Carbohydrates

Carbohydrates are the sugars, starches and cellulose: molecules made of carbon, hydrogen and oxygen that store energy, transport it, and build the walls of plant cells. This topic is the clearest example of the central idea of Topic 2: that the structure of a molecule explains its function. You need to draw α- and β-glucose, describe how glycosidic bonds are made and broken, and relate the structures of starch, glycogen and cellulose to their roles. These are examined every year, often as "explain how the structure is related to the function" questions worth 4 to 6 marks.

Monomers, polymers and macromolecules

Large biological molecules are usually built by joining many small, similar molecules together. The vocabulary for this is examined directly.

Monosaccharides

Monosaccharides are classified by the number of carbon atoms: trioses have 3 carbons (such as glyceraldehyde), pentoses have 5 (ribose and deoxyribose, in nucleic acids) and hexoses have 6 (glucose, fructose, galactose). They are sweet, soluble in water and are all reducing sugars.

Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) is the most important. It is the main respiratory substrate, the form in which sugar is carried in mammalian blood, and the monomer of starch, glycogen and cellulose. Glucose is soluble because its many hydroxyl (–OH) groups are polar and form hydrogen bonds with water.

Ring forms of α-glucose and β-glucose

In solution, glucose forms a ring of five carbon atoms and one oxygen atom. The carbons are numbered 1 to 6, starting from the carbon to the right of the ring oxygen; carbon 6 sits outside the ring as a $\text{CH}_2\text{OH}$ group. The ring can close in two ways, giving two isomers that differ only at carbon 1:

• in α-glucose the –OH on carbon 1 is below the ring;
• in β-glucose the –OH on carbon 1 is above the ring.

The arrangement of groups to learn, going round the ring from carbon 1:

Carbon	Above the ring	Below the ring
C1 (α-glucose)	H	OH
C1 (β-glucose)	OH	H
C2	H	OH
C3	OH	H
C4	H	OH
C5	$\text{CH}_2\text{OH}$ (C6)	H

Disaccharides and the glycosidic bond

Two monosaccharides join in a condensation reaction: an –OH group from each molecule reacts, a molecule of water is removed, and the two rings are left linked through an oxygen atom. This C–O–C link is a glycosidic bond, a type of covalent bond.

When two α-glucose molecules join between carbon 1 of one and carbon 4 of the other, the bond is an α(1→4) glycosidic bond and the product is maltose:

$$\text{α-glucose} + \text{α-glucose} \longrightarrow \text{maltose} + \text{water}$$ $$\ce{C6H12O6 + C6H12O6 -> C12H22O11 + H2O}$$

Disaccharide	Monosaccharides	Bond	Reducing?	Where it matters
maltose	α-glucose + α-glucose	α(1→4)	yes	product of starch digestion by amylase
sucrose	α-glucose + β-fructose	(1→2)	no	form in which sugar is transported in phloem
lactose	β-galactose + glucose	β(1→4)	yes	sugar in milk

Sucrose is a non-reducing sugar because the glycosidic bond links carbon 1 of glucose to carbon 2 of fructose, which are exactly the groups that would otherwise reduce Benedict's solution. Plants transport sugar as sucrose partly because it is less reactive: it does not take part in reactions on the way.

Hydrolysis

Hydrolysis is the reverse of condensation: a molecule of water is used to break a covalent bond. Hydrolysis of glycosidic bonds happens:

• in digestion, catalysed by enzymes (amylase hydrolyses starch to maltose; maltase hydrolyses maltose to glucose; sucrase hydrolyses sucrose);
• in the laboratory, by heating with dilute acid, which is the basis of the test for non-reducing sugars described in Testing for biological molecules. Boiling sucrose with dilute hydrochloric acid hydrolyses its glycosidic bond, releasing glucose and fructose, which are reducing sugars and so give a positive Benedict's test.

Polysaccharides

Polysaccharides are polymers of many monosaccharides joined by glycosidic bonds. They are not sugars: they are not sweet and are insoluble or only slightly soluble. Their properties depend on which isomer of glucose they are made from, which bonds join the monomers, and whether the chains are branched.

Starch: energy storage in plants

Starch is a mixture of two polysaccharides of α-glucose, stored as starch grains in chloroplasts and in storage organs such as potato tubers (in amyloplasts).

Amylose (about 20–30% of starch)

• Long, unbranched chains of α-glucose joined by α(1→4) glycosidic bonds.
• The angle of the bonds makes the chain coil into a helix, held in shape by hydrogen bonds between –OH groups within the chain.
• The helix is compact and is the structure that traps iodine in the starch test.

