1.01 Biochem Trans - Cell and Cell Membrane

7/23/2019 1.01 Biochem Trans - Cell and Cell Membrane

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LEARNING OBJECTIVES

1. Describe the structure of the cell membrane.2. Discuss the composition of the cell membrane.3. Discuss its functions.4. Describe the general features of the fluid mosaic

model of membrane structure.5. Discuss the different membrane processes and

the factors affecting them.6. Differentiate micelle from liposome.7. Describe the process of membrane assembly.

CELL MEMBRANE

Highly fluid, dynamic (exhibits rapid turnover andlateral diffusion)

Asymmetric, sheet-like structures with inner andouter surfaces

Viscous, plastic structures! Ex. Red blood cells that have to pass

through sinusoid should be pliable.

Thermodynamically stable and metabolically active

Noncovalent assemblies composed of lipids,proteins and carbohydrates.

ASYMMETRY OF CELL MEMBRANE

INSIDE-OUTSIDE SYMMETRY

Irregular distribution of proteins

External location of carbohydrates

Specific enzymes exhibit specificity of location

Phospholipids (choline containing are externalwhile amino acid containing are in the inner leaflet)

REGIONAL ASSYMETRIES

Villous borders (i.e. villi found in the smallintestin, microvilli found in the ears and

fallopian tubes) Gap junctions

Tight junctions

FUNCTIONAL PROPERTIES

1. Cell Individuality2. Selective permeability (channels, transporters and

pumps)3. Cell-cell interaction and adhesion

(i.e. exchange of materials through endocytosisand exocytosis, cell communication through thenervous system and hormones)

4. Transmembrane signaling5. Compartmentalization (organelles)6. Enzyme Localization7. Excitation-response coupling8. Energy transduction

MAJOR BODY COMPARTMENTS

Body = 60% water

1. INTRACELLULAR FLUID (ICF)

2/3 of TBW

Provides environment for the cell to ! Synthesize, store and utilize energy! Repair itself! Replicate

! Perform special functions

2. EXTRACELLULAR FLUID (ECF)

1/3 of TBW

2 compartments: PLASMA andINTERSTITIAL FLUID

delivery system of nutrients, ions, oxygenand hormones to cells

Removes waste products from the cells



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INTRACELLULAR(High in)

EXTRACELLULAR(High in)

K+ Na

+

Mg+ Ca

+

Proteins Glucose

Major anion: Phosphate Major Cation: Chloride

MEMBRANE LIPIDS

All major membrane lipids are amphipathic (haveboth hydrophobic and hydrophilic regions)

Forms a lipid bilayer that is impermeable to water-soluble molecules.

Need for channels and transporters. Lipid bilayers are formed by self-assembly driven

by hydrophobic effect

FATTY ACIDS

Saturated with straight tails (resulting to more rigidcell membranes).

Unsaturated fatty acids have kinked tails (morefluid membranes).

3 MAJOR MEMBRANE LIPIDS

1. PHOSPHOLIPIDS - lipids with phosphate groups.

A. Phosphoglycerides

Most common phospholipid Consists of a glycerol backbone to which

are attached two fatty acids in esterlinkage and a phosphorylated alcohol likeethanolamine, choline, serine, glycerol orinositol.

Fatty acids are even-numbered (16-18 Catoms) which could be saturated orunsaturated.

Simplest phosphoglyceride is phosphatidicacid.

B. Sphingomyelin

Second major class of phospholipid

Sphingosine instead of glycerol is itsbackbone

Ceramide = sphingosine + fatty acid

Hydroxyl group of sphingosine is esterifiedto phosphorylcholine

Sphingomyelin is prominent in myelinsheath

2. GLYCOSPHINGOLIPIDS - sugar containing lipids builton a backbone of ceramide.

Cerebrosides

Gangliosides

3. STEROLS

The most common of which is cholesterol

Modifies membrane fluidity

"moderator molecule"

Increases fluidity at temperatures belowTm; Limits disorder at temperatures aboveTm



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MEMBRANE PROTEINS

The major functional molecules of membranes

Amphipathic: hydrophilic regions are protruding atthe inside and outside faces of the membrane butconnected by a hydrophobic region traversing thehydrophobic core of the bilayer

Different membranes have varying proteincompositions

Functions of membrane proteins:! Enzymes! Pumps, channels, carriers! Antigens! Receptors! Structural Proteins

Types of Membrane Proteins:! Integral proteins:

Interact extensively withphospholipids

Require detergents forsolubilization

Amphipathic, globular and, incertain proteins, spans thebilayerseveral times (eg, Gproteins)

Asymmetrically distributed in cellmembrane

The orientation was conferred bythe time of insertion to the bilayerduring synthesis in the ER

Most membrane proteins fallunder integral proteins

! Peripheral proteins Do not interact directly with

phospholipids (do not requiredetergents for release)

Weakly bound to hydrophilicregions of integral proteins on oneside of the membrane

Ex: Ankyrin is bound to integral

protein Band 3; spectrin is in turnbound to ankyrin (Figure 1).

