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TRANSPORT ACROSS CELL

MEMBRANE

Department of Physiology

Mahatma Gandhi Medical College & Hospitals,

Jaipur

FUNCTIONS OF CELL

MEMBRANE

▪ TRANSPORT

▪PROTECTIVE- Forms outermost boundary of the cell organelles

▪DIGESTIVE-Takes in food and excretes out waste products.

▪SELECTIVE PERMEABILITY-

✓ Nonpolar molecules(gases like O2,CO2 and N2,lipids,hormones

and alcohol)dissolve in nonpolar regions of membrane and thus move

rapidly across membrane.

✓ Polar molecules (water soluble substances:ions,glucose,urea etc.)

have much lower solubility and thus penetrate membrane more

slowly.

▪FRAMEWORK for arrrangement of an ordered sequence of protein

molecules in functional pattern.

▪LINKS adjacent cells together by junctional complexes to form

tissues.

PASSIVE TRANSPORT

DIFFUSION

OSMOSIS

ACTIVE TRANSPORT

PRIMARY TRANSPORT

SECONDARY TRANSPORT

VESICULAR TRANSPORT

ENDOCYTOSIS

EXOCYTOSIS

TRANSCYTOSIS

TRANSPORT ACROSS CELL

MEMBRANE

PASSIVE TRANSPORT

Mechanism of transport of substances along the

gradient without expenditure of energy.

Passive transport mechanisms at cell membrane level

are:

▪ Diffusion

▪ Osmosis

DIFFUSION

Passive transport of molecules from area of high

concentration to low concentration.

Two subtypes- SIMPLE and FACILITATED

SIMPLE DIFFUSION

✓Occurs because of random thermal motion(Brownian motion) of

atoms or molecules.

✓Net movement ceases when concentration of molecules is equal

everywhere within solution ( Diffusional equilibrium)

✓Random movement of molecules continue after diffusional

equilibrium, but the concentration of molecules throughout solution

remains same.

Adapted from: Vanders textbook of Physiology

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Diffusion through membranes

Diffusion through membranes depends:

▪ Through lipid bilayer

✓ solubility of substance in lipids ex Oxygen,Nitrogen

✓ water and other lipid insoluble can also pass through

the cell membrane

▪ Through protein channels

✓

DIFFUSION THROUGH PROTEIN CHANNELS

✓Protein channels extend from extracellular to intracellular ends.

✓They are equipped with following characteristics:

•Selective permeability

•Gated mechanism

SELECTIVE PERMEABILITY: Permit only one type of ion to pass through it.

Highly selective. e.g. Sodium channels(selective for passage of Na+)

GATED MECHANISMS:

✓Ion channel exists either in open or close state which subsequently

changes the membrane permeability.

✓Three factors can alter channel protein conformation:

▪Voltage gated

▪Ligand gated

▪Mechanical gated

Adapted from: Vanders textbook of Physiology

GATED CHANNELS- VOLTAGE GATED:-

Respond to the electric potential across the cell membrane

e.g.gates in Na+ channels are located on outer side.

GATED CHANNELS- LIGAND GATED

➢Allosteric or covalent change in shape of channel protein

due to binding specific molecules(ligands).

➢Ligands may be:

•External (first messengers) e.g. Ach channels

•Internal (second messengers) e.g. intracellular Ca+2,

cAMP,Gprotein

GATED CHANNELS- MECHANICAL GATED

➢Stretching or pressure application may affect

conformation of some channel proteins.

➢Role in cell movements.

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MEDIATED TRANSPORT SYSTEMS

No. of molecules(amino acids,glucose) are polar to

diffuse through lipid bilayerand large to diffuse

through ion channels.

Passage of such molecules and nondiffusional

movements of ions is mediated by integral

membrane proteins-transporters via mediated

transport systems.

Two types exist-

▪ FACILITATED DIFFUSION

▪ ACTIVE TRANSPORT Model of mediated transport:- Vanders textbook of physiology

FACILITATED DIFFUSION-

Carrier mediated process that enables molecules,large enough,to flow

though membrane channels by simple diffusion.e.g.

-glucose transport by GLUT across intestinal epithelium.

-transport of glucose into RBC’s,muscles,adipose tissue in presence of

insulin.

Adapted from:- Guyton and Hall

▪Effect of concentration

of substance on rate of

diffusion and facilitated

diffusion.

▪This shows that

facilitated diffusion

approaches a maximum

rate called Vmax

Adapted from: Guyton and Hall

Types of facilitated diffusion

Uniport: eg Facilitated diffusion of Glucose

Symport: Secondary active transport of

glucose.

Antiport : eg- Na-K pump, Na-Ca exchanger in

muscle cells & Na-H exchanger in renal tubules.

Difference between

Simple diffusion

Specificity:

Saturation

Competition

Facilitated diffusion

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FACTORS AFFECTING DIFFUSION

Concentration Gradient

Electric Gradient

Pressure Gradient

Adapted from: Guyton and Hall

Fick’s law of diffusion

Rate of diffusion (J) is directly proportional to the

difference in the concentration of the substance in

two regions( concentration gradient C1-C2) and

cross sectional area (A) and inversely proportion to

the thickness of the membrane(T)

J=D A (C1-C2)

T

Diffusion affecting rate of diffusion

Cell membrane permeability

✓ Thickness of the membrane : inversely

✓ Lipid solubility: directly

✓ Distribution of protein channels in the cell membrane:

directly

✓ Temperature: directly

✓ Size of the molecules: inversely

✓ Area of membrane: directly

DIFFUSION: FACTORS AFFECTING PERMIABILITY

Journal of cell Physiology, 4th ed.

