Transport of molecules within a cell and out of the cell requires a complex endomembrane system. Endocytosis occurs when the cell membrane engulfs particles (dark blue) outside the cell, draws the contents in, and forms an intracellular vesicle called an endosome. This vesicle travels through the cell, and its contents are digested as it merges with vesicles containing enzymes from the Golgi. The vesicle is then known as a lysosome when its contents have been digested by the cell. Exocystosis is the process of membrane transport that releases cellular contents outside of the cell. Here, a transport vesicle from the Golgi or elsewhere in the cell merges its membrane with the plasma membrane and releases its contents. In this way, membranes are continually recycled and reused for different purposes throughout the cell. Membrane transport also occurs between the endoplasmic reticulum and the Golgi.
The Endomembrane System
Today, scientists know that the endomembrane system
includes the ENDOPLASMIC
RETICULUM(ER), GOLGI APPARATUS,
The first Endomemembrane System was
discovered by Camillo Golgi in the late 1800s.
The endomembrane system is composed of the different inter-related membrane
sacs within the cytoplasm of the cell.
"The police force of the cell"
“The cells' garbage disposal system.
It is the…
Lysosomes are spherical membranous bags that
contain enzymes (Acid Hydrolases).
size varies from 0.1–1.2 μm.
The lysosomal enzyemes are capable of digesting all
varieties of biological molecules.
I. Digestion of worn out or non functional organelles.
II. Metabolic functions
III. Degradation of nonuseful tissue.
These membrane-bound organelles contain a variety of
enzymes called hydrolases that can digest proteins,
nucleic acids, lipids, and complex sugars.
Lysosomes break down all varieties of biological
molecules into their constituent parts, which are then
Some important enzymes found within
All those hydrolytic enzymes are produced in the
endoplasmic reticulum, and to some extent in
cytoplasm are transported and processed through the
Golgi apparatus and through Golgi apparatus they
pinch off as single membrane vesicles.
In addition, vesicles that bud off from the plasma
membrane via endocytosis are also sent to lysosomes,
where their contents — fluid and molecules from the
extracellular environment — are processed. The
process of endocytosis is an example of reverse vesicle
trafficking, and it plays an important role in nutrition
and immunity as well as membrane recycling.
Lysosomal enzymes works best in an acid pH
the lysosome membrane fuses with the membrane of a
food vacuole or Phagosome (particle containing vessel)
and squirts the enzymes inside.
adalanall about lysosomeslysosomes at work.FLV
This action is called AUTOPHAGY…
adalanall about lysosomesautophagy.avi - YouTube.FLV
The digested food can then diffuse through the vacuole
membrane and enter the cell to be used for energy or
Two Important Function:
1. maintain the organelles low pH level.
2. retains the dangerous acid Hydrolases
while permitting the final products of digestion
to escape so that they can be used by the cell
The only thing that keeps the cell itself
from being digested is the membrane
surrounding the lysosomes.
When the cell is injured or deprived of
oxygen and when excessive amount of
Vitamin A are present, the lysosomal
membrane becomes fragile. Such
ruptured results in Self-Digestion of the
cell, a process is called
If the lysosomal enzymes do not reach the target it
causes Lysosomal Storage Disease..
an inherited disorder that is caused by the lack of the
enzyme hydrolase acid alpha glucosidase (GAA) that is
contained in lysosomes. GAA is responsible for
breaking down glycogen to glucose.
Inclusion cell disease (I-cell disease)
this condition is caused by the failure to tag, by
phosphorylation, all of the hydrolytic lysosomal
enzymes with mannose-6-phosphate. This malfunction
means that instead of the enzymes being transported to
the lysosome, they are instead secreted from the cell.
Peroxisomes were first discovered by Christian de
Peroxisomes , also
are membranous sacs
Peroxisomes biogenesis has been
Recent experiments indicate that
peroxisomes originate or bud off
from the endoplasmic reticulum.
Peroxisomes are about the size of lysosomes (0.5–1.5
µm) and like them are bound by a single membrane.
They also resemble lysosomes in being filled with
contains a peroxisomal targeting signal (PTS).
contain about 50 different enzymes.
have crystalline and non-crystalline inclusions.
