What do membrane proteins provide to a membrane?
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Specificity and function.
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What do membrane proteins provide to a membrane?
Specificity and function.
How do lipid-soluble molecules move across cell membranes?
Lipid-soluble molecules move readily across cell membranes, and the rate of diffusion depends on lipid solubility.
What is the role of ion channels in the membrane?
To allow specific ions to pass through the membrane.
How do water-soluble molecules cross the cell membrane?
Water-soluble molecules cross the cell membrane via channels or other transport proteins.
What is the role of carrier proteins in the membrane?
To transport specific molecules across the membrane.
What is the main factor that affects the rate of diffusion for lipid-soluble molecules?
Lipid solubility.
What determines transport through ungated channels?
Size, shape, and charge of the channel and ion.
What are the two types of gated channels?
Voltage-gated channels and chemical-gated channels.
What is the Nernst potential (equilibrium potential)?
The theoretical intracellular electrical potential that would be equal in magnitude but opposite in direction to the concentration force.
What would happen if a membrane were permeable only to K+?
K+ would diffuse down its concentration gradient until the electrical potential across the membrane countered diffusion.
What is the Nernst equation?
EMP (mV) = ±61 log (Ci / Co)
What is the principle of sodium channel operation?
The channel conducts 'all or none,' meaning it is either open or closed.
When will negatively charged molecules stop entering the cell?
When the intracellular electrical potential is equal in magnitude but opposite in direction to the concentration force.
What technique is used to study sodium channel operation?
Patch Clamp
What ions are conducted through the sodium channel?
Na+ (sodium ions)
What is primary active transport?
Molecules are 'pumped' against an electrochemical gradient at the expense of energy (ATP).
What type of energy is used in primary active transport?
Direct use of energy (ATP).
What is osmosis?
Osmosis is the net diffusion of water from pure water toward a water/salt solution.
What drives secondary active transport?
The energy stored in the electrochemical gradient of another molecule (usually Na+).
In which direction does water move during osmosis?
Water moves down its concentration gradient, from pure water toward a water/salt solution.
What type of energy is used in secondary active transport?
Indirect use of energy.
Why can a membrane channel be permeable to potassium but not to sodium, even though the potassium molecule is larger?
Carbonyl oxygens in the selectivity channel strip water molecules from the potassium molecule but not from the sodium molecule, allowing only potassium ions to permeate.
What role do carbonyl oxygens play in the selectivity of the potassium channel?
They strip water molecules from the potassium molecule, enabling it to permeate through the channel.
What are examples of impermeant molecules?
Ions, Glucose, Amino acids
What are examples of permeant molecules?
Urea, Glycerol
What does the α subunit bind in primary active transport?
ATP, 3 Na+, and 2 K+
What characteristic do permeant molecules have?
Relatively high lipid solubility
What type of transport is described as electrogenic but contributes less than 10% to the membrane potential?
Primary active transport
What is the function of antiporters in secondary active transport?
Antiporters transport substances in the opposite direction of a 'driver' ion like Na+.
What type of enzyme is Na+-K+ ATPase?
An antiporter enzyme located on the plasma membrane of all animal cells.
What does the Goldman-Hodgkin-Katz equation describe about the resting membrane potential?
The resting membrane potential is closest to the equilibrium potential for the ion with the highest permeability.
What is another name for facilitated diffusion?
Carrier-mediated diffusion.
What ions does Na+-K+ ATPase pump and in which direction?
It pumps sodium ions out of cells and pumps potassium ions into cells against electrochemical gradients.
What limits the rate of diffusion in facilitated diffusion?
The Vmax of the carrier protein.
What critical role does Na+-K+ ATPase play in cells?
It regulates osmotic balance by maintaining Na+ and K+ balance.
What happens when Na+-K+ ATPase is inhibited by ouabain?
Cells swell and burst.
Does adding more carriers affect the Vmax in facilitated diffusion?
No, adding more carriers does not affect Vmax.
How much energy does Na+-K+ ATPase require in a typical cell and in neurons?
About 1/5 of a typical cell’s energy and up to 2/3 of a neuron’s energy.
What happens to the transported molecule during facilitated diffusion?
It binds to the carrier protein, undergoes a conformational change, and is then released.
What is net diffusion?
Net diffusion is the difference between the magnitude of diffusion from side A to B and the magnitude of diffusion from side B to A.
What is membrane potential (V_m)?
A charge difference across the membrane.
How does passive diffusion of potassium (K+) and sodium (Na+) contribute to membrane potential?
Passive diffusion of K+ and Na+ leads to a charge difference across the membrane, contributing to the development of a negative membrane potential.
