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📌Definition Table

Term	Definition
Stem Cell	Undifferentiated cell capable of self-renewal and differentiation into specialised cell types.
Potency	The ability of a stem cell to differentiate into different cell types.
Totipotent	Stem cell that can form all cell types, including embryonic and extraembryonic tissues.
Pluripotent	Stem cell that can form all cell types of the body but not extraembryonic tissues.
Multipotent	Stem cell that can form a limited range of cell types related to a particular tissue or organ.
Unipotent	Stem cell that can only form one cell type but retains the ability to self-renew.
Stem Cell Niche	Microenvironment that maintains stem cell function and regulates differentiation.

📌Introduction

Stem cells are unique in their ability to both self-renew and differentiate into specialised cell types. They play a crucial role in growth, development, and repair in multicellular organisms. Potency describes the range of cell types a stem cell can produce. Different levels of potency are observed throughout development, from the zygote’s totipotency to the restricted capabilities of adult stem cells.

❤️ CAS Link: Volunteer with a blood donation and marrow registry campaign to raise awareness about stem cell therapies and their role in treating blood cancers.

📌 Properties of Stem Cells

• Self-renewal — ability to divide and produce more stem cells without differentiating.
• Differentiation — ability to become specialised cells with specific functions.
• Present in both embryos (embryonic stem cells) and certain adult tissues (adult stem cells).
• Controlled by internal genetic factors and external signals from the stem cell niche.

🧠 Examiner Tip: Always link the type of potency with an example when answering application questions.

📌 Stem Cell Niches

• Specialised microenvironments that maintain stem cell properties.
• Provide chemical signals, cell-to-cell contact, and physical support.
• Examples:
  • Bone marrow — hematopoietic stem cells produce blood cells.
  • Hair follicles — stem cells regenerate hair.
  • Intestinal crypts — stem cells replace gut lining cells every few days.

🌍 Real-World Connection: Research into recreating niches in vitro is key to improving stem cell therapies.

📌 Levels of Potency

• Totipotent — zygote and early embryonic cells; can form an entire organism including placenta.
• Pluripotent — embryonic stem cells from blastocyst; can form all body cells but not placenta.
• Multipotent — adult stem cells (e.g., hematopoietic) producing a limited set of cells.
• Unipotent — satellite cells in muscle; can only produce muscle cells.

📝 Paper 2: Data Response Tip: When interpreting stem cell potency diagrams, identify cell fate restrictions over time.

📌Definition Table

Term	Definition
Ribosome	Organelle responsible for protein synthesis, composed of rRNA and proteins.
Rough Endoplasmic Reticulum (RER)	Membrane network with ribosomes attached, involved in protein modification.
Golgi Apparatus	Organelle that modifies, sorts, and packages proteins and lipids for transport.
Vesicle	Small membrane-bound sac used to transport substances within or outside the cell.
Exocytosis	Process of vesicle fusion with the plasma membrane to release contents.

📌Introduction

Protein synthesis and vesicle transport are essential for the functioning of eukaryotic cells. These processes involve multiple organelles working together to produce, modify, and transport proteins to their final destinations.

❤️ CAS Link: Organise a hands-on classroom demonstration where students build a “protein pathway” model showing ribosomes, ER, Golgi apparatus, and vesicles.

📌 Protein Synthesis Pathway

• Free ribosomes: Produce proteins for use within the cytoplasm.
• Bound ribosomes on RER: Produce proteins for secretion or for membranes.
• RER: Modifies proteins (e.g., folding, adding carbohydrate groups).
• Signal sequences direct proteins to their correct cellular location.
• Ribosomes composed of 70S in prokaryotes, 80S in eukaryotes — a common exam point.
• Protein synthesis is tightly regulated to conserve resources and ensure accuracy.

🧠 Examiner Tip: If asked about protein synthesis, mention both transcription and translation, and link the ribosome location to the protein’s final destination.

