2. Cell Membrane

8/8/2019 2. Cell Membrane

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Faculty of Dentistry

Histology & Cell Biology

Dr. Al-Moutassem Billah Khair

Cell M embrane



Cell MembraneCell , o r p lasma, membrane is a semi-permeable thin

allow s certain substances to pass through from the outside w hilekeeping other constituents from escaping from the cell.

*Cell mem branes are found to surround the entire of the cell as well asthe nucleus or some c to lasmic or anelles.

*Cell membranes range from 7.5-10 nm in thickness and consequentlyare visible only in the electron microscope.

.*Cell membranes divide the eukaryotic cells in discrete compartmentsthat regulate intracellular and intercellular exchange.



Cell MembraneCell membrane functions:.

organelles.II . Cell membrane keeps the nutrients in the cell .

I I I . Cel l membrane provides cel l-cel l and cell -extracellular

matrix adhesion and recognition ..

substances.

V. Cell membrane accumulate the materials necessary tosustain life.

VI. Cell membrane contains different receptors.

. e mem rane orms e s e o a ac men or

cytoskeletal elements.



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Cell M embraneMembrane structure:

At the end of 19 th century i t was predicated that the cel l

mem brane was composed of lipid monolayer, but in 1920s it wasfound that the amount of extracted lipids was enough to cover.

bounded by double layer of lipids.

Later studies suggested that the cell membrane is symmetricalst ructure consist ing of a bi layer of phospholip ids loca tedbetw een tw o layers of protein. I n the 1950s, electronmicro r a hs showed that all membranes have a t rilaminarstructure after fixation in osmium tetrox ide.

The current concepts of membrane structure are based onnger c o son s u es n s. s mo e s now n as u

mosaic model.






Fluid mosaic model:

In this model, cell mem branes consist of phospholipid bilayer, avariety of proteins, carbohydrates and cholesterol molecules thatare arrange in a speci ic pattern.

Phospholipids consist of a polar, hydrophilic (water-loving) head anda non-polar, hydrophobic (w ater-hating) tail.

*The polar head is derived from glycerol at tached to nitrogenouscompound (choline, serine, etc.) by a phosphate bridge.

-& unsaturated ) each covalent ly linked to the glycerol of the polar head.

Within the membrane, l ip ids are most s table when organized into aou e ayer w t t e r y rop o c ta s recte tow ar t e center o

the mem brane and their hydrophilic heads directed outw ards.



Cell Membrane






Fluid mosaic model:Pro te in molecu les make up a lmos t ha l f o f the to ta l mass o f the

membrane and can be divided into three groups:

*In te ral roteins int rins ic : roteins that a re incor ora ted wi thinthe phospholipid bilayer.

*Peripheral proteins (extrinsic): proteins that are loosely attached.

*Transmembrane proteins: integral proteins that extend across thephospholipid bilayer to be ex posed to each surface, some functioning as

the membrane.

Many prot ein s are freely mob il e “ float ” w i thin t he plane of t hephospholipid bilayer [ f luid mosaic model ]. Other integral proteins m ay

be fixed by attachment to cytoskeletal elements.






Fluid mosaic model:

Carbohydrates are found on the external surface of membrane. Theyare either link ed to the lipids “ glycolipids” or to the prot eins

“ ” , .appears t o be involved in cell recognit ion , in t he format ion of

intercellular adhesions and in the adsorption of molecules to the cell,

for the plasma mem brane.

Cholesterol molecules are intercalated between the phospholipids inmembranes. Cholesterol prevents too close packing of the fat ty acidtails. It increases the stability of the bilayer and prevents the loss of mem brane liquidity at low tem peratures.



Cell Membrane



M embrane Transport M echanismsCell membranes are select ively permeable a llow some

substances to pass, but not others & the permeabil i ty can be.

1. Diffusion:

higher to lower concentration until the solute reaches equilibrium.

*Exchange of oxygen & nutr ients mater ia l between cel l & blood stream.Flux: The amount of materials crossing a surface in the unit of time.

*Concentration of substance ⇒ Flux ⇒

*Temperature of statement ⇒ Flux

*Size of molecules ⇒ Flux

*Surface area of membrane ⇒ Flux

*M ovement faster in gas phase than in w ater.



M embrane Transport M echanisms

Osmosis:I t is a special statement of d iffusion in which water

molecules are diffused through a permeable membrane fromregion of high w ater ratio to a region of low w ater ratio.

present in a solution. So, the water molecules move from the regioncontaining low concentration of solute particles to the region containing

.

In relat ionsh ip to cell concent ration , solut ions can be of three types:

*Iso tonic: a solut ion has a concent ra tion equal to tha t of the cel l

no c ange n ce s ze or s ape .*Hypertonic: a solution has a concentration more than that of the cell(cell w ill shrink)

*Hypotonic: a solution has a concentrat ion less than that of the cell

(cell w ill sw ell and then burst ).




2. M ediated-Transport System :The passage of molecules " too large & too polar“ through the

plasma membrane are mediated by integral proteins-Transporter orCarrier proteins.

Facilitated diffusion:

To move solute molecules from higher to lower concentrationusing transporter proteins. It is not coupled to energy derivedfrom metabolism.

To move solute molecules from lower to higher concentrationusing transporter proteins. I t needs energy derived frommetabolism.








3. Bulk transport:Cell mechanisms for moving large substances across their

membranes. These processes use mem brane-bound vesicles of various sizes to move materials from one compartment toanother.

Endocytosis:It is the process by w hich cell engu lfs large substances.

*P inocytosis: Cells ingest large quan titi es of fluid .

*

Endocytotic vesicles fuse with lysosomes, which contain enzymes thatkill and digest the engulfed food. Some endocytotic vesicles don't fuse

. ,degrading them .










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2. Cell Membrane

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