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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.