Carbohydrates

What Are Carbohydrates?

Carbohydrates are one of the three main food groups your body needs. They are organic compounds that contain three elements: carbon, hydrogen, and oxygen. The special thing about carbohydrates is that hydrogen and oxygen are always present in a 2:1 ratio, just like in water (H₂O).

When you eat eba, rice, or bread, you are eating carbohydrates. Your body breaks them down to get energy for running, studying, and all other activities. In Nigeria, most people get their energy from carbohydrate-rich foods like garri, yam, rice, and plantain.

Chemical Composition of Carbohydrates

The general formula for carbohydrates is C_x(H₂O)_y or C_xH_2yO_y. This means:

• C = Carbon atoms
• H = Hydrogen atoms (always twice the number of oxygen atoms)
• O = Oxygen atoms

For example, glucose has the formula C₆H₁₂O₆. Notice that there are 12 hydrogen atoms and 6 oxygen atoms – a 2:1 ratio.

Types of Carbohydrates

Carbohydrates are grouped into three main types based on their structure:

1. Monosaccharides (Simple Sugars)

These are the simplest carbohydrates. They cannot be broken down into smaller sugar units. Examples include:

• Glucose (C₆H₁₂O₆) – blood sugar, found in fruits and honey
• Fructose (C₆H₁₂O₆) – fruit sugar, the sweetest natural sugar
• Galactose (C₆H₁₂O₆) – found in milk

2. Disaccharides (Double Sugars)

These are formed when two monosaccharides join together. Examples include:

• Sucrose (glucose + fructose) – table sugar from sugarcane
• Maltose (glucose + glucose) – malt sugar from germinating grains
• Lactose (glucose + galactose) – milk sugar

3. Polysaccharides (Complex Carbohydrates)

These are large molecules made of many monosaccharide units joined together. Examples include:

• Starch – stored in plants (yam, cassava, rice, potatoes)
• Glycogen – stored in animals (liver and muscles)
• Cellulose – gives strength to plant cell walls

Sources of Carbohydrates in Nigeria

Nigerian foods are rich in carbohydrates. Here are common sources:

• Tubers: Yam, cassava, cocoyam, sweet potato
• Grains: Rice, maize, millet, guinea corn, wheat
• Processed foods: Garri, fufu, semovita, amala
• Fruits: Banana, plantain, mango, oranges
• Sugars: Honey, sugarcane, table sugar

Functions of Carbohydrates

Carbohydrates perform important roles in living things:

1. Energy supply: They provide energy for body activities like walking, running, and thinking. One gram of carbohydrate gives 17 kilojoules of energy.
2. Energy storage: Extra carbohydrates are stored as glycogen in animals and starch in plants.
3. Structural support: Cellulose gives strength to plant cell walls.
4. Making other compounds: The body uses carbohydrates to make fats, proteins, and DNA.

Properties of Carbohydrates

Chemical Tests for Carbohydrates

You can identify carbohydrates in the laboratory using these tests:

1. Molisch’s Test (General Test for All Carbohydrates)

Procedure: Add Molisch’s reagent to the sample, then add concentrated H₂SO₄ carefully down the side of the test tube.

Observation: A purple ring forms at the junction of the two liquids.

2. Dehydration Test

Procedure: Add concentrated H₂SO₄ to the carbohydrate sample.

Observation: The sample turns black because water is removed, leaving carbon behind.

3. Benedict’s or Fehling’s Test (For Reducing Sugars)

Procedure: Add Benedict’s or Fehling’s reagent to the sugar solution and heat.

Observation: A brick-red precipitate forms if reducing sugars like glucose are present.

Common Exam Mistakes

Practice Questions

Multiple Choice Questions

1. Which of the following is the correct general formula for carbohydrates?

a) C_xH_yO_z
b) C_xH_2yO_y ✓
c) C_xH_yO_2z
d) C_2xH_yO_y

2. A brick-red precipitate is formed when simple sugars are boiled with:

a) Concentrated H₂SO₄
b) Fehling’s reagent ✓
c) Molisch’s reagent
d) Dilute HCl

3. Which of these is a polysaccharide?

a) Glucose
b) Sucrose
c) Fructose
d) Starch ✓

4. What happens when carbohydrates are treated with concentrated H₂SO₄?

a) They turn blue
b) They turn black ✓
c) They dissolve completely
d) They turn red

Essay/Theory Questions

1. (a) Define carbohydrates and give their general formula. (3 marks)
(b) State three functions of carbohydrates in living organisms. (3 marks)
(c) Give two examples each of monosaccharides and polysaccharides. (4 marks)

Examiner’s Tip: For part (b), use the word “state” correctly – give short, direct points. Don’t write long explanations unless asked to “explain” or “describe”.

2. Explain how you would test for the presence of carbohydrates in a food sample using dehydration method. Include the observation you would make. (5 marks)

Examiner’s Tip: This question asks you to “explain” – so write clear steps with what you would see. Marks are for: naming the reagent (1 mark), procedure (2 marks), observation (1 mark), and conclusion (1 mark).

3. (a) Distinguish between monosaccharides and polysaccharides. (4 marks)
(b) Give two examples of Nigerian foods rich in carbohydrates. (2 marks)

Examiner’s Tip: “Distinguish” means show clear differences. Use a table or write: “Monosaccharides are… while polysaccharides are…”

Memory Aids

Related Topics

• Classification of Carbohydrate
• Test For Simple Sugar
• Test For Starch
• Organic Chemistry
• Food Tests