The 70S ribosome inside your mitochondria is the single most tested detail about these organelles. Aspirants who treat mitochondrial ribosomes as 80S — because they sit inside a eukaryotic cell — lose marks on a question that rewards knowing one fact: endosymbiotic origin means 70S.
Mitochondria are double-membrane-bound organelles. The outer membrane is porous (porins). The inner membrane folds into cristae, increasing surface area for the electron transport chain and oxidative phosphorylation. The matrix contains circular DNA, 70S ribosomes, and enzymes of the TCA cycle (NCERT Class 11 Biology Chapter 8, page 97). Mitochondria are called the "powerhouse of the cell" because they produce ATP via aerobic respiration.
Plastids are plant-exclusive, also double-membraned, also carrying their own circular DNA and 70S ribosomes. Three types: chloroplasts (photosynthesis; contain thylakoids stacked into grana, surrounded by stroma), chromoplasts (coloured pigments — carotenoids, xanthophylls; give colour to flowers and fruits), and leucoplasts (colourless; storage — amyloplasts for starch, elaioplasts for oils, aleuroplasts for proteins).
Both mitochondria and chloroplasts are semi-autonomous — they replicate by fission, carry their own DNA and ribosomes, yet depend on nuclear genes for most proteins. This is the structural evidence for the endosymbiotic theory: an ancestral eukaryote engulfed a prokaryote, and over time the prokaryote became a permanent organelle.
Watch-out for NEET: when a question asks about the ribosome type in mitochondria or chloroplasts, the answer is always 70S — the same as prokaryotes. Only the cytoplasmic ribosomes of eukaryotes are 80S. This confusion appears repeatedly across papers.
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 the site of oxidative phosphorylation in mitochondria?
Answer: C. The electron transport chain and ATP synthase complexes are embedded in the inner mitochondrial membrane (cristae), making it the site of oxidative phosphorylation (NCERT Class 11 Biology Chapter 8, page 102).
Why A is wrong: A — The outer membrane is porous due to porins and is not the site of ETC/oxidative phosphorylation.
Why B is wrong: B — The matrix holds the Krebs-cycle enzymes, but the electron transport chain and ATP synthase sit in the inner membrane, so oxidative phosphorylation does not happen in the matrix.
Why D is wrong: D — The inter-membrane space accumulates H⁺ ions (proton gradient), but the actual oxidative phosphorylation machinery is on the inner membrane.
Leucoplasts that store starch are specifically called:
Answer: C. Amyloplasts are the subtype of leucoplasts specialised for starch storage. Elaioplasts store oils; aleuroplasts store proteins (NCERT Class 11 Biology, Chapter 8).
Why A is wrong: A — Elaioplasts store lipids/oils, not starch.
Why B is wrong: B — Aleuroplasts store proteins, not starch.
Why D is wrong: D — Chromoplasts contain coloured pigments (carotenoids, xanthophylls) and are not leucoplasts.
Which of the following organelles contains its own circular DNA?
Answer: D. Mitochondria (and chloroplasts) are semi-autonomous organelles possessing their own circular DNA and 70S ribosomes, consistent with their endosymbiotic origin (NCERT Class 11 Biology Chapter 8, page 98).
Why A is wrong: A — Golgi apparatus is an endomembrane system organelle without its own DNA.
Why B is wrong: B — Lysosomes are membrane-bound vesicles containing hydrolytic enzymes; they lack their own genome.
Why C is wrong: C — Endoplasmic reticulum is continuous with the nuclear envelope and does not possess independent DNA.
A student isolates ribosomes from three sources: (i) E. coli cytoplasm, (ii) human liver cell cytoplasm, (iii) human liver cell mitochondria. The sedimentation coefficients of ribosomes from these sources, respectively, are:
Answer: A. E. coli is prokaryotic → 70S. Human liver cytoplasm is eukaryotic cytosol → 80S. Human liver mitochondria harbour 70S ribosomes due to endosymbiotic origin. The correct sequence is 70S, 80S, 70S (NCERT Class 11 Biology, Chapter 8).
Why B is wrong: B — This wrongly assigns 80S to E. coli ribosomes. Prokaryotes have 70S ribosomes.
Why C is wrong: C — This wrongly assigns 80S to mitochondrial ribosomes. Mitochondrial ribosomes are 70S, not 80S, because of their prokaryotic endosymbiotic ancestry (trap: 70S vs 80S ribosome confusion).
Why D is wrong: D — This wrongly assigns 80S to prokaryotic E. coli ribosomes. Prokaryotes always have 70S ribosomes.
A researcher uses chloramphenicol, an antibiotic that specifically inhibits 70S ribosomes. In a eukaryotic plant cell, which process would be directly affected?
Answer: A. Chloroplasts and mitochondria contain 70S ribosomes (endosymbiotic origin). Chloramphenicol targets 70S ribosomes specifically, so protein synthesis within these organelles is inhibited. Cytoplasmic ribosomes (80S) are unaffected (NCERT Class 11 Biology, Chapter 8).
Why B is wrong: B — Cytoplasmic ribosomes are 80S, which are resistant to chloramphenicol. This confuses ribosome types (trap: 70S vs 80S ribosome confusion).
Why C is wrong: C — DNA replication does not involve ribosomes and is unaffected by chloramphenicol.
Why D is wrong: D — Rough ER-bound ribosomes are 80S (eukaryotic cytoplasmic type) and are not targeted by chloramphenicol.
