Movement Across Cell Membrane Flashcards
3 types of movement across membrane
1) bulk flow (convection)
2) diffusion
3) transport across a barrier
Bulk flow (convection movement)
- movement of fluid/secretions and all the molecules dissolved in the substance
- pressure gradient drives bulk flow (dense molecules sink, less dense rise)
Diffusion
- passive, random thermal motion
- used concentration gradients (high–>low concentration)
- occurs in complete absence of convection
Transport across a barrier
1) solubility/diffusion across a lipid bilayer
2) carrier proteins mediate transport (proteins are extremely selective)
3) endocytosis/exocytosis
How determine rate of diffusion?
-Fick’s Law of Diffusion tells us rate of substance moving through medium
Equation for Fick’s law of diffusion?
- rate of diffusion measured in (moles/sec)
- J=DA(dc/dx)
1) J= flux (moles/sec) how many moles of a substance pass through a designated area per second
2) D= diffusion coefficient, takes into account temp, viscosity of medium, molecular size of solute (cm^2/sec)
3) A= (cm^2) area available for diffusion
4) dC/dx= the energy/driving force pushing the molecule, the concentration profile.
How does concentration gradient effect flux (rate of diffusion)?
- concentration profile denoted at dC/dx
- as the concentration gradient/profile INCREASEs, rate of flux increases
What is the diffusion coefficient in Fick’s law of diffusion proportional to?
1) D proportional to temperature
2) D inversely proportional to viscosity of liquid
3) D inversely proportional to molecule sie
Diffusion and distance? Why?
1) diffusion only efficent/sufficently rapid over short distances
2) time required to diffuse increases with the square of distance
- is why no cell is > ~20um
- why no cell is farther than ~100um from a capillary
Distance of a cell? distance from cell to capillary?
1) 10um (50msec to diffuse)
2) 100um (5 sec to diffuse)
How do substances enter/exit cell?
- need to cross the plasma membrane that separates cell from outside environment
- can do it through diffusion, active transport, or chanels
The plasma membrane?
- phopsholipid bilayer made of amphipathic phospholipids makes a thin film of oil around cell
- 7nm, extremely thin
What are amphipathic phospholipids?
- have hydrophobic & hydrophilic regions
1) hydrophilic head made of glycerol phosphate, and choline/serine
2) hydrophobic fatty acid tails (hydrocarbons) - tails are esterified to head group
Amphipathic phospholipids in water?
- in presence of H20 phospholipids spontaneously assembly into a thin film/ bilayer w/ charged heads pointing out, hydrophobic heads facing in
Where are the proteins responsible for carrying out transport functions in cell?
- within the lipid film are integral membrane proteins responsible for carrying out all specific functions of membrane
Diffusion through the lipid bilayer? (3 steps)
1) partition into lipid (most important step)
2) diffusion through lipid
3) partition into cytoplasm
What is diffusion through the bilayer largely dependent on?
- the ability of the solute to partition into the lipid core of the membrane (hydrophobic space)
- use the partial coefficient
What is the partition coefficient? Equation?
- the degree to which molecules can partition into the lipid phase
(degree to which molecules favor hydrophobic space) - Denoted as K
- S= separation
K= [S] in lipid/ [S] in water
Why is K important? What does
-k= partition coefficient, determines whether molecules (drugs) can penetrate cell membrane through lipid phase easily
K>1 enter lipid phase readily (hydrophobic molecules)
K<1 doesn’t enter lipid phase readily (hydrophilic molecules)
How K>1 molecules move into the cell?
since have high K (partition coefficient) they readily move into the hydrophobic space, accumulate and cause steep conc. gradient. Therefore readily move down gradient into the cell
Partition coefficient and drugs? What does it determine?
- K determines how fast drugs reach biological target
1) rate of absorption from GI tract (large K so can go from belly–> into specific cells)
2) whether accumulate in body fat (large K=hydrophobic=acucmulate in fat and escape metabolism/ kidney excretion)
3) persistence in body
What alter Fick’s law to determine flux per membrane area? (as opposed to just rate of diffusion)
-Ficks Law=DA(dC/dx)
- diffusion rate across a membrane:
- (J/A)= P(C(out)-C(in))
- y=mx
-P= permeability coefficient & is experimentally measured
most biological important molecules are hydrophilic (have very low K) how do they get into the cell readily?
- transported by on or more membrane proteins
- either channels, transporters or pumps
4 functions of membrane proteins?
1) gatekeepers (channels/pumps)
2) anchors (structural proteins)
3) sensors (receptors)
4) biochemical hubs (enzymes)
What are the 4 modes of membrane transit?
1) simple diffusion
2) ion channels
3) transporters
4) Pumps
Ion channel composition and movement?
- is a water filled pore
- molecules move down electrochemical gradient via diffusion
- is an integral membrane protein
Types of Transporters (3)?
1) uniporter: facilitate single molecule
2) cotransporter: use ion gradient to move 2 substances across cell
3) Exchanger: exchanges for one extracellular molecule for one intracellular molecule
ARE NOT CHANNELS
What are pumps?
- move solute from low–> high conc. (again gradient)
- ATPase
What do ion channels do?
-creates a voltage difference between inside & outside of cell which can be used for other things
Ion flow through a channel is?
1) conductive
2) passive
3) rapid
4) bidirectional based on gradient
5) highly selective
6) gated so opening is regulated
What does it mean that ion channels are “slaves to two masters”?
- that have high concentrate of K+ in the cell, low conc out side of cell. So K= wants to move out even though outside the cell is also +. Meaning that electrochemically K+ wants to enter the cell
- have electrochemical gradient (voltage across membrane) as well as concentration gradient, tend to be opposite
How do ion channels open/close (4 ways)?
1) ligand bindings extracellulary
2) phosphorylation
3) voltage/depoalrization
4) membrane deformation (stretch)
How cll membranes permeable to H20 despite its hydrophilic nature?
-aquaporins (membrane spanning protein channels) facilitate H2O movement
Aquaporin structure?
- 6 transmembrane alpha helices, 2 central loops
- form homotetramers (from 4 identical monomers)in cell membrane
- each monomer conducts H2O
- the tetramers form a pore in center only fits H2O in single file line
Uniport?
- a transporter
- facilitated diffusion
- high selective substrate binding pocket facing extracellualry.
- when ligand binds conf. change occurs, open intracellularly releases molecule
- conf. change open back up extracellular
- ligand binding/unbinding causes conf. change
outer vs inner cellular env?
1) outer= similar to sea water but diluted 3 fold (highly Na+)
2) high in K+, poor Na+
- causes a cation gradient
- is due to continuous activity of sodium-poitaasium pump
Na/K pump?
- example of active transport
- 3 Na+ into cell and 2 K+ out of the cell
Cation gradients?
are generated by Na/K ATPase pump
- store ENERGY that can be used to perform WORK
ex: secondary active transport
secondary active transport?
-use of cation ion gradients (active transport) to move molecules into the cell
Na+/ glucose pump?
- co-transporter
- moves Na+ and glucose into cell together
- relies on Na/K pump cation gradient
- Na+ moves down cation gradient into cell (negative charge)
- glucose move down concentration. gradient
