Fat Definition and Examples (Chemistry)

In chemistry and biology, fats are a type of lipid consisting of triesters of glycerol and fatty acids or triglycerides. Because they are organic compounds consisting of carbon and hydrogen atoms, they are generally soluble in organic solvents and largely insoluble in water. Fats are solid at room temperature. In food science, a fat is one of the three macronutrients, with the others being protein and carbohydrates. Examples of fats include butter, cream, vegetable shortening, and lard. Examples of pure compounds that are fats include triglycerides, phospholipids, and cholesterol.

Fat vs Lipid

In food science, the terms "fat" and "lipid" may be used interchangeably, but technically they have different definitions. A lipid is a biological molecule that is soluble in nonpolar (organic) solvents. Fats and oils are two types of lipids. Fats are lipids that are solid at room temperature. Oils are lipids that are liquid at room temperature, typically because they consist of unsaturated or short fatty acid chains.

Chemical Structure

Fats are derived from fatty acids and glycerol. As such, fats are glycerides (usually triglycerides). The three -OH groups on glycerol serve as attachment sites for fatty acid chains, with the carbon atoms linked via an -O- bond. In chemical structures, fatty acid chains are drawn as horizontal lines attached to the vertical glycerol backbone. However, the chains form zig-zag shapes. Longer fatty acid chains are susceptible to van der Waals forces that attract parts of the molecule to each other, giving fats a higher melting point than oils.

Classification and Nomenclature

Both fats and oils are classified according to the number of carbon atoms they contain and the nature of the chemical bonds formed by carbon atoms in their backbone.

Saturated fats contain no double bonds between the carbons in the fatty acid chains. In contrast, saturated fats contain one or more double bonds between the carbon atoms in the chains. If the molecule contains multiple double bonds, it is called a polyunsaturated fat. The non-carbonyl end of the chain (called the n-end or omega end) is used to define the number of the carbon on the chain. So, an omega-3 fatty acid is one in which the first double bonded carbon occurs on the third carbon from the omega end of the chain.

Unsaturated fats may be cis fats or trans fats. Cis and trans molecules are geometric isomers of one another. The cis or trans descriptor refers to whether the hydrogen atoms attached to the carbons sharing a bond are on the same side as each other (cis) or opposite sides (trans). In nature, most fats are cis fats. However, hydrogenation breaks double bonds in an unsaturated cis-fat, making a saturated trans fat. Hydrogenated trans fats (like margarine) may have desirable properties, such as being solid at room temperature. Examples of natural trans fats include lard and tallow.

Functions

Fat serves several functions in the human body. It is the most energy-dense macronutrient. It is the source of essential fatty acids. Some vitamins are fat-soluble (Vitamins A, D, E, K) and can only be absorbed with fat. Fat is stored in adipose tissue, which maintains body temperature, protects against physical shock, and serves as a reservoir for pathogens and toxins until the body can neutralize or excrete them. Skin secretes fat-rich sebum, which helps waterproof skin and keeps hair and skin soft and pliable.

Sources

• Bloor, W. R. (March 1, 1920). "Outline of a classification of the lipoids." Sage Journals.
• Donatelle, Rebecca J. (2005). Health, the Basics (6th ed.). San Francisco: Pearson Education, Inc. ISBN 978-0-13-120687-8.
• Jones, Maitland (August 2000). Organic Chemistry (2nd ed). W W Norton & Co., Inc. 
• Leray, Claude (November 5, 2014). Lipids Nutrition and Health. CRC Press. Boca Raton.
• Ridgway, Neale (October 6, 2015). Biochemistry of Lipids, Lipoproteins and Membranes (6th ed.). Elsevier Science.