Session 2: Membrane Permeability, Pumps & Transporters Flashcards
What is a semi-permeable membrane?
A layer through which only allowed substances can pass
What are the major solutes groups which move through lipid membranes? Which are permeable? Give an example for each.
- Hydrophobic molecules such as O2, CO2 & N2 = fully permeable => all gases = can pass backwards & forward = very small so an fit through polar heads of phospholipid bilayer
- Small & non-polar molecules such as water (gas), urea & NH3 = can diffuse through membranes and also go backwards
- Large uncharged polar molecules such as glucose & sucrose = cannot pass through membrane
- Ions = cannot penetrate the membrane
In terms of the speed of movement of molecules/ions, what does high and low permeability mean?
High permeability = fast
Low permeability = slow
What factors affect permeability? (5)
- Temperature
- Surface area of membrane
- Nature of membrane barrier (eg thickness)
- Size (molecular weight) of diffusing ion/molecule – larger ones diffuse more slowly
- Polarity/charge
What are aquaporins?
They are a type of pore which increase rate of osmotic movement of water
What type of diffusion occurs in aquaporin?
facillitated diffusion
What is a ligand-gated channel? How do they open?
A channel which allows regulation of selected ions across the membrane
They open when specific signalling proteins bind to them
What is a voltage-gated channel? How do they open?
A channel that uses a change in voltage across the membrane which causes a change in a conformational state that opens the gate
What are carriers?
- Solute carriers that transport solutes
- They behave like enzymes & show similar Michaelis-Menten kinetics
What are the 3 types of carriers? Describe each of them.
- Uniporter – only 1 molecule transported at a time down a concentration gradient
- Symporters - 2 types of molecules transported at the same time in the same direction
- Antiporters - 2 types of molecules transported in the opposite direction
What is passive diffusion?
- Solutes dissolves in phospholipid bilayer & diffuses across membrane then dissolves in solution at other side of membrane
- Higher conc -> lower conc
- No energy required
- driven by the concentration and electric gradient of the solute
What is facilitated diffusion?
- Solutes diffuse across membrane with assistance of membrane proteins as channels & carriers.
Where does facilitated diffusion derive energy from?
Energy sourced from a pre-existing electrochemical gradient
What is passive transport?
- substances move from an area of higher concentration to an area of lower concentration by diffusion
- no energy required
What is active transport?
transport of ions/molecules against concentration gradient using ATP derived directly/indirectly from ATP hydrolysis
