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Lysosomes
(Intracellular digestion centers)
Lysosome, subcellular organelle that is found in nearly all types of
eukaryotic cells and that is responsible for the digestion of
macromolecules, old cell parts, and microorganisms.
Structure of Lysosomes
Lysosomes are actually membranous sacs filled with enzymes. They
act as the 'digester' of the cell .Lysosomes are spherical bag-like
structures that are bound by a single layer membrane; however, their
shape and size may vary to some extent in different organisms.
Lysosomes are manufactured by the Golgi apparatus, by budding, in
the cell, and the various digestive enzymes that are present in the
lysosomes are produced in the endoplasmic reticulum. These enzymes are
then transported to the Golgi apparatus and are distributed to the
lysosomes. Some examples of enzymes present in these organelles
include nucleases, proteases, lipases, and carbohydrases. These enzymes
are used to dissolve nucleic acids, proteins, lipids and carbohydrates,
respectively. All these enzymes are typically hydrolytic (acid hydrolases)
they work best at acidic environments , and can digest cellular
macromolecules. Lysosomes are acidic, with a pH of 4.8. This acidic pH
is maintained by pumping protons, from the cytosol that has a pH of 7.2.
Although it may seem dangerous for cells to contain enzymes that can
digest most biological molecules, the contents of the cell are doubly
protected from the digestive enzymes of the lysosome. First, the enzymes
are enclosed in the lysosomal membrane and second, even if the enzymes
were to leak out of the lysosome, they would not be active at the neutral
pH of the cytosol.
Types of lysosomes:
The lysosomes are generally divided into two categories, primary and
secondary lysosomes.
Primary lysosome are formed by Golgi app., their hydrolytic enzymes
are produced in RER . One or more primary lysosomes fuse with the
vesicle containing the ingested material , forming a large vesicle a
secondary lysosome, and the latter is further divided into 3 types:
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a- Heterophagosomes: A digestive vacuole called phagosome, is formed
as a result of pinocytosis (cell drinking) or phagocytosis (cell eating). The
phagosome fuses with the primary lysosome so as to form the secondary
lysosome or heterophagosome.
b- Residual bodies: They contain indigestible material. They are formed
when the digestion is incomplete, perhaps due to the absence of some
enzymes. ), large quantities of residual bodies accumulate and are
referred to as lipofuscin, or age pigment.
c- Autophagosomes: also called autophagic vacuoles or cytolysosomes.
They contain some part of the cell in the process of digestion such as ER,
mitochondria etc. digestion of these intracellular components is described
as cellular autophagy.
Lysosomes
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Lysosomal storage diseases
Are genetic disorders in which a genetic mutation affects the activity of
one or more of the acid hydrolases. In such diseases, the normal
metabolism of specific macromolecules is blocked and the
macromolecules accumulate inside the lysosomes, causing severe
physiological damage or deformity. Ex. Tay-sachs disease, in which
normal lipid cannot be broken down in brain cells , the lipid accumulates
in the cells, resulting in mental retardation & death
Ribosomes
Ribosomes are small, dense-electron particles in cells that assembles
proteins. Ribosomes are composed of 65% ribosomal RNA and 35%
ribosomal proteins .Ribosomes consist of two subunits a small subunit
and a large subunit that bind together and work as one to translate the
mRNA into a polypeptide chain during protein synthesis . Ribosomes can
be found within the cytoplasm, either singly or in groups called
polyribosomes. Ribosomes can also be found attached to the endoplasmic
reticulum.
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Peroxisomes
Peroxisomes are small membrane-bound , self-replicating organelles
found in nearly all eukaryotic cells. Hundreds of these round organelles
can be found within a cell. Also known as microbodies, peroxisomes are
bound by a single membrane and contain enzymes that produce hydrogen
peroxide as a by-product. The enzymes decompose organic molecules
through oxidation reactions, producing hydrogen peroxide in the process.
RH2+O2 → R + H2O2
Hydrogen peroxide, a toxic molecule, is immediately broken down to
water and oxygen by another peroxisomal enzyme called catalase.
H2O2 + RH2
R+ 2H2O
Function:
Peroxisomes are involved in at least 50 different biochemical reactions
in the body. Types of organic polymers that are broken down by
peroxisomes include amino acids, uric acid, and fatty acids. Peroxisomes
in liver cells help to detoxify alcohol and other harmful substances
through oxidation. In addition to being involved in the oxidation and
decomposition of organic molecules, peroxisomes are also involved in
synthesizing important molecules. In animal cells, peroxisomes
synthesize cholesterol and bile acids (produced in the liver). Certain
enzymes in peroxisomes are necessary for the synthesis of a specific type
of phospholipid that is necessary for the building of heart and brain white
matter tissue.
Vacuoles
Vacuoles are membrane -bound organelles within some eukaryotic cells
that can serve a variety of secretory, excretory, and storage functions. In
general, vacuole functions include
1- Removing unwanted structural debris
2- Isolating materials that might be harmful or a threat to the cell.
3- Containing waste products.
4- Maintaining internal hydrostatic pressure within the cell
5- Enabling the cell to change its' shape.
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