intracellular transport
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Intracellular Transport
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Three basic modes of transport 1. Gated transport 2. Transmembrane transport
3. Vesicular transport
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Alberts
Vesicular transport
A Simplified Roadmap of Intracellular Transport
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Overview of major protein sorting pathways in eukaryotic cells
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Protein sorting
mitochondria
rough
ER
peroxisomes
cytosol
nucleus
plasma
membrane Golgi
lysosomes
nuclear
envelope
smooth
ER
secreted
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Mechanism 1: Gated Transport,
proteins enter the nucleus via nuclear pores
The nuclear envelope is a double membrane
Continuous with the ER - both compartments share the same lumen
Perforated by nuclear pores
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1. Proteins bind to nuclear transport receptors
2. Complex is guided to the pore by filaments
3. Pore opens, receptor + protein are transported in (uses GTP)
4. Receptor is shuttled back into the cytoplasm
The Mechanism of Nuclear Transport
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Alberts
Vesicular transport
A Simplified Roadmap of Intracellular Transport
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Mechanism 2: Transmembrane Transport,
protein translocation from cytoplasm to
organelle
Proteins moving from the cytosol into the ER, mitochondria, chloroplasts, or peroxisomes
Protein movement is mediated by specialized proteins termed protein translocators
Unlike passage through nuclear pores, translocation requires unfolding or co-translational transport
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Proteins are unfolded during translocation
into mitochondria
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Alberts
Vesicular transport
A Simplified Roadmap of Intracellular Transport
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Mechanism 3: Vesicular Transport
Vesicular transport delivers components between compartments in the biosynthetic-secretory and endocytic pathways.
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Vesicular transport delivers components between compartments in the biosynthetic-secretory and endocytic pathways.
Vesicular transport
Two Key Steps: 1. Sorting during vesicle formation 2. Targeting during vesicle fusion
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14
endocyto
sis
exocyto
sis
(secre
tion)
Retr
ogra
de t
raffic
Plasma membrane
lysosome
sorting endosome
recycling endosome
late endosome
trans-Golgi
medial-Golgi
cis-Golgi
endoplasmic reticulum
EGTC
secretory vesicles
TGN
Nucleus
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Cellular Membranes Endomembrane Structure % total membrane in
Hepatocyte
% total in Pancreatic
Exocrine Cell
Endoplasmic Reticulum
(smooth)
16
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What is the endomembrane system?
System of membrane-bound organelles in cells that work cooperatively together to create secretory proteins, membrane-bound proteins, or plasma membrane proteins
The endomembrane system regulates protein traffic and performs metabolic functions in the cell
The set of membranes that form a single functional and developmental unit, either being connected directly, or exchanging material through vesicle transport.
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Also involved in assembly and transportation of lipids
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Properties of endomembrane system
Present only in eukaryotic cells
Transport system for moving molecules through interior of the cell
Made of lipid bi-layer with proteins attached to either side or transversing them
Divides cell into organelles
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The endomembrane system plays a key role in the synthesis (and hydrolysis) of macromolecules in the
cell.
The various components
modify
macromolecules
for their various
functions.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 7.16
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Endomembrane System
secretory vesicles
Endoplasmic reticulum(ER)
Golgi complex
Lysosome
The endomembrane system consists of:
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The Endomembrane System
1. The endoplasmic reticulum manufactures membranes and performs many other
biosynthetic functions
2. The Golgi apparatus finishes, sorts, and ships cell products
3. Lysosomes are digestive compartments
4. Vesicles encloses and transport substances synthesized in the cell.
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Vacuoles are membrane-bound sacs with varied functions.
Food vacuoles, from phagocytosis, fuse with lysosomes.
Contractile vacuoles, found in freshwater protists, pump excess water out of the cell.
Central vacuoles are found in many mature plant cells.
Vacuoles have diverse functions in cell
maintenance
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Vacuole filling with water
Vacuole contracting
(a) Contractile vacuole in Paramecium
(b) Central vacuole in a plant cell
Central vacuole
Co
lori
zed
TEM
LM
LM
Figure 4.17 Laura Coronado Bio 10 Chapter 4
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Endoplasmic Reticulum ENDOPLASMIC RETICULUM (ER):
An extensive
tubovesicular network
where proteins and
lipids are made.
Rough ER: studded with
ribosomes, site of protein
biosynthesis
Smooth ER: site of lipid
biosynthesis
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Rough ER Smooth ER
Endoplasmic Reticulum
There are two distinct regions of ER Smooth ER, which lacks ribosomes Rough ER, which contains ribosomes
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Functions of ER
The smooth ER
Synthesizes lipids
Metabolizes carbohydrates
Stores calcium (In muscle cells, these trigger contractions)
Detoxifies poison
The rough ER
Has bound ribosomes
Produces proteins and membranes, which are distributed by transport vesicles
Smooth Rough
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The synthesis of phosphatidylcholine in the ER membrane
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Rough ER Functions
Protein and Membrane synthesis
Ribosomes covering Rough ER secrete proteins
Folded into lumen Later transported by vesicles
Ex. Insulin
Membranes made for itself are later transported to other endomembrane systems
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Synthesis of secretory proteins - review
1. N-terminal signal sequence is synthesized
2. Signal bound by SRP, complex docks with SRP receptor on ER membrane
3. Signal sequence binds to translocon, internal channel opens, inserted into translocon
4. Polypeptide elongates, signal sequence cleaved
5. ER chaperones prevent faulty folding, carbohydrates added to specific residues
6. Ribosomes released, recycle
7. C-terminus of protein drawn into ER lumen, translocon gate shuts, protein assumes final conformation
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Synthesis of secretory proteins
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Synthesis of integral membrane protein
1. internal signal sequence bound by SRP
2. SRP-protein-ribosome
complex docks with SRP
receptor, C-terminal portion
of protein cotranslationally
inserted into lumen of ER
3. Mature protein transverses ER bilayer forming integral membrane protein
NOTE: Orientation of protein within membrane dependent upon cluster of charged residues adjacent to internal signal sequence
4. Polypeptide elongates, carbohydrates added to specific residues
5. Ribosomes released, recycle, integral membrane protein produced that forms transmembrane domain
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Synthesis of membrane proteins
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PROTEIN GLYCOSYLATION IN THE ROUGH ER: During
translation, a signal sequence on membrane and secretory
proteins directs the nascent protein into the ER lumen. After the
protein has entered the ER, the glycosylation process begins.
