Pearson Edexcel International AS/A Level Biology Study Guide
Course Details, Specifications, and Assessment Overview
Course Identification: Pearson Edexcel International Advanced Subsidiary (IAS) / International Advanced Level (IAL) Biology Student Book 1.
Authors: Ann Fullick with Frank Sochacki.
Publication & Editorial Details: Published by Pearson Education Limited (2018), 80 Strand, London, WC2R 0RL. Edited by Deborah Webb and Penelope Lyons; indexed by Judith Reading; typeset and illustrated by Tech-Set Ltd, Gateshead, UK. ISBN: 978 1 2922 4484 6. Printed in Slovakia by Neografia.
Qualification Structure:
IAS course comprises Unit 1 (Topics 1 and 2) and Unit 2 (Topics 3 and 4).
IAL course comprises Units 1, 2, 3 (IAS) plus Units 4, 5, 6 (IA2).
Assessment Overview:
Unit 1 (Paper Code WBI11/01): Molecules, Diet, Transport and Health. $40\%$ of IAS, $20\%$ of IAL. 80 marks. Duration: 1 hour 30 minutes. Availability: January, June, October.
Unit 2 (Paper Code WBI12/01): Cells, Development, Biodiversity and Conservation. $40\%$ of IAS, $20\%$ of IAL. 80 marks. Duration: 1 hour 30 minutes. Availability: January, June, October.
Unit 3 (Paper Code WBI13/01): Practical Skills in Biology 1. $20\%$ of IAS, $10\%$ of IAL. 50 marks. Duration: 1 hour 20 minutes. Tests practical planning, risk management, and apparatus selection.
Assessment Objectives Weightings (IAS):
AO1 (Demonstrate knowledge and understanding): $36-39\%$.
AO2(a) (Application of knowledge and understanding): $34-36\%$.
AO2(b) (Analysis and evaluation of scientific information): $9-11\%$.
AO3 (Experimental skills, analysis, and evaluation): $17-18\%$.
Core Practical Procedures and Practical Techniques
CP1: Estimation of Concentration of Reducing Sugars and Starch:
Uses a semi-quantitative Benedict's reagent test for reducing sugars and iodine solution for starch using colour standards.
CP2: Investigation of Vitamin C Content of Food and Drink:
Uses 2,6-dichlorophenol-indophenol (DCPIP). DCPIP solution is blue and turns colourless (or pink in acidic solution) when reduced by Vitamin C ($C_6H_8O_6$).
Procedure: Titrate a known volume ($1.0\,cm^3$) of $1.0\%$ DCPIP with Vitamin C standard or juice dropwise until the blue colour disappears.
CP3: Investigation of Membrane Permeability:
Tests the effect of temperature and alcohol concentration on membrane permeability using red beetroot (Beta vulgaris) discs.
Membrane damage causes betacyanin pigment to leak into aqueous solution; absorbance is measured using a colorimeter.
CP4: Initial Rate of Enzyme-Catalysed Reactions:
Investigates the effect of temperature, pH, enzyme concentration, and substrate concentration on initial reaction rates (e.g., catalase breakdown of hydrogen peroxide or trypsin breakdown of protein).
CP5: Light Microscopy, Scale, and Graticule Calibration:
Uses a light microscope to make labelled drawings of animal and plant cells.
Uses an eyepiece graticule calibrated against a stage micrometer to measure cell dimensions.
CP6: Root Tip Squash for Observing Mitosis:
Preparation and staining of a root tip squash (e.g., Allium cepa) using acetic orcein or acetocarmine stain to highlight chromosomes and calculate the mitotic index.
CP7: Observation and Drawing of Plant Tissues:
Light microscope observation and plan diagrams of transverse sections ($T.S.$) of roots, stems, and leaves.
Identification of sclerenchyma fibres, phloem, sieve tubes, and xylem vessels.
CP8: Determination of Tensile Strength of Plant Fibres:
Measures the breaking force (tensile strength) of isolated plant fibres (e.g., flax, jute, sisal) by hanging masses until the fibre snaps.
CP9: Antimicrobial Properties of Plants and Aseptic Technique:
Tests plant extracts (e.g., garlic, mint) on agar plates inoculated with bacterial cultures using sterile filter paper discs.
Aseptic technique: autoclave equipment at $121\,^ ext{C}$ for $15\,min$ under high pressure; flame inoculating loops until red hot; work near a Bunsen burner flame to create updrafts; measure clear zones of inhibition.
Chemistry for Biologists
Ionic and Covalent Bonding:
Ionic Bonding: Formed when outer shell electrons are transferred from one atom to another, generating charged ions held by strong electrostatic forces.
Example: Sodium chloride ($NaCl$), where sodium loses an electron to become a cation ($Na^+$) and chlorine gains an electron to become an anion ($Cl^-$).
Covalent Bonding: Formed when atoms share pairs of electrons. Covalent bonds are very strong.
Polar Covalent Molecules & Dipoles: Unequal electron sharing due to electronegativity differences creates partial charges ( and ). A separation of charge across a molecule is a dipole.
Essential Inorganic Ions:
Anions:
Nitrate ($NO_3^-$): Required in plants to synthesise amino acids, proteins, and nucleic acids from photosynthetic products.
Phosphate ($PO_4^{3-}$): Required in all organisms to form ATP, ADP, DNA, RNA, and phospholipids.
Chloride ($Cl^-$): Required for nerve impulse transmission, sweating, and secretory systems in animals.
Hydrogencarbonate ($HCO_3^-$): Acts as a buffer in blood plasma to maintain pH equilibrium.
Cations:
Sodium ($Na^+$): Essential for nerve impulse conduction, sweating, and osmotic balance.
Calcium ($Ca^{2+}$): Required for calcium pectate formation in the plant middle lamella, bone formation, and muscle contraction.
Hydrogen ($H^+$): Essential for cellular respiration, photosynthesis, driving proton pumps, and pH regulation.
Magnesium ($Mg^{2+}$): Required for chlorophyll production in plants.
Physicochemical Properties of Water ($H_2O$):
Polarity and Hydrogen Bonding: Water is a polar molecule with a bond angle of $104.5^ ext{o}$. oxygen atoms attract hydrogen atoms of adjacent water molecules, forming weak electrostatic intermolecular hydrogen bonds.
Polar Solvent: Dissolves ionic substances (e.g., $NaCl$) via hydration shells (oxygen surrounds $Na^+$; hydrogen surrounds $Cl^-$) and polar covalent solutes.
Thermal Properties: High specific heat capacity due to hydrogen bonding; buffers temperature changes in aquatic habitats and organisms.
Density Anomaly: Water reaches maximum density at $4\,^ ext{C}$. Below $4\,^ ext{C}$, hydrogen bonding forces molecules into a open lattice structure, making ice less dense than liquid water, allowing it to float and insulate underlying aquatic life.
Cohesion and Adhesion: Cohesion (attraction between identical water molecules) and adhesion (attraction between water and different molecules) enable mass flow in plant transport systems.
Incompressibility & Surface Tension: Water cannot be compressed, making it ideal for hydraulic systems. High surface tension forms a thin skin at the air-water interface supporting organisms like raft spiders.
Carbohydrates: Monosaccharides, Disaccharides, and Polysaccharides
Organic Molecules and Carbon Chemistry:
Carbon atoms form four covalent bonds arranged in a tetrahedral geometry, forming chains, rings, and branched structures.
Monomers polymerise via condensation reactions to form macromolecules.
Monosaccharides (Simple Sugars):
General formula: $(CH_2O)_n$.
Triose Sugars ($n=3$, $C_3H_6O_3$): Intermediates in respiration and glycolysis.
Pentose Sugars ($n=5$, $C_5H_{10}O_5$): Ribose (in RNA) and deoxyribose (in DNA).
Hexose Sugars ($n=6$, $C_6H_{12}O_6$): Sweet, soluble isomers including glucose, fructose, and galactose.
Glucose Isomers:
-glucose: The hydroxyl group ($-OH$) on carbon-1 is positioned below the plane of the ring.
-glucose: The hydroxyl group ($-OH$) on carbon-1 is positioned above the plane of the ring.
Sorbitol: Hydrogenated glucose ($C_6H_{14}O_6$). Tastes $60\%$ sweeter than glucose but yields lower energy ($11\,kJ\,g^{-1}$ compared to $17\,kJ\,g^{-1}$ for glucose).
Disaccharides:
Formed when two monosaccharides undergo a condensation reaction with the removal of a water molecule ($H_2O$), producing a covalent glycosidic bond.
Maltose: -glucose + -glucose via a 1,4-glycosidic bond (malt sugar found in germinating seeds).
Sucrose: -glucose + fructose via a 1,4-glycosidic bond (stored in sugarcane/sugar beet, transport sugar in plants).
Lactose: -glucose + galactose via a 1,4-glycosidic bond (main milk sugar).
Polysaccharides:
Oligosaccharides: Composed of 3 to 10 monosaccharide units.
True Polysaccharides: Composed of 11 or more monosaccharide units joined by glycosidic bonds. Insoluble, compact, and osmotically inactive.
Hydrolysis: Glycosidic bonds are broken by the addition of a water molecule ($H_2O$).
Starch: Primary storage carbohydrate in plants. Composed of two -glucose polymers:
Amylose: Unbranched polymer of 200 to 5000 -glucose units joined purely by 1,4-glycosidic bonds. Spirals into a compact helix.
Amylopectin: Branched polymer of -glucose units joined by 1,4-glycosidic bonds with 1,6-glycosidic bonds forming branch points every 24 to 30 glucose units.
Glycogen: Primary storage carbohydrate in animals and fungi ("animal starch"). Similar structure to amylopectin but far more extensively branched via frequent 1,6-glycosidic bonds, allowing rapid enzymatic hydrolysis to release glucose for respiration.
Lipids and Triglycerides
Structure and Composition of Lipids:
Organic compounds composed of carbon, hydrogen, and oxygen, containing a much lower proportion of oxygen than carbohydrates. Non-polar and insoluble in water.
Contain abundant carbon-hydrogen ($C-H$) bonds. Yield approximately three times more ATP per mass than carbohydrates when oxidised.
Fatty Acids and Glycerol:
Glycerol: Propane-1,2,3-triol ($C_3H_8O_3$), containing three hydroxyl ($-OH$) groups.
Fatty Acids: Contain a carboxyl group ($-COOH$) attached to an unbranched hydrocarbon chain (typically 15 to 17 carbon atoms long).
Saturated Fatty Acids: Hydrocarbon chain contains only single carbon-carbon ($C-C$) covalent bonds (e.g., stearic acid). Pack tightly, producing solid fats at room temperature (common in animals).
Unsaturated Fatty Acids: Hydrocarbon chain contains one or more double carbon-carbon ($C=C$) covalent bonds.
