Cytology Plasma Membrane

8/6/2019 Cytology Plasma Membrane

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Department of Natural Sciences

University of St. La Salle

Bacolod City



Plasma membrane is present in both eukaryoticand prokaryotic cells.

Despite their differing functions, all biological

membranes have a common general structure:

each is a very thin film of lipid and proteinmolecules, held together mainly by noncovalent

interactions.

Too thin to be seen with the light microscope, but

in EM, the membrane is 8.5-10 nm thick and shows

the trilaminar structure of the unit membrane. Specialized techniques, such as x-ray diffraction

and freeze-fracture electron microscopy, are

needed to reveal the details of its organization.

CELL MEMBRANE or PLASMALEMMA



Membrane

proteins are

solubilized with amild detergent.

The detergent

disrupts the lipid

bilayer and brings

the proteins into

solution as

protein-lipid-

detergent

complexes. Thephospholipids in

the membrane are

also solubilized by

the detergent.

STUDYING CELL

MEMBRANES



Freeze-fracture EM provides images of both the

hydrophobic interior of the cytosolic half of the bilayer (the

P face) and the hydrophobic interior of the external half of the bilayer (the E face). After the fracturing process, the

exposed fracture faces are shadowed with platinum and

carbon, the organic material is digested away, and the

resulting platinum replica is examined in EM.



Models of Plasma Membrane Structure

Danielli and Davson model ² 2 layers of protein on

either side of a bimolecular leaflet of mixed lipid.

Fluid-mosaic model- phospholipid bilayer with

proteins floating on a ´sea of phospholipids.µ





There are 3 major classes of

membrane lipid molecules

phospholipids, cholesterol,

and glycolipids. The most abundant are the

phospholipids.

Lipid molecules in cell

membranes are amphipathic

or amphiphilic i.e., they havea hydrophilic or polar end

and a hydrophobic or

nonpolar end.



Hydrophilic molecules dissolve readily in water because

they contain charged or uncharged polar groups that can

form either favorable electrostatic interactions or

hydrogen bonds with water molecules.

Hdrophobic molecules are insoluble in water because

almost all of their atoms are uncharged and nonpolar

and therefore cannot form energetically favorable

interactions with water molecules. If dispersed in water, they force the adjacent water

molecules to reorganize into icelike cages that surround

the hydrophobic molecule.

Because these cage structures are more ordered than

the surrounding water, their formation increases freeenergy.

This free energy cost is minimized if the hydrophobic

portions of amphipathic molecules cluster together so

that the smallest number of water molecules is affected.



How hydrophilic and hydrophobic molecules interact differently

with water. (A) Because acetone is polar, it can form favorable

electrostatic interactions with water molecules, which are also

polar. Thus, acetone readily dissolves in water. (B) Insoluble

molecules such as 2-methyl propane is entirely hydrophobic &

cannot form favorable interactions with water, forcing adjacent water molecules to reorganize into icelike cage structures, which

increases the free energy. The symbol - indicates a partial

negative charge, and + indicates a partial positive charge. Polar

atoms are shown in color and nonpolar groups are shown in gray.



Lipid composition can influence the activity of

particular membrane proteins. They can determine

the physical state of the membrane. Depending on their shape, lipid molecules can form

spherical micelles, with the tails inward; or they can

form bimolecular sheets, or bilayers, with the

hydrophobic tails sandwiched between the

hydrophilic head groups. Being cylindrical, phospholipid molecules

spontaneously form bilayers in aqueous

environments.

A lipid bilayer is responsible for the fluid

characteristic of the membrane. In this energetically most-favorable arrangement, the

hydrophilic heads face the water at each surface of

the bilayer, and the hydrophobic tails are shielded

from the water in the interior.



(A) Wedge-shaped lipid molecules form micelles,

whereas cylinder-shaped phospholipid molecules

form bilayers. (B) A lipid micelle and a lipid bilayer

seen in cross section.



PHOSPHOLIPIDS: have a polar head group and 2 hydrophobic

hydrocarbon tails. The tails are usually fatty acids, differing in length

from 14-24 C. Unsaturated FA have cis-double bands, producing a

kink in the tail. Differences in length and saturation influence thepacking of phospholipids , affecting the fluidity of the membrane.



