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The Plasma The Plasma MembraneMembrane

Gateway to the CellGateway to the Cell

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Functions of Plasma Functions of Plasma MembraneMembrane

Protective barrierProtective barrier

Regulate transport in & out of Regulate transport in & out of cell cell (selectively permeable)(selectively permeable)

Allow cell recognitionAllow cell recognition

Provide anchoring sites for Provide anchoring sites for filaments filaments of cytoskeletonof cytoskeleton

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Functions of Plasma Functions of Plasma MembraneMembrane

Provide a binding site for Provide a binding site for enzymesenzymes

Interlocking surfaces bind cells Interlocking surfaces bind cells together (junctions)together (junctions)

Contains the cytoplasm (fluid in Contains the cytoplasm (fluid in cell) cell)

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PhospholipidsPhospholipids

CholesterolCholesterol

Proteins(peripheral and integral)Carbohydrates

Membrane ComponentsMembrane Components

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Composition of Membranes

• Water• Lipid

– many different types• Carbohydrate

– on lipids and proteins– Simple to very complex

• Protein– Many different types

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Membrane lipids

• Glycerophospholipids (phosphoglycerides)

• Sphingolipids: sphingophospholipids sphingoglycolipids• Cholesterol

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Phosphoglycerides

• Simplest glycerophos-pholipids is phosphatidic acid

• Different molecules are added onto the phosphate group to make the other phosphoglycerides

Phosphate

Glycerol

Fatty acids (saturated and unsaturated)

Ester linkage

Hydrophillic region

Hydrophobic region

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Phosphoglycerides (continued)

• Polar groups attached to phosphatidic acid

– Inositol Serine

– Glycerol Choline

– ethanolamine

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Phosphoglycerides (continued)

Hydrophilic region

Hydrophobic region

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Sphingolipids

• Based on Sphingosine

• Sphingosine convertrd to cermide with the condensation with a long chain fatty acid

• Sphingomyelin derived from cerimides

Fatty acyl group

Amide linkage

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Glycolipids

• Carbohydrate attached to lipid by way of beta glycosidic bond instead of phosphate ester

• Usually sphingosine derivative (by way of ceramide)– Cerebroside

• Galactocerebroside• Glucocerobroside

– Ganglioside• More complex carbohydrate structure• Contains sialic acid (an acidic

carbohydrate)

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Cholesterol

Hydrophillic region

Hydrophobic region

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Membrane proteins

• Varies from membrane to membrane– Amount– Diversity

• Classes of membrane proteins– Integral– Peripheral– Lipid anchored

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Proteins Are Critical to Proteins Are Critical to Membrane FunctionMembrane Function

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Membrane protein function

•transport

•enzymatic activity

•signal transduction

•intercellular joining

•cell-cell recognition

•ECM attachment

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Fluid mosaic model

FLUIDFLUID- because individual phospholipids and - because individual phospholipids and proteins can move around freely within the proteins can move around freely within the layer, like it’s a liquid.layer, like it’s a liquid.

MOSAICMOSAIC- because of the pattern produced by - because of the pattern produced by the scattered protein molecules when the the scattered protein molecules when the membrane is viewed from abovemembrane is viewed from above..

FLUID MOSAIC MODELFLUID MOSAIC MODEL

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• Rotation

• Flexing of fatty acyl chain

• Lateral diffusion

• Transverse (flip-flop) from one side to other

Movement of lipids in membrane

Fre

quen

tly

Infr

eque

ntly

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RBC plasma membrane

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Small molecules and larger hydrophobic molecules move through easily.e.g. O2, CO2, H2OIons, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own.

Semipermeable Semipermeable MembraneMembrane

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Types of Transport Types of Transport Across Cell Across Cell MembranesMembranes

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Simple DiffusionSimple DiffusionDiffusion is a

PASSIVEPASSIVE process which means no energy is used to make the molecules move

Molecules move from area of HIGH to LOWHIGH to LOW concentration

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Diffusion of HDiffusion of H22O Across O Across A MembraneA Membrane

High H2O potentialLow solute concentration

Low H2O potentialHigh solute concentration

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Isotonic Solution

NO NET MOVEMENT OF

H2O (equal amounts entering

& leaving)

Hypotonic Solution

CYTOLYSIS

Hypertonic Solution

PLASMOLYSIS

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Three Forms of Transport Across the MembraneThree Forms of Transport Across the Membrane

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Passive Passive TransportTransport

Simple DiffusionSimple Diffusion

Doesn’tDoesn’t require require energyenergy

Moves Moves high to lowhigh to low concentrationconcentration Example: Example: OxygenOxygen or or waterwater diffusing into a diffusing into a cell and cell and carbon carbon dioxidedioxide diffusing out diffusing out.

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Passive Passive TransportTransport

Facilitated diffusion

Doesn’t require energy

Uses transport proteins to move high to low concentrationExamples: Examples: GlucoseGlucose or or amino acidsamino acids moving moving from blood into a from blood into a cell.cell.

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Types of Transport Types of Transport ProteinsProteins

•Channel proteins are embedded in the cell membrane & have a pore for materials to cross

•Carrier proteins can change shape to move material from one side of the membrane to the other

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Channels that are proteinsCellular channels usually consist of large protein complexes with multiple transmembrane -helices.

