Diffusion, Transport, and Cellular Signaling

what two properties of particles influence ability to permeate the cell without assistance:

1. solubility in lipid

2. size of particle

which particles are typically soluble in lipid?

• uncharged or very nonpolar particles

• fatty acids

list particle components from most soluble to least soluble

1. hydrophobic molecules

2. small, uncharged polar molecules

3. large, uncharged polar molecules

4. ions

diffusion

• unassisted membrane transport

• uniform spreading out of molecules due to random intermingling

• molecules move from high concentration to low concentration

2 types of diffusion drive passive movement

1. concentration gradient

2. electrical gradient

van’t hoff equation

delta G = RT ln ([S]0 / [S]i)

does not tell you if diffusion will occur, just the energy differences between 2 solutions. tells us if diffusion CAN occur

• G = gibbs free energy

• R = 8.31 J/molK

• ln[S] = natural log of solute concentration (in moles)

[S]0 = outside solute concentration

[S]i = inside solute concentration

when free energy (G) is negative, the reaction is:

spontaneous

moves DOWN concentration gradient

Fick’s Law of Diffusion

Q = (delta C x B x A) / (MW² x delta X)

directly proportional variables:

• delta C = concentration gradient

• B = lipid solubility

• A = membrane surface area

inversely proportional variables:

• MW = molecular weight

• delta X = membrane thickness

1/MW² = D (diffusion coefficient)

D/delta X = P (permeability)

Q = delta C x P x A

osmosis

passive diffusion of water down a gradient

hydrostatic pressure

pressure exerted by the fluid on either side of the membrane

• pushes back on the side with the lower volume

• “pushing pressure”

osmotic pressure

a measure of the tendency of the osmotic flow of water into a solution

• “pulling pressure”

• water will move across a membrane until the opposing hydrostatic pressure is great enough to stop it

tonicity of a solution and the three types

tonicity determines whether a cell surrounded by solution remains the same size/shrinks/swells

• isotonic = cell remains the same

• hypostonic = cell swells (tonicity lower)

• hypertonic = cell shrinks (higher tonicity)

example of substances that required assisted transport

large/insoluble molecules

• Na+

• K+

• water

• glucose

• sucrose

unassisted membrane transport vs assisted transport

• Unassisted transport is passive and does not require energy or protein channels

  • diffusion and osmosis

  • moves DOWN gradient

• Assisted transport requires energy to move a substance across the membrane - substances cannot freely diffuse on their own

  • carrier-mediated transport and vesicular transport

What are the three characteristics of carrier-mediated transport that determine what kind of material can be transported

1. specificity

2. saturation

3. competition

carrier-mediated transport saturation

limit in amount of substance that can be transported

• transport maximum (Tmax)

• rate of transport is linear until maximum is reached

• all binding sites are occupied and no more movement can occur

carrier-mediated transport competition

closely related compounds may be able to compete for binding sites on the carrier

• if similar compounds are present, diffusion may decrease

elements of active transport

• moves a substance against its concentration gradient

• carrier-mediated, requires a protein carrier

• 2 forms, both require ATP

two forms of active transport

1. primary active transport

2. secondary active transport

what is primary active transport?

• requires the direct use of ATP to move a molecule across the membrane

• Na+/K+ pump

steps of an Na+/K+ pump

1. pump has 3 high-affinity sites for Na+ and 2 for K+ when exposed to ICF

2. when 3 Na+ from the ICF bind, it splits ATP into ADP and a phosphate group

   1. this phosphate group then binds to the carrier protein

3. phosphorylation causes the pump to change confirmation, so the Na+ sites are exposed to the other side

   1. Na+ is released into the ECF

   2. affinity of Na+ binding sites greatly decreases

4. change in shape in step 3 exposes the K+ sites to the ECF and greatly increases the affinity for them

5. when 2 K+ from the ECF bind, it releases that attached phosphate group

   1. dephosphorylation causes the pump to revert to its original shape

6. K+ is released into the ICF as affinity of the K+ sites decreases and affinity to Na+ sites increases

   1. cycle restarts!

what is secondary active transport? describe in detail

• uses ATP to build an ion concentration gradient, which then powers the movement of the molecule across a bilayer

• The carrier protein is not directly phosphorylated by ATP

  • uses the molecule movement down a gradient to drive the movement of another molecule AGAINST its own gradient

happens in two ways:

1. symport (cotransport) - both molecules go into the cell in the same direction

2. antiport (counter transport) - opposite movement of molecules

types of assisted membrane transport for small to medium molecules

1. carrier mediated

2. primary active transport

3. secondary active transport

4. facilitated diffusion

types of assisted membrane transport for large molecules

1. vesicular transport

   1. phagocytosis

   2. endocytosis