Membrane Transport Part 2 Flashcards
Osmosis is the
Movement of water
__ allow water to move more rapidly across the membrane
Water channels (small amounts of water can pass through membrane because small enough but need channel for larger volume)
Net water movement is baised on
which has solute
ALWAYS low to high (or hypo to hyper)
fluid and electrolyte balance
(electrolytes are Cl Na K)
maintaining flow of fluid
hypotonic
low concentration of solute
Hypersonic
high concentration of solute
Aquaporin is a ____
not an actual porin - is not betta pleated stands
is a water channel (of alfa helices)
Aquaporin Allows
rapid passage of water, while blocking passage of ions
water movement with aquaporin
water must move in a single file line
Pore is too narrow for hydrated ions to enter
(Na+, K+, Cl-, Ca2+) (interact with water - bulky - fully hydrated)
how aquaporin Blocks Hydronium Ion
Hydronium ions (H3O+) diffuse through water fast, relay mechanism requires making and breaking of bonds between adjacent water molecules
polar inner lining (hydrophilic)
two Asparagene ameno acids in center of channel - blocks H from going all the way through/block H bonding relay
Net water movement is always
low to high
hyop to hyper
hyponatremia
level of Na+ in blood is too low
water rushes into cells and they swell, skull bocks from expanding and they burst
treat by giving electrolytes to offset imbalance
Ion chanels
and selectivity
do not have binding sites inside of the chanel
must interact with wall itself
ions must be dehydrates and they energy used to remove can be valced out if favorable interaction with slelectivity filter (if too small, not balance interactions)
voltage gated
sences change in voltage and opens/closes
ligand gated
interacellular ligand - often ions themselves
extracellular ligand - any molecule that binds to the chanel
(both not going through channel, binds and then confromational change)
mechanicaly gated
strssor
ex. streching of lipid bilayer, sound, light etc.
gating of ion channels
voltage gated
ligand gated (intra and extracellular)
Mechanically gated
charge of neuron membrane
inside is more negatively charged than the outside.
This is largely due to negatively charged proteins inside the cell and greater permeability of membrane to K+ (leaky K+ channels, K+ ions move down gradient out of cell, without simulation).
resting membrane potental
Steady voltage difference across the neuron’s plasma membrane
Voltage gated Na+Channels
How detects differences of charge across membrane
Tetramer of 4 identical polypeptide chains
Each domain has a voltage sensor (contains unusual transmembrane helix with many positively charged amino acids)- interact with more negatively charged extracllular matrix
Na channel activation
Voltage gated Na+ channels open in response to a stimulus
A small amount of Na+ enters cell down its electrochemical gradient.
Influx of positive charge depolarizes membrane further, opens more Na+ channels (self amplification, positive feedback), from -70mV to +40 mV.
At this point, net electrochemical driving force for Na+ is zero, cell would come to new resting state (with all Na+ channels open). - Na chnnel incativation prevents this
Na+ Channel Inactivation
VG Na+ Chanels have three conformaitons
inactivated - gate closes - prevents cell from going through spasm - is more energeticly favorable
Action ptental
Na chanels closed, gets to threshold, na open, na inactivated
repolarization vg K channels open - during this time Na cannot open
Na closes, hyperpolarazation vg k channels close
vg na k pump restores Na and K gradeitnds (from hyperpolarized)
Repolarization
Voltage gated K+ channels open, K+ move out of cell-. Restoration of negative membrane potential is called repolarization.
Efflux of K+ causes an overshoot of resting membrane potential, hyperpolarizing the neuron.
K channels are very slow to close
Absolute regractory period
no ammount of simulation can caue anoterh AP
Na channels are inactivated and cannnot open, durring repolarization
Relative refractory period
must be a greater than normal stimulation to cause an AP
durring hyperpolarization Na is closed so it can open but need greater stimulation to reach threshold
Channelrhodopsins
Photosensitive ion channels: Open in response to light
Can be expressed in almost any cell type: introduced in neurons. When neurons exposed to blue light, channelrhodopsin channels open, depolarizing (Na in) and activating cells
Optogenetic control of aggression in a living mouse
Chemical Synapse
When action potential arrives at presynaptic site, depolarization of membrane opens voltage gated Ca2+ channels
Ca2+ influx triggers release of neurotransmitters, which are stored in synaptic vesicles and released by exocytosis
Differences between ion channels and aquaporin
- Ion selectivity
- ion channels are not conounously open.
