13 Biological membranes, membrane transport. Electric properties of membranes Flashcards
What is the permeability constant?
The permeability constant is the amount of material transported through the membrane per unit surface in unit time due to unit concentration difference.
Give the equation describing the flux of material transported across a membrane due to concentration difference between two compartments separated by the membrane!
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
What does it mean that phospholipids are amphipathic molecules?
They consist of hydrophobic and hydrophilic parts.
What are the possible forms of motion of a lipid molecule in a membrane?
-lateral diffusion
-rotational diffusion
-transmembrane flip-flop
-flexible motion of fatty acid side chains.
What are the possible phase states of biological membranes?
-gel phase
-liquid crystal/ fluid phase
What changes in the properties of the fatty acid side chains increase the transition temperature of a membrane?
-increase in the saturation of fatty acid carbon-carbon bonds
-increase in the length of the fatty acid side chains
How does cholesterol change the fluidity of lipid membranes?
Below the phase transition temperature it usually increases, above the phase transition temperature it decreases.
What is the magnitude of the lateral diffusion constant of lipids and proteins in biological membranes?
lipids: 10-8-10-9 cm2/s
proteins: 10-9-10-12 cm2/s.
Which methods can be used to measure the lateral diffusion of proteins in biological membranes?
- Fluorescence Recovery After Photobleaching (FRAP)
- Single Particle Tracking (SPT)
- Fluorescence Correlation Spectroscopy (FCS)
For what kind of molecules is the permeability of biological membranes the highest?
For small, uncharged, apolar molecules, e.g. O2, N2.
For what kind of molecules is the permeability of biological membranes the highest?
For small, uncharged, apolar molecules, e.g. O2, N2.
What are the extra- and intracellular concentrations of Na+, K+ and Ca2+ ions?
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
What is passive transport?
Transport processes driven by the electrochemical potential gradient which do not need active metabolism are called passive transport.
What is active transport?
Transport processes that results in material flow against the electrochemical potential gradient at the expense of energy utilization are called active transport.
What is active transport?
Transport processes that results in material flow against the electrochemical potential gradient at the expense of energy utilization are called active transport.
What is primary active transport?
The transporter pumps ions/molecules across the membrane against their electrochemical gradient using energy from ATP hydrolysis.
What is secondary active transport?
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.
What is secondary active transport?
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.
What is simple diffusion across biological membranes?
The passive transport of small and lipid-soluble molecules across the membrane.
What is facilitated diffusion?
The passive transport of membrane impermeant ions/molecules across the membrane aided by transport proteins (ion channels or carriers) molecules.
What are the properties of facilitated or carrier- mediated diffusion?
-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.
What kind of ion channels do you know based on their mode of activation?
-ligand gated ion channels
-voltage gated ion channels
-second messenger gated ion channels
-mechanical deformation (stretch) activated ion channels.
What is the function of ionophores?
They promote the selective transport of ions through lipid membranes.
How can ionophores be grouped according to the mechanism of ion transport?
- carrier ionophores
- channel forming ionophores
- List the factors contributing to the maintenance of resting membrane potential!
- diffusion potential
- Donnan potential
- active transport by ion pumps.
Write the Nernst equation!
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.
What is the equilibrium potential of an ion?
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.
Are the ions on the two sides of the membrane of a living cell in thermodynamic equilibrium? Why?
No. There is no thermodynamic equilibrium because the net passive flux of a given ion is not zero at the resting membrane potential.
Are the ions on the two sides of the membrane of a living cell in thermodynamic equilibrium? Why?
No. There is no thermodynamic equilibrium because the net passive flux of a given ion is not zero at the resting membrane potential.
Describe the relationship between net fluxes of major permeating ions at the resting membrane potential!
JNa+JK+JCl=0, where Jx is the flux of ion x.
Give the definition and unit of flux!
Flux (J) is the amount of transported items across unit cross section area per unit time. Unit: mol/(m2s)
Give the definition and unit of flux!
Flux (J) is the amount of transported items across unit cross section area per unit time. Unit: mol/(m2s)
Give the Goldmann-Hodgkin-Katz equation!
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.
Give the definition and unit of conductivity!
G (conductivity)=1/R (where R is the resistance) Unit: 1/omhios =siemens (S).
Give the current carried by ion x through the cell membrane if the membrane potential is Em!
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.
Define the action potential and interpret the ionic basis of its generation.
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.
Define the action potential and interpret the ionic basis of its generation.
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.
How do the concentrations of Na+ and K+ in the intra- and extracellular spaces change during a single action potential?
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.
What causes the abrupt membrane potential changes during an action potential in a neuron?
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.
What is the role of the Na+/K+ ATPase in the membrane potential changes during a single action potential? Why?
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.
What is the role of the Na+/K+ ATPase in the membrane potential changes during a single action potential? Why?
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.
What is the principle of voltage-clamp?
The membrane potential is held at a controlled value independently of ion currents through the membrane.
List the methods that can be used for measuring the membrane potential of living cells!
-optical methods with membrane potential sensitive dyes;
-electrophysiological methods (microelectrode, current- clamp)
What kind of currents can be measured by patch- clamp?
-single channel currents (eg. cell-attached configuration)
-ion currents flowing through the entire membrane of the cell (whole-cell configuration).
What are the most important features of ion channels?
-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
What is the range of the voltages corresponding to the R waves of an ECG?
millivolts
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.
