Monosaccharides and Disaccharides
Laura Armstrong & Joe Wolfensohn
Teachers


Contents
Recall Questions
This topic requires prior knowledge of carbohydrates. You can test your knowledge on these below.
What is a monosaccharide?
A monosaccharide is a single sugar molecule
What elements are carbohydrates made of?
Carbon, hydrogen and oxygen
What type of reaction produces a glycosidic bond?
Condensation
Topic Explainer Video
Check out this @JoeDoesBiology video that explains monosaccharides or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Monosaccharides
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Definition: The monomer of carbohydrates, containing carbon, hydrogen, and oxygen in a ratio of (CH₂O)n.
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Monosaccharides are soluble in water and serve as fundamental components in metabolism and biosynthesis.
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Triose sugars contain 3C, Pentose sugars contain 5C and Hexose sugars contain 6C.
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Examples:
- Hexoses
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α-glucose (used in energy storage and respiration)
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β-glucose (used in structural molecules like cellulose)
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Fructose (found in fruits, an alternative energy source)
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Galactose (part of lactose, important in milk digestion)
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- Pentoses
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Ribose (used to make nucleic acids: RNA)
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Deoxyribose (used to make nucleic acids: DNA)
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- Hexoses
Structure of Glucose:
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- Both α- and β-glucose have six carbon atoms (hexose sugars).
- α- and β-glucose are isomers of each other.
- The carbons are numbered starting clockwise from the O in the ring.
- The difference between α- and β-glucose is the position of the hydroxyl (-OH) group on carbon 1.
The hydroxyl group (OH group) is below carbon 1.
The hydroxyl group (OH group) is above carbon 1.
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You need to be able to recognise and draw both of these isomers.
Exam Tip
Use ABBA (Alpha Below, Beta Above) To remember the difference between α- and β-glucose
Disaccharides
- Definition: Carbohydrates formed when two monosaccharides join via a glycosidic bond.
- Examples:
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Maltose = α-glucose + α-glucose (important in digestion and germination)
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You need to be able to draw the structure of maltose.
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- Sucrose = glucose + fructose (common table sugar, transported in plants)
- Lactose = glucose + galactose (found in milk, important for infant nutrition)
- You do not need to be able to draw sucrose and lactose from memory, however, when given the monosaccharides you should be able to show how they join together in a condensation reaction to form a glycosidic bond and the subsequent disaccharides.
- Formation & Breakdown:
- Condensation Reaction: Two monosaccharides join, forming a glycosidic bond, with water produced.
- Hydrolysis Reaction: Water is added to break the glycosidic bond, separating the monosaccharides. Enzymes like maltase, sucrase, and lactase help catalyse these reactions in living organisms.
- The glycosidic bond can form on any of the carbons. At A-level the important ones are the 1,4-glycosidic bond (which forms between carbon 1 of one monosaccharide and carbon 4 of the next) and the 1,6-glycosidic bond (which forms between carbon 1 of one monosaccharide and carbon 6 of the next)
- The diagram above shows the formation of a 1-4 glycosidic bond. Many of these bonds will produce a straight chain of monosaccharides.
Key Terms
- Glycosidic Bond: The covalent bond between monosaccharides, crucial for carbohydrate formation.
- Isomers: Molecules with the same formula but different structures (e.g., α- and β-glucose). These differences influence their biological roles.
Exam Tip
State the bond that is formed (glycosidic) between monosaccharides.
Maltose is formed from two glucose molecules. Describe how this disaccharide is formed and broken down in biological systems. (5 marks)
- Maltose forms when two α-glucose molecules join in a condensation reaction.
- A glycosidic bond forms between carbon 1 and 4.
- A molecule of water is released in this reaction.
- Maltose is broken down by hydrolysis, using water to break the glycosidic bond.
- The enzyme maltase catalyses the hydrolysis of maltose into two glucose molecules.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!