Which of the following is true of passive transport?
Answer: A
Correct answer: it requires a gradient
Passive transport is driven by the concentration or electrochemical gradient and by nothing else - no ATP, no carrier necessarily - so it stops when the gradient is gone. Endocytosis is an active, energy-consuming process, and passive transport moves solutes as well as water.
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A cell exposed to a hypertonic environment will _____by osmosis:
Answer: B
Correct answer: lose water
A hypertonic environment has the higher solute concentration outside, so water moves out of the cell, down its own gradient. In a walled bacterial or plant cell the membrane pulls away from the wall - plasmolysis. Bursting is what happens in a hypotonic solution.
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Active transport of a substance across a membrane requires:
Answer: B
Correct answer: the expenditure of ATP
Active transport moves a substance against its gradient, which cannot happen spontaneously, so it must be paid for - directly by hydrolysing ATP, or indirectly by coupling to an ion gradient that ATP maintains. Needing a favourable gradient is the mark of passive transport, not of this.
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Psychrophiles would be expected to grow:
Answer: C
Correct answer: at refrigeration temperatures
Psychrophiles are cold-loving, growing best below about 15°C and often at 0°C. Their enzymes and membranes are adapted to stay flexible in the cold, which is why they spoil refrigerated food while mesophiles cannot.
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Superoxide ion is toxic to strict anaerobes because they lack:
Answer: C
Correct answer: dismutase
Oxygen chemistry produces the superoxide radical, and the enzyme that disposes of it is superoxide dismutase, which converts it to hydrogen peroxide and oxygen. Strict anaerobes lack it, so superoxide accumulates and destroys their cell components - which is why oxygen is lethal to them. (Catalase then removes the peroxide, and most anaerobes lack that too.)
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The time required for a cell to undergo binary fission is called the:
Answer: C
Correct answer: generation time
The generation time (doubling time) is the interval in which one cell becomes two, or a population doubles. It varies enormously - about 20 minutes for E. coli in rich medium, some 20 hours for Mycobacterium tuberculosis - and it is what sets the slope of the exponential phase of the growth curve.
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In a viable plate count, each ___represents a ___from the sample population.
Answer: B
Correct answer: colony, cell
The assumption behind a viable plate count is that each colony grew from a single cell that could reproduce. So counting the colonies and multiplying by the dilution gives the number of viable cells in the original sample - and the result is reported as colony-forming units, since a clump would also give one colony.
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_________is another term for biosynthesis.
Answer: B
Correct answer: anabolism
Anabolism is biosynthesis: small precursors assembled into the macromolecules the cell needs, using the energy and reducing power that catabolism supplies. Metabolism is the two halves together.
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Catabolism is a form of metabolism in which _____molecules are converted into ________molecules.
Answer: A
Correct answer: large, small
Catabolism degrades: large molecules are broken into small ones, and the energy released is captured as ATP. It is the source of both the energy and the precursors that anabolism goes on to spend.
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An enzyme ______ the activation energy required for a chemical reaction.
Answer: C
Correct answer: lowers
An enzyme lowers the activation energy - the barrier a reaction must climb before it can proceed - by binding the substrate in a way that stabilises the transition state. What it does not change is the equilibrium or the overall free energy change: it makes the reaction faster, not more favourable.
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