What Are the Three Common Categories of Lipids?

Lipid-rich egg yolks are excluded from many diets.
••• extreme coloured egg's close-up image by Stasys Eidiejus from Fotolia.com

Lipids are a broad group of organic compounds that play important roles in living organisms, including cell membrane structure and chemical signals, and that are used to store energy. These compounds are typically insoluble in water, termed "hydrophobic," due to the large number of nonpolar bonds within their structures. Three common categories of lipids are triglycerides (fats and oils), diglycerides (phospholipids) and steroids.

Triglycerides

Triglycerides, commonly referred to as fats and oils, possess long chains of fatty acids attached to a glycerol group and serve as thermal insulation, energy storage for cells, and form protective layers for tissues and organs. The glycerol group contains three carbon atoms, with the fatty acid attached to each carbon. The fatty acids are long chains of hydrocarbons that are hydrophobic, making the resulting fat insoluble in water despite the hydrophilic nature of the glycerol. The fatty acids may be further characterized as saturated, monounsaturated or polyunsaturated, depending on the bonds between the carbon atoms in the fatty acid.

Saturated fatty acids have single bonds between all the carbon atoms and are thus saturated with the maximum number of possible hydrogen atoms. Monounsaturated fatty acids have a single double bond between two carbon atoms, producing a bend in the chain and reducing the number of hydrogen atoms compared to a saturated fatty acid. Polyunsaturated fatty acids have multiple double bonds between the carbon atoms of the fatty acid.

Diglycerides

Diglycerides, or phospholipids, consist of only two fatty acids attached to the glycerol group and a phosphate group attached to the third carbon atom of the glycerol. This arrangement of atoms produces a hydrophilic head on the molecule and two long hydrophobic tails. Phospholipids form the lipid bilayer of cell membranes, as the phospholipids on each membrane layer arrange themselves with the hydrophilic heads on the surface of the membrane and the hydrophilic tails form the interior of the membrane.

Steroids

Unlike triglycerides and diglycerides, steroids do not contain fatty acids. Instead, steroids are composed of four joined rings of carbon atoms with additional groups attached to the sides of the ring, depending on the specific steroid. Cholesterol is a frequently mentioned steroid that plays an important role in the body in the structure of cell membranes. It is also a precursor to the formation of hormones, including estrogen and testosterone, which also are steroids. However, high levels of LDL cholesterol can lead to plaque deposits in blood vessels and potentially to high blood pressure and heart attacks.

Related Articles

You've Gotta Try This Fun, Easy Way to Grow Crystals...
Facts About Lipids
What Are the Atoms That Make Up Lipids?
What Are the Processes by Which Macromolecules Are...
Why Are Lipids Insoluble in Water?
Which Type of Lipid Is Classified as a Ring Structure?
Physical & Chemical Properties of Lipids
What Types of Organic Molecules Make Up a Cell Membrane?
Why Can the Surface of the Plasma Membrane Be Described...
Defining Characteristics of Lipid Molecules
What Is a Hydrocarbon Chain's Relationship to Fats...
The Most Common Organic Molecules in Cells
What Are the Chemical Names of the Four Macromolecules?
What Structural Role Do Phospholipids Play in Cells?
What Are the Functions of Triglyceride Phospholipid...
Definition of Molecular Bonds
What Are the Polymers of Lipids?
The Function of Macromolecules
What Is the Difference Between a Monosaccharide and...
The Differences Between Monosaccharides & Polysaccharides
Define Carbon Skeleton