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I've read the question about cell membranes breaking apart, which is close to what I'm asking, though I'm trying to probe a bit deeper.

I understand that there are hydrophobic forces keeping the cell membrane together, and I'm roughly picturing the strength of something like water surface tension. With this intuition, I tried to consider a few examples:

  1. I've read the idea that the cytoskeleton can help give a cell structure. But if a cell has some kinetic energy (say, red blood cells in the bloodstream) and runs into something else (say, the arterial wall), are the hydrophobic forces holding the cell membrane together strong enough to prevent the cytoskeleton from puncturing the membrane?
  2. Similarly, there are videos of bacteria with their flagella traveling at what seem to be pretty significant speeds (relative to their size) - are hydrophobic forces truly enough to hold the cell membrane together in the face of that kinetic energy? Are they a lot stronger than my intuition gives them credit for at the tiny scales of cells?
  3. Hydrophobic forces also seem to require being in solution - the obvious counterexample would be our skin cells, which clearly are not fully in solution, but even if that were the case, if hydrophobic forces were the bulk of the forces, then wouldn't any two cells merge the moment they made physical contact with each other?

This train of thought leads to the titular question and thought experiment:

Dropping a glass of water will result in the water going everywhere. If I were to jump up and then land, and hydrophobic forces are holding the cell membrane together, why doesn't my cell membrane fall apart from that application of kinetic energy?

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  • $\begingroup$ 1. RBCs repel endothelium because both have a resting membrane potential. When they are forced to pass through narrow passages as in the spleen, their cytoskeleton allows them to deform. 2. Glycocalyx prevents cells from fusing together. 3. Extracellular matrix has hyaluronic acid which gives resilience and deformability. 4. Surface cells die and leave keratin behind (used to be in their cytoplasms) which protects against abrasion and keeps the matrix inside. $\endgroup$ Dec 29, 2023 at 19:35

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The radius of an average human cell is approx $19$ micrometer. That gives it a a(n) (circular) area of $2000$ micrometer² (grossly exaggerating) = $2000 \times \left(\frac{1}{1000}\right)^2 = 0.00002$ cm²

If you fell on your buttocks on an area roughly $2$ palm spans, that would be an area of roughly $1000$ cm².

How many cells have you fallen on? $\frac{1000}{0.00002} = 50,000,000$ cells.

An average person weighs $68$ kg, that's around $680$ N. This force is distributed over $50,000,000$ cells. The force per cell (pressure) = $\frac{680}{50,000,000} = 0.0000136 \approx 10^{-5}$ Newtons. That ($10^{-5}$ Newtons) is the amount of force exerted by an object that weighs $1$ microgram. Pretty sure our cells can handle that amount of force.

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  • $\begingroup$ One zero is omitted in 1/1000. In the final calculation it is repaired. $\endgroup$
    – Mercury
    Mar 9 at 17:10

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