Monosaccharides, disaccharides and polysaccharides; glucose and ribose
Living things are built from a small number of kinds of molecule, used over and over.
Carbohydrates are the first: they supply the glucose that cells respire, store energy in compact forms, and build the walls that hold plants upright.
This section sorts carbohydrates into three groups and teaches the three sugar structures you must be able to draw.
Biological Molecules can be set on all three papers (Papers 1, 2 and 3).
In recent series Paper 2 has drawn on it most, often through multiple-choice items, while Paper 3 has reached it through enzymes, carbohydrates and mineral ions, so none of these ideas can be left to one paper.
Monomers, polymers and the three groups
A monomer is a small molecule that can be joined to others like it.
A polymer is a long chain of monomers.
For carbohydrates the monomer is a single sugar unit, a monosaccharide.
- A monosaccharide is one sugar unit. Glucose, fructose, galactose and ribose are monosaccharides. They are small, soluble in water and taste sweet.
- A disaccharide is two monosaccharides joined together. Sucrose, lactose and maltose are disaccharides. They are still soluble and sweet.
- A polysaccharide is many monosaccharides joined into a long chain, often hundreds or thousands of units. Starch (made of amylose and amylopectin), glycogen and cellulose are polysaccharides. They are large, insoluble or barely soluble, and not sweet.
The difference that matters in an exam is the number of sugar units: one, two or many.
Size then explains the properties.
A small monosaccharide dissolves and lowers the water potential of a cell; a huge polysaccharide does not, which is why cells store sugar as polysaccharide (the next two sections explain this).
Group
Sugar units
Examples you need
Soluble in water?
Monosaccharide
1
glucose, fructose, galactose, ribose
yes
Disaccharide
2
sucrose, lactose, maltose
yes
Polysaccharide
many (hundreds to thousands)
starch (amylose and amylopectin), glycogen, cellulose
no, or very little
Hexoses and pentoses
Monosaccharides are named by the number of carbon atoms they contain.
A hexose has six carbons; a pentose has five.
Glucose, fructose and galactose are hexoses, with the formula C₆H₁₂O₆.
Ribose is a pentose, C₅H₁₀O₅.
All monosaccharides have the general formula (CH₂O)ₙ, so a hexose has n = 6 and a pentose n = 5.
Glucose, fructose and galactose share one molecular formula but differ in how their atoms are arranged.
Molecules like this are isomers.
The arrangement changes the shape, and the shape decides which enzymes act on the sugar and what it can be built into.
The structure of glucose: alpha and beta
In water, almost all glucose molecules close into a ring of five carbon atoms and one oxygen atom.
The carbons are numbered 1 to 6 around the molecule: carbon 6 is the CH₂OH group that sticks out of the ring above carbon 5.
The diagram below shows the two ring forms.
The two forms differ in one place only: the positions of the H and the OH on carbon 1.
- In alpha-glucose (α-glucose) the OH on carbon 1 is below the ring, on the opposite side to the CH₂OH.
- In beta-glucose (β-glucose) the OH on carbon 1 is above the ring, on the same side as the CH₂OH.
Everything else is identical: OH below the ring on carbon 2, above on carbon 3 and below on carbon 4.
That one flip has large consequences.
Enzymes that join α-glucose cannot join β-glucose, and chains of β-glucose can line up side by side and bond to each other, while chains of α-glucose coil.
Starch and glycogen are made of α-glucose; cellulose is made of β-glucose.
So "which glucose?"
is the first question to ask about any polysaccharide.
A useful way to remember it: in beta, the OH on carbon 1 is above the ring.
Drawing glucose for the marks
You must be able to draw both forms from memory.
A drawing earns its marks for:
1. a ring of five carbon atoms and one oxygen, with the oxygen between carbon 5 and carbon 1;
2. the CH₂OH on carbon 5, above the ring;
3. an H and an OH on each of carbons 1 to 4, the OH groups on carbons 2, 3 and 4 placed correctly;
4. the H and OH on carbon 1 the right way round for the form asked.
Every bond line must reach an atom.
Common slips are drawing alpha when beta is asked, putting an OH on carbon 5 in place of the H, leaving the ring open, and adding a water molecule beside the drawing.
Ribose, the pentose in RNA and ATP
Ribose is a five-carbon sugar.
Its ring has four carbon atoms and one oxygen; the fifth carbon is a CH₂OH group above carbon 4.
The diagram below shows ribose.
Ribose is part of every RNA nucleotide and of ATP.
Deoxyribose, the sugar in DNA, is the same molecule with one oxygen missing: carbon 2 carries H and H in place of H and OH ("deoxy" means "without an oxygen").