Monounsaturated: Contains a single $C=C$ double bond.
Polyunsaturated: Contains two or more $C=C$ double bonds (e.g., linoleic acid, an essential dietary fatty acid). Kinks in the chain prevent tight packing, producing liquid oils at room temperature (common in plants).
Triglyceride Synthesis (Esterification):
Formed via condensation reactions between one molecule of glycerol and three fatty acid molecules.
An ester bond ($-O-CO-$) forms between each hydroxyl group of glycerol and the carboxyl group of a fatty acid, releasing three molecules of water ($3H_2O$).
A single triglyceride molecule contains exactly 6 oxygen atoms.
Proteins: Structure, Function, and Classification
Amino Acid Structure:
Monomers of proteins. Contain a central carbon attached to an amino group ($-NH_2$), a carboxyl group ($-COOH$), a hydrogen atom ($-H$), and a variable side chain ($R$ group).
20 naturally occurring amino acids (e.g., glycine where $R=-H$; cysteine where $R=-CH_2-SH$).
Peptide Bond Formation:
Amino acids join via condensation reactions between the amino group of one amino acid and the carboxyl group of another, forming a peptide bond ($-CO-NH-$) and releasing $H_2O$.
Dipeptides consist of two amino acids; polypeptides consist of long chains (from 100 to thousands of amino acids).
Intramolecular and Intermolecular Structural Bonds:
Hydrogen Bonds: Electrostatic attractions between oxygen of carboxyl groups and hydrogen of amino groups. Weak individually, but abundant; broken easily by temperature or pH changes.
Disulfide Bonds: Strong covalent bonds formed by oxidation reactions between sulfur-containing $R$ groups of cysteine molecules.
Ionic Bonds: Strong bonds formed between oppositely charged $R$ groups deep within protein molecules.
Levels of Protein Structure:
Primary Structure: The specific linear sequence of amino acids in a polypeptide chain linked by covalent peptide bonds.
Secondary Structure: Regular 3D repeating folding patterns held by hydrogen bonds between the peptide backbone components:
-helix: Right-handed spiral coil.
-pleated sheet: Polypeptide chains folded into parallel or anti-parallel pleats.
Random coil: Irregular secondary arrangements.
Tertiary Structure: Complex 3D folding of the secondary structure into spherical or globular shapes, held by interactions between $R$ groups (hydrogen, disulfide, ionic bonds, hydrophobic/hydrophilic interactions).
Quaternary Structure: The 3D spatial arrangement of two or more polypeptide chains working as a functional protein complex (e.g., haemoglobin).
Classification of Proteins:
Fibrous Proteins: Long, parallel polypeptide chains with cross-linkages forming tough fibres. Little or no tertiary structure. Insoluble in water. Structural roles (e.g., collagen, keratin).
Collagen: Found in tendons, bone matrix, skin, and cartilage. High tensile strength (comparable to steel). Composed of three -chains (each up to 1000 amino acids long, with repeating sequences of glycine-proline-hydroxyproline) twisted into a unique triple helix held by hydrogen bonds. Assembles into fibrils and fibres.
Globular Proteins: Folded into compact, spherical 3D tertiary/quaternary structures. Hydrophobic $R$ groups face inwards; hydrophilic $R$ groups face outwards, forming colloidal suspensions in water. Metabolically active (e.g., enzymes, antibodies, hormones, haemoglobin).
Haemoglobin: Conjugated globular protein with quaternary structure consisting of 4 polypeptide chains (, 574 amino acids total) bound by disulfide bonds. Each chain surrounds an iron-containing ($Fe^{2+}$) prosthetic haem group, enabling each haemoglobin to bind up to 4 $O_2$ molecules.
Conjugated Proteins: Proteins incorporating a non-protein prosthetic group.
Lipoproteins: Proteins conjugated with lipids (LDLs diameter; HDLs diameter).
Glycoproteins: Proteins conjugated with carbohydrates (e.g., mucus, synovial fluid).
Principles of Circulation and Mass Transport Systems
Limits of Diffusion and $sa:vol$ Ratio:
Single-celled and tiny multicellular organisms rely on simple diffusion because diffusion distances are tiny, metabolic demands are low, and their surface area to volume ratio ($sa:vol$) is large.
As organisms increase in size, $sa:vol$ decreases, diffusion distances increase, and simple diffusion across the outer surface becomes insufficient to supply internal cells with nutrients and $O_2$ or remove waste ($CO_2$, urea).
Mass Transport Systems:
Systems that move materials in bulk flow within a fluid medium over long distances to overcome diffusion limits.
Essential features: exchange surfaces, system of transport vessels, directional movement mechanisms, force/pump to move fluid, suitable fluid medium, adaptation of transport rate to demand.
Types of Circulatory Systems:
Open Circulatory System: Blood/hemolymph pumps into large open body cavities (hemocoel) bathing tissues directly (e.g., insects).
Closed Circulatory System: Blood remains enclosed entirely within blood vessels under pressure.
Single Circulation (Fish):
Heart Gill capillaries (oxygenation) $ ightarrow$ Systemic capillaries (body tissues) $ ightarrow$ Heart.
Blood passes through the heart once per complete circuit. Blood pressure drops significantly at the gill capillaries, resulting in low-pressure flow to body tissues.
Double Circulation (Birds and Mammals):
Involves two separate circuits:
Pulmonary Circulation: Deoxygenated blood from heart lungs $ ightarrow$ oxygenated blood back to heart.
Systemic Circulation: Oxygenated blood from heart body tissues $ ightarrow$ deoxygenated blood back to heart.
Blood passes through the heart twice per complete circuit. Allows high pressure in the systemic circuit for rapid oxygen delivery while maintaining low pressure in the pulmonary circuit to protect delicate alveolar capillaries.
The Roles and Components of Mammalian Blood
Composition of Blood:
Plasma: $>50\%$ of blood volume. Fluid medium carrying digested food (glucose, amino acids), excretory products (urea, $CO_2$), chemical messengers (hormones), plasma proteins (albumin, fibrinogen, prothrombin), and heat.
Erythrocytes (Red Blood Cells): ( in females, in males). Produced in bone marrow. Non-nucleated when mature, biconcave disc shape giving high $sa:vol$ ratio. Packed with haemoglobin molecules per cell. 120-day lifespan.
Leucocytes (White Blood Cells): $4000-11000\,cells/mm^3$. Nucleated cells involved in immune defence and inflammatory responses.
Platelets: $150000-400000/mm^3$. Anucleate cell fragments derived from bone marrow megakaryocytes; essential for blood clotting.
Oxygen Transport and Haemoglobin:
Reversible oxygen binding:
Cooperative Binding: Binding of the first $O_2$ alters the 3D conformation of haemoglobin, making it progressively easier for subsequent $O_2$ molecules to bind.
Oxygen Dissociation Curve: Sigmoidal (S-shaped) curve. High partial pressure of oxygen ($pO_2$) in lungs causes high saturation (). Low $pO_2$ in respiring tissues causes rapid release/dissociation of $O_2$. Resting tissues release of oxygen carried.
Carbon Dioxide Transport:
$5\%$ dissolved directly in plasma.
$10-20\%$ bound to amine groups of haemoglobin forming carbaminohaemoglobin.
Most $CO_2$ enters erythrocytes and reacts with water, catalysed by carbonic anhydrase:
Carbonic acid ($H_2CO_3$) dissociates into $H^+$ and hydrogencarbonate ($HCO_3^-$). $HCO_3^-$ diffuses out of the erythrocyte into plasma. To maintain electrical neutrality, chloride ions ($Cl^-$) move into the erythrocyte (Chloride Shift). $H^+$ ions are buffered by haemoglobin forming haemoglobinic acid ($HHb$).
Bohr Effect:
Increased $pCO_2$ or decreased pH in respiring tissues reduces haemoglobin's affinity for $O_2$, shifting the dissociation curve down and to the right, releasing more $O_2$ where metabolic demand is high.
Fetal Haemoglobin:
Fetal haemoglobin has a higher affinity for oxygen than adult maternal haemoglobin at all partial pressures, allowing the fetus to extract $O_2$ from maternal blood across the placenta. Curve is shifted to the left.
Elephant Seal Adaptations:
Deep-diving elephant seals have double the blood volume of land mammals, the haemoglobin concentration, and the myoglobin concentration in muscle tissue.
Blood Clotting Mechanism:
Platelets contact exposed sub-endothelial collagen at a wound site, break open, and release serotonin (causes smooth muscle vascular constriction) and thromboplastin.
Thromboplastin (an enzyme), in the presence of $Ca^{2+}$ ions and Vitamin K, catalyses the conversion of inactive plasma protein prothrombin into active enzyme thrombin.
Thrombin catalyses the conversion of soluble plasma protein fibrinogen into insoluble fibrin threads.
Fibrin forms a mesh that traps erythrocytes and platelets, forming a clot that contracts into a protective scab.
Anatomy and Function of Blood Vessels
Arteries & Arterioles:
Carry blood away from the heart at high pressure.
Thick walls containing an inner smooth endothelium, a middle layer of elastic fibres and smooth muscle, and an outer layer of tough collagen fibres.
Elastic recoil of elastic fibres maintains high diastolic blood pressure between heart contractions.
Arterioles contain a higher proportion of smooth muscle, which contracts (vasoconstriction) or relaxes (vasodilation) to regulate peripheral resistance and direct blood flow.
Capillaries:
Site of exchange between blood and tissue fluid.
Microscopic vessels forming dense capillary beds. Diameter (just wide enough for a single erythrocyte to pass, slowing flow rate).
Walls consist of a single layer of squamous endothelial cells ( thick); no muscle, elastic, or collagen tissue.
Veins & Venules:
Carry blood back towards the heart at low pressure.
Large lumen with thin walls containing low amounts of smooth muscle and elastic tissue, and an outer collagen layer. Act as a blood reservoir ($>50\%$ of blood volume).
Venous return is facilitated by skeletal muscle pumps (contraction of surrounding muscle squeezes veins) and one-way semilunar valves (formed from infoldings of the endothelium) to prevent backflow.
Main veins: Superior vena cava (drains head/upper body) and Inferior vena cava (drains lower body).
The Mammalian Heart and the Cardiac Cycle
Cardiac Anatomy:
Four-chambered double pump made of specialized, myogenic, non-fatiguing cardiac muscle. Divided into left and right sides by a thick muscular septum.
Coronary arteries branch off the aorta to supply oxygenated blood directly to the cardiac muscle tissue.
Contains respiratory pigment myoglobin, which has a higher oxygen affinity than haemoglobin.
Chambers and Valves:
Right Atrium: Receives deoxygenated blood from superior and inferior vena cavae at low pressure.
