Cell Compartmentalization

7/28/2019 Cell Compartmentalization

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CELL

COMPARTMENTALIZATION


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COMPARTMENTALISATION IN EUKARYOTIC

CELLS

Cellular compartments in cell biology comprise allof the closed parts within the cytosol of a

eukaryotic cell, usually surrounded by a singleor double lipid layer membrane.

Major compartments in eukaryotic cellsare cytosol (gray), endoplasmic reticulum, Golgiapparatus, nucleus, mitochondrion, endosome,lysosome, and peroxisome. These are isolated fromrest of the cell by atleast one layer of membrane.


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MAJOR CELLULAR COMPARTMENTS

The nuclear compartment comprisingthe nucleus

The intercisternal space Organelles

The cytosol


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LOCATION OF COMPARTMENTS

Characteristic distributions depend on interactions of theorganelles with the cytoskeleton and with one another.

Golgi apparatus is located close to the nucleus, whereas thenetwork of ER tubules extends from the nucleus throughoutthe entire cytosol.

For protein synthesis, all the organs used for it are relativelynear one another, the nucleolus makes the ribosomes whichsynthesize the proteins, the rough endoplasmic

reticulum (rough ER) is near the nucleus as well. The Golgibody is also near the rough ER for packaging andredistributing.


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FUNCTIONS

Isolation of important cell functions.

Intracellular pH, different enzyme

systems, and other differences areisolated. This enables the cell to carry outdifferent metabolic activities at the same

time. Provides cell with functionally

specialized aqueous spaces.


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Continued.

Increases surface area.

Compartmentalization allows

eukaryotic cells to perform otherwiseincompatible chemical reactionssimultaneously.


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TOPOLOGICAL RELATIONSHIP

BETWEEN ORGANELLES


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SPECIALISATION OF MEMBRANE

FUNCTIONS

Development of thylakoid vesicles in proplastids.

In the process of differentiating into chloroplasts,specialized membrane patches form and pinch offfrom the inner membrane of the proplastid.

The vesicles that pinch off form a newspecialized compartment, the thylakoid, thatharbors all of the chloroplast's photosyntheticmachinery.


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EVOLUTIONARY ORIGINS


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DNA molecule is attached to an invagination of theplasma membrane.

Such an invagination could have rearranged to form

an envelope around the DNA.

This envelope is presumed to have eventuallypinched off completely from the plasma membrane,producing a nuclear compartment surrounded by adouble membrane.

The nuclear compartment is topologicallyequivalent to the cytosol


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MEMBRANE TRAFFICKING

Flow of membrane material betweenendomembrane compartments and theplasmalemma

Trafficking mainly by 3 mechanism- SIMPLE DIFFUSION- FACILITATED DIFFUSION

- ACTIVE TRANSPORT


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SIMPLE DIFFUSION

Unassisted movement down the gradient

Limited to small or nonpolar molecules.

Ex:02 ,CO2 & lipids

Exception: water molecules


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FACILITATED DIFFUSION

Protein mediated movement down thegradient

Transport of large, polar molecules mediatedby carrier proteins

Ex: Na+ , K , Ca+2 , Cl- etc.


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ACTIVE TRANSPORT

Protein-mediated movement Up the gradient

May be powered by ATP hydrolysis, light

energy, electrochemical potential of an iongradient

Ex: ATP powered Na+/K+ pump.


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VESICULAR TRANSPORT

RER to cis Golgi

Modified in Golgi (glycosylation,

phosphorylation)

Sorted at trans Golgi network into

Lysosomal (endocytosis)

Regulated (exocytosis)

constitutive (exocytosis)


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Vesicle formation and

transport Capturing cargo molecules

Vesicle coat

- clathrin- COPI- COPII

Vesicle docking

-Surface markers called SNAREs , attach withtarget molecule SNAREs and fusion of bothmembrane occurs.


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EXOCYTOSIS

Cell releases intracellular molecules

Molecules transportation by vesicles

Vesicles fuses with plasma membrane

5 steps involved


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EXOCYTOSIS STEPS

Vesicle trafficking

Vesicle tethering

Vesicle docking

Vesicle priming

Vesicle fusion


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Final secretion by two process:

- Constitutive Secretion- continuous secretion

- Regulated Secretion

- secreted in response to a specific signal


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ENDOCYTOSIS

Endocytosis is a process bywhich cells absorb molecules (such as proteins) byengulfing them.

It is used by all cells of the body because mostsubstances important to them arelarge polar molecules that cannot pass throughthe hydrophobic plasma or cell membrane


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Pathways for endocytic

processes

Phagocytosis

Pinocytosis

Receptor-mediated endocytosis


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Phagocytosis

Phagocytosis is the process of taking in particles such asbacteria, parasites, dead host cells, and cellular andforeign debris by a cell.

