Lecture 4: Diffusion Potentials Flashcards
DIFFUSION IF ISOTONIC CELL
- Is there a concentration gradient?
- Is the membrane permeable?
- what moves?
- Is there a concentration gradient?
- Yes out for potassium and organics
- In for sodium and chloride
- No for water
- Is the membrane
permeable? - Only to water
- Nothing moves
ISOSMOTIC AND ISOTONIC
DIFFUSION IF OSMOSIS OF CELL
- Is there a concentration gradient?
- Is the membrane permeable?
- what moves?
- Is there a concentration gradient?
- Yes for K, organics Na+ & Cl-
- Yes for water into cell
- Is the membrane permeable?
- Only to water
- Water enters cell, swells, may lyse
HYPOSOMOTIC AND HYPOTONIC
DIFFUSION IF UNCHARGED
- Is there a concentration gradient?
- Is the membrane permeable?
- what moves?
Is there a concentration gradient?
- Yes in for urea
- No for water at the
start
- Is the membrane permeable?
- Yes to urea and water
- Urea enters the cell
**ISOSMOTIC AND HYPOTONIC
Understanding Iso-Hypo-Hyper…Isotonic and Hyposmotic
- Iso- means the same
- Hypo- means less than
- Hyper- means more than
- Isotonic = the same tonicity
- Hyposmotic = less osmolarity.
Ion movement in cells
if cell inside = 140mM K+ and 150mM A-
Outside = 140 mM Na+ and 110 mM Cl-
*If permeable to both Na+ and Cl- they would enter same as urea
*What if only permeable to sodium not to chloride?
*Na+ has a charge, accumulation of charge generates a voltage
*A VOLTAGE WILL MOVE CHARGES i.e. ion movement depends on concentration and voltage
A Contraction is signaled electrically
HOW? Example?
- Cells are electrically charged.
- Nerves and muscle use changes in charge to
send electrical signals called Action Potentials. - Cardiac Muscle cells uses action potentials to co-ordinate contraction across the heart and activate contraction.
ACTION POTENTIALS - where is this going?
Nerve cells, sensory receptor, muscle, neurotransmitters, kidney+saliva+glands+mucous glands, gut? 6
- Nerve cells use change in potential to carry signals.
- Sensory receptor cells use change in potential to provide sensation, hearing ,vision etc
- Muscle uses change in potential to signal contraction.
- Release of neurotransmitters from nerves triggered by change in potential
- Electrical currents used to move salts in kidney, saliva glands, mucous glands.
- Absorption of food in gut linked to ion currents using co-transport.
What is Charge?
- Charge (q), units coulombs (C).
- Charge is to electricity like mass is to gravity.
- Positive change repels positive charge and attracts negative charge.
Valence? equation?
Valance is charge on an ion given as z
z= − (qion/qelectron)
q = charge
Faradays constant?
Faradays constant (F) is charge per mole (96,485 C/mol).
charge of n moles of ions with valance z Equation.
q=zFn is charge of n moles of ions with valance z
What is Voltage? 4
- Voltage (electrical potential, V) units volts (V)
- Measures electrical potential that makes charges
move. - Put two positive changes together makes a voltage that forces them apart.
- Voltage is a bit like pressure in a fluid.
What is a Current?
- Current (i) unit ampere (A)
- Flow rate of charge one A is 1 C/s
- Current is like flow in liquids
i = q/t
Understanding Charge separation:
diagram
* Na+ and Cl- (left side)
Na+ selective membrane
Na+ moves to right side through membrane
OVERTIME
LEFT IS NEGATIVELY CHARGED WHILST RIGHT UIS POSITIVELY CHARGED.
- Diffusion gradient for sodium and chloride
- Only permeable to sodium
- Cl- stays behind, Na+ crosses membrane
- Membrane now polarised, negative on left, positive on right
Understanding Charge Separation:
Selectively permeable membrane, ion movement…4
- Selectively permeable membrane
- One ion moves causing charge separation
- Cells develop electrical charge and voltage across membrane
- Ion movement now depends on voltage and concentration
Membrane Potential Qualitative:
* Is there a concentration gradient?
* is the membrane permeable
* Charged?
* Repels?
diagram*
outside: Cl- and Na+
150 mM NaCl = 3000 mOsmol/L
Permeable to Sodium and Water
ISOSMOTIC AND ISOTONIC
- Is there a concentration gradient?
- Yes in for Na+ and Cl-
- Is the membrane permeable?
