The significance of this new 
instrument did not become 
apparent until years later. 
For several decades, quite a 
few biochemists studied the 
chemical reactions of the cell 
without realizing that the 
microscopists could actually 
see many of the particles they 
were analyzing. Finally, in the 
1950’s, the two groups began 
to edge closer together. By that 
time, electron-microscopic 
techniques had been refined. 
As the microscopists and 
biochemists began to 
communicate, there was an 
avalanche of discoveries 
about the world within the 
cells of animals and plants. A 
whole new vocabulary had to 
be developed for the cellular 
structures that the electron 
microscope and ultracentrifuge 
revealed. Cell biologists 
began to define many of the 
general characteristics that 
cells share, and to discern the 
mechanisms that cells use to 
make proteins and other vital 
molecules. Understanding 
these mechanisms, in turn, 
enabled them to begin to 
identify specific steps in 
critical biochemical pathways. 
Some of these pathways will 
be mentioned in the following 
sections on specific cellular 
organelles. 
Ho w a centrifuge is used 
to isolate cell components. 
To separate the various 
particles in cells, bio- 
chemists begin by placing 
whole cells in a solution 
and then breaking the 
cells with a pestle or 
with high-frequency 
sound waves (a). 
The mixture is then 
filtered to remove 
unbroken cells. At this 
point, the cell organelles 
and fragments are 
free-floating (b). 
As the sample is spun in 
the centrifuge at increas- 
ingly higher speeds and 
force, the organelles 
begin to settle to the 
bottom depending on 
their size and density. 
First to settle, out are 
pellets of nuclei (c). 
a. 
b. 
d. 
Crush and fdter Spin at low speed Spin at higher 
and force speed and force 
Pestle 
Whole cell 
Spin at highest 
speed and force 
O’ 
o% 
# A A. 
Pellets 
of nuclei 
O'- 
i-q 
O.-: 
Lysosome 
Mitochondrion 
At forces greater than 
10.000 times that of 
gravity, pellets of 
mitochondria and 
lysosomes sink to the 
bottom (d). 
Finally , at very high 
speeds and at forces 
100.000 times that of 
gravity, the very lightest 
particles and organelles 
begin to settle out (e). 
Fragments of 
endoplasmic 
reticulum and 
other light 
particles 
