Definitive Study Guide for IGCSE/O-Level BiologyAly Wael - Dr MRS GRENCharacteristics of Living Organisms and Cellular Structure Taxonomy: The seve.

Characteristics of Living Organisms and Cellular Structure

MRS GREN Taxonomy: The seven fundamental characteristics shared by all living organisms are: 1. Movement: A change in position. 2. Respiration: Chemical reactions within cells that break down nutrient molecules to release energy. 3. Sensitivity: The ability to detect and respond to changes in the environment. 4. Growth: A permanent increase in dry mass by increasing the number and/or size of cells. 5. Reproduction: Process of producing new offspring of the same species. 6. Excretion: The removal of metabolic waste products from the body. 7. Nutrition: The intake of food materials for energy and growth.
Cellular Hierarchy: * Cells: The smallest building unit of any body. The human body contains approximately $7.2 \times 10^9$ cells. * Tissue: A group of cells that are similar in function and work together to perform a specific task. * Organ: A structure made up of different tissues working together to perform a shared function. * Organ System: A group of organs with related functions working together to perform body functions.

Cell Types and Organelles

Structure of Animal Cells: 1. Cell Membrane: A partially permeable barrier that controls what enters and exits the cell and encloses cell contents. 2. Nucleus: The control center containing genetic material including DNA, genes, alleles, and chromosomes. 3. Mitochondria: The site of aerobic respiration for the release of energy in the form of $ATP$ . 4. Rough Endoplasmic Reticulum (RER): Site where ribosomes are attached. 5. Ribosomes: Site of protein synthesis. 6. Small Temporary Vacuole: Stores and transports substances. 1. Cytoplasm: Acts as a solvent and the site for many chemical reactions; holds organelles in place.
Structure of Plant Cells: Contains all animal cell organelles plus: 1. Cell Wall: Made of cellulose; provides mechanical support and protection, preventing the cell from bursting. 2. Chloroplasts: Contain chlorophyll (green pigment) which absorbs sunlight for photosynthesis; often contain starch grains for food storage. 3. Large Permanent Vacuole: Contains cell sap (solution of sugars, water, and salts). It exerts turgor pressure against the cell wall to maintain shape.
Fungi Characteristics: * Usually multicellular and contain nuclei. * Lack chlorophyll. * Reproduce via spores (asexual). * Cell walls are made of chitin (not cellulose). * Store energy as glycogen granules.
Prokaryotes (Bacteria) Characteristics: * Often unicellular. * Lack a nucleus; contain circular DNA instead. * Cell walls made of peptidoglycan ( $murein$ ). * Lack mitochondria. * Contain plasmids: small circular DNA loops containing genes of importance.
Protoctista Characteristics: * Can be multicellular (Kelps) or unicellular (algae). * Contain a nucleus (eukaryotes). * Some have cell walls and chloroplasts (like algae); others lack them and feed on other organisms (like Malaria).
Viruses: Not considered living organisms as they do not perform functions like respiration or excretion until they infect a living organism. They consist of a protein coat for protection and multiply inside host cells.

Specialized Cells and Differentiation

Developmental Process: Human formation begins with the fusion of a sperm and egg to form a zygote ( $2n$ ). Mitotic division forms an embryo consisting of identical embryonic stem cells.
Specialized Cells: * Red Blood Cells (RBCs): Adapted for oxygen transport with a biconcave shape to increase surface area. They contain Hemoglobin to bind oxygen. They lack a nucleus and mitochondria to provide more space for hemoglobin. * Intestinal Cells: Develop microvilli to increase surface area for nutrient absorption. * Nerve Cells: Specialized to transmit electrical messages. * Muscle Cells: Rich in mitochondria to provide energy for movement. * Placenta: Contains stem cells capable of differentiating into types needed for fetal development.