- Yes to water and Na+
- Accumulation of positive charge makes cell positive.
- Cell positive repels sodium
ISOSMOTIC AND ISOTONIC
Understanding Charge Separation: EXPLAIN
Diagram:
Beaker
left side: NEGATIVELY CHARGED WITH Na + and Cl right side: POSITIVELY CHARGED with Na+ and Cl-
movement
Chemical Gradient ——>
<——– Electrical gradient
- Sodium entry down diffusion gradient will make right side positive.
- At equilibrium positive potential will repel sodium as fast as it enters
- Sodium entry stops
Understaning GPE… equation?
KE …equation?
- Pendulum has gravitational potential energy at top of swing
- Falling turn GPE into kinetic energy
- Kinetic energy turned back to GPE on upswing
GPE=mg* Δ h
KE =1/2mv^2
1/2mv2 =mgΔh
v2 =(2mgΔh) / m
v= (2g*Δh)^(1/2)
Understanding Nernst Equation:
diffusion? concentration gradients? voltage? AND EQUATIONS
- Molecules diffuse from high to low concentration.
- Concentration gradients have chemical potential energy.
𝜇𝑥 =𝑛𝑅𝑇 × 𝑙𝑛 ([𝑥]1/ [𝑥]2) - Voltage generates electrical potential energy
𝐸𝑃𝐸 = 𝑞 × 𝑉
𝜇𝑒 =𝑧𝐹𝑛×𝑉
Nernst Equation at Equilibrium = 3 and equations
ue =−ux
znFV = −nRT * ln [x]1/ [x]2
1. At equilibrium energy per mole moved by diffusion is equal and opposite energy per mole moved by voltage.
- Nernst equation gives membrane potential at equilibrium.
1 V = − RT/zF * ln [x]1/[x]2 - Assumes only one ion is permeable
Nernst Equation
V = − (61/2) * log [x]1/log[x]2
At 37°C using log and in mV
At 20°C using log and in mV
1 V = −( 58/2) * log [x]1/[x]2
Membrane Potential Quantitative
Diagram
15 Na+ mM inside
Outside Cl- and Na+
150nM Na+ outside
permeable to Sodium only
Mammal cell at body temp
- Sodium will enter cell so answer must be positive.
- V=-61/z log [Na+]in / [Na+]out
- = - 61/1 log(15/150)
- = - 61 log (0.1)
- = - 61 (-1) = 61 mV
Equilibrium Potentials..Ve, K+, Na+, Cl-
Ve is equilibrium potential.
- Voltage is only that ion is permeable
- For K+ = -61 log (140/5) ~ -90mV
- For Na+ =-61 log (10/145) ~ +70 mV
- For Cl- = -61/-1 log(6/106) ~ -75 mV
Resting Membrane Potential. 6.
- At rest most cells are permeable to potassium
- Much less permeable to sodium or chloride.
- Resting membrane potential is resting voltage from inside to out.
- Typical resting potential -70 mV most cells
- Increased permeability to x
- Potential moves toward V for x.
neuron….Resting Membrane Potential
- Measurement close to predicted.
- Error because not just permeable to potassium
- Capacitance (C, unit Faradays (F)) is relationship between charge and voltage.
C = V/q
Capacitance for a typical cell ~1.3x10-11 F.
q = CV
* q=1.3x10-11(F)x0.07(V) = 9.1x10-13 C
n =9.1x10-13(C)/96 500(C/mol)
= 9.4x10-18 mol of K+ move out.
n =0.140(mol/L) x 4x10-12(L) = 5.6x10-13mol of K+ in cell
q = zFn
n=q/zF
- i.e. only 0.0017% of cells K+ moves out
- Concentrations change by nmol not mmol
K+… Concentration and Electrical gradient for K+…. at Resting Membrane Potential
Large organic anions in cells cannot leave
- At rest potassium leaves down concentration gradient.
- Negative resting membrane potential develops
- Membrane potential stops K+ leaving cell
what next?
- What happens if more than one ion is permeable?
- Or if the ion permeability changes
- What about cells that pump ions by active transport
- Electrogenic potentials generated by active transport Na+ Cl-
SUMMARY OF THIS LECTURE**
- Selectively permeable membranes can produce charge separation.
- Charge separation leads to a membrane voltage
- Nernst equation gives the equilibrium potential if the
membrane is only permeable to one ion.
*Changing concentration gradient will change equilibrium potential
*Resting membrane potential due to potassium permeability
- Very few ions move to generate resting potential, no change in concentration
