In this final passage of the Chem/Phys section, we take a look at lift force, potential weight, and the importance of paying attention to units.
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[02:24] Passage 10 (Questions 53-56]
If vesicles are deformed while being transported through the body, vesicle membranes exhibit a tank-treading motion. During movement, a force capable of lifting these non-spherical buoyant vesicles away from the substrate may result. To understand the role of vesicles may have in heart disease, researchers designed synthetic lipid vesicles to mimic erythrocytes and subjected them to shear flow close to a vessel wall. The aim was to study the effect of vesicle buoyancy, size, and volume.
Unilamellar vesicles are prepared by electro formation, 1,2-dioleoyl-sn-glycero-3-phosphocholine was dissolved in chloroform methanol solution. The solution was then spread on the conductive faces of two glass plates coated with transparent indium tin oxide (ITO). The plates were connected to a 10 Hz alternating current (AC), and the resulting 1.5 V potential difference was maintained between the plates for 3 hours.
Concentrations of solutions were altered to cause density variations. Finally, vesicles were diluted in a glucose solution of higher osmolality, but lower buoyancy. Osmosis produced partially deflated, flaccid deformable objects. Vesicles settled on a substrate, and their buoyancy was controlled and varied.
Figure one shows partial results and data for selected vesicles.
Note: They created these vesicles trying to mimic red blood cells, and they put them in a hypertonic solution. So it pulled some of the water out. They became flat tires if you will, and they want to see how these are going to act under these scenarios.
The explanation gives us the shaped diagram of vesicles underflow of one vesicle for different flow rates. So we have different rates of movement of blood flow that we have going on here.
And we can see that under condition one, it seems like it’s stuck to the plate so you can see the reflection there. But then as the flow rate increases, it starts to kind of detach. So we see the buoyancy by increasing the rate of flow within this.
Deflated vesicles detach and lift off from the substrate when the wall shear rate is higher than a minimum value, gc. This minimum depends upon the viscosity (h), the density of the vesicles in solution, the vesicle radius (R).
When the vesicles are unbound from the substrate, they hover at a distance (D) from the wall, which increases upon increasing the shear rate. At a D value sufficient to establish equilibrium there is a hydrodynamic lift force (fl), which is equal to the apparent weight (P) of the vesicle.
Note: They’re telling you that they’re looking at buoyancy, like some buoyant forces. That’s where the apparent weight comes in. Apparent weight is how much something weighs when it’s underwater.
[09:34] Question 53
What is the lift force for the vesicle reported in Table 1 that would be the last to detach from the vascular wall?
- 0.2 pN
- 1.53 pN
- 68.7 Pa/m
- 121.9 Pa/m
Thought Process:'Pay attention to units.'Click To Tweet
We’re looking for a force here which is measured in Newton. And C and D are in Pascal. And Pascal is a Newton/m2. In this case, C and D, are going to end up being Newtons/m3. It’s force divided by volume, which is not a force.
So even if you have no idea what’s going on, if you know that force is a Newton, then the answer has got to be A or B.
The answer here is B because it’s the apparent weight. That’s how much it’s going to wait to lift it which is going to acquire 153 N to lift. And A is going to require just 0.2 N so it’s going to be a whole lot easier.
Correct Answer: B
[14:33] Question 54
According to the results presented by the experiments, the relationship between the lift force and the vesicle radius would be best described as:
Based on what we know from the last question, the lift force is equal to the apparent weight. And so we know the equation for the apparent weight or P because it was given, which is:
P = (4pR3/3) Drg
Since the radius here is cubed, there is that exponential relationship. So the relationship between the lift force and the vertical radius would be described as exponential.
Just looking straight at the equation. If the radius doubles, the apparent weight is going to be eight times bigger, because it’s raised to the third power.
You can also answer this by looking at the table. As we go from the first row to the fourth row because the radius there goes from 8.7 to 27.3, which is about three times bigger. 9 x 3 is 27 so it’s close enough. Now, the apparent weight goes from point 0.2 to 13. That is not three times bigger. It’s way bigger so it’s definitely an exponential relationship.
Going over the answer choices, it would be inverse if it was the opposite. If one doubled and the other doubled, it’s linear. Sigmoidal would be increasing slowly first and then increasing very quickly, and then increasing slowly again. And if you look at the table it seems the apparent weight just keeps increasing a lot. But it never leveled out.
Correct Answer: C
[16:47] Question 55
When the researchers connected the solution-filled glass plates of the flow chamber to the AC generator, the ITO-coated plates most likely functioned as:
- a resistor.
- a capacitor.
- a galvanic cell
- an electrolyte cell.
Going through a resistor is something you’re going to have trouble getting current through. But the current will go through it.
A capacitor is building up charges on two different areas.
A galvanic cell is a battery that releases energy.
An electrolyte cell is a battery that we are inputting energy into or that we’re recharging.
So as they go through this, we talked about these plates. And there’s a potential difference between them. That means that there is a difference in charge from one plate to the other. And we’re creating an electric field voltage here.
So the answer here is a capacitor. In fact, there’s a type of capacitor called a parallel plate capacitor, which is exactly what this is as we have the two glass plates and distance between them.
That being said, there’s got to be some form of power source here, and the power source could be a galvanic cell. But the passage tells us that we connect this to this alternating current like outside of this. And electrochemical cells tend to produce direct currents, not alternating currents.
Important note: This is another example of how the MCAT likes to smash multiple topics together. This is a passage mostly about fluid dynamics. But now all of a sudden, there’s a question here about circuits.
Correct Answer: B
[16:47] Question 56
According to the experimental procedure, which of the following describes the physical properties of indium tin oxide?
- Electrically conducting
- Solid at standard temperature
- II only
- III only
- I and III only
- II and III only
It’s spread on these plates. And then we’re running water in between the plates to see how well the vesicles lift off of that.
It’s probably going to be solid because if it was a liquid, it would probably just wash away. And so we’re coating this with something here. Another thing to note is that we’re dealing with a tin oxide, which is going to be a metal. It’s an oxidized metal but it’s still metal. It’s going to be like a nail that’s rusted.
Given the fact that this is a metallic oxide, it definitely makes sense for it to be electrically conducting, and probably a solid. Also note that in the passage, it said transparent ITO, so you can immediately eliminate Roman Numeral I based off that.
Correct Answer: B
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