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 transport Simple diffusion. Osmosis Osmotic pressue Osmolarity and tonicity Composition of body fluids Facilitated diffusion

Membrane transport active passive Simple diffusion Vesicular transport Active transport Facilitated diffusion Primary Active transport Secondary Active transport osmosis Sodium Potassium pump Co transport Counter transport

Membrane Transport Unassisted Transport No need of carrier (Simple diffusion and osmosis) Assisted Transport - Carrier mediated transport (Facilitated diffusion and active transport) - Vesicular transport

Diffusion Occurs along Concentration gradient Random movement of molecules. Down concentration gradient. To restore steady state .

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

Simple diffusion Occurs through the lipid bilayer or through the protein channels

Carrier proteins Channel protein Facilitated diffusion Simple diffusion Active transport diffusion

Fick’s law of diffusion ∆C x P x A = Net rate of diffusion (Q) 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.

Figure 3.16 Page 76 Membrane (permeable to both water and solute) H2O moves down concentration gradient H2O Solute Solute moves down concentration gradient Higher H2O concentration, lower solute concentration Side 1 Side 2 • Water concentrations equal • Solute concentrations equal • No further net diffusion • Steady state exists = Water molecule = Solute molecule

Membrane (permeable to H2O but impermeable to solute) Side 1 Side 2 H2O moves down its concentration gradient H2O Solute unable to move Higher H2O concentration, lower solute concentration Side 1 Side 2 Original level of solutions • Water concentrations equal • Solute concentrations equal • No further net diffusion • Steady state exists = Water molecule = Solute molecule

• Water concentrations not equal • Solute concentrations not equal Membrane (permeable to H2O but impermeable to solute) Hydrostatic (fluid) pressure difference Original level of solutions H2O Osmosis Hydrostatic pressure Pure water Lower H2O conc higher solute conc • 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 = Water molecule = Solute molecule

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

Interstitial fluid (ISF) Are identical Except that ISF lacks (PP) plasma proteins Presence of PP only in plasma creates colloid osmotic pressure =25 mmHg Interstitial fluid (ISF) Plasma Cell Blood vessel Extracellular fluid (ECF) (=ICF & Plasma) Intracellular fluid (ICF)

Extracellular Intracellular fluid fluid IONIC COMPOSITION OF ECF & ICF ARE COMPLETELY DIFFERENT BUT osmolarity is the same as number of particles /L is the same Plasma membrane

Concentrations of ions in body fluids (mM/liter) Extracellular fluid intracellular Ion Plasma interstitial Na+ 145 14 K + 4.2 140 Ca + + 2.5 0! cl - 110 4 Proteins- <0.1 45

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.

Figure 3.19 (1), Page 79 Step 1 Molecule to be transported Concentration gradient ECF (High) Conformation X of carrier (binding sites exposed to ECF) Molecule to be transported binds to carrier Plasma membrane ICF (Low) Carrier molecule Figure 3.19 (1), Page 79

Figure 3.19 (2), Page 79 Step 2 Conformation Y of carrier Conformation X of carrier On binding with molecules to be transported, carrier changes its conformation Figure 3.19 (2), Page 79

Figure 3.19 (3) Page 79 Step 3 Direction of transport ECF Conformation Y of carrier (binding sites exposed to ICF) Transported molecule detaches from carrier ICF Figure 3.19 (3) Page 79

Figure 3.19 (4) Page 79 Step 4 ECF Conformation X of carrier (binding sites exposed to ECF) After detachment, carrier reverts to original shape ICF Figure 3.19 (4) Page 79

Click to view animation. Carrier Mediated Click to view animation.

CHARACTERISTICS OF CARRIER MEDIATED TRANSPORT Specificity Competition Saturation

Figure 3.20 Page 80 Simple diffusion down concentration gradient Carrier-mediated transport down Concentration gradient (facilitated diffusion) Tm Rate of transport of molecule into cell Low High Concentration of transported molecules in ECF

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