Human Physiology: Endomembrane system

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Human PhysiologyEndomembrane system
BY DR BOOMINATHAN Ph.D. M.Sc.,(Med. Bio,
JIPMER), M.Sc.,(FGSWI, Israel), Ph.D (NUS,
SINGAPORE) PONDICHERRY UNIVERSITY III
Lecture 9/August/2012

Source Collected from different sources on the
internet-http//koning.ecsu.ctstateu.edu/cell/cell
.html
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Learning objectives
1.The structure and function of
endoplasmic reticulum(ER) 2.The structure
and function of Golgi complex 3.The
structure and function of lysosomes .
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Introduction

• The Compartmentalization in Eukaryotic Cells
• Membranes divide the cytoplasm of eukaryotic
  cells into distinct compartments.
• Three categories in eukaryotic cells
• 1.the endomembrane system endoplasmic
  reticu- lum, Golgi complex, Lysosomes.
• 2.the cytosol.
• 3.mitochondria, peroxisomes and the nucleus.
• Membrane-bound structures (organelles) are
  found in all eukaryotic cells.

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Introduce

• Endomembrane System The structural and
  functional relationship organelles include
  endoplasmic reticulum ,Golgi complex, lysosome,
  secretory vesicles.
• Membrane-bound structures (organelles) are found
  in all eukaryotic cells.

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Endomembrane System
endoplasmic reticulum(ER)
Golgi complex
lysosome
secretory vesicles
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Endomembrane System
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Section I Endoplasmic Reticulum
1.1 The Structure of ER The endoplasmic
reticulum is a network of flattened sacs and
branching tubules that extends throughout the
cytoplasm in plant and animal cells. These sacs
and tubules are all interconnected by a single
continuous membrane so that the organelle has
only one large, highly convoluted and complexly
arranged lumen (internal space).
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The Sructure of ER
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1.2 The types of ER
The ER comes in two forms.
Rough endoplasmic reticulum(RER)
Smooth endoplasmic reticulum(SER)
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1.2 The types of ER
1.2.1 The rough ER The ER are covered with
ribosome .Their rough appearance under electron
microscopy led to their being called rough ER .
RER has ribosomes on the cytosolic side of
continuous, flattened sacs.
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1.2 The types of ER
1.2.1 The rough ER The outer membrane of the
nucleus is always studded with ribosomes and is
continuous with rough ER membrane . The lumen of
RER is connected to nuclear envelope .
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1.2 The types of ER
1.2.2 The smooth ER The parts are free of
ribosomes and are called smooth ER (SER). SER is
an interconnecting network of tubular membrane
elements.
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1.2 The types of ER

• Rough and smooth ER differ not only in
  structure, but also in function.

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1.3 The functions of the RER
1.3.1 Proteins synthesized on ribosomes of RER
The ribosomes assemble amino acids into
protein units, which are transported into the
lumen of rough endoplasmic reticulum for further
processing. These proteins may be either
transmembrane proteins, which become embedded in
the membrane of the endoplasmic reticulum, or
water-soluble proteins, which are able to pass
completely through the membrane into the lumen.
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1.3 The functions of the RER
Those proteins that reach the inside of the
endoplasmic reticulum are folded into the correct
three-dimensional conformation. Chemicals, such
as carbohydrates or sugars, are added, then the
ER either transports the completed proteins to
areas of the cell where they are needed, or they
are sent to the Golgi apparatus for further
processing and modification.
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1.3 The functions of the RER

• 1.3.1 Proteins synthesized on ribosomes of
  rER include
• Secretory proteins
• integral membrane proteins
• soluble proteins of organelles.

