Comprehensive Edexcel IGCSE Biology Revision and Study Guide

Characteristics and Classification of Living Organisms

The Unifying Characteristics of Living Organisms

To be classified as a living organism, an entity must fulfill eight specific life processes. These criteria are summarized by the acronym MRS C GREN:

• Movement: An action by an organism or part of an organism causing a change of position or place.

• Respiration: Chemical reactions in cells that break down nutrient molecules to release energy in the form of ATP.

• Sensitivity: The ability to detect or sense stimuli in the internal or external environment and make appropriate responses.

• Control: The ability to control internal conditions to maintain a constant internal environment (homeostasis).

• Growth: A permanent increase in size and dry mass by an increase in cell number, cell size, or both.

• Reproduction: The processes that make more of the same kind of organism.

• Excretion: Removal from organisms of toxic materials, waste products of metabolism, and substances in excess of requirements.

• Nutrition: Taking in of materials for energy, growth, and development.

If an agent does not carry out all eight of these life processes, it is defined as non-living or dead. Viruses are prime examples of non-living biological particles because they do not carry out metabolic reactions or reproduce independently.

Nutrition and Metabolic Strategies

Organisms obtain food to provide energy for essential life processes such as movement, respiration, and excretion.

• Autotrophic Nutrition: Autotrophs (auto = self, trophic = feeding) create their own complex organic food molecules from simple inorganic substances using light energy via photosynthesis.

• Heterotrophic Nutrition: Heterotrophs (hetero = different, trophic = feeding) cannot synthesize their own food and must consume other living or dead organisms to obtain energy, breaking down complex organic compounds via digestion.

Classification of Eukaryotic Organisms

Eukaryotic organisms possess cells containing a distinct nucleus bounded by a nuclear membrane, as well as membrane-bound organelles. Living organisms in the domain Eukarya are grouped into four primary eukaryotic kingdoms:

1. Kingdom Animalia (Animals)

• Multicellular organisms.

• Cells possess a distinct nucleus and membrane, but lack cell walls and chloroplasts.

• Incapable of photosynthesis; feed heterotrophically on organic materials made by other organisms.

• Carbohydrates are stored internally as glycogen.

• Possess nervous coordination and are capable of locomotion.

2. Kingdom Plantae (Plants)

• Multicellular organisms.

• Cells possess distinct nuclei, cell walls made of cellulose, and chloroplasts containing chlorophyll.

• Feed autotrophically via photosynthesis.

• Store carbohydrates as starch or sucrose.

• Lack nervous coordination.

3. Kingdom Fungi

• Usually multicellular (e.g., Mucor, mushrooms), though some are unicellular (e.g., yeast).

• Multicellular fungi are arranged into thread-like structures called hyphae, which contain multiple nuclei and form a network called a mycelium.

• Cell walls are made of chitin rather than cellulose.

• Do not contain chloroplasts and cannot photosynthesize.

• Feed via saprotrophic nutrition: secreting extracellular digestive enzymes onto decaying organic matter and absorbing the resulting simple nutrients.

• Store carbohydrates as glycogen.

4. Kingdom Protoctista (Protoctists)

• Microscopic, predominantly single-celled organisms, though some form larger aggregate colonies or filaments.

• A highly diverse group containing organisms with animal-like features (e.g., Amoeba, Plasmodium) or plant-like features containing chloroplasts and cell walls (e.g., Chlorella).

Prokaryotic Organisms: Kingdom Prokaryotae

Prokaryotic organisms (bacteria) differ fundamentally from eukaryotes:

• Always single-celled and substantially smaller than eukaryotic cells.

• Lack a membrane-bound nucleus; genetic material floats freely in the cytoplasm as a single circular strand of DNA.

• Possess small circular rings of DNA called plasmids.

• Possess a cell membrane, cytoplasm, and a rigid non-cellulose cell wall.

• Lack mitochondria and other membrane-bound organelles.

• Examples include Lactobacillus bulgaricus (rod-shaped, used in yogurt production) and Pneumococcus (spherical pathogen causing pneumonia).

Pathogens

A pathogen is any microorganism that causes disease in another organism. Pathogens span multiple groups:

• Pathogenic Bacteria: Mycobacterium tuberculosis causes tuberculosis (TB) in human lungs; Mycobacterium bovis infects cattle and can transmit to humans.

• Pathogenic Fungi: Black Sigatoka infects banana plant leaves, destroying photosynthetic tissue and leaving black streaks; Tinea pedis causes athlete's foot in humans.

• Pathogenic Protoctists: Plasmodium falciparum is transmitted by mosquitoes and causes severe malaria in humans.

• Viruses: Non-cellular parasitic particles smaller than bacteria. They carry a protein coat surrounding either DNA or RNA, infect every type of living organism, and can only reproduce by hijacking a host cell's metabolic pathways.

  • Tobacco Mosaic Virus (TMV): Causes leaf discoloration in tobacco and tomato plants by destroying chloroplasts, lowering crop yield.

  • Human Immunodeficiency Virus (HIV): Attacks human immune system lymphocytes, leading to Acquired Immunodeficiency Syndrome (AIDS).

  • Influenza Virus: Causes flu in humans by infecting airway epithelial lining cells.

Biological Molecules and Enzyme Catalysis

Structure of Biological Molecules

Organic molecules contain carbon bound to hydrogen and other elements. The three main classes of biological macromolecules are carbohydrates, lipids, and proteins.

Carbohydrates

• Contain the chemical elements Carbon (C), Hydrogen (H), and Oxygen (O).

• Monosaccharides: Simple single sugars, e.g., Glucose ($C_6H_{12}O_6$) and Fructose.

• Disaccharides: Two monosaccharides joined together, e.g., Maltose (two glucose molecules).

• Polysaccharides: Insoluble polymers formed from long chains of monosaccharides, e.g., Starch, Glycogen, and Cellulose.

