Permeability Barriers (2) Flashcards
What type of molecules can’t cross lipid bilayers? Give 2 examples of each.
- Large uncharged polar: Glucose/Sucrose
- Ions: H+/Na+
What types of molecules can cross lipid bilayers. Give 2 examples of each.
- Hydrophobic: O2/CO2
- Small uncharged polar: H20/urea
What 2 things is passive transport dependent on?
- Permeability
- Concentration gradient.
Passive transport increases linearly with what?
- Concentration gradient.
What are some of the roles of transport in cells?
- Maintenance of ionic composition
- Maintenance of Intracellular pH
- Regulation of cell volume
- Concentration of fuels and building blocks
- Extrusion of waste products
- Generation of ion gradients necessary for the electrical excitability of nerve and muscle.
What is ‘ping pong’ transport?
- Substrate enters carrier protein
- Carrier protein changes shape to release substrate on other side of membrane.
What is facilitated diffusion?
- Channel proteins are open or closed.
- When open, allows large insoluble molecules to cross membrane.
- Opens in response to a ligand binding.
Give examples of ligand gated ion channels.
- Nicotinic acetylcholine receptor (Ach for Na+)
- ATP sensitive K+ channel (ATP for K+)
What are voltage gated ion channels?
- Membrane depolarisation allows channels to open
What are the differences between passive and active transport?
- Passive: down conc gradient, -delta G (release of energy)
- Active: against conc gradient, +delta G (requires energy)
What does active transport allow?
- Transport of molecules against an unfavourable conc and or electrical gradient.
- Energy is supplied directly or indirectly from ATP.
What are the Extracellular amounts of Na/Cl/Ca/K?
- Na: 145mM
- Cl: 123mM
- Ca: 1.5mM
- K: 4mM
What are the Intracellular values for Na/Cl/Ca/K?
- Na: 12mM
- Cl: 4.2mM
- Ca: 1x10^-7M
- K: 155mM
What does primary active transport mean?
- Gets its energy from ATP directly.
What is co-transport?
- More than one type of ion or molecule may be transported on a membrane transporter per reaction cycle.
What are the different types of transport through a membrane?
- Uniport: Only one molecule transported in one direction
- Symport: Two molecules transported in the same direction
- Antiport: Two molecules transported, one in each direction.
What is the Na/K pump?
- Antiport, primary active transport.
- Plasma membrane associated pump
- P type ATPase: ATP phosphorylates aspartate, producing a phosphoenzyme intermediate.
- The Na/K pump is made up of two subunits, what are these and what are their roles?
- Alpha: K/Na/ATP ouabin binding site
- Beta: Glycoprotein directs pump to surface.
What proportion of BMR is used for the Na/K pump?
- 25%
What is the role of K being pumped through the membrane?
- Responsible for membrane potential
What are the two types of Ca transport?
- Uniport: Ca-Mg ATPase.
- High affinity, low capacity
- Antiport: Na-Ca exchanger.
- Low affinity, high capacity.
What are the main two sodium transport systems?
- Na-H+ exchange: inward flow of Na down conc gradient, causing cell alkalisation by removing H+ (antiport)
- Na-glucose co-transport: entry of Na provides energy for entry of glucose against conc gradient (symport)
What is the role of transport proteins in diarrhoea?
- CFTR is overstimulated by protein kinase A
- Cl outflow increased so does H2O as it follows.
What are the intracellular and extracellular Ca concentrations?
- Intracellular: 50-100 nM
- Extracellular: 2mM
What are the 6 different ways of controlling resting Ca?
- Na/K pump using ATP
- Ca ion channel
- NCX (Na/Ca exchange)
- PMCA (Ca/H exchange)
- SERCA (Ca/H exchange in sarcoplamic reticulum)
- Ca facilitated transport (mitochondria)
How can NCX work in both directions?
- As Na enters this causes depolarisation, membrane potential reverses mode of operation.
How is the NCX pump affected in ischaemia?
- Decrease in ATP
- Na pump inhibited
- Na accumulates, cell depolarises
- NCX reverses, Na is exchanged for Ca
- High Ca levels is toxic causing cell damage.
What are the ways in which cell pH is controlled?
- Acid extrusion: Na/H exchange -NHE
Na dependent Cl/HCO3 exchanger -NBC - Base extrusion: Cl/HCO3 exchange (Cl in) -AE
How is cell volume regulated?
- Transport of osmotically active ions (Na/K/Cl/organic osmolytes)
- Water follows
- Cell swelling (extrude ions)
- Cell shrinkage (influx ions)
- Conductive and co-transport systems to maintain electroneutrality
Outline how bicarbonate reabsorption occurs in the proximal tubule.
- NaHCO3 splits into Na and HCO3-
- Na is transported into the tubule from the lumen by NHE (Na/H)
- Na pump, pumps Na into the capillary. (K opposite direction, but diffuses back into capillary)
- In the proximal tubule H20 and CO2 from metabolism are combined to form H2CO3
- This splits into HCO3- and H+
- HCO3- is transported into the capillary by an AE pump (Cl the other way)
- H+ is used in the NHE pump to be transported to the lumen.
- H+ and HCO3- combine to form H2CO3
- This can be changed into CO2 and H2O by carbonic anhydrase
- These can be transported into the tubule.
What are the mechanisms to resist cell swelling?
- Conductive: equal ion transport (K+ and Cl- efflux)
- Co-transport: K+ and Cl- efflux (co-transport) also amino acid and H2O out.
- Co-transport: Cl- and HCO3- exchange and K+/H+ exchange. H+ and HCO3- combine to form H2CO3 -> H20 and CO2 which both leave the cell.
What are the mechanisms to resist cell shrinkage?
- Co-transport: Na/K and 2Cl influx. Na and organic osmolytes influx and Na/Cl influx. -> H2O influx
- Co-transport: CO2 and H2O influx combine to form H2CO3. H2CO3 splits into H+ and HCO3-. Cl/HCO3 exchange (Cl influx, HCO3- out) H+/Na+ exchange (H+ efflux)
- Na/Ca transported in. H2O in.
