Mechanisms of membrane transport Flashcards
What are the main points of membrane transport?
• Cells exchange molecules with the environment to live and grow
• Plasma membrane (barrier, hydrophobic interior) blocks passage of water soluble substances
• specialised membrane transport proteins span the lipid bilayers
• Main types:
- transporters (use energy to move specific substrates across membrane- usually up conc grad)
- channels (allow passive transport of specific substrates across membrane down conc grad)
What is facilitated transport?
• If the measured permeability of a solute is much higher than expected then transport is facilitated in some way
• usually involves a transport protein
• 2 types:
- passive (down conc grad)
- active (up conc grad)
Transporters are required for movement of molecules across membranes, how do they do this?
• each transporter is selective
• each membrane contains a set of different transporters
• functions share some general principles
What is the difference between channel mediated transport and transporter mediated transport?
while they are both passive, channel acts as pore (hydrophobic hole through membrane) and can be opened and close- is faster. Transporters go through conformational changes (in shape).
in actuve transport, uses protein that goes through conformational change in present of atp
What are the principles of passive transport?
• No energy input required to move solutes down their conc gradient
• passive protein transporters act to increase permeability of membrane
• allows communication between two sides of membrane
• two principle types: channels & transport proteins
How do gap junctions allow neighbouring cells to share solutes?
‘flower shape’
Very big channels 2-4nm.
Joins two a-joining cells together- make cytoplasm a single solution via hemi channels. (connexin forms hexamers that form hemi channels- and hemi channels from neighbouring cells connect to form channel)
- allows amino acids, ions, monosaccharides etc..
this allows electrical, ionic and second messenger communication to move rapidly through tissues
What if function of aquaporins?
increase water permeability of cell membranes
What are aquaporins?
They are tetramer of identical subunits (channel pool noodle cylindrical shaped)
- form central pore (0.28nm diameter)
- allows water but not charged ions to pass through
- critical for maintenance of osmotic balance in cells (ie kidneys: for resorption of water during urine formation)
What are the main properties of ion channels?
• Distinguish them from simple holes in membrane
• Ion selectivity- allow some ions to pass but not others
• Depends on diameter, shape and distribution of charged amino acids
• Not continuously open to control flow of ions
• gated- specific stimulus switch between open and closed states/change of conformation
What are the properties of non gates ion channels?
• ‘pores’ allow specific ions to pass
• bi directional net movement down conc gradient
• no energy required
• aka ‘leak channels’
• exists for all principle inorganic ions (Na+, K+, Ca2+, Cl-)
• most cells have more K+ leak channels than Na+
How do non gated ion channels allow specificity?
oxygen atoms present around pore/channel to allow for no entropic change from the ion bonded with surrounding oxygens- mirrors the ions in water.
size
How do gated ion channels allow specificity?
• some open in response to environment (membrane potential, signalling molecules, mechanical signals)
• key molecules in neuronal and neuromuscular signalling
- acetylcholine receptors allow communication from nerve to muscle)
- voltage gated Na+ channels allow signals to travel down nerves
How do voltage gated channels function?
They convert electric signals into chemical signals
How do transmitter gated ion channels function?
Converts chemical signals back to electrical signals
How does the acetylcholine receptor in membrane of muscle cells work?
has open and closed formation (from gate). Opens when acetylcholine bonds to binding site and allows Na+ into cytoplasm of cell