Types of phospholipids: phosphoglycerides have a glycerol

backbone; sphingolipids contain sphingosine instead of glycerol.



The same forces that drive phospholipids to form

bilayers also provide a self-healing property.

A small tear in the bilayer creates a free edge withwater; because this is energetically unfavorable,

the lipids spontaneously rearrange to eliminate the

free edge. (In eukaryotic plasma membranes, larger

tears are repaired by the fusion of intracellularvesicles.)



The only way for a

bilayer to avoid

having free edges is

by closing in on

itself and form a

sealed

compartment.The closed structure

is stable because it

avoids the exposure

of the hydrophobic

hydrocarbon tails towater, which would

be energetically

unfavorable.



Cholesterol reduces membrane fluidity at moderate temperatures by

reducing phospholipid movement,

but at low temperatures it hinderssolidification by disrupting the

regular packing of phospholipids.

Phospholipids very rarely migrate from

the monolayer on one side to that onthe other ("flip-flop´). In contrast, lipid

molecules readily exchange placeswith their neighbors within a monolayer

giving rise to a rapid lateral diffusion.



The structure of cholesterol. Cholesterol is represented (A) by a

formula, (B) by a schematic drawing, and (C) as a space-filling model.

Eukaryotic plasma membranes contain up to one

molecule of for every

phospholipid molecule which enhance thepermeability-barrier properties of the lipid bilayer.



� They orient themselves in the

bilayer with their hydroxyl

groups close to the polar head

groups of the phospholipidmolecules.

� In this position, their rigid,

platelike steroid rings interact

with and partly immobilize

those regions of the hydro-

carbon chains closest to the polar head groups.� By decreasing the mobility of the first few CH2 groups of the

hydrocarbon chains of the phospholipid molecules,

cholesterol makes the lipid bilayer less deformable in this region

and thereby decreases the permeability of the bilayer to small

water-soluble molecules.� Although cholesterol tends to make lipid bilayers less fluid, at the

high concentrations found in most eukaryotic plasma

membranes, it also prevents the hydrocarbon chains from coming

together and crystallizing. In this way, it inhibits possible phase

transitions.



Spur cell anemia is caused

by too much cholesterol in

RBC membrane. It is

common in alcoholics with

cirrhosis of liver. Serum

cholesterol gets very high

and goes into the outer

leaflet of RBC membrane.

This expands the outerleaflet and produce spurs.

The membrane loses

flexibility and cells

become round and spur

covered. The cells get

trapped in spleen

capillaries and are

destroyed. Total RBC

count decreases resulting

to anemia.



Almost all glycolipids are derived from glycerol, as are most

phospholipids; in animal cells they are produced from serine, e.g.

phospholipid sphingomyelin. 3 uncharged sugars: Gal =

galactose; Glc = glucose, GalNAc = N- acetylgalactosamine

GLYCOLIPIDS: (A)

Galactocerebroside,

a neutral glycolipidbecause the sugar

that forms its head

group is uncharged.

(B) A ganglioside

always contains one

or more negativelycharged sialic acid

residues (also called

N-acetylneuraminic

acid, or NANA)

shown in (C).



� Glycolipids are confined to the exposed apical surface,

where they help protect the membrane against the

harsh conditions such as low pH and degradativeenzymes.

� Charged glycolipids, such as gangliosides, alter the

electrical field across the membrane and the

concentrations of ions especially Ca+2 at the

membrane surface.� They function in cell-recognition processes, in which

membrane-bound carbohydrate-binding proteins

(lectins) bind to the sugar groups on both glycolipids

and glycoproteins in the process of cell-cell adhesion

� Mutant mice that are deficient in all of their complex

gangliosides show no obvious abnormalities, although

the males cannot transport testosterone normally in

the testes and are consequently sterile.



Tay-Sach·s disease is a fatal

inherited condition that occurs

when harmful quantitiesof gangliosides accumulate in

the nerve cells of the brain

because of deficiency in -N-

acetylhexosaminidase A, an

enzyme which catalyzes thebiodegradation of

gangliosides, eventually

leading to the premature death

of those cells.

One of the symptoms of Tay-

Sach·s disease is the reddish

dot in the retina of the eye

which was first noticed byWarren Tay in the year 1881.

