Monosaccharides, Disaccharides, and Polysaccharides

Carbohydrates are polyhydroxy aldehydes and ketones and their polymers: monosaccharides are the simple sugars (such as glucose C₆H₁₂O₆), disaccharides consist of two monosaccharides joined by a glycosidic bond (sucrose, lactose, maltose), and polysaccharides are long chains (starch, glycogen, cellulose). In brief: two glucose units form maltose with the loss of one water molecule (2 × 180.16 − 18.02 = 342.30 g/mol), and thousands of glucose units form starch, glycogen or cellulose, which differ in the type of linkage and branching.

Carbohydrates are the main source of energy in most diets and the most abundant organic compounds on Earth, since cellulose is the main structural component of plants. Their properties depend on the size of the molecules and on the way in which the units are linked. This article describes the three groups, the ring structures of sugars, the main disaccharides and polysaccharides in tables, tests for reducing sugars, and examples including mass calculations for hydrolysis.

What are monosaccharides?

Monosaccharides are the simplest sugars. They have the general formula (CH₂O)_n, with n = 3 to 7, and contain a carbonyl group and several –OH groups. An aldose has an aldehyde group (glucose, galactose, ribose) and a ketose has a ketone group (fructose). They are named by the number of carbons: trioses, pentoses (ribose, C₅H₁₀O₅) and hexoses (glucose, fructose, galactose, C₆H₁₂O₆).

Glucose, fructose and galactose are isomers of each other (the same formula, different structure). Glucose, the main fuel of cells, is called blood sugar (normal fasting concentration about 4–6 mmol/L, or 70–100 mg/dL). Fructose is the sweetest natural sugar and is found in fruit and honey. Galactose is part of lactose. Ribose and deoxyribose are components of RNA and DNA. Most natural sugars are of the D series, in which the highest-numbered stereocenter has the same configuration as D-glyceraldehyde.

What are the ring forms?

In water, glucose exists mainly as a six-membered ring (a pyranose), formed when the hydroxyl group on carbon 5 attacks the carbonyl carbon (C1), making a cyclic hemiacetal. The new –OH group on C1 (the anomeric carbon) can point down (α-glucose) or up (β-glucose) relative to the ring. In solution, the two anomers interconvert through the open-chain form until an equilibrium is reached with about 36 % α and 64 % β (and a trace of the open chain, about 0.002 %). Fructose forms five-membered (furanose) rings. The differences between α and β linkages explain the difference between starch and cellulose.

What are disaccharides?

A disaccharide is formed by the condensation of two monosaccharides: the –OH group of the anomeric carbon of one reacts with an –OH group of the other to give a glycosidic bond (an acetal link), with the loss of water. The reaction is reversed by hydrolysis with acid or an enzyme.

Sucrose is not a reducing sugar, because the anomeric carbons of both units are used in the glycosidic bond, and there is no free aldehyde or ketone form. In lactose and maltose, one anomeric carbon is free. Lactose must be broken down by the enzyme lactase in the small intestine; adults with a lactase deficiency suffer from lactose intolerance.

What are polysaccharides?

Polysaccharides are polymers of many monosaccharide units, with molar masses of from thousands to millions. Their general formula is (C₆H₁₀O₅)_n when made from glucose.

• Starch is the energy store of plants. It is a mixture of amylose (a linear chain of glucose units joined by α-1,4 bonds, coiled into a helix, which gives a blue-black color with iodine) and amylopectin (branched, with α-1,6 links every 24–30 units).
• Glycogen is the energy store of animals, stored in the liver and muscles. It has a structure like amylopectin but is more highly branched (a branch every 8–12 units), so that many chain ends are available for fast release of glucose.
• Cellulose is the structural polysaccharide of plant cell walls, a linear chain of glucose units joined by β-1,4 bonds. Every other glucose is turned upside down, so that the chains are straight and form hydrogen bonds with neighboring chains into strong fibers. Humans lack the enzyme to hydrolyze β-1,4 bonds, so cellulose is dietary fiber for us, but ruminants and termites digest it with the help of microorganisms.
• Chitin is the polymer of N-acetylglucosamine in the shells of crustaceans and in insects and fungi.

Comparison of polysaccharides

How can sugars be detected?

• Benedict’s or Fehling’s test: reducing sugars (glucose, fructose, maltose, lactose) reduce blue Cu²⁺ to a brick-red precipitate of Cu₂O on heating. Sucrose gives no reaction unless it is first hydrolyzed.
• Iodine test: starch gives a blue-black color, because iodine molecules (as I₃⁻ and I₅⁻) fit inside the amylose helix. Glycogen gives a red-brown color, and cellulose does not react.
• Tollens’ test: reducing sugars give a silver mirror.

Worked examples

Example 1: mass of a disaccharide

Two glucose molecules (180.16 g/mol each) condense to maltose with the loss of one water: 2 × 180.16 − 18.02 = 342.30 g/mol, which corresponds to the formula C₁₂H₂₂O₁₁.

Example 2: hydrolysis of starch

How much glucose is obtained from the complete hydrolysis of 100 g of starch? The repeat unit C₆H₁₀O₅ has a mass of 162.14 g/mol, and takes up one water to give C₆H₁₂O₆ (180.16 g/mol). Mass of glucose = 100 × 180.16 / 162.14 = 111 g. The extra 11 g come from the water that is added.

Example 3: the size of a polymer

An amylose molecule has a molar mass of about 100,000 g/mol. The number of glucose units is n = 100,000 / 162.14 = 617. Cellulose chains have 3000 to 10,000 units (M = 500,000 to 1.6 million g/mol).

Example 4: hydrolysis of sucrose

C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose). From 34.2 g of sucrose (0.100 mol), 0.100 mol of each monosaccharide is obtained: 18.0 g of glucose and 18.0 g of fructose. The mixture is called invert sugar.

Example 5: energy from carbohydrates

The complete oxidation of glucose releases 2803 kJ/mol, which is 2803 / 180.16 = 15.6 kJ/g. Carbohydrates in food are usually quoted at about 17 kJ/g (4 kcal/g). The glycogen reserve of a person (about 400–500 g in the muscles and liver) contains about 7–8 MJ, enough for roughly a day of low-level activity or for one to two hours of intense exercise.

How was carbohydrate chemistry developed?

Emil Fischer worked out the structures and the stereochemistry of the sugars between 1884 and 1894, with phenylhydrazine as a reagent, and was the first to show that glucose, fructose and mannose are related, which brought him the Nobel Prize in Chemistry in 1902. Walter Haworth proposed the ring (Haworth) formulas in the 1920s and received the Nobel Prize in 1937 for his work on carbohydrates and vitamin C. Hermann Staudinger showed in the 1920s that starch and cellulose are long chains, and not aggregates of small molecules.

Which mistakes are common?

• Saying that all carbohydrates are sweet: starch and cellulose are not.
• Mixing up the α and β linkages: they make starch digestible and cellulose not.
• Forgetting the loss of water: the mass of a disaccharide is not the sum of the masses of the monosaccharides.
• Thinking that sucrose is a reducing sugar: both anomeric carbons are tied up.
• Assuming that the ring form is fixed: the anomers interconvert in solution.