A lysosome is a
membranous sac of hydrolytic enzymes that an animal cell uses to digest
macromolecules. Lysosomal enzymes work best in the acidic environment found in
lysosomes. If a lysosome breaks open or leaks its contents, the released
enzymes are not very active because the cytosol has a neutral pH. However,
excessive leakage from a large number oflysosomes can destroy a cell by autodigestion.
Hydrolytic
enzymes and lysosomal membrane are made by rough ER and then transferred to the
Golgi apparatus for fur· ther processing. At least some lysosomes probably
arise by bud· ding from the trans face ofthe Golgi apparatus (see Figure
6.13). Proteins of the inner surface of the lysosomal membrane and the
digestive enzymes themselves are thought to be spared from destruction by
having three-dimensional shapes that protect vulnerable bonds from enzymatic
attack.
Lysosomes carry
out intracellular digestion in a variety of circumstances. Amoebas and many
other protists eat by engulfing smaller organisms or other food particles, a
process called phagocytosis (from the Greek phagein, to eat, and kytvs,
vessel, referring here to the cell). The food vacuole formed in this
way then fuses with a lysosome, whose en· zymes digest the food (Figure 6.14a, bottom). Digestion products, including simple sugars,
amino acids, and other monomers, pass into the cytosol and become nutrients for
the cell. Some human cells also carry out phagocytosis. Among them are
macrophages, a type of white blood cell that helps defend the body by engulfing
and destroying bacteria and other invaders (see Figure 6.14a, top ).
Lysosomes also
use their hydrolytic enzymes to recycle the cell's own organic material, a
process called autophagy. During autophagy, a damaged organelle or small
amount of cytosol becomes surrounded by a double membrane, which is of unknown
origin, and a lysosome fuses with the outer membrane of this vesicle (Figure
6.14b). The lysosomal enzymes dismantle the enclosed material, and the organic
monomers are returned to the cytosol for reuse. With the help of lysosomes, the
cell continually renews itself. Ahuman liver cell, for example, recycles half
of its macromolecules each week.
The cells of
people with inherited lysosomal storage diseases lack a functioning hydrolytic
enzyme normally present in lysosomes. The lysosomes become engorged with
indigestible substrates, which begin to interfere with other cellular
activities. In Tay-Sachs disease, for example, a lipid-digesting enzyme is missing
or inactive, and the brain becomes impaired by an accumulation of lipids in the
cells. Fortunately, lysosomal storage diseases are rare in the general
population.
