X Membrane and Transport

7/27/2019 X Membrane and Transport

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Membrane Structure and Function

-plasma membrane acts as a barrier between cells

and the surrounding.

-plasma membrane is selective permeable

-consist of lipids, proteins and carbohydrates

-major lipids are phospholipid which is amphipathic

= contain both hydrophobic and hydrophilic region


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The Fluid Mosaic Model :current model of membrane

1972, S.J.Singer and G.Nicolson proposed this

model: the membrane is amosaic of proteinmolecules bobbing in a

fluid bilayer ofphospholipid.

- membrane is held together by hydrophobic interaction

-some membrane proteinsmove along cytoskeletal fibers bymotors proteins

-some membrane proteins are immobileby attachment to thecytoskelton


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Membranes are fluid

-phospholipids can drift laterally in the plane of themembrane (an average lipid molecule can diffuse the length of a large

bacterial cells (~ 2 m) in about 1 second) = lateral movement

(frequently)-also, phospholipids can migrate from the monolayeron one side to that on the other = flip-flop (rarely)


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-Temperature and lipid composition determine

fluidity of the membrane-At low temperature, membrane is less fluid andbecause the phospholipids are more closely

packed.

-Membranes rich in

unsaturated fatty acids aremore fluid that thosedominated by saturated

fatty acids because thekinks in the unsaturatedfatty acid tails preventtight packing.


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-Steroid cholesterol which is wedged between

phospholipids also effects membrane fluidity.

-At warm temperature, it makes membrane less

fluid by restraining the movement ofphospholipid.

-At low temperature, themembrane remains fluidbecause cholesterol

hinders the close packingof the phospholipids.


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Membranes are mosaic of structure andfunction

A membrane is a collage of different proteinsembedded in the fluid matrix of the lipid bilayer


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2 major populations of membrane proteins:-integral proteins -peripheral proteins

Integral proteins penetrate the

hydrophobic core of the lipid bilayer.


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Many of the integral proteins are

transmembrane proteins which completelyspan the membrane.

-The hydrophobic region of anintegral protein consist of oneor more stretches of nonpolaramino acids, usually coiledinto a-helices.-The hydrophilic parts are

exposed to the aqueoussolutions on either side of themembrane.


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Peripheral proteins are not embedded in the

lipid bilayer at all.They are appendage loosely bound to thesurface of the membrane, often to he exposed

parts of integral proteins.


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-The lipid and proteins of plasma membrane also

covalently bound to carbohydrates (~15 sugar units)= glycolipid and glycoprotein which are restricted tothe exterior surface and are important for cell-cell

recognition.

-e.g.variation ofoligosaccharideson the RBC

determineshuman bloodgroup A,B,AB andO.


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-Membrane has distinct inside and outside

faces.

-This asymmetrical

orientation begins duringsynthesis of newmembranein the endoplasmicreticulum


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Some Functions of Membrane Protiens


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Traffic across Membranes

Biological membrane is a

supramolecular structure = many molecules

ordered into a higher level of organization

with emergent properties beyond those of

the individual molecules.

The supramolecular structure of the

membrane results in selective permeability.


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Permeability of the membrane

-lipid bilayer is permeable to small nonpolarmolecules: CO2 and O2-but it is impermeable to ions and polarmolecules: glucose, sucrose, Na+, H+, Cl-

-transport proteins enable cell membranespermeable to specific ions and polar moleculesincluding H2O

Thus, the selective permeability of a membrane

depends on both the discriminating barrier of the lipid

bilayer and the specific transport proteins.


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Transport Across Membrane

- passive transport = diffusion across amembrane down the concentration gradient

- osmosis = passive transport of water

- facilitated diffusion = specific proteinsfacilitate the passive transport of water and

selected solutes- active transport = pumping of solutes againsttheir concentration gradients

- cotransport = membrane protein couples thetransport of one solute to another- exocytosis and endocytosis transport large

molecules


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Passive transport is diffusion across a

membrane

-diffusion of a substance across a biological

membrane is called passive transport

-molecules diffuse across membrane downits concentration gradient

-spontaneous process: does not need energy


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Osmosis is the passive transport of water

Hypertonic solution = solution with higherconcentration of solute

Hypotonic solution = solution with lower soluteconcentrationIsotonic solution = solution of equal solute

concentration


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U-shaped vessel with a selectively

permeable membrane separating 2 sugarsolution of different concentration.The membrane pores are too small for

sugar molecule to pass but large enough forwater

Water diffuses acrossmembrane fromhypotonic solution to the

hypertonic solution.This diffusion of water =osmosis


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Direction of osmosis is determined only

by a different in total solute concentration.Water diffuses from hypotonic to a

hypertonic even if the hypotonic solution has

more kinds of solutes.

(Water from seawater (containing a

great variety of solutes) will diffuse to a veryconcentrated sugar solution)

For isotonic solutions, water movesacross membrane in both direction at an

equal rate.


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Water balance of cells


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The protist Paramecium lives in pondwater which is hypotonic to the cell.

Its contractile vacuole forces water outof the cell as fast as it enters by osmosis.

Osmoregulation: the control of water balance

f f


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Specific proteins facilitate the passive transportof water and and selected solutes

Facilitate diffusion = diffusion of polar

molecules (including H2O) and ions with thehelp of transport proteins spanning themembrane

Model # 1: The purpletransport protein forms a

channel through whichwater molecules or aspecific solute can pass.


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Model # 2: Transportprotein alternates between

2 conformations, moving asolute across themembrane as the shape ofthe protein changes.

The proteins can transport the solute in

either direction, with the net movement being

down the concentration gradient of thesolute.

A ti t t


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Active transport:

-pumping of solutes against their concentrationgradient-require energy in the form of ATP


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The sodium-potassium pump


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Comparison between passive and active transport


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An electrogenic pump = a transport protein

that generates voltage across a membranesodium-potassium pumpproton pump transport positive charge in

the form of hydrogen ions

Voltage generatedacross membranes

can be trapped forcellular work suchas in cotansport.

Cotransport: a membrane protein couples the


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Cotransport: a membrane protein couples thetransport of 2 solutes

A substance that

has been pumpedacross a membrane

can do work as it leaksback by diffusion.Plant cells uses

hydrogen gradient generated by proton pumps

to drive active transport of amino acids, sugarand other nutrients

E t i d d t i t t f l


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Exocytosis and endocytosis: transport of large

molecules (proteins, polysaccharides)

Exotytosis = secretion of

macromolecules fromcells by the fusion of

vesicles with the plasmamembrane.


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Endocytosis = the process which cell takes inmacromolecules and particulate matter by

forming new vesicles from the plasmamembrane.

3 types of endocytosis:

phagocytosis = celular eating

pinocytosis = cellular drinking

receptor-mediated endocytosis

Phagocytosis


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Phagocytosis

The cell engulfs a particle by wrappingpseudopodia around it and packaging it within

a vacuole: food vacuole.Food vacuole fuses with lysosome to bedigested.

Pinocytosis


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Pinocytosis

The cell gulps droplets of extracellularfluid into tiny vesicles.

Pinocytosis is unspecific in the substancesit transports.

Receptor mediated endocytosis


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Receptor-mediated endocytosis

Specific receptor proteins embedded inthe membrane bind to the ligands.

e.g. transport of cholesterol into the cells.

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X Membrane and Transport