Which ions are primarily involved in creating the membrane potential?
Potassium (K+) and Sodium (Na+).
What is the direction of potassium (K+) diffusion in relation to the cell membrane?
Potassium (K+) diffuses out of the cell.
Where is Ca2+ ATPase present?
On the cell membrane and the sarcoplasmic reticulum in muscle fibers.
What is the direction of sodium (Na+) diffusion in relation to the cell membrane?
Sodium (Na+) diffuses into the cell.
What is the function of Ca2+ ATPase?
It maintains a low cytosolic Ca2+ concentration.
Where is H+ ATPase found?
In parietal cells of gastric glands (HCl secretion) and intercalated cells of renal tubules (controls blood pH).
What is the Na+ equilibrium potential (ENa)?
The electrical potential that counters net diffusion of Na+.
What is the function of H+ ATPase?
It concentrates H+ ions up to 1 million-fold.
What would happen if a membrane were permeable to only Na+?
Na+ would diffuse down its concentration gradient until the potential across the membrane countered diffusion.
What drives the diffusion of Na+ across the membrane?
The concentration gradient of Na+.
What determines the steady-state cell volume?
The concentration of impermeant particles in the extracellular fluid (e.g., Na+, K+, protein-).
What counters the diffusion of Na+ across the membrane?
The electrical potential across the membrane.
What effect do permeant particles like urea and glycerol have on cell volume?
They cause only transient changes in cell volume.
What is the relationship between membrane permeability and the time course of a change in cell volume?
Higher membrane permeability results in more transient changes in cell volume.
Which has higher membrane permeability, urea or glycerol?
Urea has higher membrane permeability than glycerol.
What are ionophores?
Lipid-soluble molecules (usually synthesized by microorganisms) that transport inorganic ions across the lipid bilayer and mediate passive transport.
What are the two types of ionophores?
Mobile ion carriers and channel formers.
What substances does the lipid bilayer act as a barrier to?
Water and water-soluble substances such as ions, glucose, H2O, and urea.
What is the structure of mobile ion carriers?
They have a hydrophobic exterior and a hydrophilic interior.
Why is the lipid bilayer a barrier to water-soluble substances?
Because the lipid bilayer is hydrophobic, preventing water-soluble substances from passing through easily.
What is the function of channel formers like gramicidin A?
They form ion-permeable pores in the cell membrane.
Where do action potentials (APs) occur during saltatory conduction?
At the nodes (Nodes of Ranvier).
Name two gases that can diffuse through the lipid bilayer.
CO2 and O2.
How do channel formers compare to mobile ion carriers in terms of ion transport?
Channel formers transport 1000 times more ions per unit time than mobile ion carriers.
Why do action potentials only occur at the nodes during saltatory conduction?
Because Na channels are concentrated at the nodes.
What characteristic of the lipid bilayer allows it to be a barrier to ions?
Its hydrophobic nature.
What is one benefit of saltatory conduction in terms of signal transmission?
Increased velocity.
What is the absolute refractory period?
A period during which an action potential (AP) is not possible due to voltage inactivation of Na channels.
What is the relative refractory period?
A period during which a greater than normal stimulus is required to elicit an action potential (AP).
What limits the maximum frequency of action potentials (APs)?
Refractory periods.
Why is an action potential not possible during the absolute refractory period?
Due to voltage inactivation of Na channels.
How does saltatory conduction contribute to energy efficiency in neurons?
It conserves energy.
What ions are involved in simple diffusion through leak channels as shown in the image?
Na+ (Sodium) and K+ (Potassium).
What is required to elicit an action potential during the relative refractory period?
A greater than normal stimulus.
What is the typical range for the resting membrane potential (Vm)?
-90 to -70 mV
In which direction does K+ move during simple diffusion through leak channels?
From inside to outside the cell.
How do cardiac glycosides like Digoxin increase cardiac contractility?
Cardiac glycosides increase cardiac contractility by inhibiting the Na+/K+ ATPase pump, leading to an increase in intracellular Na+ levels. This, in turn, reduces the activity of the Na+/Ca++ exchanger, resulting in increased intracellular Ca++ levels, which enhances cardiac contractility.
What is the equilibrium potential for potassium (EK)?
-94 mV
What is the effect of inhibiting the Na+/K+ ATPase pump on intracellular ion levels?
Inhibiting the Na+/K+ ATPase pump increases intracellular Na+ levels and reduces the activity of the Na+/Ca++ exchanger, leading to increased intracellular Ca++ levels.
In which direction does Na+ move during simple diffusion through leak channels?