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1.3 The functions of the RER
? How do the ribosomes (a membrane-bound
ribosome ) attach to the outer surface of rER
,and then the newly synthesized protein pass
completely through the membrane into the lumen of
rER?
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1.3 The functions of the RER
The Signal Hypothesis would explain completely
the mechanism that the ribosomes attach to the
outer surface of rER ,and then the newly protein
pass completely through the membrane into the
lumen of rER?
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1.3 The functions of the RER
The basic structure of the process is a Signal
Reco-gnition Particle(SRP),that lead the
ribosomes attach to the outer surface of rER ,and
then lead the newly protein pass completely
through the membrane into the lumen of rER.
SRP
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1.3 The functions of the RER

• Signal-recognition particle(SRP)1.synthesisSix
  different polypeptides complexed with a
  300-nucleotide molecule of RNA.

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1.3 The functions of the RER
2. SRP have three main active sites One that
recognizes and binds to ER signal sequence One
that interacts with the ribosome to block further
translation One that binds to the ER membrane
(docking proteinreceptor protein)
The site to recognize and bind to ER signal
sequence
The site to block further translation
The site to recognize receptor protein
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1.3 The functions of the RER

• ER signal sequence1. synthesis Typically 15-30
  amino acids.2.consist of three domains a
  positively charged N-terminal regiona central
  hydrophobic regiona polar region adjoining the
  site where cleavage from the mature protein will
  take place. A signal sequence on nascent
  seretory proteins targets them to the ER and is
  then cleaved off.SRP receptor (a binding protein
  or docking proteinreceptor protein)

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• Signal peptides and Signal patches

Figure 6-8 Two ways that a sorting signal can be
built into a protein. (A) The signal resides in a
single discrete stretch of amino acid sequence,
called a signal peptide, that is exposed in the
folded protein. Signal peptides often occur at
the end of the polypeptide chain, but they can
also be located elsewhere. (B) A signal patch can
be formed by the juxtaposition of amino acids
from regions that are physically separated before
the protein folds alternatively, separate
patches on the surface of the folded protein that
are spaced a fixed distance apart could form the
signal.
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1.3 The functions of the RER
The basic structure of the mechanism of Signal
Hypothesis 1. Signal Recognition
Particle(SRP)2. ER signal sequence 3.SRP
receptor
1
3
SRP
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• The mechanism of the Signal Hypothesis

As the signal sequence emerge from the
ribosome, they are recognized and bound by a
signal recognition particle (SRP), This binding
inhibits translation and target the complex to
the RER by binding to the SRP receptor on the ER
membrane. SRP is then released and the ribosome
binds to a protein translocation complex in the
ER membrane the signal sequence is inserted into
a membrane channel, translation is resumed and
the unfolded growing polypeptide chain is
translocated across the membrane into the ER. As
translocation proceeds, the signal sequence is
cleaved by signal peptidase and the polypeptide
is released into the ER lumen.
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Signal Hypothesis
(SRP)
mRNA
A
P
A
ribosome
signal sequence
tRNA
Channel protein???????
SRP receptor
cytoplasme
The lumen of RER
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This process shows the signal receptor particle
that associates with the large and small subunit
of the ribosome that allows binding to the
receptor on the rough endoplasmic reticulum.After
the protein is synthesized, the ribosome
dissociates into large and small subunits and the
SRP also looses its attachment to the receptor.
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1.3.2 Modification/processing of newly
synthesized proteins glycosylation in the RER
Glycosylation of newly synthesized proteins

• N-linked oligosaccharide chain is linked to the
  amide nitrogen of asparagine (Asn) (in ER)
• O-linked oligosaccharide chain is linked to the
  hydroxyl group of serine or threonine (in Golgi)

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1.3.2 N-linked glycosylation in the RER
Lipid-linked oligosaccharide chain is added to
the dolichol by glycosyltransferase,then be
transferred to linked to the N terminus of
asparagines within polypeptide by oligosaccharine
protein transferase.
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• 1.3.2 Modification/processing of newly
  synthesized proteins the folding of proteins

The lumen of rER contains These chaperones and
enzymes recognize and bind to unfolded or
misfolded proteins and give them correct
conformation
Quality control ensuring that misfolded proteins
do not leave ER.
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• 1.3.2 Modification/processing of newly
  synthesized proteins the formed of disulfide
  bonds within polypeptide

• The lumen of rER contains
• Protein disulfide isomerase ( PDI ) transfer
  incorrect disulfide bonds to correct disulfide
  bonds within polypeptide.

