13 Biological membranes, membrane transport. Electric properties of membranes

Question 1

What is the permeability constant?

Answer 1

The permeability constant is the amount of material transported through the membrane per unit surface in unit time due to unit concentration difference.

Question 2

Give the equation describing the flux of material transported across a membrane due to concentration difference between two compartments separated by the membrane!

Answer 2

Jm= -p Delta c
Jm – is the flux of the material, which is the amount of material transported through a unit membrane area in a unit time Unit: mol/(m2s)
p – membrane permeability constant, (m/s)
Delta c – concentration difference between the concentrations on the two sides of the membrane

Question 3

What does it mean that phospholipids are amphipathic molecules?

Answer 3

They consist of hydrophobic and hydrophilic parts.

Question 4

What are the possible forms of motion of a lipid molecule in a membrane?

Answer 4

-lateral diffusion
-rotational diffusion
-transmembrane flip-flop
-flexible motion of fatty acid side chains.

Question 5

What are the possible phase states of biological membranes?

Answer 5

-gel phase
-liquid crystal/ fluid phase

Question 6

What changes in the properties of the fatty acid side chains increase the transition temperature of a membrane?

Answer 6

-increase in the saturation of fatty acid carbon-carbon bonds
-increase in the length of the fatty acid side chains

Question 7

How does cholesterol change the fluidity of lipid membranes?

Answer 7

Below the phase transition temperature it usually increases, above the phase transition temperature it decreases.

Question 8

What is the magnitude of the lateral diffusion constant of lipids and proteins in biological membranes?

Answer 8

lipids: 10-8-10-9 cm2/s
proteins: 10-9-10-12 cm2/s.

Question 9

Which methods can be used to measure the lateral diffusion of proteins in biological membranes?

Answer 9

• Fluorescence Recovery After Photobleaching (FRAP)
• Single Particle Tracking (SPT)
• Fluorescence Correlation Spectroscopy (FCS)

Question 10

For what kind of molecules is the permeability of biological membranes the highest?

Answer 10

For small, uncharged, apolar molecules, e.g. O2, N2.

Question 10

For what kind of molecules is the permeability of biological membranes the highest?

Answer 10

For small, uncharged, apolar molecules, e.g. O2, N2.

Question 11

What are the extra- and intracellular concentrations of Na+, K+ and Ca2+ ions?

Answer 11

Na+ ec. 140 mM ic. 10-20 mM
K+ ec. 5 mM ic. 140-150 mM
Ca2+ ec 1-2 mM ic 10-4 mM

Question 12

What is passive transport?

Answer 12

Transport processes driven by the electrochemical potential gradient which do not need active metabolism are called passive transport.

Question 13

What is active transport?

Answer 13

Transport processes that results in material flow against the electrochemical potential gradient at the expense of energy utilization are called active transport.

Question 13

What is active transport?

Answer 13

Transport processes that results in material flow against the electrochemical potential gradient at the expense of energy utilization are called active transport.

Question 14

What is primary active transport?

Answer 14

The transporter pumps ions/molecules across the membrane against their electrochemical gradient using energy from ATP hydrolysis.

Question 15

What is secondary active transport?

Answer 15

Secondary active transport moves ions/molecules across the membrane against their gradient using energy stored in the gradient of another ion, created by a primary active transport mechanism.

Question 16

What is secondary active transport?

Answer 16

Secondary active transport moves ions/molecules across the membrane against their gradient using energy stored in the gradient of another ion, created by a primary active transport mechanism.

Question 17

What is simple diffusion across biological membranes?

Answer 17

The passive transport of small and lipid-soluble molecules across the membrane.

Question 18

What is facilitated diffusion?

Answer 18

The passive transport of membrane impermeant ions/molecules across the membrane aided by transport proteins (ion channels or carriers) molecules.

Question 19

What are the properties of facilitated or carrier- mediated diffusion?

Answer 19

-transporter proteins specifically bind the transported molecules
-the transport can be selectively inhibited
-the flux of transport can be saturated over a given concentration of the transported material.

Question 20

What kind of ion channels do you know based on their mode of activation?

Answer 20

-ligand gated ion channels
-voltage gated ion channels
-second messenger gated ion channels
-mechanical deformation (stretch) activated ion channels.

Question 21

What is the function of ionophores?

Answer 21

They promote the selective transport of ions through lipid membranes.

Question 22

How can ionophores be grouped according to the mechanism of ion transport?

Answer 22

• carrier ionophores
• channel forming ionophores

Question 23

22. List the factors contributing to the maintenance of resting membrane potential!

Answer 23

• diffusion potential
• Donnan potential
• active transport by ion pumps.

Question 24

Write the Nernst equation!

Answer 24

where U0,x (or Ex) is the equilibrium potential of the given ion,
R is the universal gas constant
T is the absolute temperature
zx is the charge of the given ion
F is the Faraday constant
[x]e and [x]i are the extra- and intracellular concentrations of the given ion, respectively.

Question 25

What is the equilibrium potential of an ion?

Answer 25

The membrane potential where the net flux of the given ion is zero, i.e. the system is in thermodynamic equilibrium for the given ion.

