PASSIVE TRANSPORT OF WATER AND

SOLUTEGROUP 13

DIFFUSION

Definition: passive process of movement of particle down concentration gradient by Brownian motion due to the particle’s kinetic energy.

Both solute and solvent undergo diffusion May occur through membrane (e.g. plasma

membrane) , may occur without membrane(e.g. diffusion of dye in water).

Concept of diffusion

Flux: Rate of material movement (amount of material crossing a surface per unit time)

One-way flux: rate of movement of a substance in one direction depending on its concentration

Magnitude and direction of diffusion

Net flux : Difference between one-way fluxes, indicate the direction with greater movement of solute in a unit time.◦ The greater the difference between the two one-way fluxes the

greater the concentration gradient.

Diffusion Equilibrium: net flux= 0◦ Two one-way fluxes equal in magnitude but

opposite direction

Magnitude and direction of diffusion

Plasma Membrane

General functions

Selective physical barrier

Involved in communication between the cell

and its environment.

Confines the products of specific cell organelles.

Regulates exchange

between cells and the

environment.

Structural support

GENERAL FUNCTIONS OF CELL MEMBRANE

Membrane Structure

Membrane Composition

Plasma Membrane

Phospolipid Bilayer

•Amphipathic molecules •Pol

ar heads (charged)

•2 long non-polar fatty acid tails.

In aqueous environmen

t

• Non-polar fatty acid chains in the middle

• Polar heads interact with the water molecules ( ECF & cytosol )

Fluid

Mosaic

Model

•No chemical bonds linking the phospolipid

• Each molecule is FREE to move independently.

MEMBRANE STRUCTURE

MEMBRANE COMPOSITION

Phospholipid Bilayer

• Formation of vesicles (by ‘pinching off’ parts of the membrane. )

Cholesterol

• Integral membrane protein

• Peripheral membrane proteins.

Proteins

INTEGRAL PROTEINTightly bound

into the membrane phospolipid.

Amphipathic molecules

PERIPHERAL

PROTEINSLoosely attached to polar regions

of the phospholipid.

Not amphipathic molecules

Most span entire membrane: Transmembrane proteins

Cytosolic surface of the membrane

5 FACTORS AFFECTING THE DIFUSSION RATE

OF SUBTANCES ACROSS THE PLASMA

MEMBRANE

1. Steepness of concentration

gradient-

2. Temperature

3. Surface area

4. Molecular size

5. Distance of travel

DIFFUSSION THROUGH THE PHOSPHOLIPID

BILAYER

i. The size of the moleculeii. Lipid solubility or polarity

of the molecule

Two properties of the solutes that will

determine this process

a)Lipid soluble/ non-polar molecules :

(e.g. Oxygen, carbon dioxide, fatty acid, steroid hormones )

- the permeability is proportional to their solubility in the interior of the lipid bilayer- when solubility increase , permeability also increases

b) Very small or non-polar molecules :- can cross directly through the lipid bilayer- non-polar molecules can dissolve in the non-polar regions of the membrane

c) Larger or polar/lipid non-soluble molecules- cross at a very slow rate or not at all - thus transferred by membrane proteins

The transport of molecules down concentration or electrochemical gradient.

Does not require input of energy. Types of passive transport:

1)Simple diffusion2)Facilitated diffusion3)Osmosis

PASSIVE TRANSPORT ACROSS PLASMA MEMBRANE

Diffusion across membrane

Capillary walls

Continuous capillaries

Fenestrated capillaries

Discontinuous capillary (sinusoids)

Characteristics

Endothelial cells form continuous tube, intercellular cleft, tight junction between endothelial cells.

Fenestrations (small pores 70-100 nm in diameter)

Unusually large fenestration, large intercellular cleft, incomplete or absence of basement membrane.

Location CNS, lungs, skin, muscle tissue

Kidney, villi of small intestine, most endocrine gland

Liver, spleen, anterior pituitary, adrenal gland.

Molecules Less than 10 nm.

