IGCSE Biology (0610) — Complete Revision Notes (Core & Extended)

<h1 class="notes-h1">IGCSE Biology (0610) — Complete Revision Notes</h1>
<h2 class="notes-h2">Cambridge Assessment International Education</h2>
<h3 class="notes-h3">Syllabus Code: 0610 | Core & Extended</h3>
<hr class="section-divider">
<p><strong>Prepared by:</strong> CBC Edu Kenya | cbcedukenya.com</p>
<p><strong>Syllabus Version:</strong> 0610 (2023–2025 and 2026 onwards)</p>
<p><strong>Coverage:</strong> All topics — Core and Extended</p>
<p><strong>Note:</strong> Original revision notes aligned to the Cambridge IGCSE Biology (0610) syllabus. Not official Cambridge materials.</p>
<hr class="section-divider">
<h2 class="notes-h2">PAPER OVERVIEW</h2>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Paper</th><th>Type</th><th>Duration</th><th>Marks</th><th>Notes</th></tr>
</thead><tbody>
<tr><td>Paper 1</td><td>Core — Multiple choice</td><td>45 min</td><td>40</td><td>40 MCQs</td></tr>
<tr><td>Paper 2</td><td>Core — Structured</td><td>1 hr 15 min</td><td>80</td><td>Short + structured</td></tr>
<tr><td>Paper 3</td><td>Extended — Multiple choice</td><td>45 min</td><td>40</td><td>40 MCQs (harder)</td></tr>
<tr><td>Paper 4</td><td>Extended — Structured</td><td>1 hr 15 min</td><td>80</td><td>Short + structured</td></tr>
<tr><td>Paper 5</td><td>Practical</td><td>1 hr 15 min</td><td>40</td><td>OR</td></tr>
<tr><td>Paper 6</td><td>Alternative to Practical</td><td>1 hr</td><td>40</td><td>Written practical skills</td></tr>
</tbody></table></div>
<p><strong>Extended candidates</strong> sit Papers 3, 4, and 5 (or 6).</p>
<p><strong>Core candidates</strong> sit Papers 1, 2, and 5 (or 6).</p>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 1: CHARACTERISTICS AND CLASSIFICATION OF LIVING ORGANISMS</h2>
<h3 class="notes-h3">1.1 Characteristics of Life (MRS GREN)</h3>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Letter</th><th>Characteristic</th><th>Description</th></tr>
</thead><tbody>
<tr><td>M</td><td>Movement</td><td>Change in position or shape</td></tr>
<tr><td>R</td><td>Respiration</td><td>Release of energy from food</td></tr>
<tr><td>S</td><td>Sensitivity</td><td>Detecting and responding to stimuli</td></tr>
<tr><td>G</td><td>Growth</td><td>Permanent increase in size and complexity</td></tr>
<tr><td>R</td><td>Reproduction</td><td>Producing offspring</td></tr>
<tr><td>E</td><td>Excretion</td><td>Removing waste products of metabolism</td></tr>
<tr><td>N</td><td>Nutrition</td><td>Obtaining and using food</td></tr>
</tbody></table></div>
<h3 class="notes-h3">1.2 Classification of Organisms</h3>
<p><strong>Five kingdoms:</strong> Animals, Plants, Fungi, Protoctists, Prokaryotes (Bacteria)</p>
<p><strong>Viruses</strong> are not classified in any kingdom — they are not living organisms. They can only reproduce inside host cells.</p>
<p><strong>The Plant Kingdom:</strong></p>
<ul class="notes-list">
<li>Ferns, mosses, conifers, flowering plants</li>
<li>All contain chlorophyll and photosynthesise</li>
<li>Cell walls made of cellulose</li>
</ul>
<p><strong>The Animal Kingdom:</strong></p>
<ul class="notes-list">
<li>Vertebrates: fish, amphibians, reptiles, birds, mammals</li>
<li>Invertebrates: insects, worms, molluscs, arachnids, crustaceans, echinoderms</li>
</ul>
<p><strong>Dichotomous keys:</strong> Used to identify organisms. At each step, choose between two features.</p>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 2: ORGANISATION OF THE ORGANISM</h2>
<h3 class="notes-h3">2.1 Cell Structure</h3>
<p><strong>Animal cell structure:</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Organelle</th><th>Function</th></tr>
</thead><tbody>
<tr><td>Cell membrane</td><td>Controls what enters/leaves the cell (selectively permeable)</td></tr>
<tr><td>Nucleus</td><td>Contains DNA; controls cell activities</td></tr>
<tr><td>Cytoplasm</td><td>Jelly-like substance; site of chemical reactions</td></tr>
<tr><td>Mitochondria</td><td>Site of aerobic respiration; produces ATP</td></tr>
<tr><td>Ribosomes</td><td>Site of protein synthesis</td></tr>
</tbody></table></div>
<p><strong>Plant cell structure (has all animal organelles PLUS):</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Organelle</th><th>Function</th></tr>
</thead><tbody>
<tr><td>Cell wall</td><td>Rigid; made of cellulose; provides support</td></tr>
<tr><td>Chloroplasts</td><td>Contain chlorophyll; site of photosynthesis</td></tr>
<tr><td>Permanent vacuole</td><td>Contains cell sap; maintains turgor pressure</td></tr>
</tbody></table></div>
<p><strong>[E] Additional organelles:</strong></p>
<ul class="notes-list">
<li>Rough endoplasmic reticulum (RER): Covered with ribosomes; transports proteins</li>
<li>Smooth endoplasmic reticulum (SER): Lipid synthesis and transport</li>
<li>Golgi apparatus: Packages and secretes proteins</li>
<li>Lysosomes: Contain digestive enzymes</li>
</ul>
<h3 class="notes-h3">2.2 Levels of Organisation</h3>
<p>Cell → Tissue → Organ → Organ system → Organism</p>
<p><strong>Examples:</strong></p>
<ul class="notes-list">
<li>Cardiac muscle cells → cardiac muscle tissue → heart → cardiovascular system → human body</li>
</ul>
<h3 class="notes-h3">2.3 Specialised Cells</h3>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Cell</th><th>Special features</th><th>Function</th></tr>
</thead><tbody>
<tr><td>Red blood cell</td><td>Biconcave disc; no nucleus; haemoglobin</td><td>Carries oxygen</td></tr>
<tr><td>Sperm cell</td><td>Long tail (flagellum); many mitochondria; acrosome</td><td>Fertilisation</td></tr>
<tr><td>Root hair cell</td><td>Long extension; large surface area</td><td>Absorbs water and minerals</td></tr>
<tr><td>Palisade cell</td><td>Many chloroplasts; near surface</td><td>Photosynthesis</td></tr>
<tr><td>Nerve cell (neurone)</td><td>Long axon; myelin sheath</td><td>Transmits impulses</td></tr>
<tr><td>Guard cell</td><td>Kidney-shaped; chloroplasts; control stomatal opening</td><td>Gas exchange in leaves</td></tr>
</tbody></table></div>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 3: MOVEMENT IN AND OUT OF CELLS</h2>
<h3 class="notes-h3">3.1 Diffusion</h3>
<p><strong>Definition:</strong> The net movement of molecules/ions from a region of <strong>higher concentration</strong> to a region of <strong>lower concentration</strong>, down a concentration gradient.</p>
<ul class="notes-list">
<li>Requires no energy (passive)</li>
<li>Continues until equilibrium (equal concentrations)</li>
