Condensation, hydrolysis and glycosidic bondsEdexcel International A Level Biology: Revision notes
Section 1
The monosaccharides
The monosaccharides in this topic are glucose, fructose and galactose. All have the molecular formula C₆H₁₂O₆, but their atoms are arranged differently. They are soluble, and each contains several hydroxyl (–OH) groups.
Monosaccharides are the building blocks (monomers) of all larger carbohydrates.
Section 2
Condensation reactions and glycosidic bonds
Two monosaccharides join in a condensation reaction. A hydroxyl (–OH) group on one sugar and a hydrogen atom on the other are removed together as a molecule of water. The two sugars become linked through an oxygen atom, a glycosidic bond (C–O–C).
monosaccharide + monosaccharide → disaccharide + H₂O
The same reaction repeated many times builds a polysaccharide. Each new glycosidic bond releases one water molecule.
Do not say 'a water molecule is added' for condensation. Condensation removes water; hydrolysis adds it.
Section 3
Three disaccharides
Learn which monosaccharides make each disaccharide:
- Maltose = glucose + glucose
- Sucrose = glucose + fructose
- Lactose = glucose + galactose
Sucrose is the form in which sugar is transported in the phloem. Lactose is the sugar in milk. Maltose is produced when starch is digested.
Section 4
Polysaccharides
Polysaccharides such as starch and glycogen are polymers of glucose, built by many condensation reactions. They are large and insoluble.
Starch is a store in plants and glycogen a store in animals. Using one large molecule in place of thousands of glucose molecules has almost no effect on the water potential of a cell.
Section 5
Hydrolysis
Hydrolysis is the reverse of condensation. A molecule of water is added across a glycosidic bond, which breaks it and releases the monosaccharides. Enzymes catalyse these reactions, e.g. amylase (starch to maltose), maltase (maltose to glucose) and lactase (lactose to glucose and galactose).
disaccharide + H₂O → monosaccharide + monosaccharide
Worked example: an unbranched chain of n monosaccharide units has n − 1 glycosidic bonds. Forming it releases n − 1 water molecules, and hydrolysing it completely uses n − 1 water molecules. For a chain of 100 glucose units, 99 water molecules are involved.
For a chain, the number of bonds is one fewer than the number of units. Branches in a polysaccharide change this, so only apply n − 1 to unbranched chains.
Must Know
- Glucose, fructose and galactose join by condensation reactions that form glycosidic bonds and release water
- Maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose
- Polysaccharides form by repeated condensation
- Hydrolysis adds water to break glycosidic bonds
- Unbranched chain of n units has n − 1 bonds
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Condensation, hydrolysis and glycosidic bonds
- Sucrose is the main sugar transported in the phloem of a sugar-cane plant. The plant makes sucrose in its leaves from two monosaccharides.Describe how a molecule of sucrose is formed from its two monosaccharides.2 marks
- Milk contains the disaccharide lactose. A man with lactose intolerance has too little of the enzyme lactase in his small intestine, so lactose in milk is not digested properly.Explain how lactase breaks down lactose, referring to the bond and to water.2 marks
- During germination, a barley seed releases amylase, which digests the starch stored in the seed. The embryo needs glucose for respiration. Starch is an unbranched polysaccharide in this example, made of glucose units joined by glycosidic bonds.Explain how amylase and maltase together convert starch into glucose.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).