A Level Biology Practice Exam

Glycoproteins are distinguished by their structure, which includes what?

1. Sugar molecules attached to nucleic acids

2. Carbohydrate molecules attached to extrinsic membrane proteins

3. Fatty acids linked to amino acids

4. Free carbohydrates existing independently

The correct answer is: Carbohydrate molecules attached to extrinsic membrane proteins

Glycoproteins are indeed distinguished by their structure, which consists of carbohydrate molecules covalently attached to extrinsic (or peripheral) membrane proteins. These carbohydrates can be simple sugars or more complex oligosaccharides, and their attachment to proteins plays a significant role in cell-cell recognition, signaling, and providing structural stability to the protein. This key relationship between sugars and proteins is crucial for various biological processes, such as immune responses and cellular communication. In contrast, other indicated options do not describe the correct structure of glycoproteins. For instance, the first option refers to sugar molecules being attached to nucleic acids, which does not pertain to glycoproteins. The third option suggests a connection between fatty acids and amino acids, which is more characteristic of lipoproteins or other types of macromolecules. Lastly, the fourth option about free carbohydrates existing independently misses the definition of glycoproteins, as the defining feature is the attachment of carbohydrates to proteins rather than their independent existence.