<li>Examples: oxygen into cells, carbon dioxide out of cells, absorption of digested food in gut</li>
</ul>
<p><strong>Factors affecting rate of diffusion:</strong></p>
<ul class="notes-list">
<li>Steeper concentration gradient → faster diffusion</li>
<li>Higher temperature → faster diffusion (more kinetic energy)</li>
<li>Larger surface area → faster diffusion</li>
<li>Shorter diffusion distance → faster diffusion</li>
<li>Smaller molecules → faster diffusion</li>
</ul>
<h3 class="notes-h3">3.2 Osmosis</h3>
<p><strong>Definition:</strong> The net movement of <strong>water molecules</strong> through a <strong>selectively permeable membrane</strong>, from a region of <strong>lower solute concentration</strong> (higher water potential) to a region of <strong>higher solute concentration</strong> (lower water potential).</p>
<ul class="notes-list">
<li>Requires no energy (passive)</li>
</ul>
<p><strong>Effects on cells:</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Solution</th><th>Animal cell</th><th>Plant cell</th></tr>
</thead><tbody>
<tr><td>Hypotonic (dilute)</td><td>Swells/bursts (lysis)</td><td>Becomes turgid (swells but wall prevents burst)</td></tr>
<tr><td>Isotonic</td><td>No change</td><td>No change</td></tr>
<tr><td>Hypertonic (concentrated)</td><td>Shrinks (crenation)</td><td>Plasmolysis (vacuole shrinks, membrane pulls away from wall)</td></tr>
</tbody></table></div>
<p><strong>Turgor pressure:</strong> The pressure exerted by water on the cell wall when a plant cell is fully swollen. Supports non-woody plants.</p>
<p><strong>Worked Example:</strong> A potato chip is placed in a concentrated sugar solution. Explain what happens.</p>
<ul class="notes-list">
<li>Water in the potato cells has a higher water potential than the solution outside</li>
<li>Water moves out of the cells by osmosis</li>
<li>Cells lose water, vacuole shrinks, the potato chip becomes <strong>soft and limp</strong> (flaccid/plasmolysed)</li>
</ul>
<h3 class="notes-h3">3.3 Active Transport</h3>
<p><strong>Definition:</strong> Movement of molecules or ions from a region of <strong>lower concentration</strong> to <strong>higher concentration</strong>, <strong>against</strong> the concentration gradient, using <strong>energy</strong> from respiration.</p>
<ul class="notes-list">
<li>Requires ATP (energy)</li>
<li>Uses carrier proteins in the membrane</li>
<li>Examples: absorption of mineral ions by root hair cells; glucose absorption in intestine</li>
</ul>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 4: BIOLOGICAL MOLECULES</h2>
<h3 class="notes-h3">4.1 Carbohydrates</h3>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Type</th><th>Examples</th><th>Functions</th></tr>
</thead><tbody>
<tr><td>Simple sugars (monosaccharides)</td><td>Glucose, fructose, galactose</td><td>Immediate energy source</td></tr>
<tr><td>Disaccharides</td><td>Sucrose, maltose, lactose</td><td>Transport form of sugar</td></tr>
<tr><td>Polysaccharides</td><td>Starch, glycogen, cellulose</td><td>Energy storage; structural</td></tr>
</tbody></table></div>
<p><strong>Testing for sugars:</strong></p>
<ul class="notes-list">
<li><strong>Reducing sugars</strong> (glucose, maltose): Benedict's test → orange/brick-red precipitate when heated</li>
<li><strong>Non-reducing sugars</strong> (sucrose): Negative Benedict's → hydrolyse with HCl → neutralise → repeat Benedict's</li>
<li><strong>Starch:</strong> Iodine test → blue-black colour</li>
</ul>
<h3 class="notes-h3">4.2 Proteins</h3>
<ul class="notes-list">
<li>Made from <strong>amino acids</strong> joined by peptide bonds</li>
<li>Functions: enzymes, antibodies, haemoglobin, structural proteins (collagen)</li>
<li><strong>Testing for protein:</strong> Biuret test — add sodium hydroxide then copper sulfate → purple colour</li>
</ul>
<h3 class="notes-h3">4.3 Fats and Oils (Lipids)</h3>
<ul class="notes-list">
<li>Made from <strong>fatty acids and glycerol</strong></li>
<li>Functions: energy storage, insulation, cell membranes, fat-soluble vitamins</li>
<li><strong>Testing for fat:</strong> Emulsion test — shake in ethanol, pour into water → milky-white emulsion</li>
</ul>
<h3 class="notes-h3">4.4 Enzymes</h3>
<p><strong>Definition:</strong> Biological catalysts that speed up chemical reactions without being used up.</p>
<p><strong>Lock and key model:</strong></p>
<ul class="notes-list">
<li>Each enzyme has an <strong>active site</strong> with a specific shape</li>
<li>Only a molecule with the complementary shape (substrate) can fit = <strong>enzyme-substrate complex</strong></li>
<li>Products are released; enzyme is unchanged and reused</li>
</ul>
<p><strong>Factors affecting enzyme activity:</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Factor</th><th>Effect</th></tr>
</thead><tbody>
<tr><td>Temperature</td><td>Rate increases up to optimum (~37°C in humans); above optimum, enzyme denatures (active site changes shape permanently)</td></tr>
<tr><td>pH</td><td>Each enzyme has an optimum pH; away from optimum, activity decreases; extreme pH causes denaturation</td></tr>
<tr><td>Substrate concentration</td><td>Rate increases as substrate increases (more collisions); levels off when all active sites are occupied</td></tr>
<tr><td>Enzyme concentration</td><td>Rate increases as more enzymes are available; levels off when substrate is limiting</td></tr>
</tbody></table></div>
<p><strong>Worked Example:</strong> Explain why a fever above 42°C is dangerous.</p>
<ul class="notes-list">
<li>Enzymes have an optimum temperature of approximately 37°C</li>
<li>Above 42°C, the high temperature causes the enzyme's active site to change shape (denature)</li>
<li>The substrate can no longer fit the active site</li>
<li>Metabolic reactions stop → cells cannot function → organ failure</li>
</ul>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 5: NUTRITION</h2>
<h3 class="notes-h3">5.1 Photosynthesis</h3>
<p><strong>Word equation:</strong> Carbon dioxide + Water → Glucose + Oxygen</p>
<p><strong>Symbol equation:</strong> 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light and chlorophyll)</p>
<p><strong>Conditions needed:</strong></p>
<ul class="notes-list">
<li>Light (energy source)</li>
<li>Chlorophyll (absorbs light)</li>
<li>Carbon dioxide (enters through stomata)</li>
<li>Water (absorbed by roots)</li>
</ul>
<p><strong>Uses of glucose in plants:</strong></p>
<ul class="notes-list">
<li>Respiration (energy)</li>
