Biological Molecules and Enzymes

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your WAEC exam.

The Four Major Biological Molecules:

Molecule	Monomer/Building Block	Function	Example
Carbohydrates	Monosaccharides	Energy source, storage	Glucose, starch, glycogen
Proteins	Amino acids	Enzymes, structure, transport	Haemoglobin, collagen
Lipids	Fatty acids + glycerol	Energy storage, insulation	Fats, oils, phospholipids
Nucleic acids	Nucleotides	Genetic information storage	DNA, RNA, ATP

Carbohydrates:

Type	Examples	Structure	Use
Monosaccharide	Glucose, fructose	Single sugar (C₆H₁₂O₆)	Immediate energy
Disaccharide	Sucrose, maltose, lactose	Two sugars joined	Transport in plants
Polysaccharide	Starch, glycogen, cellulose	Many sugars	Energy storage

Glucose Ring Form:

• α-glucose: OH on C1 below ring (forms starch, glycogen)
• β-glucose: OH on C1 above ring (forms cellulose)

Proteins: 20 amino acids, general structure: NH₂-CH(R)-COOH

Essential amino acids (must be from diet): Valine, leucine, isoleucine, phenylalanine, methionine, threonine, tryptophan, lysine

Lipids:

• Fats and oils (triglycerides): 1 glycerol + 3 fatty acids
• Saturated fats: no double bonds in fatty acids (solid at room temp)
• Unsaturated fats: double bonds present (liquid at room temp)
• Phospholipids: Cell membrane structure

⚡ WAEC Tip: Lipids are soluble in organic solvents (chloroform, ether) but NOT in water. This is how you can test for them — the emulsion test.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Food Tests:

Test	Substance	Positive Result	Negative Result
Iodine	Starch	Blue-black	Remains brown/orange
Benedict’s	Reducing sugar (glucose)	Orange/brick red precipitate	Blue (no change)
Biuret	Protein	Purple/violet	Blue (no change)
Ethanol emulsion	Lipids	Milky white emulsion	Clear
DCPIP	Vitamin C	Blue → Colourless	No change

Benedict’s Test for Sucrose: Sucrose is non-reducing. First hydrolyse with HCl, then neutralise with NaHCO₃, then perform Benedict’s test.

Enzymes - Biological Catalysts:

Key Properties:

1. Speeds up reactions without being consumed
2. Lower activation energy
3. Specific to one substrate (lock and key model)
4. Reusable
5. Affected by temperature and pH

Enzyme Nomenclature:

• Name usually ends in “-ase”
• Amylase breaks down amylose (starch)
• Lipase breaks down lipids
• Protease breaks down proteins
• Trypsin breaks down proteins in the small intestine

Mechanism - Lock and Key Model:

• Enzyme has active site (specific shape)
• Substrate fits into active site like a key in a lock
• Product is released, enzyme is unchanged

Induced Fit Model (Updated):

• Enzyme active site is not rigid
• Changes shape slightly to fit substrate better
• More accurate explanation

Factors Affecting Enzyme Activity:

Factor	Effect
Temperature	Activity increases to optimum (~37°C for human enzymes), then denatures
pH	Each enzyme has optimum pH (pepsin ~2, trypsin ~8)
Substrate concentration	Rate increases until enzyme is saturated
Enzyme concentration	Rate proportional to enzyme at unsaturated conditions

Denaturation:

• Irreversible change in protein structure
• Causes: Heat, extreme pH, heavy metals, alcohol
• Denatured enzyme cannot function (active site altered)

⚡ Common Mistake: Students say high temperature “kills” enzymes. Enzymes are not alive. Heat causes denaturation — the hydrogen bonds and other weak forces holding the 3D shape break, and the protein unfolds irreversibly.

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for serious exam preparation.

