01Monosaccharides
Simple sugars like glucose, fructose and galactose (often C₆H₁₂O₆) are fuel and building blocks. In water they mostly form rings. Glucose comes in two ring forms — α and β — that differ only in whether the –OH on carbon 1 points down (α) or up (β). That tiny difference changes everything downstream.
02Linking sugars: glycosidic bonds
Two monosaccharides joined by dehydration synthesis form a disaccharide (glucose + fructose = sucrose; glucose + galactose = lactose). The covalent link is a glycosidic linkage. Long chains are polysaccharides.
03Structure decides function
Starch (plants) and glycogen (animals) are made of α-glucose. α-linkages make the chain coil into a helix — compact and easy for enzymes to break, perfect for energy storage. Glycogen is more branched than starch, so enzymes can release glucose from many ends at once — fast energy for muscles and liver.
Cellulose (plant cell walls) is made of β-glucose. β-linkages flip every other glucose, producing straight, unbranched chains that lie side by side and hydrogen-bond into tough fibers — perfect for structure. Most animals lack enzymes to hydrolyze β-linkages, which is why cellulose is 'fiber' you can't digest.
Chitin — a β-linked polysaccharide with nitrogen groups — forms insect exoskeletons and fungal cell walls.
04Notebook box
α-glucose → starch / glycogen → storage. β-glucose → cellulose / chitin → structure.
Worked example: Cows digest grass but humans cannot. Explain at the molecular level. → Grass is mostly cellulose with β-glycosidic linkages. Microbes in the cow's gut produce enzymes that hydrolyze β-linkages; human enzymes (like amylase) only fit α-linkages.