membrane function - ion gradients

why are there ion gradients across cell membranes

to maintain cell volume

• gives rise to the membrane potential

purpose of nernst equation

• translates the chemical gradient in mM to an electrical gradient mV

osmoregulation

• major limiting factor in evolution

• challenges met in vertebrates with gills and or kidneys

• so cells are subject to an extracellular environment

extracellular environment

• total ion composiiton - 296.5mM

• each ion generates an osmotic potential

  • extracellular is 296.5 and intracellular must be the same

• change in these values causes movement by osmosis and change of cell volume

what happens if inc of extracellular osmolarity 

• hypertonic extracellular

• cell volume will decrease

what happens if dec extracellular osmolarity

• hypotonic

• increase in cell volume

why do volume changes not reach theoretical values

• osmotically inactive volume - 15-30% does not respond

• cell volume regulatory mechanisms are activated before cell change is complete

regulatory volume increase mechanism

• due to accumulation of K+ and Cl-

• water follows

• transport driven by Na+ gradient with ATPase

regulatory volume decrease mechanism

• extrusion of potassium and chloride

• transport driven by K+ and or Cl- gradient

• most cells use ion channels or some cells use cotransporter

what happens with chloride ions in most neurons and alpha cells

• chloride ions are extruded by active K+ and Cl- cotransporters

• opening of chloride ions causes Cl- influx and results in hyperpolarisation of Vm

• eg GABAa and glycine receptor inhibition

what happens to chloride in most cells

• in most cells chloride ions are accumulated

• opening of chloride ion channels results in chloride efflux

• causes depolarisation of Vm

changes in intracellular osmolarity

• imbalance in ion efflux and influx in transporting epithelia

• accumulation of metabolites in metabolically active cells eg liver and pancreatic beta cells

what allows brain tumour (glioma) metastasis

• dec cell volume from loss of potassium and chloride allows glioma cells to squeeze between other cells

chlorotoxin 

• chloride ion channel blocker

• inhibits glioma cell migration 

• used to identify tumour cells in surgery