Question 26

Are the ions on the two sides of the membrane of a living cell in thermodynamic equilibrium? Why?

Answer 26

No. There is no thermodynamic equilibrium because the net passive flux of a given ion is not zero at the resting membrane potential.

Question 26

Are the ions on the two sides of the membrane of a living cell in thermodynamic equilibrium? Why?

Answer 26

No. There is no thermodynamic equilibrium because the net passive flux of a given ion is not zero at the resting membrane potential.

Question 27

Describe the relationship between net fluxes of major permeating ions at the resting membrane potential!

Answer 27

JNa+JK+JCl=0, where Jx is the flux of ion x.

Question 28

Give the definition and unit of flux!

Answer 28

Flux (J) is the amount of transported items across unit cross section area per unit time. Unit: mol/(m2s)

Question 28

Give the definition and unit of flux!

Answer 28

Flux (J) is the amount of transported items across unit cross section area per unit time. Unit: mol/(m2s)

Question 29

Give the Goldmann-Hodgkin-Katz equation!

Answer 29

Um (Em) is the resting membrane potential (diffusion potential);
R is the universal gas constant;
T is the absolute temperature;
F is the Faraday constant;
[x]0 and [x]i are the extra- and intracellular concentrations of ion x, respectively;
px is the permeability of the cell membrane for ion x.

Question 30

Give the definition and unit of conductivity!

Answer 30

G (conductivity)=1/R (where R is the resistance) Unit: 1/omhios =siemens (S).

Question 31

Give the current carried by ion x through the cell membrane if the membrane potential is Em!

Answer 31

where Ix is the current;
Gx is the conductivity of the membrane for ion x (G=1/R); Em is the actual membrane potential;
Ex is the equilibrium potential of ion x.

Question 32

Define the action potential and interpret the ionic basis of its generation.

Answer 32

The action potential is a characteristic, time-dependent change in the membrane potential as a result of time- and membrane potential-dependent changes in the ion permeabilities of the membrane. In the depolarization phase the Na+ permeability is dominant, the subsequent repolarization is the result of a concurrent decrease in Na+ permeability and an increase in K+ permeability.

Question 33

Define the action potential and interpret the ionic basis of its generation.

Answer 33

The action potential is a characteristic, time-dependent change in the membrane potential as a result of time- and membrane potential-dependent changes in the ion permeabilities of the membrane. In the depolarization phase the Na+ permeability is dominant, the subsequent repolarization is the result of a concurrent decrease in Na+ permeability and an increase in K+ permeability.

Question 34

How do the concentrations of Na+ and K+ in the intra- and extracellular spaces change during a single action potential?

Answer 34

Due to the short duration of opening of Na+ and K+ channels the concentrations of Na+ and K+ change negligibly during a single action potential.

Question 35

What causes the abrupt membrane potential changes during an action potential in a neuron?

Answer 35

Changes in the permeabilities for Na+ and K+ and the consequent miniature inward Na+ and outward K+ currents, without significant changes in the concentration gradients, lead to the abrupt membrane potential changes in a neuronal action potential.

Question 36

What is the role of the Na+/K+ ATPase in the membrane potential changes during a single action potential? Why?

Answer 36

The Na+/K+ ATPase does not have any role in the membrane potential changes in a single action potential, because:
(i) it is too slow to transport enough ions in a couple of milliseconds to change ion concentrations significantly or carry significant net charge,
(ii) the concentration gradients of Na+ and K+ do not change in a single action potential, so these concentration gradients do not need to be restored.

Question 36

What is the role of the Na+/K+ ATPase in the membrane potential changes during a single action potential? Why?

Answer 36

The Na+/K+ ATPase does not have any role in the membrane potential changes in a single action potential, because:
(i) it is too slow to transport enough ions in a couple of milliseconds to change ion concentrations significantly or carry significant net charge,
(ii) the concentration gradients of Na+ and K+ do not change in a single action potential, so these concentration gradients do not need to be restored.

Question 37

What is the principle of voltage-clamp?

Answer 37

The membrane potential is held at a controlled value independently of ion currents through the membrane.

Question 38

List the methods that can be used for measuring the membrane potential of living cells!

Answer 38

-optical methods with membrane potential sensitive dyes;
-electrophysiological methods (microelectrode, current- clamp)

Question 39

What kind of currents can be measured by patch- clamp?

Answer 39

-single channel currents (eg. cell-attached configuration)
-ion currents flowing through the entire membrane of the cell (whole-cell configuration).

Question 40

What are the most important features of ion channels?

Answer 40

-selective permeability: the channel is permeable only to a certain ion species;
-gating: appropriate trigger/signal/event causes a conformational change in the channel protein resulting in the transition among different states (closed, open, inactivated) of the channels

Question 41

What is the range of the voltages corresponding to the R waves of an ECG?

Answer 41

millivolts

Question 42

What is the meaning of an ECG curve in the case of unipolar and bipolar leads?
Unipolar: It is the potential difference between an exploring and a reference electrode.
Bipolar: It is the potential difference between two exploring electrodes.