OSMOSIS:-

Passive flow of solvent across selectively permeable membrane

from solution containing low concentration of solutes to high

concentration of solutes.

OSMOTIC PRESSURE:-

Minimum pressure which when applied to side of high

concentration of solutes prevents osmosis.

Adapted from :-Guyton and Hall

Tonicity of fluids:-

Tonicity of fluid with respect to plasma(290mOsm).

Tested in RBC membrane. Thus,

•ISOTONIC

•HYPOTONIC

•HYPERTONIC

Adapted from; jpeg images.com

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ACTIVE TRANSPORT

Transport of substances against chemical/electric

gradient(uphill movement)

Expenditure of energy by breakdown of ATP.

Active transport is of two types-

▪ PRIMARY ACTIVE TRANSPORT e.g. Na+-K+ ATPase

pump, Ca+2 pump.

▪ SECONDARY ACTIVE TRANSPORT e.g. cotransport,

countertransport

PRIMARY ACTIVE TRANSPORT:-

Hydrolysis of ATP by transporter provides energy for

primary active transport

Adapted from:- Vanders textbook of Physiology

Primary active transport in Na+-K+ ATPase pump SECONDARY ACTIVE TRANSPORT:-

Uses electrochemical gradient across cell membrane as its

energy source,rather than phosphorylation of transport

molecule by ATP

Adapted from :-Vanders textbook of Physiology

Secondary active transport:- Na- glucose cotransport

Adapted from: Guyton& Hall

Secondary active transport-

Sodium-amino acid

cotransport

▪Occurs in epithelial cells of

intestinal tract and renal

tubules

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Secondary active

transport-

Sodium-H+ counter

transport

▪Occurs in proximal tubules

of kidney

Movement of solutes across cell memb. involving membrane proteins.

Adapted from: Vanders textbook of Physiology

VESICULAR TRANSPORT

Transport of macromolecules which can neither pass

through cell membrane by diffusion nor by active

transport.

Three subtypes:

ENDOCYTOSIS

EXOCYTOSIS

TRANSCYTOSIS

ENDOCYTOSIS:

Process of transport into cell by infolding of cell

membrane. Three subtypes:

PHAGOCYTOSIS

PINOCYTOSIS

RECEPTOR MEDIATED

ENDOCYTOSIS

PINOCYTOSIS (cell drinking):

Process of engulfing liquid substances by infolding of cell

membrane

Adapted from : Guyton and Hall

PHAGOCYTOSIS (cell eating):

Process of engulfing of solid particles. Three steps are

involved:

1.Attachment

2.Engulfment

3.Degradation

Adapted from: Guyton and Hall

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DIGESTION OF VESICLES BY LYSOSOMES

Adapted from: Guyton & Hall

RECEPTOR MEDIATED ENDOCYTOSIS: (POTOCYTOSIS)

Substance to be transported binds to special receptor

protein present on cell surface. Eg vitamins

Adapted from ; Vanders textbook of Physiology

EXOCYTOSIS:

Substances are expelled from the cell without passing through cell

membrane. Requires energy and Ca

Two pathways of secretion:

•Nonconstituve pathway – Golgi Apparatus to secretory vesicles'

(Regulated secretion)

•Constituve pathway

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TRANSCYTOSIS (CYTOPEMISIS):

Vesicular transport within the cell.

Three steps involved:

1. Vesicle formation

2. Vesicle transportation

3. Docking in the cell

VARIOUS PROTEINS INVOLVED IN VESICULAR

TRANSPORT

•AP-1clathrin

•AP-2clathrin

•CO-PI and CO-PII

•Dynamin

•Docking protein

TRANSPORT ACROSS EPITHELIA

Movement of substances from one side of epithelium

to other.

Occurs in body cavities.

Occur in two ways:

▪ Transport through cell proper

▪ Transport through tight junction

INTERCELLULAR COMMUNICATION

Intercellular communication mediated by messengers are of

following types:

Adapted from: Grey’s anatomy

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Prominent parts of intercellular connections are CELL

ADHESION MOLECULES (CAMs).

TYPES OF CAMs-

▪Integrins

▪Adhesion molecules of IgG subfamoly

▪Cadherins

▪Selectins

TYPES OF INTERCELLULAR JUNCTIONS:

1. TIGHT JUNCTIONS

▪ Zona occludens

▪ Present at apical

surface of epithelium

E.g.- BBB, Blood renal

barrier.

Adapted from; Vanders textbook of Physiology

2. ADHERENS JUNCTION

▪Zonula adherens

▪Cell membranes of adjacent

cells are separated by 15-

20nm space.

Two types:

•Desmosomes – adherens

junctions present on both

apposing membranes.

•Hemidesmosomes – junctions

are seen on one of adjacent

cells.

Adapted from :- Vanders textbook of Physiology

3.GAP JUNCTION

▪Nexus

▪Present on lateral surface of

two adjacent membranes.

▪Each half of channel is

surrounded by 6subunits of

proteins.

▪Reduces intercellular space

from 15-20nm to2-3nm.

e.g.-cardiac muscle and other

smooth muscle cells.

Juxtaglomerular apparatus in

kidney.

Adapted from: Vanders textbook of Physiology

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