Classification and Binomial Nomenclature

Scientific Principles: Classification involves grouping organisms to facilitate study. * Morphology and Anatomy: Use of external features (morphology) and internal features (anatomy). * DNA Sequencing: Modern method where similarity in DNA base sequences indicates corelated species sharing a recent common ancestor.
Binomial Nomenclature: Every organism has a two-part scientific name: 1. Genus: First part; must begin with a capital letter (e.g., $Homo$ ). 2. Species: Second part; must begin with a small letter (e.g., $sapiens$ ).
Hierarchical Order: $Kingdom$ → $Phylum$ → $Class$ → $Order$ → $Family$ → $Genus$ → $Species$ ( $KPCOFGS$ ).
Five Kingdoms: Animalia, Plantae, Fungi, Prokaryote, Protoctista.
Phylum Arthropods: Features include jointed legs, exoskeleton, and segmented bodies. No backbone. * Insects: Three pairs of jointed legs, one or two pairs of wings, breathe through tracheae. Body parts: Head, Thorax, Abdomen. * Arachnids: Four pairs of jointed legs, breathe through book lungs. Body parts: Cephalothorax, Abdomen. * Crustaceans: More than four pairs of legs ( $5$ or $6$ ), breathe through gills. Often have claws. * Myriapods: Segmented body with one or two pairs of legs per segment.
Other Invertebrate Phyla: * Nematoda: Roundworms with simple structures. * Annelida: Segmented worms with ring-like segments. * Mollusca: Soft, slimy bodies often protected by a calcium carbonate shell; use a muscular foot for locomotion.
Phylum Vertebrates: 1. Fish: Gills, streamlined bodies, scaly wet skin. 2. Amphibia: Moist skin, no scales; aquatic larvae (gills) and terrestrial adults (lungs). External ear drum and webbed digits. 3. Reptiles: Dry scaly skin; eggs have leathery/rubbery shells. 4. Birds: Feathers, beaks, wings, hard-shelled eggs, and scaly legs. 5. Mammals: Fur/hair, mammary glands for milk, give birth to live young. Feature external ear pinnae and internal ear drums. Have different types of teeth.

Movement In and Out of Cells

Diffusion: Net movement of molecules from high concentration to low concentration down a concentration gradient. It is a passive process. * Factors Increasing Rate: Increased temperature (more kinetic energy), steeper concentration gradient, increased surface area to volume ratio, shorter diffusion distance, and smaller molecule size.
Osmosis: Diffusion of water molecules from higher water potential to lower water potential through a partially permeable membrane. * Hypertonic Solutions (Low Water Potential): Animal cells shrink/shrivel; plant cells become flaccid then plasmolysed (cell membrane detaches from cell wall). * Hypotonic Solutions (High Water Potential): Animal cells swell and may burst; plant cells become turgid (cell wall resists pressure to prevent bursting).
Active Transport: Movement of molecules from low to high concentration against the gradient. Requires energy ( $ATP$ ) from mitochondria and uses carrier proteins embedded in the cell membrane.

Biological Molecules and Nutrition

Composition: Human cells are $70\%$ water and $30\%$ organic molecules.
Proteins: Made of Amino Acids ( $C, H, O, N$ ). Vital for growth and tissue repair. * Deficiencies: * Kwashiorkor: Edema (pot belly due to water retention); caused by protein deficiency despite sufficient energy. * Marasmus: Severe malnutrition resulting in muscle wasting; caused by overall calorie/energy deficit.
Carbohydrates: Made of $C, H, O$ (Ratio of $H:O$ is $2:1$ ). Primary energy source. * Monosaccharides: Simple units (glucose, fructose, galactose). * Disaccharides: Two sugars (maltose, lactose, sucrose). * Polysaccharides: Multiple units (starch in plants, glycogen in animals, cellulose in cell walls).
Lipids (Fats): Made of Fatty acids and glycerol ( $C, H, O$ ). Used for insulation and long-term energy storage.
Essential Vitamins and Minerals: * Vitamin A: Vision pigments in retina. Deficiency causes night blindness. Sources: carrots, milk, eggs. * Vitamin C: Healthy skin, blood vessels, and gums; aids healing. Deficiency causes Scurvy (bleeding gums). Sources: citrus fruits, guava. * Vitamin D: Calcium absorption in bones. Deficiency causes Rickets (bowing bones) or Osteoporosis. Sources: fish, cheese, sunlight. * Iron: Needed for hemoglobin in RBCs to transport oxygen. Deficiency causes Anemia (fatigue). * Calcium: Strong bones and teeth; muscle/nerve function. Deficiency causes brittle bones and cramps. * Fiber: Stimulates peristalsis (digestion) and prevents constipation and colon cancer.
Balanced Diet Definition: Consuming a variety of all food nutrients (protein, carbs, lipid, minerals, vitamins, fibers, water) in appropriate proportions relative to age, sex, and activity level.
Food Tests: * Starch: Iodine solution (Orange/Brown → Blue/Black). * Reducing Sugars: Benedict’s reagent + Heat (Blue → Green/Yellow/Orange/Brick Red). * Fat: Alcohol emulsion test (Clear → Milky/Cloudy white). * Protein: Biuret test ( $Potassium\,hydroxide$ + $Copper\,sulfate$ ) (Blue → Purple). * Vitamin C: DPCP (Blue → Colorless/Clear).

