A Level Biology Practice Exam 2025 - Free A Level Biology Practice Questions and Study Guide

Starch is a polysaccharide that is primarily composed of alpha glucose molecules. In starch, the linkage between the glucose units occurs through alpha-1,4-glycosidic bonds, which give the molecule a helical shape. This structural formation is crucial for its function as a storage form of energy in plants. Alpha glucose is characterized by the specific orientation of the hydroxyl group on the first carbon atom, which allows for these types of bonds to form effectively.

The other types of glucose mentioned, such as beta glucose, gamma glucose, and delta glucose, do not participate in the formation of starch. Beta glucose, for instance, is found in cellulose, where it forms beta-1,4-glycosidic bonds, resulting in long, straight chains that contribute to cell wall structure. Gamma and delta glucose are not commonly referenced in biological contexts, and their structures do not play roles in significant polysaccharide formations like starch. Hence, it is alpha glucose that specifically contributes to the formation of starch in plants.