cell membrane.drmamtasingh

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CELL MEMBRANE

DR. MAMTA SINGHPGT-1

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INDEXINTRODUCTIONHISTORYSTRUCTURECOMPOSITIONTRANSPORT PROCESSES PASSIVE TRANSPORT ACTIVE TRANSPORT FUNCTIONS CONCLUSION BIBLIOGRAPHY

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THE CELL:“BUILDING BLOCK OF

LIFE”

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Basic structural & functional unit of living organism.

Smallest unit of life that is capable of carrying out all the activities necessary for life.

Formed by a cell body and a membrane covering the cell body called cell membrane.

CELL

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Cell body : Cytoplasm and nucleus

Cytoplasm consists of various membrane bound cell organelles which can be single layered or double layered.

Cell membrane is double layered.

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CELL MEMBRANEAlso called as plasma membrane or

plasmalemma

Separates the cell from its surroundings.

Thickness : 7.5nm

Exhibits semi permeability .

Bilayered

2 Outer thin electrondense layers (2.5-3nm)

Separated by an electron lucent Intermediate zone (3.5-4nm)

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HISTORYPfeffer (1877) gave the term plasma

membrane.

Bilaminar structure: Jim Danielli and Hugh Davson (1935)

Fluid Mosaic Model : S.J Singer and J.L Nicolsan (1972)

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The Fluid-Mosaic Membrane Model

FLUID MOSAIC MODEL- S.J. Singer and G.L. Nicolson (1972) is the most accepted model.

States that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it.

A fluid mosaic model

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STRUCTURE

Composed of a phospholipid bilayer with a

collage of many different proteins, lipids

and carbohydrates.

Phospholipids are the most abundant

lipids in the plasma membrane.

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CompositionLipids

◦Phospholipid bilayer◦Cholesterol

Proteins◦Integral ◦Peripheral◦Transmembranous

Carbohydrates◦Glycoproteins and glycolipids

Lipid BilayerSheet of phosholipids.

The hydrophilic phosphate heads point “out” to the water on either side of the bilayer

The hydrophobic tails point “in” to the core of the bilayer.

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Phospholipid Structure

Composed of 1 glycerol molecule, 2 fatty acids and 1 phosphate group.

Amphipathic molecule: Both hydrophilic and hydrophobic regions are present.

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Phospholipid Structure

Fatty

Aci

d

Fatty

Aci

d

Glycerol

Polar groupGlycerol

Fatty

Aci

d

Fatty

Aci

d

Fatty

Aci

d

Change from Triglyceride to Phospholipid

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Cholesterol

Structural Integrity

Cholesterol

Functions of membrane lipids

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LIPIDS form the framework of biological membranes.

Anchor soluble proteins. Store energy.

Carry information as intracellular hormones

MEMBRANE PROTEINS

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• A protein molecule that is attached to, or associated with the membrane of a cell or an organelle.

Membrane proteins can be divided into several categories:1. Integral Proteins2. Peripheral Proteins 3. Transmembranous Proteins

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• Integral membrane proteins -which are permanently bound to the lipid bilayer.

• Peripheral membrane proteins- that are temporarily associated with lipid bilayer or with integral membrane proteins. Cell surface identity markers.

• Transmembrane proteins - some of the protein run throughout the lipid bilayer

Carrier Proteins•Move specific molecules through the membrane one at a time. Channel proteins

•Channel proteins extend through the bilipid layer.•They form a pore through the membrane that can move molecules in several ways.

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Outside

Plasmamembrane

InsideTransporter Signal Transduction

Cell surface receptorEnzyme

Cell surface identitymarker

Attachment to thecytoskeletonCell adhesion

Functions of Plasma Membrane Proteins

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MEMBRANE CARBOHYDRATES

Usually branched molecules of 15 or less sugar units.

Some are bonded to lipids: Glycolipids.Most are bonded to proteins:

Glycoproteins.Function: Cell-cell recognition.

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TRANSPORT

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MEMBRANE TRANSPORT

Collection of mechanisms that regulate the passage of solutes via plasma membrane.

Membrane structure- well suited for transport of substances in and out of cell.

(selectively permeable).

