ICE 5 Review Session Marty and Aaron November
ICE 5 Review Session Marty and Aaron November 9, 2014
Diffusion It is energetically favorable for molecules to move down their concentration gradient. ΔGrxn But why? Entropy increases (the system is less ordered). Depending on the identity of the particles, ethalpy might decrease (e. g. , if these particles had opposite charges or interacted favorably with each other).
Crossing membranes A lipid vesicle filled only with water is placed in a 1 M solution of sucrose. Which of the following would occur? A. B. C. D. E. F. G. Sucrose will enter the vesicle Sucrose will exit the vesicle The vesicle will shrivel The vesicle will swell A and D B and C None of these Large, polar molecules (e. g. , sucrose) cannot cross the membrane, but small polar molecules can (e. g. , water). As a result, water moves down its concentration gradient and leaves the vesicle, causing it to shrink and shrivel. Sucrose: 1 M Exterior Interior Sucrose: none
Why don’t ions cross the membrane? Na+ ΔHrxn >> 0 Hydrated ion is too large to cross the membrane. Na+ +
How to get an ion across a membrane… The antibiotic valinomycin enables K+ ions to passively diffuse across the cell membrane. Based on its structure, propose a mechanism for how valinomycin allows K+ ions to cross membranes. Valinomycin Space-filling Sticks Valinomycin works a lot like the potassium channel. It folds into a shape that surrounds the potassium ion and interacts with it via ion-dipole interactions with its carbonyl groups. This allows it to displace the water molecules that it would otherwise bind. The molecule contains non-polar “side chains” like those found in valine and alanine, allowing it to be soluble in the hydrophobic membrane.
Membrane potential Consider a large vesicle with 150 m. M K+ on its exterior and 10 m. M K+ on its interior. What would the sign of the membrane potential become if K+ channels embedded in such a vesicle were to open? Is the movement of K+ through the ion channel passive or active? K+ enters the vesicle because its concentration is higher outside the vesicle. It moves down it’s concentration gradient, making this passive transport. K+ = 10 m. M As K+ enters the vesicle, the interior of the vesicle becomes more positive and the exterior of the vesicle becomes more negative. Therefore, the membrane potential becomes positive. K+ = 150 m. M
Membrane potential Consider a large vesicle with initial concentrations of 150 m. M K+ outside and 10 m. M K+ inside. Potassium channels are opened and time passes such that the membrane potential stops changing. Will there be a higher concentration of potassium inside or outside of the vesicle? K+ = ? m. M There will be a greater concentration of potassium outside of the vesicle. If potassium were uncharged and diffused down its concentration gradient, the concentration inside and outside of the vesicle would become equal. However, potassium is positive, and the membrane potential becomes more and more positive as potassium enters the vesicle. As a result there would be some point at which the membrane potential would become so positive that it would prevent additional potassium from entering the vesicle, even though there is still a greater concentration on the outside of the vesicle.
Membrane potential A certain cell has the following intra- and extracellular concentrations of various salts. Intracellular Extracellular KCl Na. Cl 150 m. M 15 m. M 200 m. M Ca. Cl 2 0. 1 μM 1 m. M Which ion channels, when opened, would make the cell’s membrane potential negative? Inside: Outside: K+ Na+ Ca 2+ Cl- 150 m. M 15 m. M 10 m. M 200 m. M 0. 1 μM 1 m. M ~160 m. M 217 m. M Negative Positive Negative
Secretory pathway
ER-resident protein: Requires a second signal sequence, a “retention sequence”
lysosome-resident protein: Requires a second signal sequence, a “retention sequence”
A mutation to which region(s) of a protein would most likely result in a membrane-bound ER-resident protein localizing to the plasma membrane? Mutant protein Normal protein Step that diverges = where the mutation must be A: ER-retention sequence
A mutation to which region(s) of a protein would most likely result in a membrane-bound ER-resident protein being secreted? Mutant protein A: ER-retention sequence AND the transmembrane domain Normal protein Steps that diverge = where the mutation must be
A mutation to which region(s) of a protein would most likely result in a membrane-bound ER-resident protein localizing to the cytosol? Mutant protein A: ER-signal sequence (AND the transmembrane domain… just because a TMD may result in aggregating for a cytosolic protein, but that wouldn’t be a required part of the answer) Normal protein Step that diverges = where the mutation must be
- Slides: 31