Surface tension berkeley review

[fullset]

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On page 69, example 7.9b:

A sealed vessel contains two chambers separated by a phospholipid bilayer membrane, as show in A. After completion of an experiment on the vessel, the membrane distorts, as shown in B. This distortion may have occurred because the pressure on the right side of the vessel:

A. increased, and the membrane's surface tension increased.
B. increased, and the membrane's surface tension decreased.
C. decreased, and the membrane's surface tension increased.
D. decreased, and the membrane's surface tension decreased.

I thought that the answer is A, because the membrane looks to be going toward a spherical shape, but the answer listed is B.

 

[BerkReviewTeach]

On page 69, example 7.9b:

A sealed vessel contains two chambers separated by a phospholipid bilayer membrane, as show in A. After completion of an experiment on the vessel, the membrane distorts, as shown in B. This distortion may have occurred because the pressure on the right side of the vessel:

A. increased, and the membrane's surface tension increased.
B. increased, and the membrane's surface tension decreased.
C. decreased, and the membrane's surface tension increased.
D. decreased, and the membrane's surface tension decreased.

I thought that the answer is A, because the membrane looks to be going toward a spherical shape, but the answer listed is B.

From their answer explanations: "The membrane is being forced to the left, so pressure on the right side must have increased relative to the pressure on the left side of the membrane. This eliminates choices C and D. Because the molecules of membrane are being stretched apart, they are feeling less of an attractive force towards one another. This means that the membrane’s surface tension must have decreased, making choice B a better explanation than choice A."

If you think about it, it would be easier to poke a hole in the membrane after the the pressure buildup, so it must have less surface tension.

 

[pineappletree]

That's a tricky question, I got the same thing wrong...



Would the "hole test" work for most cases??
=P

 

[Lokhtar]

Would the "hole test" work for most cases??
=P

That.....doesn't sound right.

 

[fizzgig]

If you think about it, it would be easier to poke a hole in the membrane after the the pressure buildup, so it must have less surface tension.

how does surface tension relate to wall tension, because i thought a larger balloon popped more easily exactly because tension was higher, not lower? as i stretch a rubber band (same amount of material over a larger distance) i have to pull harder as the band extends, not less hard.

it seems like surface tension is force tangent to surface and wall tension is the direct pressure-opposing force that points radially? i guess i am just missing how they connect...

http://hyperphysics.phy-astr.gsu.edu/hbase/ptens.html#bal
so for a given pressure, the wall tension of a vessel has to go UP if the radius goes up (harder to hold a pressure in with a larger radius, so need stronger or thicker vessel wall)...

and the surface tension eqn is the same really (P=Tsurf*k_rad, k_rad is rad of curvature), so ok...

then, for a unit area, if pressure (force, since P=F/A) increases and surface tension stays the same, you need higher curvature to deal with it. that works for both the surface tension and wall tension equation.

in this problem the area of the membrane increased as it stretched. pressure went up, and for a unit area the pressure force is opposed by fewer molecules and their spread out intermolecular forces. it makes sense that Tsurf would go down based on that idea, but the curvature also changes.

P=Tsurface*k_rad -- if P went up and k_rad went up, how do you know if Tsurface went down? for each unit area P and Tsurf must balance or the membrane would continue to move, but if the radius of curvature increased enough you could have the same surface tension as before...

ok i'm thoroughly frustrated... time for bed.. thanks for any insights :/

 

[fullset]

I'll send this link to BerkReviewTeach.

 

[Swagster]

how does surface tension relate to wall tension, because i thought a larger balloon popped more easily exactly because tension was higher, not lower? as i stretch a rubber band (same amount of material over a larger distance) i have to pull harder as the band extends, not less hard.

I thought surface tension was for liquids and that it measures how hard it is to rupture their surface. It seems like the balloon example deals with a solid and the restoring force between bonded molecules. Greater tension caused by stretching would put them closer to their breaking point, reducing the necessary external pressure to rupture the rubber membrane. I could be totally wrong, but it seems like rules for liquids are not the same as rules for solids. JAT (just a thought)!

 

[dblue10]

From their answer explanations: "The membrane is being forced to the left, so pressure on the right side must have increased relative to the pressure on the left side of the membrane. This eliminates choices C and D. Because the molecules of membrane are being stretched apart, they are feeling less of an attractive force towards one another. This means that the membrane’s surface tension must have decreased, making choice B a better explanation than choice A."

If you think about it, it would be easier to poke a hole in the membrane after the the pressure buildup, so it must have less surface tension.

I'm still a bit confused about this question but I think have an idea of how it works, if someone can confirm.

So, in 7.5a, we are asked what happens when you increase the surface tension of a cell wall, and the answer is that it becomes more spherical. In 7.5b, we are told that the membrane is distorted and then asked about the surface tension AFTER the distortion. So, yeah, after the pressure is changed on the right side, surface tension decreases for the reason in the book.

I guess another way of looking at this is that in 7,5a, the cell membrane is not attached to anything so if surface tension increases, it should become more spherical. But in 7,5b, since the membrane is attached at two unmovable, unstretchable ends, the ballooning out just means that tension decreases because of less attractive forces between molecules.

Did that make sense? Is that a right interpretation?

Thanks!

 

[Temperature101]

I thought A was the answer as well. I still dont understand why B is the answer even after BerkReviewTeach explanation.

 

[Dylan4081]

 

[DDW]

The correct answer is definitely A.

Surface tension depends on a difference between attractive forces at the surface and in the bulk of a fluid. In a phospholipid bilayer, there is only two layers and no "bulk fluid", so the deformation of a phospholipid bilayer actually has nothing to do with surface tension. It's really just a poor choice of examples.

I did some quick research and in real life, biophysicists model the deformation of phospholipid bilayer membranes using elastic (i.e. hooke's law) deformation. As predicted by hooke's law (F=-kx), the tension in the membrane increases as deformation increases.

This increase in membrane elastic tension should also be apparent by looking at conservation of forces. Since the membrane is not accelerating, the net force must be zero. To achieve zero net force, there must be a force opposing the increased pressure on the right side. If this opposing force was an increase in pressure on the left side, then there would be no deformation of the membrane. The only other possible opposing force would be the tension created in the membrane.

When talking about surface tension, you should really be thinking about a difference in attractive forces between two materials (i.e. adhesive and cohesive forces). Don't let this problem confuse you.