CDS (II) 2022 General Knowledge · Question 17 of 120

Correct — B, Nucleus and chloroplasts are surrounded separately by their own membrane. A plant cell is eukaryotic, and the defining feature of a eukaryote is that its genetic material sits inside a nucleus bounded by a nuclear envelope. The chloroplast is a separate organelle with its own boundary, and in fact a double one — an outer and an inner membrane, with a third system of thylakoid membranes stacked into grana inside it. The two organelles are not connected and neither one lends its membrane to the other, which is exactly what the option states. The point matters biologically: keeping the genome in one compartment and photosynthesis in another is what lets a cell run incompatible chemistry in the same volume at the same time.

• (a)Nucleus does not need a membrane but chloroplasts are surrounded by their own membrane. — Describes a cell that does not exist among plants. A nucleus without a membrane is the nucleoid of a bacterium, and bacteria have no chloroplasts at all.
• (c)Nucleus is surrounded by membrane but chloroplasts do not need membrane. — Reverses the error. An unbounded chloroplast would spill its chlorophyll and its stroma enzymes into the cytoplasm; the proton gradient that drives ATP synthesis in photosynthesis depends on an intact membrane to hold it.
• (d)Both nucleus and chloroplasts are not surrounded by any membrane. — Denies the compartmentation that makes a cell eukaryotic. If this were true, a plant cell would be a prokaryote, which it is not.

Cells fall into two classes on this exact question. A prokaryotic cell — bacteria and cyanobacteria — has its DNA lying free in the cytoplasm as a nucleoid and has no membrane-bound organelles. A eukaryotic cell has a true nucleus wrapped in a double nuclear envelope pierced by pores, and a set of organelles each with its own membrane: mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and in plants the chloroplast and the large central vacuole. Chloroplasts and mitochondria are doubly unusual in carrying their own circular DNA and their own ribosomes, which is the evidence behind the view that they were once free-living bacteria taken up by an ancestral cell.

Four options that differ only in which organelle is granted a membrane are really one question: is a plant cell eukaryotic? Answer that and only one option survives, since the other three each strip a membrane from something. It is worth being precise about the wrapping rather than merely remembering that it exists. The nuclear envelope is two membranes with pores that let RNA and proteins pass. The chloroplast has two membranes as well, and a separate internal thylakoid system where the light reactions happen while the stroma around it runs the carbon reactions. That internal geography is what the next few questions in this paper go on to test.

• A eukaryotic cell has a nucleus bounded by a double nuclear envelope with pores; a prokaryotic cell has a nucleoid with no membrane.
• The chloroplast is bounded by two membranes and contains a third, internal thylakoid membrane system stacked into grana.
• Chloroplasts and mitochondria carry their own DNA and ribosomes.
• Plant cells additionally have a cell wall of cellulose outside the plasma membrane and a large central vacuole.
• Photosynthesis occurs in the chloroplast and aerobic respiration in the mitochondrion.

• Treating the cell wall as the plant cell's membrane; they are different structures with different permeability.
• Forgetting that the nuclear envelope and the chloroplast envelope are each double, not single.
• Assuming an organelle with its own DNA is independent of the nucleus; most chloroplast proteins are still coded by nuclear genes.

As a statements item on which structures are membrane-bound, as a labelled plant-cell diagram, or as an organelle-to-function match.

• practice — not a real PYQ

  Which one of the following is present in a plant cell but absent in an animal cell?

  • (a)Mitochondrion
  • (b)Golgi apparatus
  • (c)Chloroplast
  • (d)Ribosome

  Answer(c) Chloroplast — mitochondria, Golgi bodies and ribosomes occur in both; chloroplasts and a cellulose cell wall are plant features.

• practice — not a real PYQ

  The genetic material of a bacterial cell lies in a region that is

  • (a)bounded by a single membrane
  • (b)bounded by a double membrane
  • (c)not bounded by any membrane
  • (d)enclosed within the chloroplast

  Answer(c) not bounded by any membrane — the bacterial nucleoid has no envelope, which is what makes the cell prokaryotic.