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MCAT Sample Questions : Biological Sciences

Passage V

Scientists have hypothesized that mitochondria evolved from aerobic heterotrophic bacteria that entered and established symbiotic relationships with primitive eukaryotic anaerobes. Many structural and functional similarities between mitochondria and present-day bacteria support this hypothesis. They are approximately the same size, reproduce by similar means, and contain non-histone-bound DNA. They contain the tRNAs, ribosomes, etc., necessary for transcription and translation, and they show some similarities in base sequences of rRNAs.

In addition, the inner membranes of mitochondria have enzymes and transport systems similar to those on the plasma membranes of bacteria. One similar system is the electron transport system (ETS). Electron transport in both mitochondria and bacteria is accomplished using three large protein complexes, each composed of multiple polypeptides (Figure 1).

Figure 1 Electron transport across inner mitochondrial membrane

Hydrogen atoms and electrons donated from NADH are passed between components of the electron transport chain and eventually reduce oxygen to form water. This chain of events creates both a pH gradient and an electrical potential across the membrane. The protons are thought to move down the pH gradient, interacting with the enzyme ATP synthetase. This results in the production of ATP from ADP and phosphate.

Following are some sample questions on this passage:

  1. According to the hypothesis described in the passage, the bacteria that entered primitive eukaryotic cells were able to carry out which of the following functions that the primitive eukaryotic cells could NOT?

    1. Glycolysis
    2. Krebs cycle and electron transport
    3. Cell division
    4. Transcription and translation

    Answer: B

    Explanation: The primitive eukaryotic cells are described in the passage as anaerobes, so it is likely that they could breakdown sugars by glycolysis.That means answer choice A is incorrect.The bacteria are described as aerobic, so the passage suggests that the ability to use oxygen was acquired with the bacteria.Electron transport in mitochondria and bacteria is described and the electron transport system is needed for eukaryotes to use oxygen.The acquisition of aerobic bacteria as partners would have provided them with the ability to carry out aerobic metabolism through use of the Krebs cycle and electron transport.Answer choice B is therefore the correct choice.The anaerobic, eukaryote-precursor cells must have engaged in cell division and the processes of transcription and translation, so answer choices C and D are incorrect.

  2. Most proteins in present-day mitochondria are made by cytoplasmic ribosomes from mRNA transcribed from nuclear genes. Can this fact be reconciled with the hypothesis described in the passage?

    1. Yes; the transfer of genes from symbionts to the eukaryotic nucleus could have occurred during the last billion years of evolution.
    2. Yes; this difference from bacteria is unimportant, because the many similarities between bacteria and mitochondria provide sufficient evidence in favor of the hypothesis.
    3. No; the fact that mitochondrial proteins are made in the cytoplasm is convincing evidence that mitochondria do not have a bacterial origin.
    4. No; because bacteria can make all their own proteins and mitochondria cannot, this disproves the hypothesis.

    Answer: A

    Explanation: The endosymbiotic theory described in the passage is widely accepted today, so there must be some way of using the theory to reconcile the fact that mitochondrial proteins are made in the cytoplasm.We know, for example, that lateral transfer of genes from one genome to another is widespread among living organisms.It would have been an easy matter for mitochondrial genes to be acquired by the host cell nucleus and to disappear from the mitochondrial genome.It is likely that this exchange would have resulted in a net gain in efficiency for the composite organism.Answer choice A is therefore the most plausible answer.Despite many similarities between mitochondria and bacteria, the question of cytoplasmic synthesis of mitochondrial proteins is relevant to the question of the origin of mitochondria and needs to be answered.The fact that mitochondria do have all the machinery (ribosomes, t-RNA, etc.) for protein synthesis eliminates answer choice C from consideration

  3. The chemical gramicidin inserts into membranes and creates an artificial pathway for proton movement. Based on Figure 1, if mitochondria are treated with gramicidin, the rate of ATP synthesis will most likely:

    1. increase, because of increased proton movement back into the mitochondria.
    2. decrease, because of a decreased rate of hydrogen-atom donation by NADH.
    3. decrease, because the proton gradient will rapidly reach equilibrium.
    4. not be altered, because sufficient protons will remain between the membranes to generate ATP.

    Answer: C

    Explanation: Hydrogen ions (H+) are protons.The provision of a channel for proton flow across the membrane would allow hydrogen ions to flow across the membrane until equilibrium had been achieved between the concentrations on each side of the membrane.Because ATP synthesis is driven by a flow of hydrogen ions down a concentration gradient, ATP production will decrease and eventually stop as equilibrium is established (not increase as suggested in answer choice A or remain unchanged as suggested in answer choice D).The decrease has nothing to do with the rate of hydrogen ion donation by NADH, answer choice B.Answer choice C is the correct answer.

  4. According to the hypothesis in the passage, what is the most likely explanation for the origin(s) of the two mitochondrial membranes?