Lipids (Fats and Oils)

• Contain Carbon (C), Hydrogen (H), and Oxygen (O).

• Basic unit is a triglyceride, composed of one glycerol molecule chemically bonded to three fatty acid chains.

• Divided into fats (solid at room temperature) and oils (liquid at room temperature).

Proteins

• Contain Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), and sometimes Sulfur (S).

• Composed of long polymers of amino acids folded into unique three-dimensional shapes that dictate their specific function (e.g., enzymes, hemoglobin, keratin, antibodies).

Practical Food Tests

Before testing solid food samples, prepare a liquid sample by crushing the specimen with a pestle and mortar, mixing with distilled water, and filtering the mixture.

Test for Reducing Sugars (Glucose)

• Reagent: Benedict's solution.

• Procedure: Add Benedict's reagent to the sample in a test tube and heat in a boiling water bath at $80\text{--}100^\text{o}C$ for $5\text{ minutes}$.

• Result: Positive test changes from blue to green, yellow, or brick-red precipitate.

Test for Starch

• Reagent: Iodine solution.

• Procedure: Add a few drops of iodine solution directly to the sample.

• Result: Positive test changes from orange-brown to blue-black.

Test for Proteins

• Reagent: Biuret reagent (contains sodium hydroxide and copper(II) sulfate).

• Procedure: Add Biuret solution to the liquid sample and mix.

• Result: Positive test changes from blue to violet/purple.

Test for Lipids (Ethanol Emulsion Test)

• Reagent: Ethanol and distilled water.

• Procedure: Dissolve the food sample in $4\text{ cm}^3$ of ethanol, shake thoroughly, then pour the ethanol layer into an equal volume ($4\text{ cm}^3$) of cold distilled water.

• Result: Positive test produces a milky-white cloudy emulsion.

Enzymes as Biological Catalysts

Enzymes are biological catalysts made of protein that speed up metabolic chemical reactions without being permanently altered or consumed.

• Mechanism of Action: Enzymes possess a specifically shaped active site that is complementary in shape to a specific substrate molecule.

• Collision Theory: Random thermal motion causes the enzyme and complementary substrate to collide, forming an enzyme-substrate complex.

• Product Formation: The enzyme catalyzes the cleavage or synthesis of bonds, releasing products that no longer fit the active site. The enzyme remains unchanged and ready to process further substrates.

Factors Affecting Enzyme Function

Temperature

• Low Temperatures: Low kinetic energy results in fewer collisions between substrate and active site per unit time, causing slow reaction rates.

• Optimum Temperature: The temperature at which the rate of reaction is maximum (typically $37^\text{o}C$ in humans). High kinetic energy maximizes successful collisions.

• Above Optimum: Thermal energy disrupts the hydrogen and ionic bonds maintaining the tertiary protein structure of the enzyme. The active site loses its specific complementary shape (denaturation). Substrates can no longer bind, and catalytic activity permanently stops.

Practical Investigation: Temperature and Amylase Activity

Amylase breaks down starch into maltose.

1. Add $5\text{ cm}^3$ of starch solution to a test tube and heat to a designated temperature (20\text{^\text{o}C}, 30\text{^\text{o}C}, 40\text{^\text{o}C}, 50\text{^\text{o}C}, 60\text{^\text{o}C}) using a water bath.

2. Add drops of iodine solution to individual wells of a spotting tile.

3. Add $2\text{ cm}^3$ of amylase solution to the starch, mix, and start a timer immediately.

4. Every minute, transfer a drop of the reaction mixture to a fresh iodine well.

5. Record the time taken for iodine to remain orange-brown (indicating complete digestion of starch).

pH

• Deviation from an enzyme's optimum pH (e.g., pH 2 for stomach pepsin, pH 7-8 for intestinal amylase) alters charges on amino acids and breaks ionic/hydrogen bonds in the protein structure.

• Extreme pH values cause complete denaturation of the active site.

Cell Structure, Differentiation, and Transport

Subcellular Organelles

Eukaryotic cells contain specialized intracellular structures called organelles:

• Nucleus: Contains genetic material (DNA) organized into chromosomes; controls cell activity.

• Cytoplasm: Gel-like fluid where cellular metabolic reactions occur.

• Cell Membrane: Partially permeable phospholipid bilayer containing proteins; regulates the entry and exit of substances.

• Ribosomes: Microscopic structures responsible for protein synthesis.

• Mitochondria: Site of aerobic respiration where ATP is generated.

• Cell Wall (plants, fungi, bacteria): Rigid outer layer (cellulose in plants, chitin in fungi) that provides structural support and prevents osmotic lysis.

• Chloroplasts (plants, some protoctists): Contain chlorophyll to absorb light energy for photosynthesis.

• Permanent Vacuole (plants): Filled with cell sap (water, sugars, salts) to maintain cell turgor.

Levels of Cellular Organization

Living organisms feature hierarchical organization:

1. Organelles: Specialized subcellular components performing specific functions within a cell.

2. Cells: Basic structural and functional units of living organisms.

3. Tissues: Groups of cells with similar structures working together to perform a specific function.

4. Organs: Structures made from a group of different tissues working together to perform specific body functions.

5. Organ Systems: Groups of organs with related functions working together to perform body functions.

Cell Differentiation and Specialization

Cell differentiation is the process by which undifferentiated cells develop specific structural adaptations to perform specialized functions. Gene expression dictates which organelles and structures develop.

• Ciliated Epithelial Cell: Has hair-like extensions (cilia) that beat synchronously to move mucus and trapped dust/pathogens out of the trachea and bronchi.

• Nerve Cell (Neuron): Elongated cells with insulated fatty myelin sheaths to conduct high-speed electrical impulses over long distances.

• Red Blood Cell (Erythrocyte): Lacks a nucleus to maximize space for hemoglobin; possesses a biconcave disc shape to maximize surface area to volume ratio for oxygen diffusion.

