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NCERT tells you that eukaryotic ribosomes are 80S, made of 60S and 40S subunits, and that prokaryotic ribosomes are 70S, made of 50S and 30S. Add the numbers and they do not match. The reason lies in how S is measured, which goes back to a machine built in Uppsala a century ago. This post covers The Svedberg, his ultracentrifuge, and how to calculate what an S value means.
Who was The Svedberg?
His Nobel Prize facts page gives his full name as Theodor Svedberg, born on 30 August 1884 in Fleräng, Sweden, and died on 25 February 1971 in Örebro. He won the 1926 Nobel Prize in Chemistry "for his work on disperse systems", meaning colloids: tiny particles spread through a liquid.
According to his Nobel biography, he spent his whole career at Uppsala University, where he became Professor of Physical Chemistry in 1912. His 1908 doctoral thesis gave strong support to the theory of Brownian motion developed by Einstein and Smoluchowski, and so to the existence of molecules. He later worked on nuclear chemistry and, during the Second World War, on a method for making synthetic rubber.
How did the ultracentrifuge develop?
Heavy particles settle under gravity, but molecules are too small: they stay spread out. Svedberg's answer was to replace gravity with a much stronger centrifugal field. His Nobel lecture of 1927 traces the steps:
With Nichols at the University of Wisconsin, he built a centrifuge that reached only about 150 times gravity.
In 1924, with Rinde, he built one reaching about 7,000 times gravity at up to 12,000 revolutions per minute. They called it the ultracentrifuge.
In 1925 and 1926, with Lysholm, he built one reaching about 100,000 times gravity at 42,000 revolutions per minute.
The samples were watched while spinning, by measuring how much light they absorbed or by photographing them.
What did it show about proteins?
In the lecture Svedberg reports that 80 to 90 per cent of the haemoglobin studied consisted of molecules of weight 68,000, and concludes that haemoglobin is "a substantially homogeneous chemical individual". In other words, a protein is a definite molecule of fixed size, not a loose cluster of varying size. A 2018 review by Stephen Harding lists this as one of the great discoveries made with the method, citing Svedberg and Fåhraeus's 1926 paper.
What does the S in 70S mean?
A spinning particle quickly reaches a steady speed. The sedimentation coefficient s is that speed divided by the centrifugal acceleration, as a Nature Communications paper on the method defines it:
s = v / (ω²r)
Velocity divided by acceleration has units of seconds. Values for molecules are tiny, so a review by Planken and Cölfen gives the convenient unit: 1 S = 1 × 10⁻¹³ s. The NCERT Class 11 chapter on the cell calls S "Svedberg's Unit" and says it is "indirectly a measure of density and size".
Because s depends on shape and on the drag the particle feels, not only on its mass, two subunits joined together do not sediment at the sum of their separate rates. That is why 50S + 30S gives 70S.
A worked example: how fast does a ribosome sink?
Use these sample numbers: a rotor spins at 60,000 revolutions per minute, and the sample sits 6.5 cm from the axis.
Angular speed: ω = 60,000 × 2π ÷ 60 = 6,283 rad/s.
Centrifugal acceleration: ω²r = (6,283)² × 0.065 = 3.948 × 10⁷ × 0.065 = 2.57 × 10⁶ m/s², about 260,000 times g.
For a 70S ribosome, s = 70 × 10⁻¹³ s, so v = s ω²r = 7.0 × 10⁻¹² × 2.57 × 10⁶ = 1.8 × 10⁻⁵ m/s.
Time to sink 1 cm: 0.01 ÷ 1.8 × 10⁻⁵ ≈ 560 s, a little over 9 minutes.
For a 30S subunit, v = 3.0 × 10⁻¹² × 2.57 × 10⁶ = 7.7 × 10⁻⁶ m/s, so 1 cm takes about 1,300 s, nearly 22 minutes.
Common mistakes to avoid:
Adding S values of subunits. They are rates, not masses.
Calling S a unit of mass or of density. It is a unit of time, 10⁻¹³ s.
Using revolutions per minute directly. Convert to radians per second first.
What can a student take from Svedberg's way of working?
Svedberg needed a field of force that did not exist, so he built it, and then improved it step by step, from 150 to 7,000 to 100,000 times gravity. Each machine answered questions the last one could not.
The same steady approach works for a hard chapter. If units and fields feel tangled, the Fermi Concept Library covers every concept in physics, chemistry and biology, one idea at a time. For ribosomes in detail, read our post on Venkatraman Ramakrishnan and the ribosome, and for the NCERT chapter, our guide to the cell as the unit of life. Svedberg's former assistant also won a Nobel Prize, in our post on Arne Tiselius and electrophoresis.
Next step: repeat the worked example for an 80S ribosome and compare its time with the 70S value.