Phagocytosis occurs after the foreign body or a bacterialcell comes near a plasma membrane of the cell

For example, it has bound to molecules called"receptors" that are on the surface of the phagocyte.


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Cells that uses

PhagocytosisSeveral types of cells in the immune system engulf microorganismsvia phagocytosis.

Neutrophils. Neutrophils are abundant in the blood, quicklyenter tissues, and phagocytize pathogens in acute inflammation.

Macrophages. Macrophages are closely related to monocytes inthe blood. These longer-lived cells predominate in chronicinflammation. They also release some important inflammatoryparacrines. (See below.)

Dendritic Cells. Phagocytosis in these cells is important for theelaboration of a specific immune response rather than for directly

destroying the pathogens. B Lymphocytes. A small amount of phagocytosis in these cells is

often necessary in order for them to develop into cells thatrelease antibodies


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Phagocytosis in steps

Phagocytosis begins with the neutrophil or macrophageflowing around the pathogen and engulfing it so that itwinds up enclosed in a phagosome (phagocytic vesicle).

The next step is the fusion oflysosomes with thephagosome. The result is called a phagolysosome.

Killing of microbes within a phagolysosome.

oxygen radicles in membrane of phagolysosome

antimicrobial proteins in lysosomes

Hydrogen ion transport

The final step is release of molecules and unwantedmolecules that comes insides the cell during ingestion


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Pinocytosis

It is the process of engulfing the liquid food.

It requires energy in the form of ATP

It is primarily used for absorption ofExtracellular Fluids.

In contrast to phagocytosis, generates verysmall vesicles.

It is non- specific


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Pinocytosis

In this, smallparticles arebrought into thecell, forming aninvagination, and

then suspendedwithinsmall vesicles (pinocytotic vesicles) thatsubsequently fuse

with lysosomes tohydrolyze, or tobreak down, theparticles


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Receptor mediated

endocytosis Also called clathrin-dependent endocytosis, It is a process by which cells internalize molecules

(endocytosis) by the inward budding of plasmamembrane vesicles containing proteins with receptor sites

specific to the molecules being internalized. Mechanism-After the binding of a ligand to plasma

membrane-spanning receptors, a signal is sent through themembrane, leading to membrane coating, and formation of amembrane invagination. The receptor and its ligand are

then opsonized in clathrin-coated vesicles. Once opsonized,the clathrin-coated vesicle uncoats (a pre-requisite for thevesicle to fuse with other membranes) and individual vesiclesfuse to form the early endosome


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Function of RME

Used for the specific uptake of certainsubstances required by the cell (examplesinclude LDL(low density lipo-proteins viathe LDL receptor or iron via transferrin).

in the downregulation of transmembranesignal transduction. The activated receptor

becomes internalised and is transported tolate endosomes and lysosomes fordegradation.


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Transcytosis

Transcytosis occurs as membrane-bound carriers

selectively transport materials between one part

of the cell and another in order to maintain

unique environments on either side of the cell.

Epithelial cells use transcytosis for immune

defense, nutrient absorption, and plasma

membrane biogenesis There are two types of transcytosis differing in

mechanisms of vesicle formation and majorproteins


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Clathrin-Mediated Transcytosis

Clathrin, a protein located onboth apical and basal surfaces ofthe epithelial cells, lines thesevesicles. Clathrin-mediatedtranscytosis is a way for thesecells to sort through the cargoof molecules entering the cell asone of the destinations of thesevesicles is the Golgi. On thesurface of the cell membrane, apit forms from specific cell

receptors that are coated byclathrin. The protein clathrinspurpose is to stabilize theforming vesicle after thereceptors have bound and beginto invaginate. Clathrin achievesthis by forming a rigid matrix ofan assembling of clathrinproteins, which can laterdisassemble after the vesicle has

disassociated from the cellmembrane. Vesicles attach to theendoplasmic reticulum beforebeing "sorted" to either theapical or basal side of the cell.


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Caveolae-Mediated Transcytosis

Endothelial cells, specializedepithelium that line bloodvessels, utilize caveolae-mediated transcytosis.Caveolae are pits in theapical and basal membranesof all endothelial cells,named for their small caveshape. The major structural

component of caveolae,shown in Figure 1, iscaveolin. These vesiclestransport cargo, usuallyfluid, from the apical tobasal or basal to apicalsurfaces of the cells. Thecaveolae can merge tocreate arrangements shown

above (Figure 3), including atunnel or channel, in order tomove cargo to other side ofcell

Cell Compartmentalization

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