The high-frequency trap in prokaryotic vs eukaryotic cell comparisons: ribosome sedimentation values. Prokaryotes have 70S ribosomes (50S + 30S subunits). Eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits). The catch — mitochondria and chloroplasts also contain 70S ribosomes due to their endosymbiotic origin. A NEET question asking "which organelle has 70S ribosomes?" expects you to pick mitochondria/chloroplast, not confuse them with the cytoplasmic 80S.
Core structural differences (NCERT Class 11 Biology Chapter 8, page 90):
NEET watch-out: When a question lists features and asks you to identify which belongs to prokaryotes vs eukaryotes, the ribosome coefficient (70S vs 80S) is the most commonly tested discriminator. Remember that "S" values are not additive — 50S + 30S = 70S, not 80S — because Svedberg units measure sedimentation rate, not mass.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is a characteristic feature of prokaryotic cells?
Answer: B. Prokaryotic cells have a single circular chromosome that is not associated with histone proteins. The DNA lies in the nucleoid region without a nuclear membrane (NCERT Class 11 Biology Chapter 8, page 102).
Why A is wrong: A is wrong because a membrane-bound nucleus is a defining feature of eukaryotic cells. Prokaryotes have a nucleoid region instead (trap: confusing nucleoid with nucleus).
Why C is wrong: C is wrong because prokaryotic ribosomes are 70S, not 80S. The 80S ribosomes are found in eukaryotic cytoplasm.
Why D is wrong: D is wrong because endoplasmic reticulum is a membrane-bound organelle found only in eukaryotic cells. Prokaryotes lack all membrane-bound organelles.
The ribosomes found in mitochondria of eukaryotic cells are:
Answer: D. Mitochondrial ribosomes are 70S, similar to prokaryotic ribosomes. This supports the endosymbiotic theory — mitochondria evolved from engulfed prokaryotes (NCERT Class 11 Biology Chapter 8, page 102).
Why A is wrong: A is wrong because 80S ribosomes are found only in the eukaryotic cytoplasm, not inside mitochondria. This is the core 70S vs 80S ribosome confusion trap.
Why B is wrong: B is wrong because 100S ribosomes do not exist in standard cell biology. This is a fabricated distractor.
Why C is wrong: C is wrong because 60S is a subunit of the eukaryotic 80S ribosome, not a complete ribosome type. No organelle has '60S ribosomes' as its functional unit.
Svedberg units (S) are not additive because they measure:
Answer: A. Svedberg units measure the rate of sedimentation during ultracentrifugation, which depends on size, shape, and density — not just mass. Hence 50S + 30S subunits together sediment as 70S, not 80S (NCERT Class 11 Biology Chapter 8).
Why B is wrong: B is wrong because if S measured molecular weight, the values would be additive (50 + 30 = 80). The non-additivity specifically demonstrates that S measures sedimentation behavior, not mass.
Why C is wrong: C is wrong because the number of rRNA molecules is a discrete count unrelated to Svedberg units. The 70S ribosome has 3 rRNAs; this does not explain non-additivity of S values.
Why D is wrong: D is wrong because diameter is a linear measurement. Svedberg units reflect sedimentation coefficient, which is a composite of shape, size, and density during centrifugation.
Prokaryotic and eukaryotic plasma membranes are structurally similar, yet prokaryotes lack membrane-bound organelles such as mitochondria and a dedicated secretory system. Given this, which function must the prokaryotic plasma membrane and its infoldings (mesosomes) take on that in eukaryotes is instead handled by dedicated organelles?
Answer: C. NCERT states that mesosomes — extensions of the plasma membrane in the form of vesicles, tubules and lamellae — help in cell wall formation, DNA replication and distribution to daughter cells, and also help in respiration and secretion by increasing the surface area of the plasma membrane and its enzymatic content. Since prokaryotes have no mitochondria or separate secretory organelles, these functions are taken over by the plasma membrane's own infoldings instead (NCERT Class 11 Biology Chapter 8, pages 90–91).
Why A is wrong: A is wrong because the genetic material of a prokaryote lies naked in the nucleoid region, not stored inside any membrane structure — mesosomes are infoldings of the plasma membrane, not a substitute nucleus.
Why B is wrong: B is wrong on two counts: prokaryotic ribosomes are 70S, not 80S, and mesosomes are not described as ribosome-housing structures — this answer confuses the 70S/80S ribosome trap with the separate function of mesosomes.
Why D is wrong: D is wrong because mesosomes are infoldings of the existing plasma membrane, not a separate membrane-bound compartment; NCERT lists their role in DNA replication and distribution to daughter cells as one of several surface-area-linked functions, not as nuclear-style compartmentalisation.
A student observes a cell with the following features: peptidoglycan cell wall, 70S ribosomes, circular DNA, and no membrane-bound organelles. The cell is most likely:
Answer: C. All listed features — peptidoglycan cell wall, 70S ribosomes, circular DNA, absence of membrane-bound organelles — are characteristic of bacterial (prokaryotic) cells. Plant cells have cellulose walls and 80S cytoplasmic ribosomes; fungi have chitin walls; animal cells lack cell walls entirely (NCERT Class 11 Biology Chapter 8, page 102).
Why A is wrong: A is wrong because plant cells have cellulose cell walls (not peptidoglycan), 80S cytoplasmic ribosomes, linear DNA with histones, and membrane-bound organelles like chloroplasts.
Why B is wrong: B is wrong because fungal cells have chitin cell walls (not peptidoglycan), 80S cytoplasmic ribosomes, and membrane-bound organelles including a nucleus.
Why D is wrong: D is wrong because animal cells have no cell wall at all, possess 80S cytoplasmic ribosomes, linear DNA with histones, and numerous membrane-bound organelles.
