Biology · Cell: The Unit of Life · NEET
This is the number-one doubt and NTA loves it. The 'S' stands for Svedberg unit, which measures how fast a particle settles (sediments) in a centrifuge, NOT its mass. Sedimentation speed depends on both size AND shape, so the values are NOT simply added. When the 50S and 30S subunits join, the combined shape sediments at 70S (not 80S). Same logic for eukaryotes: 60S + 40S = 80S, not 100S. Just memorise the final pairs; never add the subunit numbers.
70S ribosomes are found in ALL prokaryotes (bacteria, cyanobacteria, mycoplasma). 80S ribosomes are found in the cytoplasm of eukaryotes (plant, animal, fungi cells). The important NEET twist: eukaryotes ALSO contain 70S ribosomes inside their mitochondria and chloroplasts. So a plant cell has both 80S (cytoplasm) and 70S (in mitochondria + chloroplast).
70S ribosome (prokaryotic) = 50S (larger subunit) + 30S (smaller subunit). 80S ribosome (eukaryotic) = 60S (larger subunit) + 40S (smaller subunit). NEET 2025 asked this exact fact. Wrong distractor values you must reject: 60S+20S, 60S+30S, 50S+20S — all fake. Only 60S+40S and 50S+30S are correct.
No. Ribosomes are NON-membrane-bound organelles — they have no membrane at all. They are made of ribosomal RNA (rRNA) and proteins only; they do NOT contain DNA. NCERT says they are 'not surrounded by any membrane'. This makes them the only organelle present in both prokaryotes and eukaryotes (NEET 2026 tested this).
A ribosome is a ribonucleoprotein — made of rRNA plus proteins (the eukaryotic 80S ribosome has about 80 different proteins; NEET 2023 asked this). Its job is protein synthesis (translation). In prokaryotes ribosomes lie free in the cytoplasm or attach to the plasma membrane; in eukaryotes they lie free OR attach to the endoplasmic reticulum to form rough ER (RER).
80S is the bigger, heavier ribosome (eukaryotic). 70S is smaller (prokaryotic). Higher S value = settles faster = generally larger/heavier here. But remember S is sedimentation, so use this only as a rough guide, not for adding subunits.
From the statements given below choose the correct option: A. The eukaryotic ribosomes are 80S and prokaryotic ribosomes are 70S. B. Each ribosome has two sub-units. C. The two sub-units of 80S ribosome are 60S and 40S while that of 70S are 50S and 30S. D. The two sub-units of 80S ribosome are 60S and 20S and that of 70S are 50S and 20S. E. The two sub-units of 80S are 60S and 30S and that of 70S are 50S and 30S.
How many different proteins does the ribosome consist of?
Which of the following nucleic acids is present in an organism having 70S ribosomes only?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
S stands for Svedberg unit. It measures the sedimentation rate — how fast a particle settles when spun in an ultracentrifuge. It depends on size and shape, not just mass, which is why subunit values do not add up simply.
Yes. Ribosomes are the only organelle present in both. Prokaryotes have 70S ribosomes. Eukaryotes have 80S in their cytoplasm, plus 70S inside their mitochondria and chloroplasts.
No. Ribosomes have no membrane. They are non-membrane-bound organelles made of rRNA and protein, and they contain no DNA.
NCERT states prokaryotic ribosomes are about 15 nm by 20 nm and are associated with the plasma membrane of the cell.
Because they are semi-autonomous organelles believed to have evolved from ancient prokaryotes (endosymbiosis). They keep their own circular DNA and 70S ribosomes, just like bacteria — a favourite NEET point.
