Fatty acids are lipids that contain long-chain hydrocarbons with a carboxylic acid functional group ending.
Fats are just one type of lipid, a category of molecules united by their inability to mix well with water. Lipids tend to be hydrophobic, nonpolar, and made up mostly of hydrocarbon chains. Fatty acids are generally considered to be insoluble in water and longer hydrocarbon chains have a lower solubility.
The different varieties of lipids have different structures and correspondingly diverse roles in organisms. A fat molecule consists of two parts: a glycerol backbone and three fatty acid tails. A typical fatty acid consists of a carboxyl group attached to a long hydrocarbon chain containing 12–18 carbons, though some may have as few as 4 or as many as 36. While many fatty acids are found in fat molecules, some are also free in the body, and they are considered a type of lipid in their own right.
In addition to differing in length, fatty acids also differ in their degree of unsaturation. Fatty acids with hydrocarbon chains that contain only single bonds (C-C) are called saturated fatty acids because they have the greatest number of hydrogen atoms possible and are, therefore, “saturated” with hydrogen. Fatty acids with hydrocarbon chains containing at least one double bond (C=C) are called unsaturated fatty acids because they have fewer hydrogen atoms. If there is just one double bond in a fatty acid, it’s monounsaturated, while if there are multiple double bonds, it’s polyunsaturated.
The double bonds in unsaturated fatty acids, like other types of double bonds, can exist in either a cis or a trans configuration. In the cis configuration, the two hydrogens associated with the bond are on the same side, while in a trans configuration, they are on opposite sides. A cis double bond generates a kink or bend in the fatty acid, which means that while saturated fatty acids have a straight, flexible carbon backbone, unsaturated fatty acids have “kinks” in their carbon skeleton because each double bond causes a rigid bend of the carbon skeleton. Take a look at the examples in the figure below. These differences in saturated versus unsaturated fatty acid structure result in different properties for the corresponding lipids in which the fatty acids are incorporated. For example, lipids containing saturated fatty acids are solids at room temperature, whereas lipids containing unsaturated fatty acids are liquids. This is because saturated fatty acids tails are straight, so fat molecules with fully saturated tails can pack tightly against one another. This tight packing results in fats that are solid at room temperature (have a relatively high melting point).
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Key Points
• Fatty acids are long-chain hydrocarbons with a carboxylic acid functional group.
• The relatively long nonpolar hydrocarbon chains makes them hydrophobic. They are generally considered to be insoluble with increasing chain length making them less soluble.
• Fatty acids with no double bonds are saturated; those with double bonds are unsaturated.
Key Terms
Fatty acids: Lipids that contain a carboxylic acid functional group attached to a long-chain hydrocarbon tail.
Lipids: A type of biomolecule that is insoluble in water and tend to be nonpolar, hydrophobic, and have hydrocarbon chains.
Carboxylic acid: A molecule containing a functional group consisting of a carbonyl and a hydroxyl functional group (-CO.OH).
Hydrophobic: A physical property of a molecule that describes how it is repelled by water.
Nonpolar: A molecule is nonpolar when there is equal sharing of electrons between atoms (in a diatomic molecule) or when polar bonds are symmetrically arranged.
Fat: A type of lipid that consists of a glycerol backbone and three fatty acid tails.
Glycerol: A small organic molecule with three hydroxyl (OH) groups.
Saturated fatty acids: Fatty acids with hydrocarbon chains containing only single bonds (C-C).
Unsaturated fatty acids: Fatty acids with hydrocarbon chains containing at least one double bond (C=C).
Monounsaturated: Fattys acids with only one double bond in the hydrocarbon tail.
Polyunsaturated: Fatty acids with multiple double bonds in the hydrocarbon tail.
Cis configuration: When both hydrogens associated with a double bond are on the same side.
Trans configuration: When the two hydrogens associated with a double bond are on opposite sides.