Lecture 3 - Membrane Permeability And Transport Of Ions And Small Molecules Flashcards
What is a semi-permeable membrane?
A layer through which only ALLOWED substances can pass through
Normally, what is the charge distribution either side of a membrane?
Outside = POSITIVE
Inside = NEGATIVE
What is meant by solute equilibrium?
When the concentration of solute is equal on either side of the membrane
What is the main phospholipid in erythrocytes?
Phosphatidylserine
What are the 4 classes of molecules that may try and cross the phospholipid bilayer?
Hydrophobic molecules
Small uncharged polar molecules
Large uncharged polar molecules
Ions
What is the phospholipid bilayer permeable to and describe their relative permeability (passive diffusion)?
Hydrophobic molecules = very high permeability
Small uncharged polar molecules (high permeability)
What molecules is the phospholipid bilayer not permeable to and what is their relative permeability?
Large uncharged polar molecules = low permeability
Ions = extremely low permeability
Give some examples of hydrophobic molecules:
Why do they have a very high permeability?
Oxygen
Carbon dioxide
Nitrogen
VERY SMALL so can pass through small gaps
Benzene
Give some examples of small uncharged polar molecules
H2O
Urea
Glycerol
Give some examples of large uncharged polar molecules:
Glucose
Sucrose
(Sugars and large amino acids)
NORMALLY REJECTED
Give some examples of ions:
Na+
K+
Ca2+
Mg2+
Cl-
HCO3-
H+
(NORMALLY REJECTED)
What is passive transport?
Transport that does not require energy to take place, particles flow in direction of concentration gradient
What 2 factors does rate of passive transport depend on?
Permeability of molecule
Concentration gradient
Using Ficks law, how does rate of passive transport change as concentration gradient increases?
Increases linearly
What is facilitated diffusion?
Where a membranes protein is required for transport across the membrane to occur
What sort of relationship exists between the concentration of highly permeable substances and the rate of simple diffusion?
Directly proportional
What limits the rate of facilitated diffusion when concentration of substance is increased?
Number of available transporter proteins
After certain conc, Vmax is reached since transporters get fully saturated
What are the methods water uses in order to enter cells?
Passive diffusion
AQUAPORINS
How does water travel during passive transport into a cell?
Uses entire surface of cell membrane
Bidirectional flow
Water crosses to reach solute equilibrium
What are aquaporins?
Integral membrane proteins with aqueous centre
How does water enter into cells via aquaporins?
Rapid movement so equilibrium reached very quickly
Bidirectional flow
What is passive transport?
Solute moves by diffusion down concentration or electrochemical gradients using NO ENERGY
What is active transport?
Solute moves across the membrane against its concentration gradient using the energy from the cell
Does simple diffusion use membrane proteins?
No
What membrane proteins can facilitated diffusion happen across?
Channel proteins
Carrier proteins
What type of transport is facilitated diffusion?
PASSIVE TRANSPORT
What type of membrane proteins carry out active transport?
Carrier proteins
What are the 2 types of membrane channels/pores?
Gated or Non gated
What type of protein channel/pore is conditionally open?
Gated ion channels
Why type of channel is always open?
Non gated ion channels
What are the 2 types of gated ion channel?
Ligand gated ion channel
Voltage gated ion channel
What is the property to the centres of the channel proteins that allow hydrophilic molecules through?
The centres = hydrophilic
What is bidirectional transport and what type of channel does this occur across?
Substances can move back and forth
Non gated pores (always open)
Give an example of facilitated diffusion via a a gated ion channel (ligand)
Ligand gated ion channels at neuromuscular junctions
Ach binds to channel causing conformational change opening channel
Na+ can pass through depolarising membrane
Where is the example of ATP sensitive K+ ion channels for facilitated diffusion applicable?
B cell of pancreas
How does facilitated diffusion across the ATP sensitive K+ channel occur in the B cell?
Influx of ATP in cell
ATP binds to the ATP sensitive K+ ligand gated ion channel
Conformational change closes the channel
K+ accumulates in cell since it cant leave
Causes depolarisation (more +)
How does the Ca2+ voltage gated ion channel in the B cell work?
Depolarisation of cell as a result of build up of K+ makes inside of cell more positive
This causes conformational change opening the channel causing Ca2+ influx
What ion is the CFTR channel protein a channel for?
Cl-
Chloride ions
How does the CFTR channel normally function to produce normal mucus?
Chloride ions transported onto apical membrane of cell via CFTR
Chloride draws 5 water molecules out of cell onto apical surface
Produces correct consistency mucus
What condition does a mutated CFTR gene cause?
Cystic fibrosis
What happens when the CFTR protein is mutated in cystic fibrosis?
Cl- not pout on cell surface
Water not drawn out so mucous membrane snot moistened
THICK STICKY MUCUS = HARD TO BREATHE
What toxin does cholera produce?
Protein kinase A
How does cholera cause diarrhoea?
Toxin protein kinase A Phosphorylates CFTR causing it to always be open
Cl- constantly moved through CFTR into lumen of gut, lots of water constantly drawn into gut = diarrhoea
How do carrier proteins carry out facilitated diffusion?
Molecule binds causing conformational change (closes to outside of cell)
Molecule released causing conformational change (opening to inside of the cell)
Why can only a limited number of substances by moved via facilitated diffusion through a carrier protein?
The speed of the conformational changes can not be changed
What are the 3 types of carrier protein transporters?
Uniporter
Cotransporters :
- Symporters
- Antiporters
What is a uniporte?
Carrier protein that only moves 1 substance at a time
What is the definition of uniport?
Only 1 molecule transported at a timer per cycle down the concentration gradient (channel or carrier protein)
What is co-transport?
2 or more substances transported on a membrane transporter per reaction cycle
What is symport?
2 or more substance moving in the same direction
What is antiport?
2 or more substances move in the opposite directions
What type of transport is Na+/K+ ATPase?
What is its role?
Cotransport
Antiport
Maintains cell concs of Na+ and K+
What type of transport is Na+/Ca2+ exchange?
What is its role?
Cotransport
Antiport
Expels Ca2+ from cell during cell recovery (repolarisation)
What type of transport is SGLT (Na+/glucose transporter)?
Symport
What type of transport is Na+/H+ exchanger?
What is its role?
Antiport
Alkaliniisation of cell since it removes H+
What are the general roles of transport process in the body?
Maintain ionic comp
Maintain intracellular pH
Regulates cell vol
COnc of fuels
Expulsion of waste and toxic substance
Generating ion gradients for muscles and nerves