<li>Converted to starch (storage)</li>
<li>Made into cellulose (cell walls)</li>
<li>Made into amino acids (with mineral salts → proteins)</li>
<li>Made into fats/oils (seeds)</li>
</ul>
<p><strong>Limiting factors for photosynthesis:</strong></p>
<ul class="notes-list">
<li>Light intensity: More light → more photosynthesis (up to a limit)</li>
<li>CO₂ concentration: More CO₂ → more photosynthesis (up to a limit)</li>
<li>Temperature: Rate increases with temperature up to enzyme optimum</li>
</ul>
<p><strong>Leaf adaptations for photosynthesis:</strong></p>
<ul class="notes-list">
<li>Large, flat surface — maximises light absorption</li>
<li>Thin — short diffusion distance for CO₂</li>
<li>Palisade cells near surface — many chloroplasts</li>
<li>Stomata — allow gas exchange</li>
<li>Veins — supply water, remove products</li>
</ul>
<p><strong>[E] The two stages of photosynthesis:</strong></p>
<ul class="notes-list">
<li>Light-dependent stage: in thylakoid membranes; water split → O₂ released; ATP produced</li>
<li>Light-independent stage (Calvin cycle): in stroma; CO₂ fixed; glucose produced using ATP</li>
</ul>
<h3 class="notes-h3">5.2 Mineral Ion Requirements in Plants</h3>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Mineral ion</th><th>Function</th><th>Deficiency symptom</th></tr>
</thead><tbody>
<tr><td>Nitrate (NO₃⁻)</td><td>Making amino acids and proteins</td><td>Stunted growth; yellow older leaves</td></tr>
<tr><td>Magnesium (Mg²⁺)</td><td>Making chlorophyll</td><td>Yellowing of leaves (chlorosis)</td></tr>
<tr><td>Phosphate (PO₄³⁻)</td><td>Making DNA, cell membranes, ATP</td><td>Poor root growth; purple leaves</td></tr>
<tr><td>Potassium (K⁺)</td><td>Enzyme function; guard cell operation</td><td>Yellow-brown leaf edges</td></tr>
</tbody></table></div>
<h3 class="notes-h3">5.3 Human Nutrition</h3>
<p><strong>A balanced diet requires:</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Nutrient</th><th>Function</th><th>Sources</th></tr>
</thead><tbody>
<tr><td>Carbohydrates</td><td>Energy</td><td>Rice, bread, potatoes</td></tr>
<tr><td>Proteins</td><td>Growth and repair</td><td>Meat, fish, eggs, beans</td></tr>
<tr><td>Fats</td><td>Energy storage; insulation</td><td>Butter, oils, nuts</td></tr>
<tr><td>Vitamins</td><td>Various metabolic functions</td><td>Fruits, vegetables</td></tr>
<tr><td>Minerals</td><td>Structural and metabolic</td><td>Dairy, leafy vegetables</td></tr>
<tr><td>Water</td><td>Solvent; transport; temperature regulation</td><td>Drinks, fruits</td></tr>
<tr><td>Dietary fibre</td><td>Prevents constipation; peristalsis</td><td>Whole grains, vegetables</td></tr>
</tbody></table></div>
<p><strong>Vitamin C (ascorbic acid):</strong></p>
<ul class="notes-list">
<li>Needed for healthy connective tissue</li>
<li>Deficiency → scurvy (bleeding gums, slow wound healing)</li>
<li>Source: citrus fruits</li>
</ul>
<p><strong>Vitamin D:</strong></p>
<ul class="notes-list">
<li>Needed for calcium absorption</li>
<li>Deficiency → rickets (soft, curved bones)</li>
<li>Source: sunlight, oily fish, fortified foods</li>
</ul>
<p><strong>Iron:</strong></p>
<ul class="notes-list">
<li>Needed to make haemoglobin</li>
<li>Deficiency → anaemia (fewer red blood cells; fatigue)</li>
<li>Source: red meat, leafy vegetables</li>
</ul>
<p><strong>Calcium:</strong></p>
<ul class="notes-list">
<li>Needed for strong bones and teeth; muscle contraction; blood clotting</li>
<li>Deficiency → weak bones</li>
<li>Source: dairy products, leafy greens</li>
</ul>
<h3 class="notes-h3">5.4 Human Digestive System</h3>
<p><strong>Digestion:</strong> Breaking down large, insoluble food molecules into small, soluble ones that can be absorbed.</p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Structure</th><th>Role</th><th>Enzymes/Secretions</th></tr>
</thead><tbody>
<tr><td>Mouth</td><td>Chewing; starch digestion begins</td><td>Salivary amylase (starch → maltose)</td></tr>
<tr><td>Oesophagus</td><td>Peristalsis moves food to stomach</td><td>None</td></tr>
<tr><td>Stomach</td><td>Churns food; protein digestion begins</td><td>Pepsin (protease); HCl (pH 2; kills bacteria; activates pepsin)</td></tr>
<tr><td>Small intestine (duodenum)</td><td>Main digestion</td><td>Pancreatic juice: amylase, lipase, proteases; bile from liver</td></tr>
<tr><td>Small intestine (ileum)</td><td>Absorption of digested food</td><td>Intestinal enzymes</td></tr>
<tr><td>Large intestine</td><td>Water absorption</td><td>None</td></tr>
<tr><td>Rectum/Anus</td><td>Storage and egestion</td><td>None</td></tr>
</tbody></table></div>
<p><strong>Bile:</strong></p>
<ul class="notes-list">
<li>Made in liver, stored in gall bladder</li>
<li>NOT an enzyme — it emulsifies fats (breaks into small droplets = larger surface area for lipase)</li>
<li>Alkaline — neutralises stomach acid</li>
</ul>
<p><strong>Absorption in the ileum:</strong></p>
<ul class="notes-list">
<li>Villi and microvilli greatly increase surface area</li>
<li>Rich blood supply (capillaries + lacteals)</li>
<li>Thin wall — short diffusion distance</li>
<li>Glucose and amino acids → blood capillaries → portal vein → liver</li>
<li>Fatty acids and glycerol → lymph vessels (lacteals) → lymphatic system</li>
</ul>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 6: RESPIRATION</h2>
<h3 class="notes-h3">6.1 Aerobic Respiration</h3>
<p><strong>Word equation:</strong> Glucose + Oxygen → Carbon dioxide + Water (+ energy)</p>
<p><strong>Symbol equation:</strong> C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ ATP)</p>
<ul class="notes-list">
<li>Occurs in <strong>mitochondria</strong></li>
<li>More efficient than anaerobic respiration</li>
<li>All organisms can respire aerobically</li>
</ul>
<h3 class="notes-h3">6.2 Anaerobic Respiration</h3>
<p><strong>In animals (and humans):</strong></p>
<p>Glucose → Lactic acid (+ small amount of energy)</p>
<ul class="notes-list">
<li>Occurs during intense exercise when O₂ is insufficient</li>
<li>Lactic acid causes muscle fatigue/cramp</li>
<li>Oxygen debt: extra O₂ needed after exercise to break down lactic acid</li>
</ul>
<p><strong>In yeast and some plants:</strong></p>
<p>Glucose → Ethanol + Carbon dioxide (+ small amount of energy)</p>
<ul class="notes-list">
<li>Used in brewing (ethanol) and bread-making (CO₂ makes bread rise)</li>
</ul>
<h3 class="notes-h3">6.3 Gas Exchange in Humans</h3>
<p><strong>The lungs:</strong></p>
<ul class="notes-list">
<li>Alveoli are the site of gas exchange</li>
