Permeability Barriers (2) Flashcards Preview

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What types of molecules can cross lipid bilayers. Give 2 examples of each.

- Hydrophobic: O2/CO2
- Small uncharged polar: H20/urea


What type of molecules can't cross lipid bilayers? Give 2 examples of each.

- Large uncharged polar: Glucose/Sucrose
- Ions: H+/Na+


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.