Tricuspid Valve (Atrioventricular Valve): Three flaps. Prevents backflow from right ventricle to right atrium during ventricular contraction. Anchored by tendinous cords (heartstrings).
Right Ventricle: Thin-walled ventricle; pumps deoxygenated blood through the pulmonary semilunar valve into the pulmonary artery to the lungs at low pressure.
Left Atrium: Receives oxygenated blood from lungs via pulmonary veins.
Bicuspid Valve / Mitral Valve (Atrioventricular Valve): Two flaps. Prevents backflow from left ventricle to left atrium.
Left Ventricle: Very thick muscular wall ( thicker than right ventricle); generates high pressure to pump oxygenated blood through the aortic semilunar valve into the aorta to the systemic circulation.
The Cardiac Cycle (Total duration at $70\,bpm$):
Atrial Systole ($0.1\,s$): Atria contract, pressure in atria exceeds pressure in ventricles, forcing AV valves open and driving blood into ventricles.
Ventricular Systole ($0.3\,s$, starts $0.13\,s$ after atrial systole): Ventricles contract from the apex upwards, increasing ventricular pressure. AV valves snap shut ('lub' sound) preventing backflow. Ventricular pressure exceeds arterial pressure, forcing semilunar valves open, driving blood into aorta and pulmonary artery.
Diastole ($0.4\,s$): Atria and ventricles relax. Pressure in ventricles falls below arterial pressure; semilunar valves snap shut ('dub' sound) preventing backflow into ventricles. Passive filling of atria and ventricles begins.
Pathophysiology of Atherosclerosis and Cardiovascular Diseases
Atherosclerosis Sequence of Events:
Endothelial Damage: High blood pressure or toxins from tobacco smoke damage the delicate endothelial lining of an artery.
Inflammatory Response: White blood cells (macrophages) move into the artery wall.
Cholesterol Accumulation: Macrophages accumulate low-density lipoproteins (LDLs) and cholesterol, forming a fatty deposit called an atheroma.
Plaque Formation: Calcium salts and fibrous tissue build up over the atheroma, creating a hard, rigid plaque.
Narrowing & Loss of Elasticity: The plaque bulges into the lumen, narrowing it, restricting blood flow, increasing friction, and raising blood pressure further (positive feedback).
Pathological Consequences of Atherosclerosis:
Aneurysm: Blood accumulates behind a narrowed plaque-blocked area, putting high pressure on the arterial wall, causing it to bulge and potentially burst, leading to fatal internal haemorrhage.
Hypertension Damage: Elevated blood pressure damages kidney filtering capillaries (forcing protein into urine) and retinal blood vessels (causing blindness).
Angina: Partial occlusion of coronary arteries by plaques. During exertion, heart muscle receives insufficient $O_2$ and respires anaerobically, causing severe chest, arm, and jaw pain.
Myocardial Infarction (Heart Attack):
Rupture of a plaque triggers blood clotting (thrombosis). A clot in the coronary artery is a coronary thrombosis.
Completely blocks blood flow to a region of cardiac muscle, permanently starving it of $O_2$ and nutrients, leading to muscle tissue death (necrosis).
Stroke: Interruption of blood supply to the brain due to a blockage (thrombus/embolus) or capillary rupture/haemorrhage, leading to rapid brain cell death and neurological deficits.
Cardiovascular Risk Factors, Epidemiology, and Study Design
Risk, Correlation, and Causation:
Risk: The mathematical probability that an event will occur.
Correlation: A statistical link where two variables change together, which does not prove that one causes the other.
Causation: When a specific variable directly brings about an effect.
Epidemiological Study Design Principles:
Large Sample Size: Reduces sampling error and ensures results are statistically representative.
Control of Variables: Isolates the variable under investigation.
Longitudinal Studies: Track the same group of individuals over many years (e.g., Münster Heart Study on $10856$ men; Framingham Study; National Children's Study on $100000$ children; UAE Healthy Future Study).
Meta-Analysis: Combines data from multiple independent studies to increase statistical power.
Validity, Precision, and Reliability: Valid (tests what it claims), Precise (minimal spread in measurements), Reliable (repeatable by independent researchers).
Non-Modifiable Risk Factors for CVD:
Genetics: Inherited tendencies towards easily damaged arteries, hypertension, or high blood cholesterol (e.g., Swedish twin study of $21000$ pairs).
Age: Arteries lose elasticity and narrow naturally over time.
Gender: Men under 50 are at significantly higher risk than pre-menopausal women, as oestrogen confers protective effects against plaque formation.
Modifiable (Lifestyle) Risk Factors for CVD:
Smoking: Nicotine and toxins damage endothelium, cause vasoconstriction, raise blood pressure, and alter blood lipoprotein ratios.
Inactivity: Regular exercise lowers blood pressure, reduces LDLs, raises HDLs, prevents obesity/diabetes, and stabilises plaques (e.g., Harvard study of $10269$ graduates showed $23\%$ lower mortality in active men).
Hypertension: Blood pressure consistently $>140/90\,mmHg$ increases endothelial damage risk.
Diet & Obesity: High intake of saturated fats and salt, and low intake of fruit/vegetables increase risk.
Dietary Impacts, Lipoproteins, Antioxidants, and Health Indicators
Obesity Indicators:
Body Mass Index (BMI):
Categories: Underweight ($<18.5$), Ideal ($18.5-25$), Overweight ($25-30$), Obese ($30-40$), Morbidly Obese ($>40$).
Limitations: Does not distinguish between muscle mass and fat mass, underestimating fat in elderly and overestimating in athletes.
Waist-to-Hip Ratio:
Obesity thresholds: $>0.9$ in males; $>0.85$ in females.
Superior predictor of CVD risk compared to BMI as it directly reflects abdominal fat storage.
Lipoproteins and Cholesterol:
Low-Density Lipoproteins (LDLs): Formed from saturated fats, cholesterol, and protein. Transport cholesterol from liver to tissues. High blood LDL levels saturate cell receptors, leaving excess cholesterol in the blood, promoting atheroma formation.
High-Density Lipoproteins (HDLs): Formed from unsaturated fats, cholesterol, and protein. Transport cholesterol from tissues back to the liver for excretion/breakdown, reducing plaque formation.
LDL:HDL Ratio: Healthy ratio is . High ratio strongly correlates with increased coronary heart disease (CHD) mortality.
Dietary Antioxidants:
Antioxidants (Vitamins A, C, E) neutralize free radicals that damage cell membranes and vessel linings.
Longitudinal data ($84000$ women, $42000$ men) showed higher fruit/vegetable intake ($>5$ portions/day) correlates with reduced CHD risk.
Vitamin C case study: Eastern Finland study ($1605$ men) initially linked Vitamin C deficiency to higher heart attack rates ($13.2\%$ vs $3.8\%$), but a 2016 meta-analysis in Int. J. Mol. Sci. confirmed Vitamin C supplements do not reduce CVD risk and high doses may be harmful.
Trehalose Case Study:
Trehalose ($C_{12}H_{22}O_{11}$, -D-glucopyranosyl $ imes 2$): Non-reducing disaccharide synthesized by desert plants (e.g., Selaginella lepidophylla), insects, and fungi.
Protects cellular membranes and proteins against desiccation and oxidative stress (anhydrobiosis). Used in dry eye syndrome treatments and investigated for Alzheimer's and Huntington's treatment.
Pharmacological Treatments for Cardiovascular Disease
Antihypertensives:
Diuretics: Increase urine output by kidney, reducing blood volume, lowering cardiac output and blood pressure.
Beta Blockers: Block -receptors for adrenaline/noradrenaline on the heart, slowing heart rate and reducing force of contraction.
Sympathetic Nerve Inhibitors: Prevent sympathetic nervous stimulation, keeping peripheral arteries dilated.
ACE Inhibitors: Block the production of angiotensin (a hormone that constricts blood vessels), keeping vessels dilated.
Risks: Dizziness, faintness, falls, fatigue, coughs, ankle swelling, impotence.
Statins:
Inhibit HMG-CoA reductase (the liver enzyme responsible for synthesizing cholesterol), reducing LDL production and raising HDL ratios.
Risks: Muscle aches, joint pain, nausea, rare fatal muscle inflammation (rhabdomyolysis), and rare liver damage ().
Plant Stanols and Sterols:
Naturally occurring sterols/stanols structurally similar to cholesterol. $2\,g\,day^{-1}$ intake reduces intestinal cholesterol absorption, lowering blood LDL levels by and CHD risk by .
Anticoagulants and Platelet Inhibitors:
Warfarin: Anticoagulant that interferes with Vitamin K synthesis, inhibiting prothrombin production, making blood clot less readily. Requires strict monitoring to avoid internal haemorrhage.
Aspirin & Clopidogrel: Platelet inhibitory drugs that reduce platelet stickiness and aggregation. Risk: irritation of stomach lining and gastrointestinal bleeding.
Cell Membrane Structure: The Fluid Mosaic Model
Phospholipid Bilayer:
Formed from amphipathic polar lipids. Glycerol bound to two fatty acid tails (hydrophobic, non-polar) and one phosphate head (hydrophilic, polar, negative charge).
In aqueous environments, phospholipids assemble into a bilayer: hydrophilic heads interact with the aqueous cytoplasm and extracellular fluid; hydrophobic tails face inwards away from water, forming a $7-10\,nm$ core.
Fluid Mosaic Model (Singer and Nicholson, 1972):
Fluid: Individual phospholipid molecules move laterally within their monolayer, giving flexibility.
Mosaic: Proteins float within or span the fluid lipid matrix like a mosaic.
Cholesterol: Intercalates between phospholipid tails, regulating membrane fluidity and stability, reducing permeability to small water-soluble ions.
Membrane Proteins:
Integral / Transmembrane Proteins: Span the entire bilayer (e.g., channel proteins, carrier proteins).
Peripheral Proteins: Bound to inner or outer surface.
Glycoproteins & Glycolipids: Carbohydrate side chains attached to proteins or lipids on the outer surface; act as cell recognition sites, receptors for hormones, and cell adhesion markers.
Evolution of the Cell Membrane Model:
Overton (1890s): Observed lipid-soluble substances enter cells fastest; proposed membrane is lipid.
Langmuir (1917): Formed monolayer of phospholipids on water using a Langmuir-Blodgett trough.
Gorter and Grendel (1925): Extracted lipids from red blood cells; found monolayer area was double the cell surface area; proposed a phospholipid bilayer.
Davson and Danielli (1935): Proposed a "sandwich" model: lipid bilayer coated on both sides by solid protein layers.
Robertson (1950s): Electron microscopy showed three-line "unit membrane" structure.
Singer and Nicholson (1972): Freeze-fracture electron microscopy revealed embedded proteins within the bilayer, disproving Davson-Danielli and establishing the Fluid Mosaic Model.