That missing oxygen is one of the differences between DNA and RNA that the papers ask about.
Used again in: the next two sections, where glucose units are joined into starch, glycogen and cellulose; the DNA and RNA sections later in this unit, where ribose and deoxyribose are the sugars of the nucleotides; and Topic 5, Energy for Biological Processes, where glucose is respired and ribose is part of ATP.
- Carbohydrate groups (what you need to know): Know the difference between monosaccharides, disaccharides and polysaccharides.
- Sugar structures (what you need to know): Know the structure of the hexose glucose (alpha and beta) and the pentose ribose.
- Monosaccharide: a single sugar unit, the monomer of carbohydrates, such as glucose, fructose, galactose or ribose.
- Disaccharide: two monosaccharides joined by a glycosidic bond, such as sucrose, lactose or maltose.
- Polysaccharide: a polymer of many monosaccharides joined by glycosidic bonds, such as starch, glycogen or cellulose.
- Hexose / pentose: a monosaccharide with six / five carbon atoms.
- Isomers: molecules with the same molecular formula but a different arrangement of atoms.
- Every mark is for one creditworthy point, and the scheme lists more points than there are marks, so any correct points up to the total score.
- You do not need the scheme's exact words: ACCEPT lines allow other correct wording, IGNORE lines show extras that neither gain nor lose, DO NOT ACCEPT lines show the near misses that score nothing.
- A right answer written beside a wrong one, or two answers that contradict each other, scores zero. Commit to one answer.
- Calculations: the correct answer with no working gains full marks, but working earns method marks when the final answer is wrong. A rounding or form instruction ("two significant figures", "standard form") is a separate mark.
- Explain means reasons: a cause linked to an effect in the stem's context. Describing what happens under Explain stops at one or two marks.
- Compare and contrast needs at least one similarity and one difference, each written as a pair in one sentence ("Both ...", "X has ... whereas Y has ..."). Evaluate needs at least one strength and one weakness. Justify is an improvement plus its reason; an improvement with no reason scores zero.
- Determine always needs a number from the data. State, Give and Name need one precise term or fact, in words, not a formula. Analyse the data to ... means use the figures given, then do the second verb.
- Parts marked with an asterisk (*) are levels-marked: judged on how many data sets or ideas you cover and how well you link them, not on a count of points.
Draw the structure of a molecule of beta-glucose.
(2)
A drawing that shows the following:
- a ring of five carbon atoms and one oxygen atom, the O linking carbon 1 to carbon 5; CH₂OH above the ring on carbon 5; an H and an OH correctly placed on each of carbons 2, 3 and 4 (1)
- the OH on carbon 1 above the ring, on the same side as the CH₂OH, and the H on carbon 1 below (1)
Weaker answer: a ring with the OH on carbon 1 drawn below the ring.
This is alpha-glucose and scores the first mark only.
A ring with no oxygen, or with lines that end in mid-air, loses the first mark as well.
State one structural difference between alpha-glucose and beta-glucose.
(1)
- In alpha-glucose the OH on carbon 1 is below the ring, whereas in beta-glucose it is above the ring (on the same side as the CH₂OH) (1)
Weaker answer: "Beta-glucose has its OH group the other way up."
This scores 0: it does not say which carbon, so it could describe any of the four OH groups.
Always name carbon 1.
Galactose is a hexose.
Two monosaccharides join to make a disaccharide, and one molecule of water is removed.
Calculate the molecular formula of a disaccharide made from two hexoses.
(1)
1. Two hexoses: 2 × C₆H₁₂O₆ = C₁₂H₂₄O₁₂.
2. Remove one water, H₂O: C₁₂H₂₄O₁₂ − H₂O = C₁₂H₂₂O₁₁ (1)
Answer: C₁₂H₂₂O₁₁.
This is the formula of sucrose, lactose and maltose alike.
They are isomers of each other, so the formula alone cannot tell you which one you have.
Explain why glucose is described as both a hexose and a monosaccharide.
(2)
- it has six carbon atoms (C₆H₁₂O₆), so it is a hexose (1)
- it is a single sugar unit that cannot be hydrolysed into smaller sugars, so it is a monosaccharide (1)
Weaker answer: "Glucose is small and sweet."
This scores 0.
Being small and sweet is a property of all simple sugars; it does not say what the two terms mean.
When you draw glucose, draw the ring first, put the CH₂OH on carbon 5, then fill in carbons 4, 3 and 2 (OH down, up, down), and only then decide carbon 1.