Amylopectin (about 70–80%)

• Chains of α-glucose joined by α(1→4) glycosidic bonds, with branches joined by α(1→6) glycosidic bonds, roughly every 25–30 glucose units.

Glycogen: energy storage in animals and fungi

Glycogen is stored as tiny granules in liver and muscle cells.

• Like amylopectin: α-glucose with α(1→4) bonds and α(1→6) branches.
• But more highly branched (a branch roughly every 8–12 glucose units), with shorter chains, so it is even more compact and has many more ends.

How starch and glycogen are suited to storage

Glycogen's greater branching suits animals, which have higher and more rapidly changing metabolic rates than plants: muscle cells can mobilise glucose very quickly during exercise.

Cellulose: the structure of plant cell walls

Cellulose is the most abundant organic molecule on Earth. It is made of β-glucose joined by β(1→4) glycosidic bonds.

Because the –OH on carbon 1 of β-glucose is above the ring, while the –OH on carbon 4 is below, two β-glucose molecules can only form a 1→4 bond if every alternate molecule is rotated by 180° (flipped over). This has a major consequence: the chain is straight, not coiled, and has –OH groups projecting on both sides.

From molecule to wall:

1. Each cellulose molecule is a long, straight, unbranched chain of several thousand β-glucose units.
2. Many chains lie parallel to each other, and hydrogen bonds form between the –OH groups of neighbouring chains. One hydrogen bond is weak, but there are enormous numbers of them, so collectively they are very strong.
3. Groups of about 60–70 cross-linked chains form microfibrils.
4. Microfibrils bundle together into fibres (macrofibrils), embedded in a matrix of other polysaccharides (such as hemicelluloses and pectins).
5. In the cell wall, the fibres are laid down in layers, with the fibres in each layer running in a different direction.

Comparing the polysaccharides

Feature	Amylose	Amylopectin	Glycogen	Cellulose
Monomer	α-glucose	α-glucose	α-glucose	β-glucose
Glycosidic bonds	α(1→4)	α(1→4) and α(1→6)	α(1→4) and α(1→6)	β(1→4)
Branching	none	some (every 25–30 units)	many (every 8–12 units)	none
Shape	helix	branched	highly branched, compact	straight chains
Hydrogen bonds	within the chain (holding the helix)	within the molecule	within the molecule	between parallel chains
Found in	plants	plants	animals and fungi	plant cell walls
Function	energy store	energy store	energy store	structural support

Practice questions

Answers

1. (a) A relatively simple molecule used as a basic building block (subunit) for the synthesis of a polymer. (b) A polymer made of many monosaccharides joined by glycosidic bonds.
2. In α-glucose the –OH on carbon 1 is below the ring; in β-glucose it is above the ring.
3. Glycosidic bond; condensation (water removed).
4. (a) Two α-glucose. (b) Glucose and fructose. (c) Glucose and galactose.
5. In sucrose the glycosidic bond links carbon 1 of glucose and carbon 2 of fructose, which are the groups that would reduce Benedict's (copper(II) ions); in maltose, carbon 1 of the second glucose is free (not involved in the bond), so it can act as a reducing group.
6. Amylose is unbranched, amylopectin is branched; amylose has only α(1→4) bonds, amylopectin has α(1→4) and α(1→6) bonds; amylose coils into a helix, amylopectin does not form a simple helix (branched structure). Both are α-glucose.
7. Bonds $= 1200 - 1 = 1199$, so 1199 water molecules. $M_r = 1200 \times 180 - 1199 \times 18 = 216\,000 - 21\,582 = 194\,418$.
8. Glucose is soluble, so large amounts would lower the water potential of the cell, causing water to enter by osmosis (cells could burst or be damaged); glucose could diffuse or be transported out of the cell; glucose is reactive. Starch/glycogen are insoluble, compact and have no osmotic effect.
9. Similarities: both polysaccharides of glucose with (1→4) glycosidic bonds. Differences: glycogen α-glucose, cellulose β-glucose; glycogen branched with α(1→6) bonds, cellulose unbranched; glycogen compact and coiled, cellulose straight chains; in cellulose alternate glucose units inverted; hydrogen bonds between cellulose chains form microfibrils, glycogen does not form fibres; glycogen is an energy store, cellulose structural.
10. Cellulose is made of β-glucose joined by β(1→4) bonds; alternate units are inverted so chains are straight and unbranched; parallel chains are held together by many hydrogen bonds; forming microfibrils and fibres with high tensile strength; fibres laid in layers in different directions; so the wall resists the outward pressure of the expanding protoplast (pressure from the vacuole), preventing further water entry and bursting.