First stage
Biology
Lec-6
14/12/2015
.د
انعام
Lysosomes
(Intracellular digestion centers)
The lysosomes ( intercellular digestion centers), are round or
vacuolar structures which filled with dense material and are
bounded by single unit membrane. Size and shape of lysosomes
vary from cell to cell and time to time. They contain at least 60
hydrolytic destructive enzymes. These enzymes work under
acidic medium and hence they are collectively called as acidic
hydrolases. A lysosome may contain one or more of these
enzymes: proteases, lipases, phosphatase, glycosidases, nucleases
and sulphatases.
They digest excess or worn out organelles, food particles, and
engulfed viruses or bacteria. The membrane surrounding a
lysosome prevents the digestive enzymes inside from destroying
the cell. Lysosomes fuse with vacuoles and dispense their
enzymes into the vacuoles, digesting their contents. At pH 4.8,
the interior of the lysosomes is more acidic than the cytosol
(pH 7.2) .
The hydrolytic enzymes in lysosomes, are produced in the
endoplasmic reticulum, and transported and processed through
the Golgi apparatus. The Golgi apparatus produces lysosomes by
budding. Each acid hydrolase is then target to a lysosome by
phosphorylation.
The lysosomes are generally divided into two categories,
primary and secondary lysosomes and the latter is further
divided into 3 types:
(1)
a- Heterophagosomes: also called as secondary lysosomes. They
contain acid hydrolyases. A digestive vacuole called
phagosome, is formed as a result of pinocytosis (cell drinking) or
phagocytosis (cell eating). The phagosome fuses with the primary
lysosom so as to form the secondary lysosome or heterophagy.
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
(2)
Ribosome
Ribosome 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 .
There are different type of ribosome;70s ribosome, 55s ribosome
and 80s ribosome.
Peroxisomes
Peroxisomes are small membrane-bound , self-replicating
organelles found in nearly all eukaryotic cells ,that function to rid
the cell of toxic substances . They have a single lipid bilayer
membrane that separates their contents from the cytosol.
(3)
Unlike lysosomes, which are formed in the secretory pathway,
peroxisomes usually self-replicate by enlarging and then
dividing, although there is some indication that new ones may
be formed directly.
Peroxisomes contain oxidative enzymes, such as catalase, D-
amina acid oxidase and ureate oxidase. Certain enzymes within
the peroxisome, by using molecular oxygen, remove hydrogen
atoms from specific organic substrates (labeled as R), in an
oxidative reaction, producing hydrogen peroxide (H
2
O
2
, itself).
RH
2
+ O
2
R+ H
2
O
2
The name peroxisome refers to the hydrogen peroxide produced
as a by-product of the activities of the oxidative enzymes in the
microbody. Hydrogen peroxide is dangerous of cells because of
its violent chemical reactivity.
However peroxisomes also contain
the enzyme catalase, which breaks down hydrogen peroxide into
harmless water and oxygen and uses this H
2
O
2
to oxidize other
substrates, including phenols, formic acids, formaldehyde and
alcohol, by means of the peroxidation reaction:
H
2
O
2
+ RH
2
R+ 2H
2
O
Thus eliminating the poisonous hydrogen peroxide in the process.
This reaction is important in liver and kidney cells where the
peroxisomes detoxify various toxic substances that enter the
blood. In addition, when excess H
2
O
2
accumulates in the cell,
catalase converts it to H
2
O through this reaction:
2H
2
O
2
2H
2
O+ O
2
(4)
A major function of the peroxisome is the breakdown of fatty
acid molecules, in a process called beta-oxidation.
Vacuoles and
Vesicles
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.
Vesicles are basic tool of the cell for organizing metabolism,
transport, enzyme storage, as well as being chemical reaction
chambers. Many vesicles are made in Golgi apparatus ,but also in
the endoplasmic reticulum ,or are made from the plasma
membrane.
(5)