Final answer:
Sugars are often depicted as pentagons in their cyclic forms, such as the furanose ring structure found in fructose. This representation, called the Haworth projection, is used to depict the stable ring structures sugars adopt in aqueous solutions, with the shape being either a pentagon or hexagon based on the sugar involved.
Step-by-step explanation:
The sugar molecule is often depicted as a pentagon because in their cyclic forms, certain sugars such as fructose adopt a five-membered ring structure which resembles a pentagon in the simplified representation. This five-membered ring is known as a furanose ring, a reference to the compound furan that has a similar structure. When sugars like glucose and galactose form six-membered rings, these are depicted as hexagons. The representation of these molecules has a basis in the Haworth projection, a type of structural formula that shows the cyclic form of sugars, where carbon atoms are at the corners of the pentagon or hexagon and functional groups are positioned above or below the plane of these polygons based on their orientation in the Fischer projection.
The reason that sugars are shown in these cyclic forms is because in aqueous solutions, monosaccharides typically exist in these stable ring structures rather than as straight chains, which is a result of a chemical reaction between the carbonyl group and a hydroxyl group on the sugar's carbon chain resulting in the formation of a hemiacetal. The specific shape of the sugar molecule, pentagon or hexagon, depends on the particular monosaccharide and the specific atoms involved in forming the ring structure.