Maltose, Sucrose, Lactose: Glycosidic Bonds
How two monosaccharides join by condensation to form a glycosidic bond, and which pairs make the three disaccharides: maltose, sucrose and lactose. Includes the 4-mark exam question, a worked model answer and the examiner trap on naming the bond.
Chapters (9)
Exam question [4 marks]
Sucrose and lactose are disaccharides. Describe how a molecule of sucrose is formed, and name the monosaccharides that make up lactose.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Sucrose is formed from glucose and fructose | "a glucose molecule and a fructose molecule", "glucose + fructose", "alpha-glucose and beta-fructose" | "glucose and glucose"; "glucose and galactose"; "two sugars" with no names |
| 2 | In a condensation reaction / water is released (eliminated) | "condensation", "loses a molecule of water", "water removed / given off" | "water is used"; "hydrolysis"; "dehydration" alone with no water released |
| 3 | A glycosidic bond forms between the (two) monosaccharides | "glycosidic link", "covalent glycosidic bond", "1,2 glycosidic bond"; phonetic spellings (e.g. "glycosydic") | "peptide bond"; "ester bond"; "hydrogen bond"; "covalent bond" alone |
| 4 | Lactose is made of glucose and galactose | "galactose + glucose", either order | "glucose and fructose"; "glucose and glucose"; "galactose" alone |
Marks 1 to 3 describe sucrose formation (the sugars, the reaction, the bond); mark 4 tests lactose. The commonest lost marks are swapping the partners of sucrose and lactose, and naming the wrong bond. "Condensation" and "water released" are the same mark point; do not award twice. Maltose is not asked here, so any mention of it earns no mark and loses none.
Model answer (4/4)
Sucrose is formed from a glucose molecule and a fructose molecule. They join in a condensation reaction, in which a molecule of water is released. A glycosidic bond forms between the two monosaccharides. Lactose is made of glucose and galactose.
Examiner trap
- Mixing up which monosaccharides make sucrose and lactose
- Saying 'peptide bond' or 'ester bond' instead of glycosidic bond
Key terms
- monosaccharide
- disaccharide
- glycosidic bond
- maltose
- sucrose
- lactose
- glucose
- galactose
- fructose
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:24Monosaccharides: glucose, galactose, fructose
Single sugar units are called monosaccharides. They are the monomers that bigger carbohydrates are built from. You need three of them: glucose, galactose and fructose. Glucose is the one you will see everywhere. It is the main sugar your cells use for energy. Galactose and fructose are close relatives, with the same atoms arranged a little differently. Fructose is the sugar you find in fruit, and its ring has five sides rather than six.
When two monosaccharides join, the result is a disaccharide. Di means two.
1:01The glycosidic bond
Now, how do two monosaccharides join? By a condensation reaction. A molecule of water is released, and the two sugars are linked by a glycosidic bond. That bond is a single oxygen, acting as a bridge between the two rings. The water comes from an OH on one sugar and a hydrogen from the other. And as you saw in the last lesson, adding water back, by hydrolysis, breaks the bond and splits the sugar apart again.
Learn the name. Glycosidic bond. Not peptide, not ester.
1:36Maltose: glucose + glucose
Join two glucose molecules and you get maltose. Glucose plus glucose equals maltose. That is the easy one. You meet maltose when seeds break down their starch to germinate. Remember: maltose is just two glucose molecules.
1:52Sucrose: glucose + fructose
Join glucose and fructose, and you get sucrose. That is ordinary table sugar, the sugar plants carry around in their sap. Same reaction, same bond, water released. Only the partners have changed.
2:07Lactose: glucose + galactose
Join glucose and galactose, and you get lactose, the sugar in milk. Notice that every one of these three disaccharides contains glucose. The partner is what makes each one different. And you need to know all three pairings by heart.
2:22A memory trick for the three
Here is a memory trick. Maltose is the double: glucose and glucose. Sucrose is the sugar you spoon on fruit, and fructose is the fruit sugar. Sucrose, fructose. Lactose is the milk sugar, and the word galactose comes from the Greek word for milk. Lactose, galactose. Every pair includes glucose, so you only ever have to remember the partner.
3:14Examiner trap and recap
The trap. Writing peptide bond or ester bond. Those belong to proteins and fats. And do not swap the partners in sucrose and lactose.
Recap. Two monosaccharides join by condensation, with a glycosidic bond. Next up, glucose has two forms, alpha and beta, and that one small difference explains starch and cellulose.
Watch first
This lesson builds on:
- Condensation and Hydrolysis Reactions · AQA 7402 · 3.1.1
Keep going
More from 3.1 Biological molecules
- Alpha vs Beta Glucose: Starch vs Cellulose · AQA 7402 · 3.1.2
- Benedict's and Iodine Tests Explained · AQA 7402 · 3.1.2
- Condensation and Hydrolysis Reactions · AQA 7402 · 3.1.1
- Triglycerides: Saturated vs Unsaturated Fats · AQA 7402 · 3.1.3
All 21 3.1 Biological molecules lessons Whole playlist on YouTube
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-002.