• Root Hair Cell: Possesses long hair-like cytoplasmic projections to drastically increase surface area for water absorption by osmosis and mineral ion absorption by active transport.

• Palisade Mesophyll Cell: Columnar shape tightly packed with high concentrations of chloroplasts situated near the upper leaf surface to maximize light absorption for photosynthesis.

Stem Cells in Medicine

A stem cell is an undifferentiated cell capable of dividing indefinitely by mitosis and giving rise to specialized cell types via differentiation.

• Embryonic Stem Cells: Derived from early embryos (e.g., 5-day blastocyst); totipotent or pluripotent, capable of differentiating into almost any cell type in the body.

• Adult Stem Cells: Found in adult tissues such as bone marrow; multipotent, restricted to differentiating into specific cell lineages (e.g., blood cells).

• Meristem Cells: Unspecialized plant stem cells situated in root and shoot tips that retain the ability to differentiate into any plant tissue throughout the plant's entire lifespan.

Mechanisms of Substance Transport

Diffusion

• The net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient as a result of random particle motion.

• Passive process (requires no cellular metabolic energy).

Osmosis

• The net movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) across a partially permeable membrane.

• Effects on Animal Cells:

  • Hypertonic (concentrated) solution: Water leaves the cell by osmosis, causing it to shrink (crenation).

  • Hypotonic (pure water) solution: Water enters the cell by osmosis, causing the cell membrane to stretch and burst (lysis).

• Effects on Plant Cells:

  • Hypertonic solution: Water leaves the vacuole, cell membrane pulls away from the cell wall (plasmolysis), making the tissue flaccid.

  • Hypotonic solution: Water enters the vacuole, pushing the cytoplasm against the cell wall, generating turgor pressure (turgid).

Active Transport

• The movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration against a concentration gradient.

• Requires energy in the form of ATP generated by aerobic respiration in mitochondria, utilizing carrier proteins embedded in the membrane.

Factors Influencing the Rate of Transport

1. Surface Area to Volume Ratio: Larger surface areas relative to volume increase transport rates.

2. Distance: Shorter diffusion distances increase transport rates.

3. Temperature: Higher kinetic energy increases random particle movement, speeding diffusion/osmosis.

4. Concentration Gradient: Steeper concentration differences increase net diffusion rate.

Practical Investigation: Osmosis in Potato Cylinders

1. Cut uniform potato cylinders using a cork borer and measure their initial mass using a balance.

2. Place cylinders into test tubes containing sucrose solutions of varying concentrations ($0.00$, $0.25$, $0.50$, $0.75$, $1.00\text{ mol/dm}^3$).

3. Leave for 4 hours, blot dry with paper towels to remove excess surface liquid, and reweigh to determine final mass.

4. Calculate percentage change in mass:

$\text{Percentage Change in Mass} = \frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \times 100$

Plant Physiology and Nutrition

Photosynthesis

Photosynthesis is an endothermic chemical reaction by which plants synthesize glucose from light energy, carbon dioxide, and water.

Equations for Photosynthesis

• Word Equation: $\text{Carbon Dioxide} + \text{Water} \rightarrow \text{Glucose} + \text{Oxygen}$

• Balanced Chemical Symbol Equation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Environmental Limiting Factors

1. Light Intensity: Increasing light intensity increases photosynthesis rate until another factor (e.g., $CO_2$ or temperature) becomes limiting.

2. Carbon Dioxide Concentration: $CO_2$ is a substrate; increasing its concentration speeds up photosynthesis until saturated.

3. Temperature: Photosynthetic enzymes work faster with increasing kinetic energy up to the optimum temperature; thermal energy above optimum denatures photosynthetic enzymes, drastically dropping the rate.

Leaf Structural Adaptations for Photosynthesis and Gas Exchange

• Waxy Cuticle: Waterproof protective coating preventing transpiration water loss.

• Upper Epidermis: Thin and transparent layer allowing light to pass unhindered to mesophyll tissue.

• Palisade Mesophyll: Column-shaped cells densely packed with chloroplasts situated near the upper leaf surface to maximize light capture.

• Spongy Mesophyll: Loosely packed cells with vast intercellular air spaces to facilitate efficient gas diffusion ($CO_2$, $O_2$, $H_2O$ vapour).

• Vascular Bundle: Contains xylem (transports water and mineral ions from roots to shoots) and phloem (transports sucrose and amino acids from sources to sinks).

• Lower Epidermis: Contains stomatal pores regulated by pairs of guard cells.

Guard Cells and Stomatal Mechanism

• Turgid (Stoma Open): In bright light and ample water, guard cells absorb water by osmosis, swell unevenly due to thick inner cell walls, and bow outwards, opening the stomatal pore for gas exchange.

• Flaccid (Stoma Closed): In darkness or drought, guard cells lose water by osmosis, shrink, and collapse together, closing the pore to prevent desiccation.

Plant Mineral Requirements

• Nitrate Ions ($NO_3^-$): Essential source of nitrogen to synthesize amino acids, proteins, and nucleic acids. Deficiency causes stunted growth and yellow older leaves (chlorosis).

• Magnesium Ions ($Mg^{2+}$): Essential central ion required to synthesize the chlorophyll molecule. Deficiency results in yellowing between leaf veins.

Practical Investigations in Photosynthesis

Testing a Leaf for Starch

1. Boil the leaf in water for $1\text{ minute}$ to kill cell membranes.

2. Boil the leaf in hot ethanol in a water bath for $5\text{--}10\text{ minutes}$ to dissolve and extract green chlorophyll.

3. Dip the leaf in cold water to soften it.

4. Spread on a white tile and add iodine solution.

5. Starch-containing areas turn blue-black; non-photosynthesizing areas remain orange-brown.

Investigating Carbon Dioxide Necessity

De-starch a potted plant in darkness for $24\text{ hours}$. Enclose one leaf in a flask containing potassium hydroxide (absorbs atmospheric $CO_2$) and another leaf in a control flask without potassium hydroxide. Expose to light for several hours and test both leaves for starch. The leaf deprived of $CO_2$ tests negative for starch.

