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Questions and Answers
What is the equilibrium transmembrane potential for a frog muscle cell if the membrane is only permeable to K+ ions?
What is the equilibrium transmembrane potential for a frog muscle cell if the membrane is only permeable to K+ ions?
Why do the equilibrium potentials for K+ and Na+ ions have different polarities?
Why do the equilibrium potentials for K+ and Na+ ions have different polarities?
In an electrical model of a single ion channel, what does Vm=Ek signify?
In an electrical model of a single ion channel, what does Vm=Ek signify?
What happens to potassium ions when Vm is greater than Ek?
What happens to potassium ions when Vm is greater than Ek?
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What does mobility (μ) represent in the context of charged particles in an electric field?
What does mobility (μ) represent in the context of charged particles in an electric field?
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Which of the following correctly describes what happens to diffusion in the presence of an electric field?
Which of the following correctly describes what happens to diffusion in the presence of an electric field?
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In the Nernst-Plank Equation, what does the term 'drift' refer to?
In the Nernst-Plank Equation, what does the term 'drift' refer to?
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What is meant by 'concentration' in the discussed context?
What is meant by 'concentration' in the discussed context?
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What can be inferred about ion permeability when large gradients exist across a thin membrane?
What can be inferred about ion permeability when large gradients exist across a thin membrane?
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Which statement is accurate regarding the valence of different types of particles?
Which statement is accurate regarding the valence of different types of particles?
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In terms of units, what distinguishes molarity from moles?
In terms of units, what distinguishes molarity from moles?
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What can be a consequence of applying an electric field to charged particles?
What can be a consequence of applying an electric field to charged particles?
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What is the condition of net current in a system at equilibrium?
What is the condition of net current in a system at equilibrium?
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Which variable is NOT part of the Nernst equation for equilibrium potential?
Which variable is NOT part of the Nernst equation for equilibrium potential?
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In the context of Nernst equilibrium, what does the symbol 'Vm' represent?
In the context of Nernst equilibrium, what does the symbol 'Vm' represent?
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What does the logarithmic form in the Nernst equation help to express?
What does the logarithmic form in the Nernst equation help to express?
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Which variable corresponds to the concentration of ions inside the cell in the Nernst equation?
Which variable corresponds to the concentration of ions inside the cell in the Nernst equation?
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What does the 'Z' in the Nernst equation stand for?
What does the 'Z' in the Nernst equation stand for?
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At which temperature does the Nernst potential expression change, as mentioned?
At which temperature does the Nernst potential expression change, as mentioned?
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What does the term 'steady state' imply in the context of Nernst equilibrium?
What does the term 'steady state' imply in the context of Nernst equilibrium?
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In the equilibrium condition for Nernst potential, what aspect does the induced conduction current counterbalance?
In the equilibrium condition for Nernst potential, what aspect does the induced conduction current counterbalance?
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What is needed for an ion to reach Nernst equilibrium according to the equilibrium potential formula?
What is needed for an ion to reach Nernst equilibrium according to the equilibrium potential formula?
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Study Notes
Bioelectricity and Biophotonics Engineering
- Course code: WSC331
- Lecturer: Felipe Iza
- Email: [email protected]
- University: Loughborough University, UK
- Website: http://www.lboro.ac.uk/departments/meme/staff/felipe-iza
Recap of Last Lecture
- Transport phenomena
- Transport mechanisms
- Nernst-Planck Equation (drift-diffusion equation)
Some Definitions
- Concentration/units
- Molarity: mol vs molar
- Diffusion/advection/drift(conduction)
- Flux/units
Mobility/Drift
- In an electric field, charged particles experience an electrical force.
-
= sign(Z) μE (velocity is proportional to the electric field and valence) - μ: Mobility (m²/V-sec)
- Z: valence (-1 for electrons, 0 for neutrals, >0 for cations, <0 for anions)
Diffusion
- Transport from high to low concentration.
- Random motion of particles results in this transport.
- jd = − D ∇C (diffusion flux equation)
- D = D0 exp(−ΔE / RT): diffusion coefficient
Osmosis
- Solvent diffuses across a semi-permeable membrane.
- Moves from low to high solute concentration.
Tonicity
- Hypotonic: one solution has a lower solute concentration than another.
- Hypertonic: one solution has a higher solute concentration than another.
- Isotonic: both solutions have the same solute concentrations.