Mechanisms of Cell Transport: Passive and Active Transport
Summary of Transport Mechanisms:
Passive Transport: No metabolic energy ($ATP$) required; driven by kinetic energy down a concentration gradient.
Simple Diffusion: Net movement of small, non-polar, lipid-soluble molecules ($O_2, CO_2$) directly through the phospholipid bilayer.
Facilitated Diffusion: Net movement of large, polar, or charged molecules ($Na^+, K^+$, glucose, amino acids) down a concentration gradient via specific transmembrane protein channels (gated or open) or carrier proteins.
Osmosis: Net movement of free water molecules across a partially permeable membrane down a water potential gradient.
Active Transport: Movement of solutes against a concentration gradient using metabolic energy ($ATP$) via transmembrane carrier proteins (e.g., $Na^+/K^+$ pump, ATPase).
, providing energy to induce conformational changes in the carrier protein.
Evidence: occurs only in living respiring cells; stopped by respiratory poisons like cyanide; rate depends on $O_2$ and temperature.
Bulk Transport (requires $ATP$):
Endocytosis: Cell surface membrane invaginates around extracellular material, pinching off to form an intracellular vesicle. Includes Phagocytosis (ingestion of solids/cells) and Pinocytosis (ingestion of fluid).
Exocytosis: Intracellular secretory vesicles fuse with the cell surface membrane, releasing contents extracellularly.
Osmosis, Water Potential, and Plant Cell Dynamics
Water Potential ():
A measure of the potential of free water molecules to move out of a solution by osmosis. Pure water has the highest water potential, defined as $0\,kPa$. Adding solutes lowers water potential (makes it negative).
Water moves from an area of higher water potential (less negative) to an area of lower water potential (more negative).
Osmotic Solutions:
Isotonic: Solution has the same osmotic concentration/water potential as cell contents.
Hypotonic: Solution has a higher water potential (lower solute concentration) than cell contents.
Hypertonic: Solution has a lower water potential (higher solute concentration) than cell contents.
Effects on Animal Cells:
In hypotonic solution: water enters by osmosis, cell swells and bursts (haemolysis in RBCs).
In hypertonic solution: water leaves by osmosis, cell shrivels (crenation).
Effects on Plant Cells:
In hypotonic solution: water enters vacuole by osmosis; cytoplasm swells against rigid cellulose cell wall, generating hydrostatic pressure until inward pressure potential equals solute potential; cell becomes turgid ($1500\,kPa$).
In hypertonic solution: water leaves vacuole by osmosis; cytoplasm shrinks away from cell wall. The point where $50\%$ of cells show cytoplasm pulling away from cell wall is incipient plasmolysis. Complete shrinkage is plasmolysis.
Gas Exchange Principles, Fick's Law, and the Human Respiratory System
Fick's Law of Diffusion:
Adaptations of Gas Exchange Surfaces:
Large surface area.
Very thin exchange barrier (short diffusion distance).
Steep concentration gradient maintained by ventilation and rich blood supply.
Moist and permeable surfaces.
Anatomy of Human Respiratory System:
Nasal Cavity: Large SA, rich blood supply, ciliated epithelium and mucus-secreting goblet cells to warm, humidify, and filter air.
Trachea & Bronchi: Airways kept open by incomplete rings of C-shaped cartilage. Lined with pseudostratified ciliated columnar epithelium and goblet cells.
Bronchioles: Narrower tubes with smooth muscle to regulate airflow.
Alveoli: alveoli in adult lungs, providing $40-75\,m^2$ SA. Made of single layer of thin, flat squamous epithelial cells. Surrounded by capillary network (walls 1 cell thick). Diffusion distance . Coated with lung surfactant (phospholipid) to reduce surface tension and prevent alveolar collapse.
Ventilation Mechanism:
Inhalation (Active): External intercostal muscles contract, pulling ribcage up and out; diaphragm contracts and flattens. Thoracic volume increases, pressure drops below atmospheric, air flows in.
Normal Exhalation (Passive): External intercostals relax, ribs move down/in under gravity; diaphragm relaxes and domes; elastic recoil of lung tissue. Thoracic volume decreases, pressure rises above atmospheric, air flows out.
Forced Exhalation (Active): Internal intercostal muscles contract, pulling ribs down/in; abdominal muscles contract, forcing diaphragm up.
Asthma and $FEV_1$:
Asthma causes airway inflammation, smooth muscle constriction, and excess mucus production.
Measured using $FEV_1$ (Forced Expiratory Volume in 1 second).
Biological Catalysts: Enzyme Properties and Kinetics
Enzyme Fundamentals:
Globular proteins that act as biological catalysts, increasing reaction rates ($10^8$ to $10^{26}$ times) without being used up or altering reaction products.
Lower the activation energy required to reach the transition state.
Intracellular Enzymes: Catalyse reactions inside cells (e.g., DNA polymerase, catalase).
Extracellular Enzymes: Secreted by cells to catalytically act externally (e.g., digestive enzymes, lysozyme).
Metabolism: Anabolic (synthesis) + Catabolic (breakdown) reactions.
Models of Enzyme Action:
Lock-and-Key Hypothesis: Substrate key fits into rigid complementary 3D active site lock forming an enzyme-substrate complex.
Induced-Fit Hypothesis: Active site is flexible. Binding of substrate induces conformational changes in active site, bringing catalytic groups into optimal alignment to strain bonds and lower activation energy.
Enzyme Kinetics and Factors Affecting Activity:
Molecular Activity / Turnover Number: Substrate molecules converted per minute per enzyme molecule (e.g., catalase ).
Enzyme & Substrate Concentration: Rate increases linearly with substrate concentration until enzyme active sites become saturated ($V_{max}$ reached).
Temperature:
Temperature coefficient ($Q_{10}$):
Between $0\,^ ext{C}$ and $40\,^ ext{C}$, .
Above $40\,^ ext{C}$, thermal agitation breaks weak hydrogen and ionic bonds holding 3D tertiary structure, causing denaturation and altering active site shape.
Thermophilic bacteria enzymes operate up to $85\,^ ext{C}$ due to abundant internal hydrogen and disulfide bonds.
pH: Deviations from optimal pH alter ionization of amino acid $R$ groups and break ionic/hydrogen bonds, altering active site shape.
Structure of Nucleic Acids: DNA, RNA, and Mononucleotides
Mononucleotide Structure:
Composed of three components joined by condensation reactions:
A 5-carbon pentose sugar (ribose in RNA, deoxyribose in DNA).
A nitrogen-containing organic base.
A phosphate group ($PO_4^{3-}$).
Bases:
Purine Bases (two rings): Adenine ($A$) and Guanine ($G$).
Pyrimidine Bases (one ring): Cytosine ($C$), Thymine ($T$, in DNA only), and Uracil ($U$, in RNA only).
Polynucleotide Formation:
Mononucleotides join via condensation reactions forming strong covalent phosphodiester bonds between carbon-3 of the pentose sugar of one nucleotide and the phosphate group of the next, forming a sugar-phosphate backbone.
DNA Double Helix Structure:
Two antiparallel polynucleotide strands running in opposite directions ($5'$ to $3'$ and $3'$ to $5'$), twisted into a double helix ($2\,nm$ width, 10 base pairs per turn).
Sugar-phosphate backbones on the outside; nitrogenous bases point inwards.
Complementary Base Pairing: Held together by hydrogen bonds between purines and pyrimidines:
Adenine ($A$) pairs with Thymine ($T$) via 2 hydrogen bonds.
Cytosine ($C$) pairs with Guanine ($G$) via 3 hydrogen bonds.
RNA Structure:
Single-stranded polynucleotide containing ribose sugar and Uracil ($U$) instead of Thymine ($T$). Much shorter than DNA.
DNA Replication and the Meselson-Stahl Experiment
Models of DNA Replication:
Conservative Replication: Original double helix stays intact and acts as a whole template for an entirely new double helix.
Semiconservative Replication: Original double helix unzips; each original strand acts as a template for a new complementary strand, producing two identical double helices, each containing one original and one newly synthesised strand.
Meselson and Stahl Experiment (1950s):
Cultured E. coli in medium containing heavy isotope of nitrogen ($^{15}N$) for many generations until all DNA contained $^{15}N$.
Transferred E. coli to medium containing light isotope ($^{14}N$) and allowed them to replicate.
Isolated DNA and separated by density gradient centrifugation:
Generation 0: Single heavy band ($^{15}N-^{15}N$).
Generation 1: Single hybrid intermediate band ($^{15}N-^{14}N$), refuting conservative model.
Generation 2: One hybrid intermediate band ($^{15}N-^{14}N$) and one light band ($^{14}N-^{14}N$), confirming semiconservative model.
Mechanism of DNA Replication:
DNA Helicase: Unzips double helix by breaking hydrogen bonds between complementary base pairs, exposing single-stranded templates.
DNA Polymerase: Aligns free DNA nucleotides along template strands via complementary base pairing ($A-T, C-G$) and catalyses phosphodiester bond formation.
DNA Ligase: Joins breaks or Okazaki fragments in the sugar-phosphate backbone.
The Genetic Code, Transcription, and Translation
Nature of the Genetic Code:
Gene: A sequence of bases on a DNA molecule coding for the sequence of amino acids in a polypeptide chain.
Triplet Code / Codon: Three consecutive DNA or mRNA bases code for a single specific amino acid ($4^3 = 64$ possible codons for 20 amino acids).
Non-Overlapping: Each base in the sequence is read once only as part of a single codon.
Degenerate / Redundant: Most amino acids are coded for by more than one codon (protects against point mutations).
Universal: Same codons code for same amino acids across almost all living organisms.
Start and Stop Codons: $TAC$ (DNA) / $AUG$ (mRNA) is the start codon coding for methionine; three stop codons ($UAA, UAG, UGA$ on mRNA) terminate translation.
Transcription (in Nucleus):
DNA helicase unzips DNA gene region, exposing sense (coding) and antisense (template) strands.
Free RNA nucleotides align along the antisense (template) strand via complementary base pairing ($A-U, T-A, C-G, G-C$).
RNA Polymerase links RNA nucleotides with phosphodiester bonds to synthesize a single-stranded pre-mRNA molecule.
Pre-mRNA leaves nucleus via nuclear pores into cytoplasm.
Translation (at Ribosomes in Cytoplasm):
Small subunit of $80S$ ribosome attaches to mRNA at the start codon ($AUG$).
Transfer RNA (tRNA) molecules (clover-leaf shape held by hydrogen bonds, carrying a specific amino acid at $3'$ end and a 3-base anticodon at opposite loop) align opposite mRNA codons via complementary base pairing.
Ribosomal enzymes catalyse peptide bond formation between adjacent amino acids.