Human Physiology: Digestion, Respiration, and Gas Exchange

Human Balanced Diet and Nutrition

A balanced diet provides all seven essential food groups in correct proportions:

1. Carbohydrates: Primary energy source (e.g., rice, potatoes).

2. Proteins: Growth and tissue repair (e.g., meat, fish, eggs).

3. Lipids: Long-term energy storage and thermal insulation (e.g., butter, oils).

4. Dietary Fibre: Bulk roughage ensuring proper intestinal peristalsis (e.g., whole grains, vegetables).

5. Vitamins:

   • Vitamin A: Photoreceptor pigment formation in retina; deficiency causes night blindness.

   • Vitamin C: Collagen synthesis for skin, gums, and blood vessels; deficiency causes scurvy.

   • Vitamin D: Intestinal calcium absorption; deficiency causes rickets.

6. Minerals:

   • Calcium: Bone and teeth mineralization; deficiency causes osteoporosis.

   • Iron: Central component of hemoglobin for oxygen transport; deficiency causes anemia.

7. Water: Solvent for cellular metabolic reactions and solvent medium for transport.

Human Alimentary Canal and Digestion

Digestion transforms large insoluble macromolecular structures into small soluble molecules for absorption.

• Mouth: Mechanical digestion via mastication; chemical digestion of starch via salivary amylase.

• Oesophagus: Muscular tube pushing food bolus to the stomach via peristalsis (rhythmic, alternating contractions of circular and longitudinal muscle layers).

• Stomach: Churns food mechanically; secretes gastric juice containing hydrochloric acid ($ ext{pH } 1.5 ext{--}2.0$, kills ingested bacteria and provides optimum pH) and pepsin (protease hydrolyzing proteins to peptides).

• Duodenum (First section of small intestine): Receives bile from the gall bladder and pancreatic juice from the pancreas containing amylase, trypsin (protease), and lipase.

• Bile: Produced in the liver, stored in the gall bladder. Functions to:

  1. Neutralize acidic stomach chyme to provide an alkaline $ ext{pH } 8 ext{--}9$ in the small intestine.

  2. Emulsify large lipid droplets into smaller droplets, drastically increasing surface area for lipase action (mechanical process).

• Ileum (Second section of small intestine): Completes digestion via membrane-bound enzymes (maltase, peptidases). Absorbs digested nutrients into blood/lymph.

• Large Intestine (Colon and Rectum): Absorbs excess water from undigested residue, forming faeces stored in the rectum before egestion via the anus.

Adaptations of the Small Intestine for Absorption

The ileum is modified for rapid absorption of soluble monomers:

• Exceptionally long with highly folded internal walls.

• Densely covered in millions of microscopic finger-like projections called villi lined with microvilli.

• Epithelial Wall: One cell thick, reducing diffusion distance.

• Capillary Network: Dense network maintaining steep concentration gradients for glucose and amino acids.

• Lacteal: Central lymphatic vessel absorbing and transporting fatty acids and glycerol.

Practical Investigation: Energy Content of Food (Calorimetry)

1. Measure $25\text{ cm}^3$ of water into a boiling tube held by a clamp stand; measure initial temperature ($T_1$).

2. Weigh the initial mass ($m_{\text{food}}$) of a dry food sample (e.g., peanut, popcorn).

3. Ignite the sample in a Bunsen burner flame and hold $2\text{ cm}$ beneath the water until completely burned.

4. Record the final maximum water temperature ($T_2$).

5. Calculate energy content per gram:

$\text{Energy Transferred per Gram (J/g)} = \frac{\text{Mass of Water (g)} \times \text{Temperature Rise } (^\text{o}\text{C}) \times 4.2\text{ J}/(\text{g}^\text{o}\text{C})}{\text{Mass of Food Sample (g)}}$

Cellular Respiration

Cellular respiration is a metabolic chemical reaction occurring in all living cells to break down glucose and generate chemical energy stored as ATP (adenosine triphosphate).

Aerobic Respiration

• Requires molecular oxygen; complete oxidation of glucose releasing large amounts of ATP ($32\text{--}38\text{ ATP per glucose}$).

• Word Equation: $\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water}$

• Balanced Chemical Symbol Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

Anaerobic Respiration

• Occurs in the absence of molecular oxygen; incomplete breakdown of glucose yielding significantly less ATP ($2\text{ ATP per glucose}$).

• In Animals (Muscle cells during vigorous exercise): $\text{Glucose} \rightarrow \text{Lactic Acid}$ Accumulation of lactic acid lowers muscle cellular pH, inducing enzyme inefficiency and muscle fatigue. Lactic acid is later oxidized in the liver, creating an oxygen debt.

• In Plants and Fungi (Yeast - Fermentation): $\text{Glucose} \rightarrow \text{Ethanol} + \text{Carbon Dioxide}$

Practical Investigation: Demonstrating Respiration Waste Products

Respiration produces $CO_2$ and heat energy. Use hydrogencarbonate indicator solution:

• Red at atmospheric $CO_2$ levels.

• Yellow at high $CO_2$ levels (acidic).

• Purple at low $CO_2$ levels (alkaline).

Human Gas Exchange System

Mechanism of Ventilation

• Inhalation (Inspiration):

  1. External intercostal muscles contract, pulling the rib cage up and out.

  2. Diaphragm contracts and flattens downward.

  3. Thoracic cavity volume increases, causing internal pressure to decrease below atmospheric pressure.

  4. Air is drawn into the lungs down a pressure gradient.

• Exhalation (Expiration):

  1. External intercostal muscles relax, allowing the rib cage to move down and in.

  2. Diaphragm relaxes, returning to its domed shape.

  3. Thoracic cavity volume decreases, causing internal pressure to increase above atmospheric pressure.

  4. Air is forced out of the lungs.

Adaptations of Alveoli for Gas Exchange

• Vast Surface Area: Millions of rounded alveoli create a huge surface area to volume ratio.