Protein Structure:

Level	Description	Bonds/Forces
Primary	Amino acid sequence (peptide bonds)	Covalent (peptide)
Secondary	Alpha helix, beta pleated sheet	H-bonds between peptide groups
Tertiary	3D folding of entire polypeptide	H-bonds, ionic bonds, hydrophobic interactions, disulfide bridges
Quaternary	Multiple polypeptide subunits	Same forces as tertiary

Examples:

• Haemoglobin: 4 polypeptide subunits (2 α, 2 β) + haem group
• Collagen: Triple helix of alpha chains
• Antibodies: Two heavy + two light chains

Enzyme Classification:

Class	Type of Reaction	Example
1. Oxidoreductases	Oxidation-reduction	Dehydrogenases
2. Transferases	Transfer groups	Transaminases
3. Hydrolases	Hydrolysis reactions	Amylase, lipase
4. Lyases	Add/remove groups (no hydrolysis)	Decarboxylases
5. Isomerases	Rearrange structure	Epimerases
6. Ligases	Join molecules, requires ATP	DNA ligase

Enzyme Inhibition:

Competitive Inhibition:

• Inhibitor resembles substrate
• Competes for active site
• Can be overcome by increasing substrate concentration
• Example: Malonate competes with succinate for succinate dehydrogenase

Non-competitive Inhibition:

• Inhibitor binds to a site other than active site (allosteric site)
• Changes shape of enzyme
• Cannot be overcome by increasing substrate
• Example: Cyanide inhibits cytochrome c oxidase (important enzyme in respiration)

Allosteric Regulation:

• Molecule binds to allosteric site
• Causes conformational change
• Can be activator or inhibitor
• Feedback inhibition: end product inhibits pathway enzyme

Cofactors and Coenzymes:

Type	Definition	Example
Cofactor	Non-protein chemical required for activity	Zn²⁺ in carbonic anhydrase
Coenzyme	Organic cofactor, loosely bound	NAD⁺, FAD, coenzyme A
Prosthetic group	Tightly bound cofactor	Haem in haemoglobin

Vitamins as Coenzymes:

Vitamin	Coenzyme	Function
B1 (Thiamine)	Thiamine pyrophosphate	Decarboxylation
B2 (Riboflavin)	FAD, FMN	Electron carrier
B3 (Niacin)	NAD⁺	Dehydrogenation
B5 (Pantothenic acid)	Coenzyme A	Acetyl group transfer
B6 (Pyridoxine)	Pyridoxal phosphate	Transamination
B12 (Cobalamin)	Methylcobalamin	Methyl group transfer
C (Ascorbic acid)	-	Collagen synthesis, antioxidant

ATP - The Energy Currency:

• Adenosine triphosphate
• High-energy phosphate bonds
• ATP → ADP + Pi releases energy (~30.6 kJ/mol)
• Used in: muscle contraction, active transport, biosynthesis, cell division

Nucleic Acid Structure:

DNA (Deoxyribonucleic Acid):

• Double helix (Watson and Crick, 1953)
• Sugar = deoxyribose
• Bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
• A pairs with T (2 H-bonds)
• G pairs with C (3 H-bonds)
• Antiparallel strands

RNA (Ribonucleic Acid):

• Single stranded (usually)
• Sugar = ribose
• Bases: A, U (instead of T), G, C
• Types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal)

DNA Replication:

• Semi-conservative (each new DNA has one old, one new strand)
• Enzyme: DNA polymerase (adds nucleotides 5’→3’)
• Needs primer

Protein Synthesis:

1. Transcription: DNA → mRNA (in nucleus)
2. Translation: mRNA → Protein (at ribosome)

⚡ WAEC Previous Year Pattern:

Year	Question	Concept
2023	Food test procedure	Benedict’s test
2022	Enzyme specificity	Lock and key model
2021	Protein structure levels	Bonds in each level

Water Properties (Why water is important for life):

1. Excellent solvent (ionic and polar substances dissolve)
2. High specific heat capacity (temperature buffer)
3. High latent heat of vaporisation (cooling)
4. Cohesion (surface tension, capillary action)
5. Density of ice < density of liquid water (ice floats)

⚡ Exam Strategy: For food test questions, describe the test procedure accurately: add reagent, heat (if needed), observe colour change. For enzyme questions, explain why the rate changes with temperature or pH by referring to denaturation or changes in kinetic energy.

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