Which definition best describes a macromolecule?

A macromolecule is accurately defined as a very large molecule. This is because macromolecules are typically composed of numerous atoms and often consist of repeating subunits, leading to their substantial size. Common examples of macromolecules include proteins, nucleic acids (like DNA and RNA), carbohydrates, and lipids. Each of these plays critical roles in biological processes and structures due to their large molecular weights and complex architectures.

The other options do not capture the essence of macromolecules effectively; for instance, describing a macromolecule as a very small molecule contradicts the fundamental characteristic of size. Similarly, stating it is comprised solely of nucleotides is too narrow, as macromolecules can also be composed of amino acids or sugar units, depending on the type. Lastly, defining a macromolecule as a substance with a simple structure undermines its complexity and the intricate functions it performs in biological systems. Thus, the most accurate description is that of a very large molecule, which aligns with the true nature of what macromolecules represent in biology.