module 5: electrochemical gradients

Diffusion

Movement of molecules from a region of high concentration to low concentration.

Facilitated Diffusion

Diffusion through a membrane via protein channels.

Entropy

A measure of disorder (thermodynamics); diffusion increases entropy.

Ionic Gradient

Difference in ion concentration across a membrane.

Primary Active Transport

• Movement of ions against concentration gradient using ATP

• pump proteins

• eg sodium-potassium pump

Secondary Active Transport

• Uses energy from one ion gradient to move another ion

• carrier proteins

• eg sodium-calcium exchanger

Ion Channels

• Proteins that allow ions to cross membranes via facilitated diffusion.

• eg potassium channels

Why do ions spread out?

Molecules in liquids are in constant motion due to thermal agitation

average center-to-center distance between water molecules?

2.8 angstroms (Å) - short distances between each other so doesn’t take long for water molecules before they start bumping into each other (psec)

what did Adolf Fick show

• demonstrated that the number of molecules moving across an interface is proportional to the area of the interface and the concentration gradient

• ie:

• more molecules will move across a surface (like a membrane) if:

  1. The surface area is larger – More space for molecules to pass through.

  2. The concentration difference is bigger – More molecules on one side means a stronger push to the other side.

key idea behind Fick’s Law

Diffusion depends on the interface area and the concentration gradient

what Einstein show regarding diffusion?

Diffusion happens due to the random movement of molecules

How does diffusion depend on the number of dimensions?

In one dimension, movement is slower. In three dimensions, molecules diffuse further because they have more space to move => less chance of colliding

What is the biological significance of diffusion dimensions?

• One-dimensional diffusion happens along DNA.

• Two-dimensional diffusion happens in membranes.

• Three-dimensional diffusion happens in cytosol or extracellular fluid.

How does dimensions relate to catalysts?

Catalysts provide a two-dimensional surface that increases the chances of molecules interacting

Why do signaling molecules in membranes have higher chances of interacting?

They are confined to a two-dimensional plane, which increases the probability of collisions.

Electrophoresis

• Movement of charged particles in an electric field

• electrophoretic movement either adds to or subtractis from diffusion

Electrochemical Gradient (+ equation)

• Total gradient combining concentration and electrical forces

• determines the direction in which ions move in

• (total) electrochemical gradient = gradient caused by diffusion - gradient caused by electrochemical movement

Ohm's Law

V=IR; relationship between voltage

Current (I)

Flow of ions per second.

Resistance (R)

Opposition to ion flow through a channel.

Voltage (V)

Potential difference across a membrane.

Conductance

Reciprocal of resistance

relationship between current and resistance

if you have 2 boxes of same voltage, the current can be different as it’s determined by resistance

factors which determine rate at which ions move across the membrane

• size of electrochemical gradient

  • strongly negative on one side of membrane => ions diffuse into the cell (high electrochemical gradient)

  • weakly negative on one side of the membrane => low concentration gradient so less ion movement (low electrochemical gradient)

• nature of the ion

  • sodium ion channels have higher electrochemical gradient that chloride ion channels (see image)

• number of open ion channels

  • many open channels leads to quicker ion movement into the cell compared to less open channels

• properties of the ion channel

  • selectivity of plasma membrane

  • permeability of plasma membrane