Carbohydrates

Chemical Composition

Carbohydrates consist of three elements:

General formula: C(H₂O)n - water to carbon ratio

Types of Carbohydrates

Simple Carbohydrates (Monosaccharides and Disaccharides)

• Single sugar units or two joined together
• Sweet-tasting, soluble in water
• Examples: glucose, fructose, sucrose

Complex Carbohydrates (Polysaccharides)

• Long chains of monosaccharide units
• Not sweet-tasting, insoluble in water
• Examples: starch, glycogen, cellulose

Functions in Living Organisms

1. Energy Source and Storage

• Glucose: Immediate energy source for cellular respiration
• Starch: Energy storage in plants (broken down to glucose when needed)
• Glycogen: Energy storage in animals (mainly in liver and muscles)

2. Structural Support

• Cellulose: Major component of plant cell walls
• Provides rigidity and structural integrity to plants
• Humans cannot digest cellulose (lack cellulase enzyme) - dietary fibre

Fats and Oils (Lipids)

Chemical Composition

Fats and oils consist of three elements:

Structure: Glycerol + 3 fatty acids = fat molecule

Types of Fats

Saturated Fats

• No double bonds between carbon atoms
• Straight molecular chains
• Solid at room temperature
• Examples: butter, lard, animal fats

Unsaturated Fats

• One or more double bonds between carbon atoms
• Kinked molecular chains
• Liquid at room temperature (oils)
• Examples: vegetable oils, fish oils

Functions in Living Organisms

1. Long-Term Energy Storage

• More energy per gram than carbohydrates (9 kcal/g vs 4 kcal/g)
• Compact storage form for long-term needs
• Examples: adipose tissue in animals, seed oils in plants

2. Insulation and Protection

• Thermal Insulation: Subcutaneous fat layer reduces heat loss
• Cushioning: Protects organs from mechanical damage
• Example: fat around kidneys, heart, and intestines

3. Membrane Structure

• Phospholipids form cell membrane bilayer
• Hydrophilic heads face water; hydrophobic tails face inward

Proteins

Chemical Composition

Proteins consist of four elements:

Building Blocks: Amino Acids

Amino Acid Structure

All amino acids contain:

• Amino group (-NH₂)
• Carboxyl group (-COOH)
• Hydrogen atom (-H)
• Variable R group (side chain) - determines amino acid properties

20 Standard Amino Acids

• Different R groups create different amino acids
• All share same backbone structure
• Linked by peptide bonds to form polypeptide chains

DNA Structure

• Double helix shape
• Complementary base pairing: Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G)
• Sugar-phosphate backbone on outside
• Nitrogenous bases on inside

Functions of Proteins

Chemical Tests for Biological Molecules

1. Starch Test (Iodine Test)

Procedure

1. Add 2 drops of iodine solution (brown/yellow)
2. Observe colour change

Result

• Positive: Turns blue-black
• Negative: Remains brown/yellow

Application

• Test for presence of starch in food samples
• Plants store glucose as starch

2. Reducing Sugars Test (Benedict’s Test)

Procedure

1. Mix 2cm³ food sample with Benedict’s reagent (blue)
2. Heat in boiling water bath for 5 minutes
3. Observe colour change

Result

• Negative: Remains blue (no reducing sugar)
• Positive: Green → Yellow → Orange → Brick red precipitate
• Intensity of colour indicates concentration

What is a Reducing Sugar?

• Glucose, fructose, maltose, lactose
• Have free aldehyde or ketone groups
• Can donate electrons (hence “reducing”)

Important Note

• Sucrose is NOT a reducing sugar (disaccharide with glycosidic bond)

3. Protein Test (Biuret Test)

Procedure

1. Mix 2cm³ food sample with 2cm³ water (if needed)
2. Add 2 drops of Biuret reagent (dilute copper sulfate in alkaline solution)
3. Observe colour change

Result

• Negative: Remains blue
• Positive: Turns purple/violet

Important Note

• Test detects peptide bonds (protein structure)
• Amino acids (individual, no peptide bonds) give negative result
• Proteins must be hydrolysed to amino acids to give negative test

4. Fats/Oils Test (Emulsion Test)

Procedure

1. Add 2cm³ ethanol to food sample
2. Shake vigorously for 30 seconds
3. Add 2cm³ water
4. Observe

Result

• Negative: Clear/yellow solution
• Positive: Milky white/cloudy emulsion

Why it Works

• Fats dissolve in ethanol
• When water added, fat precipitates as emulsion
• Cloudiness indicates fat presence

5. Vitamin C Test (DCPIP Test)

Procedure

1. Add DCPIP solution (blue) to food sample
2. Measure amount needed to decolourise

Result

• Positive: DCPIP decolourises from blue to colourless
• Amount of vitamin C present measured by volume needed

Principle

• Vitamin C (ascorbic acid) is a reducing agent
• Reduces blue DCPIP to colourless form

Comparison Summary

Typical Biological Examples

Carbohydrates in Action

• Starch: Potato, rice, wheat - major energy stores in plants
• Glycogen: Liver, muscles - immediate glucose release on demand
• Cellulose: Cell walls - structural support in plants

Fats in Action

• Animal fats: Butter, meat - insulation, energy storage
• Plant oils: Olive, sunflower - liquid energy storage in seeds
• Membrane lipids: Phospholipid bilayer in all cells

Proteins in Action

• Enzymes: Digestive enzymes in stomach/intestine
• Haemoglobin: Oxygen transport in red blood cells
• Antibodies: Immune defence against bacteria/viruses
• Collagen: Structural support in skin, bones, tendons