    1. Both inner and outer membranes were derived from folding of the prokaryotic plasma membrane.
    2. Both inner and outer membranes were derived from invagination of the eukaryotic plasma membrane.
    3. The inner membrane was derived from folding of the prokaryotic plasma membrane, and the outer from invagination of the eukaryotic plasma membrane.
    4. The inner membrane was derived from invagination of the eukaryotic plasma membrane, and the outer from folding of the prokaryotic plasma membrane.

    Answer: C

    Explanation: When a eukaryotic cell ingests a particle in the process known as endocytosis, the particle ends up in the cytoplasm inside a vesicle, a hollow membrane bounded intracellular organelle.The membrane of the vesicle is formed from a closed-up patch of membrane detached from the cell's own plasma membrane.Prokaryotes also have their own plasma membranes.It seems likely that the two membranes of the mitochondrion consist of an inner prokaryote-derived membrane and an outer eukaryote-derived one, this arrangement having been established when some early cell engulfed a bacterium through the process of endocytosis.Answer choice C describes this arrangement best.Because the primitive eukaryotic-precursor cell did not use electron transport, their membranes lacked the complicated protein machinery for this process.The inner membrane of mitochondria contains this machinery while the outer one lacks it.Answer choices B and D both suggest eukaryotic membranes make up the inner membrane, so they must be wrong.If answer choice A was true, the electron transport proteins would be found in both membranes of mitochondria

  5. Which of the following pieces of evidence most strongly supports the hypothesis of mitochondrial origin described in the passage?

    1. Mitochondria have fewer genes than typical bacterial cells have.
    2. Mitochondria contain hundreds of different enzymes.
    3. The diameters of mitochondria and typical present-day bacteria are approximately equal.
    4. Nitrogen-fixing bacteria live symbiotically inside the cells of present-day plants.

    Answer: D

    Explanation: Answer choice D, is the correct answer.It presents an example of bacteria inhabiting eukaryotic cells that is analogous to the hypothesized relationship between mitochondria and their eukaryote hosts.Because it shows that such a relationship is possible, it provides stronger evidence than the other answer choices in support of the theory presented in the passage.That the number of genes in a mitochondrion is less than that of the typical bacterial cell (answer A) does not support the hypothesis in the passage, it refutes it.That mitochondria have hundreds of different enzymes (answer B) neither supports the theory or refutes it since the number in bacteria and mitochondria are not compared.That mitochondria have diameters that are the same as bacteria is dubious (answer C) since both mitochondria and bacteria are very diverse in their sizes and shapes.

  6. To support the symbiotic hypothesis presented in the passage, mitochondria should be similar to bacteria in which of the following ways?

    1. They should use 80S ribosomes.
    2. They should be incapable of binary fission.
    3. They should have circular DNA.
    4. They should be capable of anaerobic respiration.

    Answer: C

    Explanation: Answer choice C is the correct answer because it states that the fact that mitochondria and bacteria both have circular DNA is a reason to think they are related.Such a characteristic is likely to have been highly conserved over time since it involves the basic material of life.Changes in the topology of the DNA molecule would involve substantial changes in the way the molecule replicated and the way it was transcribed.The fact that both have circular DNA supports the symbiotic hypothesis presented in the passage.Alternative choices A and B presented are not actually true characteristics of bacteria.Bacteria do have 70S ribosomes, not the 80S ribosomes of eukaryotes as stated in answer choice A and they do reproduce by binary fission, which choice B denies.The loss of ability to carry out anaerobic respiration (answer D) might be a product of the long history of association with the host cell.It is not unusual for symbiotic organisms to lose abilities that are compensated for by host functions.

  7. The chemical valinomycin inserts into membranes and causes the movement of K+ into the mitochondria. Based on Figure 1, if mitochondria are treated with valinomycin, the rate of ATP synthesis in the mitochondria will most likely:

    1. decrease, because the K+ will compete with protons at the active site on ATP synthetase.
    2. decrease, because movement of K+ into the mitochondrial compartments will disrupt proton movement into the intermembrane space.
    3. increase, because the net positive charge in the mitochondria will cause increased movement of protons into the intermembrane space.
    4. increase, because the additional positive charge will further activate ATP synthetase

    Answer: B

    Explanation: Any disruption of mitochondria is likely to decrease ATP production since they are a major cellular source of that molecule.Answer choices C and D can not be right because they propose an increase in ATP production.There is no information in the question to suggest that valinomycin will cause K+ to compete with H+ for an active site on ATP synthetase.Furthermore, one would suspect that ATP or precursor molecules such as phosphate and ADP would occupy the active site on the ATP synthase molecule.Answer choice A, therefore, does not seem plausible.The question does provide the information that valinomycin increases the flow of K+ across the membrane.An influx of another positively charged ion into the compartment would disrupt the electrochemical gradient responsible for the necessary flow of protons.Answer B, therefore, seems more plausible than any other choice

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