Lecture 3 - Ion Channels, Membrane and Action Potential

Question 1

cell membranes - what are they, what do they separate and why, what do they envelop and why

Answer 1

-semi-permeable barriers
-separate cytoplasm from ECF (allows them to differ in composition)
-envelop organelles = allows organelles to become discrete compartments that perform specialized biochemical functions

Question 2

what makes up the cell or plasma membrane

what can diffuse through it, example, what type of diffusion

Answer 2

-its a thin lipid bilayer (~8nm)
-has hydrophilic heads on the outside
-hydrophobic interior

-small uncharged molecules that are relatively lipid soluble can diffuse through the lipid bilayer (steroids)

-simple diffusion (passive)

Question 3

what is more than half of the mass of the cell membrane composed of?

Answer 3

proteins - some molecules pass through these by passive or active transport mechanisms

Question 4

simple diffusion (binding, energy, gradient)

Answer 4

-doesnt require binding of the particule to a carrier
-cant move substance up a gradient
-doesnt consume energy

Question 5

what are ion channels, example of what, formed by what, diffusion, how can ions get through, how many ions

Answer 5

-they are defined as pore forming membrane proteins that allow for passage of ions

-example of a protein that allows passive transport.
-are water filled pores formed by membranes
-allow diffusion across membrane DOWN a concentration gradient
-usually only allows ONE TYPE of ion (ie. Na, K, Cl)
-some channels are inherently leaky (ie. K), others can be opened and closed by gates

Question 6

voltage gated channels - opens in response to what, parts, example

Answer 6

-opens in response to change in transmembrae potential (voltage to open a channel varies between different channels)
-activation gate = opens channel
-inactivation gate = closes; creates recovery period
-example = voltage gated Na channel along axons

Question 7

chemically gated ligand channels - how do they work, example

Answer 7

gates are controlled by binding of ligands/chemicals to the channel or an associated protein nearby in the membrane (chemical/ligand gated calcium channels such as the acetylcholine receptor)
-molecule (ligand) binds to receptor to open gate
-no inactivation gates

Question 8

mechanically gated channels - how are they controlled, what else are they sometimes called

Answer 8

these gates are controlled by mechanical deformation to the membrane channel
-so they open in response to mechanical force
-sometimes called stretch activate channels. they rely on sensory info to know what is happening in the physical environment

Question 9

what type of ion channel turns touch or pressure into an electrical signal that neurons can transmit

Answer 9

mechanically gated channels or stretch activated channels

Question 10

what is primary active transport (how does it work), where is it active, mediated by what, how does it move

Answer 10

-movement of substances (ions,molecules) AGAINST their concentration gradient
- consumes metabolic energy (ATP; key characteristic)
-usually mediated by an enzyme (Na/K ATPase, Na/H ATPase)
-highly active in cells that need to move ions (neurons, kidney, muscle, intestine) or which require a charge on the membrane

Question 11

Na/K ATPase - what moves in and out of the cell and how many

Answer 11

Moves Na out of the cell and K into the cell (3 Na out, 2 K in)

Question 12

secondary active transporters - rely on what, examples, done by what type of proteins, energy, gradient, what is transported?

Answer 12

-rely on primary active transport to produce ion concentration gradients
-ie co transporters and counter transporters
-co transporters use this “stored” energy to move their molecules (AA, glucose) against their concentration gradient
-indirect consumption of ATP (rely on energy consumption of primary active transporters)
-usually done by symporter or antiporter proteins

Question 13

what do cell membranes act as a barrier to

Answer 13

chemical movement

Question 14

what can act as transporters

Answer 14

integral membrane proteins

Question 15

types of passive diffusion and active diffusion

Answer 15

passive = simple or faciliated

active = primary or secondary

Question 16

what is important in establishing the electrochemical gradient in cells

Answer 16

active transport of Na/K

Question 17

why are ion channels important in the nervous system

Answer 17

they help produce electrical changes that transmit information rapidly

Question 18

membrane poteintal - what was discovered, what does the Na/K ATPase estsblish, what is the exception