Ribosome moves along mRNA strand until a stop codon is reached, releasing the completed polypeptide chain.
Multiple ribosomes move along same mRNA simultaneously, forming a polysome.
Gene Mutations and Genetic Disorders
Gene Mutations:
A permanent change in the DNA base sequence. Occurs spontaneously during DNA replication ( mutation rate per base).
Point Mutations:
Substitution: One base replaced by another (may alter 1 amino acid or have no effect due to degeneracy).
Deletion: One base removed, causing a frameshift mutation altering all subsequent codons.
Insertion: One base added, causing a frameshift mutation.
Chromosomal vs Whole-Chromosome Mutations:
Chromosomal Mutations: Changes in positions or arrangements of whole genes on a chromosome.
Whole-Chromosome Mutations: Loss or duplication of an entire chromosome (e.g., Down syndrome = Trisomy 21).
Sickle Cell Disease Pathophysiology:
Point mutation in the -globin gene: $6 ext{th}$ codon $GAG$ (coding for glutamic acid, polar) is mutated via single base substitution to $GUG$ (coding for valine, non-polar).
Deoxygenated sickle haemoglobin polymerises into rigid rods, distorting erythrocytes into rigid sickle shapes.
Sickled cells lodge in capillaries, causing blockages, severe pain, tissue necrosis, and anaemia.
Patterns of Monohybrid Inheritance and Pedigree Analysis
Genetic Terminology:
Genotype: The genetic make-up of an organism with respect to specific alleles.
Phenotype: Physical and biochemical characteristics expressed as a result of genotype-environment interaction.
Homozygote: Individual with two identical alleles at a locus (homozygous dominant $AA$, homozygous recessive $aa$). True-breeding.
Heterozygote: Individual with two different alleles at a locus ($Aa$).
Dominant Allele: Expressed in phenotype whether present in homozygous or heterozygous state.
Recessive Allele: Expressed in phenotype only when present in homozygous state ($aa$).
Codominance: Both alleles in a heterozygote are fully expressed in the phenotype without blending (e.g., ABO blood groups $I^A I^B$).
Monohybrid Crosses:
Cross between two heterozygotes () yields a theoretical $1:2:1$ genotype ratio ($1AA : 2Aa : 1aa$) and a $3:1$ phenotype ratio ().
Test Cross: Breeding an individual with a dominant phenotype ($A?$) with a homozygous recessive individual ($aa$) to determine if the dominant parent is homozygous ($AA$) or heterozygous ($Aa$).
Pedigree Diagrams:
Symbols: Squares = males; Circles = females; Shaded = affected; Half-shaded = carriers.
Recessive conditions can skip generations and be carried by unaffected parents.
Sex Linkage and Sex-Linked Genetic Disorders
Sex Determination:
Autosomes: 22 pairs of non-sex chromosomes in humans.
Sex Chromosomes: 1 pair ($XX$ in females = homogametic; $XY$ in males = heterogametic).
The $Y$ chromosome is tiny ( base pairs, 78 genes) and carries the $SRY$ gene (sex-determining region Y) triggering testis development. The $X$ chromosome is large ( base pairs, $800-1200$ genes).
Mechanism of Sex Linkage:
Genes located on the $X$ chromosome that lack a corresponding allele on the shorter $Y$ chromosome are sex-linked.
Males ($XY$) are hemizygous for X-linked genes. A single recessive mutant allele on a male's $X$ chromosome will always be expressed in his phenotype.
Females ($XX$) require two copies of the recessive allele to express the condition; single-copy females are asymptomatic carriers.
Sex-Linked Disorders:
Red-Green Colour Blindness: Recessive X-linked mutation affecting retinal photoreceptors. Affects $7-8\%$ of males and $<1\%$ of females.
Haemophilia A: Recessive X-linked mutation resulting in missing Clotting Factor VIII. Affects male births. Causes severe bleeding; treated with recombinant Factor VIII.
Pathophysiology and Inheritance of Cystic Fibrosis
Molecular Etiology:
Autosomal recessive genetic disease caused by mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene on chromosome 7 (most common mutation is , a 3-base deletion).
Normal CFTR protein is a transmembrane $Cl^-$ channel (1480 amino acids) that pumps $Cl^-$ out of epithelial cells, inhibiting $Na^+$ channels. Water follows $Cl^-$ out by osmosis, keeping surface mucus runny.
Mutant non-functioning CFTR: $Cl^-$ ions cannot leave cells, $Na^+$ channels are uninhibited so $Na^+$ enters cells, water moves out of mucus into cells by osmosis, creating abnormally thick, sticky mucus.
Multi-System Effects:
Respiratory System: Thick mucus blocks bronchioles, reducing alveolar ventilation and surface area. Mucus traps bacteria; cilia cannot sweep it away, causing chronic bacterial infections, coughing, and lung damage.
Digestive System: Mucus blocks the pancreatic duct, preventing pancreatic digestive enzymes (lipases, proteases, amylases) from reaching the duodenum. Causes malabsorption, malnutrition, and failure to thrive. Trapped enzymes cause pancreatic auto-digestion and diabetes.
Reproductive System: Cervical mucus in females is too thick for sperm penetration; blocked oviducts. Males lack or have blocked vas deferens (), causing infertility.
Sweat Glands: CFTR normally reabsorbs $Cl^-$ in sweat ducts; mutant CFTR causes high $NaCl$ concentration in sweat (salty sweat).
Genetic Screening, Diagnosis, and Ethical Implications
Types of Genetic Screening:
Carrier Testing: Identifies asymptomatic carriers of recessive alleles (e.g., CF, Tay-Sachs) in individuals with a family history.
Neonatal Screening: Blood test on newborn babies (e.g., PKU/phenylketonuria: deficiency in phenylalanine hydroxylase leading to toxic buildup of phenylalanine; treated with low-phenylalanine diet to prevent brain damage).
Prenatal Screening:
Amniocentesis: Performed at pregnancy. $20\,cm^3$ amniotic fluid withdrawn; fetal cells cultured for for karyotyping. Miscarriage risk: $0.5-1\%$.
Chorionic Villus Sampling (CVS): Performed at pregnancy. Sample of embryonic placental tissue (chorionic villi) withdrawn. Faster results than amniocentesis. Miscarriage risk: $0.5-1\%$.
Preimplantation Genetic Diagnosis (PGD): Used alongside IVF. Embryos grown to 8-cell stage; a single cell is removed and screened. Only embryos free of mutant alleles are implanted into the uterus.
Ethical and Social Implications:
Risk of false positives and false negatives.
Risk of procedure-induced miscarriage of healthy fetuses.
Ethical debates on termination of pregnancy, selective implantation, disability discrimination, designer babies, and emotional stress.
Ethical principles: Respect for autonomy, Beneficence, Non-maleficence, Justice. Assisted by genetic counsellors.
Microscopy, Cell Ultrastructure, and Organelles
Microscopy & Resolution:
Light Microscope: Uses light and glass lenses. Max magnification . Resolution ($200\,nm$). Can observe living or dead specimens.
Electron Microscope: Uses electron beams and electromagnetic lenses in a vacuum. Resolution .
Transmission Electron Microscope (TEM): Electrons pass through thin specimen sections; produces 2D high-resolution ultrastructural images (max mag ).
Scanning Electron Microscope (SEM): Electrons bounce off specimen surface; produces 3D surface images at lower resolution than TEM.
Magnification Formula: where , , .
Organelles of Eukaryotic Cells:
Nucleus: diameter. Enclosed by double nuclear envelope with nuclear pores. Contains chromatin (DNA + histones) and nucleolus (dense area producing rRNA and ribosomes).
Mitochondria: . Double membrane: smooth outer membrane; inner membrane folded into cristae (large SA for electron transport chain). Central fluid matrix containing $70S$ ribosomes, circular DNA, and respiratory enzymes. Site of aerobic respiration/ATP synthesis.
Centrioles: Pair of cylindrical bundles of 9 triplets of protein microtubules (). Organize spindle fibres during cell division in animal cells.
Ribosomes: $80S$ in eukaryotic cytoplasm ($60S + 40S$ subunits); $70S$ in prokaryotes, mitochondria, and chloroplasts ($50S + 30S$ subunits). Site of protein synthesis.
Lysosomes: Membrane-bound vesicles containing acidic hydrolytic enzymes. Break down worn-out organelles, endocytosed materials, or induce apoptosis (programmed cell death).
Protein Synthesis, Modification, and Secretion Pathway
Endoplasmic Reticulum (ER):
Rough Endoplasmic Reticulum (RER): 3D network of fluid-filled membrane cavities (cisternae) continuous with nuclear envelope, coated externally with $80S$ ribosomes. Synthesises, folds, and transports proteins.
Smooth Endoplasmic Reticulum (SER): Tubular membrane network lacking ribosomes. Synthesises, stores, and transports lipids and steroids (e.g., testosterone, cholesterol).
Golgi Apparatus:
Stacks of flattened, membrane-bound sacs (cisternae) surrounded by vesicles.
Modifies proteins received from RER (e.g., adding carbohydrate chains to form glycoproteins like mucus), packages modified proteins into vesicles for secretion (exocytosis), or forms lysosomes.
Secretion Pathway Sequence:
Gene transcribed into pre-mRNA in nucleus spliced to mature mRNA $ ightarrow$ exits nuclear pore.
mRNA attaches to $80S$ ribosome on RER $ ightarrow$ translated into polypeptide chain which enters RER lumen.
Polypeptide folded in RER $ ightarrow$ packaged into RER transport vesicle.
Vesicle buds off RER, travels along cytoskeleton, and fuses with cis-face of Golgi apparatus.
Protein modified as it passes through Golgi cisternae $ ightarrow$ packaged into secretory vesicle at trans-face.
Secretory vesicle moves to cell surface membrane, fuses with it, releasing protein via exocytosis.
Prokaryotic Cell Structure, Classification, and Gram Staining
Prokaryotic Ultrastructure:
Cell Wall: Made of peptidoglycan (murein), a net-like polymer of polysaccharide chains cross-linked by short peptides. Prevents osmotic lysis.
Capsule / Slime Layer: Outer gelatinous layer (starch, protein, glycolipid); protects against desiccation and phagocytosis.
Pili: Hair-like protein projections for adhesion to surfaces/host cells and conjugation (plasmid transfer).
Flagella: Helical rotatable rods made of flagellin protein; rotated by a motor in the membrane for locomotion ($100\,rev\,s^{-1}$).
Cell Surface Membrane: Lacks sterols. Site of respiratory enzymes and mesosomes.
Nucleoid: Unenclosed region containing a single, highly folded, circular DNA strand (no histones).
Plasmids: Small, independent loops of double-stranded DNA coding for non-essential advantageous traits (e.g., antibiotic resistance).