📌 Golgi Apparatus and Protein Modification

• Cis face: Receives proteins from RER in transport vesicles.
• Cisternae: Flattened membrane sacs where proteins are modified (e.g., glycosylation, phosphorylation).
• Trans face: Ships proteins in vesicles to final destinations.
• Proteins are sorted for lysosomes, the plasma membrane, or secretion.
• Golgi also processes lipids and produces some polysaccharides.

🌍 Real-World Connection: Defective Golgi function can cause Congenital Disorders of Glycosylation (CDG), leading to severe developmental issues.

📌 Vesicle Formation and Transport

• Vesicles bud from donor membranes (e.g., RER, Golgi) and fuse with target membranes.
• Coat proteins (like clathrin) help shape vesicles and select cargo.
• Vesicle movement is facilitated by motor proteins along cytoskeletal tracks.
• Exocytosis: Secretion of hormones, enzymes, or neurotransmitters.
• Endocytosis: Uptake of external materials, which may then be processed by lysosomes.
• Essential for rapid and regulated material exchange.

📝 Paper 2: Data Response Tip: In vesicle-related diagrams, trace the protein pathway step-by-step and clearly identify the role of each organelle.

📌Definition Table

Term	Definition
Cristae	Folds of the inner mitochondrial membrane that increase surface area for ATP production.
Matrix	Fluid-filled space inside mitochondria containing enzymes, ribosomes, and mitochondrial DNA.
Thylakoid	Flattened membrane-bound sac in chloroplasts where light-dependent reactions occur.
Stroma	Fluid-filled space in chloroplasts containing enzymes, ribosomes, and DNA.
Photosystem	Protein-pigment complex in thylakoid membranes that captures light energy.

📌Introduction

Mitochondria and chloroplasts are double-membrane organelles responsible for ATP production through respiration and photosynthesis respectively. Their structures are highly adapted to maximise efficiency in energy conversion.

❤️ CAS Link: Create an interactive 3D model of mitochondria and chloroplasts for a school science fair, explaining how each adaptation aids energy production.

📌 Structure and Adaptations of Mitochondria

• Double membrane: Outer membrane allows molecule entry; inner membrane is highly selective and folded into cristae.
• Cristae: Increase surface area for the electron transport chain and ATP synthase.
• Matrix: Contains enzymes for the Krebs cycle, ribosomes for protein synthesis, and mitochondrial DNA for enzyme production.
• Intermembrane space: Allows accumulation of protons for chemiosmosis.
• Small size: Increases surface area-to-volume ratio for faster diffusion of substrates and products.
• Dynamic nature: Can change shape and number to meet cell energy demands.

🧠 Examiner Tip: Always link the structure of cristae to increased ATP production capacity in mitochondria-related questions.

📌 Structure and Adaptations of Chloroplasts

• Double membrane: Controls exchange of substances with cytoplasm.
• Thylakoids: Contain photosystems, electron carriers, and ATP synthase for the light-dependent stage of photosynthesis.
• Grana: Stacks of thylakoids increase surface area for light absorption.
• Stroma: Contains enzymes for the Calvin cycle, ribosomes, and chloroplast DNA.
• Pigments: Chlorophyll a, chlorophyll b, and carotenoids absorb different wavelengths of light.
• Interconnected thylakoid membranes: Efficient transport of energy and products between light-dependent and light-independent reactions.

🌍 Real-World Connection: Chloroplast adaptation research has been used in developing high-efficiency crop plants through genetic modification.

📌 Shared Features of Mitochondria and Chloroplasts

• Both have double membranes, DNA, and 70S ribosomes — enabling production of some proteins independently of the nucleus.
• Both generate ATP through chemiosmosis using electron transport chains.
• Both maintain proton gradients across internal membranes.
• Endosymbiotic theory explains their origin from free-living prokaryotes.
• Presence of multiple copies of DNA and ribosomes allows rapid synthesis of proteins for energy conversion.

