Transport of Molecules Across Membranes Flashcards
What types of molecules are able to cross a pure lipid bilayer? By what mechanism do they cross?
- Gases & small polar / uncharged molecules
- Passive diffusion
What types of molecules are NOT able to cross a pure lipid bilayer?
- Ions
- Charged molecules
- Large uncharged / polar molecules
What TWO factors influence the movement of charged molecules across a membrane?
- Concentration gradient (chemical)
- Electrical Potential (voltage)
What does the Electrochemical gradient determine?
The energetically favourable direction of movement of a molecule
What equation gives the equilibrium potential for two solutions separated by a membrane that is permeable only for a given ion (X+/-)?
Nernst Equation
(don’t need to know the equation)
TRUE OR FALSE:
When ΔG is negative, there is net movement of a molecule INTO the cell?
True
What are the THREE broad classes of transport proteins?
- ATP-powered Pumps
- Transporters /Carriers
- Ion Channels
What type of transport do ATP-powered Pumps facilitate?
Active Transport
What type of transport do Transporters / Carriers facilitate?
- Facilitated Diffusion
OR - Coupled Transport / Secondary Active Transport
What type of transport do Ion Channels facilitate?
Facilitated Diffusion
What are the THREE classes of ATP-powered Pumps?
- P class
- V class
- ABC superfamily
What are the THREE classes of Ion Channels?
- Voltage gated
- Ligand gated
- Non-gated (resting)
What are the THREE classes of Transporters / Carriers?
- Uniporters
- Symporters
- Antiporters
What reaction powers ATP-powered Pumps?
ATP hydrolysis
* ATP –> ADP + Pi
TRUE OR FALSE:
ATP-powered Pumps transport molecules WITH their electrochemical gradient?
False.
They transport AGAINST the gradient
What is Active Transport?
Coupling of ATP hydrolysis and “uphill” transport of ions or small molecules
Does Active Transport provide high or low rates of transport?
Low
What type of molecule do P-Class ATP-powered Pumps transport?
Ions
What type of molecule do V-Class ATP-powered Pumps transport?
Protons
Where in the cell are V-class pumps found?
Lysosomes and Vacuoles
* generate acidic compartments
What type of molecules do ABC superfamily pumps transport?
- Ions
- Sugars
- Amino acids
- Phospholipids
- Peptides
What are the domains of an ABC superfamily pump?
Two transmembrane and two ATP binding ‘core’ domains
What does ‘ABC’ stand for in ABC superfamily pumps?
ATP Binding Cassette
What is the structure of the P-class pump?
2x α and 2x β subunits
What happens to the Ion during transport through a P-class pump?
Phosphorylated on a conserved Aspartate in α subunits
P for Phosphorylated and P-class
What are TWO examples of P-class pumps?
- Na+ / K+ ATPase - pumps K+ IN and Na+ OUT of cell
- Ca++ ATPases in PM and ER - pump Ca++ OUT of cytosol
What is the mechanism of action for Na+ / K+ ATPase?
Hydrolyis of 1 ATP per cycle coupled to transport of 3 Na+ OUT of cell and 2 K+ INTO cell
How many conformational states does Na+ / K+ ATPase have?
2
E1 & E2
* unidirectional transport
What are the affinities for Na+ and K+ in the E1 state?
3 HIGH affinity Na+
and
2 LOW affinity K+
sites on Cytosolic side
What are the affinities for Na+ and K+ in the E2 state?
3 LOW affinity Na+
and
2 HIGH affinity K+
sites on Extracellular side
TRUE OR FALSE:
V-class proton pumps run the in SAME direction as ATP Synthase?
False.
They run in the OPPOSITE direction
Where are the V0 and V1 components of a V-class proton pump located respectively?
- V0 located in MEMBRANE
- V1 located in CYTOSOL
Which subunits hydrolyse ATP to provide energy to pump protons across the membrane in V-class proton pumps?
β subunits of V1
What is the pH of a Lysosome?
4.5 - 5
What is the mechanism of action for V-class proton pumps to acidify intracellular compartments?
- Protons are charged (cations) so pumping protons in is electrogenic
- Charge builds up across membrane to prevent acidification of compartment
- Anions are transported in the same direction to counteract
What are the domains of ABC superfamily pumps?
2x Transmembrane domains
and
2x Cytosolic ATP-binding domains
What protein is an example of an ABC superfamily pump?
Multi-drug resistance transport proteins (MDR1 & 2)
Are MDR1 & 2 over or under expressed in tumours?
Overexpressed
* higher concentration of drug required to kill tumour cells
What mechanism does MDR1 & 2 use to transport molecules across the membrane?
Flippase
What molecule does the Cystic Fibrosis Transmembrane Regulator (CFTR) transport?
Cl- ions
Mutation of which ABC gene causes a rare disorder called Tangiers Disease?
ABC A1
Mutation of which ABC gene causes X-linked Adrenoleukodystrophy (ALD)?
ABC D1
