ring. Five of the atoms comprising the ring would be carbons, and one would be an oxygen. This formation is strain free; therefore, it would be most easily formed. Choice D would yield a seven-membered ring, which is less stable than choice C.
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