General Interview Questions - Water & Membranes

Describe the structure of lipid bilayer and compare it to the structure of cell membrane.

What is meant when we say that a molecule is hydrophilic or hydrophobic? Describe how each type of molecule interacts with water molecules.
Explain how it is that energy is stored in a concentration gradient.
What is diffusion and what causes it? How is osmosis different from or the same as diffusion?
What makes a chemical bond polar? What is a H-bond?
How do hydrophilic molecules pass through biological membranes?
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General importance
1. Hydrogen-bonding between water molecules is the cause of water's unique physiochemical properties. A water molecule can interact with four neighboring water molecules.
2. Molecules that cannot make H-bonds are insoluble in water; the larger such a molecule, the more insoluble. Such molecules are termed hydrophobic.
3. Lipids are a diverse class of molecules grouped based on their insolubility in water.  Of particular importance in biology are the amphipathic lipids, which contain two domains, one capable of making H-bonds, the other not. The ability to make H-bonds makes a region hydrophilic.
4. When dispersed into aqueous solvent, amphipathic lipids can self-assemble into higher order structures such as micelles and bilayers. In these states, the lipid's hydrophilic domain interacts with water while its hydrophobic domain(s) are removed from contact with water.
5. The primary boundary layer of a cell, the plasma membrane, is based on the ability lipids to self-assemble.
6. The plasma membrane poses a barrier to the movement of hydrophilic molecules into and out of the cell
7. Proteins within the plasma membrane regulate molecular movements into and out of the cell.
8. Within the plasma membrane is a concentrated solution of proteins and other small and macromolecules, the cytoplasm.
9. Energy can be stored in the form of chemical gradients across the membranes.
10. The high concentration of cytoplasmic components leads to osmotic effects across the plasma membrane.

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