Monosaccharides, disaccharides and polysaccharides

The chemistry of carbohydrates: general formula and classification

Carbohydrates are organic macronutrients composed exclusively of carbon (C), hydrogen (H), and oxygen (O) atoms. Their general empirical formula is (CH₂O)_n, where n is the number of carbon atoms — hence the name "carbohydrate" (literally, hydrates of carbon). In practice, monosaccharides follow this formula precisely (e.g. glucose: C₆H₁₂O₆), while larger carbohydrates deviate slightly because water molecules are released during condensation reactions.

The QCAA classifies carbohydrates into two broad groups based on molecular complexity:

• Simple carbohydrates — monosaccharides and disaccharides, which are rapidly digested and absorbed, producing a comparatively fast rise in blood glucose.
• Complex carbohydrates — polysaccharides (starch, glycogen, dietary fibre), which are larger chain molecules that take longer to digest (or are not digested at all in the case of fibre).

An alternative classification distinguishes carbohydrates by their degree of polymerisation: monosaccharides (1 monomer unit), disaccharides (2 units), oligosaccharides (3–9 units), and polysaccharides (10 or more units). In the QCAA Food & Nutrition syllabus the focus is on the three principal categories — monosaccharides, disaccharides, and polysaccharides — and their distinct chemical structures and nutritional roles.

Carbohydrates are synthesised in plants via photosynthesis, where solar energy converts CO₂ and H₂O into glucose: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This makes plant foods (grains, legumes, fruits, vegetables) the primary dietary source of carbohydrates for Australians. The Australian Dietary Guidelines recommend carbohydrates provide approximately 45–65% of total daily energy intake.

Monosaccharides: glucose, fructose and galactose

Monosaccharides are the simplest carbohydrate units and the fundamental building blocks from which all larger carbohydrates are constructed. They cannot be hydrolysed into smaller sugar molecules. All three nutritionally significant monosaccharides share the molecular formula C₆H₁₂O₆ — they are hexoses — but differ in the spatial arrangement of their atoms, making them structural isomers.

• Glucose — The body's primary fuel molecule and the reference standard for the glycaemic index (GI = 100). Glucose exists predominantly in a six-membered ring form called pyranose. It is found in fruits, honey, and as the product of starch digestion. All cells, particularly brain cells and red blood cells, depend on glucose for energy via glycolysis and the citric acid cycle. Blood glucose is regulated by the hormones insulin (lowers) and glucagon (raises), produced by the pancreatic islets of Langerhans.
• Fructose — The sweetest naturally occurring monosaccharide (approximately 1.5× sweeter than sucrose). Fructose forms a five-membered ring (furanose form) in solution. It is abundant in fruit, honey, and high-fructose corn syrup (HFCS) used in processed foods. Unlike glucose, fructose is metabolised almost entirely in the liver and does not directly stimulate insulin secretion, which has implications for metabolic health. Excessive fructose consumption (particularly from added sugars) is associated with hepatic lipogenesis, contributing to non-alcoholic fatty liver disease (NAFLD) — a growing concern in Australian public health.
• Galactose — Rarely found free in nature; it is produced by the digestion of lactose (milk sugar). Galactose differs from glucose only in the orientation of the hydroxyl (–OH) group on carbon 4, making it an epimer of glucose. Galactose is converted to glucose in the liver. Individuals with galactosaemia, a rare autosomal recessive metabolic disorder, lack the enzyme galactose-1-phosphate uridyltransferase, causing toxic accumulation of galactose if dairy is consumed — a significant dietary restriction managed from infancy.

Applied example: A Queensland adolescent consuming a banana (contains free glucose and fructose) and a glass of milk (provides galactose via lactose) has ingested all three dietary monosaccharides. After absorption through intestinal epithelial cells (all three via the GLUT and SGLT1 transporters), they travel via the portal vein to the liver for metabolic processing.