📌Definition Table

Term	Definition
Organelle	A specialised subunit within a cell with a specific function, usually membrane-bound.
Compartmentalisation	Separation of cellular activities into different organelles or areas of the cell.
Cytoplasm	Fluid matrix inside the cell containing organelles and dissolved substances.
Cytoskeleton	Network of protein filaments providing shape, support, and transport pathways.
Cell Fractionation	Laboratory process to separate organelles by centrifugation based on size/density.

📌Introduction

Eukaryotic cells are highly organised, with distinct compartments called organelles. This compartmentalisation allows specialisation of functions, prevents interference between incompatible processes, and maintains optimal conditions for reactions. It is one of the defining differences between prokaryotic and eukaryotic cells.

❤️ CAS Link: Organise a cell biology lab activity for younger students, using microscopes to compare plant and animal cell structures and identify organelles.

📌 Advantages of Compartmentalisation

• Isolates harmful substances, e.g., digestive enzymes in lysosomes.
• Allows different parts of the cell to maintain different environments (pH, ion concentration).
• Localises enzymes and substrates for efficiency.
• Enables simultaneous but incompatible processes (e.g., aerobic respiration and photosynthesis in plant cells).
• Provides flexibility — organelle numbers can change based on cell needs.
• Increases surface area for metabolic reactions (e.g., cristae in mitochondria).

🧠 Examiner Tip: In questions comparing prokaryotic and eukaryotic cells, always link the presence of membrane-bound organelles to compartmentalisation benefits.

📌 Specialisation of Organelles

• Nucleus: Stores genetic information, coordinates cell activities.
• Mitochondria: ATP production via aerobic respiration.
• Chloroplasts: Photosynthesis in plant and algal cells.
• Endoplasmic reticulum (ER): Smooth ER synthesises lipids; Rough ER modifies proteins.
• Golgi apparatus: Modifies, packages, and distributes proteins and lipids.
• Lysosomes: Digestive enzymes for breakdown of waste and pathogens.
• Peroxisomes: Detoxification and fatty acid breakdown.

🌍 Real-World Connection: Defective lysosomes cause Tay-Sachs disease, leading to lipid accumulation in neurons.

📌 Techniques for Studying Organelles

• Cell fractionation separates organelles by density using ultracentrifugation.
• Electron microscopy reveals fine structural details.
• Staining techniques highlight specific organelles.
• Advances in imaging (e.g., cryo-electron microscopy) allow 3D reconstruction of organelles in near-native states.
• Immunofluorescence labelling can identify proteins in specific organelles.

📝 Paper 2: Data Response Tip: When explaining the hydrothermal vent hypothesis, always include energy source, protection from UV, and chemical gradients for full marks

📌Definition Table

Term	Definition
Cell Adhesion Molecule (CAM)	Protein on the cell surface that binds to other cells or the extracellular matrix.
Extracellular Matrix (ECM)	Network of proteins and carbohydrates outside cells providing support and communication.
Tight Junction	Cell junction forming a seal to prevent leakage between cells.
Desmosome	Strong cell junction linking cytoskeletons of adjacent cells.
Gap Junction	Channel linking cytoplasm of two cells for molecule/ion exchange.

📌Introduction

Membranes are not just passive barriers — they actively participate in cell adhesion, communication, and coordination of activities between cells. Proteins and carbohydrates embedded in or attached to membranes facilitate these interactions, which are essential for tissue formation, signalling, and maintaining homeostasis.

❤️ CAS Link: Volunteer in a biology outreach program explaining how cancer research targets cell adhesion to prevent metastasis.

📌 Cell Adhesion and the Extracellular Matrix

• Cell adhesion molecules (CAMs) anchor cells to each other and the ECM.
• CAMs interact with cytoskeleton and signalling pathways.
• Extracellular matrix (ECM) is composed of collagen, glycoproteins, and proteoglycans.
• ECM provides structural support and regulates cell behaviour.
• ECM composition varies by tissue type and function.
• Changes in CAM expression are linked to developmental processes and diseases.

