Week 2 ELM 5: Electrochemical Gradients

Diffusion

The movement of molecules down a concentration gradient, from regions of high concentration to regions of low concentration.

Concentration Gradient

The difference in concentration of a substance across a space.

Entropy

A measure of disorder; diffusion increases entropy as molecules become more dispersed (2nd law of thermodynamics)

Lipid Bilayers

Cell membranes composed of lipids (hydrophobic) that prevent ions from freely crossing.

Facilitated Diffusion

Diffusion across a membrane facilitated by molecules that punch holes in the membrane.

Primary Transporters (Pumps)

Proteins like the sodium pump that actively transport ions across the cell membrane. Use energy (ATP) to move ions against their concentration gradient

Secondary Transporters (Carriers)

Proteins like the sodium-calcium exchanger that couple the movement of one ion to the movement of another. Use electrochemical gradient of one ion to move another ion

Ion Channels

Proteins that form pores in the cell membrane, allowing specific ions to pass through, down their electrochemical gradient. They can be opened by voltage changes or ligand binding

Adolf Fick

Scientist who showed the number of molecules (N) moving across an interface is proportional to the area of the interface (A) and the concentration gradient.

Diffusion Coefficient

A proportionality constant that describes how quickly ions diffuse.

Albert Einstein

Scientist who explained diffusion as a random walk of molecules and described how far a molecule will diffuse over time. He said it depends on whether the molecule moves in one, two, or three dimensions.

Electrophoretic Movement

Ion movement under the influence of an electric field.

Electrochemical Gradient

The total gradient that considers both the concentration gradient and the electrical gradient. Formula: gradient caused by diffusion - gradient caused by electrophoretic movement

Ohm's Law

The principle stating that Current (I) = Volts (V) / Resistance (R) or Current (I) = Volts (V) x Conductance. (Conductance = 1/R)

Conductance

The measure of how easily ions flow through a channel; it is the inverse of resistance (1/R).

Selectivity

The property of an ion channel that determines which ions can pass through (e.g., a sodium-selective channel).

Permeability

The property of an ion channel related to how easily ions pass through. A more permeable channel allows ions to pass more easily.

Molecular motion

Molecules move short distances (Angstroms) for short times (picoseconds) before colliding with each other. Water molecules have an average center-to-center distance ( r ) of about 2.8 Angstroms

1 dimension diffusion in biology

Movement along DNA

2 dimension diffusion in biology

Movement across cellular membrane or within a plane of the membrane surface.

3 dimension diffusion in biology

Movement in liquid (cytosol, extracellular fluid)

Dimension that allows molecules to diffuse further

3D

Dimension that allows molecules to have a greater chance of bumping in to each other

2D

4 key factors that affect the RATE at which ions move across the membrane

• The size of the electrochemical gradient (High EC gradient = high rate of movement)

• The nature of the ion (Negative ion doesn’t want to travel into negative environment)

• Number of open ion channels (more ion channels = more movement)

• Properties of the ion channel (Selectivity; won’t let other ions in, less flow rate, Permeability; more permeable channel, higher flow rate)

What controls the DIRECTION ions move (in or out of the cell)

Electrochemical gradient

Current

Flow of ions

Voltage

Potential difference (You can’t get ion flow unless you have a potential difference)

Relationship between current and resistance (when V is the same)

Inverse. When voltage remains the same, Low resistance = big current (lots of flow). High resistance = low current (little flow)