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A PRE-FORMED
PRECURSOR
OLIGOSACCHARIDE
IS TRANSFERRED
EN BLOC
TO PROTEINS
IN THE ER
Alberts
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PROTEIN GLYCOSYLATION IN THE ROUGH ER
Alberts
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SOME PERIPHERAL MEMBRANE PROTEINS
AQUIRE A COVALENTLY ATTACHED
GLYCOPHOSPHATIDYLINOSITOL (GPI)
ANCHOR IN THE ER
Alberts
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Transport from the ER through the Golgi apparatus
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Golgi Apparatus
Made of cisternae Cis and Trans faces
Cis serves as bridge w/ER Trans makes vesicles for transport to other cell regions
Modifies proteins Cisternae between cis and trans faces
Works in partnership with the ER Receives, refines, stores, and distributes chemical products of the cell
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12/28/2014 41
Function
1. First modification of lipids and proteins (Modifies the N-linked oligosaccharides and adds O-linked oligosaccharides)
2. Storage and packaging of materials that will be exported from the cell.
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OLIGOSACCHARIDE CHAINS ARE PROCESSED
IN THE GOLGI APPARATUS
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Production of complex oligosaccharides
Modification of the N-linked oligosaccharides is done by enzymes in the lumen of various Golgi compartments.
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Two possible models
explaining the organization
of the Golgi apparatus and
the transport of proteins from
one cisterna to the next
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Transport from the trans Golgi nextwork to Lysosomes
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The Lysosomes
The structure of the lysosome:
Discovered in 1950 by Rene . De .Duve, a Lysosome is a tiny
membrane-bound organelle found in the cytoplasm of all
eukaryotic cells containing various acid hydrolytic enzymes
that can digest every kind of biological molecule.
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The structure of the lysosome
Lysosomes is common in
animal cells but rare in plant.
Marker enzyme: acid
phosphatase.
Lysosomes are highly heterogeneous Shape and size But all have acid hydrolases The stomach of a cell
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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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Biogenesis of Lysosomes
1. A phosphate attached to the mannose residue.
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.
3.After it binds to the receptor, it begins to bud and a coat made of clathrin
forms around the bud (to strengthen it).
4.It moves away to fuse with a late endosome .
5.The phosphate is removed and hydrolase is dissociated from the receptor.
6.The receptor is then recycled back to the Golgi complex .
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Biogenesis of Lysosomes
RER Cis golgi
network Trans golgi
network
Golgi apparatus
Lysosomal hydrolase precursor
Addition of phosphate
Mannose-6-phosphate(M-6-P)
Mature
lysosomal
hydrolase
ATP ADP+Pi
H+
PH=5
Binding to
M6P receptor
From
RER
Mature lysosomes
Dissociation at
acidic pH
Removal of
phosphate
Late endosome
M6P receptor in
budding vesicle
Receptor-dependent
transport
Clathrin coat
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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.
Primary Lys.
Second Lys
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 bacteriumor worn organalles and so on.
Phagosome is a kind of secondary
lysosomes licked up food or bacterium.
Autophagosome is a kind of secondary
lysosomes licked up ageing organelles.
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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D. The Functions of Lysosomes
The functions of Lysosomes
Lysosomes are involved in four major cell functions:
1.Heterophagy;
2.Autophagy;
3.The extracellular digest;
4. Autocytolysis;
All major classes of macromolecules are degraded in lysosomes
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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.
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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The functions of Lysosomes
Autophagy 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.
The functions of Lysosomes
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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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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Transport into the cell from the plasma membrane endocytosis
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Endocytosis: process of taking in liquids or larger molecules into a cell by engulfing in a vesicle;
requires energy
Endocytosis
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Phagocytosis by a macrophage Phagocytosis by a neutrophil
Pinocytosis: cell drinking; Phagocytosis: cell eating
pseudopods
One macrophage and two red blood cells
Phagocytosis: large particle, >250nm
Pinocytosis: fluid, liquid, 100 nm
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ENDOCYTOSIS IS IMPORTANT FOR
CELLS TO:
1. Import selected extracelluar molecules (i.e.
receptor-mediated endocytosis).
2. Regulate levels of membrane proteins on the
cell surface (i.e. receptor down-regulation).
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Transport from the trans Golgi network to the cell exterior: exocytosis
Exocytosis: fusion of vesicles with the plasma membrane
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Exocytosis
Exocytosis: process of releasing substances out of a cell by fusion of a vesicle with the membrane
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Exocytosis of
secretory vesicles
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Two mechanisms of secretion
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Three pathways of protein sorting in the trans Golgi network
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