MEMBRANE CARBOHYDRATES

Occur in association with lipids or proteins:! Glycolipids! Glycoproteins

Mostly found on the external membrane surface Functions:

! Receptors

! Antigens! Confers negative charge to cell (as

glycocalyx)

RBC cystoskeletal proteinsspectrin and ankyrin with emphasis on theirinteractions.

FLUID MOSAIC MODEL

Universally accepted description of membranestructure coined by singer and Nicolson (1972)

Lipid bi-layer with embedded proteins (trilaminar

under electron microscope) “icebergs” (proteins) floating in a “sea” of

phospholipids Membrane lipids = fluid part Membrane proteins = mosaic part Membranes undergo changes from stiff (gel or

crystalline) to fluid state Proteins and lipids undergo rapid redistribution

(“lateral diffusion”)



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MEMBRANE FLUIDITY critical to its function; some cells have acquired

the ability to vary the fluidity of their membranesas needed

Factors that affect membrane fluidity:

1. Lipid composition longer and more saturated fatty acid chains exhibit

higher transition temperature Unsaturated fatty acids create a kink (bend) Prevents fatty acids from packing together as

tightly

Decreases the melting temperature (increasing thefluidity) of the membrane

2. Temperature Transition Temperature (Tm) - temp at which

structure undergoes transition from ordered todisordered state* high temperatures = more fluid* low temperatures = hydrophobic side chainsbecome aligned --> stiff structure

3. Cholesterol moderator molecule

T above Tm, ↓ fluidity due to its rigid structure(condensing effect)

T below Tm, ↑ fluidity (induces disorder)

Importance of Membrane Fluidity Permeability to water and other hydrophilic

molecule increases Lateral mobility of integral proteins increases*

! especially for transport and receptorproteins

ARTIFICIAL MEMBRANES & OTHER SPECIAL

MEMBRANE STRUCTURES

MICELLES

Small aggregates of amphipathic molecules thatform a monolayer with

! Hydrophobic regions – shielded fromH2OHydrophilic regions – immersed inH2O

The arrangement of the two regions depends onthe chemical environment where the micelle is

located. In a polar environment, the hydrophilic region is

facing the solution and the hydrophobic regionsare situated in the interior of the micelle.Conversely, in a non-polar environment, thehydrophilic region is found in the interior while thehydrophobic region faces the solution (inversemicelle).

CLINICAL APPLICATION OF MICELLES

Formed when bile acids, which are amphipathic,associate with products of lipid digestion

Bile acids – forms micelles that assist in thedigestion and absorption of fat and vitamins A, D,E, and K

LIPOSOMES: ARTIFICIAL MEMBRANES Vesicles that are surrounded by a lipid bilayer Consists of phospholipids that can be natural or

synthetic in origin



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Uses:! Lipid content can be varied for the

examination of varying lipid compositionon certain functions (i.e. transport)

! Study of factors that affect protein andenzyme function

! May be used for specific drug delivery andgene therapy

TIGHT JUNCTIONS

Located below the apical surface of epithelial cells Prevent diffusion of macromolecules between

them

Composed of proteins occludin, claudins Route for paracelullar transport

GAP JUNCTIONS

Low resistance connections between the cells

Allow the movement of ions and small moleculesbetween the cells

Adjacent cells are coupled electrically Connexon – functional unit of gap junction The alignment of a connexon of one cell with the

connexon of other cell forms a channel



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LIPID RAFTS

Dynamic areas of the exoplasmic leaflet of thelipid bilayer enriched in cholesterol, sphingolipids,and proteins

Involved in and enhances signal transduction byclustering the elements of signaling systems

SIGNAL TRANSDUCTION

Signal transmission across the membranes Biochemical signals from hormones,

neurotransmitters bind to receptors in cellmembrane

Through the generation of signaling moleculesmthe information is transmitted to the cytoplasm.

Signaling molecules:! Cyclic nucleotides! Calcium! Diacylglycerol! Phosphoinositides

TRANSPORT SYSTEM

Transfer of solutes and information across membranes.

