LYSOSOMES
Lysosomes are the membrane-bound
vesicular organelles found throughout the cytoplasm. The lysosomes are
formed by Golgi apparatus. The enzymes synthesized in rough endoplasmic reticulum
are
processed and packed in the form of
small vesicles in the Golgi apparatus. Then, these vesicles are
pinched off
from Golgi apparatus and become the lysosomes. Among the organelles of the
cytoplasm, the
lysosomes have the thickest covering
membrane. The membrane is formed by a bilayered lipid material. It has many
small granules which contain hydrolytic enzymes.
Types of Lysosomes
Lysosomes are of two types:
1. Primary lysosome, which
is pinched off from Golgi apparatus. It is inactive in spite of having hydrolytic
enzymes
2. Secondary lysosome, which is the
active lysosome. It is formed by the fusion of a primary lysosome with phagosome
or endosome (see below).
Functions of
Lysosomes
Lysosomes are often called ‘garbage system’ of the cell because
of their degradation activity. About 50 different hydrolytic enzymes, known as
acid hydroxylases
are present
in the lysosomes, through which lysosomes execute their functions.
Important lysosomal
enzymes
1. Proteases, which hydrolyze the
proteins into amino acids
2. Lipases, which hydrolyze the lipids
into fatty acids and glycerides
3. Amylases, which hydrolyze the
polysaccharides into glucose
4. Nucleases, which hydrolyze the
nucleic acids into mononucleotides.
Mechanism of
lysosomal function
Lysosomal functions involve two
mechanisms:
1. Heterophagy: Digestion
of extracellular materials engulfed by the cell via endocytosis
2. Autophagy:
Digestion of intracellular materials such as worn-out cytoplasmic organelles.
Specific
functions of lysosomes
1. Degradation of macromolecules
Macromolecules are engulfed by the
cell by means of endocytosis (phagocytosis, pinocytosis or receptormediated
endocytosis. The macromolecules such as bacteria, engulfed by the cell via
phagocytosis are called phagosomes or vacuoles. The other macromolecules taken inside
via pinocytosis or receptor-mediated endocytosis are called endosomes. The primary lysosome fuses with the
phagosome or endosome to form the secondary lysosome. The pH in the secondary
lysosome becomes acidic and the lysosomal enzymes are activated. The bacteria
and the other macromolecules are digested and degraded by these enzymes. The
secondary lysosome containing these degraded waste products moves through
cytoplasm and fuses with cell membrane. Now the waste products are
eliminated by exocytosis.
2. Degradation of worn-out
organelles
The rough endoplasmic reticulum wraps
itself around the worn-out organelles like mitochondria and form
the vacuoles called autophagosomes. One primary lysosome
fuses with one autophagosome to form the
secondary lysosome. The enzymes in the
secondary lysosome are activated. Now, these enzymes digest the contents of
autophagosome.
3. Removal of excess secretory
products in the cells
Lysosomes in the cells of the
secretory glands remove the excess secretory products by degrading the
secretory granules.
4. Secretory function – secretory
lysosomes
Recently, lysosomes having secretory
function called secretory lysosomes are found in some of the
cells, particularly in the cells of
immune system. The conventional lysosomes are modified into
secretory
lysosomes by combining with secretory
granules (which contain the particular secretory product of the cell). Examples
of secretory lysosomes:
i. Lysosomes in the cytotoxic T
lymphocytes and natural killer (NK) cells secrete perforin and
granzymes, which destroy both
viral-infected cells and tumor cells. Perforin is a pore-forming
protein that initiates
cell death. Granzymes belong to the family of serine proteases (enzymes that
dislodge the peptide bonds of the
proteins) and cause the cell death by apoptosis
ii. Secretory lysosomes of melanocytes
secrete melanin
iii. Secretory lysosomes of mast cells
secrete serotonin, which is a vasoconstrictor substance
and inflammatory
mediator.
PEROXISOMES
Peroxisomes or microbodies are the
membrane limited vesicles like the lysosomes. Unlike lysosomes,
peroxisomes are pinched off from
endoplasmic reticulum and not from the Golgi apparatus. Peroxisomes
contain some
oxidative enzymes such as catalase, urate oxidase and
Damino acid
oxidase.
Functions of
Peroxisomes
Peroxisomes:
i. Breakdown the fatty acids by means
of a process called betaoxidation:
This is the major
function of
peroxisomes
ii. Degrade the toxic
substances such as hydrogen peroxide and other metabolic products by means
of detoxification.
A
large number of peroxisomes are present in the cells of liver, which
is the major
organ for detoxification.
Hydrogen peroxide is formed
from poisons or alcohol, which enter the
cell. Whenever hydrogen peroxide is
produced in the cell, the peroxisomes are ruptured and
the oxidative enzymes are released.
These oxidases destroy hydrogen peroxide and the
enzymes which are necessary for the
production of hydrogen peroxide
iii. Form the major site
of oxygen utilization in the cells
iv. Accelerate gluconeogenesis from
fats
v. Degrade purine to
uric acid
vi. Participate in the formation of
myelin
viii. Play a role in the formation of
bile acids.
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