Enzymes

Definition: Biological catalysts made of protein that lower activation energy without being used up.
Mechanism: "Lock and Key" model. The substrate fits into the complementary active site of the enzyme to form an enzyme-substrate complex before being converted to products.
Factors Affecting Activity: * Temperature: Rate increases until optimal temperature ( $approx\,37-40\,^{\circ}\text{C}$ ), then decreases drastically as the enzyme denatures (active site changes shape). * pH: Every enzyme has an optimal pH. Deviations lead to denaturation. * Concentration: Higher substrate or enzyme concentration increases the rate until all active sites are saturated.
Industrial Applications: * Biological Washing Powders: Contain proteases to digest insoluble protein stains into soluble amino acids. * Pectinase: Used to break down plant cell pectin to extract more juice from fruit and make it clearer. * Lactase: Used to produce lactose-free milk by hydrolyzing lactose into glucose and galactose. Often immobilized in alginate beads to be reused and keep products enzyme-free.

Human Digestive System

Processes: * Ingestion: Taking food into the mouth. * Mechanical Digestion: Breaking food into smaller pieces without chemical change (teeth, stomach churning, emulsification by bile). * Chemical Digestion: Breakdown of large insoluble molecules into small soluble molecules using enzymes. * Absorption: Nutrients passing through the intestinal lining into the blood. * Assimilation: Absorbed food becoming part of the cells/body. * Egestion: Removal of undigested waste as feces.
Alimentary Canal Components: * Teeth: Incisors (biting), Canines (tearing), Premolars/Molars (grinding). * Stomach: Gastric juice contains Pepsin (protease), $HCl$ (pH $2$ for pepsin and killing bacteria), and Mucus (protection). * Duodenum: Mixes food with pancreatic juice (Amylase, Trypsin, Lipase) and bile (emulsifies fats to increase surface area). Contains $NaHCO_3$ to neutralize stomach acid. * Ileum (Small Intestine): Adapted for absorption with villi/microvilli, thin walls ( $1\,cell\,thick$ ), rich blood supply, and lacteals for fat products. * Liver: Regulates blood glucose (storage as glycogen), performs deamination (breaking down amino acids to urea), and detoxifies toxins.
Cholera: Caused by $Vibrio\,cholerae$ , which releases toxins that move chloride ions into the intestinal lumen, causing water to follow by osmosis, leading to severe watery diarrhea/dehydration. Treated with Oral Rehydration Therapy (ORT).

Immunity and Disease

Pathogen: A disease-causing microorganism (virus, bacteria, fungi).
Body Barriers: * Mechanical: Skin and nose hairs. * Chemical: Stomach acid ( $HCl$ ) and mucus.
Immune Response: * Phagocytosis: Phagocytes (WBCs) engulf and digest pathogens using enzymes. * Antibody Production: Lymphocytes (B-cells) recognize foreign antigens and produce complementary antibodies. * Agglutination: Antibodies cause pathogens to clump together to be easily destroyed. * Active Immunity: Long-term; memory cells are formed. Can be natural (infection) or artificial (vaccines). * Passive Immunity: Short-term; pre-made antibodies are given (mother to baby via placenta/milk or medical injection). No memory cells formed.
Autoimmune Diseases: Immune system attacks its own antigens (e.g., Type 1 Diabetes attacking pancreatic beta cells).
HIV/AIDS: Virus destroys T-helper lymphocytes, crippling the immune system and leading to secondary infections.

Transportation and Circulatory System

Blood Components: * Plasma: Fluid transporting glucose, CO2, urea, and hormones. * Red Blood Cells: Transport oxygen; contain hemoglobin. * White Blood Cells: Phagocytes and lymphocytes for defense. * Platelets: Fragments responsible for clotting.
Blood Clotting: Damaged site → Platelets release Thrombokinase → converts Prothrombin to Thrombin → converts soluble Fibrinogen to insoluble Fibrin (forms a mesh/scab).
Blood Vessels: * Arteries: Thick walls, narrow lumen, high pressure, no valves (except semi-lunar). Carry blood away from the heart. * Veins: Thin walls, wide lumen, low pressure, contain valves to prevent backflow. Carry blood to the heart. * Capillaries: One cell thick; site of gas and nutrient exchange.
The Heart: Four chambers; Left side (LORD: Left Oxygenated, Right Deoxygenated). Left ventricle is thicker as it pumps blood to the whole body. * Coronary Heart Disease (CHD): Blockage of coronary arteries (fatty deposits). Risk factors: diet, smoking, stress, lack of exercise. Prevention: lifestyle changes; Treatment: Aspirin, Angioplasty (stents), Bypass surgery.

