Biological molecules

What you'll learn

Biological molecules form the foundation of all living organisms and are a core component of the CIE IGCSE Biology syllabus. This topic covers the structure, function and identification of carbohydrates, proteins, lipids and vitamins, alongside the food tests that detect them. Understanding these molecules is essential for questions on nutrition, enzyme action, and cellular processes throughout the exam.

Key terms and definitions

Carbohydrates — organic molecules made of carbon, hydrogen and oxygen, with the general formula (CH₂O)ₙ, functioning as energy sources and structural components in cells.

Proteins — large molecules composed of long chains of amino acids, essential for growth, repair, enzymes and antibodies.

Lipids — fats and oils made from glycerol and fatty acids, used for energy storage, insulation and cell membrane structure.

Monosaccharides — simple sugars like glucose and fructose that cannot be hydrolysed into simpler sugars.

Polysaccharides — complex carbohydrates formed from many monosaccharide units joined together, such as starch, glycogen and cellulose.

Enzymes — biological catalysts made of protein that speed up specific chemical reactions in living organisms.

Amino acids — small molecules containing carbon, hydrogen, oxygen and nitrogen that link together to form proteins.

Glycerol — a three-carbon alcohol molecule that combines with fatty acids to form lipids.

Core concepts

Structure and function of carbohydrates

Carbohydrates exist in three main categories based on their molecular complexity. Monosaccharides are single sugar units including glucose (C₆H₁₂O₆), fructose and galactose. Glucose is particularly important as the primary respiratory substrate in cells, releasing energy during aerobic respiration.

Disaccharides form when two monosaccharides join through a condensation reaction, releasing water. Examples include:

• Maltose = glucose + glucose
• Sucrose = glucose + fructose
• Lactose = glucose + galactose

Polysaccharides are polymers containing hundreds or thousands of monosaccharide units:

• Starch — the storage carbohydrate in plants, found in seeds, potatoes and rice. Made of amylose (unbranched chains) and amylopectin (branched chains).
• Glycogen — the storage carbohydrate in animals, stored primarily in liver and muscle cells. Highly branched for rapid glucose release.
• Cellulose — a structural polysaccharide forming plant cell walls, providing strength and rigidity. Humans cannot digest cellulose but it provides dietary fibre.

Structure and function of proteins

Proteins are polymers constructed from 20 different amino acids. Each amino acid contains an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom and a variable R group attached to a central carbon atom. The R group determines the amino acid's properties.

Amino acids link through peptide bonds formed during condensation reactions. The sequence and number of amino acids determine each protein's unique structure and function.

Key functions of proteins include:

• Structural — collagen in skin and tendons, keratin in hair and nails
• Catalytic — all enzymes are proteins that catalyse specific reactions
• Transport — haemoglobin carries oxygen in red blood cells
• Defence — antibodies combat pathogens in the immune system
• Hormonal — insulin regulates blood glucose concentration

Protein structure depends on the specific sequence of amino acids. Heat and extreme pH can denature proteins, permanently changing their shape and destroying their function. This explains why enzymes stop working at high temperatures.

Structure and function of lipids

Lipids are composed of one glycerol molecule bonded to three fatty acid chains through condensation reactions, forming triglycerides. Unlike carbohydrates, lipids contain proportionally less oxygen.

Fatty acids may be:

• Saturated — no carbon-carbon double bonds, solid at room temperature (animal fats like butter)
• Unsaturated — contain one or more carbon-carbon double bonds, liquid at room temperature (plant oils like olive oil)

Functions of lipids:

• Energy storage — lipids store twice as much energy per gram as carbohydrates, forming adipose tissue
• Insulation — subcutaneous fat reduces heat loss in mammals
• Protection — fat surrounds delicate organs like kidneys
• Cell membranes — phospholipids form the bilayer structure of all cell membranes
• Waterproofing — waxy cuticles on plant leaves prevent water loss

Vitamins and minerals

While not organic macromolecules, vitamins and minerals are essential nutrients required in small amounts.

Vitamin C (ascorbic acid):

• Needed for healthy connective tissue and gums
• Deficiency causes scurvy (bleeding gums, slow wound healing)
• Sources include citrus fruits, tomatoes, green vegetables

Vitamin D (calciferol):

• Required for calcium absorption and bone formation
• Deficiency causes rickets in children (soft, deformed bones)
• Synthesised in skin exposed to sunlight; found in fish oils and eggs

Calcium:

• Essential for bone and tooth formation, muscle contraction and blood clotting
• Deficiency causes weak bones and poor tooth development
• Sources include dairy products and green vegetables

Iron:

• Component of haemoglobin in red blood cells
• Deficiency causes anaemia (fatigue, pale skin, shortness of breath)
• Sources include red meat, liver, and fortified cereals

Food tests for biological molecules

CIE IGCSE Biology requires knowledge of four standard biochemical tests:

Benedict's test for reducing sugars (glucose, maltose, fructose):

1. Add 2 cm³ of Benedict's reagent to food sample in test tube
2. Heat in water bath at 80-100°C for 5 minutes
3. Positive result: colour change from blue → green → yellow → orange → brick red
4. The intensity of colour indicates sugar concentration

Iodine test for starch:

1. Add several drops of iodine solution to food sample
2. Positive result: colour changes from yellow-brown to blue-black
3. No colour change indicates absence of starch

Biuret test for protein:

1. Add 2 cm³ of sodium hydroxide solution to food sample
2. Add several drops of copper sulfate solution
3. Mix gently
4. Positive result: colour changes from blue to purple/lilac
5. Negative result remains blue

Ethanol emulsion test for lipids:

1. Add 2 cm³ of ethanol to food sample in test tube
2. Shake thoroughly to dissolve lipids
3. Pour solution into test tube of water
4. Positive result: cloudy white emulsion forms
5. Negative result remains clear

These tests appear frequently in practical exam questions and structured paper questions requiring students to identify unknown substances.

