Cell membrane

CELL MEMBRANE /

PLASMA MEMBRANE

Department of Biochemistry, KMC, Duwakot Tuesday, April 21,

2015

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Tuesday, April 21, 2015 2

All membrane shares

common features: flexibility,

self-sealing, selective

permeability, all appear

trilaminar in appearance and

are mostly 3-8 nm thick.

Overview of cell membrane

Tuesday, April 21, 2015

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Are flexible, self-sealing, and selectively permeable to polar solutes.

Both prokaryotes and eukaryotes have same classes of chemical components.

Similar in structural organization.

Major differences in: lipids, proteins and carbohydrate but not in physiochemical interaction.

Have trilaminar appearance under electron microscope.

Cell membrane


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Cell membranes act as selective barrier

Membrane forms compartments


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Plasma membrane


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Plasma membrane is involved in cell communication, import and export of

materials and cell growth and cell motility.

Function of cell membrane inside the cell Vs on the surface?

Role of cell inside the cell Vs outside the cell?

Membranes composition


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Membrane structure

composed of lipids, proteins and carbohydrates.

Major lipids in membranes are Phospholipids, Glycosphingolipids, and Cholesterol.

Each type of membrane have characteristic lipids and proteins.

Each type of membrane have

characteristic lipids and proteins


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Relative proportion of lipids and proteins vary with type of

membrane.

E.g.: Myelin sheath of neurons: primarily of lipids.

PM of bacteria, mitochondria, chloroplast: more protein than lipids.

Cholesterol & Cardiolipin content in Plasma membrane Vs

Inner mitochondrial membrane?

GLYCOPHORIN

So, what is the function of sugar moiety attached to

these surface glycoproteins?

Fluid mosaic model for membrane structure

according to Singer and Nicolson


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All membranes share some fundamental proper ties.

Amphipathic lipid aggregates that form

in water 10

Depending on the precise conditions and the nature of lipids

three types of lipid aggregation are formed when mixed with

water.

The lipid bilayer is 3nm (30 Å) thick

Lysophospholipids, Glycerophospholipids, sphingolipids & sterols

Amphipathic compounds in aqueous

solution


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Proteins of cell membrane


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Form channels for movements of ions and small molecules

Serve as transporters for large molecules

Types: 1. Peripheral, 2. Integral protein, 3.

Amphitropic protein

Peripheral, Integral and amphitropic

proteins


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Membrane proteins can be

distinguished by the

condition required to

disassociate them from the

membrane.

Many membrane proteins span lipid

bilayer


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Transbilayer disposition of

glycophorin in the erythrocyte.

Types of transmembrane protein


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Type I

E.g.: LDL-

receptor

Type II

E.g.: Transferrin

Type III

E.g.: Glucose

transporter

Types of transmembrane protein


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Transmembrane protein


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Bacteriorhodopsin

Fluidity of the membrane


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Fluidity of the membrane depends on its lipid

composition

Longer and saturated chain less fluid

Unsaturated fatty acid chain increases fluidity

Cholesterol modifies fluidity of the membrane

Transport of materials and

information across the membrane


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Classification of transporter


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Membrane transport of small

molecules


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Passive Active

Simple Facilitated

Via various

transporter via ion channels

Active Vs Passive transport


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In Summary


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The net diffusion of substance depends on the

following:

1. Its concentration gradient across the membrane.

2. The electric potential across the membrane

3. Permeability coefficient of the substance for the

membrane.

4. The hydrostatic pressure gradient across the

membrane.

5. Temperature.

Transport pathways through cell membrane,

and basic mechanism of transport


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Three general classes of transport system


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Schematic representation of types of

transport system


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Facilitated diffusion Vs Active

transport


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Similarities

Involves carrier proteins, have specificity for ions, sugars and

amino acids.

Resembles substrate-enzyme reaction.

Have specific binding site for the solute.

Carrier is saturable, so it has maximum rate of transport

(Vmax)

There is binding constant for solute, so whole system have

Km.

Structurally similar inhibitor blocks transport.

Facilitated diffusion Vs Active

transport


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Differences

Facilitated diffusion can operate bidirectionally while

active transport is usually unidirectional.

Active transport always occur against electrical and

chemical gradient and so it requires energy.

Kinetics of Facilitated diffusion Vs

Passive diffusion 29

Solute concentration

Ping-Pong model explains facilitated

diffusion 30

Facilitated diffusion is determined by:

1. Concentration gradient across membrane.

2. Amount of carrier available.

3. Rapidity of solute-carrier interaction.

4. The rapidity of the conformational change for both and loaded and the

unloaded carrier.

Active transport


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Energy is required.

Energy comes from: hydrolysis of ATP, electron movement, and light.

Pump used: Na+-K+-ATPase

Glucose transport involves several

mechanisms 32

Transport of glucose into myocyte by GLUT4

is regulated by insulin (Facilitated diffusion)


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Summary of transport types


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1. Harper’s Biochemistry, 25th. Edition

2. Lehninger’s Biochemistry, 7th. Edition

3. Essential Cell Biology, 3rd. Edition

References


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Cell membrane

Science

proteins of cell membrane

type of membrane

function of cell membrane

overview of cell membrane

membrane structure

cell membranes

cell vs

membrane forms compartments