Respiration and Gas Exchange

Aerobic Respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}$ .
Anaerobic Respiration: * Humans: Partial breakdown producing Lactic Acid; causes muscle pain. * Yeast: producing Ethanol and $CO_2$ (fermentation).
Breathing Mechanism: * Inhalation: Diaphragm flattens, external intercostals contract, ribcage moves up/out, thorax volume increases, pressure decreases, air rushes in. * Inhaled vs. Exhaled Air: Inhaled air ( $21\%\,O_2$ , $0.04\%\,CO_2$ ); Exhaled air ( $16\%\,O_2$ , $4\%\,CO_2$ ).

Coordination and Homeostasis

Nervous System: Fast coordination. * Reflex Arc: Receptor → Sensory Neuron → Relay Neuron (CNS) → Motor Neuron → Effector (Muscle/Gland). * Synapse: Gap between neurons. Neurotransmitters diffuse from the presynaptic knob to postsynaptic receptors.
Human Eye: * Iris Reflex: In bright light, circular muscles contract and radial muscles relax (pupil constricts). * Accommodation: Focus on near objects (Ciliary muscles contract, suspensory ligaments slacken, lens becomes round/convex).
Endocrine System: Slower, chemical messengers (hormones). * Adrenaline: "Fight or flight"; increases heart/breathing rate, releases glucose. * Blood Glucose Control (Negative Feedback): * High Glucose: Pancreas releases Insulin → Liver converts glucose to glycogen. * Low Glucose: Pancreas releases Glucagon → Liver converts glycogen to glucose.
Excretion and Osmoregulation: * Anti-Diuretic Hormone (ADH): Controls water reabsorption in the kidney collecting ducts. * Ultrafiltration: High pressure in the Glomerulus forces small molecules into the Bowman's capsule. * Selective Reabsorption: Glucose and amino acids are reabsorbed in the proximal convoluted tubule via active transport.

Reproduction and Life Cycles

Human Reproduction: * Menstrual Cycle: Controlled by FSH (follicle development), Estrogen (builds lining), LH (ovulation), and Progesterone (maintains lining). * Fertilization: Occurs in the fallopian tube. Zygote implants in the endometrium. * Amniotic Sac: Produces amniotic fluid to cushion the fetus and maintain temperature.
Plant Reproduction: * Pollination: Transfer of pollen from anther to stigma (Insect vs. Wind pollination). * Seed Germination Requirements: Water, Oxygen, and Warmth (WOW).
Cell Division: * Mitosis: Two genetically identical diploid ( $2n$ ) cells; for growth/repair. * Meiosis: Four genetically non-identical haploid ( $n$ ) cells; for gametes.

Ecology and Environment

Energy Flow: Only $10\%$ of energy is passed to the next trophic level; $90\%$ is lost as heat, excretion, or undigested parts.
Food Pyramids: Pyramids of Number, Biomass, and Energy.
Human Impact: * Deforestation: Leads to soil erosion, habitat loss, and global warming. * Eutrophication: Fertilizer leaches into rivers → algae growth blocks light → plants die → bacteria decompose and use oxygen → fish suffocate. * Global Warming: Greenhouse gases (CO2, Methane, CFCs) trap infrared radiation.

Biotechnology and Genetic Engineering

Antibiotics: Kill bacteria specifically, not viruses. Penicillin produced by the fungus $Penicillium$ in a fermenter.
Genetic Engineering: Using enzymes (restriction enzymes and DNA ligase) to insert human genes (like insulin) into bacterial plasmids to mass-produce substances for medicinal use.

Questions & Discussion

Specific Inquiry: Rhabdostyla Freshwater Adaptation: * Question: Why does the organism remove excess water? * Response: Because the freshwater has a higher water potential than the cell, water enters the cell via osmosis. The contractile vacuole must remove this water to prevent the cell from bursting.
Specific Inquiry: Protease Activity in Quinoa Seeds: * Observation: Enzyme activity peaks at an optimal pH ( $suggested\,as\,pH\,4.0\,in\,the\,graph$ ) and stops at extremes due to denaturation.
Specific Inquiry: Translocation of Sucrose: * Experiment: Results showed that if the phloem is removed (ringing), translocation of sugars from source (leaf) to sink (root) is blocked, proving phloem is responsible for food transport.