• Ultra-Thin Diffusion Barrier: Wall of alveolus and capillary wall are each single layers of flattened squamous cells (distance $< 1 ext{ }\bar{ ext{m}} ext{m}$).

• Moist Surface: Gases dissolve in a thin moisture layer for rapid diffusion.

• Dense Capillary Network: Continuous blood flow maintains steep concentration gradients for $O_2$ and $CO_2$.

Pathological Consequences of Cigarette Smoking

• Nicotine: Addictive drug that narrows blood vessels (vasoconstriction) and increases heart rate, elevating blood pressure.

• Carbon Monoxide ($CO$): Binds irreversibly to hemoglobin to form carboxyhemoglobin, permanently reducing oxygen-carrying capacity.

• Tar: Carcinogen that mutates DNA in respiratory epithelial cells causing lung cancer; paralyzes and destroys cilia, leading to mucus accumulation and chronic bronchitis.

• Emphysema: Frequent infections cause phagocytes to secrete elastase enzyme, destroying elastic fibers in alveolar walls. Alveoli burst and fuse into large damaged chambers with reduced surface area, causing severe breathlessness.

Transport Systems in Plants and Animals

Transport in Plants

Multicellular plants require specialized vascular transport tissues:

• Xylem: Transports water and dissolved inorganic mineral ions unidirectionally from roots to shoots in dead, hollow tubes reinforced with lignin.

• Phloem: Transports soluble organic nutrients (sucrose and amino acids) bidirectionally from source tissue to sink tissue (translocation).

Water Absorption and Transpiration Stream

1. Water enters root hair cells by osmosis down a water potential gradient established by active transport of mineral ions.

2. Water travels across the root cortex to the xylem.

3. Water evaporates from spongy mesophyll cell surfaces into sub-stomatal air spaces and diffuses out open stomata (transpiration).

4. Evaporation creates a tension force pulling cohesive water molecules continuously up xylem vessels (transpiration pull).

Factors Influencing Transpiration Rate

• Light Intensity: High light opens stomata, increasing rate.

• Temperature: High temperature increases kinetic energy of water molecules, increasing evaporation and rate.

• Wind Speed: High airflow blows away saturated water vapor at leaf surface, maintaining steep diffusion gradients and increasing rate.

• Humidity: High external humidity decreases the concentration gradient, decreasing rate.

Measuring Transpiration Rate: Bubble Potometer

A bubble potometer measures water uptake by a cut plant shoot as an indicator of transpiration rate.

$\text{Rate of Transpiration} = \frac{\text{Distance Traveled by Air Bubble (mm)}}{\text{Time (min)}}$

Human Blood and Circulatory System

Components of Human Blood

• Plasma: Pale yellow aqueous fluid transporting carbon dioxide, digested nutrients (glucose, amino acids), urea, hormones, and heat energy.

• Red Blood Cells (Erythrocytes): Biconcave cells loaded with hemoglobin to transport oxygen; lack nuclei to maximize hemoglobin capacity.

• White Blood Cells (Leukocytes): Immune defense cells.

  • Phagocytes: Engulf and digest pathogens non-specifically using lytic enzymes (phagocytosis).

  • Lymphocytes: Produce specific Y-shaped protein antibodies complementary to specific surface antigens on pathogens. Antibodies neutralize pathogens or cause agglutination.

• Platelets: Cell fragments that initiate blood clotting by releasing enzymes converting soluble fibrinogen into insoluble fibrin threads, forming a mesh that traps red blood cells.

Vaccination and Long-Term Immunity

1. A harmless form of a pathogen (dead, attenuated, or antigenic fragment) is injected into the body.

2. Antigens trigger specific lymphocytes to synthesize complementary antibodies and divide rapidly by mitosis.

3. A population of long-lived memory cells remains in circulation.

4. Upon subsequent infection by the live pathogen, memory cells recognize the antigen immediately, producing massive quantities of specific antibodies rapidly (secondary immune response), eliminating the infection before symptoms occur.

Human Heart and Cardiac Cycle

The human heart is a muscular double-pump operating at two pressures:

• Right Side: Receives deoxygenated blood from the body via the vena cava, pumps it through the right atrium and right ventricle out the pulmonary artery to the lungs.

• Left Side: Receives oxygenated blood from the lungs via the pulmonary vein, pumps it through the left atrium and thick muscular left ventricle out the aorta to the body at high pressure.

• Valves: Atrioventricular valves (tricuspid on right, bicuspid on left) prevent backflow into atria; semilunar valves prevent backflow into ventricles.

Coronary Heart Disease (CHD)

Occurs when coronary arteries supplying oxygenated blood to cardiac muscle become narrowed or blocked by atheromatous fatty plaques (cholesterol).

• Partial blockage leads to restricted blood flow and chest pain (angina).

• Complete occlusion causes lack of oxygen and glucose for aerobic respiration, leading to cardiac muscle cell death (myocardial infarction / heart attack).

• Risk Factors: High-fat diet, smoking, high blood pressure, physical inactivity, stress, genetic predisposition.

Blood Vessels Structure

• Arteries: Transport high-pressure blood away from the heart. Thick walls containing smooth muscle and elastic fibers to withstand surge pressures; narrow lumen.

• Veins: Transport low-pressure blood back to the heart. Thin muscular walls; wide lumen; contain internal valves to prevent backflow.

• Capillaries: Microscopic vessels for exchange. Walls are a single cell thick; narrow diameter allowing red blood cells to pass single file.

Homeostasis, Coordination, and Response

Principles of Coordination

• Homeostasis: Maintaining a constant internal environment (e.g., body temperature at 37\text{^\text{o}C}, blood glucose levels, water potential).

• Control System Components: Stimulus $ ightarrow$ Receptor $ ightarrow$ Coordination Centre $ ightarrow$ Effector $ ightarrow$ Response.