Which statement correctly distinguishes prokaryotic DNA from eukaryotic DNA?
Answer: D. Prokaryotic DNA is not associated with histone proteins (it is naked DNA or associated with HU proteins). Eukaryotic DNA is wrapped around histone octamers forming nucleosomes. This is a fundamental organizational difference (NCERT Class 11 Biology Chapter 8, page 102).
Why A is wrong: A is wrong because it reverses the correct relationship. Prokaryotic DNA is circular; eukaryotic DNA is linear. This is a common confusion trap where students swap the shapes.
Why B is wrong: B is wrong because it reverses the correct relationship. Eukaryotic genes typically contain introns; prokaryotic genes generally lack introns (they have continuous coding sequences).
Why C is wrong: C is wrong because both prokaryotic and eukaryotic DNA are double-stranded (dsDNA). Single-stranded DNA is found in certain viruses, not in cellular organisms.
An antibiotic that specifically targets 70S ribosomes would be effective against bacteria but would also potentially damage which eukaryotic organelle?
Answer: B. Mitochondria contain 70S ribosomes (endosymbiotic origin from prokaryotes). An antibiotic targeting 70S ribosomes would inhibit bacterial protein synthesis and could also disrupt mitochondrial protein synthesis in eukaryotic cells (NCERT Class 11 Biology Chapter 8).
Why A is wrong: A is wrong because the Golgi apparatus does not contain ribosomes. It processes and packages proteins synthesized by rough ER ribosomes (which are 80S, not 70S).
Why C is wrong: C is wrong because rough ER-associated ribosomes are 80S (eukaryotic type). An antibiotic targeting 70S ribosomes would not affect ER-bound protein synthesis.
Why D is wrong: D is wrong because lysosomes do not contain ribosomes. They are membrane-bound vesicles containing hydrolytic enzymes, with no protein synthesis machinery inside.
Consider the following cellular features:
(i) Nucleoid region
(ii) 80S ribosomes in cytoplasm
(iii) Plasmids
(iv) Cellulose cell wall
How many of the above are found in a typical bacterial cell?
Answer: C. A typical bacterial cell has (i) nucleoid region and (iii) plasmids — that is two features. Feature (ii) 80S ribosomes belong to eukaryotic cytoplasm (bacteria have 70S). Feature (iv) cellulose cell wall belongs to plant cells (bacteria have peptidoglycan). Two correct features total (NCERT Class 11 Biology Chapter 8, pages 90–91).
Why A is wrong: A is wrong because this undercounts. Both nucleoid region AND plasmids are prokaryotic features. Selecting 'one' likely means the student forgot that plasmids are characteristic of bacteria.
Why B is wrong: B is wrong because this overcounts. The student likely included 80S ribosomes (bacteria actually have 70S) or cellulose wall (bacteria have peptidoglycan, not cellulose). Only two of the four features apply.
Why D is wrong: D is wrong because this assumes all four features belong to bacteria. Cellulose walls are plant-specific and 80S ribosomes are eukaryote-specific. Only nucleoid and plasmids are bacterial.
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Pattern: Comparison-based recall with distractor confusion (from NEET pattern: unit bundle — "similar-term-confusion" distractor type)
Given
Four ribosome sources: bacterial cytoplasm, eukaryotic cytoplasm, chloroplast, mitochondria.
Required
Count of sources containing 70S ribosomes.
Concept
Prokaryotic cells and semi-autonomous eukaryotic organelles (mitochondria, chloroplasts) have 70S ribosomes. Eukaryotic cytoplasmic ribosomes are 80S.
Formula
No formula required — this is a recall + classification question.
Substitution
• (i) E. coli (prokaryote) → 70S ✓• (ii) Human liver cytoplasm (eukaryote) → 80S ✗• (iii) Spinach chloroplast (endosymbiotic organelle) → 70S ✓• (iv) Human mitochondria (endosymbiotic organelle) → 70S ✓
Calculation
Count of 70S sources = 3 (E. coli + chloroplast + mitochondria).
Final answer
Three of the four sources yield 70S ribosomes.
Common trap
The trap is including human liver cytoplasm as 70S (confusing the cell type with the organelle). Remember: the cytoplasmic ribosomes of ANY eukaryote are always 80S. The 70S ribosomes inside mitochondria and chloroplasts are a separate population from the cytoplasmic ones.
Similar NEET-style question
"Which of the following contains 70S ribosomes? (a) Cytoplasm of Amoeba (b) Chloroplast of Chlamydomonas (c) Cytoplasm of human RBC (d) Smooth ER of hepatocyte"
Answer: (b) — chloroplast ribosomes are 70S.
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Prokaryotes: no membrane-bound nucleus, smaller (1-10 μm), 70S ribosomes, no true organelles, single circular DNA, peptidoglycan cell wall (bacteria). Eukaryotes: membrane-bound nucleus, 80S ribosomes (70S in mito/chloroplast), multiple linear chromosomes, organelles.
-- NCERT Class 11 Biology, Ch. 8, p. 91These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Prokaryotes 70S; eukaryotic cytosol 80S; mitochondria + chloroplast 70S (endosymbiotic).
Question gives organelle/cell type and asks ribosome type.
Cytosolic of eukaryote = 80S; everywhere else (bacteria, mito, chloroplast) = 70S.
More in Cell: Structure and Function: 13 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
3 questions from NEET 2020, 2026. Answers verified against NTA official keys.
Which of the following statements about inclusion bodies is incorrect ?
All 69 past-paper questions from Cell: Structure and Function →
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
Test yourself on this topic with real past-paper questions:
Practice this topic →Short50S + 30S makes 70S, not 80S — why Svedberg units do not add, and what S actually measures (2:41)
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