🧠 Examiner Tip: In tissue-related questions, always link ECM and adhesion molecules to both structure and communication roles.

📌 Types of Cell Junctions

• Tight junctions: Create a watertight seal, preventing leakage (e.g., epithelial lining in gut).
• Adherens junctions: Connect actin cytoskeleton between cells for mechanical stability.
• Desmosomes: Anchor intermediate filaments; found in skin and heart muscle for strength.
• Gap junctions: Allow direct communication via channels for ions and small molecules.
• Junction types differ in protein composition and function.
• Multiple junctions often work together in the same tissue.

🌍 Real-World Connection: Mutations in desmosomal proteins can lead to skin blistering diseases like pemphigus vulgaris.

📌 Membranes in Communication

• Membrane receptors detect chemical signals (hormones, neurotransmitters) and trigger intracellular responses.
• Signal transduction pathways amplify the initial signal inside the cell.
• Membranes integrate signals from multiple sources to coordinate responses.
• Receptor location and density influence sensitivity to signals.
• Intercellular communication maintains tissue and organ homeostasis.
• Faulty signalling can lead to cancer, diabetes, or autoimmune disorders.

📝 Paper 2: Data Response Tip: When analysing diagrams of cell junctions, label the specific junction type and link it to its mechanical or communication role.

📌Definition Table

Term	Definition
Passive Transport	Movement of molecules down a concentration gradient without energy input.
Simple Diffusion	Passive movement of molecules directly through the lipid bilayer.
Facilitated Diffusion	Passive movement through a membrane via channel or carrier proteins.
Osmosis	Passive movement of water molecules through a selectively permeable membrane.
Active Transport	Movement of molecules against their concentration gradient using ATP.
Endocytosis	Bulk transport of substances into the cell via vesicle formation.
Exocytosis	Bulk transport of substances out of the cell via vesicle fusion with the membrane.

📌Introduction

Membrane transport mechanisms regulate the movement of substances into and out of cells, maintaining homeostasis. Transport can be passive (no energy required) or active (requires ATP). Bulk transport processes allow cells to move large molecules or whole particles using vesicles.

❤️ CAS Link: Design a lab demonstration for younger students showing osmosis using dialysis tubing and coloured solutions.

📌 Passive Transport

• Molecules move down their concentration gradient.
• Simple diffusion: Small, nonpolar molecules (O₂, CO₂) pass directly through the lipid bilayer.
• Facilitated diffusion: Polar molecules and ions move through channel proteins (pores) or carrier proteins (shape change).
• Rate of facilitated diffusion depends on number and availability of transport proteins.
• Osmosis: Movement of water through aquaporins or directly across bilayer; driven by solute concentration differences.
• Hypotonic, hypertonic, and isotonic solutions affect cell water balance.

🧠 Examiner Tip: For osmosis questions, always mention water potential differences rather than just “concentration.”

📌 Active Transport

• Moves molecules against their concentration gradient using ATP.
• Performed by carrier proteins (pumps) — undergo conformational changes powered by ATP hydrolysis.
• Example: Sodium-potassium pump in neurons (3 Na⁺ out, 2 K⁺ in per ATP).
• Maintains electrochemical gradients critical for processes like nerve impulses and muscle contraction.
• Active transport allows uptake of nutrients even when external concentrations are low.
• Selective and tightly regulated by the cell.

🌍 Real-World Connection: Active transport is vital for nutrient absorption in the small intestine, especially in nutrient-poor environments.

📌 Bulk Transport

Endocytosis:

• Phagocytosis: Engulfing large particles or cells.
• Pinocytosis: Uptake of extracellular fluid and small molecules.
• Receptor-mediated endocytosis: Specific uptake using membrane receptors.

Exocytosis: Vesicles fuse with membrane to release contents (e.g., neurotransmitters).

Requires ATP for vesicle movement and membrane fusion.

