Macromolecules Part 2 Notes

Phospholipids share similarities with fats, such as glycerol molecule.
Fats consist of a glycerol molecule connected to three fatty acids.
- The connection involves a carboxyl group from the fatty acid chain.
- Fats are largely nonpolar due to long hydrocarbon chains, making them interact poorly with water.
Phospholipids differ from fats by having two hydrocarbon chains instead of three.
- They maintain the glycerol molecule backbone.
- The third hydroxyl group of the glycerol connects to a phosphate group, giving it a negative charge.
- An additional head group is connected to the phosphate group, varying among different phospholipids.
- Variety exists in phospholipids due to different head groups and fatty acid tails.
Fatty acid tails can differ; one may have a kink due to a cis double bond, while the other is fully saturated.
Space-filling models illustrate the volume of a phospholipid, showing glycerol, phosphate (yellow), and oxygen molecules (red), with the head group (e.g., choline).
The head group contributes to the name of the phospholipid (e.g., phosphatidylcholine).
The glycerol, phosphate, and head group constitute the hydrophilic head, while hydrocarbon chains form the nonpolar tails.
Phospholipids are amphipathic molecules, possessing both nonpolar and polar regions.

In aqueous solutions, phospholipids self-assemble into double-layered sheets called bilayers spontaneously.
- They arrange tail-to-tail, with polar heads facing water and hydrophobic tails in the interior.
- The tails attract each other via Van der Waals forces and are shielded from water.
- Bilayers spontaneously close into sealed compartments to further shield hydrocarbon tails from water.
The cylindrical shape of phospholipids promotes bilayer formation, unlike conical fatty acids that form micelles.
Lipid micelles have hydrocarbon tails pointing inward, away from water.
In aqueous solutions, phospholipids arrange tail-to-tail, forming two monolayers which make the bilayer, with hydrophilic heads in contact with water.
Phospholipid bilayers are crucial as boundaries for cells and organelles, defining what is inside versus outside.
Spontaneous closure of the bilayer is energetically favorable, shielding hydrocarbon tails from water.

Steroids are lipids characterized by a four fused-ring structure.
Cholesterol is a type of steroid and a component of cell membranes and the backbone for steroid hormones.
Steroid hormones, such as cortisol and testosterone, act as signaling molecules between cells.
- They can be delivered directly or transported via the bloodstream.
- Despite structural similarities, slight differences in structure lead to significant functional differences.
- Receptors bind steroids with high specificity.
Cholesterol is embedded in phospholipid bilayers, with its hydroxyl group near the polar heads and the hydrophobic rings inserted alongside the phospholipid tails.
The four fused rings of the cholesterol molecule is nonpolar, except for the hydroxyl group.
Membranes have sidedness, with different phospholipid types and protein attachments on the outer versus inner monolayers.
Cholesterol molecules are positioned with the hydroxyl group near the polar heads of phospholipids and nonpolar rings inserted next to the phospholipid tails, contributing to the membrane's hydrophobic interior.