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1.3.3 The transport of the proteins

• The transport of the
• proteins contains
• 1.The formation of transport
• Vesicle (secretory proteins)
• 2.The transport of
• integral membrane proteins3.The transport
  ofsoluble proteins of organelles.

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1.3 The functions of the RER
1.Proteins synthesized on ribosomes of RER.
2.Modification and processing of newly
synthesized proteins A.glycosylation in the
RER.B. the folding of proteins.C. the formation
of disulfide bonds within polypeptide.3. The
transport of the proteins .
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1.4 Functions of the SER
The side of cytoplasm
1.It takes part in the synthesis of various
lipids phospholipids (building membranes ) fatty
acids steroids (e.g.,hormones).
The lumen of SER
flipase
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1.4 Functions of the SER

• Transport by phospholipid exchange proteins
  (PEP)SER?other organelles.

phospholipid exchange proteins (PEP)
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1.4 Functions of the SER
2.Detoxification of organic compounds in liver
cells. It contains enzymes needed to detoxify
drugs. 3.Metabolism of heparin. 4.The SER serve
as a storage place for calcium.
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• The SER serve as a storage place for calcium
• In the case of smooth endoplasmic
  reticulum in muscle cells, the tubules serve as a
  store of calcium which is released as one step in
  the contraction process of muscle. Calcium pumps
  serve to move the calcium.

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1.4 Functions of the SER
1.Synthesis of lipid 2.Detoxification of organic
compounds in liver cells 3.Metabolism of
heparin 4.The SER serve as a storage place for
calcium.
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What are the functional differences between the
RER and the SER?

• Functions of the RER
• 1.Proteins synthesized on ribosomes of RER.
  2.Modification and processing of newly
  synthesized proteins A.glycosylation in the
  RER.B. the folding of proteins.C. the formation
  of disulfide bonds within polypeptide.3. The
  transport of the proteins .

Functions of the SER 1.Synthesis of
lipid 2.Detoxification of organic compounds in
liver cells 3.Metabolism of heparin 4.The SER
serve as a storage place for calcium.
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Summary

• ER is a network of folded membranes that extend
  through the cytoplasm to the nuclear membrane.
• There are two kinds of ER, rough and smooth.
• The functions of RER include the synthesis of
  protein, modification/processing and quality
  control of newly synthesized proteins.
• The SER has functions in several metabolic
  processes.
• It takes part in the synthesis of various lipids
  , fatty acids and steroids, and also plays an
  important role in carbohydrate metabolism,
  detoxification of the cell and calcium storage.

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Self-Quiz Choosed-question
1.Where does protein synthesis take place in
eucaryotic cells ? A.granum B.nucleus C.endoplasmi
c reticulum D.golgi apparatus
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Self-Quiz
2 .The ribosomes of prokaryotic cells are
found A.in the golgi apparatus B.free-floating C.
in the nucleus D.in the endoplasmic reticulum
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Self-Quiz
3.The smooth endoplasmic reticulum is the area in
a cell where ___ are synthesized.
A.polysaccharides B.proteins C.lipids
D.DNA
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Section? The Golgi complex
The Golgi apparatus is a polarized struc-ture
consisting of an oriented stack of disc-shaped
cisternae sur-rounded by a swarm of small
vesicles.
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2.1 The structure of Golgi complex

• The Golgi complex consists of a stack of
  flattened ? vesicles and tubules.
• The Golgi apparatus has two distinct faces
• a cis ,or forming face
• a trans,or maturing face