From outside to inside the cell.
What is the equilibrium potential for sodium (ENa)?
+61 mV
What is the consequence of increased intracellular Ca++ levels in cardiac cells?
Increased intracellular Ca++ levels enhance cardiac contractility.
What type of channels do Na+ and K+ use for simple diffusion?
Leak channels.
Why is the resting membrane potential (Vm) so close to the equilibrium potential for potassium (EK)?
The membrane is far more permeable to K+ than Na+.
What is the process called when Na+ and K+ move through leak channels?
Simple diffusion.
What is the direction of sodium (Na+) movement in the transcellular transport of glucose and amino acids (AA)?
Sodium (Na+) moves from the lumen into the epithelial cell and then into the extracellular fluid.
What is the concentration gradient of sodium (Na+) in the epithelial cell during the transcellular transport of glucose and amino acids (AA)?
The concentration of sodium (Na+) is high in the epithelial cell and low in both the lumen and extracellular fluid.
What triggers the opening of voltage-gated sodium channels?
The membrane potential becoming less negative and reaching the threshold.
What role does potassium (K+) play in the transcellular transport of glucose and amino acids (AA)?
Potassium (K+) is transported into the epithelial cell from the extracellular fluid to maintain the sodium-potassium balance.
What enzyme do glycosides (e.g., digoxin) inhibit?
Na+-K+ ATPase.
What is the role of the voltage sensor in a voltage-gated sodium channel?
It detects changes in membrane potential and triggers the opening of the channel.
What is the effect of glycosides inhibiting Na+-K+ ATPase on intracellular Na+ concentration?
Increased intracellular Na+ concentration.
What is the relationship between the rate of diffusion and the concentration gradient in simple diffusion?
The rate of diffusion is directly proportional to the concentration gradient (Co - Ci).
What is the function of the selectivity filter in a voltage-gated sodium channel?
It ensures that only sodium ions can pass through the channel.
How does the inhibition of Na+-K+ ATPase by glycosides affect the Na+ electrochemical gradient?
It decreases the Na+ electrochemical gradient.
What limits the maximum rate of facilitated diffusion?
The maximum rate of facilitated diffusion is limited by Vmax.
What happens to the voltage-gated sodium channel when the membrane potential reaches the threshold?
The gate opens, allowing sodium ions to flow into the cell.
What is the formula for the Sodium Nernst Potential (E_Na)?
E_Na = -61 x log(Na_i/Na_o)
How does the rate of diffusion change with increasing concentration gradient in simple diffusion?
In simple diffusion, the rate of diffusion increases linearly with the concentration gradient.
What happens to the activity of Na+/Ca2+ antiporters when Na+-K+ ATPase is inhibited by glycosides?
The activity of Na+/Ca2+ antiporters decreases.
What is the role of the inactivation gate in a voltage-gated sodium channel?
It closes the channel after a certain period, stopping the flow of sodium ions.
What characterizes action potentials as all-or-none events?
They have a threshold voltage, usually 15 mV positive to the resting potential.
What are the given concentrations of Na_o and Na_i in the example?
Na_o = 142 mM and Na_i = 14 mM
How does the rate of diffusion change with increasing concentration gradient in facilitated diffusion?
In facilitated diffusion, the rate of diffusion increases with the concentration gradient but eventually reaches a maximum rate (Vmax).
How are action potentials initiated?
By depolarization, and they can be induced in nerve and muscle by extrinsic (percutaneous) stimulation.
What is the result of decreased activity of Na+/Ca2+ antiporters due to glycosides inhibiting Na+-K+ ATPase?
Increased intracellular Ca2+ concentration.
What is the calculated Sodium Nernst Potential (E_Na) in the example?
+61 mV
What is Vmax in the context of facilitated diffusion?
Vmax is the maximum rate of facilitated diffusion.
What are some causes of hypernatremia (increased plasma Na+)?
Increased water loss, excessive sweat loss, central or nephrogenic diabetes insipidus (decreased ADH secretion or responsiveness to ADH).
Do action potentials have constant amplitude?
Yes, action potentials do not summate; information is coded by frequency, not amplitude.
What is one of the factors that affect the net rate of diffusion?
Concentration difference (C_o - C_i).
How do cardiac glycosides increase cardiac contractility?
By increasing intracellular Ca2+ concentration.
What would the membrane potential (Vm) be if the membrane were permeable only to Na+?
+61 mV
What condition is characterized by decreased ADH secretion or responsiveness to ADH?
Central or nephrogenic diabetes insipidus.
What determines the conduction velocity of action potentials in nerve fibers?