Ribosomes: $70S$ ($50S + 30S$ subunits).
Gram Staining Technique:
Gram-Positive Bacteria: Thick peptidoglycan cell wall containing teichoic acid. Retains crystal violet-iodine complex after alcohol wash; appears purple/blue under light microscope (e.g., MRSA). Susceptible to -lactam antibiotics (penicillins) and lysozyme.
Gram-Negative Bacteria: Thin peptidoglycan wall surrounded by an outer lipopolysaccharide membrane; lacks teichoic acid. Alcohol wash dissolves outer membrane, washing out crystal violet; takes up safranin counterstain, appearing red/pink (e.g., E. coli). Resistant to penicillin.
Classification of Bacteria:
Shape: Cocci (spherical), Bacilli (rod), Spirilla (spiral), Vibrios (comma).
Respiration: Obligate aerobes (require $O_2$), Facultative anaerobes (respire with or without $O_2$), Obligate anaerobes (killed by $O_2$).
Cellular Organisation: Tissues, Organs, and Systems
Levels of Structural Organisation:
Specialised Cells Tissues $ ightarrow$ Organs $ ightarrow$ Organ Systems $ ightarrow$ Organism.
Animal Tissue Types:
Epithelial Tissue: Lines internal and external surfaces. Originate from basement membrane.
Squamous Epithelium: Flat, thin cells forming thin barriers (e.g., alveoli, capillary walls).
Cuboidal & Columnar Epithelium: Cube or column-shaped cells line tubes/glands.
Ciliated Epithelium: Contains cilia and goblet cells (mucus-secreting) to sweep mucus along (e.g., trachea, oviducts).
Compound Stratified Epithelium: Multiple layers for protection against abrasion (e.g., skin).
Connective Tissue: Supporting tissue containing collagen and matrix (e.g., bone, cartilage, packing tissue).
Muscle Tissue: Skeletal, smooth, and cardiac muscle.
Nervous Tissue: Conducts electrical impulses.
Plant Organs:
Leaf: Composed of upper/lower epidermis, palisade mesophyll, spongy mesophyll, and vascular bundles (xylem and phloem).
The Eukaryotic Cell Cycle and Mitosis
Chromosome Organisation:
Chromatin: DNA double helix wrapped around positively charged histone proteins forming nucleosomes ($10\,nm$). Coils into $30\,nm$ fibres and supercoils into $80\,nm$ dense structures during cell division.
The Cell Cycle:
Interphase: Period of growth and non-division ($90\%$ of cycle).
$G_1$ Phase (Gap 1): Cell grows, synthesises proteins and new organelles.
$S$ Phase (Synthesis): DNA replication occurs; chromosomes become double-stranded chromatids.
$G_2$ Phase (Gap 2): Organelles duplicate, energy stores ($ATP$) replenish, cell prepares for division.
Control: Governed by cyclins binding to cyclin-dependent kinases (CDKs), forming complexes that phosphorylate target proteins to drive transitions between phases.
Mitosis: Nuclear division yielding two genetically identical daughter nuclei.
Cytokinesis: Division of cytoplasm.
Stages of Mitosis:
Prophase: Chromosomes condense and become visible as two sister chromatids joined at the centromere. Nucleolus disappears; nuclear envelope breaks down. Centrioles move to opposite poles, forming spindle fibers.
Metaphase: Chromatids line up along the metaphase plate (equator), attached to spindle fibres by their centromeres.
Anaphase: Centromeres split. Sister chromatids are pulled apart to opposite poles, centromere first, by contracting spindle microtubules. Now termed individual daughter chromosomes.
Telophase: Chromosomes reach poles, uncoil into chromatin. Nuclear envelopes re-form around each set; nucleoli reappear; spindle breaks down.
Cytokinesis:
Animal cells: Ring of contractile microfilaments tightens around cell equator, forming a cleavage furrow that pinches cell into two.
Plant cells: Membrane vesicles from Golgi apparatus align at equator, fusing to form a cell plate. Cellulose deposited to build new cell walls and middle lamella.
Mitotic Index Formula:
Sexual Reproduction, Meiosis, and Genetic Variation
Gametes and Ploidy:
Diploid ($2n$): Cells containing two full matching sets of homologous chromosomes.
Haploid ($n$): Cells containing one single set of chromosomes (gametes).
Fertilisation: Fusion of two haploid gametes to form a diploid zygote ($2n$).
Meiosis Overview:
Reduction division taking place in gonads (testes/ovaries; anthers/ovaries) producing four haploid, genetically non-identical daughter cells.
Involves two successive nuclear divisions: Meiosis I and Meiosis II.
Stages of Meiosis:
Meiosis I:
Prophase 1: Chromosomes condense. Homologous chromosomes pair up forming bivalents. Crossing over (recombination) occurs at chiasmata: chromatids break and exchange non-sister genetic material.
Metaphase 1: Bivalents line up randomly on metaphase plate (Independent Assortment).
Anaphase 1: Homologous chromosomes separate to opposite poles (centromeres DO NOT split).
Telophase 1: Nuclear envelopes form; cytokinesis produces 2 haploid cells.
Meiosis II:
Prophase 2: Spindles form in both cells.
Metaphase 2: Chromosomes line up on equator.
Anaphase 2: Centromeres split; sister chromatids move to opposite poles.
Telophase 2: Nuclear envelopes re-form; cytokinesis yields 4 haploid, non-identical gametes.
Sources of Genetic Variation in Meiosis:
Crossing Over (Recombination) in Prophase 1: Swaps alleles between maternal and paternal chromatids.
Independent Assortment in Metaphase 1: Random alignment of maternal/paternal homologous chromosomes ($2^n$ possible combinations; in humans).
Random Fertilisation: Fusion of unique sperm and egg cells.
Structure, Specialisation, and Formation of Gametes
Mammalian Gamete Specialisations:
Spermatozoa (Male): long. Small, motile. Head contains condensed haploid nucleus and an acrosome (specialised lysosome with digestive enzymes). Middle piece packed with tightly coiled mitochondria for $ATP$ production. Tail (flagellum with microtubules) propels sperm.
Ovum (Female): () diameter. Large, non-motile. Contains haploid nucleus, cytoplasm packed with food reserves (proteins/lipids), surrounded by cell membrane, protective zona pellucida (glycoprotein jelly), and outer corona radiata (follicle cells).
Plant Gametes:
Alternation of generations: Diploid sporophyte produces haploid spores by meiosis; haploid gametophyte produces gametes by mitosis.
Pollen Grain (Microgamete): Produced in anthers from microspores. Contains a thick outer wall (exine) and two haploid nuclei: the tube nucleus (controls pollen tube growth) and the generative nucleus (divides by mitosis into two male nuclei).
Ovule / Embryo Sac (Megagamete): Produced in ovary from megaspores. Embryo sac contains an egg cell ($n$), two polar nuclei ($n+n$), three antipodal cells, and two synergids.
Fertilisation Mechanisms in Mammals and Flowering Plants
Mammalian Fertilisation Sequence:
Sperm travel through cervix and oviducts, undergoing acrosome maturation.
Sperm reach ovum and touch zona pellucida Acrosome Reaction: acrosome membrane fuses with sperm membrane, releasing hydrolytic enzymes via exocytosis to digest follicle cells and zona pellucida.
Single sperm membrane fuses with oocyte membrane; sperm nucleus enters cytoplasm.
Cortical Reaction: Depolarisation of membrane occurs. Cortical granules in oocyte release enzymes via exocytosis into zona pellucida, cross-linking glycoprotein jelly into a tough, impenetrable fertilisation membrane, destroying sperm-binding sites to prevent polyspermy.
Oocyte completes Meiosis II; haploid sperm and egg nuclei fuse to form diploid zygote.
Double Fertilisation in Flowering Plants:
Pollen grain lands on compatible stigma $ ightarrow$ germinates.
Pollen tube grows down style controlled by tube nucleus, secreting hydrolytic enzymes to digest style tissue.
Generative nucleus divides by mitosis inside tube to form two haploid male nuclei.
Pollen tube enters ovule through the micropyle; tube nucleus degenerates.
Double Fertilisation occurs:
One male nucleus fuses with the egg cell nucleus $ ightarrow$ diploid zygote ($2n$).
Other male nucleus fuses with the two polar nuclei $ ightarrow$ triploid primary endosperm nucleus ($3n$) (provides food store for germinating seed).
Cell Differentiation, Gene Linkage, and Polygenic Inheritance
Differential Gene Expression:
Cell differentiation occurs when specific genes are activated (expressed) while others are repressed (silenced). Active genes are transcribed into mRNA, which is translated into specific proteins that determine cell structure and function.
Multiple Alleles:
More than two alleles exist for a gene locus within a population (e.g., ABO blood groups: $I^A, I^B, I^O$).
Polygenic Traits:
Phenotypic traits controlled by multiple independent genes located at different loci (e.g., height, skin color, eye color), resulting in continuous variation.
Gene Linkage and Digenic Crosses:
Independent Assortment: Two unlinked genes on separate chromosomes cross (), yielding a $9:3:3:1$ phenotype ratio.
Gene Linkage: Genes located close together on the same chromosome are linked and tend to be inherited together as a single unit.
Dihybrid cross of tightly linked genes () yields a $3:1$ monohybrid phenotype ratio instead of $9:3:3:1$.
Recombination/crossing over in meiosis can break linkage; frequency of recombination depends on distance between linked loci.
Gene-Environment Interactions and Epigenetics
Phenotype Equation:
Examples of Gene-Environment Interaction:
Siamese Cat / Himalayan Rabbit Coat Color: Gene codes for tyrosinase (enzyme producing dark melanin pigment). Mutation renders tyrosinase heat-sensitive: denatured at core body temperature ($37\,^ ext{C}$, pale fur), active only at cooler extremities (ears, paws, nose, dark fur).
E. coli Lac Operon (Jacob and Monod):
Lactose Absent: Repressor protein binds to operator/promoter region of DNA, blocking RNA polymerase; -galactosidase gene is NOT transcribed.
Lactose Present: Lactose binds repressor protein, altering its 3D shape so it detaches from operator; RNA polymerase transcribes gene for -galactosidase enzyme.
Human Variation Studies:
Twin Studies: Identical twins (monozygotic, $100\%$ identical DNA) vs fraternal twins (dizygotic, $50\%$ shared DNA). Shows high genetic component for height ($77\%$) and IQ, but significant environmental influence on BMI and mass.
Continuous vs Discontinuous Variation:
Discontinuous Variation: Distinct qualitative categories (e.g., blood groups, sex); controlled by 1 or few genes; unaffected by environment.
Continuous Variation: Continuous quantitative range of values (e.g., height, mass); polygenic inheritance; significantly influenced by environmental factors.
