Page 32 - Biology_F5

Cytology

Functions of lysosomes engulfed, they are contained within the
The main functions of lysosome include autophagosome (autophagic vacuole).
heterophagy, autophagy and autolysis. The latter can then fuse with lysosomes,
which result in the release of lysosomal
Heterophagy acid enzymes to degrade the contents of
This is a process whereby lysosomes the autophagosome. The products of the
FOR ONLINE READING ONLY
perform the intercellular digestion of degradation include amino acids, among
materials that are gathered from outside other molecules and all of them are
the cell by endocytosis, which are released back into the cytoplasm and re-
pinocytosis (taking in liquid materials) and used in metabolism as well as in building
phagocytosis (taking in solid materials). The macromolecules.
endocytotically ingested food material is
collected in a membrane bounded vesicle
called heterophagosomes (pinosomes or Autolysis
phagosomes) and eventually fuse with Autolysis is more commonly known
lysosomes to form phagolysomes in as self-digestion, which refers to the
which the engulfed material is digested. destruction of a cell through the action of
For example, when bacteria are ingested its own enzymes. Lysosomes break down
by phagocytosis, the ingested bacteria are to release its contents (enzymes) which

packed in a phagocytic vesicle (phagosomes). digest various organelles and finally the
This vesicle fuses with a primary lysosome, entire cell in certain pathological conditions.
forming a secondary lysosome. Thereafter, This process is known as autolysis or self-
enzymes of the lysosome digest the bacterial destruction of the cell.
macromolecules.
Exocytosis or extracellular digestion
Autophagy Exocytosis is the process responsible
Autophagy allows the orderly degradation for breaking down damaged cellular
and recycling of cellular components. components and some unwanted proteins,
This process occurs when cytoplasmic
components become enclosed in a double which are expelled out of the cell. One
membrane (phagophore) to form a of the causes of aging is that, in long-
compartment known as autophagosome lived cells the types of metabolic waste
for degradation (Figure 1.14). This is the that cannot be broken down accumulates
regulation mechanism of the cell that and lysosomes may release them outside
disintegrates unnecessary or dysfunctional the cell by exocytosis. For example, the
components. Autophagy starts with the enzymes of lysosome are released during
formation of a phagophore from the replacement of the cartilage by bone during
membrane; this expands and engulfs the development. Also lysosomes of certain
molecules or “cargo” for degradation. The cells such as spermatozoa discharge their
cargo can include redundant organelles or enzymes outside the cell during fertilization
any other chemical aggregates and once process.

Form Five Student’s Book
25