<li>Adaptations of alveoli:</li>
<li>Very large surface area (~70 m²)</li>
<li>Moist surface — gases dissolve</li>
<li>Thin walls (one cell thick) — short diffusion distance</li>
<li>Rich blood supply — maintains concentration gradient</li>
</ul>
<p><strong>Mechanism of breathing:</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Action</th><th>Diaphragm</th><th>External intercostals</th><th>Ribcage</th><th>Volume</th><th>Pressure</th><th>Air moves</th></tr>
</thead><tbody>
<tr><td>Inhalation</td><td>Contracts (moves down)</td><td>Contract</td><td>Rises/expands</td><td>Increases</td><td>Decreases</td><td>Into lungs</td></tr>
<tr><td>Exhalation</td><td>Relaxes (moves up)</td><td>Relax</td><td>Falls/contracts</td><td>Decreases</td><td>Increases</td><td>Out of lungs</td></tr>
</tbody></table></div>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 7: TRANSPORT</h2>
<h3 class="notes-h3">7.1 Blood and its Components</h3>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Component</th><th>Structure</th><th>Function</th></tr>
</thead><tbody>
<tr><td>Red blood cells</td><td>Biconcave; no nucleus; haemoglobin</td><td>Carry O₂ (as oxyhaemoglobin)</td></tr>
<tr><td>White blood cells</td><td>Irregular shape; nucleus; fewer than RBCs</td><td>Immune defence: phagocytosis and antibody production</td></tr>
<tr><td>Platelets</td><td>Cell fragments; no nucleus</td><td>Blood clotting</td></tr>
<tr><td>Plasma</td><td>Pale yellow liquid</td><td>Transport: dissolved food, CO₂, urea, hormones; distributes heat</td></tr>
</tbody></table></div>
<p><strong>Haemoglobin + Oxygen ⇌ Oxyhaemoglobin</strong></p>
<ul class="notes-list">
<li>Oxygen loading in lungs (high O₂ concentration)</li>
<li>Oxygen unloading at tissues (low O₂ concentration)</li>
</ul>
<h3 class="notes-h3">7.2 The Heart and Circulatory System</h3>
<p><strong>Double circulation:</strong></p>
<ul class="notes-list">
<li>Pulmonary circulation: heart → lungs → heart (oxygenation)</li>
<li>Systemic circulation: heart → body → heart (oxygen delivery)</li>
</ul>
<p><strong>Heart structure:</strong></p>
<ul class="notes-list">
<li>Right side: pumps deoxygenated blood to lungs (pulmonary artery)</li>
<li>Left side: pumps oxygenated blood to body (aorta)</li>
<li>Valves (tricuspid, bicuspid, semilunar): prevent backflow of blood</li>
</ul>
<p><strong>Cardiac cycle:</strong></p>
<ol class="notes-list">
<li>Atria fill with blood (from veins)</li>
<li>Atria contract → blood pushed into ventricles</li>
<li>Ventricles contract → blood pushed out to arteries (valves prevent backflow)</li>
</ol>
<p><strong>Differences between blood vessels:</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Feature</th><th>Artery</th><th>Vein</th><th>Capillary</th></tr>
</thead><tbody>
<tr><td>Direction</td><td>Away from heart</td><td>Towards heart</td><td>Connecting</td></tr>
<tr><td>Wall thickness</td><td>Thick, muscular</td><td>Thin</td><td>One cell thick</td></tr>
<tr><td>Valves</td><td>No (except aorta/pulm. artery)</td><td>Yes</td><td>No</td></tr>
<tr><td>Blood pressure</td><td>High</td><td>Low</td><td>Very low</td></tr>
<tr><td>Lumen size</td><td>Small</td><td>Large</td><td>Tiny</td></tr>
<tr><td>Blood type</td><td>Oxygenated (except pulm. artery)</td><td>Deoxygenated (except pulm. vein)</td><td>Mixed</td></tr>
</tbody></table></div>
<h3 class="notes-h3">7.3 Coronary Heart Disease</h3>
<p><strong>Risk factors:</strong> Fatty diet, high cholesterol, smoking, lack of exercise, stress, high blood pressure, diabetes, genetics</p>
<p><strong>Mechanism:</strong></p>
<ul class="notes-list">
<li>Fatty deposits (atheroma) form in coronary artery walls</li>
<li>Lumen narrows → reduced blood flow to heart muscle</li>
<li>Complete blockage → heart attack (myocardial infarction)</li>
</ul>
<p><strong>Treatments:</strong> Bypass surgery, stent insertion, lifestyle changes, medication (statins)</p>
<h3 class="notes-h3">7.4 Transport in Plants</h3>
<p><strong>Xylem:</strong></p>
<ul class="notes-list">
<li>Carries water and mineral ions from roots to leaves</li>
<li>Made of dead cells forming hollow tubes (no cytoplasm)</li>
<li>Movement driven by transpiration pull (evaporation from leaves)</li>
</ul>
<p><strong>Phloem:</strong></p>
<ul class="notes-list">
<li>Carries sucrose (made in leaves) and amino acids to all parts of the plant</li>
<li>Made of living cells (sieve tubes and companion cells)</li>
<li>Movement = translocation; can be up or down the plant</li>
</ul>
<p><strong>Transpiration:</strong></p>
<ul class="notes-list">
<li>Evaporation of water from leaves through stomata</li>
<li>Creates tension that pulls water up the xylem (transpiration stream)</li>
</ul>
<p><strong>Factors affecting transpiration rate:</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Factor</th><th>Effect on transpiration</th></tr>
</thead><tbody>
<tr><td>Higher temperature</td><td>Increases (more evaporation)</td></tr>
<tr><td>More light</td><td>Increases (stomata open wider)</td></tr>
<tr><td>Higher wind speed</td><td>Increases (removes water vapour, steepens gradient)</td></tr>
<tr><td>Higher humidity</td><td>Decreases (reduces concentration gradient)</td></tr>
</tbody></table></div>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 8: EXCRETION</h2>
<h3 class="notes-h3">8.1 Excretion in Humans</h3>
<p><strong>Excretion:</strong> Removal of metabolic waste products from the body.</p>
<p>(NOT to be confused with egestion = removal of undigested food)</p>
<p><strong>Main excretory organs:</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Organ</th><th>Waste product removed</th></tr>
</thead><tbody>
<tr><td>Kidneys</td><td>Urea, excess water, excess salts</td></tr>
<tr><td>Lungs</td><td>Carbon dioxide, water vapour</td></tr>
<tr><td>Skin</td><td>Small amounts of urea, water, salts (in sweat)</td></tr>
<tr><td>Liver</td><td>Produces urea (from excess amino acids)</td></tr>
</tbody></table></div>
<h3 class="notes-h3">8.2 The Kidney</h3>
<p><strong>Functions:</strong></p>
<ul class="notes-list">
<li>Removes urea from blood</li>
<li>Regulates water content of blood (osmoregulation)</li>
<li>Regulates salt balance</li>
</ul>
<p><strong>Nephron — the functional unit of the kidney:</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Stage</th><th>Process</th><th>What happens</th></tr>
</thead><tbody>
<tr><td>Glomerulus (Bowman's capsule)</td><td>Ultrafiltration</td><td>High pressure forces small molecules into capsule: water, glucose, urea, salts. Large molecules (proteins, cells) stay in blood.</td></tr>