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TRANSPORT

PASSIVE ACTIVE BULK TRANSPORT

FACILIATED DIFFUSION

OSMOSIS

Diffusion

Simple(via lipid layer )

Channel (via protein channels)

Exocytosis Endocytosis

Primary Secondary

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Solutions and Transport-Solution – homogeneous mixture of two or

more components◦Solvent – dissolving medium◦Solutes – components in smaller quantities within a solution

• Intracellular fluid – nucleoplasm and cytosol

Extracellular fluid◦Interstitial fluid – fluid on the exterior of the cell within tissues

◦Plasma – fluid component of blood

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PASSIVE TRANSPORTAlso called as downhill movement.Along the concentration gradient

◦Substances move from [high][low]◦No energy input required.

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Diffusion: Passive movement of molecules from a

region of high concentration to a region of low concentration.

(Concentration gradient is the difference in concentration between the two regions)

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Diffusion of Dye in Water

Time 0Steep

ConcentrationGradient

Time 1Reduced


Dispersing

Time 2No


RandomDispersal

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Diffusion Processes

1. Passive Diffusion- ( A) Through the lipid layer

(B) Diffusion through protein channels

2. Facilitated transport

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Simple Diffusion through lipid layer

Nonpolar, hydrophobic molecules diffuse directly through the lipid bilayerSimple diffusion does not require the use of

transport proteins.Examples: O2, CO2, steroids

Polar, hydrophilic substances cannot pass directly through the lipid bilayer Examples: water, ions, carbohydrates

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Simple DiffusionThrough Lipid Layer

(extracellular fluid)

(cytoplasm)

Some molecules

diffuse freelyacross

Some moleculesdiffuse freely

across

(lipid soluble molecules, CO2, O2, H2O )

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Simple Diffusion Through Lipid Layer

small, nonpolar molecules

(ex. O2, CO2)

Polar molecules(ex. Glucose, water)

ions(ex. H+, Na+, K+)

LIPID-SOLUBLE WATER-SOLUBLE

LIPID-SOLUBLE

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Simple Diffusion Through Protein Channels: Channel TransportProtein channels:•Throughout the central lipid layer there are pores.

•These pores form channels for diffusion of H2O and electrolytes.

•Integral protein molecules invaginate into these pores from either surface of cell membrane.

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Types of ChannelsSome are continuously opened : Ungated Channels

Most of the channels are always closed : Gated Channels

Gated channels open only when required.

As channels are lined by protein molecules so called as Protein Channels.

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Diffusion through lipid layer

Diffusion through ungated channel

Diffusion through gated channel

Facilitated Diffusion: Passive Transport Aided by Proteins

In facilitated diffusion, transport proteins speed movement of molecules across the plasma membrane

Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane.

Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane.

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Facilitated Diffusion: Use of Carriers to Assist with Diffusion

Carrier proteinhas binding sitefor molecule

Molecule entersbinding site

Carrier protein changesshape, transportingmolecule across membrane

Carrier protein resumesoriginal shape

(Inside Cell)(Inside Cell)

(OutsideCell)

(OutsideCell)

DiffusionChannelProtein

DiffusionChannelProtein

Diff

usio

nG

radi

ent

Molecule inTransit

Molecule inTransit

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Osmosis

Diffusion of water across a differentially permeable membrane.

Water moves from [high] [low]

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“Bound” water moleculesclustered around sugar:cannot fit through pore

“Free” watermolecule: can fitthrough pore

Osmosis TerminologyTonicity is the ability of a solution to cause a

cell to gain or lose waterIsotonic solution: solute concentration is the

same as that inside the cell; no net water movement across the plasma membrane

Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water

Hypotonic solution: solute concentration is less than that inside the cell; cell gains water

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Normal RBCs

Isotonic Solution

The Effects of Osmosis

Equal movement of waterinto and out of cells

Net movement ofwater out of cells Net movement of

water into cells

Shriveled RBCs Swollen RBCs

Hypertonic Solution Hypotonic Solution

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ACTIVE TRANSPORTUPHILL TRANSPORTThe movement against concentration

gradient.

Energy is required for the process.

Occurs in case of both ions and non ions.

