What Are Lipids? Definition and Examples

Introduction to Lipids in Chemistry

Triglycerides are lipids that result from the reaction between glycerol and fatty acids.
Triglycerides are lipids that result from the reaction between glycerol and fatty acids. LAGUNA DESIGN / Getty Images

Lipids are class of naturally-occurring organic compounds that you may know by their common names: fats and oils. A key characteristic of this group of compounds is that they are not soluble in water.

Here's a look at the function, structure, and physical properties of lipids.

Fast Facts: Lipids

  • A lipid is any biological molecule that is soluble in nonpolar solvents.
  • Lipids include fats, waxes, fat-soluble vitamins, sterols, and glycerides.
  • Biological functions of lipids include energy storage, cell membrane structural components, and signaling.

Lipid Definition

A lipid is a fat-soluble molecule. To put it another way, lipids are insoluble in water but soluble in at least one organic solvent. The other major classes of organic compounds (nucleic acids, proteins, and carbohydrates) are much more soluble in water than in an organic solvent. Lipids are hydrocarbons (molecules consisting of hydrogen and oxygen), but they do not share a common molecule structure.

Lipids that contain an ester functional group may be hydrolyzed in water. Waxes, glycolipids, phospholipids, and neutral waxes are hydrolyzable lipids. Lipids that lack this functional group are considered nonhydrolyzable. Nonhydrolyzable lipids include steroids and the fat soluble vitamins A, D, E, and K.

Examples of Common Lipids

There are many different types of lipids. Examples of common lipids include butter, vegetable oil, cholesterol and other steroids, waxes, phospholipids, and fat-soluble vitamins. The common characteristic of all of these compounds is that they are essentially insoluble in water yet soluble in one or more organic solvents.

What Are the Functions of Lipids?

Lipids are used by organisms for energy storage, as a signalling molecule (e.g., steroid hormones), as intracellular messengers, and as a structural component of cell membranes. The fat-soluble vitamins (A, D, E, and K) are isoprene-based lipids that are stored in the liver and fat. Some types of lipids must be obtained from the diet, while others can be synthesized within the body. The types of lipids found in food include plant and animal triglycerides, sterols, and membrane phospholipids (e.g., cholesterol). Other lipids may be produced from carbohydrates from the diet via a process called lipogenesis.

Lipid Structure

Although there is no single common structure for lipids, the most commonly occurring class of lipids are triglycerides, which are fats and oils. Trigylcerides have a glycerol backbone bonded to three fatty acids. If the three fatty acids are identical then the triglyceride is termed a simple triglyceride. Otherwise, the triglyceride is called a mixed triglyceride.

Fats are triglycerides that are solid or semisolid at room temperature. Oils are triglycerides that are liquid at room temperature. Fats are more common in animals, while oils are prevalent in plants and fish.

The second most abundant class of lipids are the phospholipids, which are found in animal and plant cell membranes. Phospholipids also contain glycerol and fatty acids, plus the contain phosphoric acid and a low-molecular-weight alcohol. Common phospholipids include lecithins and cephalins.

Saturated Versus Unsaturated

Fatty acids that have no carbon-carbon double bonds are saturated. Saturated fats are commonly found in animals and are usually solids.

If one or more double bond is present, the fat is unsaturated. If only one double bond is present, the molecule is monounsaturated. The presence of two or more double bonds makes a fat polyunsaturated. Unsaturated fats are most often derived from plants. Many are liquids because the double bonds prevent efficient packing of multiple molecules. The boiling point of an unsaturated fat is lower than the boiling point of the corresponding saturated fat.

Lipids and Obesity

Obesity occurs when there is an excess of stored lipids (fat). While a few studies have linked fat consumption to diabetes and obesity, the vast majority of research suggests there is no link between dietary fat and obesity, heart disease, or cancer. Rather, weight gain is a consequence of excess consumption of any type of food, combined with metabolic factors.

References

  • Bloor WR (1920). "Outline of a classification of the lipids". Proc. Soc. Exp. Biol. Med. 17: 138–140. doi:10.3181/00379727-17-75
  • Leray, C. 2015. Introduction, History and Evolution. In: Lipids. Nutrition and health. Boca Raton: CRC Press.
  • Maitland, Jr Jones (1998). Organic Chemistry. W W Norton & Co Inc (Np). p. 139. ISBN 978-0-393-97378-5.
  • Vance JE, Vance DE (2002). Biochemistry of Lipids, Lipoproteins and Membranes. Amsterdam: Elsevier. ISBN 978-0-444-51139-3.