Lipid Structure and Function Notes

Lipids play key roles in various biological phenomena:
- Beer: The taste and smell of hops come from lipids called terpenes.
- Human Eyes: The ability of the human eye to respond to light relies heavily on retinal, a lipid derived from Vitamin A.
- Sperm Whales: They use spermaceti, a lipid with a density that dramatically changes with temperature, to adjust their density for deep dives (up to 3 kilometers) to hunt giant squid.
General Characteristics of Lipids:
- Insoluble in water.
- Soluble in nonpolar organic solvents.
- Diverse in structure and function, serving structural, signaling, and energy storage roles.

Lipids are major components of the phospholipid bilayer, which is crucial for cell function.
The phospholipid bilayer separates the cell interior from the surrounding environment.
Key structural lipids include phospholipids, glycerophospholipids, and sphingolipids.
Waxes are another unique class of structural lipids.

Membrane lipids are amphipathic, possessing both hydrophilic and hydrophobic regions.
- Polar Head: Hydrophilic region.
- Fatty Acid Tails: Hydrophobic region.
In aqueous solutions, these molecules spontaneously form structures:
- Liposomes
- Micelles
- Phospholipid bilayers

Composition:
- Phosphate and alcohol (polar head group).
- Hydrophobic fatty acid tails.
- Joined by phosphodiester linkages.
Fatty acids attach to a backbone to form the hydrophobic tail region.
Classification based on Backbone:
- Glycerol: Forms glycerophospholipids (phosphoglycerides).
- Sphingosine: Forms sphingolipids.
All these lipids have long-chain fatty acid tails.

Contain one or more double bonds.
Double bonds introduce kinks in the chain.
Make it difficult to stack and solidify.
Example: Olive oil.
Tend to be liquids at room temperature.
Phospholipids with unsaturated fatty acid tails create more fluid regions in the phospholipid bilayer.
Phospholipids, glycerophospholipids, and sphingolipids can have different fatty acid tails and head groups, influencing their properties at the cell membrane surface.

Glycerophospholipids (or phosphoglycerides) are phospholipids containing:
- A glycerol backbone.
- Two fatty acids bonded by ester linkages.
- A phosphodiester linkage to a highly polar head group.
Named according to their head group:
- Phosphatidylcholine: Choline head group.
- Phosphatidylethanolamine: Ethanolamine head group.
Head groups can be positively charged, negatively charged, or neutral.
Important for cell recognition, signaling, and binding.
Fatty acid chains vary in length and saturation, resulting in diverse functions.

ABO blood typing system is based on cell surface antigens, which are well-known sphingolipids.
Serve as sites of biological recognition at the cell surface.
Can be bonded to various head groups and fatty acids.
Have a sphingosine or sphinoid backbone (not glycerol).
Also have long-chain nonpolar fatty acid tails and polar head groups.
Some sphingolipids are phospholipids due to a phosphodiester linkage, while others have glycosidic linkages to sugars (glycolipids).

Ceramide:
- Simplest sphingolipid.
- Head group is a single hydrogen atom.
Sphingomyelins:
- Major class of sphingolipids that are also phospholipids (sphingophospholipids).
- Have phosphocholine or phosphoethanolamine as a head group.
- Contain a phosphodiester bond.
- Head groups have no net charge.
- Major components in plasma membranes of myelin-producing cells (oligodendrocytes and Schwann cells).
Glycosphingolipids:
- Sphingolipids with head groups composed of sugars bonded by glycosidic linkages.
- Are glycolipids (no phosphodiester linkage).
- Found mainly on the outer surface of the plasma membrane.
- Classified as:
  - Cerebrosides: Single sugar.
  - Globosides: Two or more sugars.
- Neutral glycolipids (no net charge at physiological pH).
Gangliosides:
- Glycolipids with polar head groups composed of oligosaccharides.
- Contain one or more N-acetylneuraminic acid (NANA), also called sialic acid molecules, at the terminus.
- Have a negative charge.
- Have a glycosidic linkage and no phosphate group.
- Play a major role in cell interaction, recognition, and signal transduction.

Esters of long-chain fatty acids with long-chain alcohols.
Form pliable solids at room temperature.
Function as protection for plants and animals.
In Plants: Prevent excessive evaporation and protect against parasites.
In Animals: Prevent dehydration, act as water repellents, and serve as lubricants.
Example:
- Carnauba wax (from Copernicia prunifera palm leaves) is used to coat candles and wax cars.
- Bees secrete waxes to construct shelter.