Comparison of Control Systems

• Nervous System: High-speed electrical nerve impulses along neurones; short-lived localized responses.

• Endocrine System: Chemical hormone messengers transported in blood plasma; slower acting, long-lasting, widespread target responses.

Plant Tropisms

Plants coordinate directional growth responses called tropisms via plant growth regulators (auxins).

• Phototropism: Growth response to light. Auxin synthesized in shoot tips diffuses down the shaded side of the shoot, causing cells on the shaded side to elongate faster than the illuminated side, bending the shoot toward light (positive phototropism).

• Geotropism (Gravitropism): Growth response to gravity. Shoots show negative geotropism (grow upward); roots show positive geotropism (grow downward).

Human Nervous System and Synapses

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): Sensory and motor nerves connecting CNS to organs.

• Nerve Pathway: Stimulus $ ightarrow$ Sensory Neurone $ ightarrow$ Relay Neurone (in CNS) $ ightarrow$ Motor Neurone $ ightarrow$ Effector (Muscle/Gland).

Synaptic Transmission

1. An electrical impulse arrives at the presynaptic terminal, triggering vesicles to fuse with the presynaptic membrane.

2. Neurotransmitters are released by exocytosis into the microscopic synaptic cleft.

3. Neurotransmitters diffuse across the cleft down a concentration gradient.

4. Neurotransmitters bind to complementary receptor molecules on the postsynaptic membrane, generating a new electrical impulse in the second neurone.

5. Neurotransmitters are rapidly degraded by specific enzymes to prevent continuous stimulation.

Reflex Arc

An involuntary, rapid, automatic response to a harmful stimulus that bypasses conscious brain processing to minimize tissue injury (e.g., pain withdrawal reflex).

The Human Eye

Accommodation (Focusing Mechanism)

• Focusing on Near Objects:

  1. Ciliary muscles contract (ring contracts).

  2. Suspensory ligaments slacken / loosen.

  3. Elastic lens becomes thicker / fatter (more convex).

  4. Light rays are refracted more.

• Focusing on Distant Objects:

  1. Ciliary muscles relax.

  2. Suspensory ligaments tighten / pull taut.

  3. Lens is pulled thin / flat.

  4. Light rays are refracted less.

Pupil Reflex (Light Intensity Response)

• In Bright Light: Circular iris muscles contract, radial iris muscles relax, pupil constricts (narrower) to protect retina from damage.

• In Dim Light: Radial iris muscles contract, circular iris muscles relax, pupil dilates (widens) to admit maximum light.

Human Thermoregulation and Skin Adaptations

The hypothalamus monitors core blood temperature.

• Response to High Temperature (Cooling):

  • Vasodilation: Smooth muscles in arteriole walls relax, dilating arterioles supplying skin capillaries. Increased blood flow to skin surface maximizes heat radiation to environment.

  • Sweating: Sweat glands secrete sweat; heat energy is lost as water evaporates from skin.

  • Flattening Hairs: Hair erector muscles relax, allowing hairs to lie flat, preventing an insulating air layer.

• Response to Low Temperature (Warming):

  • Vasoconstriction: Smooth muscles in arteriole walls contract, constricting skin arterioles and diverting blood flow away from surface capillaries to reduce radiative heat loss.

  • Shivering: Involuntary, rapid skeletal muscle contractions generate heat via exothermic metabolic respiration reactions.

  • Erecting Hairs: Hair erector muscles contract, pulling hairs upright to trap an insulating boundary layer of air.

Human Endocrine System

• Adrenaline (Adrenal Glands): "Fight or flight" hormone; increases heart rate, breathing rate, pupil dilation, and diverts blood to muscles.

• Insulin (Pancreas): Lowers elevated blood glucose levels by stimulating liver and muscle cells to absorb glucose and convert it into stored glycogen.

Human Excretion and Kidney Function

Excretory Organs

Excretion is the removal of metabolic toxic waste products:

• Lungs: Excrete carbon dioxide gas and water vapor from respiration.

• Kidneys: Excrete urea, excess water, and inorganic mineral ions as urine.

• Skin: Excrete water, sodium chloride ions, and trace urea via sweat.

Kidney Anatomy and Nephron Function

Each kidney contains roughly 1 million filtering units called nephrons.

1. Ultrafiltration

• High arterial blood pressure entering the glomerulus capillary bundle forces small molecules (water, glucose, amino acids, salts, urea) across the basement membrane into the Bowman's capsule to form glomerular filtrate.

• Large blood proteins and blood cells are too large to pass and remain in capillaries.

2. Selective Reabsorption of Glucose

• All filtered glucose is selectively reabsorbed back into blood capillaries from the proximal convoluted tubule via active transport against a concentration gradient.

• Epithelial cells lining the proximal convoluted tubule possess microvilli and dense concentrations of mitochondria.

3. Reabsorption of Water and Salts

• Essential mineral salts are reabsorbed by diffusion and active transport along the Loop of Henle.

• Water is reabsorbed by osmosis back into blood capillaries from the collecting duct.

Osmoregulation and ADH Feedback Loop

• Dehydration / High Blood Concentration: Hypothalamus detects low water potential, stimulates pituitary gland to release Antidiuretic Hormone (ADH) into blood. ADH increases the permeability of collecting ducts to water, causing maximum water reabsorption into blood. A small volume of concentrated urine is excreted.

• Overhydration / Low Blood Concentration: Pituitary releases less ADH, collecting duct walls become less permeable to water, less water is reabsorbed. A large volume of dilute urine is excreted.

Molecular Genetics and Inheritance

DNA and RNA Structure

• Genome: The entire genetic material of an organism.

• Gene: A section of DNA molecule that codes for a specific sequence of amino acids to build a specific protein.

• DNA: Double-stranded polynucleotide coiled into a double helix. Composed of a sugar-phosphate backbone attached to nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

  • Base Pairing Rules: A pairs with T, G pairs with C via hydrogen bonds.

