1.1.2Unit 1: Lifestyle, Transport, Genes and Health
Saccharides — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 9 of 29 Unit quiz
Part 1 of 4Monosaccharides
Structure
- Monosaccharides are single sugar units with a general formula (CH₂O)ₙ,
- such as glucose, fructose and galactose,
- which are all hexoses with six carbons.
α- and β-glucose
- The two forms of glucose differ only in the position of the –H and –OH groups on carbon 1;
- in α-glucose the –OH is below the ring.
Properties
- They are small, soluble and sweet, so they are easily transported and are used directly in respiration.
Reducing sugars
- All monosaccharides are reducing sugars because they can donate electrons to the copper(II) ions in Benedict’s reagent.
Part 2 of 4Disaccharides
Disaccharides
Maltose
- Two α-glucose molecules joined by a 1,4 glycosidic bond make maltose, which is a reducing sugar.
Sucrose
- Glucose and fructose joined together form sucrose, a non-reducing sugar that is the main transport sugar in plants.
Lactose
- Glucose joined to galactose forms lactose, the sugar found in milk.
Formation
- Each disaccharide is made by a condensation reaction that releases a molecule of water.
Part 3 of 4Polysaccharides: Starch and Glycogen
1
Polysaccharides are long polymers made of many α-glucose monomers joined by glycosidic bonds, and are insoluble, so they do not affect water potential.
2
Starch is the storage carbohydrate in plants and is a mixture of two polymers, amylose and amylopectin.
3
Amylose is an unbranched chain with 1,4 bonds that coils into a helix, making it compact for storage.
4
Amylopectin is branched, with 1,4 bonds in the chains and 1,6 bonds at branch points, so enzymes can release glucose quickly from many ends.
5
Glycogen is the animal storage carbohydrate, similar to amylopectin but with more branches, which gives rapid release of glucose for respiration.
| Polysaccharide | Found in | Bonds | Branching |
|---|---|---|---|
| Amylose | Plants | 1,4 | None, helical |
| Amylopectin | Plants | 1,4 and 1,6 | Branched |
| Glycogen | Animals, fungi | 1,4 and 1,6 | Highly branched |
Part 4 of 4Exam Checklist & Tips
1.1.2 – Key Points for the Exam
- Monosaccharide A single sugar unit, the monomer of carbohydrates.
- Glycosidic bond The covalent bond between two sugar units formed in a condensation reaction.
- Amylose An unbranched, helical starch polymer of α-glucose.
- Glycogen The highly branched storage polysaccharide of animals.
Common Mistakes
- Stating that sucrose is a reducing sugar; it must be hydrolysed before Benedict’s reagent reacts.
- Mixing up α-glucose and β-glucose, which differ in the orientation of the –OH on carbon 1.
- Saying that starch is soluble; it is insoluble, which makes it a good store.
- Giving the formula of glucose as CH₂O instead of C₆H₁₂O₆.
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