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.
