What types of substances are classified as signaling molecules?

Signaling molecules play a crucial role in cellular communication and can initiate or influence various physiological processes. The correct answer encompasses a wide range of substances that are involved in signaling pathways. Hormones such as insulin and adrenaline act as signaling molecules to regulate metabolism and other bodily functions. Small ions like calcium and potassium are vital for transmitting signals within cells, often acting as secondary messengers. Sugars, particularly those that function as glycoproteins or glycolipids, can also serve as important signaling molecules by participating in cell recognition and interaction.

The other choices are too restrictive or misaligned with the known roles of signaling molecules. For instance, only considering proteins and nucleic acids excludes many vital signaling entities, and focusing solely on neurotransmitters neglects the diverse roles that hormones and small ions play. Likewise, enzymes and structural proteins typically do not function as signaling molecules; their main roles pertain to catalysis and providing cellular structure, respectively. Therefore, the inclusive nature of the mentioned substances in the correct answer is essential for understanding the diverse mechanisms of cellular signaling.