The diameter of the fiber. Larger diameter fibers conduct faster than smaller ones.
How is the Sodium Nernst Potential (E_Na) calculated in the example?
E_Na = -61 log(14/142) = -61 log(0.1) = +61 mV
How is net diffusion related to the concentration difference?
Net diffusion is proportional to the concentration difference (C_o - C_i).
What are some causes of hyponatremia (decreased plasma Na+)?
Large water ingestion, Syndrome of Inappropriate ADH Secretion (SIADH).
What is the general rule for the conduction velocity of myelinated fibers?
Fiber diameter (in mm) x 4.5 = velocity in m/s.
What does Syndrome of Inappropriate ADH Secretion (SIADH) lead to?
Water retention, hyponatremia, and excretion of concentrated urine.
What is the general rule for the conduction velocity of unmyelinated fibers?
Square root of fiber diameter = velocity in m/s.
What does the symbol 'D' represent in the context of diffusion?
'D' represents the diffusion coefficient.
Why is the cell membrane so permeable to K+?
Due to the presence of K+ 'leak' channels and the selectivity filter that allows K+ ions to pass through easily.
What happens when there is too much ADH in the body?
It leads to water retention, hyponatremia, and excretion of concentrated urine.
What role do K+ 'leak' channels play in cell membrane permeability?
They allow K+ ions to move across the cell membrane, contributing to its high permeability to K+.
In the context of diffusion, what do C_o and C_i represent?
C_o represents the concentration outside the membrane, and C_i represents the concentration inside the membrane.
What is the function of the selectivity filter in K+ channels?
The selectivity filter ensures that only K+ ions can pass through the channel, maintaining the membrane's permeability to K+.
What happens to the net diffusion rate if the concentration difference (C_o - C_i) increases?
The net diffusion rate increases.
How does the Na+-K+ pump contribute to ion balance in the cell?
The Na+-K+ pump actively transports 3 Na+ ions out of the cell and 2 K+ ions into the cell, using ATP.
What is the ratio of Na+ to K+ ions transported by the Na+-K+ pump?
The Na+-K+ pump transports 3 Na+ ions out of the cell and 2 K+ ions into the cell.
What is the resting membrane potential for skeletal muscle fibers?
-85 to -95 mV
What is the point of communication between neurons?
Synapses.
What is the resting membrane potential for smooth muscle fibers?
-50 to -60 mV
What is the structure of ion channels?
Proteins that span the membrane and have a water-filled channel that runs through the protein.
What do most synapses involve?
Transmitter substances.
What is the resting membrane potential for astrocytes?
-80 to -90 mV
What are the properties of ion channels?
They have conducting states and non-conducting states, and the transition between states is called 'gating'.
What are the two types of synapses?
Excitatory and Inhibitory.
What cells surround the nerve axon to form a myelin sheath?
Schwann cells.
What does the term 'gating' refer to in ion channels?
The transition between conducting and non-conducting states.
What is the resting membrane potential for neurons?
-60 to -70 mV
What is an axo-dendritic synapse?
A synapse between the axon of one neuron and the dendrite of another.
What are the three types of stimuli that can cause ion channels to 'gate'?
Changes in membrane potential (voltage-gated channels), occupation of receptor (ligand-gated or receptor-operated channels), and mechanical forces (mechanosensitive channels).
What is the role of sphingomyelin in myelination?
It decreases membrane capacitance and ion flow 5,000-fold.
What is the function of myelin?
Myelin insulates axons to increase the speed of electrical transmission.
What type of ion channels are involved in action potential propagation?
Voltage-gated channels.
What initiates the propagation of an action potential?
Opening of Na+ channels generates local current that depolarizes adjacent membrane, opening more Na+ channels.
How often is the myelin sheath interrupted along the nerve axon?
Every 1-3 mm.
What is the resting membrane potential for photoreceptor cells in the dark?
-40 mV
What is the state of the membrane during rest in the context of action potential propagation?
The membrane is polarized with a negative charge inside and a positive charge outside.
What is the name of the interruptions in the myelin sheath?
Node of Ranvier.
What type of ion channels initiate action potentials?
Ligand-gated or receptor-operated channels (ROCs).
What is the resting membrane potential for photoreceptor cells in the light?
-70 mV
What is the main function of the sodium-potassium pump?
To pump sodium (Na+) out of the cell and potassium (K+) into the cell.
What happens during local depolarization in action potential propagation?
Na+ channels open, causing a local current that depolarizes the adjacent membrane.
What type of ion channels are important for hearing?
Mechanosensitive channels.
