Disaccharides, starch, cellulose and glycogen

Two sugars join

The anomeric $\text{-}\text{OH}$ of one sugar can react with an $\text{-}\text{OH}$ of another sugar. A molecule of water leaves, and an oxygen bridge is left behind. This is a glycosidic link. It is the sugar version of an acetal.

Two hexoses joined in this way have a molar mass of $2 \times 180.16 - 18.02 = 342.30\ \text{g/mol}$. The formula is $\text{C}_{12}\text{H}_{22}\text{O}_{11}$.

Three disaccharides

Sucrose is table sugar, from cane or beet. It is made of α-D-glucose and β-D-fructose. The link joins C-1 of glucose to C-2 of fructose: an α(1→2)β link. Both anomeric carbons are inside the link, so no free aldehyde or ketone can form. Sucrose is a non-reducing sugar. It does not react with Fehling's or Tollens' reagent, and it does not show mutarotation.

Maltose is made of two α-D-glucose units joined by an α(1→4) link. It comes from the partial hydrolysis of starch. One anomeric carbon is free, so maltose is a reducing sugar and it shows mutarotation.

Lactose is milk sugar. It is made of β-D-galactose and D-glucose, joined by a β(1→4) link. It is a reducing sugar too. Some adults cannot digest lactose, because they make too little of the enzyme lactase. The sugar then stays in the gut.

The rule is simple. A di- or polysaccharide is reducing only if at least one anomeric carbon is not in a glycosidic link.

Inversion of sucrose

Sucrose has a specific rotation of $+66.5$ degrees. Boil it with dilute acid, or add the enzyme invertase, and it hydrolyses to equal moles of glucose and fructose:

$\text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_{2}\text{O} \to \text{C}_{6}\text{H}_{12}\text{O}_{6} + \text{C}_{6}\text{H}_{12}\text{O}_{6}$

Glucose has $+52.7$ degrees and fructose has $-92.4$ degrees. A mixture of equal moles has a specific rotation of $\tfrac{1}{2}(52.7 - 92.4) = -19.9$ degrees. The sign changes from plus to minus. We say that the sugar has been inverted, and the mixture is invert sugar. Honey is rich in it, and that adds to the sweetness, because fructose is sweeter than sucrose.

Slide the fraction from 0 to 1. The curve crosses zero at about 0.76, whatever the concentration. At that point the right-turning and left-turning sugars cancel exactly.

Starch

Starch is the store of energy in plants. Rice, wheat and potato are rich in it. It is a mixture of two polysaccharides, both made only of α-D-glucose.

• Amylose is about 20% of starch. It is a long, unbranched chain, with all links α(1→4). It dissolves in hot water. The chain coils into a helix, and iodine fits in the helix to give the blue-black colour of the iodine test.
• Amylopectin is about 80% of starch. Its main chains are α(1→4) too, but every 24 to 30 units a side chain starts from an α(1→6) link. It is branched, and it does not dissolve in water.

Cellulose and glycogen

Cellulose makes the cell walls of plants. Cotton is almost pure cellulose. Like amylose, it is a chain of glucose with 1→4 links, but the glucose is β-D-glucose, so the links are β(1→4). This small change has large effects. Every second unit is turned over, so the chain is straight and flat. Hydrogen bonds hold the chains side by side in strong fibres. Humans have no enzyme that cuts the β links of cellulose, so we cannot digest it. (Lactase cuts the β link of lactose, but it does not act on cellulose.) Cattle can, because bacteria in their stomach make cellulase.

Glycogen is the store of energy in animals. It is found mainly in the liver and in the muscles. It is made of α-D-glucose, like amylopectin, but it is more highly branched: a branch every 8 to 12 units. When the body needs glucose, enzymes cut it from the many chain ends at once.

The three polymers compare like this:

• Amylose: α-glucose, α(1→4) links, an unbranched helix. Humans digest it.
• Amylopectin: α-glucose, α(1→4) and α(1→6) links, branched. Humans digest it.
• Glycogen: α-glucose, α(1→4) and α(1→6) links, highly branched. Humans digest it.
• Cellulose: β-glucose, β(1→4) links, straight chains in fibres. Humans cannot digest it.

What carbohydrates do

• Glucose is the fuel of cells, and about $17\ \text{kJ/g}$ of energy is released when it is oxidised.
• Starch and glycogen store glucose. Cellulose gives plants their structure.
• Ribose and deoxyribose are in RNA and DNA. You will meet them in the third course.
• Cellulose gives paper, cotton and rayon. Glucose is given by drip to patients. Sugars feed the fermentation industry.

Try it in Python

The program works out the molar mass of five sugars, then decides if each is reducing from the number of free anomeric carbons.

Practise with new numbers

Common mistakes

• Counting links wrongly. A chain of $n$ units has $n - 1$ links and loses $n - 1$ molecules of water.
• Calling sucrose a reducing sugar. Both anomeric carbons are in the link.
• Thinking starch and cellulose differ in the sugar. The sugar is glucose in both. The link is α in starch and β in cellulose.
• Saying that amylose is branched. Amylopectin is branched. Amylose is not.

Remember these

• A glycosidic link joins two sugars and releases one molecule of water.
• Sucrose: α(1→2)β, non-reducing. Maltose: α(1→4), reducing. Lactose: β(1→4), reducing.
• Sucrose gives equal moles of glucose and fructose, and the rotation changes from $+66.5$ to $-19.9$ degrees (inversion).
• Starch is about 20% amylose and 80% amylopectin. Cellulose has β(1→4) links. Glycogen is a highly branched store.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is sucrose a non-reducing sugar, while maltose is a reducing sugar?

2. Give two differences between amylose and amylopectin.

3. Starch and cellulose are both polymers of glucose. Why can we digest one and not the other?

4. How many molecules of water are released when 50 glucose units join in an unbranched chain?

References

• Nelson and Cox, Lehninger Principles of Biochemistry, 8th edition, Chapter 7 (disaccharides, starch, cellulose and glycogen).
• NCERT Chemistry for Class 12, Part II, the chapter Biomolecules (disaccharides and polysaccharides).
• OpenStax, Organic Chemistry, Chapter 25 (Biomolecules: Carbohydrates).