Electrochemical Gradients & Signaling Flashcards
Define diffusion.
A spontaneous process in which a substance moves from a region of high concentration to a region of low concentration, eventually eliminating the concentration of difference between the two regions.
How does free energy relate to the way in which an electrolyte diffuses across the plasma membrane?
The greater the difference in charge (the potential difference or voltage) between the two compartments, the greater the difference in free energy. Thus, the tendency of an electrolyte to diffuse between two compartments depends on two gradients: a chemical gradient, determined by the concentration difference of the substance in the two compartments, and the electric potential gradient determined by the difference in charge.
What two conditions must be met in order for a substance to diffuse passively across a plasma membrane?
- The substance must be present at higher concentration on one side of the membrane than the other
- The membrane must be permeable to the substance.
What two conditions can make a membrane permeable to a given solute?
- The solute can pass directly through the lipid bilayer.
- The solute can transverse an aqueous pore that spans the membrane.
Define partition coefficient.
The ratio of a substance’s solubility in a nonpolar solvent to that in water. It describes the amount of energy needed to transport a hydrophilic molecule over the hydrophobic bilayer.
Define osmosis.
The movement of water from a region of low solute to a region of high solute.
What do the terms hypertonic and hypotonic mean?
The hypertonic compartment is the compartment of higher solute concentration in relation to the hypotonic compartment, which has a lower solute concentration.
How do aquaporins keep hydrogen protons from entering the cell?
Each aquaporin subunit contains a central channel that is lined with hydrophobic amino acid residues that is highly specific for water molecules. Water molecules interact with the inner surface of the channel, reorienting the molecule so that it cannot maintain the hydrogen bonds that normally link it to the other water molecules.
Define ion channels.
Openings in the membrane that are permeable to specific ions. Each is formed by integral membrane proteins that enclose an aqueous pore.
Do ion channels require energy to run?
No; they rely on passive transport. Ions moving through these channels always move from a place of higher energy to a place of lower energy.
What does the conformational state of a voltage-gated channel rely upon?
The conformation depends on the difference in ionic charge on the two sides of the membrane.
What does the conformational state of a ligand-gated channel rely upon?
It depends on the binding of a specific molecule (the ligand), which is usually not the solute that passes through the channel.
What does the conformational state of a mechano-gated channel rely upon?
It depends on mechanical forces (such as stretch tension) that are applied to the membrane.
Why can’t Na+ ions penetrate a K+ ion pore in a KcsA channel?
Inside the pore are eight oxygen atoms with which a K+ ion temporarily coordinates with; it is a type of selectivity filter. However, while this selectivity filter is a perfect match for K+, an Na+ ion is too large. Therefore, it cannot interact optimally with the eight oxygen atoms necessary to stabilize it in the pore and overcome the higher energy barrier required to penetrate the pore.
How is the gating of molecules in a KcsA channel accomplished?
Gating is accomplished by conformational changes of the cytoplasmic ends of the inner alpha helices. In the closed conformation, the helices are straight and cross over one another to form a “helix bundle” that seals the cytoplasmic face of the pore. The helices bend at a certain point where a glycine residue is located.
Describe the two distinct domains of the six helices (S1-S6) of a eukaryotic Kv channel.
- A pore domain which contains the selectivity filter that permits the selective passage of K+ ions.
- A voltage-sensing domain consisting of helices S1-S4 that senses the voltage across the plasma membrane.
What causes the S4 helix of a K+ channel to move, and why is it important?
The S4 helix connects the voltage-sensing domain of the channel to the pore domain. S4 responds to membrane depolarization (when the change in potential becomes more positive), which initiates a series of conformational changes within the protein that opens the gate at the cytoplasmic end of the channel.
How does inactivation of a K+ channel occur?
Inactivation is accomplished by movement of a small peptide that dangles from the cytoplasmic portion of the protein. When it moves up into the mouth of the pore, the passage of ions is blocked and the channel is inactivated. Later, the inactivation peptide is released and the gate to the channel is closed.
Define facilitated transport.
Substances always diffuse from a place of high concentration to low concentration, but they do not always do this through the lipid bilayer or a channel. Sometimes the substance first binds to a membrane-spanning protein called a facilitative transporter. that facilitates the diffusion process.
How is facilitated diffusion like an enzyme-catalyzed reaction?
Like enzymes, transporters exhibit saturation-type kinetics. Unlike ion channels, which can conduct millions of ions per second, most transporters can move only hundreds to thousands of solute molecules per second across the membrane. Facilitated diffusion is important in mediating the entry and exit of polar solutes, such as sugars and amino acids.
How is a favorable gradient maintained for glucose diffusion?
The cell phosphorylates the sugar after it enters the cytoplasm, thus lowering the intracellular glucose concentration.
Why must the affinity of a protein for sodium or potassium ions change during the activity of the sodium/potassium pump?
To begin, the pump must take K+ or Na+ ions from a place of low concentration, which requires high affinity to pull it away. Then, to be able to release the ions into an area of high concentration, the protein must lessen its affinity for the ion.
How does a protein decrease its affinity for an ion after it has moved from low to high concentration in an ATPase pump?
This is achieved by ATP hydrolysis and the subsequent release of ADP, which induces a large conformational change within the protein molecule.
How does secondary active transport work?
