NCERT grounding
NCERT Class 11 Biology, Chapter 8 Cell — The Unit of Life, places ribosomes in two different sections — and that is itself an exam hook. §8.4.2 (Ribosomes and Inclusion Bodies) describes prokaryotic ribosomes as 15 nm × 20 nm particles associated with the plasma membrane, made of a 50S and a 30S subunit that together form the 70S prokaryotic ribosome. §8.5.6 (Ribosomes, within the Eukaryotic Cell section) then describes the 80S eukaryotic ribosome (60S + 40S), credits the first electron-microscope observation to George Palade in 1953, and explicitly defines the Svedberg unit as a sedimentation coefficient that is indirectly a measure of density and size. The NIOS supplement (Chapter 4, §4.3.3) reinforces this: ribosomes are 150–250 Å in diameter, made of ribonucleoproteins, occur either free in the cytoplasm or attached to the ER, and are also stored in the nucleolus.
"Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953). They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane."
NCERT Class 11, §8.5.6
Structure, sizes & function
A ribosome is not an organelle in the classical sense — it has no lipid bilayer, no enclosed lumen, and no internal compartments. It is a ribonucleoprotein particle: roughly two-thirds ribosomal RNA (rRNA) and one-third ribosomal proteins by mass, packed into two unequal subunits that dock together around an mRNA strand only during active translation. When translation ends, the subunits dissociate and float separately in the cytoplasm, ready to be recruited by the next mRNA. This cyclical assembly is part of why NCERT records ribosomes under a granular-particle heading rather than under any membranous compartment.
Each ribosome carries three functional sites on its large subunit — the A (aminoacyl), P (peptidyl) and E (exit) sites — across which charged tRNAs march as the mRNA is decoded codon by codon. The catalytic core that forms each new peptide bond is itself made of rRNA, not protein: the ribosome is therefore a ribozyme. Class 11 does not test this term in detail, but it is a recurring NEET sub-trap and a useful anchor for the molecular-basis chapter.
Two size classes: 70S and 80S
Prokaryotic ribosome
50S large + 30S small. Found in bacteria, archaea, mitochondria and chloroplasts. NCERT §8.4.2 gives the size as roughly 15 nm × 20 nm.
Eukaryotic cytoplasmic ribosome
60S large + 40S small. Found free in the cytosol and attached to the rough endoplasmic reticulum. NCERT §8.5.6.
The two size classes are nature's deepest cytological divide. Every prokaryote on earth assembles proteins on a 70S ribosome; every eukaryote uses an 80S ribosome for cytoplasmic translation. Antibiotics such as streptomycin, tetracycline and chloramphenicol exploit this difference — they bind the 70S ribosome and stall bacterial translation while leaving the 80S ribosome of the host largely untouched. The 70S–80S distinction is the single highest-yield ribosome fact on NEET, surfacing in match-the-column items and in distractors for organelle identification.
The Svedberg (S) unit — and why the numbers do not add
NCERT itself flags this gently: "Here 'S' (Svedberg's Unit) stands for the sedimentation coefficient; it is indirectly a measure of density and size." NEET examiners have repeatedly used the temptation to add S-values — the wrong answer of 80S for a prokaryotic ribosome is a popular distractor.
Figure 1. The two size classes side-by-side. A prokaryotic ribosome is 70S, built from a 50S large subunit (with A, P and E tRNA-binding sites) and a 30S small subunit that grips the mRNA. The eukaryotic cytoplasmic ribosome is 80S — a larger 60S over a 40S small subunit. S-values are sedimentation coefficients, not masses, and therefore do not add arithmetically.
Where do ribosomes live? Free, bound, organellar, nucleolar
Free ribosomes
cytosol
Floating in the cytoplasmic matrix
- Synthesise cytosolic proteins
- Also produce most nuclear, mitochondrial and chloroplast proteins encoded by nuclear DNA
- Often organised into polysomes on a single mRNA
- Identical 80S particles — only their location differs
Membrane-bound ribosomes
RER
Attached to rough endoplasmic reticulum
- Synthesise secretory proteins (exported from cell)
- Synthesise membrane proteins for plasma membrane / organelles
- Synthesise lysosomal hydrolases
- Their presence is what makes RER "rough"
Beyond the cytosol, a eukaryotic cell also runs 70S ribosomes inside its mitochondria and chloroplasts. NCERT records this twice — §8.5.5 explicitly notes that "the ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S)" — and the same is true of mitochondrial ribosomes (mitoribosomes). The presence of bacterial-style ribosomes inside these double-membrane organelles, alongside their own circular DNA, is one of the textbook pieces of evidence for the endosymbiotic theory: mitochondria and chloroplasts descend from engulfed prokaryotic ancestors, and they have retained the translation machinery of that lineage.