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2.2 The polarity of Golgi complex
The cis face is closely associated with a
transitional RER. In secretory cells ,the trans
face is the closest to the plasma membrane.The
large secretory vesicles are found in association
with the trans face of a Golgi stack. So it is
called the polarity organelle.
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Image of nucleus, endoplasmic reticulum and Golgi
apparatus.(1) Nucleus. (2) Nuclear pore. (3)
Rough endoplasmic reticulum (RER). (4) Smooth
endoplasmic reticulum (SER). (5) Ribosome on the
rough ER. (6) Proteins that are transported. (7)
Transport vesicle. (8) Golgi apparatus. (9) Cis
face of the Golgi apparatus. (10) Trans face of
the Golgi apparatus. (11) Cisternae of the Golgi
apparatus.
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2.2 The polarity of Golgi complex
The Golgi complex is compartmentalized.
Phosphorylation occurs in the Cis region. In
other regions, different types of carbohydrates
are added as a glycoprotein passes through the
cisternae. This figure illustrates the different
regions where sugars like mannose , galactose ,
etc are added. The final sorting is done in the
Trans Golgi complex. So it is called the
polarity organelle
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2.3 The functions of Golgi complex
2.3.1 Glycosylation in the Golgi complex
Golgi complex plays a key role in the assembly of
the carbohydrate component of glycoproteins and
glycolipids. O-linked oligosaccharides takes
place in Golgi complex. Oligosaccharide chain is
linked to the hydroxyl group of serine or
threonine .
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2.3.1 Glycosylation in the Golgi complex
The important role of Glycosylation 1.One
might suspect that they function to aid folding
and the transport process for example,carbohydrat
e as a marker during protein folding in ER and
the use of carbohydrate-binding lectins in
guilding ER-to-Golgi transport. 2.Limit the
approach of other macromolecules to the protein
surface, more resist to digestion by proteases.
3.Regulatory roles in signaling through the
cell-surface receptor notch, to allow these cells
to respond selectively to activating stimuli.
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2.3.2 The processing? sorting and transport in
the Golgi complex
The Golgi networks are processing and sorting
stations where proteins are modified, segregated
and then shipped in different directions.
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• Golgi complex and cells secretion

Constitutive secretion Continual,unregulated
discharge of material from the cells
Regulated secretion The discharge of products
stored in cytoplasmic granules, require
appropriate stimulus(e.g. neurotransmitter)
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Can proteins be transported back to the RER ?

Sometimes vital proteins needed in the
RER are transported along with the other proteins
to the Golgi complex. The Golgi complex has a
mechanism for trapping them and sending them back
to the rough endoplasmic reticulum. This cartoon
shows the process.
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The protein destined for secretion is red. The
blue protein must remain in the RER. The RER has
inserted a receptor protein on the membrane, it
sends to the Golgi complex in the transitional
vesicles (shown in green).  The ER protein
receptor captures all of the  protein that
carries the ER residency signal. Vesicles then
bud from the Golgi complex and move back to the
RER. The receptor can circulate and continue to
return the proteins needed by the ER.
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2.3 The functions of Golgi complex
2.3.1 Glycosylation in the Golgi complex
2.3.2 The processing? sorting and tran-sport in
the Golgi complex
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Summary
The Golgi complex consists of a stack of
flattened?vesicles and tubules.The Golgi
apparatus has two distinct facesa cis face and a
trans face. It is called the polarity organelle
.The Golgi complex receives newly synthesized
proteins and lipids from the ER and then
processing?sorting and distributes them to the
plasma membrane ?lysosomes and secretory vesicles
.Golgi is the site of O-linked glycosylation.
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Self-Quiz
1. Which cell component consists of a stack of
smooth membrane elements, through which newly
synthesized proteins travel by vesicles budding
off and fusing while they are being chemically
modified and targeted for export or other
destinations?A.cytoplasm B.cell
membrane C.Golgi body D.SER E.RER
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Self-Quiz
2.What is the function of the golgi
apparatus? A.It produces DNA molecules B.It
propels the cell C.It produces ribosomes D.It
secretes cell products
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Self-Quiz
3. What is the correct sequence of membrane
compartments through which a secretory protein
moves from synthesis to release from the cell?
A.SER ? Golgi ? RER ? cell membrane B.cell
membrane ? Golgi ? RER ? SER C.RER ? Golgi ?
cell membrane ? SER D.Golgi ? RER ? SER ? cell
membrane E.RER ? Golgi ? cell membrane
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Section ? The Lysosomes
3.1 The structure of the lysosomeDiscovered in
1950 by Rene . De .Duve, a Lysosome is a tiny
membrane-bound organelle found in the cytoplasm
of all eucaryotic cells containing various acid
hydrolytic enzymes that can digest every kind of
biological molecule.
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3.1 The structure of the lysosome
Lysosomes is common in animal cells but rare in
plant.Marker enzyme acid phosphatase.
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3.1 The structure of the lysosome