Transcriptional, Post-Transcriptional, and Post-Translational Regulation
Transcriptional Regulation (Transcription Factors):
Transcription Factors: Proteins that bind to specific DNA promoter sequences upstream of genes to stimulate or inhibit RNA polymerase binding.
Enhancer Sequences: Specific DNA regions where transcription factors bind, altering chromatin structure to make it open (active) or closed (repressed).
Post-Transcriptional Regulation (RNA Splicing):
Pre-mRNA contains coding exons and non-coding introns.
Spliceosomes excise introns and splice exons together.
Alternative RNA Splicing: Exons can be spliced together in different combinations from a single pre-mRNA transcript, producing multiple mature mRNA variants and different functional proteins from a single gene (e.g., chick inner ear hair cells produce 576 protein variants from 1 gene).
Post-Translational Regulation:
Polypeptides modified after synthesis by cleavage, phosphorylation, or addition of carbohydrate/lipid groups.
Epigenetic Mechanisms (Heritable changes in gene expression without altering DNA base sequence):
DNA Methylation: Addition of a methyl group ($-CH_3$) to cytosine at CpG sites by DNA methyltransferase. Changes DNA conformation, preventing transcription factor/RNA polymerase binding silences gene.
Histone Modification:
Histone Acetylation: Addition of acetyl groups ($-COCH_3$) to lysine $R$ groups on histones reduces positive charge, loosening DNA-histone interaction $ ightarrow$ euchromatin (active transcription).
Histone Methylation: Addition of methyl groups to histones can condense chromatin into heterochromatin $ ightarrow$ silences gene.
Non-coding RNA (ncRNA): $98\%$ of RNA. $Xist$ ncRNA coats one $X$ chromosome in female mammals, condensing it into an inactive Barr body.
Stem Cell Potency, Development, and Therapeutic Applications
Stem Cell Potency Spectrum:
Stem Cells: Undifferentiated cells capable of continuous division and differentiation into specialised cell types.
Totipotent Stem Cells: Can differentiate into any cell type, including extra-embryonic tissues (placenta/amnion) (e.g., zygote, early blastomeres up to 8-cell stage / morula).
Pluripotent Stem Cells: Can differentiate into almost all cell types of the body, but not extra-embryonic tissues (e.g., inner cell mass of blastocyst).
Multipotent Stem Cells: Can differentiate into a limited range of cell types within a specific tissue (e.g., adult bone marrow stem cells forming blood cells).
Early Embryonic Development:
Zygote undergoes cleavage (mitotic divisions without growth) Morula (solid ball of 10-30 totipotent cells, day 4) $ ightarrow$ Blastocyst (hollow ball of cells with outer trophoblast forming placenta, and inner cell mass of pluripotent stem cells, day 5-6).
Therapeutic Applications and Sources:
Embryonic Stem Cells: Pluripotent, extracted from surplus IVF blastocysts. Risk of teratoma/cancer formation and immune rejection; ethical debate regarding destruction of human embryos.
Adult Stem Cells: Multipotent, extracted from bone marrow, brain, or heart tissue. No rejection if autologous, no ethical issues with embryo destruction, but low numbers and limited potency.
Therapeutic Cloning (Somatic Cell Nuclear Transfer): Transfer nucleus from patient's adult somatic cell into enucleated donor ovum $ ightarrow$ stimulate division to blastocyst $ ightarrow$ harvest inner cell mass pluripotent stem cells $ ightarrow$ identical genetic match to patient.
Induced Pluripotent Stem Cells (iPS cells, Yamanaka 2006): Adult somatic cells (e.g., skin fibroblasts) reprogrammed back to pluripotency by introducing 4 transcription factor genes using retroviral vectors. Avoids embryo destruction and rejection risks.
Therapeutic Targets: Parkinson's disease (replacing dopamine neurones), Type 1 diabetes (replacing pancreatic -cells), Spinal cord injury, Heart tissue repair after myocardial infarction.
Plant Cell Structure, Cellulose Chemistry, and Organelles
Cellulose Chemistry and Microfibrils:
Linear, unbranched polymer of -glucose monomers linked by -1,4-glycosidic bonds.
Every alternate -glucose monomer is inverted by $180^ ext{o}$ along the chain.
Hydroxyl ($-OH$) groups project outwards on both sides of the linear chain, forming extensive cross-linking hydrogen bonds between adjacent parallel cellulose chains.
$10000$ to $100000$ cellulose chains bundle together to form rigid microfibrils, embedded in a matrix of hemicelluloses and pectin to form a high-tensile-strength composite material.
Plant Cell Wall Layers:
Middle Lamella: Outermost layer made of calcium pectate (pectin) gluing adjacent cells together.
Primary Cell Wall: Flexible layer formed during cell growth; cellulose microfibrils oriented in a parallel direction.
Secondary Cell Wall: Rigid inner layer deposited after growth stops; microfibrils laid down at different dense angles; impregnated with hemicelluloses and lignin.
Specialised Plant Organelles:
Permanent Vacuole: Large fluid-filled central space containing cell sap, surrounded by the tonoplast membrane. Maintains cell turgor pressure ($1500\,kPa$). Stores pigments (betacyanin), enzymes, waste, or secondary metabolites (digitalis).
Chloroplasts: diameter, thick double-membrane organelles. Inner membrane forms stacked thylakoid discs (grana) embedded in fluid stroma. Contains chlorophyll, $70S$ ribosomes, circular DNA, and starch grains. Site of photosynthesis.
Amyloplasts: Colourless, non-pigmented organelles that synthesize and store starch (amylose and amylopectin) in storage tissues (e.g., potato tubers).
Anatomy of Plant Stems, Transport Tissues, and Mechanical Support
Stem Anatomy (Transverse Section):
Epidermis: Outer protective layer coated with waxy cuticle.
Parenchyma: Unspecialised packing tissue forming cortex and pith; turgid cells provide support.
Collenchyma: Living cells with extra cellulose thickening at corners; provides flexible support in young stems.
Sclerenchyma: Dead cells with heavily lignified secondary walls; provides rigid support.
Sclerenchyma Fibres: Long, hollow, lignified tubes found in bundles around vascular tissue.
Sclereids: Isodiametric lignified cells.
Vascular Bundles: Arranged in a ring around the periphery of the stem.
Xylem Tissue:
Functions: Transport of water and mineral ions (transpiration stream) and mechanical support.
Structure: Continuous hollow tubes formed from dead cells arranged end-to-end; end walls completely broken down. Cell walls impregnated with lignin (spiral, annular, or reticulate patterns) which waterproofs cells and provides strength against negative tension. Contains pits (unlignified wall regions) allowing lateral water movement.
Protoxylem: First-formed xylem; incomplete lignin rings allow stretching.
Metaxylem: Mature, fully lignified xylem.
Phloem Tissue:
Function: Active transport of organic solutes / sucrose (translocation) up and down the plant.
Structure: Living tissue.
Sieve Tube Elements: Elongated cells joined end-to-end with perforated sieve plates at junctions. Microscopic cytoplasm lacks nucleus, tonoplast, and ribosomes, allowing unobstructed sap flow.
Companion Cells: Active cells connected to sieve tubes via abundant plasmodesmata. Contain dense cytoplasm, nucleus, and abundant mitochondria to generate $ATP$ for active loading of sucrose.
Plant Mineral Nutrition and Water Requirements
Functions of Water in Plants:
Photosynthesis reactant.
Turgor pressure maintenance for structural support ($1500\,kPa$).
Transport medium for xylem mineral ions and phloem sucrose.
Evaporative cooling via transpiration.
Essential Inorganic Ions:
Nitrate Ions ($NO_3^-$): Required to synthesise amino acids, proteins, enzymes, nucleic acids (DNA/RNA), ATP, and plant hormones.
Deficiency: Stunted growth, chlorosis (yellowing) of older leaves.
Calcium Ions ($Ca^{2+}$): Combines with pectin to form calcium pectate in the middle lamella, holding cell walls together; regulates membrane permeability.
Deficiency: Death of growing points (meristems), crinkled/stunted young leaves.
Magnesium Ions ($Mg^{2+}$): Central constituent of the chlorophyll molecule; activates respiratory and photosynthetic enzymes.
Deficiency: Interveinal chlorosis (yellowing between veins) of older leaves, reduced growth.
Phytoremediation:
Use of hyperaccumulating plants to extract toxic heavy metals (e.g., nickel, lead, cadmium) from contaminated soils (e.g., Streptanthus polygaloides accumulating up to $1\%$ dry mass nickel).
Utilization of Plant Starch, Fibres, and Bioplastics
Plant Fibres:
Long, tough sclerenchyma and xylem fibre bundles with high tensile strength (resistance to breaking under tension).
Extracted via retting (microbial decay of soft surrounding parenchyma) or chemical processing.
Uses: Textiles, clothing (cotton), ropes/twine (flax, jute, hemp, sisal), paper (lignified wood pulp).
Sustainability & Renewable Resources:
Plant materials are renewable (can be grown continuously) and carbon neutral (carbon released on burning/decomposition equals $CO_2$ absorbed during growth).
Replacing non-renewable, oil-based petrochemical plastics with plant-based alternatives reduces fossil fuel depletion and plastic pollution.
Bioplastics:
Plastics derived from renewable biological sources (starch, cellulose) rather than petroleum.
Thermoplastic Starch: Starch extracted from potatoes/maize mixed with gelatin; used for drug capsules and food packaging.
Cellulose-Based Plastics: Made from wood pulp (e.g., Cellophane).
Polylactic Acid (PLA): Biodegradable bioplastic made from fermented maize/sugar beet starch; used for 3D printing, bottles, and cups.
Poly-3-hydroxybutyrate (PHB): Stiff biopolymer used in car parts and bank notes.
Benefits: Biodegradable (broken down by decomposers), renewable.
Drawbacks: High production costs, land-use competition with food crops.
Plant Antimicrobial Properties, Pharmacology, and Drug Testing Protocols
Plant Antimicrobial Defences:
Plants synthesise secondary metabolites (e.g., antiseptics, phenols like gossypol in cotton) to kill invading bacterial and fungal pathogens.
Bacterial Culturing and Aseptic Technique:
Bacteria reproduce rapidly by binary fission under optimal conditions (warmth, moisture, nutrients, $O_2$).
Aseptic Technique: Autoclave glassware/media at $121\,^ ext{C}$ for $15\,min$; flame inoculating loops; work beside a Bunsen burner flame to create updrafts; seal petri dishes with tape (not completely airtight to prevent anaerobic growth); incubate at $25\,^ ext{C}$ in schools to prevent human pathogen growth.
Core Practical 9: Place plant extract filter paper discs on agar lawn inoculated with bacteria. Measure diameter/area of clear zones of inhibition to assess antimicrobial effectiveness.