The establishment of concentration gradients provides a means by which free energy can be stored in a cell. The potential energy stored by these gradients can be used to transport other solutes.
Describe how the movement of glucose across the apical plasma membrane illustrates the idea of cotransport and secondary active transport.
Polysaccharides are hydrolyzed in the intestine, and the glucose that results needs to be moved into the epithelial cells of the intestine. Sodium ion concentration is kept low within the the epithelial cells , so the tendency for those sodium ions to diffuse back across the apical plasma membrane is “tapped” by the epithelial cells to drive the cotransport of glucose molecules into the cell against a concentration gradient. The sodium ion and glucose are coupled together to pass through the membrane.
Define terminal knob.
A terminal knob is the specialized site where impulses are transmitted from neuron to target cell.
How do K+ ions affect the membrane potential?
Thinking in terms of concentration, K+ ions want to flow out of the cell into a place of lower concentration. However, when they do so through K+ leak channels, they leave behind a negative charge. This favors the retention of K+ ions within the cell. This conflict of interest between concentration and voltage results in an equilibrium of K+ ions.
Define depolarization in the context of a membrane.
Depolarization refers to the increase in positivity in the membrane voltage, which causes a decrease in polarity between the two sides of the membrane: depolarization.
Describe what happens when the threshold for a stimulus has been reached.
The threshold refers to the depolarization of the membrane up to a point around - 50 mV. The change in voltage causes the voltage-gated sodium channels to open. As a result, sodium ions diffuse freely into the cell. The increased permeability of the membrane to sodium ions and movement of positive charge into the cell causes the membrane to reverse briefly, becoming positive at about +40 mV.
What is the relationship between the diameter of an axon and the speed with which an action potential can travel down it?
The greater the diameter of the axon, the less the resistance to current flow and the more rapidly an action potential can be conducted. The speed of conduction increases with the square root of the increase in diameter.
Define saltatory conduction.
A saltatory conduction is the mechanism by which action potentials are generated at nodes of Ranvier and conducted from node to node.
What is the synaptic cleft?
A space of about 20 to 50 nm between a neuron and its target cell.
What are synaptic vesicles?
Synaptic vesicles are found in the terminal knobs of the branches of an axon. They serve as storage sites for neurotransmitters.
Describe the sequence of events of synaptic transmission.
When an impulse reaches a terminal knob, its depolarization induces the opening of a number of voltage-gated Ca2+ channels, which diffuse into the terminal knob of the neuron and greatly increase the concentration of Ca2+. This causes the release of neurotransmitter molecules into the synaptic cleft. These neurotransmitter then diffuse across the gap and selectively bind to receptor molecules in the postsynaptic membrane.
What two effects can a neurotransmitter molecule have on its target cell membrane?
- The bound transmitter can trigger the opening of cation-selective channels in the membrane, leading to an influx of sodium ions and a less negative membrane potential. This depolarization excites the cell, making it more likely to generate its own action potential.
- The transmitter can trigger the opening of anion-selective channels, leading to an influx of chloride ions and a more negative membrane potential. Hyperpolarization of the postsynaptic membrane makes it less likely the cell will generate an action potential.
What is the resting electrical potential of a typical cell?
About 70 mV.
State Ohm’s Law.
V = IR
Voltage = current x resistance.
What do patch/clamp recordings measure, and how do they do so?
Patch/clamp recordings measure the movement of ions in ion channels. A cell is attached to an electrode, which clamps onto a tiny portion of its membrane. This causes covalent bonds to form between the glass of the electrode and the membrane, creating a tight seal against the current. This gives the researcher the ability to measure the current and number of ions that flow through voltage-gated channels.
What two characteristics of the selectivity filter in a K+ channel made it designed specifically for K+ ions?
- The size of the channel; it is stabilized specifically to fit potassium ions–no others.
- The pore R groups. Negatively-charged O of carbonyl groups interacts directly with the + ions. This replaces the normal water of hydration, energetically favoring ion passage.
In reference to K+ channels, what is the water of hydration, and why must it be removed?
The water of hydration refers to the sphere of water molecules that surround the potassium ion (because water is so polar). This must be stripped away in order to fit through the pore. The only energetically favorable way to strip away the oxygens of the water is to replace them with oxygens from the amino acid residues of the pore.
Briefly describe the process of generating an action potential.
- Voltage change reaches the threshold.
- Fast voltage-gated Na+ channels open first; Na+ enters the cell; depolarization.
- Na+ channels then inactivate in a time-dependent fashion; Na+ current stops.
- Slow voltage-gated channels open second; K+ exits the cell; hyperpolarization.
- The cycle repeats.
What is the vesicle hypothesis?
Vesicles containing a chemical signal fuse with the plasma membrane to deliver quantal packets of neurotransmitter.
How does a freeze slammer work and what does it aim to study?
A freeze slammer is a machine designed to “stop time” at the neuromuscular junction in order to study it. The nerve is electrically stimulated and then stomped down onto a platform to rapidly freeze the synapse.
Briefly describe the way an action potential is transmitted at the synapse.
- Action potential arrives; terminal depolarized.
- Ca2+ influx through voltage-gated channels.
- Ca2+ triggers vesicle fusion; secretion of transmitter into cleft.
- Transmitter binds receptor; opens ligand-gated ion channels.
