lecture 3: ions, fluids, gradients, channels Flashcards
oversimplified view of the body
- a bag of salty water containing a collection of cells organised into tissues and organs
extracellular concentration
high in sodium, low in potassium
intracellular concentration
low in sodium, high in potassium
what is the exchange of substances between the cell and its environment regulated by
cell membrane
features of the cell membrane
- plasma membrane is a barrier to the free movement of ions
- it is composed largely of lipids and proteins
- the lipid portions of the membrane are hydrophobic
- transport proteins (channel/carrier) facilitate ion movement across the plasma membrane
what are the different types of ion channels
may typically be “open” (leak channels), or they be opened in response to a stimulus (gated channels)
what does normal cell function require
continuous regulated movement of water and solutes (incl. ions) into and out of cells
what do transport proteins and ion channels do
- they are embedded in the membrane and provide the pathway for translocation across the membrane
- all neuronal signaling is dependent upon the functions and properties of ion channels and transporters
- channels can be open to ECF and ICF at same time, may allow high rates of solute passage
Na/K ATPase function
- maintains and establishes important ion gradients for nervous system function
ion channels 3 important properties
- selectivity = may allow only specific ions or types to transit
- conductance = how many ions can cross per unit time
- gating = external influences may act on a channel to cause a change in conductivity
channel selectivity
- ions in solution have associations with water molecules
- the hydrophobic cell membrane prevents unrestricted transit of hydrated ions
- ions may cross the membrane passively through channels
- channels vary in pore size and pore properties, they may exclude substances on basis of size or electrical charge
what is an ions size dependent on
- how big an ion is, is due to its charge density
smaller = holds less water around it - size is what gives ions their selectivity
what is ion flux through a channel measured as
electrical current
what is the magnitude of ion flux determined by
electrical and chemical driving forces for ion movement
channel gating
- many channels can be switched between conducting and non conducting states
- gating usually involves a change in protein configuration
examples of channel gating
- chemical gating
- phosphorylation gating
- voltage gating
- mechanical gating
chemical (ligand) gating
- a ligand binds to a binding site on the channel protein
- the bound channel changes its configuration, and a pore appears
- the ligand unbinds and the channel returns to its original (closed) configuration
- many neurotransmitters interact with ion channels (receptors) in this way
phosphorylation gating
- activations of second messenger pathways (often by neurotransmitter interaction with GPCRs) can later result in ion channel opening via phosphorylation (eg mAChR)
Voltage gating
- channel configuration changes as a consequence of change in membrane potential
- typically have a charged region (the voltage sensor) that triggers a change in shape when the voltage across it changes
inactivation of voltage gated channels
- many VG channels become inactivated (refractory) after opening, and must be reactivated by MP being largely restored before they can open again
- this property is fundamental to the initiation and conduction of action potentials
mechanical gating
- some channels are opened by mechanical force applied to the membrane in which the channel is embedded
- by this mechanism, membrane permeability is increased in response to physical force
- important in touch sensation, hearing and balance as well as in many body functions
transporters basic traits
- membrane carrier proteins which move substances across the membrane
- a carrier protein typically binds a solute and transports it across the membrane
- relatively low rates of solute transport
- carriers typically not open to both ICF and ECF at same time
transporters - active transport
active transport = moves substances against their concentration gradient
- primary active transport uses energy such as ATP (eg: Na-K ATPase)
- secondary active transport uses an electrochemical gradient
–> symporters move one substance down its concentration gradient and a second substance against its gradient, both in the same direction (eg Na-glucose transporter)
- antiporters move one substance down its concentration gradient and a second substance against its gradient, in opposite directions
transporters - passive transport
- may move substances by passive transport (as opposed to active transport)
- substances move down their concentration gradient = facilitated diffusion
- limited speed at which the carrier can cycle through the conformational changes to allow a solute to pass
- subject to saturation (the maximum rate at which all carriers in a cell can work)