Development of Drug Testing:
William Withering's Digitalis Soup (1775-1785):
Extracted digitalis (digoxin) from foxgloves (Digitalis) to treat dropsy (oedema).
Unsystematic trial-and-error approach: tested varying doses of powdered foxglove leaf soup on 163 patients, gradually increasing dosage until side-effects (nausea, vomiting) appeared to establish optimal therapeutic dose.
Modern Contemporary Drug Testing Protocol:
Preclinical Testing: Computer modelling, isolated cell/tissue cultures, and animal testing (rodents and non-rodents) to assess safety, toxicity, and efficacy.
Clinical Trial Phase 1: Tested on a small group (20-80) of healthy human volunteers to evaluate safety, metabolism, and side-effects.
Clinical Trial Phase 2: Tested on a small group (100-500) of patient volunteers with the target disease to assess efficacy, dosage, and side-effects.
Clinical Trial Phase 3: Large-scale trial on thousands (1000-5000+) of patient volunteers, comparing the new drug against a placebo or best existing treatment using a double-blind protocol.
Trial Features:
Placebo: Inactive dummy treatment used as an experimental control to account for the placebo effect (psychological improvement).
Double-Blind Trial: Neither the patients nor the administering doctors know who receives the active drug and who receives the placebo, eliminating bias.
Principles of Taxonomy, Binomial Nomenclature, and Classification Systems
Principles of Taxonomy:
Taxonomy is the science of describing, naming, and classifying organisms based on shared characteristics, phenotypes, and genotypes.
Homologous Structures: Structures sharing a common evolutionary origin, reflecting true ancestral relationships (e.g., mammalian pentadactyl limb).
Analogous Features: Features sharing similar functions or appearances but different evolutionary origins (convergent evolution, e.g., wings of birds and insects).
Binomial Nomenclature (Linnaeus):
Two-part Latin name: Genus species (e.g., Homo sapiens, Escherichia coli).
Genus name capitalized; species name lowercase; italicized or underlined.
Taxonomic Hierarchy:
The Three-Domain System (Carl Woese, 1990s):
Based on molecular phylogeny analyzing $16S$ ribosomal RNA (rRNA) sequencing.
Bacteria (Eubacteria): Prokaryotes with $70S$ ribosomes, peptidoglycan cell walls, ester-linked unbranched lipids, 1 RNA polymerase, formylmethionine initiator tRNA.
Archaea (Archaebacteria): Prokaryotes lacking peptidoglycan, with ether-linked branched membrane lipids, complex RNA polymerases, methionine initiator tRNA, $70S$ ribosomes. Include extremophiles.
Eukaryota: Eukaryotes with membrane-bound organelles, $80S$ ribosomes, no peptidoglycan, 3 RNA polymerases, methionine initiator tRNA.
The Six Kingdoms:
Archaebacteria
Eubacteria
Protista (unicellular eukaryotes, algae)
Fungi (chitin walls, heterotrophic, saprophytic)
Plantae (cellulose walls, autotrophic, chloroplasts)
Animalia (no cell walls, heterotrophic, multicellular)
Species Concepts, Molecular Phylogeny, and Domains of Life
Species Concepts:
Morphological Species Concept: Defines species based purely on visible physical appearance and anatomical characteristics. Limitations: sexual dimorphism, phenotypic plasticity, convergent evolution.
Biological / Reproductive Species Concept: A group of organisms with similar characteristics that can interbreed to produce fertile offspring, with gene flow between individuals. Limitations: geographical isolation, asexual reproduction (bacteria), fossil organisms, fertile inter-species hybrids.
Ecological Species Concept: Defined by ecological niche occupied.
Mate-Recognition Species Concept: Defined by unique fertilisation and courtship behaviours.
Genetic Species Concept: Defined by degree of DNA sequence similarity.
Molecular Phylogeny Techniques:
Gel Electrophoresis: Separates DNA/RNA fragments or proteins by size and electrical charge.
DNA / RNA / Protein Sequencing: Comparing base sequences of specific genes (e.g., $16S$ rRNA) or amino acid sequences of conserved proteins (e.g., cytochrome c, fibrinogen, blood pigments) to establish evolutionary divergence.
Bioinformatics: Use of software and computing algorithms to store, organize, and analyze vast genomic and proteomic datasets.
Biodiversity, Endemism, and Ecological Hotspots
Biodiversity Definitions:
The variety of living organisms, including species richness, relative species abundance, genetic diversity, and ecosystem diversity.
Species Richness: The total number of different species present in a defined habitat.
Relative Species Abundance: The relative numbers / evenness of individuals of each species present.
Endemism:
An endemic species is one that evolves in geographical isolation and is found naturally in only one specific geographic location (e.g., koalas in Australia, lemurs in Madagascar, Devils Hole pupfish in Nevada).
Biodiversity Hotspots:
Geographic areas with exceptionally high species richness and high levels of endemism that are severely threatened by human habitat destruction (occupy $15.7\%$ of land, house $77\%$ of terrestrial vertebrates).
High biodiversity supported by stable climate, high ecosystem productivity, and rapid growth/mutation rates.
Quantifying Biodiversity and Assessing Genetic Diversity
Diversity Index Formula: where , , .
A higher $D$ value indicates greater biodiversity, high species evenness, and a stable, resilient ecosystem.
Assessing Genetic Diversity:
Gene Pool: The total collection of all alleles for all genes present in a population at a given time.
Heterozygosity Index:
A higher heterozygosity index indicates greater genetic diversity and population health. Low index indicates severe inbreeding (e.g., cheetahs $0.0004-0.014$).
Ecological Niches and Evolutionary Adaptations
Ecological Niche:
The precise role and position of an organism within its habitat, including its interactions with biotic and abiotic factors, resource utilization, and trophic relationships.
Categories of Adaptation:
Anatomical Adaptations: Structural features of the body (e.g., camel's broad padded feet, long eyelashes, split upper lip; sundew's sticky leaf hairs).
Physiological Adaptations: Internal biochemical and metabolic mechanisms (e.g., seal's diving bradycardia where heart rate slows and blood flow is diverted to brain/heart; camel's $30\%$ dehydration tolerance, concentrated urine, thermoregulation; $Arthrobotrys$ fungus osmotic constriction rings).
Behavioural Adaptations: Learned or instinctive actions (e.g., lizards orienting to sun for thermoregulation; camels huddling together or sitting on legs early in morning; social hunting).
Population Genetics, Allele Frequencies, and the Hardy-Weinberg Principle
Allele Frequencies:
The proportion of a specific allele relative to all alleles for that gene in a population.
Selection pressures (e.g., introduction of warfarin poison) favor advantageous alleles, changing allele frequencies via natural selection.
Hardy-Weinberg Equations:
For a gene with two alleles ($A$ and $a$):
Let $p =$ frequency of dominant allele ($A$).
Let $q =$ frequency of recessive allele ($a$).
Genotype frequencies in the population:
$p^2 =$ frequency of homozygous dominant genotype ($AA$).
$2pq =$ frequency of heterozygous genotype ($Aa$).
$q^2 =$ frequency of homozygous recessive genotype ($aa$).
Conditions for Hardy-Weinberg Equilibrium:
No mutations occur.
Mating is completely random.
Population size is infinitely large.
Population is completely isolated (no immigration/emigration / no gene flow).
No natural selection (all genotypes have equal fitness/fertility).
Deviations from Equilibrium: Caused by selection pressures, non-random mating, small population size, migration, or mutations, driving evolutionary change.
Reproductive Isolation, Speciation, and Genetic Drift
Speciation Process:
Formation of a new species when a population becomes reproductively isolated, preventing gene flow. Differing selection pressures drive divergent natural selection until populations can no longer interbreed to produce fertile offspring.
Isolating Mechanisms:
Geographical Isolation: Physical barriers (rivers, mountains, oceans) divide a population.
Ecological Isolation: Populations occupy different microhabitats within the same area.
Seasonal / Temporal Isolation: Reproductive timing (flowering/mating seasons) shifts.
Behavioural Isolation: Courtship rituals or display changes.
Mechanical Isolation: Anatomical changes in genitalia or flower structure prevent mating/pollination.
Types of Speciation:
Allopatric Speciation: Occurs when populations are geographically separated. Leads to adaptive radiation (rapid diversification to fill vacant niches, e.g., Darwin's finches on Galapagos, Australian marsupials).
Sympatric Speciation: Occurs in the same geographic area without physical separation, driven by behavioural, mechanical, or temporal isolation.
Genetic Drift Phenomena:
Population Bottleneck: A catastrophic event (disaster, disease, hunting) dramatically reduces population size, shrinking the gene pool and altering allele frequencies by chance (e.g., northern elephant seals reduced to 20 individuals; cheetahs).
Founder Effect: A small number of individuals isolate from a main population to establish a new population. The new gene pool reflects only the alleles of the founders, altering allele frequencies (e.g., high prevalence of Ellis-van Creveld syndrome among Amish in Pennsylvania founded by Samuel King's group in 1744; carriers, ).
Conservation Strategies: In-Situ, Ex-Situ, and Sustainable Management
Human Threats to Biodiversity:
Habitat destruction, deforestation, pollution, climate change, overexploitation of biological resources.
Conservation Methods:
In-Situ Conservation: Protecting species within their natural habitats (e.g., national parks, nature reserves, marine protected areas in Costa Rica covering $>25\%$ of land). Preserves natural ecological interactions and evolutionary processes.
Ex-Situ Conservation: Protecting species outside their natural habitats.
Seed Banks: Seeds collected, screened via X-rays, dried, and stored at $-20\,^ ext{C}$ to $-40\,^ ext{C}$ at low relative humidity (e.g., Millennium Seed Bank at Kew storing $80000$ species). Preserves plant genetic diversity compactly and cheaply. Lifespan doubles per $5\,^ ext{C}$ drop in temperature.
Field Gene Banks & Tissue Culture: Used for plants with recalcitrant seeds that cannot be frozen (e.g., International Potato Centre storing $4100$ potato clones).
Captive Breeding Programmes (Zoos): Breeding endangered animals in controlled environments. Uses studbooks to track genetic pedigrees and manage artificial insemination to maximize genetic diversity and prevent inbreeding depression (e.g., black rhinos, California condors, Przewalski's horses).
Frozen Ark: Freezes DNA samples and tissue cells of endangered animal species.
Sustainable Management:
Balancing human resource needs with conservation.
Sustainable Forestry: Selective logging and replanting instead of slash-and-burn clearing.
Sustainable Agriculture: Crop rotation, organic fertilisers, biological pest control.
Ecotourism: Sustainable tourism providing local economic benefits while protecting ecosystems (e.g., Costa Rica $6\%$ GDP from ecotourism).