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Movement of Water and Solutes Across Cell Membrane (I)

Study Guyton Ch. 4

Simple diffusionOsmosis

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

• 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 travel5. 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 ()• Osmotic pressure depends on the number (molar

concentration) of particles in solution = 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 TransportStudy 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