Cell Fractionation
Background
Studies
of the properties of specific subcellular organelles
requires that the
organelles be separated from the cells, purified and concentrated. In this laboratory
exercise cellular organelles will be separated by differential centrifugation.
During centrifugation, objects move in a circle at a steady angular velocity and
are subjected to an outward directed force, 'F'. The magnitude of this force is given by
the formula
F =
(02r
where
'(0'
is the angular velocity in radians per second and 'r' is the radial distance, in
centimeters, of the particle from the axis of rotation. A 'radian' (rad) is an arc whose
length is equal to its radius. Therefore the circumference of any circle contains
21t
radians.
Force applied to particles is expressed as 'Relative Centrifugal
Force' (RCF)
since the force applied is compared to the earth's gravitational force.
RCF =
centrifugal force
=
gravitational force
(02r
__
980
cm/see/sec
The operating speed of a centrifuge is usually expressed in revolutions per
minute (rpm). These values can be converted into RCF by first converting rpm to
'(0'
and then to RCF.
(0
=
21t
x n revolutions x 1 min
1 revolution
1 minute
60 sec
where 'n' = number of rpm
1t
=
22
7
=
0.105
n / sec
Then,
RCF=
(02r
__
980
cm/see/sec
=
(0.105
n / sec)2r
980
cm/see/sec
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 Fall '10
 DorrisTerry
 Centrifuge, Cytosol, sucrose solution, pelleting mitochondria*

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