• RNA: Single-stranded polynucleotide containing ribose sugar; contains Uracil (U) instead of Thymine.

Protein Synthesis

Stage 1: Transcription (Nucleus)

1. DNA double helix unwinds at the specific gene location.

2. RNA polymerase reads the template strand and builds a complementary single-stranded messenger RNA (mRNA) molecule utilizing free RNA nucleotides ($A-U$, $G-C$).

3. mRNA leaves the nucleus via nuclear membrane pores into the cytoplasm.

Stage 2: Translation (Ribosomes)

1. mRNA binds to a ribosome in the cytoplasm.

2. Transfer RNA (tRNA) molecules carrying specific amino acids arrive at the ribosome.

3. The triplet anticodon on tRNA binds to its complementary codon on the mRNA strand.

4. Amino acids are linked together by peptide bonds to build a growing polypeptide protein chain until a stop codon is reached.

Inheritance and Key Genetic Terminology

• Allele: Alternative variations of a gene.

• Phenotype: The observable physical characteristics of an organism.

• Genotype: The combination of alleles present in an organism's genome.

• Dominant Allele: Allele expressed in the phenotype if present ($BB$ or $Bb$).

• Recessive Allele: Allele expressed in phenotype only when homozygous ($bb$).

• Homozygous: Possessing two identical alleles for a gene ($BB$ or $bb$).

• Heterozygous: Possessing two different alleles for a gene ($Bb$).

• Codominance: Both alleles are expressed equally in the phenotype (e.g., ABO Blood Groups: $I^A$ and $I^B$ are codominant, yielding blood type AB).

• Polygenic Inheritance: Characteristics controlled by multiple interacting genes, yielding continuous variation (e.g., human height, eye color).

Monohybrid Crosses and Genetics Diagrams

Monohybrid inheritance is the inheritance of a single gene characteristic.

Example: Pea Plant Height ($T = ext{Tall}$, $t = ext{Short}$)

Crossing two heterozygous tall pea plants ($Tt imes Tt$):

• Genotypic Ratio: $1 ext{ }TT : 2 ext{ }Tt : 1 ext{ }tt$.

• Phenotypic Ratio: $3 ext{ Tall} : 1 ext{ Short}$ ($75 ext{\%}$ probability tall, $25 ext{\%}$ probability short).

Nuclear Division: Mitosis vs Meiosis

• Diploids ($2n$): Cells containing 23 homologous chromosome pairs ($46$ total in humans).

• Haploids ($n$): Cells containing 1 set of single chromosomes ($23$ total in human gametes).

Mitosis

• Nuclear division yielding two genetically identical diploid daughter cells.

• Functions: Growth, tissue repair, asexual reproduction.

• DNA replicates prior to division; duplicated chromosomes line up individually at the equator, sister chromatids are pulled apart.

Meiosis

• Nuclear division yielding four genetically distinct haploid gametes.

• Functions: Gamete production for sexual reproduction.

• Involves two successive divisions: homologous pairs separate in division one; chromatids separate in division two. Generates vast genetic variation through crossing over and random assortment.

Sex Determination in Humans

Human somatic cells contain $22$ pairs of autosomes and $1$ pair of sex chromosomes ($XX$ in females, $XY$ in males). Fathers pass either an $X$ or $Y$ chromosome in sperm ($50 ext{\%}$ chance male, $50 ext{\%}$ chance female).

Mutations and Evolution

• Mutations: Rare, random changes in DNA base sequences.

  • Types: Insertion (shifts reading frame), Deletion (shifts reading frame), Substitution (alters single codon).

• Mutagens: Ionizing radiation (gamma, X-rays, UV light) and chemical mutagens (tobacco tar) drastically increase mutation frequencies.

• Natural Selection (Darwin's Theory):

  1. Genetic variation exists within a population due to random mutations.

  2. Environmental selection pressures favor individuals possessing advantageous alleles.

  3. Advantageous individuals have higher survival rates (survival of the fittest) and reproduce successfully.

  4. Advantageous alleles are passed to offspring, increasing in frequency over generations.

• Antibiotic Resistance: Excessive antibiotic application selects for resistant mutant bacteria. Non-resistant bacteria die, resistant mutants survive, replicate rapidly, and spread resistance.

Reproduction in Plants and Animals

Sexual vs Asexual Reproduction

• Asexual Reproduction: Reproduction involving one parent, no gamete fusion, producing genetically identical clone offspring via mitosis.

• Sexual Reproduction: Fusion of two haploid nuclei from male and female gametes (fertilization) producing genetically unique diploid zygote offspring.

Plant Reproduction

Insect-Pollinated vs Wind-Pollinated Flowers

• Insect-Pollinated: Large bright petals, sweet nectar, scented, sticky pollen, internal enclosed stamen and stigma.

• Wind-Pollinated: Small dull green petals, no nectar or scent, light smooth airborne pollen produced in huge quantities, feathery exposed stigmas and long dangling filaments hanging outside the flower.

Plant Fertilisation and Fruit Formation

1. Pollen lands on a compatible sticky stigma (pollination).

2. A pollen tube digests its way down through the style into the ovary.

3. The male pollen nucleus travels down the tube and enters the ovule micropyle, fusing with the female ovum nucleus (fertilisation).

4. The fertilised ovule becomes a seed; the surrounding ovary wall develops into a fruit.

Germination Requirements

Germination requires three abiotic conditions (WOW):

1. Water: Rehydrates seed tissues, activates metabolic enzymes.

2. Oxygen: Required for aerobic respiration to generate ATP for seedling growth.

3. Warmth: Provides optimum temperature for metabolic enzyme activity.

Human Reproductive Physiology

Menstrual Cycle Hormonal Control

• FSH (Pituitary): Stimulates egg follicle maturation in ovaries; stimulates oestrogen secretion.