The disease is caused by the genetic mutation of HEXA

gene on chromosome 15 which provides instructions for

making the GM2 activator protein, a cofactor required for normal function of the -N-acetylhexosaminidase A.



Different mixtures of lipids are found in the membranes of

cells of different types, as well as in the various

membranes of a single eucaryotic cell. Functions include:

1. Certain membrane proteins accumulate in lipid rafts

which are specialized areas in membranes where some

lipids (primarily sphingolipids and cholesterol) and

proteins (green) are concentrated. Rafts concentrate

them for transport in small vesicles or to enable the

proteins to function together, such as when they convert

extracellular signals into intracellular ones.



Caveolae- special type of lipid rafts specialized for

endocytosis and compartmentalization of signal

transduction at the cell surface. One pathway is

through formation of vesicles that bud off from the PM

and travel to endosomes via microtubules. Another

pathway is through cyclic budding at the cell surface

to deliver small molecules and ions. The 3rd type form

tubules from the cell surface to the center.



2. Some membrane-bound enzymes require specific

lipid head groups in order to function.

3. The head groups

of some lipids

form docking sites for specific

cytosolic proteins.



4.Some extracellular signals that act through membrane

receptor proteins activate phospholipases that cleave

selected phospholipid molecules in the plasmamembrane, thereby generating fragments that act as

intracellular signaling molecules.

� For example, when animal cells undergo programmed

cell death, or apoptosis, phosphorylated inositol

phospholipids act as binding sites that recruit specificproteins from the cytosol to the membrane.

� Phosphatidylserine, which is normally confined to the

cytosolic monolayer of the plasma membrane lipid

bilayer, rapidly translocates to the extracellular

monolayer� The phosphatidylserine exposed on the cell surface

serves as a signal to induce neighboring cells, such as

macrophages, to phagocytose and digest the dead cell.



Some functions of membrane phospholipids in cell signaling. (A)

Extracellular signals can activate PI 3-kinase, which phosphorylatesinositol phospholipids in the plasma membrane. Signaling molecules

then bind to these phosphorylated lipids & are thus recruited to the

membrane, where they can interact and help relay the signal into the

cell. (B)Other extracellular signals activate phospholipases that cleave

phospholipids. The lipid fragments then act as signaling molecules to

relay the signal into the cell.



Freeze-fracturing splits the bilayer, showing proteins to be entirely within

the lipid bilayer (integral/ transmembrane/ intrinsic), or projecting from

the outer surface to varying degrees (peripheral/ extrinsic).

PROTEINS





1.Transmembrane, integral or intrinsic proteins extend

through the lipid bilayer, with part of their mass on either

side. are amphiphatic molecules possessing one or more

hydrophobic regions that exhibit an affinity for the

hydrophobic interior of the lipid bilayer; hence these

molecules are difficult to remove from membranes

They are notoriously difficult to crystallize; few have beenstudied in their entirety by x-ray crystallography.

DNA cloning and sequencing techniques, have revealed

their amino acid sequences, and it is often possible to

predict from an analysis of the protein's sequence which

parts of the polypeptide chain extend across the lipidbilayer.

Proteins are capable of lateral movement within the

bilayer; apical to basolateral movement is hindered by cell

junctions, or their attachment to cytoskeletal elements.



2.Peripheral or extrinsic proteins function on

only one side of the lipid bilayer; are often

associated exclusively with either the lipidmonolayer or a protein domain on that side.

Some of these are anchored to the cytosolic

surface by an amphipathic helix that

partitions into the cytosolic monolayer of the

lipid bilayer through the hydrophobic face of

the helix.

� For example, some of the proteins involved in

intracellular signaling are bound to the

cytosolic half of the plasma membrane by

one or more covalently attached lipid groups.



Peripheral proteins of RBC plasma membrane are

spectrin and ankyrin, and a protein called band.

These proteins are bound to the inner surface of PMand form as a meshwork that support the PM and

help maintain the shape of RBC. Gene mutations for

these proteins result to RBC shape anomalies.



� How a membrane protein associates with the lipid

bilayer reflects the function of the protein.

� Only transmembrane proteins (e.g. cell-surfacereceptors can function on both sides of the bilayer

or transport molecules across it.



Various ways in which membrane proteins associate with the lipid bilayer.

