Due to friction with molecules of the surrounding solution, sedimenting particles in a mixture reach a terminal velocity when subjected to ultracentrifugation. The sedimentation coefficient (S) for a particle is defined as the ratio of terminal velocity (vt) to the centrifugal acceleration applied to the particle (ac). S = vt / ac
If each Svedberg unit (S value) is defined as 10-¹³ seconds, approximately how long would it take for a free 50S ribosomal subunit to travel the length of a 9 cm ultracentrifuge tube subjected to a centrifugal force of 50,000 times gravity (g)? Recall that g is approximately 10 m/s².

Question

Quizplus · Accepted Answer

The terminal velocity for the 50S particle is calculated as:
vt = S × ac = (50S × 10-¹³ s/S) × (50,000 × 10 m/s²) = 2.5 × 10-⁶ m/s = 0.9 cm/h
It would therefore take about 10 hours for the 50S particle to travel about 9 cm.

Due to Friction with Molecules of the Surrounding Solution, Sedimenting