• Oestrogen (Ovaries): Thickens uterus lining; inhibits FSH and stimulates LH surge.

• LH (Pituitary): Triggers ovulation (day 14 egg release); causes empty follicle to form corpus luteum.

• Progesterone (Corpus Luteum): Maintains thick vascularized uterus lining. If fertilisation fails, progesterone levels plummet, causing menstruation.

Pregnancy Support Structures

• Placenta: Interface where fetal and maternal capillaries lie adjacent. Allows diffusion of $O_2$, glucose, amino acids to fetus, and removal of $CO_2$ and urea. Secretes progesterone during pregnancy.

• Amniotic Fluid: Fluid within the amniotic membrane enveloping the fetus, cushioning it against physical impacts.

Ecology, Ecosystems, and Human Environmental Impacts

Ecological Key Definitions

• Population: Group of individuals of the same species living in the same habitat at the same time.

• Community: All the populations of different species living and interacting in a habitat.

• Habitat: The physical place where an organism lives.

• Ecosystem: A distinct self-sustaining unit consisting of all interacting biotic communities and abiotic environmental factors.

Investigating Populations: Quadrats

Quadrats are square frames ($1 ext{ m}^2$) placed randomly (using grid coordinates from a random number generator) to sample non-motile plant/animal populations, determining population density or species percentage cover.

Trophic Energy Transfer and Food Webs

• Trophic Levels: Producer $ ightarrow$ Primary Consumer $ ightarrow$ Secondary Consumer $ ightarrow$ Tertiary Consumer.

• Energy Losses: Only roughly 10% of biomass/energy transfers to the next trophic level.

• Loss Mechanisms: Energy lost as heat during respiration, unconsumed animal tissues (bones, hair), and undigested egested faeces.

$\text{Efficiency of Energy Transfer (\%)} = \frac{\text{Energy in Higher Trophic Level}}{\text{Energy in Lower Trophic Level}} \times 100$

Biotic Interdependence

Population dynamic changes in one species directly affect interdependent species in a food web.

Biogeochemical Cycles

The Carbon Cycle

Process converting carbon between atmospheric $CO_2$ and organic biological matter:

• Photosynthesis: Removes atmospheric $CO_2$.

• Respiration: Releases $CO_2$ back to atmosphere.

• Decomposition: Fungi/bacteria decompose dead material, releasing $CO_2$.

• Combustion: Burning fossil fuels releases $CO_2$.

The Nitrogen Cycle

• Nitrogen-Fixing Bacteria: Convert atmospheric $N_2$ gas into ammonium compounds.

• Nitrifying Bacteria: Convert ammonium into nitrites and then nitrates ($NO_3^-$) for plant absorption.

• Decomposers: Convert nitrogenous proteins/urea into ammonia.

• Denitrifying Bacteria: Convert soil nitrates back into atmospheric $N_2$ gas in anaerobic conditions.

Human Environmental Impacts

Atmospheric Pollution

• Sulfur Dioxide ($SO_2$): Combustion of fossil fuels releases $SO_2$, dissolving in clouds to form acid rain, lowering pH of lakes and leaching aluminum ions, causing fish mortality and defoliation of trees.

• Enhanced Greenhouse Effect: Excessive $CO_2$, methane, nitrous oxides, and CFCs trap infrared radiation in the atmosphere, leading to global warming, ice melting, sea level rise, ocean acidification, and habitat loss.

Water Pollution

• Eutrophication: Agricultural nitrate fertilizers leach into aquatic systems, causing algal blooms. Sunlight is blocked, submerged plants die, decomposers consume dead matter, consuming dissolved oxygen via respiration. Aquatic animals suffocate and die.

Deforestation

Causes loss of biodiversity, soil erosion (roots no longer anchor soil), leaching of soil minerals, disturbance of evapotranspiration water cycles, and elevated atmospheric $CO_2$ levels.

Applied Biotechnology and Food Production

Micropropagation (Plant Tissue Culture)

1. Explants are cut from a desirable parent plant and surface-sterilised.

2. Explants are cultured in vitro on sterile nutrient agar containing glucose, mineral salts, and plant growth hormones (auxins/cytokinins).

3. Cells divide into a mass called a callus, which develops roots and shoots into plantlets.

4. Allows rapid, year-round commercial cloning of disease-free plants with desirable traits.

Animal Cloning Techniques

• Adult Cell Cloning (e.g., Dolly the Sheep):

  1. Nucleus removed from an unfertilised egg cell (enucleation).

  2. Diploid nucleus from an adult somatic body cell inserted into enucleated egg.

  3. Electric shock applied to stimulate mitotic cell division into an embryo.

  4. Embryo implanted into the uterus of a surrogate mother.

Industrial Biotechnology

• Bread Production: Yeast respires anaerobically producing carbon dioxide gas bubbles trapped in gluten dough, causing bread to rise.

• Yogurt Production: Lactobacillus bacteria ferment lactose sugar in pasteurized milk into lactic acid, lowering pH, coagulating milk proteins, and thickening milk into yogurt.

• Industrial Fermenters: Vessels equipped with temperature water jackets, pH probes, sterile air bubblers, and stirring paddles to optimize bacterial growth conditions.

Recombinant DNA and Genetic Engineering

1. Target gene identified and cut out of human DNA using a specific restriction enzyme, leaving complementary single-stranded sticky ends.

2. Bacterial plasmid vector isolated and cut open with the same restriction enzyme.

3. Human gene and opened plasmid mixed with DNA ligase enzyme, joining sticky ends to form a recombinant plasmid.

4. Recombinant plasmid inserted into host bacteria (transgenic organism).

5. Bacteria cultured in industrial fermenters to produce human proteins (e.g., human insulin).

Selective Breeding

Humans select individual animals or plants showing desirable characteristics (e.g., disease resistance, high milk yield) and breed them over successive generations. Inbreeding can reduce the genetic pool, increasing the risk of inheriting harmful recessive genetic defects.