Some of these "single-pass" and "multipass" transmembrane proteins havea covalently attached fatty acid chain inserted in the cytosolic lipid

monolayer (1). Most are thought to extend across the bilayer as (1) a single

a helix, (2) as multiple a helices, (3) as a rolled-up sheet or a barrel. Others(5) are attached to the bilayer by a covalently attached lipid chain either a

fatty acid chain or a prenyl group in the cytosolic monolayer, (6) via an

oligosaccharide linker, or to phosphatidylinositol in the noncytosolicmonolayer. (7, 8) Many proteins are attached to the membrane only by

noncovalent interactions with other membrane proteins.



The cell coat is made up of the oligosaccharide side chains

of glycolipids and integral membrane glycoproteins and the

polysaccharide chains on integral membrane

proteoglycans. In addition, adsorbed glycoproteins and

adsorbed proteoglycans contribute to the glycocalyx in

many cells. Note that all of the carbohydrate is on the

noncytosolic surface of the membrane.



It plays a role in the selective uptake of

substances, recognition of self, a cellular ID

system.

It confers an electrostatic charge on the cell

surface, important in cell-to-cell adhesion.

� Disturbances in the cell coat and surface charge

maybe responsible for the failure of cancer cellsto adhere to one another.

� Histogenesis is achieved by modulating cell-

adhesion molecules (CAMs) which are integral

protein-carbohydrate (e.g., sialic acid)complexes.

� Antigens are located within the cell coat.



Human ABO blood-

group antigens.

These antigens are

oligosaccharidechains covalently

attached to

glycolipids or glyco-

proteins in the

plasma membrane.

The terminal oligo-saccharide sugars

distinguish the 3

antigens. The

presence or absence

of the glycosyl-

transferases that addgalactose (Gal) or N-

acetylgalactosamine

(GalNAc) toO antigen

determine a person·s

blood type.





TRANSPORT MECHANISMS



1. Passive transport down an electrochemical gradient

occurs spontaneously, either by simple diffusion

through the lipid bilayer or by facilitated diffusion

through channels and passive carriers.



A model of how a conformational change in a carrier protein

could mediate the PASSIVE TRANSPORT of a solute. The carrier

protein can exist in 2 conformational states: in state A, the bindingsites for solute are exposed on the outside of the lipid bilayer; in

state B, the same sites are exposed on the other side of the

bilayer. The transition between the two states can occur randomly,

completely reversible and does not depend on whether the solute

binding site is occupied.



2. Active transport requires an input of metabolic energy and

is always mediated by carriers that harvest metabolic

energy to pump the solute against its electrochemical

gradient.

� An electrochemical gradient combines the membrane

potential and the concentration gradient, which can work

additively to increase the driving force on an ion across

the membrane or can work against each other.





The sodium-potassium pump: a specific case of active transport



http://highered.mcgraw-

hill.com/olc/dl/120068/bio03.swf



transports the solute in (or out) and the co-

transported solute the opposite direction.


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dl/120068/bi

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Co-transport

� Glucose is

pumped into the

cell through the

apical domain of

the membrane bya Na+ -poweredglucose symport.

� Glucose passes

out of the cell by

passive transport

mediated by adifferent glucosecarrier protein in

the basal and

lateral membrane

domains.

Transcellular transport of glucose across an intestinal epithelial cell



4.Signal-Reception- messenger molecules are

membrane-bound or may pass through

membrane channels to mediatecommunication among cells.

� G-proteins- activated by the binding of a 1st

messenger molecule to a membrane receptor.

� The GTP-G protein exchanges act onmembrane-bound enzyme effectors, which in

turn activate a 2nd messenger.

� This triggers a cascade of molecular

reactions that lead to changes in cell behavior(e.g., LH activates G5 adenylyl cyclase

increased synthesis of estrogen and

progesterone).





The gating of ion channels. Different kinds of stimuli open ion

channels. Mechanically gated channels often have cytoplasmic

extensions that link the channel to the cytoskeleton. Ion Channel Diseases:

Myasthenia Gravis ² Muscle weakness due to autoantibodies

against the acetylcholine receptor

Cystic Fibrosis ² Defect in the Cl- channel CFTR ² leads to

excessive phlegm and static infections



when transport is out of the cell.



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.mcgraw-














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Cytology Plasma Membrane