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ABOUT AUTHOR

GUL MUNEERS/O ABDUL RAZZAQUE

BIOCHEMISTRY

BS PART IIUNIVERSITY OF SINDH

2K10/BCH/23

PROUD TO BE BIOCHEMIST

WE EXPLORE THE SECRETS OF LIFE

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PLASMA MEMBRANE

By GUL MUNEER

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What is plasma membrane?

Outermost layer (animal) Thickness is 5-8 nm Selectively permeable Serve as outer boundary Allows some substances to cross more easily than others Made of Phospholipids, proteins & conjugated molecules Separate and protect cell from external environment Provide connecting system b/w cell & its environment Also called cell membrane

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How could you define plasma membrane?

An outermost envelope surrounding the cell that separates and protects the cell from the external environment and provides a connecting system B/w the cell and its environment is called plasma membrane.

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STRUCTURE OF PLASMA

MEMBRANE

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Structure of the Plasma Membrane

The plasma membrane is composed of two layers of phospholipids back-to-back.

Phospholipids are lipids with a phosphate attached to them.

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Phospholipids Fatty acid tails

hydrophobic Phosphate group head

hydrophilic Arranged as a bilayer

Fatty acid

Phosphate

Phospholipids, glycolipids, cholesterol are amphipathic lipids containing hydrophobic and hydrophillic ends

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Phospholipid bilayer

polarhydrophilicheads

nonpolarhydrophobictails

polarhydrophilicheads

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Sandwitch model OR

Danielli- Davson Model

Proposed by Davson and Danielle in 1935“Cell membrane is lipid bilayer sandwitched

B/w two monomolecular protein layers”

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Fluid mosaic model

In 1972, S.J. Singer & G. Nicolson proposed Fluid

mosaic model.

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What is Fluid mosaic model?

“Cell membrane is lipid bilayer in which proteins are partially

embeded like Floating iceburgs in sea”

The fluid mosaic model describes the plasma membrane as a flexible boundary of a cell. The phospholipids move within

the membrane.

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OVERVIEW

Lipid molecules are present in a fluid state capable of rotating and moving.

The proteins occur as a ‘mosaic’ of discontinuous particles that penetrate deeply into and even through the lipid

sheet.Globular proteins are irregularly

embedded in the lipid bilayer.

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Membrane proteins

Membrane proteins are categorized into two groups:

1- Extrinsic (peripheral) membrane proteins2- Intrinsic (integral) membrane proteins

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Extrinsic membrane protein

Proteins loosely associated with membrane surface Located entirely outside of the lipid bilayer Either on the extracellular or cytoplasmic surface Also called Peripheral membrane proteins: Example: Cytochrome C of Mitochondria Cell surface identity marker (antigens)

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Intrinsic membrane proteins

Directly incorporated within the lipid bilayer Tightly bound to lipid bilayer Provides channel for the water-soluble substances Also called Integral membrane proteins Example: Transmembrane protein Transport proteins Channels, permeases (pumps)

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Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid

bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

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Proteins domains anchor molecule Within membrane

nonpolar amino acids

hydrophobic anchors protein

into membrane

On outer surfaces of membranepolar amino acids

hydrophilicextend into

extracellular fluid & into cytosol

Polar areasof protein

Nonpolar areas of protein

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NH2

H+

COOH

Cytoplasm

Retinalchromophore

Nonpolar(hydrophobic)a-helices in thecell membrane H+

Porin monomer

b-pleated sheetsBacterialoutermembrane

proton pump channel in photosynthetic bacteria

water channel in bacteria

function through conformational change = shape change

Examples

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Classes of amino acidsWhat do these amino acids have in

common?

nonpolar & hydrophobic

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Classes of amino acidsWhat do these amino acids have in

common?

polar & hydrophilic

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Association of membrane proteins with the lipid bilayer Transmembrane Proteins

1. A single α-helix2. Multiple α-helices3. Rolled up β-sheet

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4.α- helix (hydrophobic face) embedded in lipid bilayerthe.

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Cytoplasmic side

5. Protein covalently attaches lipid chain– fatty acid chain or prenyl group(cytoplasmic side)6. Protein attaches phosphatidylinositol viaan oligosaccharide linker7, 8. Noncovalent interaction betweenproteins

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Plasma Membrane : Membrane ProteinsFunctional classificationTransmembraneProteins

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Plasma Membrane : Membrane ProteinsFunctional classification Transmembrane Proteins

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Plasma Membrane : Membrane ProteinsFunctional classification Peripheral Proteins(only one side of the membrane)

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Chemical composition

Composed of Lipids, Proteins and Carbohydrates. Actual components differs from tissue to tissue.

