Chapter 4Chapter 4 Cell Cell MembranesMembranes

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4.1Cell membrane structure 4.1.1 Model structure 4.1.2 Composition of the Cell Membrane 4.1.3 Fluidity of membrane 4.1.4 Asymmery membrane 4.1.5 Membrane function 4.1.6 Cell cytoskeleton interaction with cellmembrane

4.1.1 stucture of Cell 4.1.1 stucture of Cell MembraneMembrane

Every cell is encircled by a membrane and most Every cell is encircled by a membrane and most cells contain an extensive intracellular membrane cells contain an extensive intracellular membrane system. Membranes fence off the cell's interior system. Membranes fence off the cell's interior from its surroundings. Membranes let in water, from its surroundings. Membranes let in water, certain ions and substrates and they excrete certain ions and substrates and they excrete waste substances. waste substances. They act to protect the cellThey act to protect the cell. .

Without a membrane the cell contents would Without a membrane the cell contents would diffuse into the surroundings, information diffuse into the surroundings, information containing molecules would be lost and many containing molecules would be lost and many metabolic pathways would cease to work:metabolic pathways would cease to work:

The cell would die! The cell would die!

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Cell MembranesCell Membranes::

Surround all cells Surround all cells Fluid-like composition…like soap Fluid-like composition…like soap

bubblesbubbles Composed of:Composed of:

• Lipids in a bilayerLipids in a bilayer• Proteins embedded in lipid layer Proteins embedded in lipid layer

(called transmembrane proteins)(called transmembrane proteins)• And, Proteins floating within the lipid And, Proteins floating within the lipid

sea (called integral proteins)sea (called integral proteins)• And Proteins associated outside the And Proteins associated outside the

lipid bilayer (peripheral). lipid bilayer (peripheral).

The Cell The Cell MembraneMembrane

The Cell MembraneThe Cell Membrane

Liposome Liposome 脂质体脂质体

artificial lipid vesicle• Model of membrane study• Carrier of drug• Use in Gene treatment

Cell MembraneCell Membrane: :

At Very High Magnification & in color

Cell MembraneCell Membrane

Plasma membranePlasma membrane Endomembrane Endomembrane

systemsystem• Nuclear membraneNuclear membrane• Endoplasmic Endoplasmic

reticulumreticulum• VessiclesVessicles• Golgi apparatusGolgi apparatus

MitochondronMitochondron

CholoplastCholoplast

4.1.2 Molecular 4.1.2 Molecular structure model of cell structure model of cell membranemembrane

1 sandwich model 1 sandwich model Danielli and DavisonDanielli and Davison ，， 19301930 Protein-lipid –protein Protein-lipid –protein 2 Robertson2 Robertson ，， 1950 suugest unit membrane 1950 suugest unit membrane

model model structure of dark-brightstructure of dark-bright －－ dark under dark under

electron microscopeelectron microscope 33 Fluid Mosaic Membrane modelFluid Mosaic Membrane model 19721972 ，， Singer and Nicolson suggestSinger and Nicolson suggest two-dimensional liquid, emphasize two-dimensional liquid, emphasize

fluidity,asymmetricalfluidity,asymmetrical 非对称的非对称的 membranemembrane

Fluid Mosaic MembraneFluid Mosaic Membrane

Diversity of Molecules make a MozaicDiversity of Molecules make a Mozaic

MozaicMozaic Integral proteins embedded in membraneIntegral proteins embedded in membrane Amphipathic!Amphipathic!

Hydrophilic

Hydrophobic

Hydrophilic

CM is composed of A) LipidsCM is composed of A) Lipids

B) ProteinsB) Proteins

C) CarbohydratesC) Carbohydrates

4.1.3 Composition of the Cell Membrane4.1.3 Composition of the Cell Membrane

细胞膜的化学组成

A Membrane lipid

PhospholipidsPhospholipids

Glycolipid

Cholesterol

The chemical nature of The chemical nature of the Phosphate-Nitrogen the Phosphate-Nitrogen chain, and the small chain, and the small glycerol part of the glycerol part of the molecule are polar, while molecule are polar, while the fatty acid parts of the the fatty acid parts of the molecule are non polarmolecule are non polar

CM is essentially a double CM is essentially a double layer (bilayer) of a type of layer (bilayer) of a type of lipid called a phospholipidlipid called a phospholipid

PhospholipidsPhospholipids

The dual nature of the phospholipid The dual nature of the phospholipid molecule, the cell membrane is formed of a molecule, the cell membrane is formed of a double later of these molecules, arranged double later of these molecules, arranged “tail-to-tail” = phospholipid bilayer“tail-to-tail” = phospholipid bilayer

Tails

Head

Head

Phospholipid BilayerPhospholipid Bilayer

amphipathic

H

HO

H

HO

HHO

hydrophobic

hydrophilic

H2O

H2O

Organization of the cell membrane relies on water!