A fourth location matters for NEET: the nucleolus. The nucleolus is the factory where ribosomal RNA is transcribed and where the large and small subunits are partially assembled before being exported through the nuclear pores into the cytoplasm. This is why NEET 2018 Q.104 ("which is true for nucleolus") had the correct answer "site for active ribosomal RNA synthesis", and why dividing cells (with high protein demand) have larger, more numerous nucleoli.
Polysomes — many ribosomes, one mRNA
A single mRNA molecule does not have to be read by only one ribosome at a time. In both prokaryotes and eukaryotes, several ribosomes load sequentially onto the same transcript, spaced like beads on a string, each one translating a few codons behind the next. The resulting structure is called a polysome (or polyribosome). NCERT introduces this term in §8.4.2: "Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome. The ribosomes of a polysome translate the mRNA into proteins."
Figure 2. A polysome (polyribosome). Multiple ribosomes load sequentially onto a single mRNA. Each ribosome carries a polypeptide chain whose length reflects how long it has been translating: the leading ribosome (left) has the longest chain, the newly attached ribosome (right) has only just begun. The result is parallel production of many copies of the same polypeptide from one transcript.
In bacteria, where there is no nuclear envelope, polysomes can begin translating an mRNA even while RNA polymerase is still transcribing it from DNA — coupled transcription and translation. Eukaryotes cannot do this because the nucleus physically separates the two events, but polysomes still operate in the cytoplasm and on the RER.
Why ribosomes matter — the function in one line
George Palade · first observation
NCERT §8.5.6: "Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953)." Palade later received the Nobel Prize in 1974 for his broader cell-fractionation and structural work.
Functionally, ribosomes are the site of protein synthesis — the single highest-priority fact on this subtopic. They translate the codons of an mRNA into a sequence of amino acids, catalyse peptide bond formation through their rRNA, and release the finished polypeptide so it can fold (often with chaperones) into a functional protein. Every other structural detail on this page exists to support, modify or locate that one process.
Worked examples
A eukaryotic cell is treated with a drug that inhibits 70S ribosomes but not 80S ribosomes. Which of the following will most directly be affected?
(a) Cytosolic protein synthesis (b) Mitochondrial protein synthesis (c) RER-mediated secretion (d) Histone production
Answer: (b) Mitochondrial protein synthesis. Inside a eukaryotic cell, only mitochondria and chloroplasts use 70S ribosomes; everything else — cytosol, RER, nucleolus output — uses 80S. A drug specific to 70S will cripple mitochondrial (and in plant cells, chloroplastic) translation while leaving cytosolic and RER translation intact. This is also the mechanistic basis of why certain antibiotics (e.g. chloramphenicol) can cause mitochondrial side-effects in eukaryotes.
Match the subunit on the left with its assembled ribosome on the right.
A. 30S subunit — i. 80S ribosome
B. 40S subunit — ii. 70S ribosome
C. 60S subunit — iii. 70S ribosome
D. 50S subunit — iv. 80S ribosome
Answer: A–ii, B–i, C–iv, D–iii. The prokaryotic 70S = 50S + 30S; the eukaryotic 80S = 60S + 40S. The smaller subunits (30S, 40S) carry the mRNA-binding decoding centre; the larger subunits (50S, 60S) carry the peptidyl-transferase site that forms peptide bonds.
A student writes: "Since a prokaryotic ribosome is 50S + 30S, and a eukaryotic ribosome is 60S + 40S, the eukaryotic ribosome must be heavier by 20S." Identify the error.
Answer: Two errors. (1) S-values are sedimentation coefficients, not masses, so subtracting them does not give a mass difference. (2) S-values are not additive in the first place — the assembled ribosomes are 70S and 80S, not 80S and 100S. The eukaryotic ribosome is heavier and larger than the prokaryotic one, but "20S heavier" is a meaningless quantity.
Which of the following are correctly paired? (i) Free ribosome — cytosolic protein synthesis; (ii) Membrane-bound ribosome — secretory protein synthesis; (iii) Mitochondrial ribosome — 80S; (iv) Nucleolus — rRNA synthesis.
Answer: (i), (ii) and (iv) are correct. Pair (iii) is wrong — mitochondrial ribosomes are 70S, not 80S. This 70S signature is one of the cornerstones of the endosymbiotic theory.