Answer 18

the relationship between ion concentration difference across any semi permeable membrane and the electric potential that results

-Na/K ATPase establishes a difference in concentration of sodium and potassium across the cell membrane

-but the membrane remains permeable to K so it can diffuse through (via leaky ion channels). this partly separates positive and negative ions because SOME of the K diffuses across the membrane

-therefore, any flux of K across the membrane is determined by the relative concentration gradient AND the electrical potential, creating an electrochemical balance/electrochemical gradient

Question 19

what does the nernst equation tell you?
what is the result of the equation called

Answer 19

its the equation that allows one to calculate the charge on a membrane if you know the concentration of permeable ions or the concentration of ions if you know the charge

the charge is called resting membrane potential

Question 20

-what happens when neuron membranes are at rest
-what is the resting membrane potential at this time

Answer 20

-they are permeable to K (through leaky K channels) and constantly maintain a gradient through the Na/K ATPase activity. the membrane is impermeable to Na when at rest so Na doesnt contribute to the charge

• -91mV is the charge on the membrane due to the difference in K concentration inside vs outside

Question 21

the charge on neurons is primarily the result of what

Answer 21

the electrochemical gradient between K conc inside and outside the cell

Question 22

what happens if the interior of the cell becomes LESS negative? (so becomes more positive)

Answer 22

electrochemical gradient will drive K out, charge will decrease (become more negative) as dictated by nernst equation

Question 23

what happens if the cell interior becomes MORE negative

(K levels drop)

Answer 23

electrochemical gradient will pull K in, charge will rise (becomes less negative)

Question 24

overall mechanism of K in resting membrane potential

Answer 24

K moves to restore the RMP in neurons when the charge changes…. to maintain a -91mV

Question 25

RMP and chloride in muscle - what does it determine

Answer 25

Cl determines the RMP in skeletal muscle…. -88mV

Question 26

RMP and sodium - RMP influenced by what, at rest, specific circumstances

Answer 26

-at any given instance in time, the RMP is influences ONLY by ions that can cross the membrane
-at rest, the neuron membrane is NOT permeable to Na so it has no direct effect, but it is permeable to K which determines the RMP
-but under specific circumstances the membrane potential becomes far more permeable to Na - this is called excitation or depolarization and results in an AP (opening of Na channels on membrane)

Question 27

what is the threshold in which voltage gated channels open

Answer 27

-55mV

Question 28

graded (local) potential - background info

Answer 28

-the membrane potential changes as ions move in/out
- this change in resting membrane potential is called a graded potential

Question 29

graded potential - approaching excitation

Answer 29

-graded potential may ultimately become large enough to cause voltage gated Na channels to open
-threshold value is usually -55m
once the voltage gated channels open, Na rushes in cell causing an AP as a chain reaction

Question 30

what is the all or none principle

Answer 30

below threshold - nothing happens
above threshold - AP occurs

once started, there is no way to lessen or strengthen an AP

Question 31

what happens during excitation (AP)

Answer 31

the membrane suddenly becomes vastly more permeable to Na. therefore the membrane potential quickly moves toward the equilibrium potential for Na (+65-+75mV)
-this depolarizes the membrane briefly in a wave that moves along then length of an axons

Question 32

why is the AP so brief

Answer 32

because Na stops entering the cell. this is because Na channels inactivate ~1millisecond after they open

Question 33

how is the RMP re-established

Answer 33

based on K concentration (transient hyperpolarization because K is actually lower than normal); in the long ter, the Na/K ATPase pumps “out” the Na

the normal charge is reestablished

Question 34

what does the refractory period do

Answer 34

it ensure the AP will only travel in one direction

Question 35

saltatory conduction and what that means for AP

Answer 35

AP doesnt occur at myelinated regions (internodes) along axons, only at nodes. as a result the charge rapidly diffuses along the inner surface of the membrane and ““refreshes/restrengthens” by stmulating a new round of opening of voltage gated channels at each node