1.4: CARBOHYDRATES

Monosaccharides, Polysaccharides, and Their Functions

Monosaccharides [simple sugars such as glucose and fructose] serve as the monomers of all carbohydrates. Each monosaccharide contains carbon, hydrogen, and oxygen, typically in a ratio near 1:2:1. These small, ring-shaped molecules provide cells with a rapid source of energy because their bonds are easily broken during cellular respiration.

When two or more monosaccharides join together, they form larger carbohydrates through covalent bonds created by dehydration synthesis [a reaction that removes a water molecule to link monomers]. The reverse process, hydrolysis, breaks these bonds by adding water back, releasing individual monosaccharides for use. This is the same bond-forming and bond-breaking logic that applies to all biological polymers.

Polysaccharides [complex carbohydrates built from many monosaccharide subunits] can be either linear or branched, and their shape directly determines their function:

Because glycogen is highly branched, it has many exposed ends where enzymes can quickly detach glucose molecules — this is why animals can mobilize stored energy rapidly. Starch is less branched, reflecting the slower energy-release needs of plants. Cellulose, with its rigid linear chains, resists digestion by most organisms, making it ideal as a structural material rather than an energy source.

Illustrative ExampleCellulose, starch, and glycogen all consist of glucose monomers, yet their distinct bonding patterns and levels of branching give them very different biological roles.

Out of ScopeThe specific molecular structures (e.g., alpha vs. beta linkages, ring conformations) of carbohydrate polymers. Do not memorize this for the exam.

MisconceptionStudents often assume all polysaccharides serve the same purpose because they share the same monomer (glucose). Structure determines function — linear chains provide structural support, while branched chains allow rapid energy access.

Exam TipWhen asked about carbohydrates, connect the degree of branching to the specific function before naming the molecule.

Examiner InsightAP free-response questions frequently ask students to connect monomers to polymers and explain how dehydration synthesis and hydrolysis apply across all macromolecule categories — not just carbohydrates.

Exam TipPractice writing one sentence that links monomer → covalent bond → polymer → function for each macromolecule class.

Exam TipAnnotate the direction of each reaction and note that dehydration synthesis builds polymers while hydrolysis breaks them down.