Drawing in this order stops you swapping alpha and beta by accident, which is the commonest way a molecule drawing loses its last mark.
- Swapping alpha and beta. It is tempting because both are "glucose" and the change is one group. Learn the rule: beta has the OH on carbon 1 above the ring, on the same side as the CH₂OH.
- Saying a disaccharide is "two sugars" without saying they are joined. Two loose glucose molecules are not maltose; the glycosidic bond is what makes it a disaccharide.
- Calling ribose a hexose because, like glucose, it closes into a ring. Ribose has five carbons in all: four in its ring with one oxygen, and the fifth in the CH₂OH group. Count every carbon, not the ring atoms.
- Writing that polysaccharides are "too big to move". Say they are insoluble, so they do not lower the water potential of the cell; size alone is not the reason that earns the mark.
- Using "glucose" for every sugar. Name the sugar the question gives: fructose and galactose have the same formula but a different structure.
Check your understanding
1. Which row of the table correctly describes alpha-glucose and beta-glucose? (1)
Alpha-glucose: OH on carbon 1
Beta-glucose: OH on carbon 1
A
above the ring
below the ring
B
below the ring
above the ring
C
below the ring
below the ring, H above
D
on carbon 5
on carbon 1
Total [1]
2. Which of the following is a pentose? (1)
- A fructose
- B galactose
- C maltose
- D ribose
Total [1]
3. Nectar collected by honeybees contains sucrose, glucose and fructose. In the hive, the bees add an enzyme that splits sucrose into glucose and fructose, and then they evaporate most of the water.
(a) Give the group of carbohydrate to which sucrose belongs.
(1)
(b) State why glucose and fructose are described as isomers.
(1)
(c) Explain why a solution containing equal masses of sugar has more dissolved molecules after the sucrose is split.
(2)
(d) Glucose and fructose are both hexoses.
Calculate the number of hydrogen atoms in 25 molecules of fructose.
(1)
Answer ..............................
Total [5]
4. Draw the structure of a molecule of alpha-glucose. (2)
Total [2]
5. A student wrote: "Ribose and deoxyribose are both hexoses because they are found in nucleic acids." Explain whether the student is correct. (2)
Total [2]
6. A food scientist analysed three sugar samples, P, Q and R. The table shows the results.
Sample
Molecular formula
Number of sugar units after complete hydrolysis
P
C₅H₁₀O₅
1
Q
C₁₂H₂₂O₁₁
2
R
(C₆H₁₀O₅)ₙ
about 2 000
(a) Give the group of carbohydrate to which each sample belongs.
(2)
(b) Explain why sample R would be more suitable than sample Q as a storage molecule in a cell.
(2)
Total [4]
Answers
1. The only correct answer is B (1). A is incorrect because the positions are reversed. C is incorrect because in beta-glucose the OH on carbon 1 is above the ring. D is incorrect because carbon 5 carries CH₂OH in both forms, not an OH.
2. The only correct answer is D (1). A is incorrect because fructose is a hexose (six carbons). B is incorrect because galactose is a hexose. C is incorrect because maltose is a disaccharide.
3. (a) disaccharide (1) (b) they have the same molecular formula (C₆H₁₂O₆) but a different arrangement of atoms / structure (1) (c) An explanation that makes reference to the following:
- each sucrose molecule is hydrolysed into two monosaccharide molecules, one glucose and one fructose (1)
- so the number of dissolved sugar molecules (per unit volume) increases / almost doubles (1) (d) 300 (1). Each fructose molecule is C₆H₁₂O₆, with 12 hydrogen atoms; 25 × 12 = 300.
4. A drawing that shows the following:
- a ring of five carbon atoms and one oxygen atom, the O linking carbon 1 to carbon 5; CH₂OH above the ring on carbon 5; an H and an OH correctly placed on each of carbons 2, 3 and 4 (1)
- the OH on carbon 1 below the ring and the H above (1) DO NOT ACCEPT a water molecule drawn beside the structure.
5. An explanation that makes reference to the following:
- the student is not correct: both are pentoses / five-carbon sugars (1)
- deoxyribose has one less oxygen atom than ribose (H in place of OH on carbon 2), but the number of carbons is the same (1)
6. (a) P monosaccharide, Q disaccharide, R polysaccharide (all three = 2 marks; two correct = 1 mark) (b) An explanation that makes reference to the following:
- R is a polysaccharide, so it is insoluble / does not dissolve (1)
- so it does not lower the water potential of the cell / has no osmotic effect, so water does not enter the cell by osmosis (1) ACCEPT R is a large molecule that cannot leave the cell / cross the membrane.