WEEK 3 (Electric properties of plasma membranes) Flashcards
What are the properties of membrane potential?
- Cells are slightly -ve
- Hydrophobic bilayer is a poor conductor
- Ion concentration difference is based on ions
- Ion channels are involved in membrane transport
What is membrane potential (Vm)?
The voltage difference across a cell plasma membrane
What is an example of a non-excitable cell?
Epithelial cell
What are examples of excitable cells?
Neuron cells
Muscle cells
What establishes membrane potential?
Asymmetric distribution of ions across the membrane forming an ion concentration gradient as well as selective ion channels for K+, Na+ and Cl-
What is the Nernst equation?
The Nernst equation defines the electrochemical equilibrium between a concentration gradient and an electrical potential, for one particular ion.
The electrical potential that exactly balances a concentration gradient is called the reversal potential. The Nernst equation reveals how the reversal potential of an ion depends on the inside and outside concentrations of this ion:
z
R T F
Erev = RT ln [X]out zF [X]in
positive or negative charge
= 8.31 Joule/mol K◦
≈ 310 K at body temperature
= 96480 Coulomb/mol
RT ≈ 2576 Joule/mol ≈ 0.6kCal/mol
What is Coulomb’s law equation and what does it state?
F = k \frac{q_1q_2}{r^2}
F = electric force
k = Coulomb constant
q_1, q_2 = charges
r = distance of separation
- As charges increase so does the electric force
- As distance increases, the electric force decreases
What is the Electric field formula and what does it state?
E = F q test = k | Q | r 2
- As distance increases, the electric field decreases
- As electric force increases, the electric field increases
Describe the cell membrane in terms of capacitors/resistors
The membrane capacitors represent the insulating portion of the cell membrane, and the membrane resistors represent open ion channels that allow charge to move across the membrane. At the onset of the injected current, all of the injected charge initially flows onto the membrane capacitance.
In __________________ cells the membrane potential (Vm) is the same as the resting membrane potential (Vrest)
non-excitable
What is required to establish a membrane potential?
A semi-permeable membrane
What does the Goldman-Hodgkin-Katz equation calculate?
The resting potential
The resting potential of a cell is a compromise between the reversal potentials of all ions to which the membrane is permeable.
What is the Electrochemical driving force acting on ions equation and what does it state?
When an ion is not at its equilibrium, an electrochemical driving force (VDF) acts on the ion, causing the net movement of the ion across the membrane down its electrochemical gradient. The driving force is quantified by the difference between the membrane potential and the ion equilibrium potential (VDF = Vm − Veq.)
What is the correlation between the sign of driving force and the direction of ion flow for cations (+ve)?
SIGN OF DRIVING FORCE: +
DIRECTION OF ION FLOW: outward
SIGN OF DRIVING FORCE: 0
DIRECTION OF ION FLOW: no net flow
SIGN OF DRIVING FORCE: -
DIRECTION OF ION FLOW: inward
What is the correlation between the sign of driving force and the direction of ion flow for anions (-ve)?
SIGN OF DRIVING FORCE: +
DIRECTION OF ION FLOW: inward
SIGN OF DRIVING FORCE: 0
DIRECTION OF ION FLOW: no net flow
SIGN OF DRIVING FORCE: -
DIRECTION OF ION FLOW: outward
What two systems working in conjunction with each other are responsible for extracellular K+ and Na+ homeostasis?
Urinary systems (Kidneys) & Endocrine system
How is Na+ transported across the membrane?
Active transport via ATPase by secondary active transport
How is glucose transported across the membrane?
Facilitated transport via permease and secondary transport against concentration gradient
What are the different stages of action potential?
Depolarisation
Repolarisation
Hyperpolarisation
Resting Potential
Describe the different stages of an action potential?
- Depolarisation to threshold and beyond as Na+ gated channels are activated causing an influx of Na+ ions
- Overshoot occurs as there is a further activation of Na+ channels and Na+ permeability increases which causes the potential inside the membrane to become +ve
- Peak of the action potential; Na+ channels inactivate and K+ channels activate releasing K+ ions out of the cell
- Repolarisation occurs
- Hyperpolarisation occurs where K+ channels close and inactivate
- Sodium-potassium pump releases 3 Na+ out and 2 K+ which re-establishes the electrochemical gradient and brings the cell back to resting potential
Where are Na+ and K+ channels located?
Ranvier nodes/Nodes of Ranvier
Describe the anatomy of the neuron
Dendrites
Cell body
Axon
Axon terminals
Axons are covered with myelin sheaths constructed from Schwann cells. In between the myelin sheaths lie the nodes of ranvier in which the action potentials “jump” across which increases the speed of conduction
Oligodendrocytes are cells that make up the myelin sheaths of which nervous system?
Central nervous system
Oligodendrocytes are cells that make up the myelin sheaths of which nervous system?
Central nervous system
Define Repolarisation of membrane
Restoration of the difference in charge between the inside and outside of the cell membrane following depolarisation
What is the driving force of osmosis?
Increase in entropy generated by the movement of free water molecules
What is the major role of lipids?
Storing energy, signaling and acting as structural components of the cell membrane
Define Threshold potential
The critical level to which membrane potential must be depolarised to initiate an action potential
How are hydrophilic and hydrophobic side chains of amino acids distributed in peripheral membrane proteins in terms of interaction with aqueous environment?
The hydrophilic core blocks the diffusion of hydrophilic ions and polar molecules
