Membrane Transport Flashcards
What is the transport of inorganic ions and small molecules across the membrane required for? (3)
- Regulation of intracellular ion concentrations
- Uptake of nutrients
- Excretion of metabolic waste products
Does passive transport require energy?
no (driven by electrochemical gradients/conc)
What are the types of passive transport?
Simple diffusion
Facilitated diffusion
Whats the difference between simple and fac?
In fac membrane proteins are involved
What is the electrochemical gradient?
The combined force of conc gradient and membrane potential (membrane potential only applies to charged molecules)
What is the membrane potential?
The difference in charge between the inside and the outside of a cell
What affects ability of a solute to cross the membrane by simple diffusion? (4)
- conc gradient
- electrochem gradient (ions don’t cross)
- hydrophobicity (polar molecules don’t tend to cross bilayer)
- size
What types of membrane protein are involved in fac diffusion?
- channels
- transporters
What are the channels and transporters required for in fac?
to transport inorganic ions and small molecules across the membrane (passively along conc/electrochem grad)
What are channels required for?
To transport ions (membranes impermeable to them)
What are 4 features of ion channels?
- exhibit ion selectivity
- driven by conc/electrochem grad
- V fast (10^7 molecules per sec)
- Regulated (open + close in response to stimulus)
What is one of the most common ion channel?
K+ channel
What are K+ leak channels
K+ ion channels that are constantly open (not gated)
How many times more effective are K+ channels than Na+ channels?
10,000 fold more effective
Describe how a K+ ion moves through a K+ channel
- Due to being in a high conc a hydrated K+ ion enters the channel where there are negatively charged amino acids at the pore entrance
- As it passes through it gets dehydrated
- The K+ ion passes through the selectivity filter which is lined with carbonyl oxygens of amino acids
- As the K+ leaves the channel it gets rehydrated and moves out into the are of lower electrochem gradient
How is the K+ ion channel selective?
- Anions prevented by entering due to negative amino acids at entrance
- Hydrated cations including K+ are too big to pass through selectivity filter + must be dehydrated (requires energy). In K+ the energy lost by dehydration is regained by dehydrated K+ ions interacting with carbonyl oxygens of amino acids lining selectivity filters (act as surrogate water molecules) (Na+ would only be able to interact with 2 O molecules in contrast to K+ 4 (energetically unfavourable))
What are 5 features of glucose transporters (Gluts)
- uniporters (only transport glucose)
- expressed by most cells
- many isoforms
- 12 pass membrane spanning proteins
- alternate between 2 conformations
What happens when someone has Glut1 deficiency syndrome?
- seizures
- microcephaly
- retarded development
What 2 transporters are involves in active transport?
- ATP-driven pumps
- coupled receptors
What is the role of ATP driven pumps?
To move solutes against their conc/electrochem gradient by expending energy (primary active transport)
What are 7 features of the (Na+K+ pump)
- expressed by all cells
- up to 30 million of them on some cells
- Hydrolyses ATP (primary active transport
- Utilises 30% of energy available in most cells (more in nerve cells)
- Continually expels Na+ from the cell in exchange for K+)
- Maintains k+ and Na+ electrochem gradients
- Stabilises osmotic balance so that secondary active transport can’t occur
How does the Na+K+ pump work?
- 3Na+ ions bind, the pump hydrolyses ATP and is phosphorylated
- Na+ dependent phosphorylation causes the pump to change conformationally and Na+ is transferred across the membrane and released
- 2K+ ions bind and the pump is dephosphorylated
- K+ dependent dephospho rylation causes the pump to return to normal conformation and K+ is transferred across the membrane and released
What is the role of coupled receptors?
To move solutes against their conc/electrochem gradient by coupling their transport to the Na+ gradient created by Na+K+ATPase (does not depend directly on the hydrolysis of ATP)
What are 2 features of the Na+/glucose symporter?
- oscillates between two conformations
- binding is cooperative (Na+ binding increases affinity for glucose)
Where is glucose most likely to bind to and why?
extracellular side where Na+ conc is higher (glucose more likely to enter cells then leave them)
Why are glucose symporters required for absorption of glucose in the GI tract?
There are tight junctions so glucose can’t pass through paracellular pathway
What do tight junctions prevent?
Paracellular movement
Name the 2 functions of tight junctions and explain.
Barrier-prevents things from moving through paracellular pathway
Fence-Prevents proteins diffusing through the basal domain to the apical domain and vice versa
How is glucose absorbed
- Na+ grad created by Na+/K+ATPase
- Na+/ glucose symporter transports glucose into the epithelial cells (against conc grad) using Na+ electrochem gradient
- Glut2 transports glucose out of cells (along conc grad) and into blood supply (fac diffusion)
What does glucose absorption depend on?
The asymmetric distribution of transporters in epithelial cells
Na+/glucose symporter must be in apical membrane
Glut2 must be in basolateral membrane
Where is the Na+/Ca+ anti porter expressed by?
Cardiomyocytes in heart muscles
Is the conc Ca+ conc greater in the interstitial fluid or in the cardiomyocytes?
Interstitial fluid
What is the role of the antiporter in cardiomyoctes?
To use Na+ electrochem gradient to transport Ca2+ out of the cell against its gradient (causes muscle relaxation)
How is muscle contraction caused?
By a rise in intracellular Ca2+
How would you treat heart failure?
By preventing Ca2+ being transported out of cell (increase strength of heartbeat)
- ouabain and digoxin inhibit Na+K+ATPase
- This inhibition increases intracellular Na+ and a reduction in Na+ gradient
- So effectiveness of anti porter reduced so fewer Ca2+ transported out of the cell
What is the structure of an aquaporin?
- tetrameric complex
- made up of four monomers (each acting as a water channel)
- Each monomer has a six pass membrane spanning protein
What is the role of the aquaporin?
to allow passage of water whist preventing the passage of ions