Peroxisomes stand out
major sites of oxygen utilization
Detoxify harmful or toxic substance like alcohol and
Disarm oxygen free radicals such as superoxide radical
by converting them to hydrogen peroxide (H2O2).
And reduce Hydrogen peroxide to water and oxidizes
organic compounds (CH2)
H2O2 + CH2 2H2O+ C
Breakdown (by oxidation) of excess fatty acids.
Breakdown of hydrogen peroxide (H2O2), a
potentially dangerous product of fatty-acid
oxidation. It is catalyzed by the enzyme catalase.
Participates in the synthesis of cholesterol. One of
the enzymes involved is reductase.
Participates in the synthesis of bile acids.
Participates in the synthesis of the lipids used to
Functions of the peroxisomes in the
Lack of peroxisomes or dysfunction may lead to:
A mutation in peroxin 2 (Pex2), a peroxisomal membrane
protein involved in import process, causes one form of
It is a rare, congenital (present at birth) disorder characterized
by the reduction or absence of peroxisomes (cell structures
that rid the body of toxic substances) in the cells of the liver,
kidneys, and brain. Zellweger syndrome is one of a group
of genetic disorders called peroxisomal diseases that affect
brain development and the growth of the myelin sheath,
the fatty covering-which acts as an insulator-on nerve fibers
in the brain
Adrenoleukodystrophy (ALD) is a rare,
inherited metabolic disorder that afflicts the
young boy Lorenzo Odone, whose story is told in
the 1993 film "Lorenzo's oil." In this disease, the
fatty covering (myelin sheath) on nerve fibers in
the brain is lost, and the adrenal gland
degenerates, leading to progressive neurological
disability and death.
transport vesicles play a central role in the
traffic of molecules between different
Vesicular transport is thus a major cellular
activity, responsible for molecular traffic
between a variety of specific membrane-
the formation of a vesicle by budding from the membrane.
The cytoplasmic surfaces of transport vesicles are coated
Three kinds of coated vesicles
1. clathrin-coated vesicles
are responsible for the uptake of extracellular molecules from
the plasma membrane by endocytosis as well as the transport of
molecules from the trans Golgi network to lysosomes.
2. COPII-coated vesicles-
bud from the ER and carry their cargo forward along the
secretory pathway, to the Golgi apparatus.
3. COPI-coated vesicles
bud from the ER-Golgi intermediate compartment or the
Golgi apparatus and function in the retrieval pathways that
serve to retain resident proteins in the Golgi and ER.
are composed of two types of protein complexes
clathrin and adaptor proteins.
Clathrin plays a structural role by assembling into a
basketlike lattice structure that distorts the
membrane and drives vesicle budding.
The binding of clathrin to membranes is mediated by
a second class of proteins, called adaptor proteins.
COPI- and COPII-coated vesicles
composed of distinct protein complexes, which
function analogously to clathrin and
adaptor proteins in vesicle budding
The fusion of a transport vesicle with its target
involves two types of events
1.the transport vesicle must specifically recognize
the correct target membrane.
2. the vesicle and target membranes must fuse,
thereby delivering the contents of the vesicle to
the target organelle.
Two groups of proteins are targeted to the ER:
Proteins that are completely translocated into lumen of the
ER. These are soluble proteins destined for secretion, or
for the lumen of another organelle.
Proteins that are inserted into membranes, and
hence are only partially translocated into the
endoplasmic reticuluum. These proteins may be
destined for ER, another organelle, or the plasma
Lysosomal proteins from the rough
endoplasmic reticulum (ER) must be further
begins in the
where a core
e is added to
The hydrolases are then
packaged into transport vesicles
and transferred to the cis-
cisterna of the Golgi apparatus.
Inside the cis-cisterna of the
Golgi, the core
One of the mannose residues
in the oligosaccharide receives
a phosphate by way of two
The modification of the core
oligosaccharide on the hydrolase
enzyme results in the creation of an
mannose 6-phosphate signal unique
to proteins destined for the
lysosome. Other signals target other
proteins for different destinations.
Upon reaching the trans-
Golgi, the M6P portion of the
hydrolase binds to M6P
receptors embedded in the
The membrane of the trans-
Golgi then buds off into a
vesicle containing the
receptors and the bound