LYSOSOMES AND PEROXISOMES, THE CELL'S "DIGESTIVE SYSTEM" 
hen a white 
blood cell 
engulfs a 
bacterium 
and destroys 
it, the white blood cell’s 
lysosomes do most of the 
work. They fuse with the 
vesicle of engulfed 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’ membranes 
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 indigestible 
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 using 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 had risen 
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 
organelle. He also knew that 
he had used a relatively gentle 
method of homogenization, 
which could have allowed the 
unknown organelle to remain 
intact. Presumably, it 
released its contents later. 
De Duve’s biochemical 
approach, for which he 
shared the Nobel Prize 
with Claude and 
Palade in 1974, 
was soon supple- 
mented by electron 
microscopy. But it 
proved difficult to 
identify the new 
particles, since, 
unlike other 
organelles, lysosomes 
vary in shape from 
cell to cell. Finally, in 
An electron micrograph 
showing two small lyso- 
somes and one large 
lysosome. These 
organelles contain 
enzymes capable of 
breaking (lawn various 
substances. 
