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How do monounsaturated and polyunsaturated fatty acids differ?

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A composite diagram shows the structural formulas, space-filling models, and simplified depictions of lipid molecules including stearic acid and phosphatidylcholine, and how these assemble into a micelle and lipid bilayer, with labels identifying hydrophilic heads and hydrophobic tails. — Lipid Structure and properties of two representative lipids. Both stearic acid (a fatty acid) and phosphatidylcholine (a phospholipid) are composed of chemical groups that form polar “heads” and nonpolar “tails.” The polar heads are hydrophilic, or soluble in water, whereas the nonpolar tails are hydrophobic, or insoluble in water. Lipid molecules of this composition spontaneously form aggregate structures such as micelles and lipid bilayers, with their hydrophilic ends oriented toward the watery medium and their hydrophobic ends shielded from the water.

Monounsaturated and polyunsaturated fatty acids are both types of unsaturated fats, characterized by the presence of carbon-carbon double bonds in their hydrocarbon chains, which means they contain fewer hydrogen atoms than saturated fats. They differ primarily in the number of these double bonds present in each molecule.123

Key Differences

Number of Double Bonds: Monounsaturated fatty acids contain exactly one double bond per molecule. In contrast, polyunsaturated fatty acids contain two or more double bonds.123
Structure and Properties: The presence of these double bonds, particularly in the cis configuration common in nature, prevents the molecules from stacking closely together. This structural characteristic generally results in lower melting points compared to saturated fats, causing these fatty acids to be liquid at room temperature. As the degree of unsaturation increases—meaning more double bonds are present—the melting point typically decreases further.2413
Sources and Function: Monounsaturated fats are commonly found in olive, peanut, and canola oils. Polyunsaturated fats are found in sources such as fish, soybeans, and various plant oils. Certain polyunsaturated fatty acids, such as linoleic and alpha-linolenic acid, are considered essential fatty acids because the human body cannot synthesize them and must obtain them through diet.124