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Bio/Biochem Lesson 2 Questions
#1 Posted : Thursday, June 03, 2021 1:31:55 AM
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Hi! I have a few questions below:

1. Why is it that micelles can form spontaneously in an aqueous environment, while lipid bilayers cannot?
2. Is the “electrical” part of an electrochemical gradient specifically referring to the direction that a positively charged molecule wants to move (BB2 Classroom Companion)?
3. I don’t quite understand what dynamic equilibrium is referring to in terms of membrane transport. In particular, what does it mean when we say that osmosis stops “…when osmotic pressure is equal to hydrostatic pressure”?

Thank you so much!

Karen He
#2 Posted : Thursday, June 03, 2021 2:47:26 PM
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Hello Karen!

1. The hydrophobic effect is the driving force of the formation of micelles and the bilayer membrane. For larger and bulkier lipids that contain thicker hydrocarbon components, these structures will form the bimolecular sheet (also called the lipid bilayer). The reason is that the two fatty acyl chains of a phospholipid or a glycolipid are too bulky to fit into the interior of a micelle. ... The formation of bilayers instead of micelles by phospholipids is of critical biological importance. A micelle is a limited structure, usually less than 20 nm (200 Å) in diameter.

2. electrical part of the gradient is the drive of charge. So if the inside of a cell has a positive charge, negative molecules will want to come in down its electrical gradient. Chemical is the concentration of molecules- so is there is not a lot of Na in the cell, this will drive Na into the cell down its chemical gradient.

3. Dynamic equilibrium meaning the lipid bilayer is moving. Phospholipids in the lipid bilayer can either move rotationally, laterally in one bilayer, or undergo transverse movement between bilayers. Lateral movement is what provides the membrane with a fluid-structure. '

4. This is sort of a different concept. Osmotic pressure refers to the pressure based on the concentration of proteins/molecules- if there is little proteins in the blood (someone who is malnourished) there would be little osmotic pressure, so small amount of water in the blood being driven from osmotic pressure into the cell/capillaries. Hydrostatic pressure is the water driving pressure, so someone with high blood pressure will have high hydrostatic pressure and will push lots of fluid into the cell/capillaries. When these 2 forces are = osmosis would stop, but typically hydrostatic outweigh osmotic so you get constant fluid movement/osmosis.

Hope this helps!
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