Importance of transport systems:

Cell membrane is selective Cell membrane receives and transmits signals to

and from other cells

Transport System according to Direction of Movement

1. UNIPORT moves one type of substance bi-

directionally (ie. Glucose transported intocell through influence of insulin.)

2. COTRANSPORT

SYMPORT - moves two solutes in thesame direction (ie. Glucose transportedtogether with Na

+)

ANTIPORT - moves two solutes inopposite directions (ie. Na

+ (in) and Ca

++

or H+ (out) and Cl

-HCO3 exchanger in

RBC membrane)

PASSIVE TRANSPORT

Some molecules can passively traverse the bilayerdown electrochemical gradients by simplediffusion or by facilitated diffusion

Unlike active transport, passive transport does notrequire energy because it does not constitutesmovement against an electrochemical gradient.

SIMPLE DIFFUSION

Transport across the membrane down an

electrochemical gradient No need for energy Passive flow of a solute from a higher to lower

conc. due to random thermal movement The difference between facilitated diffusion (FD) is

that FD is mediated by a specific proteintransporter

Movement via simple diffusion is limited by: Thermal agitation of that specific molecule Concentration gradient across the membrane Solubility of that solute (permeability coefficient) in

the hydrophobic core of the membrane bilayer. Involves kinetic energy of molecules

Factors affecting net diffusion:

1. Concentration across the membrane – solutesmove from high to low concentration

2. Electrical potential across the membrane – solutesmove toward the solution that has the oppositecharge (the inside of the cell is usually negative)

3. Permeability coefficient of the substance for themembrane (lipid solubility)



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4. Hydrostatic pressure gradient across themembrane – higher pressure will increase the rateand force of the collision between the moleculesand the membrane

5. Thickness of membrane – the thinner themembrane the greater the rate of diffusion

6. Temperature – increased temperature willincrease particle motion and thus increase thefrequency of collisions between external particlesand the membrane

7. Distance8. Number of channels

FACILITATED DIFFUSION

Involves either certain transporters or ion channels.

ION CHANNELS for water soluble substances (ions) that cannot

just simply permeate the membrane Permeability depends upon:’

! Size ! extent of hydration ! charge density of the ion ! there are specific channels for each ion ! activity of some channels are regulated by

neurotransmitters function can be impaired by disease/mutations channels can be “gated” Specific channels for Na

+, K

+, Ca

2+, and Cl

- have

been identified

Membranes of nerve cells contain ion channelsthat are responsible for the generation of actionpotential

Activity of some ion channels is controlled byneurotransmitters

Ion Channel Gating

1. Voltage Gating channels open or close in response to changes in

membrane potential ex. Sodium Channels

2. Ligand Gating a specific molecule or chemical binds to a receptor

which opens the channel ex. Use of neurotransmitters like Ach

3. Mechanical Gating channel respond to mechanical stimuli (pressure

and touch)

AQUAPORINS water channels found in certain cells : RBC, distal

tubules and collecting ducts of renal nephrons tetrameric membrane proteins 5 distinct aquaporins : AP-1 to AP-5 mutation in AP-2 is the cause of nephrogenic

Diabetes Insipidus



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CARRIER MEDIATED TRANSPORT

Facilitated diffusion and active transport are similar in thefollowing:

1. Involve carrier proteins2. Show specificity for ions, sugars and amino acids3. Carriers exhibit maximum transport (Vmax) or

saturability4. There is binding constant (Km) for the solute

The rate of facilitated diffusion, a uniport system, can besaturated. Many facilitated diffusion systems arestereospecific are driven by the transmembrane

electrochemical gradient.

The rate of movement in passive diffusion is directlyproportionate to solute concentration. The process issaturable, however, when carriers are involved (carrier-mediated diffusion). The concentration at half maximal

velocity is equal to the binding constant (K m) of the carrierfor the solute. (V max, maximal rate.)

FACILITATED DIFFUSION

a uniport system explained by the “PING PONG” mechanism

! ping state = carrier is exposed to highconcentrations of solute. Molecules of thesolute bind to specific sites on the carrierprotein

! pong state = carrier is exposed to a lowerconcentration of solute. Solute isdischarged (released from binding) whereit goes to the side of the membrane thatfavors the new equilibrium

in the ping state, the conformation of the proteinexposes the binding site to high concentration ofsolute, the molecules of the solute bind to specific

sites on the carrier protein the binding will cause a conformational change

(pong state) on the carrier protein which exposesthe binding site to the side of lower soluteconcentration, the solute is discharged from thecarrier to achieve equilibrium

empty carrier reverts back to original conformation(ping state again) completing the cycle

The rate at which solutes enter a cell by facilitateddiffusion is determined by:

1. concentration gradient across membrane2. amount of carrier available (key control step)3. rapidity of solute-carrier interaction4. rapidity of conformational change for both the

loaded and unloaded carrier5. presence of certain hormones : Insulin, GH and

glucocorticoids

Hormones can regulate facilitated diffusion by changingthe number of transporters available.(i.e. insulin increasesglucose transport in fat and muscle by recruiting glucose

transporters (GLUT) from an intracellular reservoir.)



