Still not understanding TBR's surface tension questions

[keikoblue2]

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I read the previous threads and am still confused. On pg 65 in the Physics Fluids and Solids chapter, question 7.5b asks what caused the phospholipid bilayer to distort (it looks like it stretched from a flat membrane to a more spherical one). It says since the molecules of the membrane are being stretched apart, the molecules feel less attracted to each other, and thus, surface tension decreases.

Ok, cool, I get that. But then for question 7.5a, the answer seems contradictory to the reasoning behind b's explanation.

7.5a- "increasing the surface tension of a cell surface will..." make the cell's shape more spherical. Explanation: "When surface tension increases on a lipid, its surface decreases in area (although volume does not change). The resulting shape always tends towards spherical."

Using the logic from part b, when surface tension increases, its molecules are brought closer together so they feel more attracted to each other. This would compress, instead of stretch, the cell membrane, making the cell flatten. I think I follow the answers' logic by themselves, but if I use the rationale behind one answer for the other question, the answers contradict. Help?

 

[athockey7]

when you have increased surface tension then particles will come closer together no matter what right..

so for part b it is clear that line should become more like a line instead of curved and far apart when there is not as much surface tension..

now when you increase the surface tension in part a for the sphere, imagine several lines of particles acting just like example 2 coming closer together... but since it is happening all over the sphere it takes a more spherical shape..

clearly i am spit balling. dont take anything i just said as fact.. but does it kinda make sense what i said?

 

[keikoblue2]

when you have increased surface tension then particles will come closer together no matter what right..

so for part b it is clear that line should become more like a line instead of curved and far apart when there is not as much surface tension..

now when you increase the surface tension in part a for the sphere, imagine several lines of particles acting just like example 2 coming closer together... but since it is happening all over the sphere it takes a more spherical shape..

clearly i am spit balling. dont take anything i just said as fact.. but does it kinda make sense what i said?

For part B, the cell membrane goes from flat to a more curved shape. And the book says it's because surface tension decreased. But for part a, it's saying an increase in surface tension will make the cell more curved. I'm so confused 🙁 Thank you, though.

 

[Dylan4081]

Same here lol it contradicts itself in the readings. In 7.5 b The picture goes from a flat surface to a more rounded surface and it says the "surface tension decreases." So confused.

 

[brood910]

Why did you bump two posts for the same question?

Anyways, you are missing the fact that this problem is about MEMBRANE, not liquids.
YES. Increasing surface tension OF liquids will shrink their surface area and thus forming a sphere.

This problem has nothing to do with liquids. Just think about it. If you stretch something, molecules will be less likely to attach to one another. As pressure difference is made, the membrane will be pushed towards the left + the membrane will be stretched. Thus, less intermolecular forces = less surface tension.

 

[inasensegone]

I read the previous threads and am still confused. On pg 65 in the Physics Fluids and Solids chapter, question 7.5b asks what caused the phospholipid bilayer to distort (it looks like it stretched from a flat membrane to a more spherical one). It says since the molecules of the membrane are being stretched apart, the molecules feel less attracted to each other, and thus, surface tension decreases.

Ok, cool, I get that. But then for question 7.5a, the answer seems contradictory to the reasoning behind b's explanation.

7.5a- "increasing the surface tension of a cell surface will..." make the cell's shape more spherical. Explanation: "When surface tension increases on a lipid, its surface decreases in area (although volume does not change). The resulting shape always tends towards spherical."

Using the logic from part b, when surface tension increases, its molecules are brought closer together so they feel more attracted to each other. This would compress, instead of stretch, the cell membrane, making the cell flatten. I think I follow the answers' logic by themselves, but if I use the rationale behind one answer for the other question, the answers contradict. Help?

Brood is pretty spot on (as usual).

I just wanted to point out that the problem with your logic is that you're assuming that because liquids have a more spherical shape with higher surface tension, the increase in spherical shape of the membrane must be caused by higher surface tension. This is a classical logical fallacy (I believe it's called affirming the consequent).