LYSOSOMES AND 
PEROXISOMES, 
THE CELL'S 
"DIGESTIVE SYSTEM" 
When a white blood cell engulfs a 
bacterium and destroys it, the white 
cell's lysosomes do most of the work. 
They fuse with the vesicle of en- 
gulfed material and release digestive 
enzymes to break up the material. 
Similarly, when a cell takes in large 
molecules of food, enzymes in the 
lysosomes break the food down into 
smaller and simpler products that the 
cell can use. These products diffuse 
through the lysosomes' memoranes 
and go into the rest of the cell, 
where they serve as building blocks 
for various structures, until nothing is 
left inside the lysosomes but indigest- 
ible material and the lysosomes 
become what are called residual 
bodies. In some cells, the residual 
bodies then migrate to the cell 
surface and eject the undigested 
material into the external environment. 
Lysosomes were discovered by a 
Belgian researcher, Christian de Duve, 
in 1949, when he homogenized some 
animal cells and separated them into 
various components by running them 
through an ultracentrifuge. After one- 
of these components had been left 
standing for a few days, de Duve 
noticed that the level of a certain 
enzyme in it rose dramatically. Since 
this enzyme had not attacked any part 
of the cells before they were ground 
up, he reasoned that it must have been 
kept segregated within the cell — 
probably inside some kind of organ- 
elle. He also knew that he had used 
a relatively gentle method of homog- 
enization, which could have allowed 
An electron micrograph showing 
two small lysosomes and one large 
lysosome. These organelles contain 
enzymes capable of breaking down 
various substances. 
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