Organic Chemistry MCAT Practice Exam

Which feature differentiates saturated fatty acids from unsaturated fatty acids?

1. The presence of double bonds

2. The length of the carbon chain

3. The number of hydroxyl groups

4. The type of ester bond formed

The correct answer is: The presence of double bonds

Saturated fatty acids are characterized by the absence of double bonds between the carbon atoms in their hydrocarbon chains. This means that every carbon atom in a saturated fatty acid is fully "saturated" with hydrogen atoms, leading to a straight-chain structure. In contrast, unsaturated fatty acids contain one or more double bonds in their carbon chains. These double bonds introduce kinks or bends in the structure, preventing the molecules from packing closely together, which often results in lower melting points. The presence of double bonds is crucial because it fundamentally alters the physical properties of the fatty acids. For instance, unsaturated fatty acids tend to be liquid at room temperature (like olive oil), while saturated fatty acids are typically solid (like butter). This distinction is significant in biological systems, influencing fluidity in cell membranes and overall metabolic processes. The other aspects mentioned, such as the length of the carbon chain, the number of hydroxyl groups, and the type of ester bond formed, do not specifically differentiate saturated from unsaturated fatty acids. While the length of the carbon chain and functional groups can vary, they do not inherently determine the saturated or unsaturated status of a fatty acid. Thus, the defining feature that differentiates these two classes of fatty acids is indeed the