Lecture 3 Membrane Transport

By Dr Nazish

A 24 years old boy was severely injured in a road traffic accident and

had excessive blood loss. He was brought to the emergency

department. On examination he had a very weak pulse and his blood

pressure was 80/50mmHg.His blood group was B+ve

LECTURE OBJECTIVES

Types of membrane transportSimple diffusion.

OsmosisOsmotic pressue

Osmolarity and tonicityComposition of body fluids

Facilitated diffusion

Membrane transport

Simple diffusion Active

transport

Facilitateddiffusion

osmosis

PrimaryActive

transport

SecondaryActive

transport

SodiumPotassium pump

Co transport

Countertransport

Vesicular transport

passive active

Membrane Transport

Unassisted TransportNo need of carrier

(Simple diffusion and osmosis)Assisted Transport

- Carrier mediated transport(Facilitated diffusion and active transport)

- Vesicular transport

Diffusion Occurs along Concentration gradientConcentration gradient

• Random movement of molecules.• Down concentration gradient. • To restore steady state .

Ions can diffuse down electrical gradient Ions can diffuse down electrical gradient ((A difference in charge )

Simple diffusion

• Occurs through the lipid bilayer or through the protein channels

Channel protein

Carrier proteins

Simple diffusion

Facilitated diffusion

Activetransportdiffusion

Fick’s law of diffusion

Net rate of diffusion (Q)∆C x P x A

MW x ∆X=

By Fick’s law of diffusion: Factors affecting the rate of diffusion.• the magnitude of the concentration gradient (∆C) • the permeability of the plasma membrane to a

substance. ( P)• the surface area of the membrane across which

diffusion takes place. (A)• the molecular weight of a substance. (MW)• the distance through which diffusion takes place

(∆X)

Simple Diffusion

It is the movement of the molecules down the concentration, electrical or pressure gradient with the use of the kinetic motion of the molecules.

Osmosis: the net diffusion of water down its own concentration gradient.

Membrane (permeable to both water and solute)

Higher H2O concentration,lower solute concentration

H2O moves down concentration gradient

Solute moves down concentration gradient

• Water concentrations equal• Solute concentrations equal• No further net diffusion• Steady state exists

Side 1 Side 2

= Water molecule

= Solute molecule

Figure 3.16 Page 76

H2O

Solute

= Water molecule

= Solute molecule

Membrane (permeable to H2O but impermeable to solute)

Higher H2O concentration,lower solute concentration

H2O moves down its concentration gradient

• Water concentrations equal• Solute concentrations equal• No further net diffusion• Steady state exists

Solute unable to move

Side 1 Side 2

Side 1 Side 2

Originallevel ofsolutions

H2O

= Water molecule

= Solute molecule

Membrane (permeable to H2O but impermeable to solute)

Pure water Lower H2O conchigher solute conc

Originallevel ofsolutionsH2O

• Water concentrations not equal• Solute concentrations not equal• Tendency for water to diffuse by osmosis into side 2 is exactly balanced by opposing tendency for hydrostatic pressure difference to push water into side 1• Osmosis ceases• Opposing pressure necessary to completely stop osmosis is equal to osmotic pressure of solution

Hydrostatic(fluid)pressuredifference

Osmosis

Hydrostatic pressure

Osmotic pressure

Pressure required to stop osmosis is called as osmotic pressure.

osmolarity

• It is the number of osmoles per liter of the solution.

• Osmolarity of human body is 300 mosmoles/litre

Cell

Interstitial fluid (ISF)

Blood vessel

Plasma

Extracellular fluid (ECF) (=ICF & Plasma)

Intracellular fluid (ICF)

Are identicalExcept thatISF lacks(PP) plasma proteinsPresence of PPonly in plasma creates colloidosmotic pressure=25 mmHg

IONIC COMPOSITION OF ECF & ICF ARE

COMPLETELY DIFFERENT

BUTosmolarity is the same as number

of particles /L is the same

Pla

sma

mem

bra

ne

Extracellular fluid

Intracellular fluid

Concentrations of ions in body Concentrations of ions in body fluids (mM/liter)fluids (mM/liter)

Extracellular fluidExtracellular fluid intracellulaintracellularr

IonIon Plasma Plasma interstitiainterstitiall

NaNa++ 145145 1414

K K ++ 4.24.2 140140

Ca Ca + ++ + 2.52.5 0!0!

cl cl -- 110110 44

ProteinsProteins-- 1414 <0.1<0.1 4545

Tonicity of a solution is the effect the solution has on cell volume.

• An isotonic solution has the same concentration of nonpenetrating solutes as normal body cells.

• A hypotonic solution has a lower concentration of nonpenetrating solutes compared to normal

body cells.• A hypertonic solution has a higher concentration

of no penetrating solutes compared to normal body cells.

A 24 years old boy was severely injured in a road traffic accident and had excessive blood loss.

He was brought to the emergency department. On examination he had a very weak pulse and his blood

pressure was 80/50mmHg.His blood group was 0-ve.

Doctors were not able to find out his matching blood group in emergency.

What type of fluid imbalance do you expect in this patient.What will happen to the volume and shape of his body

cells.What would be the effects on osmolarity of the ECF and ICF

of the patient.What type of the fluid would be given to the paient to save

his life.

ASSISTED TRANSPORT MECHANISMS

Facilitated diffusion is a type of assisted transport.

• By this process a substance moves from a higher to a lower concentration.

• Unlike simple diffusion, facilitated diffusion requires a carrier molecule.

• Glucose is transported into cells by facilitated transport.

Step 1

Conformation X ofcarrier (binding sitesexposed to ECF)Molecule to betransported binds tocarrier

Molecule to betransported

Concentrationgradient

Plasmamembrane

Carrier molecule

(Low)

(High)ECF

ICF

Figure 3.19 (1), Page 79

Step 2

On binding withmolecules to betransported, carrierchanges itsconformation

Conformation X of carrier Conformation Yof carrier

Figure 3.19 (2), Page 79

Step 3

Conformation Y ofcarrier (binding sitesexposed to ICF)Transported moleculedetaches from carrier

Direction oftransport

ECF

ICF

Figure 3.19 (3)Page 79

Step 4

ECF

ICF

Conformation X ofcarrier (binding sitesexposed to ECF)After detachment,carrier reverts tooriginal shape

Figure 3.19 (4)Page 79

Click to view animation.

Carrier Mediated

Animation

CHARACTERISTICS OF CARRIER MEDIATED TRANSPORT

• Specificity

• Competition

• Saturation

Simple diffusion down concentration gradient

Rate oftransportof moleculeinto cell

Concentration of transportedmolecules in ECF

Carrier-mediated transport down Concentrationgradient(facilitated diffusion)

Low High

Tm

Figure 3.20Page 80

Thank youAnd

Love you all

Thank youAnd

Love you all