Which substance is found in RNA molecules but not in DNA molecules?

The substance that is found in RNA (ribonucleic acid) molecules but not in DNA (deoxyribonucleic acid) molecules is the sugar ribose.

Explanation:

1. Sugar Composition: RNA contains ribose as its sugar component, while DNA contains deoxyribose. The difference between these two sugars is the presence of an oxygen atom. Ribose has an -OH (hydroxyl) group attached to the second carbon in the sugar ring, whereas deoxyribose lacks this hydroxyl group and instead has a hydrogen atom at this position, hence the prefix "deoxy."

2. Nucleotide Structure: Nucleotides, the building blocks of RNA and DNA, consist of a sugar, a phosphate group, and a nitrogenous base. The presence of ribose in RNA contributes to its structure and function, making it generally more reactive and less stable than DNA. This makes RNA suitable for various roles in the cell, such as acting as a messenger (mRNA), a structural component (rRNA), or a catalyst (some types of RNA are catalytic, known as ribozymes).

In summary, the distinct presence of ribose in RNA, compared to the deoxyribose of DNA, is a fundamental difference between these two essential biomolecules.