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ACTIVE TRANSPORT

Transport is away from thermodynamic equilibrium (energyrequiring).

1. Primary active transport Requires energy from light, electron movement

or ATP hydrolysis

energy for this process represents 30 40% ofenergy expenditure of the cell

Ex: Na+

K+

ATPase Is the transport of a solute across a membrane in

the direction of increasing concentration, and thusrequires energy (frequently derived from the

hydrolysis of ATP); a specific transporter (pump) isinvolved

The Na+

K+

ATPase of the plasma membrane Is a KeyEnzyme in regulating intracellular concentrations of Na

+ K

+.

Na+ K

+ ATPase pump moves three Na

+ ions from the

inside of the cell to the outside and brings two K+ ions from

the outside to the inside for every molecule of ATPhydrolyzed to ADP by membrane associated ATPase

(Mg2+

is a cofactor)ouabain and digitalis, inhibit the Na

+ K

+ ATPase by

binding to the extracellular domain.

2. Secondary active transport – “piggy-back”

OSMOSIS

the net flow of solvent across a semipermeablemembrane from an area of LOWER SOLUTECONCENTRATION to an area of HIGHERSOLUTE CONCENTRATION

due to a semipermeable membrane that onlyallows the solvent to pass

affected by osmotic pressure



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Osmotic Pressure minimum pressure required to negate or reverse

osmosis. force or pressure is applied on the side of the

membrane with higher solute concentration topush the solvent back to the area with low soluteconcentration

determined by the number of particles per unitvolume of fluid

volume may increase or decrease toaccommodate equilibrium if non-penetrating soluteis concerned

CELLULAR TRANSPORT OF MACROMOLECULES

involves vesicle formation with or from the plasmamembrane (PM)

Two process:! endocytosis! exocytosis

I. ENDOCYTOSIS uptake of large molecules

! molecules taken up by the cell whenhydrolyzed yield nutrients

provides a mechanism for regulating the content ofcertain membrane component (e.g. hormone

receptors) responsible for DNA transfection (entry of DNA

into the cell) ! DNA from one cell transfecting a different

cell, altering the latter’s function andphenotype

! uses Ca2+

(Ca2+

stimulates endocytosisand precipitates DNA, making DNA abetter object for endocytosis)

involves macromolecules: proteins,polysaccharides and polynucleotides

Requires the following: ! energy ! Ca

2+

! contractile elements/proteins(microfilament system)

Types of endocytosis

1. PINOCYTOSIS cellular uptake of fluid and fluid content

Two types of Pinocytosis:

a) Fluid-phase pinocytosis

also called “cell drinking”

is a nonselective process – norequirements

uptake of a solute thru small vesicleformation that is proportionate to its

concentration in the ECF (extracellularfluid)

is an active process (requires ATP)

b) Absorptive pinocytosis

is a receptor-mediated selective processfor the uptake of macromolecules

high affinity receptors permit the selectiveconcentration of ligands from the medium,minimize the uptake of fluid or solubleunbound macromolecules, and increasethe rate at which specific molecules enterthe cell

involves clathrin-coated pits! cell membrane with

invaginations/pits where thereceptors for a particular moleculecan be found

! underneath the pits in thecytosolic side is a contractileprotein called clathrin

may be a mechanism through whichcertain viruses enter the cell causingdiseases

! HIV – affects T cells! Hepatitis – liver cells

! Poliomyelitis – motor neurons e.g. LDL receptors – cholesterol

Receptors that can recognizecarbohydrate moieties – extracellularglycoproteins

! Galactosyl receptors – asialoglycoproteins

! mannose 6-phosphate moietyreceptors – acid hydrolases



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Additional Information about Clathrin

has a three-limbed structure (triskelion)

each limb made of one light and one heavy chainof clathrin

polymerization of clathrin into a vesicle! directed by assembly particles composed

of four adapter proteins that interact to thereceptors ensuring the selectivity ofuptake

! involves PIP2 (phosphatidylinositol 4.5-bisphospate) for vesicle assembly

! dynamin – binds and hydrolyzes GTP forthe pinching off of clathrin-coated vesiclesfrom the cell surface