Membrane LipidsMembrane Lipids Composed largely of phospholipids Composed largely of phospholipids Phospholipids composed of….glycerol Phospholipids composed of….glycerol

and two fatty acids + POand two fatty acids + PO44 group group P-Lipids are polar molecules… P-Lipids are polar molecules…

P-Lipids are represented like this

Membrane lipids Membrane lipids 膜脂膜脂 磷脂磷脂 糖脂糖脂 胆固醇胆固醇

Membrane LipidsMembrane Lipidsform a Bilayerform a Bilayer

Outside layer

Inside Layer

Phospholipid Phospholipid Molecule ModelMolecule Model

glycerol

fatty acids (hydrophobic 疏水的 )

phosphate (hydrophilic 亲水的 )

Quiz Quiz

If Phospholipids are polar, which end If Phospholipids are polar, which end seeks out water and which avoids seeks out water and which avoids water? water?

B Membrane ProteinsB Membrane Proteins Various shapes and sizes, depending on Various shapes and sizes, depending on

function in the membranefunction in the membrane 2 main kinds:2 main kinds:

1) Peripheral Proteins1) Peripheral Proteins 2) Integral 2) Integral ProteinsProteins

Membrane ProteinsMembrane Proteins

IntegralIntegral: embedded within : embedded within

bilayerbilayer

PeripheralPeripheral: reside outside : reside outside

hydrophobic region of lipidshydrophobic region of lipids

Text pg. 80

Peripheral ProteinsPeripheral Proteins

HydrophobicHydrophobic (water-loving) (water-loving)

Are attached to Are attached to inner and outerinner and outer surface of membranesurface of membrane

In contact wit hydrophilic heads of In contact wit hydrophilic heads of phospholipids and watery phospholipids and watery interior/exterior of cellinterior/exterior of cell

Integral ProteinsIntegral Proteins Have both hydrophilic Have both hydrophilic

and hydrophobic areasand hydrophobic areas Penetrate completely Penetrate completely

through or partially through or partially into the phospholipid into the phospholipid bilayerbilayer

Hydrophobic areas Hydrophobic areas contact watery interior contact watery interior & exterior of cell& exterior of cell

Hydrophobic areas are Hydrophobic areas are embedded inside the embedded inside the bilayer to avoid bilayer to avoid contact with watercontact with water

Membrane ProteinsMembrane Proteins

Text pg 80

Integral membrane Integral membrane proteinsproteins

Peripheral Peripheral membrane proteinsmembrane proteins

Integral

Membrane Protein FunctionsMembrane Protein Functions

Every cell membrane is a unique, two-sided work surface

Control factor of membrane protein Control factor of membrane protein movementmovement

Cytoskeleton Cytoskeleton 细胞骨架细胞骨架 Protein Protein Congregation Congregation 蛋白的聚集蛋白的聚集

Interlink with extracelluarInterlink with extracelluar 细胞与外侧结构细胞与外侧结构的连结的连结

C CarbohydratesC Carbohydrates Polar – Polar – hydrophilichydrophilic

Found attached to Found attached to outer surfaceouter surface of of membrane onlymembrane only

Carbohydrate + protein = Carbohydrate + protein = glycoproteinglycoprotein

Carobhydrate + lipid = Carobhydrate + lipid = glycolipidglycolipid

Review of last lesson Review of last lesson

Which component do take Which component do take part in forming CM? part in forming CM?

A)A)LipidsLipidsB)B)ProteinsProteins

C) CarbohydratesC) Carbohydrates

what is prominent what is prominent point of every point of every components?components?

two-dimensional liquid, two-dimensional liquid, emphasize fluidity,emphasize fluidity,asymmetrical membraneasymmetrical membrane

Please give conclusion of the fluid mosaic model

The dual nature of the phospholipid The dual nature of the phospholipid molecule, the cell membrane is formed of a molecule, the cell membrane is formed of a double later of these molecules, arranged double later of these molecules, arranged “tail-to-tail” = phospholipid bilayer“tail-to-tail” = phospholipid bilayer

Tails

Head

Head

The chemical nature of The chemical nature of the Phosphate-Nitrogen the Phosphate-Nitrogen chain, and the small chain, and the small glycerol part of the glycerol part of the molecule are polar, while molecule are polar, while the fatty acid parts of the the fatty acid parts of the molecule are non polarmolecule are non polar

CM is essentially a double CM is essentially a double layer (bilayer) of a type of layer (bilayer) of a type of lipid called a phospholipidlipid called a phospholipid

PhospholipidsPhospholipids

How many types Could membrane protein divide into? What is special points of every type membrane protein?