How many sodium ions are pumped out of the cell by the sodium-potassium pump?
Three sodium ions (3 Na+).
How does the action potential propagate along the membrane?
The local current spreads, opening more Na+ channels and depolarizing adjacent sections of the membrane.
How many potassium ions are pumped into the cell by the sodium-potassium pump?
Two potassium ions (2 K+).
What is the sequence of events in the propagation of an action potential?
Rest, local depolarization (stimulation), and propagation (current spread).
What is the net driving force on K+ ions?
20 mV
What molecule provides the energy for the sodium-potassium pump to function?
ATP (Adenosine Triphosphate).
How does an action potential respond to stimulus strength?
It is independent of stimulus strength (all or none).
What is the net driving force on Na+ ions?
135 mV
How is a subthreshold potential change related to stimulus strength?
It is proportional to stimulus strength (graded).
What happens to ATP during the active transport of Na+ and K+?
ATP is converted to ADP (Adenosine Diphosphate).
Which way do the ions diffuse when permeability increases?
Ions diffuse in the direction of their electrochemical gradient.
What type of disease is Multiple Sclerosis (MS)?
An immune-mediated inflammatory demyelinating disease of the CNS.
Where is sodium (Na+) pumped in the sodium-potassium pump process?
Sodium (Na+) is pumped out of the cell.
What effect does increasing Na+ or K+ permeability (or extracellular concentration) have on Vm?
Increasing Na+ or K+ permeability (or extracellular concentration) affects the membrane potential (Vm) by altering the net driving force on the ions.
Where is potassium (K+) pumped in the sodium-potassium pump process?
Potassium (K+) is pumped into the cell.
What is the net driving force on any ion?
The net driving force on any ion is the difference in millivolts between the membrane potential (Vm) and the equilibrium potential for that ion (Eion).
Does an action potential change in magnitude as it propagates?
No, it is propagated unchanged in magnitude.
How does a subthreshold potential change propagate?
It is not propagated but decremental with distance.
Can summation occur with action potentials?
No, summation is not possible.
Can summation occur with subthreshold potential changes?
Yes, subthreshold potential changes exhibit summation.
What happens to the post-synaptic cell during hyperpolarization?
The post-synaptic cell is hyperpolarized, which depresses excitability and is inhibitory.
What is an inhibitory post-synaptic potential (IPSP)?
An IPSP is a hyperpolarization of the post-synaptic cell that depresses excitability.
What is depolarization?
Depolarization is the process of the membrane potential becoming less negative (moving towards 0 mV).
What is hyperpolarization?
Hyperpolarization is the process of the membrane potential becoming more negative than the resting potential.
What does overshoot mean in the context of membrane potential?
Overshoot refers to the membrane potential becoming positive, moving above 0 mV.
What is repolarization?
Repolarization is the process of the membrane potential returning towards the resting potential after depolarization.
Can IPSPs summate?
Yes, IPSPs can summate.
What is excitability in the context of membrane potentials?
Excitability refers to the ability of a cell to respond to stimuli and generate action potentials.
What results from increases in membrane permeability?
EPSPs (excitatory post-synaptic potentials) and IPSPs (inhibitory post-synaptic potentials) result from increases in membrane permeability.
What is secondary active transport?
It involves the use of an electrochemical gradient (usually for sodium).
What is the threshold in the context of action potential generation?
The threshold is the membrane potential level that must be reached for an action potential to be generated.
What should you consider when thinking about which ions are involved in synaptic responses?
You need to consider the equilibrium potentials of the ions.
How are protein cotransporters classified?
They are classified as symporters or antiporters.
What do symporters do in secondary active transport?
They transport substances in the same direction as a 'driver' ion like Na+.
What is the permeability coefficient of water in an artificial lipid bilayer?
10^-2 cm/sec
Give an example of a symporter in secondary active transport.
Na+ and glucose (Na+ gluc).
Which substances have a permeability coefficient of 10^-4 cm/sec in an artificial lipid bilayer?
Urea
What is the role of Na+ in secondary active transport?
Na+ acts as a 'driver' ion in symporters, facilitating the transport of other substances.
How does the permeability of water compare to that of glucose in an artificial lipid bilayer?
Water has a much higher permeability (10^-2 cm/sec) compared to glucose (10^-8 cm/sec).
What is the general trend of permeability coefficients for substances in an artificial lipid bilayer?
Permeability decreases from water to ions like Cl-, K+, and Na+.
What does the upper record represent in synaptic responses?
The upper record is from the presynaptic cell.
What does the lower record represent in synaptic responses?
The lower record is from the postsynaptic cell.