Involves cytoskeleton for vesicle transport.

Enables rapid and large-scale material exchange.

Critical in immune responses (e.g., macrophage engulfing bacteria).

📝 Paper 2: Data Response Tip: In transport graph questions, always relate trends to concentration gradients, protein saturation, or ATP availability for full marks.

📌Definition Table

Term	Definition
Amphipathic	Molecule with both hydrophilic and hydrophobic regions.
Phospholipid	Lipid with two fatty acids and a phosphate group attached to glycerol.
Integral Protein	Membrane protein embedded within the lipid bilayer, often spanning the membrane.
Peripheral Protein	Membrane protein loosely bound to the surface of the membrane.
Glycolipid	Lipid with carbohydrate chain attached, involved in cell recognition.
Fluid Mosaic Model	Model describing the membrane as a dynamic structure with proteins embedded in a fluid lipid bilayer.

📌Introduction

The plasma membrane is a selectively permeable barrier that controls the entry and exit of substances, enabling compartmentalisation and communication. Its amphipathic phospholipid bilayer forms the basic structure, with embedded proteins, cholesterol, and carbohydrates providing additional functions.

❤️ CAS Link: Lead a microscopy demonstration for junior students, showing onion epidermis or cheek cells stained to highlight membranes.

📌 Lipid Bilayer and Amphipathic Nature

• Composed of phospholipids arranged with hydrophilic heads outwards and hydrophobic tails inwards.
• Amphipathic nature creates a barrier to polar molecules and ions.
• Bilayer provides flexibility and allows for self-healing.
• Fatty acid composition (saturated vs unsaturated) affects fluidity.
• Cholesterol interspersed in the bilayer moderates fluidity — reduces movement at high temperatures, prevents solidification at low temperatures.
• Membrane asymmetry — different lipid composition in inner vs outer leaflet.

🧠 Examiner Tip: Always connect phospholipid amphipathic nature to selective permeability in long-answer questions.

📌 Membrane Proteins

• Integral proteins: Span the bilayer, often involved in transport, receptors, and enzyme activity.
• Peripheral proteins: Attached to membrane surface; often act in cell signalling or as structural anchors.
• Transport proteins: channel proteins (passive transport) and carrier proteins (active/facilitated transport).
• Receptor proteins: bind to specific signalling molecules, triggering responses.
• Enzymatic proteins: catalyse reactions at the membrane surface.
• Adhesion proteins: help cells stick together in tissues.

🌍 Real-World Connection: Mutations in membrane transport proteins can cause diseases such as cystic fibrosis.

📌 Carbohydrates in Membranes

• Glycolipids: Lipid + carbohydrate chain; aid in cell recognition and stability.
• Glycoproteins: Protein + carbohydrate chain; function in signalling, immune recognition, and adhesion.
• Carbohydrate chains extend outward into the extracellular space forming the glycocalyx.
• Important in tissue formation and immune defence.
• Pathogens may exploit glycocalyx for attachment to host cells.
• Serve as antigens in blood groups (A, B, AB, O).

📝 Paper 1 Tip: If asked to label a membrane diagram, always include phospholipid bilayer, integral proteins, peripheral proteins, cholesterol, glycoproteins, and glycolipids for maximum marks.

📌Definition Table

Term	Definition
Structural Protein	Protein providing mechanical support and strength (e.g., collagen).
Functional Protein	Protein performing dynamic roles like catalysis, transport, or signalling.
Immunoglobulin	Antibody protein involved in immune defence.
Hormonal Protein	Protein that acts as a chemical messenger (e.g., insulin).
Rhodopsin	Light-sensitive receptor protein in the retina.

📌Introduction

Specialised proteins perform highly specific roles, often crucial to survival. These functions range from maintaining structural integrity to enabling complex biochemical reactions, immune defence, sensory perception, and hormonal regulation. Their unique structures allow them to operate efficiently in a wide variety of environmental and physiological conditions.