4.1.3 Evidence for the Fluid Mosaic 4.1.3 Evidence for the Fluid Mosaic Model (Cell Fusion)Model (Cell Fusion)

Phospholipid Phospholipid Molecule ModelMolecule Model

glycerol

fatty acids (hydrophobic 疏水的 )

phosphate (hydrophilic 亲水的 )

Experimental Observation of Experimental Observation of FluidityFluidity

Fluorescence recovery after photobleaching FRAP)

Control of Membrane FluidityControl of Membrane Fluidity

PlantsPlants• unsaturated fatty unsaturated fatty

acidsacids• type of fatty acid type of fatty acid

adjusted seasonallyadjusted seasonally Animals- CholesterolAnimals- Cholesterol

• buffers membrane at buffers membrane at hi temphi temp

• prevents membrane prevents membrane from crystalizing at from crystalizing at low templow temp

Influence factor of membrane Influence factor of membrane fluidityfluidity

Fatty acids lengthFatty acids length

Saturation of fatty acidsSaturation of fatty acids

Temperature Temperature Cholesterol concentration

4.1.4 Asymmetry of membrane

Membrane lipid asymmetry: distribution

Membrane protein asymmetry :directional signal

4.1.5 membrane function 4.1.5 membrane function

CM is material exchange gateway of the cell

In essence membranes are In essence membranes are essential for the integrity and essential for the integrity and function of the cell.function of the cell.

FunctionsFunctions

1) Transport raw materials into the cell from the 1) Transport raw materials into the cell from the surrounding surrounding EExtra xtra CCellular ellular FFluid (luid (ECFECF))

e.g – Oe.g – O22,H,H22O,glucose,aaO,glucose,aa

2) Transport manufactured products and wastes 2) Transport manufactured products and wastes out out of the cell into the ECFof the cell into the ECF

e.g. – hormones, enzymes, COe.g. – hormones, enzymes, CO22

3) Prevent entry of unwanted matter into the 3) Prevent entry of unwanted matter into the cellcell

e.g. – bacteria, poisonse.g. – bacteria, poisons

4) Prevents the escape of matter needed within 4) Prevents the escape of matter needed within the cell to perform cellular functionsthe cell to perform cellular functions

e.g. – keep cell parts, needed e.g. – keep cell parts, needed enzymes, nutrients inside cellenzymes, nutrients inside cell

Membrane FunctionsMembrane Functions be protective ,material exchage gatewaybe protective ,material exchage gateway regulate transport in and out of cell or regulate transport in and out of cell or

organelleorganelle allow selective receptivity and signal allow selective receptivity and signal

transduction by providing transmembrane transduction by providing transmembrane receptors that bind signaling moleculesreceptors that bind signaling molecules

allow cell recognitionallow cell recognition

subdivides cell into compartments with subdivides cell into compartments with

different internal conditionsdifferent internal conditions

allows for different conditions between allows for different conditions between

inside and outside of cellinside and outside of cell

allows release of substances from cell via allows release of substances from cell via

vesicle fusion with outer membranevesicle fusion with outer membrane

Form special structure on cell surface Form special structure on cell surface

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Membrane function Membrane function conclusion of text book conclusion of text book

provide anchoring sites for cytoskeletal provide anchoring sites for cytoskeletal components. This allows the cell to maintain its components. This allows the cell to maintain its shape and perhaps move to distant sites.shape and perhaps move to distant sites.

provide a stable site for the binding and catalysis provide a stable site for the binding and catalysis of enzymes.of enzymes.

regulate the fusion of the membrane with other regulate the fusion of the membrane with other membranes in the cell via specialized junctions membranes in the cell via specialized junctions

provide a passageway across the membrane for provide a passageway across the membrane for certain moleculescertain molecules

allow directed cell or organelle motilityallow directed cell or organelle motility

Cellular MembranesCellular Membranes

REVIEWREVIEW Importance of MembranesImportance of Membranes Fluid Mosaic modelFluid Mosaic model Membrane StructureMembrane Structure Membrane function Membrane function