Control of channel gating is a form of allosteric regulation. Conformational changes associated with channel opening may be regulated by:

Voltage Binding of a ligand (a regulatory

molecule) Membrane stretch (e.g., via link to

cytoskeleton)

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Channels cycle between open & closed conformations.

When open, a channel provides a continuous pathway through the bilayer, allowing flux of many ions.

Gramicidin is an example of a channel.

closed

conformationchange

open

Ion Channels

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ionophores

Antibiotics of bacterial origin facilitate the movement of ions across membranes called ionophores

Ionophores :carriers and channels.

valinomycin (a carrier)gramicidin (a channel).

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Valinomycin is a carrier for K+.

It is a circular molecule, made up of 3 repeats of the sequence shown above.

N C H C OHC

C H

C H 3H 3C

O

C N

C H

C H 3H 3C

OHC

C H

C H 3H 3C

C O C H

C H 3

C

O

H

O

H

3

V a lin o m y c in

L -v a l in e D -h y d ro x y - D -v a l in e L - la c t ic i s o v a le r ic a c id a c id

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Valinomycin is highly selective for K+ relative to Na+.

The ionophore valinomycin is a uniport carrier.

Valinomycin

O O O

O O

Hydrophobic

O

K+

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Gramicidin A

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An open channel forms when two gramicidin molecules join end to end to span the membrane.

Gating (opening & closing) of a gramicidin channel is thought to involve reversible dimerization.

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Classes of

carrier proteins

Uniport (facilitated diffusion) carriers mediate transport of a single solute.

An example is the GLUT1 glucose carrier.

Uniport Symport Antiport

A A B A

B

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A gradient of one substrate, usually an ion, may drive uphill (against the gradient) transport of a co-substrate.

It is sometimes referred to as secondary active transport.

E.g: glucose-Na+ symport, in plasma membranes of some epithelial cells

Symport (cotransport) carriers bind two dissimilar solutes & transport them together across a membrane.

Transport of the two solutes is obligatorily coupled.

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Facilitated DiffusionFacilitated Diffusion• SomeSome Carrier Carrier

proteinsproteins do not do not extend through extend through the membrane.the membrane.

• They They bond and bond and drag moleculesdrag molecules through the through the lipid bilayer lipid bilayer and release and release them on the them on the opposite sideopposite side

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Carrier ProteinsCarrier Proteins• Other carrier Other carrier

proteins proteins change shapechange shape to move to move materials materials across the cell across the cell membranemembrane

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Active TransportActive Transport

Requires energy or ATP

Moves materials from LOW to HIGH concentration

AGAINST concentration gradient

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Active transportActive transport

Examples: Pumping Na+ (sodium ions) out and K+ (potassium ions) in against strong concentration gradients.

Called Na+-K+ Pump

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Sodium-Potassium Sodium-Potassium PumpPump

3 Na+ pumped in for every 2 K+ pumped out; creates a membrane

potential

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Transport of macromolecules• Exocytosis~ secretion of

macromolecules by the fusion of vesicles with the plasma membrane

• Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane

• phagocytosis• pinocytosis• receptor-mediated endocytosis

(ligands)

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ExocytosisExocytosis

Molecules are Molecules are moved outmoved out of the cell by of the cell by vesiclesvesicles that that fusefuse with the plasma membrane. with the plasma membrane.

- moving things out.

This is how many This is how many hormoneshormones are secreted and how are secreted and how nerve cellsnerve cells communicate with one another communicate with one another.

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PinocytosisPinocytosis

Most Most commoncommon form of endocytosis form of endocytosis. Takes in Takes in dissolveddissolved molecules as a molecules as a vesiclevesicle.

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PinocytosisPinocytosis

• Cell forms an Cell forms an invaginationinvagination

• Materials Materials dissolve in dissolve in waterwater to be to be brought into brought into cellcell

• Called Called “Cell “Cell Drinking”Drinking”

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Receptor-Mediated EndocytosisReceptor-Mediated Endocytosis

Some Some integral proteinsintegral proteins have have receptorsreceptors on their surface to on their surface to recognize & take in recognize & take in hormones, hormones, cholesterolcholesterol, etc., etc.

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Endocytosis – Phagocytosis Endocytosis – Phagocytosis

Used to Used to engulf large particlesengulf large particles such such as food, as food, bacteriabacteria, etc. into vesicles, etc. into vesicles

Called Called “Cell Eating”“Cell Eating”

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ExocytosisExocytosis The opposite of endocytosis is exocytosis. The opposite of endocytosis is exocytosis. Large moleculesLarge molecules that are manufactured in that are manufactured in

the cell are the cell are releasedreleased through the cell through the cell membranemembrane..

Inside Cell Cell environment

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Specialized Transport• Transport proteins• Facilitated diffusion~

passage of molecules and ions with transport proteins across a membrane down the concentration gradient

• Active transport~ movement of a substance against its concentration gradient with the help of cellular energy

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Movement of compounds across membranes

• Passive diffusion– Affected by

• Size of molecule• Composition of molecule• Temperature• Lipid composition

– Not mediated by proteins– Molecules move down a concentration

gradient– Unimportant for most polar molecules

S S

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Pore (a.k.a. Channel) Proteins

• Actively regulated to control flow across membranes

• Very specific for molecules able to pass through channel

• Rate of movement of solutes is usually greater than for transporters

Fig. 9.30

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•Barriers, Pores, pumps and gates.mht