Module 4: Electrochemical Gradients

Simple Diffusion

• Movement down a concentration gradient

• Molecules move spontaneously from regions of high concentration to low concentrations.  

  • Concentration gradient is dissipated.

• Increase in entropy – 2^ndlaw of thermodynamics

Simple Diffusion and The Presense of A Membrane

• Ions unable to cross lipid bilayer.

• Ionic concentration can massively differ on either side of the membrane.

  • E.g. Ca2+ across a membrane

• Gradient present but isn’t dissipated due to the presence of a bilayer/ membrane.

• If concentrations are different, a gradient form.

Facillitated Diffusion

• Presence of a hole can allow the movement of ions across the membrane, down and concentration gradient.

• Provides a pathway from one side to another.

  • Allows the gradient to dissipate, and equilibrium to be reached.

    • Same Ca2+ conc on both sides

• Pumps, carriers, and ion channels can act as this pathway.

Why Do Ions Spread Out

• Molecules in a liquid are in constant motion due to thermal agitation.

• For water molecules average centre-to-centre distance (r) is 2.8 A

• Water molecules are only able to move short distances before bumping into one another.

What Did Adolf Fick Show About Molecules Moving Across a Membrane

• The n.o molecules moving across an interface/membrane is proportional to:

  • The Concentration gradient

  • The area of diffusion

What Was Einteins View of Diffusion

• Showed that diffusion was due to a random walk of molecules

• Question how far a diffusing molecule will be from its starting point after a given time

  • Found that it was dependent on whether the molecule moved in 1,2 or 3 dimension

• Created 3 equations and found hat the time taken to move a distance is lower when in 3 dimensions - can move faster and further

1D Diffusion

• Diffusion that governs the movement along DNA

  • e.g. transcription factors and ribosome

2D Diffusion

• Diffusion that governs movement in a membrane.

  • Molecules confined to a place.

3D Diffusion

• Diffusion that governs the movement of a liquid

  • Cytosol, ECF

Consequences of Einsteins Equation

• Molecules can diffuse further if they diffuse in 3D rather than 2D

  • I.e. molecules have a greater chance of bumping into each other if they move in 2 rather than 3D dimensions

Implications of 2D/3D Diffusion

• Important for enzymes - catalysts that work by providing a 2D surface that brings molecules closer together allowing them to interact and bump into each other

• Signalling molecules in membranes have more chances of interacting eg. GCPRs - 2D is more confined

  • Molecules have more range if not bound to membrane (3D)

Electrophoresis

• Movement of a charged substance in an electrical field

Factors Affecting The Movement Of Ions

• Presence of an electrical field

• When an electrical field is applied to ions, +ve ions are attracted to negative electrode; and -ve ions are attracted to the positive electrode.

  • Current gives rise to an uneven distribution of ions

Electrophoretic Movement

• Ion movement is under the influence of an electrical field

  • Can aid or hinder diffusion

What Happens To Na+ In The Presence of A Negative Electrode

• Instead of its natural tendency to spread out they:

  • Cluster around the electrode

  • Can move acorss the membrane/ area

Total Gradeint

• The gradient cause by diffusion - the gradient caused by electrophoretic movement

• It is equal to the electrochemical gradient

Na+ Entry Into A Cell

• Cells are negative inside due to a low [Na+]

• The opening of a Na ion channel allows Na+ to diffuse in - has a strong driving force of a strong electrochemical gradient

• It is pushed from one side of the membrane to another

• Electrochemical gradient and the diffusion gradient both aid the movement of ions into the cells

Factors Affecting The Movement of Ions Through A Channel

• Determined by an electrochemical gradient

• Rate depends on:

  • Size of the electrochemical gradient

  • Nature of ion

  • N.o open ion channels

  • Properties of ion channel

Current

• The flow of ions from one area to another

  • Big - many ions flowing per second

  • Small - few ions flowing per second

Voltage

• Potential difference/ gradient between 2 compartments

• The larger the concentration difference - the larger the potential difference

• Ions are unable to flow without a potential difference

  • No difference in concentration

Resistance

• Related to the width of the path

  • Low - wide path

  • High - narrow path

Relationship Between Curret and Resistance

• Low resistance; high current flowing

• High resistance, low current flow at the same voltage

Ohms Law

• Current (I) = Volts (V)/ Resistance (R)

Rearranged Form of Ohms Law:

• Current = Volts (V) x Conductance

  • Conductance 1/R

Effect of The Electrochemical Gradient on the Rate of Ion Movement

• Has an intuitive influence on the rate of ion flow

• Electrochemical gradient = potential difference

  • The higher the potential difference the higher the rate of movement from one compartment to another

• Top Cell: High conc of Na+ outside cell, with a large negative charge inside the cell

  • Concentration and electrochemical gradient favour the movement of Na+ through the ion channel

• Bottom Cell: Lower Conc of Na+ outside cell/ decrease in negative charge inside the cell

  • Reduced electrochemical gradient and reduced rate of movement of ions across the membrane.

Na+

• Has a high concentration outside the cell; negative charge inside the cell

  • High electrochemical gradient - ion readily moves into the cell from the cytoplasm

Cl-

• Has a high concentration outside the cell; negative charge inside of the cell

  • High concentration gradient – favours Cl- movement into the cell

  • The electrical gradient is reduced – favours movement out of the cell.

    • If the -ve charge is large – no movement of Cl- inside the cell.

Effect of the N.o Open Ion Channels

• The more ion channels open, the faster the rate of ion movement across the membrane

  • Directly proportional to n.o. ion channels open

Properties of Ion Channels

• 2 main properties

  • Selectivity and permeability

Selectivity

• Ion channels only let specific types of ions through

• Selectivity filter ensures the correct ions pass through the channel

  • E.g. Na+ channel only lets Na+ through

Permeability

• Difference in the rate of ion passage through the channel

  • Will be selective to the specific ion but will have differing permeabilities - some greater than others

• Linked to Ohm’s Law

  • The greater the ______ the lower the resistance

Ion Channel Diversity

• Many different types, with each being coded for by different genes.

• Have different properties due to different amino acid sequences.

  • Can differ in the rate of ion passage through the channel