Carbohydrates: sugars, starch and cellulose: WASSCE Chemistry

Rice, cassava, yams, bananas, sugar cane and the cotton in your uniform are all made largely of carbohydrates. Plants build them by photosynthesis, and we break them down for energy. This lesson covers what carbohydrates are, how they are classified into sugars and polysaccharides, and the properties and uses of the main ones.

Learning objectives

By the end of this lesson you should be able to:

define carbohydrates and state their general formula
describe how carbohydrates are formed in plants
classify carbohydrates as monosaccharides, disaccharides and polysaccharides, with examples
compare the properties of sugars and polysaccharides
describe starch, cellulose and glycogen and their uses

What carbohydrates are

Definition

Carbohydrates are compounds of carbon, hydrogen and oxygen in which hydrogen and oxygen are usually in the ratio 2 : 1, as in water. General formula: Cₓ(H₂O)ᵧ.

Examples: glucose C₆H₁₂O₆ = C₆(H₂O)₆; sucrose C₁₂H₂₂O₁₁ = C₁₂(H₂O)₁₁. (They are not really "hydrates of carbon"; the name comes from the formula.)

Photosynthesis: green plants make glucose from carbon(IV) oxide and water, using energy from sunlight absorbed by chlorophyll:

6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2

\begin{gathered}6\text{CO}_2 + 6\text{H}_2\text{O} \\[6pt] \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2\end{gathered}

Plants then join glucose units into starch (for storage) and cellulose (for cell walls).

Classification

Glucose as a single ring (monosaccharide), maltose or sucrose as two rings joined by an oxygen link (disaccharide), and starch or cellulose as a long chain of rings (polysaccharide)

Figure 1. Carbohydrates are built from sugar units. WASSCE Tutor diagram.

Monosaccharides (simple sugars)

Glucose, C₆H₁₂O₆ (grape sugar): in ripe fruits, honey and blood; the main fuel for respiration.
Fructose, C₆H₁₂O₆ (fruit sugar): in fruits and honey; the sweetest sugar. Glucose and fructose are isomers.
Galactose, C₆H₁₂O₆: part of milk sugar.

Disaccharides

Two monosaccharide units joined by condensation, with loss of water:

2\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow \text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O}

\begin{gathered}2\text{C}_6\text{H}_{12}\text{O}_6 \\[6pt] \rightarrow \text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O}\end{gathered}

Disaccharide	Made of	Found in
Sucrose (cane sugar)	Glucose + fructose	Sugar cane, table sugar, many fruits
Maltose (malt sugar)	Glucose + glucose	Germinating grain (malt)
Lactose (milk sugar)	Glucose + galactose	Milk

Polysaccharides

Hundreds or thousands of glucose units joined by condensation: (C₆H₁₀O₅)ₙ.

Polysaccharide	Where found	Role
Starch	Cassava, rice, maize, yams, potatoes, wheat	Energy store in plants; main food carbohydrate
Cellulose	Plant cell walls; cotton, wood, paper	Structure; dietary fibre (roughage)
Glycogen	Liver and muscles of animals	Energy store in animals ("animal starch")

Humans cannot digest cellulose (we lack the enzyme cellulase), but it helps food move through the gut. Cattle and goats digest it with the help of bacteria in their stomachs.

Properties of sugars and polysaccharides

Property	Sugars (mono- and disaccharides)	Polysaccharides
Taste	Sweet	Not sweet
Solubility in water	Soluble	Insoluble (starch forms a colloid in hot water)
Appearance	Crystalline solids	Powders or fibres; not crystalline
Molecular size	Small	Very large (macromolecules)
Reducing power	Glucose, fructose, maltose and lactose are reducing sugars; sucrose is not	Non-reducing

Uses of carbohydrates

Food and energy: respiration releases energy from glucose:

\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} \qquad \Delta H = -2800 \text{ kJ/mol}

\begin{gathered}\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \\[6pt] \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} \\[6pt] \begin{aligned}\Delta H &= -2800 \text{ kJ/mol}\end{aligned}\end{gathered}

Glucose: drips for patients, energy drinks, sweets
Sucrose: sweetening, preserving jams
Starch: food; laundry starch for stiffening clothes; adhesives; making glucose syrup and ethanol by fermentation
Cellulose: cotton cloth, paper, timber; making rayon fibres and cellulose ethanoate film

Worked example

Percentage of carbon in glucose. (C = 12, H = 1, O = 16)

Mᵣ of C₆H₁₂O₆ $= 72 + 12 + 96 = 180$
% C $= 72 \div 180 \times 100 = 40\%$

In Sierra Leone

Most of the energy in a Sierra Leonean diet comes from starch: rice at almost every meal, and cassava eaten as gari, fufu or boiled roots. Sugar cane is grown and chewed fresh; its juice is mostly sucrose. Near Makeni, a large plantation has grown sugar cane to ferment into fuel ethanol, turning a carbohydrate into a fuel.

Common mistake

Sucrose is not a reducing sugar, even though both of its units (glucose and fructose) are. The groups that would do the reducing are used up in the link between the two units.

Key points

Carbohydrates, Cₓ(H₂O)ᵧ, contain C, H and O; plants make them by photosynthesis.
Monosaccharides (glucose, fructose), disaccharides (sucrose, maltose, lactose) and polysaccharides (starch, cellulose, glycogen).
Units join by condensation (losing water); hydrolysis splits them.
Sugars are sweet, soluble and crystalline; polysaccharides are not sweet and insoluble.
Starch stores energy in plants; cellulose builds cell walls; glycogen stores energy in animals.

WASSCE examination focus

WAEC tests this

Expect to define carbohydrates and classify them with examples and formulas, to compare sugars with starch, to name the units that make up sucrose, maltose and starch, and to state uses of starch and cellulose. Objective questions often ask which sugar is not reducing (sucrose).

Quick check

Give the general formula of carbohydrates.
Name the two monosaccharides that make up sucrose.
State two differences between glucose and starch.
Why can cattle digest grass, but humans cannot digest cellulose?

Answers:

Cₓ(H₂O)ᵧ.
Glucose and fructose.
Any two: glucose is sweet, starch is not; glucose is soluble, starch is insoluble; glucose is crystalline, starch is not; glucose is a small molecule, starch a macromolecule; glucose is a reducing sugar, starch is not.
Bacteria in the stomachs of cattle produce cellulase, the enzyme that breaks down cellulose; humans do not produce it.

Carbohydrates: sugars, starch and cellulose