What monosaccharide serves as the building block for starch?

The correct choice is glucose because it serves as the primary building block for starch, which is a polysaccharide. Starch is composed of long chains of glucose molecules linked together by glycosidic bonds. These chains can be either linear, as in amylose, or branched, as in amylopectin.

Glucose is a six-carbon sugar (hexose) that is widely utilized by plants and animals for energy. In plants, it is produced through photosynthesis and then polymerized to form starch, which acts as an energy reserve. When needed, starch can be broken down back into glucose units, enabling organisms to access the stored energy.

Other monosaccharides like galactose and fructose do not form starch, as they serve different roles and structures in biological systems. Galactose is primarily involved in the synthesis of lactose, while fructose is used in various metabolic pathways but does not contribute to starch synthesis. Sucrose, on the other hand, is a disaccharide made up of glucose and fructose and is not a monosaccharide itself, thus making it unsuitable as a building block for starch.