Biochemical Properties and Functions of Lipids

Properties of Fatty Acids

Fatty acids are amphipathic molecules, meaning they have two distinct polar areas: a carboxyl group with hydrophilic character and a nonpolar, hydrophobic carbon chain. They react with alcohols to form esters and can be hydrolyzed, releasing water and forming sodium or potassium salts.

Melting Point and Chain Length

The degree of unsaturation and the aliphatic chain length determine the melting point. This increases with the length of the chain as van der Waals interactions with other chains increase. The presence of double bonds causes the molecules to bend, which favors a decrease in the melting point by reducing the number of interactions with other chains.

Fats and Waxes

Fats and waxes are saponifiable lipids because they undergo alkali hydrolysis reactions, also known as saponification.

Triacylglycerols and Glycerides

Triacylglycerols are the most abundant fats and may consist of three identical fatty acids, such as tripalmitin. These molecules are nonpolar and insoluble in water. Types include vegetable fats and waxes of animal origin. They are esters of long-chain fatty acids and a monoalcohol. Because both ends of the chain are hydrophobic in nature, they are insoluble in water and perform functions of protection and coating for substances like animal hair and feathers.

Saponifiable Lipids: Phospholipids

Phospholipids, also called phosphoglycerides, are made of glycerin and are amphipathic molecules with a polar, hydrophilic region. They are the primary components of biological membranes and can form the following structures:

• Micelles: Formed by polar heads exposed to the aqueous phase while the interior interacts with itself.
• Bilayers: Structures that separate two aqueous media and form monolayers, exposing aliphatic chains to the air and polar head groups to the water.
• Liposomes: Formed by bilayers of phospholipids that enclose an internal compartment containing water.

Unsaponifiable Lipids and Steroids

Unsaponifiable lipids are derived from linear hydrocarbons and include various unsaturated cyclic molecular associations.

Cholesterol and Steroid Hormones

Cholesterol is a vital part of the plasma membrane in animal cells and joins blood lipoproteins to maintain membrane fluidity against fluctuations in temperature. Steroid hormones include sex hormones, such as testosterone in men and estrogen and progesterone in women. Additionally, the adrenal cortex secretes hormones like aldosterone and cortisol.

Bile Acids and Digestion

Bile acids, including cholic acid and deoxycholic acid, are components found in bile. They form salts that act as detergents in the small intestine, causing a fat emulsion that is subsequently degraded by the action of intestinal lipases.