A chloroplast is about 5 micrometres long and a mitochondrion about 1 micrometre. How many times longer is the chloroplast?
Answer: C
Dividing 5 by 1 gives five. Comparing volumes rather than lengths would give a much larger factor, since volume scales with the cube of the linear dimension. Chloroplasts are correspondingly bulkier organelles than mitochondria.
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State whether the following is true or false.
Mitochondrial ribosomes are more similar to bacterial ribosomes than to those of the surrounding cytosol.
Answer: D
They resemble the bacterial 70S type, which is among the strongest evidence for the endosymbiotic origin. It also explains why certain antibiotics have side effects on human cells that other drugs do not. Some aminoglycosides damage hearing through exactly this resemblance.
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Cyanide blocks respiration by inhibiting
Answer: A
Stopping the chain halts proton pumping, so the gradient collapses and ATP synthesis fails. Oxygen still enters the cell freely; what fails is the ability to use it. Death follows quickly because the brain and heart depend on aerobic respiration.
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Consider the following statements about chemiosmosis.
1. A proton gradient stores energy across a membrane.
2. ATP synthase couples proton flow to ATP synthesis.
3. The gradient is established across the outer mitochondrial membrane.
Answer: B
The first two are the core of the theory. The third names the wrong membrane, since the outer one is made permeable by porins and can hold no gradient at all. Everything of consequence happens across the inner membrane.
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Oxygen's role at the end of the electron transport chain is to
Answer: C
Without a final acceptor the carriers would remain reduced and the chain would stop, which is why respiration fails without oxygen. Combining with electrons and protons yields water as the product. This is the water formed as the end product of aerobic respiration.
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State whether the following is true or false.
A mitochondrion can make ATP in the complete absence of oxygen.
Answer: D
The chain backs up without a terminal electron acceptor, so no protons are pumped and the gradient collapses. Cells then fall back on glycolysis in the cytosol, which yields far less ATP per glucose molecule. The shortfall is why anaerobic exercise cannot be sustained for long.
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Heteroplasmy in mitochondrial genetics refers to a cell containing
Answer: A
Because a cell holds many mitochondria, a mutation may be present in only some of them. The proportion determines whether symptoms appear, which is why mitochondrial disease varies so much in severity. Two tissues in one person can differ in their mutation load.
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Assertion (A): Two siblings can inherit the same mitochondrial mutation and yet differ greatly in how severely they are affected.
Reason (R): The proportion of mutant mitochondria passed on varies from one egg to another.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason names heteroplasmy. A cell carries many mitochondrial genomes, and a mutation is usually present in only a fraction of them; that fraction is partitioned unevenly among the eggs a mother produces. Since disease appears only above a threshold proportion, siblings inheriting different loads can differ from unaffected to severely affected - which is why mitochondrial disorders are so difficult to counsel about.
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A tissue with a high energy demand, such as cardiac muscle, is characteristically rich in
Answer: C
Both the number of mitochondria and the folding within each rise with demand, since ATP output depends on inner membrane area. Muscle that works continuously shows this most clearly of all. Cardiac muscle never rests, so its demand is continuous rather than episodic.
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A muscle cell contains 2,000 mitochondria and doubles that number during endurance training. If each makes 30 units of ATP a second, what is the increase in output per second?
Answer: D
The extra 2,000 mitochondria at 30 units each add 60,000 units a second. Training increases capacity by building more of the machinery rather than by making each unit faster. Endurance training works largely by expanding this mitochondrial capacity.
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