<tr><td>Proximal convoluted tubule</td><td>Selective reabsorption</td><td>ALL glucose reabsorbed; some water and salts reabsorbed</td></tr>
<tr><td>Loop of Henle [E]</td><td>Concentration mechanism</td><td>Creates high-salt medulla; involved in water reabsorption</td></tr>
<tr><td>Collecting duct</td><td>Osmoregulation</td><td>Water reabsorbed under ADH control</td></tr>
</tbody></table></div>
<p><strong>ADH (anti-diuretic hormone) [E]:</strong></p>
<ul class="notes-list">
<li>Produced by pituitary gland</li>
<li>Released when blood water potential falls (dehydration)</li>
<li>Makes collecting duct more permeable → more water reabsorbed → concentrated urine</li>
<li>Low ADH (e.g., after drinking lots of water) → dilute, large-volume urine</li>
</ul>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 9: CO-ORDINATION AND RESPONSE</h2>
<h3 class="notes-h3">9.1 The Nervous System</h3>
<p><strong>Central nervous system (CNS):</strong> Brain + spinal cord</p>
<p><strong>Peripheral nervous system:</strong> All other nerves</p>
<p><strong>Types of neurones:</strong></p>
<ul class="notes-list">
<li><strong>Sensory neurone:</strong> Carries impulses FROM receptors TO CNS</li>
<li><strong>Motor neurone:</strong> Carries impulses FROM CNS TO effectors (muscles or glands)</li>
<li><strong>Relay (connector) neurone [E]:</strong> Connects sensory to motor neurones inside CNS</li>
</ul>
<p><strong>Synapse:</strong></p>
<ul class="notes-list">
<li>Gap between two neurones</li>
<li>Nerve impulse reaches presynaptic knob → releases <strong>neurotransmitter</strong> chemicals</li>
<li>Neurotransmitter diffuses across gap → binds to receptors on postsynaptic membrane → new impulse generated</li>
</ul>
<p><strong>Reflex arc:</strong> Quick, automatic, involuntary response.</p>
<ul class="notes-list">
<li>Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector → Response</li>
<li>Bypasses brain (faster response)</li>
</ul>
<p><strong>Examples of reflexes:</strong> Knee-jerk, pupil light reflex, blinking, withdrawing hand from pain</p>
<h3 class="notes-h3">9.2 The Eye [E]</h3>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Structure</th><th>Function</th></tr>
</thead><tbody>
<tr><td>Cornea</td><td>Refracts (bends) light — most refraction</td></tr>
<tr><td>Lens</td><td>Fine focus; changes shape</td></tr>
<tr><td>Iris</td><td>Controls amount of light entering (pupil size)</td></tr>
<tr><td>Retina</td><td>Contains rod and cone cells; converts light to nerve impulses</td></tr>
<tr><td>Fovea (yellow spot)</td><td>Area of greatest cone concentration; sharpest vision</td></tr>
<tr><td>Blind spot</td><td>Where optic nerve leaves; no receptor cells</td></tr>
<tr><td>Optic nerve</td><td>Carries impulses to brain</td></tr>
</tbody></table></div>
<p><strong>Pupil reflex:</strong></p>
<ul class="notes-list">
<li>Bright light → pupil constricts (radial muscles relax, circular muscles contract)</li>
<li>Dim light → pupil dilates (radial muscles contract, circular muscles relax)</li>
</ul>
<p><strong>Accommodation:</strong></p>
<ul class="notes-list">
<li>Near object → lens becomes fat (more convex) → greater refraction</li>
<li>Far object → lens becomes thin (less convex) → less refraction</li>
</ul>
<h3 class="notes-h3">9.3 Hormones</h3>
<p><strong>Hormones</strong> are chemical messengers secreted by endocrine glands, carried in blood to target organs.</p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Hormone</th><th>Gland</th><th>Effect</th></tr>
</thead><tbody>
<tr><td>Insulin</td><td>Pancreas (β cells)</td><td>Lowers blood glucose; promotes glycogen storage</td></tr>
<tr><td>Glucagon</td><td>Pancreas (α cells)</td><td>Raises blood glucose; promotes glycogen breakdown</td></tr>
<tr><td>Adrenaline</td><td>Adrenal glands</td><td>Prepares body for "fight or flight"; increases heart rate, blood glucose</td></tr>
<tr><td>Oestrogen</td><td>Ovaries</td><td>Female sexual development; thickens uterus lining</td></tr>
<tr><td>Testosterone</td><td>Testes</td><td>Male sexual development; sperm production</td></tr>
<tr><td>ADH</td><td>Pituitary gland</td><td>Water reabsorption in kidneys</td></tr>
<tr><td>Auxin</td><td>Shoot tips</td><td>Cell elongation; phototropism and gravitropism</td></tr>
</tbody></table></div>
<p><strong>Comparison of nervous and hormonal systems:</strong></p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Feature</th><th>Nervous</th><th>Hormonal</th></tr>
</thead><tbody>
<tr><td>Message type</td><td>Electrical impulse</td><td>Chemical (hormone)</td></tr>
<tr><td>Speed</td><td>Very fast</td><td>Slow</td></tr>
<tr><td>Target</td><td>Specific (neurone to effector)</td><td>Widespread (blood to all cells)</td></tr>
<tr><td>Duration</td><td>Short-lived</td><td>Long-lasting</td></tr>
</tbody></table></div>
<h3 class="notes-h3">9.4 Diabetes</h3>
<p><strong>Type 1 Diabetes:</strong></p>
<ul class="notes-list">
<li>Pancreas does not produce enough insulin</li>
<li>Blood glucose remains high after meals</li>
<li>Treatment: regular insulin injections and dietary control</li>
</ul>
<p><strong>Type 2 Diabetes:</strong></p>
<ul class="notes-list">
<li>Body cells become resistant to insulin</li>
<li>Blood glucose not controlled properly</li>
<li>Risk factors: obesity, poor diet, lack of exercise</li>
<li>Treatment: diet and exercise; medication if needed</li>
</ul>
<h3 class="notes-h3">9.5 Hormones in Plants [E]</h3>
<p><strong>Auxin:</strong></p>
<ul class="notes-list">
<li>Produced in shoot tip</li>
<li>Causes cell elongation on the shaded side of stem → shoot bends towards light (phototropism)</li>
<li>In roots: high auxin concentration inhibits growth</li>
</ul>
<p><strong>Tropisms:</strong></p>
<ul class="notes-list">
<li><strong>Phototropism:</strong> Growth response to light</li>
<li>Shoots: positive (grow towards light)</li>
<li>Roots: negative (grow away from light)</li>
<li><strong>Gravitropism (geotropism):</strong> Growth response to gravity</li>
<li>Shoots: negative (grow away from gravity = upward)</li>
<li>Roots: positive (grow towards gravity = downward)</li>
</ul>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 10: REPRODUCTION</h2>
<h3 class="notes-h3">10.1 Cell Division</h3>
<p><strong>Mitosis:</strong></p>
<ul class="notes-list">
<li>Produces 2 genetically identical daughter cells</li>
<li>Same number of chromosomes as parent cell</li>
<li>For: growth, repair, asexual reproduction</li>
