Carbohydrates

Carbohydrates are the most abundant organic substances in nature: about 80 % of the dry mass of plants is carbohydrate. They are made in green plants by photosynthesis from carbon dioxide and water using solar energy, and they are the main energy source for living things: oxidising 1 g of carbohydrate releases about 17 kJ. Carbohydrates underpin food (bread, rice, sugar, fruit), textiles (cotton, linen), and the paper and timber industries. Cotton fibre is almost pure cellulose, so this topic is especially important for Uzbekistan. Exam questions cover the dual properties of glucose, hydrolysis of di- and polysaccharides, and calculations on fermentation and photosynthesis.

§1. Classification and structure

• Monosaccharides — simple sugars that cannot be hydrolysed: hexoses — glucose, fructose, galactose C₆H₁₂O₆; pentoses — ribose C₅H₁₀O₅ and deoxyribose C₅H₁₀O₄ (in RNA and DNA).
• Disaccharides — give two monosaccharide molecules on hydrolysis: sucrose, maltose, lactose C₁₂H₂₂O₁₁.
• Polysaccharides — natural polymers of hundreds or thousands of monosaccharide units: starch, glycogen, cellulose (C₆H₁₀O₅)ₙ.

By functional group, monosaccharides are aldoses (with an aldehyde group — glucose, galactose, ribose) or ketoses (with a ketone group — fructose). They combine the properties of polyhydric alcohols and carbonyl compounds.

§2. Glucose

Glucose (grape sugar) is an aldehyde-alcohol: its open-chain form has five –OH groups and one –CHO group. In solution it exists mainly as six-membered rings (α and β forms), which interconvert via the open chain. It is a colourless, sweet, very soluble crystalline solid; human blood keeps it at a constant level, and it is the brain’s main fuel.

Fermentation is breakdown by microbial enzymes:

Lactic fermentation occurs in making yoghurt, kefir and sauerkraut; butyric fermentation is also known. In the body, glucose is oxidised to release energy: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.

Worked example

What volume of CO₂ (STP) is released by alcoholic fermentation of 36 g of glucose?

1. n(C₆H₁₂O₆) = 36/180 = 0.2 mol.
2. C₆H₁₂O₆ → 2CO₂: n(CO₂) = 0.4 mol.
3. V = 0.4·22.4 = 8.96 L.

§3. Fructose and other monosaccharides

Fructose (fruit sugar) is a ketose, an isomer of glucose and the sweetest natural sugar; honey and fruit are rich in it. Its ring form is five-membered (four carbons and one oxygen). Because it has no aldehyde group, glucose and fructose can be told apart with bromine water: glucose decolourises it, fructose does not. As a polyhydric alcohol, fructose also gives a deep blue solution with Cu(OH)₂. In alkaline solution fructose partly isomerises to glucose, so it may react slowly with ammoniacal silver oxide as well.
Ribose and deoxyribose are pentoses found in nucleic acids; galactose is part of milk sugar.

Worked example

Choose the statements that apply to fructose: 1) a hexose; 2) its ring form is five-membered; 3) one aldehyde group and five OH groups; 4) it is hydrolysed; 5) a ketose; 6) an aldose.

1. Fructose is C₆ → a hexose; its ring is five-membered; it has a ketone group → a ketose.
2. It has no aldehyde group and, as a monosaccharide, is not hydrolysed.
3. Answer: 1, 2, 5.

§4. Disaccharides

• Sucrose (beet or cane sugar) = glucose + fructose units; it has no free aldehyde group, so it is a non-reducing sugar — no silver mirror — though as a polyhydric alcohol it gives a blue solution with Cu(OH)₂.

• Maltose (malt sugar) = 2 glucose; lactose (milk sugar) = glucose + galactose — both are reducing sugars and give a silver mirror.

Sugar is extracted from sugar beet or sugar cane with hot water; the juice is purified with lime and CO₂, evaporated and crystallised. The glucose–fructose mixture formed by hydrolysis of sucrose is called invert sugar.

§5. Polysaccharides

• Starch consists of α-glucose units: amylose (linear) and amylopectin (branched); it is the storage carbohydrate of plants (potatoes, wheat, rice). In hot water it forms a paste; with iodine it gives a deep blue-black colour. Hydrolysis is step-wise: starch → dextrins → maltose → glucose.
• Glycogen is the storage carbohydrate of animals and humans (liver, muscles), similar to amylopectin but more branched.
• Cellulose is a linear polymer of β-glucose units and the main component of plant cell walls; insoluble in water and not digested by humans. Each unit has 3 –OH groups: [C₆H₇O₂(OH)₃]ₙ.

Starch and cellulose are not isomers: their degrees of polymerisation and structures (α- and β-links) differ.

Worked example

What mass of glucose can be obtained from 1 kg of starch (100 % yield)?

1. (C₆H₁₀O₅)ₙ + nH₂O → nC₆H₁₂O₆; unit 162, glucose 180.
2. m = 1000·180/162.
3. m ≈ 1111 g.

Worked example

What mass of cellulose trinitrate forms when 16.2 g of cellulose is completely nitrated?

1. Units: n = 16.2/162 = 0.1 mol.
2. Trinitrate unit [C₆H₇O₂(ONO₂)₃]: M = 162 − 3·17 + 3·62 = 297.
3. m = 0.1·297 = 29.7 g.

§6. Photosynthesis and the importance of carbohydrates

Photosynthesis is endothermic: solar energy is stored as chemical bond energy. It releases oxygen into the atmosphere and produces almost all organic matter on Earth. Respiration is the reverse process and releases energy. Cotton, paper and wood are cellulose; starch and sugar are staple foods; glucose is given in medicine as a nutrient drip; fermentation is the basis of bread-making, dairy products and ethanol production.
Carbohydrate metabolism in the body: dietary starch is hydrolysed to glucose by enzymes (amylases) in saliva and the intestine; glucose is absorbed into the blood and carried to the cells. Excess glucose is stored in the liver and muscles as glycogen and broken down again to glucose when needed. Blood glucose is regulated by the hormone insulin; a lack of it causes diabetes. Humans cannot digest cellulose, but it is useful dietary fibre; microbes in the stomachs of ruminants can break it down.

Check yourself

1. Which does not give a silver mirror: glucose, maltose or sucrose?

2. Which reagent detects starch?

3. What mass of glucose forms when 34.2 g of sucrose is hydrolysed?

4. How many moles of silver does 1 mol of glucose give in the silver mirror test?

5. How many hydroxyl groups are in each cellulose unit?

6. Write the equation for photosynthesis.

7. How many moles of oxygen are needed to oxidise 0.5 mol of glucose completely in the body?

8. Which reagent distinguishes glucose from fructose?

Common mistakes

• Calling sucrose a reducing sugar.
• Saying starch and cellulose are isomers.
• Classing fructose as an aldose: it is a ketose.
• Taking 1 mol of CO₂ from alcoholic fermentation of glucose: 2 mol form.
• Writing that monosaccharides are hydrolysed: they are the simplest carbohydrates.