Final answer:
The molecule synthesized in the smooth ER that is a sphingosine linked to an acid by an amide bond is a lipid, specifically sphingolipids such as sphingomyelin.
Step-by-step explanation:
The molecule synthesized in the smooth endoplasmic reticulum (SER) that is linked to an acid by an amide bond is a lipid. Specifically, this refers to sphingolipids, a class of lipids that includes compounds such as sphingomyelin. Sphingomyelin is composed of a fatty acid linked to sphingosine, an amino alcohol, by an amide bond. This complex structure, when a sphingosine is linked to a fatty acid, is known as ceramide. The subsequent addition of a phosphocholine group to ceramide forms sphingomyelin.
In contrast, the rough endoplasmic reticulum (RER) is responsible for the synthesis of proteins with the help of ribosomes attached to its surface. Proteins synthesized in the RER can be destined for cell membranes or for secretion outside the cell. Carbohydrates and nucleic acids are synthesized through different cellular pathways and are not primarily associated with the activity of the SER.