It requires carrier protein (it binds to the substances being transported)

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TYPES OF ACTIVE TRANSPORT-

According to the source of energy1. PRIMARY ACTIVE TRANSPORT2. SECONDARY ACTIVE TRANSPORT(coupled

transport)

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PRIMARY ACTIVE TRANSPORTThe energy is derived directly from

breakdown of ATP. e.g.- sodium potasium pump.

Na+-K+ pump : A transport process that pumps 3 Na ions outward through the cell membrane and at the same time pumps 2 K+ ions from outside to the inside.

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This pump is responsible for maintaining the Na+ and K+ concentration differences across the cell membrane as well as for establishing a negative electrical potential inside the cells.

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The Sodium-potassium Pump One type of active transport system

2. Na+ binding stimulatesphosphorylation by ATP.

1. Cytoplasmic Na+ binds to the sodium-potassium pump.

6. K+ is released and Na+

sites are receptive again; the cycle repeats.

3. Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside.

4. Extracellular K+ binds to the protein, triggering release of the Phosphate group.

5. Loss of the phosphaterestores the protein’s original conformation.

P

EXTRACELLULARFLUID

Na+

CYTOPLASM

[Na+] low[K+] high

Na+

Na+

Na+

Na+

Na+

P ATP

Na+

Na+

Na+

P

ADP

K+

K+

K+

K+ K+

K+

[Na+] high[K+] low

P i

P i

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Secondary active transport- When sodium is transported by a carrier

protein, another substance is also transported by the same protein either in the same direction or in the opposite direction.

Thus transport of sodium is coupled by transport of other substance.

Can be of 2 types1. Co- transport : By Symport 2. Counter Transport: By Antiport

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SODIUM CO-TRANSPORTA. Na+ and glucose from ECF bind to

carrier protein.B. Conformational changes in Carrier

proteinC. Na+ and glucose released into ICF

SODIUM COUNTERTRANSPORTA. Na+ from ECF and H+ from ICF bind

with CPB. Conformational changes occur in CPC. Na+ enters ICF and H+ enters ECF

DiffusionFacilitated diffusion

Passive transport

ATP

Active transport

Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.

BULK TRANSPORTBulk transport occurs by endocytosis and

exocytosis.

Small molecules and water enter or leave

the cell through the lipid bilayer or by

transport proteins.

Large molecules, such as polysaccharides

and proteins, cross the membrane via

vesicles.

ENDOCYTOSIS

In endocytosis, the cell takes in

macromolecules by forming vesicles from

the plasma membrane

Endocytosis is a reversal of exocytosis,

involving different proteins.

• Three types of endocytosis:

◦Phagocytosis (“cellular eating”): Cell

engulfs particle in a vacuole

◦Pinocytosis (“cellular drinking”): Cell

creates vesicle around fluid

◦Receptor-mediated endocytosis:

Binding of ligands to receptors triggers

vesicle formation

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Endocytosis


Pinocytosis

vesicle containingextracellularfluid)(cytoplasm)

12 3

cell


(cytoplasm)

1

23 4

coatedpit forming

Receptor-mediatedEndocytosis

Phagocytosis

coatedvesicle

nutrientsreceptors

food particle

pseudopodparticle enclosedin food vacuole

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Process of Phagocytosis

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Receptor-mediated Endocytosis I

Begins with a shallowpit in plasma membrane

Pit is coated withprotein

(cytoplasm)


coatedpit

proteincoating

extracellular particlesbound to receptors

plasma membrane

Pit deepens

aa bb

0.1 Micrometer

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Receptor-mediated Endocytosis II

Pit deepens further andbegins to pinch off

Eventually becomes acoated vesicle

ddcc

0.1 Micrometer

coatedvesicle

Exocytosis

In exocytosis, transport vesicles migrate to

the membrane, fuse with it, and release

their contents

Many secretory cells use exocytosis to

export their products

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Exocytosis

cytoplasm

1

Vesicle


plasma membrane

2

SecretedMaterial

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MEMBRANE FUNCTIONS: SUMMARY

Protective functionSelective permeabilityAbsorptive functionExcretory functionExchange of gasesHomeostasisMaintenance of shape and size of cell

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CONCLUSION

Important limiting structureProtects the cellOrganises the chemical activities of cellMaintains a balance between cell and

outside

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cell membrane.drmamtasingh

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