The genetic material of a typical bacterium is
Answer: C
The nucleoid is a region of the cytoplasm rather than a compartment, since no membrane separates it. Plasmids may be present in addition, but they carry accessory genes such as those for antibiotic resistance rather than the genes essential for life.
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Plasmids differ from the bacterial chromosome in that plasmids
Answer: D
Correct answer: replicate independently of the chromosome
A plasmid is a small circular DNA molecule with its own origin of replication, so it is copied and maintained separately from the chromosome. It carries genes that are useful but not essential - antibiotic resistance, toxins, conjugation machinery - which is why a cell can be cured of a plasmid and live. That independence is also what makes plasmids the standard vector in genetic engineering.
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Which structure is present in a plant cell but absent from an animal cell?
Answer: A
A large central vacuole and chloroplasts are the other plant-specific features. The remaining three are common to plant and animal cells alike, which is why none of them can serve to tell the two kinds apart under a microscope.
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Centrioles are typically found in
Answer: B
Higher plants organise their spindles without centrioles, which is why their absence is a useful marker. Animal cells use them to form the centrosome and also the basal bodies from which cilia and flagella grow, so they have roles beyond division.
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State whether the following is true or false.
Both plant and animal cells possess mitochondria.
Answer: C
Plants respire as animals do and need the same ATP, so they carry mitochondria as well as chloroplasts. The common misconception that photosynthesis replaces respiration in plants mistakes two distinct processes for alternatives.
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A cube-shaped cell has sides of 2 micrometres. What is its ratio of surface area to volume?
Answer: D
Surface area is six times four, which is 24, and volume is two cubed, which is 8, giving a ratio of 3 to 1. For any cube the ratio is simply 6 divided by the length of the side, which makes the effect of size on the ratio easy to see.
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If that cell doubled its side length to 4 micrometres, what would the ratio become?
Answer: A
Six divided by the new side length of 4 gives 1.5. Doubling the size therefore halves the surface available for each unit of volume, which is precisely the constraint that keeps cells small and forces large organisms to be multicellular.
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Assertion (A): Cells are generally very small.
Reason (R): A larger cell has less surface area available for each unit of its volume.
Answer: B
Both are true and the reason gives the constraint: exchange happens across the surface while demand scales with volume, so growth beyond a certain size starves the interior. Large cells such as eggs get round it by being metabolically quiet.
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State whether the following is true or false.
Doubling the diameter of a spherical cell doubles its surface area to volume ratio.
Answer: C
It halves the ratio instead. Surface area rises with the square of the radius while volume rises with the cube, so the ratio falls as size increases. This is the whole reason cells stay small, and why large organisms add cells rather than enlarging them.
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A neuron can be extremely long despite the surface-to-volume constraint because it is
Answer: D
Length alone does not create the problem; bulk does. An axon a metre long may be only a micrometre across, so every point within it remains close to the membrane through which exchange happens. Shape matters here more than overall length.
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