Energy Storage and Lipids

Triacylglycerols are a class of lipids used for energy storage.
Lipids are a good way to store energy because:
- Fatty acids are more reduced than sugars.
- Oxidation of triacylglycerols yields twice the energy per gram as carbohydrates.
  - This makes them a more energy-dense storage mechanism than polysaccharides like glycogen.
- Triacylglycerols are hydrophobic, so they don't draw in water.
- They don't require hydration for stability, decreasing their weight compared to hydrophilic polysaccharides.
In cold temperatures, lipids provide energy storage and insulation, helping to retain body heat and reduce the energy needed to maintain a constant internal temperature.

Composed of three fatty acids bonded by ester linkages to glycerol.
It is rare for all three fatty acids to be the same in naturally occurring triacylglycerols.
They are nonpolar and hydrophobic, which makes them insoluble in water.
The polar hydroxyl groups of glycerol and polar carboxylates of fatty acids are bonded together, decreasing polarity.
Triacylglycerol deposits can be observed in cells as oily droplets in the cytosol.
- They serve as depots of metabolic fuel.
- They can be recruited when the cell needs additional energy or when other fuel supplies are low.
Adipocytes are special cells in animals that store large amounts of fat.
- They are found under the skin, around mammary glands, and in the abdominal cavity.
Triacylglycerol deposits are also found in seeds as oils.
Triacylglycerols travel bidirectionally in the bloodstream between the liver and adipose tissue.
The physical characteristics of triacylglycerols are determined by the saturation or unsaturation of the fatty acid chains.

Free fatty acids are unesterified fatty acids with a free carboxylate group.
They circulate in the blood bonded noncovalently to serum albumin.
Fatty acids make up soap, which can be produced through saponification.
Saponification is the ester hydrolysis of triacylglycerol using a strong base (traditionally lye, i.e., sodium or potassium hydroxide).
The result is the basic cleavage of the fatty acid, leaving the sodium salt of the fatty acid and glycerol.
- The fatty acid salt is soap.

Soaps act as surfactants, lowering the surface tension at the surface of a liquid and acting as detergents or emulsifiers.
Aqueous solutions and oil will remain in separate phases unless a soap is added.
Adding soap causes the two phases to combine into a single phase forming a colloid.
This occurs due to the formation of micelles: tiny aggregates of soap with hydrophobic tails turned inward and hydrophilic heads turned outward, shielding hydrophobic tails and allowing for solvation.
Nonpolar compounds can dissolve in the hydrophobic interior of the water-soluble micelle.
- Cleaning agents can dissolve both water-soluble and water-insoluble messes and wash them away.
Micelles are important in the body for the absorption of fat-soluble vitamins (A, D, E, and K).
Lecithins, fatty acids, and bile salts secreted by the gallbladder form micelles that can increase the surface area available for lipolytic enzymes.

Structural functions of lipids:
- Phospholipids are the primary component of the phospholipid bilayer and other membrane lipids.
- Terpenes structure and function.
- Signaling lipids.
Steroid hormones.
Fat-soluble vitamins and their functions.
Energy storage in the form of triacylglycerols.
Acid-base chemistry knowledge in the formation of soap.