A. Fluid-phase Pinocytosis formation of invaginations which will become larger untilthe two sides of the membrane fuse, sealing the neck ofthe PM at the original site of invagination forming a fluid-filled vesicles

B. Absorptive Pinocytosis formation of invaginations bringing the receptor inside itfor digestion (called internalization) some receptors are digested like that of insulin receptor(that is why people consuming too much sweets developType II diabetes); some are returned back to themembrane like that of LDL receptor

Fate of the vesicles after the invagination acted upon by lysosomes producing primaryphagolysosomes that will eventually become secondary

phagolysosomes (contain hydrolytic enzymes) which willdigest the contents of the vesicles digested materials will be converted to amino acids,simple sugars and nucleotides transported out of thevesicles to be used by the cell

What happen to the PM after Endocytosis? PM getting less and less

*endocytosed large materials in the expense of their PM have to add phospholipids and proteins OR change themembrane itself membrane assembly

2. PINOCYTOSIS

also called “cell eating”

involves ingestion of large particles: wholecells (bacteria), particles (viruses) andcellular debris

involves specialized cells! macrophages – for more and

prolong infection; can engulf anentire cell

! neutrophils – present in CBC; foracute inflammation

during severe infections, macrophagesingest a large volume of their cellmembrane through this process

can ingest their whole CM in 30 mins

ingest 25% of their volume per hour

internalize 3% of its PM each minute! leads to membrane assembly

mechanism like that of absorptivepinocytosis involves formation of pitsand primary and secondaryphagolysosomes

Can the CM become too thick? opposite of endocytosis exocytosis



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II. EXOCYTOSIS

release of macromolecules to the exterior(products coming from the cell going to theoutside)

from the site of production (ribosomes) that isattached to the ER transported to Golgiapparatus for processing, packaging andformation of vesicles

! when there’s signal, vesicles will fuse withthe PM and extrude its contents outside

signal for initiation is often via a hormone bindingto cell surface receptors increasing Ca

2+

! Ca2+

triggers exocytosis

Three fates of molecules released thru exocytosis:

1. attach to cell surface to become peripheralproteins (e.g. antigen)

2. may become a part of extracellular matrix(collagen, GAGs/glycosaminoglycans)

3. may enter ECF and signal other cells(hormones and enzymes)

*Exocytosis involves the contact of two inside-surface(cytoplasmic side) monolayers, whereas endocytosisresults from the contact of two outer-surface monolayers.

MEMBRANE, LIPID AND PROTEIN ASSEMBLY

MEMBRANE ASSEMBLY

Both lipids and proteins are inserted independentlyin membranes

Lipids and proteins turnover independently and atdifferent rates

Topogenic sequences (signal N terminal orinternal or stop) are important in determining thestructure of proteins in membranes

Final sorting of many membrane proteins occur inthe trans golgi

Specific sorting sequences guide proteins toparticular organelles

! (Ex: mannose-6-PO4 guides hydrolasesdestined for lysosomes while KDEL [Lys- Asp-Glu-Leu] specify proteins for theEndoplasmic Reticulum)

LIPID ASSEMBLY

Enzymes responsible reside in the cisternae ofEndoplasmic Reticulum

Phospholipids self-assemble as they are

synthesized into thermodynamically stable bilayers

Lipid vesicles migrate and fuse with Golgi Apparatus membrane which in turn fuse withPlasma Membrane

PROTEIN ASSEMBLY

Explained by the SIGNAL HYPOTHESIS

“The signal hypothesis proposes thatproteins destined for secretion, whichinvolves the movement of the proteinacross a biological membrane, are

originally manufactured with an initialsequence of amino acids that may ormay not present in the mature protein.”

Requires Endoplasmic Reticulum--> Golgi Apparatus--> Vesicles --> Plasma Membrane

The information for both modes of translocation isencoded in the protein in the form of a short-livedsequence extension (signal sequence).

Additional information resides in the ribosome inthe case of co-translational translocation, whichproceeds via a ribosome--membrane junction.

Translocation is mediated by specific receptors(ribosome and/or signal receptors) which arerestricted in their location to distinct cellularmembranes.



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2 kinds of proteins :

those synthesized by membrane bound ribosomes(secreted proteins and integral proteins) thatcontain a SIGNAL PEPTIDE at their N-terminal

those synthesized by free ribosomes (cytosolicproteins, extrinsic proteins in the inner PlasmaMembrane leaflet) that lack signal peptide

1.01 Biochem Trans - Cell and Cell Membrane

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Transcript of 1.01 Biochem Trans - Cell and Cell Membrane