• Lysosome membrane 1.H-pumps internal proton
  is kept high H- concentration by H-ATPase
  2.Glycosylated proteins may protect the
  lysosome from self-digestion. 3.Transport
  proteins transporting digested materials.

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3.2 Biogenesis of Lysosomes
1. A phosphate attached to the mannose residue.
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3.2 Biogenesis of Lysosomes
2.This mannose-6 phosphate forms a sorting signal
that moves through the cisternae to the trans
region where it binds to a specific receptor.
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3.2 Biogenesis of Lysosomes
3.After it binds to the receptor, it begins to
bud and a coat made of clathrin forms around
the bud (to strengthen it).
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3.2 Biogenesis of Lysosomes
4.It moves away to fuse with a late endosome .
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3.2 Biogenesis of Lysosomes
5.The phosphate is removed and hydrolase is
dissociated from the receptor.
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3.2 Biogenesis of Lysosomes
6.The receptor is then recycled back to the Golgi
complex .
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Biogenesis of Lysosomes
Late endosome
Lysosomal hydrolase precursor
Receptor-dependent transport
Addition of phosphateMannose-6-phosphate(M-6-P)
ATP
ADPPi
From RER
Clathrin coat
H
???
PH5
Binding to M6P receptor
Removal of phosphate
Mature lysosomal hydrolase
Dissociation at acidic pH
Cis golgi network
Trans golgi network
M6P receptor in budding vesicle
RER
Golgi apparatus
Mature lysosomes
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3.3 The types of lysosomes

• Primary lysosome
• Secondary lysosomes
• Phagosome
• Autophagosome
• residual bodies
• (lipofuscin)

Primary Lys.
Second Lys
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3.3 The types of lysosomes
Primary lysosome are newly formed by budding from
the Golgi complex,and therefore have not yet
encountered substrate for digestion and with acid
Hydrolytic enzymes inactive.
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3.3 The types of lysosome
Secondary lysosomes result from the repeated
fusion of primary lysosomes with a variety of
membrane bounded substrates and active hydrolytic
enzymes within the lysosomes. The bounded
substrates may be food ?bacterium?or worn
organalles and so on.
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3.3 The types of lysosome
Phagosome is a kind of secondary lysosomes licked
up food or bacterium. Autophagosome is a kind of
secondary lysosomes licked up ageing organelles.
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3.3 The types of lysosome
The secondary lysosomes digest the contents of
phagocytic or autophagic vesicles to form
residual bodies that either undergo exocytosis or
are retained in the cell as lipofuscin granules.
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3.4 The functions of Lysosomes

• Lysosomes are involved in five major cell
  functions1.Heterophagy2.autophagy3.The
  extracellular digest4. Autocytolysis

