Membranes (Unit 1) Flashcards
plasma membrane
a lipid bilayer that is a barrier to water and water-soluble substances; gases move through easily
Relative extracellular/intracellular concentration of: Na+
high extracellular, low intracellular
Relative extracellular/intracellular concentration of: K+
low extracellular, high intracellular
Relative extracellular/intracellular concentration of: Ca2+
high extracellular, low intracellular
Relative extracellular/intracellular concentration of: Cl-
high extracellular, low intracellular
Relative extracellular/intracellular concentration of: PO4-
low extracellular, high intracellular
Relative extracellular/intracellular concentration of: Proteins
low extracellular, high intracellular
substances high extracellular:
Na+, Ca2+, Cl-
substances low extracellular:
K+, PO4-, Proteins
how substance can get through membrane:
simple diffusion (through membrane or pore), facilitated diffusion, active transport
Diffusion (passive transport) characteristics
- Occurs down a concentration gradient
- Through lipid bilayer or involves a protein “channel” or “carrier”
- No additional energy required
Active Transport characteristics
• Establishes concentration gradient and continues to operate against a concentration
gradient
• Involves a protein “carrier” • Requires ENERGY (ATP)
Rate of diffusion is governed by:
• Amount of substance available
• Velocity of kinetic motion
• Number and sizes of
openings in the membrane through which the molecules or ions can move
ungated channels
Transport is determined by size, shape, and charge of channel and ion
Gated channels types
voltage-gated and ligand-gated
Voltage-gated
gate opens and closes depending on voltage (membrane potential) (e.g., voltage-gated Na+ channels)
Ligand-gated
gate opens and closes depending on binding of a chemical (e.g., nicotinic acetylcholine receptor channels)
simple diffusion
either through membrane directly (if lipid) or channel
facilitated diffusion
still taking advantage of gradient, but needs carrier protein
channel proteins
selectively permeable, can be opened and closed depending on a certain stimulus
aquaporins
select for water, made very narrow, water passes easily but other things too large
how are ion channels selectively permeable to specific ions?
ion get hydrated and the channels pull the hydration shell off of the ion – only works for the selected ion
patch-clamp technique
can measure channel activity and movement of ions in their native context; micropipette suctions to a cell and creates a seal (the patch) where electrical current can be recorded
rate of diffusion for simple diffusion
linear
rate of diffusion for facilitated diffusion (carrier proteins)
looks like log function then plateaus; due to the saturation of carrier proteins the rate eventually hits its Vmax
factors that affect net rate of diffusion
- chemical concentration gradient
- electrical (charged substances)
- pressure gradient (applies to fluid filtration)
osmotically-active particles
exert attraction with water
osmotic pressure
the pressure required to stop movement of water by osmosis across a selectively permeable membrane
process of active transport through entire cell layers
The brush border on the luminal surfaces of the cells is permeable to both sodium ions and water. Therefore, sodium and water diffuse readily from the lumen into the interior of the cell. Then, at the basal and lateral membranes of the cells, sodium ions are actively transported into the extracellular fluid of the surrounding connective tissue and blood vessels.
secondary active transport
depends on carrier protein that penetrates cell membrane; utilizes the energy stored due to gradient from a different substance
types of secondary active transport
- co-transport
2. counter-transport
co-transport and example
type of secondary transport that has ions moving in the same direction; example is sodium-glucose
counter-transport and example
type of secondary transport that has ions moving in opposite direction across membrane; example is Na+-Ca2+ exchanger
membrane potential and abbreviation
difference in charge across membrane; abbreviated Vm (voltage membrane)
where does membrane potential come from?
- activity of sodium-potassium pump (electrogenic)
2. diffusion of ions across membrane
leak channel
not gated, allows free passage of ions
Ek
potassium equilibrium potential (diffusion potential)
active transport through entire cell sheet: locations
- intestinal epithelium (important for food getting to blood)
- epithelium of renal tubules
- epithelium of all exocrine glands
- epithelium of the gallbladder
- membrane of the choroid plexus of the brain
ENa
sodium equilibrium potential (diffusion potential)
diffusion potential
Electrical charge that develops across a selectively permeable membrane when ions diffuse down their concentration gradient
membrane potential of potassium
-94 mV
membrane potential of sodium
+61 mV
measuring membrane potential
taking glass electrode with small tip to puncture nerve cells; detects movement of ions of cell fluid and read on voltage meter
where is the polarization in a cell?
we see the polarization right at the cell membrane
origin of resting membrane potential
- Na+-K+ pump (quarter to a third)
2. diffusion of K+ through leak channels
action potential in neurons
rapid changes in membrane potential that spread rapidly along nerve cell membrane; change in membrane potential that elicits an action
Three phases of action potential
- cell at rest (-90mV, cell unstimulated)
- depolarization
- repolarization
depolarization stage
membrane suddenly permeable to Na+ ions causing rapid diffusion of Na+ ions into the cell – this neutralizes the cell (or can get slightly positive)
repolarization stage
sodium channels close and potassium channels open which reestablishes the normal negative resting membrane potential
resting membrane potential
the difference in membrane charge at rest
Hodgkin and Huxley
figured out that you could impale axon to measure cell potential and also cause artificial action potential; 1st to record electrical activity in neuron (action potential)
1st people to record electrical activity in neuron and record action potential
Hodgkin and Huxley
action potentials that look different
- Certain excitable cells’ APs do not repolarize immediately—there is a plateau during the depolarization phase
- Certain excitable cells’ APs show rhythmicity—spontaneous self-induction at a specific rate
axon
central core of the nerve fiber, membranes of axon is the membrane that conducts action potential
axoplasm
filling of axon center, viscid intracellular fluid
myelin sheath
surrounds axon, often much thicker than axon
node of Ranvier
located 1-3 mm along length of myelin sheath, 2-3 micrometer long area of uninsulation where ions can easily flow through the axon membrane between EC and IC fluid inside the axon; at juncture between two successive Schwann cells
Schwann cell function
insulate nerve fibers
saltatory conduction advantages
improved velocity (jumping for point to point) and conserved energy due to insulation
acute local potential
local depolarization
acute subthreshold potential
an acute local potential that fails to elicit an action potential
absolute refractory period
the period during which a second action potential cannot be elicited, even with a strong stimulus
absolute refractory period length of time for large myelinated fiber
1/2500 second