❤️ CAS Link: Organise a science outreach event where you present real-life applications of specialised proteins in medicine and industry, such as insulin therapy and antibody testing.

📌 Structural Proteins

• Provide mechanical strength, elasticity, and protection.
• Collagen: Found in connective tissue, high tensile strength due to triple-helix structure.
• Spider Silk: Stronger than steel by weight; flexible and elastic.
• Often repetitive amino acid sequences for regular structure.
• Insoluble in water — ideal for long-term structural roles.
• Used in biomedical materials like sutures and tissue scaffolds.

🧠 Examiner Tip: When comparing structural proteins, mention both strength and flexibility where applicable (e.g., spider silk vs. collagen).

📌 Functional Proteins in Metabolism and Transport

• Rubisco: Catalyses carbon fixation in photosynthesis; most abundant enzyme on Earth.
• Haemoglobin: Transports oxygen in red blood cells via iron-containing haem groups.
• Myoglobin: Stores oxygen in muscles for rapid release during activity.
• Enzyme activity depends on tertiary and quaternary structure.
• Denaturation or mutation can impair function (e.g., sickle-cell anaemia).
• Transport proteins often undergo conformational changes to move substances.

🌍 Real-World Connection: Artificial haemoglobin research aims to develop blood substitutes for transfusions in emergencies.

📌 Immune and Hormonal Proteins

• Immunoglobulins: Antibodies that recognise and bind to specific antigens.
• Diverse variable regions allow recognition of millions of pathogens.
• Insulin: Regulates blood glucose; produced by β-cells in the pancreas.
• Hormonal proteins often act at low concentrations but have large effects.
• Signalling proteins (e.g., cytokines) coordinate immune responses.
• Protein hormones bind to receptors, triggering signal cascades.

📝 Paper 2: Data Response Tip: In function-based questions, name the protein and explicitly link its structure to its function — generic descriptions lose marks.

📌Definition Table

Term	Definition
Primary Structure	The sequence of amino acids in a polypeptide chain.
Secondary Structure	Local folding of the polypeptide into α-helices or β-pleated sheets, stabilised by hydrogen bonds.
Tertiary Structure	The overall 3D shape of a single polypeptide, determined by R-group interactions.
Quaternary Structure	The association of two or more polypeptide chains into a functional protein.
Denaturation	Loss of protein’s native structure (and function) due to changes in pH, temperature, or chemicals.
Globular Protein	Spherical, soluble protein with functional roles (e.g., enzymes).
Fibrous Protein	Long, insoluble protein with structural roles (e.g., collagen).

📌Introduction

Protein structure is hierarchical, progressing from a simple amino acid sequence to complex, functional 3D shapes. These structures are stabilised by various chemical interactions, and even minor changes can alter a protein’s function or cause denaturation. The specific folding of proteins determines their biological role — from catalysis to structural support.

❤️ CAS Link: Create a protein structure model exhibition using materials like clay or 3D-printed parts to illustrate primary through quaternary levels.

📌 Levels of Protein Structure

• Primary: Unique sequence of amino acids; determines higher structures and function.
• Secondary: α-helices and β-pleated sheets formed by hydrogen bonds between backbone atoms.
• Tertiary: 3D folding due to R-group interactions — hydrogen bonds, ionic bonds, hydrophobic interactions, disulphide bridges.
• Quaternary: Multiple polypeptides combining (e.g., haemoglobin with four subunits).
• Each level is critical; mutations in the primary sequence can disrupt the entire structure.
• Proteins can be structural (fibrous) or functional (globular).

🧠 Examiner Tip: In long-answer questions, describe at least one type of bond stabilising each structural level for full marks.

📌 Factors Affecting Protein Stability

• Temperature: High heat breaks hydrogen bonds, causing denaturation.
• pH: Changes alter ionic bonds and disrupt folding.
• Chemical Agents: Organic solvents, detergents can disrupt hydrophobic interactions.
• Salt Concentration: High salt can precipitate proteins by disrupting water interactions.
• Some proteins can refold after mild denaturation; others cannot.
• Extremophiles possess proteins stable in extreme conditions.

