What Is a Lipid Molecule?

Organic Chemistry of Fats, Phospholipids, Waxes & Steroids

What are the different kinds of lipids? What is the difference between saturated and unsaturated fats? Read on and discover.

Inorganic and Organic Molecules

Inorganic molecules are essentially substances that don’t have carbon-hydrogen (C-H) bonds. Organic molecules are those substances that contain carbon-hydrogen bonds and are found in living things.

The major classes of organic molecule include carbohydrates, proteins, lipids and nucleic acids.

Lipids

Lipids are molecules that are hydrophobic or insoluble in water because the non-polar, covalent bonds linking carbon and hydrogen aren’t attracted to the polar bonds of water. The four major categories of lipids include fats, phospholipids, waxes and steroids

Fats

Fats and oils are made from two kinds of molecules:

glycerol (a type of alcohol)
three fatty acids (so known as triglycerides)

Saturated fats

These types of fat are mostly derived from animal sources. There are single bonds between the carbons in their fatty acid tails, and all carbons are bonded to maximum number of hydrogens that is possible. Therefore the hydrocarbon chains in these fatty acids are fairly straight and packed closely together. This is why saturated fats are solid at room temperature.

Unsaturated fats (oils)

These types of fat are mostly from plant sources. There are double bonds between some of the carbons in the hydrocarbon tail, causing bends or “kinks.” These kinks in the hydrocarbon tails prevent the tails from packing closely together. This is why unsaturated fats are liquid at room temperature.

Phospholipids

These fats have a hydrophobic, “water hating,” hydrocarbon tails and hydrophilic, "water loving" phosphate groups on the end. This means that phospholipids are soluble in both water and oil.

Our cell membranes are made mostly of phospholipids arranged in a double layer with the tails from both layers facing inward and the heads facing outward, called the lipid bilayer.

Waxes

These molecules are esters of alcohol which are insoluble in water and difficult to hydrolyze, or break down. Wax forms protective and waterproof layers on some plants, bacteria, animal fur and integuments of insects.

Steroids

The central core of a cholesterol molecule, consisting of four fused rings, is shared by all steroids. Our bodies make about 2 g of cholesterol per day, and that makes up about 85% of blood cholesterol, while only about 15% comes from dietary sources.

Cholesterol is precursor to our sex hormones and Vitamin D. Our cell membranes contain a lot of cholesterol which helps to keep the membrane flexible and fluid even when our cells are exposed to cooler temperatures.

Additional Organic Chemistry Resources

For more information on organic molecules and cell biology, see Science Prof Online and the Organic Chemistry Help page or look to additional Suite101 articles including, What Is a Carbohydrate, Nucleotides and Nucleic Acids, Amino Acids & Proteins and What are Organic Molecules.

Butter is a lipid saturated fat.	Lanosterol Steroid Molecule	Butter a Lipid Saturated Fat
Chemical Stucture & Polarity of a Phospholipid	Honeycomb of Beeswax, a Nonpolar Lipid Molecule