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Movement of Water and Solutes
Across Cell Membrane (I)
Study Guyton Ch. 4
Simple diffusion
Osmosis
Facilitated Transport Passive (or facilitated diffusion)
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Introduction
• Knowledge and understanding of the material in ppt outline on Cell
membranes is a prerequisite for this series of slides.
• This presentation is on Diffusion, Osmosis and facilitated
transport passive (also called Facilitated diffusion)
•
• This presentation is contains 16 slides
• The next slide summarizes some of the general functions of
membrane proteins (from Marieb’s Human Anatomy and
Physiology)
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Some Function(s) of Membrane Proteins
(from a physiology text by Marieb)
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Simple Diffusion
(see next slide for illustration)
•
•
Diffusion (simple) may be defined as the net flow of solutes down a
concentration gradient until equilibrium is reached.
In diffusion, the movement of the solute across the membrane:
 Is imited to Small and nonpolar molecules and
 Is Not assisted by membrane proteins
 For some specific examples (see next slide)
•
Remember solute criteria for selection by lipid bilayer (Lecture notes!)
 Size of the solulte (MW)
 Partition coefficient of the solute
 Polarity or charge
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•
As illustrated in the next slide, notice that membrane is impermeable to larger
uncharged but polar solutes like AA, Glucose, Nucleotides and to ions.
Transport of these solutes is mediated by membrane proteins (#6)
Diffusion is driven by kinetic energy (thermal energy of motion)
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Relative Permeability of Synthetic
Bilayers to Some Solutes (Fig. 12.2)
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Mechanisms by Which Solutes & H2O Move
Across Cell Membranes
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Rate of Diffusion: Fick’s Law of
Diffusion
• For the diffusion of a solute (S) from the extracellular
fluid (ecf) to the intracellular fluid (icf), the rate of
diffusion is given by the formula:
 Vinward = P∆S
 Where ∆S = [S]ecf - [S]icf
 and P is the permeability constant of the solute. P depends
on solute’s partition coefficient and molecular weight
• Note that units for rate of diffusion are in
 moles sec-1 cm-2 of membrane surface
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Factors That Influence Rate of Diffusion
•
Six factors will affect/influence the rate of diffusion:
1. Temperature (T)
2. Magnitude of the solute concentration gradient across the
membrane
3. Surface area of the membrane across which diffusion occurs
4. Thickness of the membrane or distance across which the solute
must travel
5. Partition coefficient of the solute (PC)
1. Examples: urea vs methylurea vs dimethylurea
6. MW of the solute
•
Diffusion is inversely proportional to the thickness of the
membrane and to the square root of the solute’s MW
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Osmosis
• Osmosis may be defined as the net flow of H2O molecules across
semipermeable membrane from compartment with lower solute
concentration towards a compartment with higher solute
concentration, until equilibrium is reached
• The driving force for osmosis is Osmotic pressure (p)
• Osmotic pressure depends on the number (molar concentration) of
particles in solution
p = RT (kic)
 Where “R” is the gas constant, “T” is the temperature in Kelvin, “k” is the
osmotic coefficient of the solute, “i” is the number of ions formed by dissociation
of a solute molecule, “c” is the molar concentration in moles/L
 R = 0.08206 L-atm/K-mole and K = 273 + Co
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Osmostic Swelling & Bursting of a Cell
in Hypotonic Solution
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Transport Proteins & Diffusion
• Facilitated diffusion is a type of transport process that is mediated by
membrane proteins
• These proteins are given such names as permeases, transporters,
carriers or channels. They have the following characteristics:




They are integral multipass membrane proteins
Some* are allosteric (alternate between two conformational states)
They have specific binding site(s) for one or more solutes
They are specific for a single solute or for a small group of closely
related solutes (examples?)
 They have a given Km and the transport process that they mediate
follows Michaelis-Menten kinetics
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Facilitated Diffusion: Definition &
Characteristics
• Facilitated diffusion is the net flow of solutes down a concentration
gradient (for uncharged solutes) or an electrochemical gradient
(for charged solutes) until equilibrium is reached (compare with
simple diffusion).
• It is mediated by carriers or by channel proteins
• The direction of solute flow is dictated by the direction of the solute
gradient, not by the protein
• It is exergonic, therefore has a negative G and requires no input of
energy
• It follows Michalis- Menten kinetics, meaning that
 The process has a Vmax, has a Km and can be competitively inhibited
by structurally related solutes.
• The next slide gives some formulas that will be useful for you to
know during the term when we cover these topics.
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Some Useful Formulas
• For transport of solutes into the cell
 Soutside - Sinside
• Free Energy change ( E) for non-polar solutes
 G = Go + RT ln [S]inside/ [S]outside
 Where Go is the free energy change calculated under standard
conditions when concentrations are at equilibrium
 [S] is the prevailing solute concentration inside and outside the cell
 R is the gas constant = 1.987 Cal/mole-K
 K = 273 + Centigrade
• Free Energy change ( E) for charged solutes
 G = Go + { RT ln [S]inside/ [S]outside } + zFVm
 Where z is the charge on the ion, Vm is the membrane potential and F
is Farraday’s constant = 23,062 cal/mole-V
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Types of Facilitated Transport
Study Guyton Ch. 4
• In facilitated transport, cells use transport proteins called:
• Uniporters and Cotransporters (or coupled
transporters)
• Uniporters transport a single solute
• Cotransporters simultaneously transport two or more
solutes. Categories of cotransporters include:
 Symporters: For transport of solutes in the same direction
 Antiporters: For transprt of solutes in opposite direction
• These will be discussed in the next PPT along with Active
transport and types of channel proteins.
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The End Diffusion and Osmosis
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