Lipids of cell membrane are Phospholipids Glycolipids Sterol Cholesterol

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Why carbohydrates are not inserted into the biological membrane?

The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.

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Membrane carbohydrates Play a key role in cell-cell

recognitionability of a cell to distinguish one cell

from anotherAntigens (MHC)

important in organ & tissue development

basis for rejection of foreign cells by immune system

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Four major phospholipids found in mammalian plasma membraneThere are many ‘minor’ phospholipids exists, too.

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CholesterolUnique to plasma membraneStabilize membrane

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CholesterolUnique to plasma membraneStabilize membrane

Cholesterol

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Movement across the

Cell Membrane

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Many Functions of Membrane Proteins

Outside

Plasmamembrane

Inside

Transporter Cell surfacereceptor

Enzymeactivity

Cell surface identity marker

Attachment to thecytoskeletonCell adhesion

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Transport proteins allow needed substances or waste materials to move through the plasma membrane.

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Aquaporins Water moves rapidly into & out of

cells evidence that there were water

channels

1991 | 2003

Peter AgreJohn Hopkins

Roderick MacKinnonRockefeller

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Diffusion 2nd Law of Thermodynamics

governs biological systems universe tends towards disorder (entropy)

Diffusion movement

fromhighlowconcentration

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Diffusion Move from HIGH to LOW

concentration“passive transport”no energy needed

diffusion osmosis

movement of water

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Diffusion through phospholipid bilayer What molecules can get through directly?

fats & other lipids

inside cell

outside cell

lipid

salt

aa H2Osugar

NH3

What molecules can NOT get through directly?

polar molecules H2O

ions salts, ammonia

large molecules starches, proteins

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Channels through cell membrane Membrane becomes semi-

permeable with protein channels specific channels allow specific

material across cell membrane

inside cell

outside cell

sugaraaH2O

saltNH3

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The diffusion of water across a selectively permeable membrane is called osmosis.

Osmosis: Diffusion of WaterOsmosis: Diffusion of Water

Regulating the water flow through the plasma membrane is an important factor in maintaining homeostasis within a cell.

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Managing water balance Cell survival depends on balancing

water uptake & loss

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Passive TransportPassive TransportWhen a cell uses no energy to move particles across a membrane passive transport occurs.

Concentration gradientPlasma

membrane

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Passive Transport by proteinsPassive Transport by proteins

Passive transport of materials across the membrane using transport proteins is called facilitated diffusion.

Plasma membrane

Channel proteins

Concentration gradient

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Facilitated Diffusion

Diffusion through protein channels channels move specific molecules across

cell membrane no energy needed

“The Bouncer”

open channel = fast transport

facilitated = with help

high

low

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Passive transport by proteinsPassive transport by proteins

The movement is with the concentration gradient, and requires no energy input from the cell.

Concentration gradient

Plasma membrane

Step 1 Step 2

Carrier proteins

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Active TransportActive Transport

Movement of materials through a membrane against a concentration gradient is called active transport and requires energy from the cell.

Plasma membrane

Concentration gradient

Carrier proteins

Cellular energy

Step 1 Step 2

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Active Transport Cells may need to move molecules against

concentration gradient shape change transports solute from

one side of membrane to other protein “pump” “costs” energy = ATP

“The Doorman”

conformational change

ATP

low

high

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Active transportMany models & mechanisms

ATP ATP

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Getting through cell membrane Passive Transport

Simple diffusion diffusion of nonpolar, hydrophobic molecules

lipids high low concentration gradient

Facilitated transport diffusion of polar, hydrophilic molecules through a protein channel

high low concentration gradient Active transport

diffusion against concentration gradient low high

uses a protein pump requires ATP

ATP

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Transport summarysimplediffusion

facilitateddiffusion

activetransport

ATP

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How about large molecules? Moving large molecules into & out

of cell through vesicles & vacuoles Endocytosis ( 2 Types)

1. phagocytosis = “cellular eating”2. pinocytosis = “cellular drinking”

exocytosis

exocytosis

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ENDOCYTOSIS

- Cell membrane surrounds the substances by infolding in the form of vacuole or forms a vesicle.

Two Types of Endocytosis

Phagocytosis – engulf solid particles

Pinocytosis – engulf liquid material

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Transport of Large ParticlesTransport of Large Particles

Endocytosis is a process by which a cell surrounds and takes in material from its environment.

Endocytosis Exocytosis

Digestion

Nucleus

Wastes

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Transport of Large ParticlesTransport of Large Particles

Exocytosis is the expulsion or secretion of materials from a cell.

Endocytosis Exocytosis

Digestion

Nucleus

Wastes

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THE END

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THANK YOU