Lecture 2 (Membrane Potentials)

Question 1

Describe the membrane lipid bilayer

Answer 1

• Low electric conductivity
• Acts like an insulator
• Separates the EXTRACELLULAR FLUID and CELL CYTOPLASM and accumulation of opposite charges on both surfaces of membrane

Question 2

Biological membranes are _________________

Answer 2

Capacitors

a device used to store electric charge

Question 3

Describe the components of a Capacitor

Answer 3

• Made up of 2 close conductors separated by a DIELECTRIC MATERIAL
• Plates accumulate electric charge when connected to a power source
• One plate accumulates positive charge and the other accumulates negative charge

Question 4

What is a Capacitor?

Answer 4

An electronic component that stores electric charge

Question 5

What is the Capacitance?

Answer 5

The amount of electric charge that is stored in the capacitor at voltage of 1 Volt

Measured in units of FARAD (F)

Question 6

What is the equation for Capacitance?

Answer 6

C = Q / V

C = Capacitance in farad (F)
Q = Electric charge in coulombs (C) stored in the capacitor
V = Voltage between the capacitor’s plates in volts (V)

Question 7

_________ _________ membrane is an insulator (dielectric) which has a capacitance

Answer 7

Lipid bilayer

Question 8

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

Answer 8

F = k \frac{q_1q_2}{r^2}
F = electric force
k = Coulomb constant
q_1, q_2 = charges
r = distance of separation

• As charge increases so does the electric force
• As distance increases, the electric force decreases

Question 9

What is the constance Ke equal to?

Answer 9

1/(4πε0) where ε0 is a constant known as the PERMITTIVITY OF A VACUUM

Question 10

What is ε?

Answer 10

• Permittivity (characterizes the tendency of the atomic charge in an insulating material to distort in the presence of an electric field)
• A measure of how easily a medium is polarised by an electric field
• Dielectric constant of the medium

Question 11

Is permittivity temperature dependent?

Answer 11

YES

Question 12

What does the dielectric constant of water being very high reflect?

Answer 12

Water’s exceptional ability to conduct electricity

Question 13

Explain why water has a high permittivity

Answer 13

The polar molecules of water have -ve ends facing +ve charge and +ve ends facing -ve charge -> Orientation creates small electric fields that oppose the electric fields of +ve and -ve charge in the medium -> This reduces the FORCE & POTENTIAL ENERGY between +ve and -ve charge

Question 14

_________ is directly proportional to voltage and inversely proportional to resistance

Answer 14

Current

Question 15

What is the equation to calculate current?

Answer 15

I = V / R

I = current (amperes)
V = electromotive force (volts)
R = resistance (ohms)

Question 16

A flow rate of how many electrons per second is an electrical current of 1 Ampere (Amp)?

Answer 16

6.24 X 10^18

Question 17

Describe the electric model of the membrane

Answer 17

• Contains CAPACITANCE (lipid bilayer) and OHMIC (ions’ protein channels) RESISTANCE
• Capacitance and Ohmic resistance defines the SELECTIVE PERMEABILITY to ions and determines ions’ non-equal redistribution on both sides of membrane
• Formation of chemical and electrical gradients and potential difference

Question 18

What is Membrane potential (V) measured by?

Answer 18

Difference in potential between the inside minus the outside (V INSIDE - V OUTSIDE)

Question 19

What does Resting potential determine?

Answer 19

Energy storage (Em) essential for regulation of cell homeostasis

Question 20

What happens if the membrane is permeable just for potassium ions?

Answer 20

K+ will be transported from high concentration area to low concentration area -> Charge separation occurs and a voltage develops across the membrane

Question 21

What does the Diffusion force depend on?

Answer 21

Ions’ concentration in both compartments of the container

Question 22

What does the Electrostatic force depend on?

Answer 22

Ions’ valency and the difference of potentials between membrane surfaces

Question 23

What is the Nernst equation?

Answer 23

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

Question 24

What does the Goldman-Hodgkin-Katz equation calculate?

Answer 24

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 25

What is Donnan’s equilibrium?

Answer 25

It refers to the uneven distribution of charged particles on one side of a semi-permeable membrane.

However, if there is an IMPERMEABLE SOLUTE in one of the solutions, the concentration doesn’t equalise. The concentration of the solution with impermeable solutes remains high even at equilibrium.

Question 26

The flow of molecules and ions between a cell and its environment is regulated by which effect?

Answer 26

Donnan effect

Question 27

`What is the mechanism that prevents cells from swelling and rupturing?

Answer 27

The Na+ K+ pump

It prevents cells from rupturing by continuously pushing out excess ions

Question 28

What is the oncotic pressure of blood responsible for?

Answer 28

Non-equal distribution of ions in blood and interstitial space