WEEK 3 (Electric properties of plasma membranes)

Question 1

What are the properties of membrane potential?

Answer 1

• Cells are slightly -ve
• Hydrophobic bilayer is a poor conductor
• Ion concentration difference is based on ions
• Ion channels are involved in membrane transport

Question 2

What is membrane potential (Vm)?

Answer 2

The voltage difference across a cell plasma membrane

Question 3

What is an example of a non-excitable cell?

Answer 3

Epithelial cell

Question 4

What are examples of excitable cells?

Answer 4

Neuron cells
Muscle cells

Question 5

What establishes membrane potential?

Answer 5

Asymmetric distribution of ions across the membrane forming an ion concentration gradient as well as selective ion channels for K+, Na+ and Cl-

Question 6

What is the Nernst equation?

Answer 6

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

Question 7

What is Coulomb’s law equation and what does it state?

Answer 7

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

Question 8

What is the Electric field formula and what does it state?

Answer 8

E = F q test = k | Q | r 2

• As distance increases, the electric field decreases
• As electric force increases, the electric field increases

Question 9

Describe the cell membrane in terms of capacitors/resistors

Answer 9

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.

Question 10

In __________________ cells the membrane potential (Vm) is the same as the resting membrane potential (Vrest)

Answer 10

non-excitable

Question 11

What is required to establish a membrane potential?

Answer 11

A semi-permeable membrane

Question 12

What does the Goldman-Hodgkin-Katz equation calculate?

Answer 12

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.

Question 13

What is the Electrochemical driving force acting on ions equation and what does it state?

Answer 13

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.)

Question 14

What is the correlation between the sign of driving force and the direction of ion flow for cations (+ve)?

Answer 14

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

Question 15

What is the correlation between the sign of driving force and the direction of ion flow for anions (-ve)?

Answer 15

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

Question 16

What two systems working in conjunction with each other are responsible for extracellular K+ and Na+ homeostasis?

Answer 16

Urinary systems (Kidneys) & Endocrine system

Question 17

How is Na+ transported across the membrane?

Answer 17

Active transport via ATPase by secondary active transport

Question 18

How is glucose transported across the membrane?

Answer 18

Facilitated transport via permease and secondary transport against concentration gradient

Question 19

What are the different stages of action potential?

Answer 19

Depolarisation
Repolarisation
Hyperpolarisation
Resting Potential

Question 20

Describe the different stages of an action potential?

Answer 20

1. Depolarisation to threshold and beyond as Na+ gated channels are activated causing an influx of Na+ ions
2. 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
3. Peak of the action potential; Na+ channels inactivate and K+ channels activate releasing K+ ions out of the cell
4. Repolarisation occurs
5. Hyperpolarisation occurs where K+ channels close and inactivate
6. Sodium-potassium pump releases 3 Na+ out and 2 K+ which re-establishes the electrochemical gradient and brings the cell back to resting potential

Question 21

Where are Na+ and K+ channels located?

Answer 21

Ranvier nodes/Nodes of Ranvier

Question 22

Describe the anatomy of the neuron

Answer 22

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

Question 23

Oligodendrocytes are cells that make up the myelin sheaths of which nervous system?

Answer 23

Central nervous system

Question 23

Oligodendrocytes are cells that make up the myelin sheaths of which nervous system?

Answer 23

Central nervous system

Question 24

Define Repolarisation of membrane

Answer 24

Restoration of the difference in charge between the inside and outside of the cell membrane following depolarisation

Question 25

What is the driving force of osmosis?

Answer 25

Increase in entropy generated by the movement of free water molecules

Question 26

What is the major role of lipids?

Answer 26

Storing energy, signaling and acting as structural components of the cell membrane

Question 27

Define Threshold potential

Answer 27

The critical level to which membrane potential must be depolarised to initiate an action potential

Question 28

How are hydrophilic and hydrophobic side chains of amino acids distributed in peripheral membrane proteins in terms of interaction with aqueous environment?

Answer 28

The hydrophilic core blocks the diffusion of hydrophilic ions and polar molecules