Monosaccharides

Monosaccharides are the simplest sugars: single sugar units, and the monomers that every larger carbohydrate is built from. Glucose, galactose and fructose are the common ones, and all three have the formula C₆H₁₂O₆. Ribose and deoxyribose, the sugars in RNA and DNA, are monosaccharides too. Two monosaccharides join to make a disaccharide, and many join to make a polysaccharide such as starch.

What is a monosaccharide?

In AQA's A-level specification, monosaccharides sit alongside amino acids and nucleotides as examples of monomers, the small units that join to make polymers. AQA names glucose, galactose and fructose as the common monosaccharides.

OpenStax gives the general formula of carbohydrates as (CH₂O)ₙ, where n is the number of carbon atoms. In monosaccharides, n is usually between 3 and 7. So a six-carbon sugar is (CH₂O)₆, which is C₆H₁₂O₆.

Types of monosaccharides

Monosaccharides are named by how many carbon atoms they have:

They can also be sorted by their functional group. OpenStax explains that a sugar with an aldehyde group is an aldose and one with a ketone group is a ketose. Glucose and galactose are aldoses; fructose is a ketose.

Examples of monosaccharides

Deoxyribose has one fewer oxygen atom than ribose, which is what "deoxy" refers to. AQA's specification describes each DNA nucleotide as deoxyribose, a phosphate group and a base, and each RNA nucleotide as ribose, a phosphate group and a base. ATP is also built on ribose.

Structure: isomers and rings

Glucose, galactose and fructose share one formula, C₆H₁₂O₆, with a different arrangement of atoms in each. OpenStax calls them structural isomers.

Monosaccharides can be drawn as a straight chain, but OpenStax notes that in solution they are usually rings. Glucose forms a six-membered ring, while fructose and ribose form five-membered rings. In the glucose ring, the OH group on carbon 1 can point down (α-glucose) or up (β-glucose). That small difference decides whether glucose ends up in starch and glycogen or in cellulose: see alpha and beta glucose.

Properties of monosaccharides

• Soluble in water. OpenStax explains that their many hydroxyl (OH) groups make them generally soluble in water. That is how glucose dissolves in blood plasma and travels round the body.
• Small enough to be absorbed. Digestion breaks bigger carbohydrates down to monosaccharides, which are taken up from the gut.
• Reducing sugars. All aldoses are reducing sugars because of their aldehyde group. Fructose, a ketose, still reacts, because in alkaline solution it rearranges into aldoses.

The Benedict's test

Benedict's solution contains blue copper(II) ions. AQA's practical method is to add a few drops of Benedict's solution to the sample and put the tube in a water bath at a minimum of 80 °C for about 5 minutes. A reducing sugar turns the copper ions into red copper oxide, so the colour moves away from blue towards orange and red: the more reducing sugar, the bigger the change.

AQA's handbook lists glucose, lactose and maltose as reducing sugars, and sucrose as a non-reducing sugar that gives no positive result. Sucrose only reacts once it has been hydrolysed into glucose and fructose, which is the idea behind the A-level test for non-reducing sugars.

From monosaccharides to disaccharides

Two monosaccharides join in a condensation reaction, which forms a glycosidic bond and releases one molecule of water. A hydrolysis reaction uses a water molecule to break the bond again. These are the three disaccharides AQA names:

Many glucose units joined by condensation make the polysaccharides: starch and glycogen from α-glucose, cellulose from β-glucose.

Worked examples

Example 1: is it a monosaccharide?

A sugar has the formula C₅H₁₀O₅. Does it fit (CH₂O)ₙ? With n = 5, (CH₂O)₅ = C₅H₁₀O₅, so yes: it is a pentose. Ribose has this formula. Deoxyribose, C₅H₁₀O₄, doesn't fit the pattern exactly, because it has lost an oxygen.

Example 2: relative molecular mass

Using C = 12, H = 1 and O = 16, a hexose C₆H₁₂O₆ has Mr = 6 × 12 + 12 × 1 + 6 × 16 = 180. Joining two hexoses removes one water (Mr 18), so a disaccharide has Mr = 2 × 180 − 18 = 342, whichever two hexoses it is made of.

Example 3: three sugars joined

Three hexoses join in a chain. How many glycosidic bonds form, and what is the formula? Three units in a row need two bonds, so two water molecules are removed: 3 × C₆H₁₂O₆ − 2 × H₂O = C₁₈H₃₂O₁₆. For any chain of n units, there are n − 1 bonds and n − 1 waters lost.

Common mistakes

• Calling sucrose a monosaccharide. Sucrose, maltose and lactose are disaccharides: two monosaccharides joined.
• Saying all sugars are reducing sugars. Sucrose isn't: it gives a negative Benedict's test until it is hydrolysed.
• Mixing up condensation and hydrolysis. Condensation joins and releases water; hydrolysis splits and uses water.
• Thinking the same formula means the same molecule. Glucose, fructose and galactose are all C₆H₁₂O₆ but are different isomers.
• Forgetting deoxyribose is a monosaccharide. It is a pentose, like ribose, with one less oxygen.

Biological molecules come up in the first year of A-level Biology, so try today's A-level Biology: Year 1 short timed paper. To see glucose being used, read about cellular respiration and photosynthesis.

Read next

Biology · Biological moleculesAlpha glucose and beta glucose: structure and differencesThe structure of alpha glucose and beta glucose, the one difference between them, how glycosidic bonds form, and why it matters for starch and cellulose.