🌍 Real-World Connection: Enzymes in hot-spring bacteria (e.g., Taq polymerase) are stable at high temperatures and vital for PCR technology.

📌Globular vs. Fibrous Proteins

• Globular: Compact, soluble, dynamic roles (enzymes, hormones, transport). Example: insulin, haemoglobin.
• Fibrous: Long, insoluble, structural roles (collagen in connective tissue, spider silk).
• Differences arise from amino acid sequences and folding patterns.
• Fibrous proteins often have repetitive sequences for strength.
• Globular proteins often have hydrophobic cores and hydrophilic surfaces.
• Structural type relates directly to biological role.

📝 Paper 2: Data Response Tip: When asked to compare protein structures, link shape to function — e.g., haemoglobin’s quaternary structure to oxygen transport.

📌Definition Table

Term	Definition
Amino Acid	Organic molecule containing an amino group, carboxyl group, hydrogen, and R-group.
Peptide Bond	Covalent bond formed between two amino acids during condensation.
Polypeptide	Chain of amino acids linked by peptide bonds.
Essential Amino Acid	Amino acid that must be obtained from the diet because the body cannot synthesise it.
Non-Essential Amino Acid	Amino acid that can be synthesised by the body.
Condensation Reaction	Chemical reaction that joins monomers, releasing water.

📌Introduction

Proteins are polymers of amino acids, each with unique R-groups that determine the protein’s shape and function. The sequence of amino acids is coded for by mRNA, meaning protein variety is directly linked to genetic diversity. Different combinations of the 20 amino acids allow for millions of possible protein structures and functions.

❤️ CAS Link: Run a school science club workshop where students model amino acids and build polypeptides with ball-and-stick kits to demonstrate condensation reactions.

📌 Amino Acids as Protein Monomers

• Amino acids have a central carbon atom bonded to an amino group (–NH₂), carboxyl group (–COOH), hydrogen, and a variable R-group.

• The R-group determines chemical properties (polar, nonpolar, acidic, basic).
• There are 20 naturally occurring amino acids in the genetic code.
• Some amino acids have unique features (e.g., cysteine forms disulphide bonds).
• Classification: essential (must be obtained in diet) and non-essential (synthesised by the body).
• Amino acids are amphoteric — can act as both acids and bases.

🧠 Examiner Tip: When drawing amino acids, always show the central carbon with four bonds — missing the hydrogen or R-group loses accuracy marks.

📌 Formation of Peptide Bonds

• Formed in condensation reactions between the carboxyl group of one amino acid and the amino group of another.
• Results in a covalent peptide bond and release of a water molecule.
• Reaction catalysed by ribosomes during translation.
• Hydrolysis reactions break peptide bonds with the addition of water.
• Sequence of amino acids (primary structure) is determined by the mRNA codon sequence.
• Small peptides (dipeptides, tripeptides) can have biological functions (e.g., hormones, neurotransmitters).

🌍 Real-World Connection: The artificial synthesis of human insulin involves producing the exact amino acid sequence coded by the human INS gene, then folding it to match natural insulin function.

📌 Diversity of Polypeptides

• Polypeptides can be as short as a few amino acids or over 30,000 amino acids long.
• Even small changes in sequence can cause major functional changes (e.g., sickle-cell haemoglobin mutation).
• Different combinations of amino acids give rise to proteins with unique shapes and chemical properties.
• Proteins may be made of a single polypeptide or multiple polypeptide subunits.
• Post-translational modifications (e.g., glycosylation, phosphorylation) further increase diversity.
• Structural diversity enables proteins to act in catalysis, signalling, transport, structural support, and defence.

📝 Paper 2: Data Response Tip: If shown amino acid diagrams, identify where the peptide bond forms — examiners often award an easy diagram mark here.