Which of the following cell organelle is not bounded by membrane ?

Question

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Correct — option (2), the ribosome. Read the stem first: it asks which organelle is NOT bounded by a membrane, and 'not' is printed in bold in the English column. The question is inverted, so the three organelles that DO have membranes are to be discarded and the one that does not is the answer. The ribosome is that one. NCERT's Class 11 biology puts it plainly: 'Ribosomes are non-membrane bound organelles found in all cells — both eukaryotic as well as prokaryotic.' A ribosome is not a compartment at all but a machine — two subunits built from ribosomal RNA and proteins, which come together on a messenger RNA strand and join amino acids into a polypeptide chain. Because it has no enclosing membrane it can sit freely in the cytoplasm, attach to the outer face of the endoplasmic reticulum to make the rough endoplasmic reticulum, or exist inside mitochondria and chloroplasts, and it is the one organelle common to prokaryotic and eukaryotic cells alike — bacteria have no nucleus, no mitochondria and no endoplasmic reticulum, but they have ribosomes. Now the three that are eliminated. The chloroplast is a double-membrane organelle, with an outer and an inner membrane and a further internal membrane system of thylakoids stacked into grana, where the light reactions of photosynthesis occur. Mitochondria are also double-membraned, the inner membrane thrown into folds called cristae that carry the electron transport chain; they are the site of cellular respiration and the cell's chief source of ATP. Lysosomes are single-membrane sacs filled with hydrolytic enzymes, and the membrane is not incidental to them but essential, since it keeps digestive enzymes away from the rest of the cell — which is also why a ruptured lysosome can digest its own cell and why lysosomes are nicknamed the suicide bags of the cell. Three membrane-bound organelles and one that is not: the row that names the ribosome is option (2).

Why the other options are wrong — (a) The chloroplast is one of the most heavily membraned structures in the cell, so it cannot answer a question asking for the organelle without a membrane. It has two enclosing membranes, an outer and an inner, and inside the stroma it carries a third and separate membrane system — flattened sacs called thylakoids, stacked into piles called grana and joined by stroma lamellae. The chlorophyll that absorbs sunlight and the machinery of the light reactions are embedded in the thylakoid membranes, and the Calvin cycle then runs in the stroma outside them. A candidate arrives at this option almost always by failing to invert the stem: reading 'which organelle is bounded by membrane' rather than 'not bounded', the chloroplast is a perfectly good answer, which is exactly why it is offered. Chloroplasts also share with mitochondria the double membrane, their own circular DNA and their own 70S ribosomes, the features on which the endosymbiotic theory of their origin rests.  (c) Mitochondria are double-membrane organelles, and the structure of those two membranes is the point of them. The outer membrane is smooth and permeable; the inner is folded into cristae, which multiply its surface area and carry the components of the electron transport chain, while the matrix enclosed by it holds the enzymes of the citric acid cycle. Cellular respiration depends on the inner membrane being intact, because the proton gradient that drives ATP synthesis can only be maintained across a closed membrane — so a mitochondrion without membranes would not merely be a different organelle, it would not work at all. Like the chloroplast, the mitochondrion carries its own circular DNA and its own 70S ribosomes, which is the evidence behind the endosymbiotic theory. The option is offered because 'powerhouse of the cell' is the phrase every candidate recalls first when organelles are mentioned, and a hurried reader who has not inverted the stem may reach for the most familiar name in the list.  (d) Lysosomes are bounded by a single membrane, and for them the membrane is a matter of survival rather than of structure. They contain hydrolytic enzymes — proteases, lipases, nucleases and carbohydrases — which digest worn-out organelles, engulfed particles and, when a cell must be dismantled, the cell itself. Those enzymes work best in the acidic interior the lysosomal membrane maintains, and the membrane's other function is to keep them away from the cytoplasm they would otherwise destroy; a ruptured lysosome digests its own cell, which is the origin of the nickname 'suicide bags'. This option can attract a candidate who half-remembers that lysosomes are simple in appearance, or who confuses them with ribosomes because the names are similar in sound and length — a genuine and surprisingly common slip. The two are unrelated: the lysosome is a membrane-bound digestive sac, the ribosome an unbounded assembly of RNA and protein that builds new polypeptides.

Concept: Eukaryotic cell organelles sort neatly into three groups by their membranes, and holding the three lists is the most efficient preparation for this whole area. Double-membrane structures: the nucleus, whose envelope is perforated by nuclear pores; the mitochondrion, with its cristae; and the plastids, of which the chloroplast is the photosynthetic form. Single-membrane structures: the endoplasmic reticulum, rough where ribosomes stud it and smooth where they do not; the Golgi apparatus, which modifies, sorts and packages what the endoplasmic reticulum sends it; lysosomes; vacuoles, very large in plant cells; and the microbodies such as peroxisomes and, in plants, glyoxysomes. Non-membranous structures: ribosomes; centrioles and the centrosome that organises the spindle; the cytoskeleton of microtubules, microfilaments and intermediate filaments; the nucleolus inside the nucleus, which is the site of ribosomal RNA synthesis; and the chromosomes themselves. The prokaryotic cell makes the same point from the other direction — a bacterium has no nucleus and none of the membrane-bound organelles, but it does have ribosomes, and it must, because no cell can live without making proteins. Ribosomes differ in size between the two: 80S in the eukaryotic cytoplasm, made of 60S and 40S subunits, and 70S in prokaryotes and also inside mitochondria and chloroplasts, made of 50S and 30S. That last fact matters twice over — it is the evidence for the endosymbiotic origin of those two organelles, and it is why several antibiotics can attack bacterial ribosomes without harming our own.

State PCS 2021 Q57 — Science & Technology PYQ with Answer

State PCS 2021 Q57 — Science & Technology PYQ with Answer