Recap from Previous Lecture- Nernst-Planck equation
- Chemists count moles j = −D∇C + CZF∇Φ/RT (moles/m² sec)
- Engineers measure currents J = −DFZ∇C + CZF∇Φ/RT (A/m²)
Today's Lecture
- Transport phenomena across cell membranes
- Cell Membrane (revisited)
- Transport mechanisms
- Nernst potential
- Parallel-conductance model
- Resting Potential
Transmembrane Potential
- Vm = Φi − Φe (+/- 100 mV) (membrane potential)
- Phospholipid bilayer + proteins, channels, pumps = Selective permeable
Equivalent circuit of a passive membrane
- Intracellular and Extracellular medium are conductors.
- The membrane is a lossy dielectric.
- The Membrane capacitance is Cm = ε0εrA/d (Capacitance = permittivity*area/distance)
- A large and important value for biological systems.
Pumps and Channels
- Large polar molecules can't cross a phospholipid bilayer, so pumps and channels are required to move them in and out of the cell.
- Pumps move ions against their concentration gradient (require energy).
- Channels allow ion's flow with their concentration gradient (no energy required).
Membrane Conductance
- The cell has many channels, but each operates discretely; macroscopically, the membrane exhibits a particular ion conductance.
Channels: Single ion permeability
- Some channels are permeable to certain ions while impermeable to others.
- P+ permeates, Q+ doesn't.
- [P+]i > [P+]e ⇒ Diffusion is favored.
Channels: Single ion permeability (continued)
- Diffusion of one ionic species in an ideal channel leads to the creation of an electric field.
- The electric field opposes diffusion and eventually no net flow occurs.
Nernst equilibrium
- In equilibrium, net current = 0
- Jp = −DpF Zp(∇Cp + CpZp∇Φ)/RT = 0
- Gradients perpendicular to the membrane – 1D.
Nernst Equilibrium (continued)
- Veq = (RT / ZpF) ln(Cp,e / Cp,i) = 23mV ln (Cp,e / Cp,i)
- For T ≈ 17°C or 25mV ln (Cp,e / Cp,i)
- Equilibrium transmembrane potential
Nernst Potential
- Equilibrium transmembrane potential of an ion.
- Potential where induced conduction current counterbalances diffusion current.
- Electrical measure of diffusion strength.
Examples (Frog Muscle Cell)
- Intracellular and extracellular concentrations of K+, Na+, Cl-.
Answers & Notation
- Transmembrane potential, given permeability of just Potassium or Sodium ions, calculated using Nernst potential formula.
p-n Junction
- Nernst potential is equivalent to the built-in voltage in a p-n junction.
Channel – Electrical model (single ion)
- Interior/cytoplasm, exterior/medium models.
- Vm = Ek ⇒ Equilibrium, Im = 0
- Vm > EK ⇒ Ik > 0, K+ flows outwards
- Vm < EK ⇒ Ik > 0, K+ flows inwards
Multiple ions: Parallel conductance model
- In reality, more than one ion passes the membrane at a time, and their currents combine to produce a net current across the membrane.
Resting potential
- Each ion's Nernst potential is different.
- Resting potential = transmembrane potential making total current = 0.
- Weighted average of each ion's Nernst potential (Goldman-Hodgkin-Katz equation).
Example (Squid axon)
- Intracellular and extracellular concentrations of K+, Na+, and Cl-
- Conductivities of those ions
- Resting potential calculation
- Determine if any net current is produced at resting potential.
- Determine if any net flow of ions occurs at resting potential.
Answer (Squid axon)
- Nernst equation for K+, Na+, Cl- calculate equilibrium potentials.
- Resting membrane potential calculated summing all potentials given by Goldman-Hodgkin-Katz relation.
Ions currents
- Vm>Ek ⇒ Ik>0 ⇒ efflux of potassium ions
- Vm<ENa ⇒ INa<0 ⇒ influx of sodium ions
- Vm<ECl ⇒ ICl<0 ⇒ efflux of chlorine ions
So far...
- Capacitive nature of bilayer (Cm).
- Transport through ion channels (diffusion and conduction (gp)).
- Nernst potential for each ion (Ep).
Today's lecture
- Review of processes/topics previously discussed.
Next Lecture
- Transport phenomena across cell membranes
- Quasineutrality
- Donnan equilibrium
- Role of pumps
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Description
Test your understanding of bioelectricity and biophotonics engineering concepts covered in course WSC331. This quiz includes topics such as transport phenomena, the Nernst-Planck equation, and definitions related to mobility and diffusion. Assess your knowledge on the principles governing the behavior of charged particles in electric fields.
