Session 2 - Membrane Permeability Flashcards
Give a few examples of molecules that can pass through a membrane?
Oxygen, Carbon Dioxide, Nitrogen, Benzene, Water, Urea, Glycerol
Give some examples of things that cannot pass through a membrane
Glucose, Sucrose, Ions; Hydrogen, Potassium, Sodium, Calcium, Chloride, Bicarbonate
What is Passive diffusion dependent on?
Permeability and conc gradient
What model do facilitated diffusion proteins show?
Ping-Pong
Describe Active Transport
Transport of molecules/ions against an unfavourable concentration gradient and/or electrical gradient
It requires the energy from the hydrolysis of ATP
Name the three types of channel protein
- Ion channel protein (gated)
- Voltage-gated ion channel
- Ligand gated ion channel
Give an example of a ligand gated ion channel - one that passes the substance into the cell and one out of the cell
Into - Nicotine ACh receptor causing Sodium ions to enter the cell
Out of - ATP sensitive Potassium ion channel
What is a Uniport protein?
A membrane protein that transports one molecule per cycle in one direction
What is a Symport protein?
A membrane protein that transports two molecules per cycle in the same direction
What is an Antiport protein?
A membrane protein that transports two molecules per cycle in opposite directions
Describe the structure of the Sodium/Potassium ATPase protein
Contains an alpha sub-unit that is responsible for binding the substrates and a Beta sub-unit which is responsible for directing the pump to the plasma membrane
What are the two major roles of the Sodium/Potassium pump?
- Setting up Sodium and potassium gradients (important for electrical excitability)
- Driving secondary active transport
What is the result of one cycle of the Sodium Potassium pump?
2 Potassium ions have been transported into the cell with 3 Sodium ions being transported out
What is the effect of Ouabain binding to the Sodium/Potassium pump?
It inhibits its action
What happens to the potassium that is transported into the cell by the Sodium/Potassium pump?
It diffuses back out of the cells by Potassium channels - these channels are responsible for setting up the resting membrane potential
Explain what PMCA does?
Expels a Calcium ion from the cells in exchange for a Hydrogen ion using the hydrolysis of ATP
(Has a high affinity but low capacity)
Explain what SERCA does?
Stores Calcium ions in the ER/SR in exchange for Hydrogen ions using the hydrolysis of ATP - very similar to PMCA in function but not location
(High affinity, low capacity)
Explain what NCX does
Expels 1 Calcium ion from the cell for 3 Sodiums ions. Uses the Sodium ion conc set up by the Na/K pump (Secondary active transport) - removes most of the cells Calcium ions
- High capacity, low affinity
What happens to NCX in Ischaemic tissues?
- ATP is not made
- Na/K pump stops
- Sodium conc increases in the cell
- NCX reverses and causes calcium conc to increase in cells
- High calcium conc is very toxic to cell
Name the two acid extruding membrane proteins
NHE and NBC
What does NHE do?
Expels Hydrogen ions from the cell and brings sodium in. Therefore it reduces the cell’s pH
What does Amiloride do?
Inhibits NHE
What does NBC do?
Brings sodium and bicarbonate ions into the cell. Removes Hydrogen ions and chloride ions. Therefore increasing the cells pH.
What does Anion Exchanger do?
Extrudes bicarbonate from the cell and brings chloride in. Therefore lowers the cells pH
How does a cell increase its volume?
Influx ions in and water will follow
How is bicarbonate reabsorbed in the proximal tube?
- Na/K pump keeps intra cellular conc of Na low
- allows NHE to pump Hydrogen ions into lumen of tube
- This then binds to bicarbonate and is brought back into the cells
- This then passes into the blood
(To pass from tube into cells and then into blood has to form water and CO2 first but then reforms in cell and blood)
Why do people with Cystic Fibrosis have thick mucus?
- Faulty CFTR protein
- Chloride enters cells instead of lumen
- Water follows and therefore doesn’t go into lumen with mucus
- Mucus is thick and viscous
What causes diarrhoea?
- CFTR overly active by increased phosphorylation by Protein Kinase A
- Increased Chloride ions in lumen
- Water follows
- Diarrhoea
What type of molecules can pass through membranes?
Small, Uncharged, polar molecules