</ul>
<p><strong>Meiosis [E]:</strong></p>
<ul class="notes-list">
<li>Produces 4 genetically different daughter cells (gametes)</li>
<li>Half the number of chromosomes (haploid)</li>
<li>Occurs in gonads (testes and ovaries)</li>
</ul>
<h3 class="notes-h3">10.2 Sexual and Asexual Reproduction</h3>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Feature</th><th>Sexual</th><th>Asexual</th></tr>
</thead><tbody>
<tr><td>Parents needed</td><td>Two</td><td>One</td></tr>
<tr><td>Gametes</td><td>Yes (sperm + egg)</td><td>No</td></tr>
<tr><td>Variation</td><td>High (genetic mixing)</td><td>None (clones)</td></tr>
<tr><td>Speed</td><td>Slower</td><td>Faster</td></tr>
<tr><td>Examples</td><td>Most animals and plants</td><td>Bacteria, some plants (runners, bulbs, cuttings)</td></tr>
</tbody></table></div>
<h3 class="notes-h3">10.3 Human Reproduction</h3>
<p><strong>Male reproductive system:</strong></p>
<ul class="notes-list">
<li>Testes: produce sperm and testosterone</li>
<li>Sperm: haploid; long tail for movement; many mitochondria; acrosome to penetrate egg</li>
</ul>
<p><strong>Female reproductive system:</strong></p>
<ul class="notes-list">
<li>Ovaries: produce eggs (ova) and oestrogen/progesterone</li>
<li>Fallopian tubes (oviducts): carry egg to uterus; site of fertilisation</li>
<li>Uterus: site of implantation and foetal development</li>
</ul>
<p><strong>Menstrual cycle (28 days):</strong></p>
<ul class="notes-list">
<li>Day 1-5: Menstruation (uterus lining shed)</li>
<li>Day 1-13: Follicle-stimulating hormone (FSH) → follicle grows → oestrogen produced</li>
<li>Day 14: Ovulation (LH surge triggers egg release)</li>
<li>Day 15-28: Progesterone maintains uterus lining; if no fertilisation → progesterone falls → lining sheds</li>
</ul>
<p><strong>Fertilisation and development:</strong></p>
<ul class="notes-list">
<li>Fertilisation: sperm meets egg in oviduct → zygote formed</li>
<li>Zygote divides by mitosis → embryo → implants in uterus wall</li>
<li>Placenta: exchange of nutrients, oxygen, CO₂, waste between mother and foetus</li>
<li>Amniotic fluid: protects foetus from physical damage</li>
</ul>
<h3 class="notes-h3">10.4 Plant Reproduction</h3>
<p><strong>Sexual reproduction:</strong></p>
<ul class="notes-list">
<li>Pollination: transfer of pollen from anther to stigma</li>
<li>Wind-pollinated: lots of small, light pollen; feathery stigmas; small petals</li>
<li>Insect-pollinated: large, colourful petals; scent; sticky pollen; sticky stigmas</li>
</ul>
<p><strong>Germination conditions:</strong> Water, oxygen, warmth (NOT light)</p>
<p><strong>Asexual reproduction in plants:</strong> Runners (strawberry), bulbs (onions), stem cuttings, tubers (potatoes)</p>
<p><strong>[E] Vegetative propagation:</strong> Taking cuttings or tissue culture allows identical (cloned) plants.</p>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 11: INHERITANCE</h2>
<h3 class="notes-h3">11.1 Key Terms</h3>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Term</th><th>Definition</th></tr>
</thead><tbody>
<tr><td>Gene</td><td>A section of DNA that codes for a protein/characteristic</td></tr>
<tr><td>Allele</td><td>Different forms of a gene</td></tr>
<tr><td>Dominant allele</td><td>Expressed when one or two copies are present</td></tr>
<tr><td>Recessive allele</td><td>Only expressed when two copies are present (homozygous)</td></tr>
<tr><td>Genotype</td><td>The alleles an organism has (e.g., Bb)</td></tr>
<tr><td>Phenotype</td><td>The observable characteristic (e.g., brown eyes)</td></tr>
<tr><td>Homozygous</td><td>Two identical alleles (BB or bb)</td></tr>
<tr><td>Heterozygous</td><td>Two different alleles (Bb)</td></tr>
<tr><td>Locus</td><td>The position of a gene on a chromosome</td></tr>
</tbody></table></div>
<h3 class="notes-h3">11.2 Monohybrid Inheritance</h3>
<p><strong>Punnett squares:</strong> Used to predict offspring genotype ratios.</p>
<p><strong>Worked Example:</strong> Tall (T) is dominant over short (t). Cross Tt × tt:</p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th></th><th>t</th><th>t</th></tr>
</thead><tbody>
<tr><td>T</td><td>Tt</td><td>Tt</td></tr>
<tr><td>t</td><td>tt</td><td>tt</td></tr>
</tbody></table></div>
<ul class="notes-list">
<li>Offspring: 50% Tt (tall), 50% tt (short)</li>
<li>Phenotype ratio: 1 tall : 1 short</li>
</ul>
<p><strong>Worked Example 2:</strong> Cross Tt × Tt:</p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th></th><th>T</th><th>t</th></tr>
</thead><tbody>
<tr><td>T</td><td>TT</td><td>Tt</td></tr>
<tr><td>t</td><td>Tt</td><td>tt</td></tr>
</tbody></table></div>
<ul class="notes-list">
<li>Offspring: 25% TT, 50% Tt, 25% tt</li>
<li>Phenotype ratio: <strong>3 tall : 1 short</strong></li>
</ul>
<h3 class="notes-h3">11.3 Codominance [E]</h3>
<p>Both alleles are expressed in the heterozygote.</p>
<ul class="notes-list">
<li>Example: ABO blood groups (A and B alleles are codominant; O is recessive)</li>
<li>Blood type AB: has both A and B antigens on red blood cells</li>
</ul>
<h3 class="notes-h3">11.4 Sex Determination</h3>
<ul class="notes-list">
<li>Females: XX (two X chromosomes)</li>
<li>Males: XY (one X, one Y)</li>
<li>All eggs contain X; sperm contain either X or Y → sex determined by sperm</li>
</ul>
<p>Expected ratio: 50% male : 50% female (in a large population)</p>
<h3 class="notes-h3">11.5 Sex-Linked Characteristics [E]</h3>
<p>The X chromosome carries genes not found on the Y chromosome.</p>
<p><strong>Haemophilia (X-linked recessive):</strong></p>
<ul class="notes-list">
<li>Xᴴ = normal clotting; Xʰ = haemophilia</li>
<li>Males (XY): only one X → if Xʰ, they have haemophilia</li>
<li>Females (XX): need two Xʰ for haemophilia; Xᴴ Xʰ = carrier (no symptoms, but can pass on)</li>
</ul>
<p><strong>Worked Example:</strong> A carrier female (Xᴴ Xʰ) × normal male (Xᴴ Y):</p>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th></th><th>Xᴴ</th><th>Y</th></tr>
</thead><tbody>
<tr><td>Xᴴ</td><td>Xᴴ Xᴴ</td><td>Xᴴ Y</td></tr>
<tr><td>Xʰ</td><td>Xᴴ Xʰ</td><td>Xʰ Y</td></tr>
</tbody></table></div>
<ul class="notes-list">
<li>25% normal female; 25% carrier female; 25% normal male; 25% haemophiliac male</li>
<li>Probability that a son is haemophiliac = <strong>50%</strong></li>
</ul>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 12: VARIATION AND EVOLUTION</h2>
<h3 class="notes-h3">12.1 Variation</h3>