Worked examples

Example 1: Identifying unknown food samples

Question: A student tested four food samples (A, B, C, D) using biochemical tests. The results are shown in the table:

Sample	Benedict's test	Iodine test	Biuret test
A	Red precipitate	Yellow-brown	Blue
B	Blue solution	Blue-black	Purple
C	Blue solution	Yellow-brown	Purple
D	Orange colour	Yellow-brown	Blue

(a) Which sample(s) contain starch? [1] (b) Which sample contains the highest concentration of reducing sugar? [1] (c) Explain why sample C did not change colour in the Benedict's test. [2]

Mark scheme answers: (a) Sample B [1 mark] (b) Sample A [1 mark] — brick red indicates highest concentration (c) Sample C contains no reducing sugars / contains only non-reducing sugars [1 mark]. Benedict's reagent only detects reducing sugars [1 mark]. Alternative: Sample C may contain sucrose which is a non-reducing sugar.

Example 2: Carbohydrate structure and function

Question: Starch and cellulose are both polysaccharides made from glucose.

(a) State two structural differences between starch and cellulose. [2] (b) Explain why humans can digest starch but not cellulose. [2] (c) Describe the role of cellulose in plants. [2]

Mark scheme answers: (a) Any two from:

• Cellulose is unbranched, starch may be branched (amylopectin) [1]
• Glucose molecules arranged differently / different type of glucose [1]
• Cellulose has stronger cross-links between chains [1]

(b) Humans have enzymes that break down starch / amylase [1]. Humans lack enzymes to break bonds in cellulose / cellulase [1].

(c) Forms (plant) cell walls [1]. Provides strength / rigidity / support / prevents bursting [1].

Example 3: Protein denaturation

Question: An investigation studied the effect of temperature on an enzyme. The enzyme was heated to different temperatures for 5 minutes, then its activity was measured. At 70°C, the enzyme showed no activity.

(a) Explain why the enzyme stopped working at 70°C. [3] (b) State whether this change is reversible. [1]

Mark scheme answers: (a) High temperature causes enzyme to denature [1]. Heat breaks bonds holding the protein structure together [1]. Active site changes shape so substrate no longer fits [1].

(b) Irreversible / permanent [1]

Common mistakes and how to avoid them

• Mistake: Stating "carbohydrates contain CHO" without specifying these are elements. Correction: Carbohydrates contain the elements carbon, hydrogen and oxygen. Chemical symbols alone are insufficient in exam answers.

• Mistake: Confusing condensation and hydrolysis reactions. Correction: Condensation joins monomers by removing water to form polymers (building large molecules). Hydrolysis breaks polymers by adding water to form monomers (breaking down large molecules). Remember: hydrolysis = hydro (water) + lysis (splitting).

• Mistake: Describing Benedict's test result as simply "positive" or "colour change." Correction: Specify the complete colour change sequence: blue to green to yellow to orange to brick red. State that brick red indicates highest sugar concentration.

• Mistake: Writing "proteins are made of amino acids" without explaining the bond. Correction: Proteins are made of amino acids joined by peptide bonds formed through condensation reactions.

• Mistake: Claiming lipids are "insoluble in water" when asked about the ethanol emulsion test. Correction: Lipids dissolve in ethanol (an organic solvent) but not in water. When ethanol solution is added to water, lipids precipitate out forming a cloudy white emulsion. This demonstrates lipids are hydrophobic.

• Mistake: Stating Benedict's reagent detects "all sugars." Correction: Benedict's reagent only detects reducing sugars (glucose, maltose, fructose). Non-reducing sugars like sucrose must first be hydrolysed with acid before testing.

Exam technique for Biological molecules

• Command word awareness: "State" requires simple facts without explanation (1 mark each). "Explain" requires reasoning or mechanism (usually 2-3 marks). "Describe" requires several linked points about what happens. For food tests, "describe" means giving the full procedure; "state the result" means giving specific colour changes.

• Food test questions: Always specify initial and final colours, not just "colour change." Include safety measures in practical questions (goggles, controlled heating). For quantitative questions, recognise that Benedict's test colour intensity indicates concentration, while iodine test is qualitative only.

• Structural comparison questions: When comparing molecules like starch and cellulose, or saturated and unsaturated lipids, give precise structural differences (branching, bond types, molecular arrangement) before discussing functional differences.

• Mark allocation patterns: One mark typically equals one distinct point. Three-mark "explain" questions usually require: the process/change [1], the mechanism/reason why [1], the consequence/effect [1]. Plan brief points before writing to match marks available.

Quick revision summary

Biological molecules include carbohydrates (monosaccharides, disaccharides and polysaccharides like starch and glycogen), proteins (chains of amino acids joined by peptide bonds), and lipids (glycerol plus three fatty acids). Four key food tests identify these molecules: Benedict's test (reducing sugars turn blue to brick red), iodine test (starch turns blue-black), biuret test (protein turns purple), and ethanol emulsion test (lipids form cloudy white emulsion). Vitamins C and D, plus minerals like calcium and iron, are essential micronutrients. High temperatures denature protein structure irreversibly, destroying enzyme function.