Here's why:

* Chemical Formula: Glucose has a specific chemical formula: C6H12O6. This means that every molecule of glucose always contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.

* Structure: Glucose has a specific arrangement of these atoms, forming a six-membered ring structure. This structure is consistent for all glucose molecules.

While glucose can exist in different forms (like alpha and beta glucose), these forms are based on slight differences in the spatial arrangement of atoms, not a change in the actual number of atoms present.

Therefore, the composition of glucose is fixed, and any deviation from the formula C6H12O6 would mean it's not glucose anymore.