D. The Functions of Lysosomes
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3.4 The functions of Lysosomes
1. Heterophagy Digestion of materical
of extracellular origin. Lysosomes pick up
foreign invaders such as bacteria, food and break
them into small pieces that can hopefully be used
again. If they pick up a really harmful invader,
they will eat it up and expel what is left of it
out of the cell so that the debris can be removed
from the body.
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3.4 The functions of Lysosomes
2.Autophagy Digestion of materical of
intracellular origin. Lysosomes also play a
key role in destroying old organelles within the
cell and thus allow them to be replaced with
fresher, more effective ones.This process is
known as autophagy and is accomplished in two
stages.
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3.4 The functions of Lysosomes
Firstly, a membrane is donated by the
endoplasmic reticulum. This membrane then
surrounds the old organelle. Secondly ,a
lysosome fuses with this membrane to form an
autophagic vacuole. The lysosome can safely enter
it's enzyme contents into this vacuole and
destroy the old organelle. The electron
micrograph shows a lysosome in the process of
destroying a membrane bound mitochondria.
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3.The extracellular digest Another function of
lysosome in the human occurs during fertilization
of the egg by the sperm. The head of the sperm
cell contains a package of lysosomal material
called the acrosome.
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The enzymes from this are released when
the sperm makes contact with an egg and they
effectively bore a hole through the cell membrane
of the egg allowing the sperm to enter.
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3.4 The functions of Lysosomes
4.Autocytolysis Lysosomes may also be important
in development.For instance , they are
responsible for the breakdown of a tadpoles tail
as the tadpole develops into a frog. In the
process ,the lysosome releases hydrolases to
cytoplasm to digest the cell of oneself.
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3.5 Lysosomes and Diseases
Autolysis A break or leak in the membrane of lys
releases digestive enzymes into the cell which
damages the surrounding tissues.For example,The
miners disease silicosis results from the uptake
of silica fibres from the dusty atmosphere of a
coal mine by macrophages and other phagocytic
cells in the lungs. These fibres then become
enclosed in the lysosomes of these cells but they
cannot be digested by the enzymes. Instead they
cause the lysosome to leak its contents in
quantities which cannot be neutralized resulting
in damage to the tissue of the lungs.
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5. Lysosomes and Diseases

• Autolysis The same process occurs in asbestos
  workers resulting in the disease asbestosis. Both
  conditions can be severely debilitating or even
  fatal. It is also thought that as we age
  lysosomes become leaky and can cause damage to
  our own tissues. Rheumatoid Arthritis is thought
  to occur partly due to damage caused to cartilage
  cells in the joints by enzymes leaked from
  lysosomes.

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5. Lysosomes and Diseases
Lysosomal storage diseases are due to the absence
of one or more lysosomal enzymes, and resulting
in accumulation of material in lysosomes as large
inclusions.1?Acid Maltase Deficiency (Lysosomal
Glycogen Storage Disease) which leads to the
accumulation of glycogen in muscle tissue.
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5. Lysosomes and Diseases
2?Tay-Sachs Disease is due to a deficiency in one
of the enzymes which breaks down certain types of
fat (lipid) called hexosaminidase A. As a result
of this deficiency, huge amounts of lipids are
deposited in neuronal (nerve) tissue and leads to
severe brain damage and nervous degeneration. The
disease is progressive and terminal resulting in
early death around 3 years of age.
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5. Lysosomes and Diseases
3?Gauchers Disease is due to the deficiency of
the lysosomal enzyme glucocerebrosidase. The
disease results in liver and spleen enlargement
and erosion of the long bones such as the femur.
If the disease manifests itself in infancy there
is also brain damage causing learning disability.
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Summary
Lysosome is A membrane-bound organelle in the
cytoplasm of most cells containing various acid
hydrolases. Lysosomes are involved in four major
cell functionsPhagocytosisAutophagy
Extracellular digestAutocytolysis. Primary lys
fuse with either phagocytic or autophagic
vesicles, forming residual bodies that either
undergo exocytosis or are retained in the cell as
lipofuscin granules.
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Choosed-question
1.The cell organelle that contains acid
hydrolases is the A.endoplasmic
reticulum B.lysosome C.golgi D.ribosome
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Choosed-question
2.Lysosomes are found in A.ribosomes B.animal
cells C.enzymes D.bacteria
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Fill-blank questions
1.Endomembrane System include ER, _______,
lysosome, secretory vesicles. 2.There are two
types of glycosylation N-linked linked to the
amide nitrogen of asparagine (Asn).These process
take place in _______O-linked linked to the
hydroxyl group serine or threonine via GalNac.
These process take place in RER and _________. 5.
Primary lysosome are newly formed by budding from
__________.
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The short-answer questions
1.what are the functions of the lysosome? 2.what
are the differences between the primary lysosome
and secondary lysosome?