<p><strong>Continuous variation:</strong> Range of values with no distinct categories (e.g., height, mass). Affected by both genes AND environment.</p>
<p><strong>Discontinuous variation:</strong> Distinct categories (e.g., blood group, sex, tongue rolling). Mainly controlled by genes.</p>
<p><strong>Causes of variation:</strong></p>
<ul class="notes-list">
<li>Genetic: mutation, sexual reproduction (meiosis)</li>
<li>Environmental: diet, exercise, sunlight, temperature</li>
</ul>
<h3 class="notes-h3">12.2 Natural Selection and Evolution</h3>
<p><strong>Charles Darwin's Theory of Natural Selection:</strong></p>
<ol class="notes-list">
<li>Organisms produce more offspring than can survive (<strong>overproduction</strong>)</li>
<li>There is variation within populations</li>
<li>Some variations make individuals better adapted to their environment</li>
<li>Better-adapted individuals are more likely to <strong>survive and reproduce</strong> ("survival of the fittest")</li>
<li>Favourable alleles are passed to offspring</li>
<li>Over many generations, the population changes → <strong>evolution</strong></li>
</ol>
<p><strong>Evolution:</strong> Change in inherited characteristics in a population over many generations.</p>
<p><strong>Evidence for evolution:</strong></p>
<ul class="notes-list">
<li>Fossil record (gradual changes over time)</li>
<li>Molecular biology (similar DNA in related species)</li>
<li>Geographical distribution (organisms evolve differently on isolated islands)</li>
<li>Selective breeding</li>
</ul>
<p><strong>Antibiotic resistance (example of natural selection):</strong></p>
<ol class="notes-list">
<li>Random mutation produces a resistant bacterium</li>
<li>Antibiotic kills non-resistant bacteria</li>
<li>Resistant bacterium survives and reproduces</li>
<li>Resistance gene passed to offspring</li>
<li>Population of resistant bacteria grows</li>
</ol>
<hr class="section-divider">
<h2 class="notes-h2">TOPIC 13: ECOLOGY AND THE ENVIRONMENT</h2>
<h3 class="notes-h3">13.1 Ecosystems and Populations</h3>
<p><strong>Ecosystem:</strong> All living organisms in an area AND their non-living environment interacting together.</p>
<p><strong>Population:</strong> All individuals of one species living in the same area at the same time.</p>
<p><strong>Community:</strong> All populations of all species living in the same area.</p>
<p><strong>Habitat:</strong> The place where an organism lives.</p>
<p><strong>Niche:</strong> The role of an organism in its ecosystem (what it eats, what eats it, where it lives).</p>
<p><strong>Adaptations:</strong></p>
<ul class="notes-list">
<li><strong>Structural:</strong> Body features (e.g., camel's hump, polar bear's thick fur)</li>
<li><strong>Behavioural:</strong> Actions (e.g., hibernation, migration)</li>
<li><strong>Physiological:</strong> Internal processes (e.g., concentrated urine in desert animals)</li>
</ul>
<h3 class="notes-h3">13.2 Food Chains and Webs</h3>
<p><strong>Food chain:</strong> Shows feeding relationships; energy flows from left to right.</p>
<ul class="notes-list">
<li>Plant (producer) → Herbivore (primary consumer) → Carnivore (secondary consumer)</li>
</ul>
<p><strong>Trophic levels:</strong></p>
<ul class="notes-list">
<li>Level 1: Producers (plants, algae — photosynthesise)</li>
<li>Level 2: Primary consumers (herbivores)</li>
<li>Level 3: Secondary consumers (carnivores)</li>
<li>Level 4: Tertiary consumers</li>
</ul>
<p><strong>Energy transfer:</strong></p>
<ul class="notes-list">
<li>Only about <strong>10%</strong> of energy passes from one trophic level to the next</li>
<li>Rest is lost as heat (respiration), waste (faeces), or not eaten</li>
</ul>
<p><strong>Pyramid of numbers vs. pyramid of biomass:</strong></p>
<ul class="notes-list">
<li>Pyramid of numbers can be inverted (one large tree supports many insects)</li>
<li>Pyramid of biomass is always the correct shape (more biomass at base)</li>
</ul>
<h3 class="notes-h3">13.3 Nutrient Cycles</h3>
<p><strong>Carbon cycle:</strong></p>
<ul class="notes-list">
<li>CO₂ removed from air: photosynthesis</li>
<li>CO₂ returned to air: respiration, combustion (burning), decomposition</li>
</ul>
<p><strong>Nitrogen cycle:</strong></p>
<ul class="notes-list">
<li>Nitrogen fixation: N₂ → ammonia (by bacteria in soil/root nodules of legumes)</li>
<li>Nitrification: ammonia → nitrites → nitrates (by nitrifying bacteria)</li>
<li>Assimilation: plants absorb nitrates → amino acids → proteins</li>
<li>Decomposition: dead organisms → ammonia (by decomposers)</li>
<li>Denitrification: nitrates → N₂ gas (by denitrifying bacteria in waterlogged soil)</li>
</ul>
<h3 class="notes-h3">13.4 Human Impact on the Environment</h3>
<p><strong>Deforestation:</strong></p>
<ul class="notes-list">
<li>Causes: agriculture, logging, urbanisation</li>
<li>Effects: habitat loss, loss of biodiversity, increased CO₂ (less photosynthesis), soil erosion, reduced water recycling</li>
</ul>
<p><strong>Greenhouse effect and global warming:</strong></p>
<ul class="notes-list">
<li>Greenhouse gases: CO₂, methane, water vapour</li>
<li>Absorb and re-emit infrared radiation → Earth warms</li>
<li>Increased by burning fossil fuels, deforestation, farming (methane from cattle/rice paddies)</li>
<li>Effects: melting ice caps, sea level rise, extreme weather, loss of habitats</li>
</ul>
<p><strong>Pollution:</strong></p>
<ul class="notes-list">
<li><strong>Water pollution:</strong> Eutrophication — fertilisers → algal bloom → algae die → bacteria decompose → use up O₂ → fish die</li>
<li><strong>Air pollution:</strong> Acid rain — burning fossil fuels → SO₂ and NO → combine with water → H₂SO₄ and HNO₃ → kills trees, acidifies lakes</li>
<li><strong>Pesticide accumulation (bioaccumulation) [E]:</strong> Toxins accumulate at each trophic level → highest concentration in top predators</li>
</ul>
<p><strong>Conservation:</strong></p>
<ul class="notes-list">
<li>Nature reserves, seed banks, captive breeding programmes, international agreements, sustainable forestry/fishing</li>
</ul>
<hr class="section-divider">
<h2 class="notes-h2">PRACTICAL SKILLS GUIDE (Paper 5 / Paper 6)</h2>
<h3 class="notes-h3">Common Practical Techniques</h3>
<p><strong>Staining cells for microscopy:</strong></p>
<ul class="notes-list">
<li>Iodine stain: stains starch blue-black; stains cytoplasm yellow-brown</li>
<li>Methylene blue: stains nuclei blue</li>
</ul>
<p><strong>Microscopy calculations:</strong></p>
<ul class="notes-list">
<li>Magnification = Image size ÷ Actual size</li>