20-100 nm. Less than 3000 nm. (allow some protein, blood cells pass through)

Capillary wall: structure & function

©John Wiley and Sons pte ltd

Water- filled channels traverse capillary wall. Location : - intercellular cleft - intracellular fused-vesicle

channels (fenestrations) The size of water-filled channels differ

according to location. Nutrients : from plasma capillary wall

interstitial fluid enter into cells. Metabolic end products : tissues across the

cells’ plasma membranes interstitial fluid across the capillary endothelium plasma.

Means of diffusion through capillary

Molecular structure of solutes.◦ Lipid-soluble substances, including oxygen

and carbon dioxide, easily diffuse through the plasma membranes of the capillary endothelial cells.

◦Transport of ions, small polar molecules and some proteins (poorly soluble in lipid) are through the water-filled channels.

◦permeability of ions and small polar molecules increase with presence of water-filled channels, diffuse at higher rate than proteins, however much lower than lipid-soluble molecule.

Factors affecting diff. rate (capillary)

Variation in size of water-filled channels.◦ The size of water-filled channels differ according

to location.◦ Continuous capillaries of the brain; have no

intercellular clefts, only tight junctions>>very small/no water-filled channel

◦ water-soluble substances, even those of low molecular weight, can gain access to or exit from the brain interstitial space only by carriermediated transport through the blood-brain barrier.

◦ liver capillaries, have large intercellular clefts as well as large fenestrae in the plasma membranes of the endothelial cells; even protein molecules can readily pass across them.

◦ functions of the liver : -synthesis of plasma proteins -metabolism of substances bound to plasma proteins.

Trans-capillary gradient across capillary wall◦ Higher trans-capillary gradient; higher rate of

diffusion.

1. Oxygen/ carbon dioxide◦ At cells: [O2] capillary plasma> [O2] interstitial

fluid, therefore : O2 diffuse from plasma capillary wall interstitial fluid enter cells.

◦ At cells: [CO2] capillary plasma <[CO2] interstitial fluid , therefore : cells interstitial fluid capillary wall plasma.

2. Metabolism ◦ Production and utilization of substance:

[nutrient] plasma > [nutrient] interstitial fluid [waste] plasma < [waste] interstitial fluid.

Substances move through the lipid bilayer without the help of membrane transport proteins.

Types of molecules: I. Small, nonpolar, hydrophobic, lipid soluble molecules.

Oxygen, carbon dioxide, nitrogen gas Fatty acids Steroids Fat soluble vitamins ( A,D,E,K) Alcohols Ammonia

II. Polar molecules Water Urea

Important for:I. Transfer of oxygen and carbon dioxideII. Absorption of some nutrientsIII. Excretion of cellular wastes

1. SIMPLE DIFFUSION

Integral membrane protein assists a specific substance across the membrane.

2 types:i. Channel-mediated facilitated diffusionii. Carrier-mediated facilitated diffusionCHANNEL-MEDIATED FACILITATED DIFFUSION

2. FACILITATED DIFFUSION

Solutes move down its concentration gradient across the lipid bilayer through a membrane channel.

Allow the passage of small, polar, inorganic ions: too hydrophilic to penetrate the nonpolar portion of lipid bilayer.

Membrane channel present:i. SPECIFIC – selective for K+ and Cl- , fewer for Na+ and

Ca2+ii. LIMITED NUMBERS- diffusion is slower than free diffusion through lipid bilayeriii. GATED MEMBRANE CHANNEL

Open and closed gated membrane channel When the gates open,ions diffuse into and out of cell

down concentration gradient Some alternate between open and closed positions,

other regulated by chemical (ligand) or electrical changes (voltage)

CARRIER-MEDIATED FACILITATED DIFFUSION Carrier protein/transporter move a solute down its concentration

gradient across the plasma membrane. No energy input required Undergo changes in shape-to transport substances to other side Net movement stops at equilibrium Carrier-mediated transport exhibits:

i. SPECIFICITY-ability to move only one type of molecule or a group of closely related molecules.

ii. COMPETITION-able to move several members of a related group of substrate but they have to compete for binding sites.

iii. SATURATION Transport maximum : limited number of transporter in

plasma membrane Reached transport maximum when all transporters

occupied-any increase in cincentration gradient does not increse transportation rate.