Organic Chemistry MCAT Practice Exam 2025 - Free MCAT Practice Questions and Study Guide

The penultimate carbon in carbohydrates, which is the second-to-last carbon atom in the carbon chain, plays a crucial role in determining the configuration of the sugar as either D or L. This designation is based on the orientation of the hydroxyl group (-OH) attached to the penultimate carbon when the sugar is drawn in the Fischer projection.

In a D-sugar, the hydroxyl group on the penultimate carbon is oriented to the right, while in an L-sugar, it is oriented to the left. This configuration is significant in biochemistry as it affects the biological activity and properties of the carbohydrate. The presence of the penultimate carbon is critical for defining these isomeric forms, which are important not only for the classification of sugars but also for their interactions in biochemical pathways.

While the other answer choices address characteristics related to carbohydrates, they do not capture the specific and defining role of the penultimate carbon in stereochemistry. For instance, the overall molecular weight is determined by the total number of atoms and their respective masses rather than the position of any single carbon. Similarly, while the penultimate carbon is near the carbonyl carbon, it does not function as a connector; instead, it influences the stereochemical configuration. Lastly, it