Phospholipid bilayers form sealed spherical vesicles in...

Phospholipid bilayers form sealed spherical vesicles in water (discussed in Chapter 11 ). Assume you have constructed lipid vesicles that contain $\mathrm{Na}^{+}$ pumps as the sole membrane protein, and assume for the sake of simplicity that each pump transports one $\mathrm{Na}^{+}$ one way and one $\mathrm{K}^{+}$ the other way in each pumping cycle. All the Nat pumps have the portion of the molecule that normally faces the cytosol oriented toward the outside of the vesicles. With the help of Figure $12-11$, determine what would happen if: A. Your vesicles were suspended in a solution containing both $\mathrm{Na}^{+}$ and $\mathrm{K}^{+}$ ions and had a solution with the same ionic composition inside them. B. You add ATP to the suspension described in (A). C. You add ATP, but the solution-outside as well as inside the vesicles-contains only Nat ions and no K $^{+}$ ions. D. The concentrations of $\mathrm{Na}^{+}$ and $\mathrm{K}^{+}$ were as in (A), but half of the pump molecules embedded in the membrane of each vesicle were oriented the other way around so that the normally cytosolic portions of these molecules faced the inside of the vesicles. You then add ATP to the suspension. E. You add ATP to the suspension described in (A), but in addition to Na' pumps, the membrane of your vesicles also contains $K^{+}$ leak channels.

Phospholipid bilayers form sealed spherical vesicles in water (discussed in Chapter 11 ). Assume you have constructed lipid vesicles that contain $\mathrm{Na}^{+}$ pumps as the sole membrane protein, and assume for the sake of simplicity that each pump transports one $\mathrm{Na}^{+}$ one way and one $\mathrm{K}^{+}$ the other way in each pumping cycle. All the Nat pumps have the portion of the molecule that normally faces the cytosol oriented toward the outside of the vesicles. With the help of Figure $12-11$, determine what would happen if:
A. Your vesicles were suspended in a solution containing both $\mathrm{Na}^{+}$ and $\mathrm{K}^{+}$ ions and had a solution with the same ionic composition inside them.
B. You add ATP to the suspension described in (A).
C. You add ATP, but the solution-outside as well as inside the vesicles-contains only Nat ions and no K $^{+}$ ions.
D. The concentrations of $\mathrm{Na}^{+}$ and $\mathrm{K}^{+}$ were as in (A), but half of the pump molecules embedded in the membrane of each vesicle were oriented the other way around so that the normally cytosolic portions of these molecules faced the inside of the vesicles. You then add ATP to the suspension.
E. You add ATP to the suspension described in (A), but in addition to Na' pumps, the membrane of your vesicles also contains $K^{+}$ leak channels.

Key Concepts

Phospholipid Bilayers

Phospholipid bilayers are the fundamental structural components of cell membranes. They form self-sealing, flexible barriers that separate the interior of a cell or vesicle from the external environment, and their hydrophobic and hydrophilic regions create the basis for compartmentalization essential in biological systems.

Membrane Protein Orientation

Membrane protein orientation refers to the directional placement of proteins within the bilayer. For functional membrane proteins such as ion pumps, the orientation determines which side of the membrane interacts with substrates, and misorientation can reverse or alter the transport activity, affecting the ion gradients across the membrane.

Active Ion Transport

Active ion transport is the process by which ions are moved across membranes using energy, often derived from ATP hydrolysis. This mechanism is essential for creating and maintaining ion gradients that drive many cellular functions, and is executed by proteins such as pumps that move ions against their concentration gradients.

ATP Hydrolysis

ATP hydrolysis is the chemical reaction that breaks down ATP into ADP and inorganic phosphate. This reaction provides the energy required for many cellular processes, including active transport mechanisms mediated by membrane pumps that move ions against their natural diffusion gradients.

Electrochemical Ion Gradients

Electrochemical ion gradients result from differences in ion concentrations and electrical charge across membranes. These gradients are crucial for various cellular functions, including electrical signaling and secondary active transport, and are established through the concerted action of active transporters such as the Na+/K+ pump.

Membrane Leak Channels

Membrane leak channels, or passive ion channels, allow ions to diffuse across the membrane down their concentration gradients. Their presence can influence membrane potential and modulate the effects of active ion transport by providing pathways for ions to move, thereby helping to stabilize or dissipate the electrochemical gradients created by pumps.

Transcript

00:01 Okay, so this is a really good question about a lipid vesicle made of a phospholipid bilayer with a sodium potassium pump.

00:10 And it seems like the sodium potassium pump has a portion of the molecule that normally faces the cytosol oriented towards the outside of the vesicle.

00:22 Okay, so this is a really good question.

00:24 So let's draw what these normally look like and then we can figure out how this looks in the vesicle.

00:29 So this is the normal cell membrane and here is the sodium potassium pump.

00:41 Normally, so this is the cytosol, okay, normally, let's see, we have sodium pumped out, outside of the cell, three sodiums, for every two potassiums coming in.

01:04 Now, let's reread.

01:06 It says that the portion that normally faces the cytosol, so this portion here, right, is going to be oriented towards the outside of the vesicle.

01:18 So that's going to be facing out, okay? so let's actually kind of like readjust our labels.

01:28 And so instead of the cytosol, this is going to be the outside of the vesicle, which is actually the cytosol.

01:40 So let's actually keep that the same way it is.

01:42 I'm going to cross it out, okay? but i am going to kind of close this loop so that we can see visually, right? so this is going to be the vesicle.

01:55 I'm not going to draw it, but imagine that all along here, we had a phospholipid bilayer, okay? and this whole structure is the vesicle and this is the inside of the vesicle.

02:12 Okay, cool.

02:14 So the only thing that i need to add to this is where the atp is going to be used.

02:24 And the atp to fuel this cellular work is always on the side with the potassium.

02:32 So this is where atp is converted to adp.

02:41 Adp, plus your inorganic phosphate.

02:45 Okay, so now we have our diagram and now let's start tackling the questions.

02:51 So one of the questions is, part a, what would happen if your vesicle was suspended in a solution containing both sodium and potassium ions and had a solution with the same ionic composition inside? okay, so this is a really good question.

03:09 So if the concentrations are equal on both sides of this and there's no atp, we would see no net movement of ions because we don't have any atp listed in the scenario to fuel any movement of ions, right? but in part b, we actually do have atp added to the solution where the vesicles are suspended.

03:45 And so what we would see is the, and let's check, the sodium being pumped into the vesicle.

03:54 So we would see an increase in the sodium concentration in the vesicle.

04:05 And we would see a decrease in the potassium concentration in the vesicle...

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