Question Set 3

Is the polar OH group of cholestrol involved in the rigidity of the phospholipids?

Not really. The rigidity as mentioned is contributed by the steriod rings. The hydroxyl group orient itself such that it is beside the polar head grps of adjacent phospholipids and thus allowing the rings to (interact and) immobilise the regions of the HC chains closer to the polar heads. =)

Next Up: Let's talk about Water Potential.

Graph on Water Potential...
For one class, I have already gone through the theory of water potential while the other awaits. But really, what usually have us scratched our heads off is the graph and that, I shall try my best to explain it. Stay with me, imagine with me and see me if need to.
Clarification for 3L: when I meant turgid today, I meant full turgor. Nothing less.

Common misunderstanding : solute potential = turgor pressure.
Solute potential within and outside cell will determine the direction of flow of water. After incipient plamoslysis in the 2nd half of the graph when surrounding medium becomes more dilute, more water will enter the cell. (note that the solute potential is increasing -> increasing dilution in the cell)
Turgor pressure is the resultant of the influx of water, determined by the difference in solute potential within and outside cell as mentioned above. The influx of water leads to the expansion of vacuole and the pressing of the cell membrane against cell wall.

Conclusion: turgor pressure is not the equivalent of solute potential and is only somewhat related.

Why is the graph of pressure potential a curve and not a straight line like solute potential.

1) Turgor pressure = pressure potential (Newton's 3rd Law). But since there is no immediate relationship between turgor pressure and solute potential, the graph of pressure potential does not have to follow a straight line.

So why is it a curve?
2) I think this is best explained by imagining blowing a balloon.
As you blow more and more air into the balloon, it becomes harder and harder to make it bigger even though you are sure you have blew in the same cheek-ful. The limitation comes from the elasticity of the balloon and past a certain limit, you are straining the bonds that keep the balloon intact and the reaction force becomes stronger. And even though the same cheekful of air is blew in, the balloon may not expand as much but the molecules are now closer in the balloon than expected.
The same idea can be applied to pressure potential graph and thus the curve. =)