Which of the following statements correctly distinguishes chromoplasts from chloroplasts?
Answer: D. Chromoplasts are plastids that contain carotenoid and xanthophyll pigments (giving colour to flowers and fruits) but lack chlorophyll. Chloroplasts contain chlorophyll and are the site of photosynthesis (NCERT Class 11 Biology, Chapter 8).
Why A is wrong: A — Chromoplasts do contain pigments (carotenoids, xanthophylls); they simply lack chlorophyll. Saying they 'lack pigments' is incorrect.
Why B is wrong: B — This reverses the roles. Chloroplasts perform photosynthesis; chromoplasts provide colour.
Why C is wrong: C — Chromoplasts are found in flowers, fruits, and some roots (e.g. carrot), not restricted to roots. Chloroplasts are found in green parts of plants, not only stems.
Mitochondria and chloroplasts are described as "semi-autonomous" organelles. Which combination of features justifies this description?
Answer: B. Semi-autonomous means these organelles have their own genetic material (circular DNA), their own protein synthesis machinery (70S ribosomes), can replicate by fission, yet still depend on nuclear genes for most of their proteins. The double membrane, circular DNA, 70S ribosomes, and fission collectively constitute the evidence (NCERT Class 11 Biology Chapter 8, page 97).
Why A is wrong: A — Both organelles are double-membraned with circular (not linear) DNA and 70S (not 80S) ribosomes. They do self-replicate by fission.
Why C is wrong: C — Both organelles possess their own circular DNA; stating 'no DNA' is factually wrong.
Why D is wrong: D — Their ribosomes are 70S, not 80S (trap: 70S vs 80S ribosome confusion). They are NOT completely independent — they require nuclear-encoded proteins, which is why they are 'semi'-autonomous, not fully autonomous.
Consider the following statements about mitochondria and plastids:
(i) Both possess double membranes.
(ii) Both contain 80S ribosomes.
(iii) Chloroplasts contain thylakoids organised into grana.
(iv) Mitochondria have cristae that increase inner membrane surface area.
(v) Leucoplasts are coloured plastids found in petals.
How many of the above statements are correct?
Answer: B. Statements (i), (iii), and (iv) are correct — three statements. Statement (ii) is wrong: both mitochondria and chloroplasts contain 70S ribosomes, not 80S (mistake: calling mitochondrial ribosomes 80S). Statement (v) is wrong: leucoplasts are colourless storage plastids; coloured plastids are chromoplasts.
Why A is wrong: A — This undercounts. Three statements are correct, not two.
Why C is wrong: C — This overcounts. Statement (ii) is false (70S, not 80S — trap: 70S vs 80S ribosome confusion) and statement (v) is false (leucoplasts are colourless).
Why D is wrong: D — Two statements are clearly false: (ii) assigns 80S to organellar ribosomes, and (v) mislabels leucoplasts as coloured.
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Given
A eukaryotic cell has the following compartments: (a) cytoplasm, (b) mitochondrial matrix, (c) chloroplast stroma. You are told the cell is a mesophyll cell of a green plant.
Required
Identify the sedimentation coefficient of ribosomes in each compartment.
Concept
Ribosomes in eukaryotic cytoplasm are 80S (composed of 60S + 40S subunits). Ribosomes inside mitochondria and chloroplasts are 70S (composed of 50S + 30S subunits), reflecting their endosymbiotic prokaryotic ancestry. Note: Svedberg units (S) are non-additive — they reflect sedimentation behaviour, not mass.
Formula/Rule
• Eukaryotic cytoplasmic ribosome → 80S• Mitochondrial ribosome → 70S (endosymbiotic origin)• Chloroplast ribosome → 70S (endosymbiotic origin)• Prokaryotic ribosome → 70S
Substitution
(a) Cytoplasm → 80S
(b) Mitochondrial matrix → 70S
(c) Chloroplast stroma → 70S
Calculation
No arithmetic required. This is a direct-recall application of the endosymbiotic ribosome rule.
Final answer
Cytoplasm: 80S; Mitochondria: 70S; Chloroplast: 70S.
Common trap
The most common error is assigning 80S to mitochondrial or chloroplast ribosomes because they are "inside a eukaryotic cell." The endosymbiotic origin overrides the host cell classification. NEET distractors routinely offer 80S for organellar ribosomes (trap: 70S vs 80S ribosome confusion; mistake: calling mitochondrial ribosomes 80S).
Similar NEET-style question
"Ribosomes found in the matrix of mitochondria and stroma of chloroplasts are: (A) 80S (B) 70S (C) 60S (D) 40S." Answer: B (70S).
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Mitochondria: double membrane; cristae increase inner SA; matrix has 70S ribosomes + circular DNA; site of TCA cycle and OXPHOS. Plastids: chloroplasts (photosynthesis), chromoplasts (pigments), leucoplasts (storage). Endosymbiotic origin.
-- NCERT Class 11 Biology, Ch. 8, p. 97These 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.
Root cause: term confusion
Mitochondrial ribosomes are 70S (endosymbiotic origin from prokaryote). Only the cytoplasmic ribosomes of eukaryotes are 80S.
More in Cell: Structure and Function: 12 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
6 questions from NEET 2024, 2026. Answers verified against NTA official keys.
Mitochondrial inner membrane encloses ______.
The plastid that stores xanthophyll is known as ______.
The DNA present in chloroplast is:
All 69 past-paper questions from Cell: Structure and Function →
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