<li>Actual size = Image size ÷ Magnification</li>
<li>Scale bar: measure scale bar on image, divide by its stated length to find magnification</li>
</ul>
<p><strong>Benedict's test (reducing sugars):</strong></p>
<ul class="notes-list">
<li>Add equal volume of Benedict's solution</li>
<li>Heat in water bath (not direct flame) for 2 minutes</li>
<li>Positive: colour change blue → green → yellow → orange → brick red</li>
</ul>
<p><strong>Biuret test (protein):</strong></p>
<ul class="notes-list">
<li>Add sodium hydroxide solution, then copper sulfate solution</li>
<li>Positive: purple/violet colour</li>
</ul>
<p><strong>Iodine test (starch):</strong></p>
<ul class="notes-list">
<li>Add a few drops of iodine solution</li>
<li>Positive: blue-black colour</li>
</ul>
<p><strong>Emulsion test (fat):</strong></p>
<ul class="notes-list">
<li>Dissolve sample in ethanol, pour into water</li>
<li>Positive: milky-white emulsion forms</li>
</ul>
<p><strong>Investigating factors affecting photosynthesis:</strong></p>
<ul class="notes-list">
<li>Method: use pondweed (Elodea), count bubbles per minute, vary light intensity/temperature/CO₂</li>
<li>Control variables: temperature, CO₂ concentration, light colour/type</li>
</ul>
<p><strong>Osmosis in potato investigation:</strong></p>
<ul class="notes-list">
<li>Cut potato chips of equal length/mass</li>
<li>Place in solutions of different concentrations</li>
<li>After 30 minutes: measure length or mass change</li>
<li>Calculate % change in mass</li>
</ul>
<h3 class="notes-h3">Graph Drawing Rules</h3>
<ul class="notes-list">
<li>Title the graph (y-axis variable vs x-axis variable)</li>
<li>Label BOTH axes with variable AND units</li>
<li>Use a suitable scale (covers more than half the grid)</li>
<li>Plot points accurately with × or ●</li>
<li>Draw a smooth best-fit curve OR a ruled straight line (state which is appropriate)</li>
<li>Do NOT join dots with zig-zag lines</li>
</ul>
<hr class="section-divider">
<h2 class="notes-h2">EXAM TECHNIQUE GUIDE</h2>
<h3 class="notes-h3">Command Words in Biology</h3>
<div class="table-wrap"><table class="notes-table">
<thead>
<tr><th>Word</th><th>Meaning</th></tr>
</thead><tbody>
<tr><td>State/Name</td><td>Give a specific word, phrase, or fact — no explanation needed</td></tr>
<tr><td>Describe</td><td>Give a detailed account (what happens, not why)</td></tr>
<tr><td>Explain</td><td>Give reasons (use "because", "so that", "this causes")</td></tr>
<tr><td>Suggest</td><td>Use your knowledge to give a plausible answer (may not have one correct answer)</td></tr>
<tr><td>Compare</td><td>Give similarities AND differences</td></tr>
<tr><td>Calculate</td><td>Work out a numerical answer — show working</td></tr>
<tr><td>Sketch</td><td>Draw a simple diagram showing main features (need not be accurate)</td></tr>
<tr><td>Draw</td><td>Accurate diagram required</td></tr>
<tr><td>Identify</td><td>Name or recognise</td></tr>
<tr><td>Evaluate</td><td>Judge the evidence — give advantages, disadvantages, and a conclusion</td></tr>
</tbody></table></div>
<h3 class="notes-h3">Common Mistakes to Avoid</h3>
<ol class="notes-list">
<li><strong>Osmosis:</strong> NEVER say "water moves from high concentration to low concentration" — say "from low solute concentration to high solute concentration" or "from high water potential to low water potential"</li>
<li><strong>Respiration vs. breathing:</strong> Respiration = chemical process in cells. Breathing = physical process of moving air.</li>
<li><strong>Excretion vs. egestion:</strong> Excretion = removal of metabolic waste. Egestion = removal of undigested food.</li>
<li><strong>Photosynthesis equation:</strong> Plants use CO₂ and release O₂ (opposite to respiration). Don't confuse the two.</li>
<li><strong>Enzymes:</strong> Say they are "denatured" when overheated, NOT "killed" (enzymes are not alive)</li>
<li><strong>Diffusion gradient:</strong> Always describe it as a CONCENTRATION gradient, not a "pressure" gradient</li>
<li><strong>Heredity:</strong> Be precise — alleles, not genes, are dominant or recessive</li>
<li><strong>Evolution:</strong> Natural selection acts on phenotypes (not genotypes)</li>
</ol>
<h3 class="notes-h3">Mark-Winning Phrases</h3>
<ul class="notes-list">
<li>For osmosis: "…by osmosis, down the water potential gradient, through the partially permeable membrane"</li>
<li>For enzymes: "…substrate fits into the active site forming an enzyme-substrate complex"</li>
<li>For natural selection: "…individuals with the advantageous characteristic are more likely to survive, reproduce, and pass on the allele to offspring"</li>
<li>For active transport: "…against the concentration gradient, requiring energy from respiration"</li>
<li>For transpiration: "…evaporation of water from leaves through stomata creates a tension that pulls water up the xylem"</li>
</ul>
<hr class="section-divider">
<h2 class="notes-h2">KEY FORMULAE AND DEFINITIONS</h2>
<pre class="code-block"><code>
FORMULAE
Magnification = Image size ÷ Actual size
Percentage change = (change ÷ original) × 100
Energy efficiency = (energy transferred to next level ÷ energy at previous level) × 100

KEY DEFINITIONS TO MEMORISE
Diffusion: net movement of molecules from high to low concentration (down a gradient)
Osmosis: net movement of WATER through a partially permeable membrane from
low solute concentration to high solute concentration
Active transport: movement against concentration gradient, using ATP
Photosynthesis: production of glucose from CO₂ + H₂O using light energy
Aerobic respiration: breakdown of glucose using oxygen to release ATP
Transpiration: evaporation of water from leaves through stomata
Natural selection: survival and reproduction of individuals best adapted to environment
Evolution: change in inherited characteristics over many generations
</code></pre>
<hr class="section-divider">
<p><em>These notes cover the complete Cambridge IGCSE Biology (0610) syllabus for Core and Extended candidates.</em></p>
<p><em>For official past papers and mark schemes, visit: cambridgeinternational.org</em></p>
<p><em>For full IGCSE Biology revision bundle and practice papers: cbcedukenya.com/igcse-biology</em></p>
<p><em>Free YouTube tutorials: youtube.com/@CBCEduKenya | Playlist: IGCSE Biology (0610)</em></p>
<p><em>CBC Edu Kenya — Complete IGCSE & CBC Revision Materials | cbcedukenya.com</em></p>
<p><em>Price: 100 KSH | M-Pesa accepted | PayPal: $2 USD</em></p>