CAMBRIDGE INTERNATIONAL AS LEVEL
AS Biology (9700)
Access structured revision notes, topical questions, and video explanations
tailored for Cambridge 9700 paper preparation.
Chapter 1: Cell Structure
Cambridge International AS Level Biology Interactive Module
Learning Outcomes & History
Welcome to Chapter 1: Cell Structure . In this interactive lesson, you will explore cellular organization, microscopy techniques, and organelle function .
- 1665 - Robert Hooke: Examined cork slices under an early light microscope and coined the word "cell" .
- 1838 - Mathias Schleiden: Suggested all plants are composed of cells.
- 1839 - Theodor Schwann: Proposed all animals are composed of cells, founding Cell Theory.
- 1855 - Rudolf Virchow: Stated that all cells arise from pre-existing cells by cell division.
- 1960s - Lynn Margulis: Proposed the Endosymbiont Theory, suggesting mitochondria and chloroplasts originated as free-living prokaryotes.
Units of Measurement in Cell Biology
| Unit | Symbol | Fraction of a Metre | Representative Structure |
|---|---|---|---|
| Millimetre | mm | 10-3 m | Small insects, tissue layers |
| Micrometre | μm | 10-6 m | Animal/Plant cells (10–40 μm), Mitochondria (~1 μm) |
| Nanometre | nm | 10-9 m | Ribosomes (~25 nm), Cell surface membranes (~7 nm) |
Microscopy, Resolution, and Magnification
Microscopy relies on two critical properties: Magnification (how much larger the image is compared to the real object) and Resolution (the ability to distinguish between two distinct points).
Interactive Magnification Calculator
Calculate Image Size (I), Actual Size (A), or Magnification (M) using I = A \ M:
Interactive Light Beam Pathway Diagram
Light vs. Electron Microscopes
| Feature | Light Microscope (LM) | Electron Microscope (TEM / SEM) |
|---|---|---|
| Source of Radiation | Visible Light (400–700 nm) | Electron Beam (Wavelength ~0.005 nm) |
| Maximum Resolution | 200 nm (1/2 wavelength of light) | 0.5 nm (TEM) / 3–20 nm (SEM) |
| Lenses Used | Glass lenses | Electromagnets |
| Specimen Condition | Living or dead | Dehydrated and dead (vacuum required) |
Interactive Cell Structure Comparison
Toggle between animal and plant cell views to explore their ultrastructure as revealed by transmission electron microscopy.
Organelles & ATP Synthesis Simulation
Organelles allow compartmentalization, enabling efficient division of labour inside eukaryotic cells.
Mitochondrion Respiration Simulation
Mitochondria carry out aerobic respiration, producing ATP on the inner membrane cristae. Click below to simulate ADP phosphorylation into ATP.
Detailed Organelle Reference
| Organelle | Structure Description | Primary Function |
|---|---|---|
| Nucleus & Nucleolus | Enclosed by double-membrane nuclear envelope with pores. Contains chromatin (DNA). | Controls cellular activities; nucleolus synthesizes ribosomes. |
| Rough ER | Flattened membrane sacs (cisternae) studded with 80S ribosomes. | Folds and transports proteins destined for membranes or secretion. |
| Smooth ER | Tubular membrane system lacking ribosomes. | Synthesizes lipids, phospholipids, and steroid hormones. |
| Golgi Body | Stack of flattened, curved sacs continuously forming and budding vesicles. | Modifies, sorts, and packages proteins (e.g., glycosylation). |
| Lysosomes | Single-membrane spherical vesicles containing hydrolytic enzymes. | Breaks down worn-out organelles, foreign material, or pathogen debris. |
| Microtubules & Centrioles | Tubulin protein dimers forming hollow cylinders (25 nm diameter). Centrioles consist of 9 triplets. | Form cytoskeleton, spindle fibers during cell division, and act as MTOCs. |
| Chloroplasts | Double-membrane envelope, stroma, thylakoid stacks (grana) with chlorophyll. | Site of photosynthesis (light-dependent stage on grana; Calvin cycle in stroma). |
Prokaryotic Cells & Viruses
Organisms are divided into prokaryotes and eukaryotes based on nuclear structure and compartmentalization. Viruses represent unique non-cellular biological entities.
Interactive Bacterium Diagram
Prokaryote vs. Eukaryote Comparison
| Feature | Prokaryotes (e.g., Bacteria) | Eukaryotes (Animals, Plants, Fungi) |
|---|---|---|
| Cell Size | Typically 0.5 – 5 μm | Up to 40 μm (1,000x to 10,000x larger volume) |
| Genetic Material | Circular DNA lying free in cytoplasm (naked) | Linear DNA enclosed in double-membrane nucleus with histones |
| Ribosomes | 70S ribosomes (~20 nm) | 80S ribosomes in cytoplasm (~25 nm) |
| Organelles | No non-membrane or double-membrane bound organelles | Extensive membrane-bound organelles (ER, Golgi, Lysosomes, Mitochondria) |
| Cell Wall | Peptidoglycan / Murein | Cellulose (plants) or Chitin (fungi); absent in animals |
Non-Cellular Viruses
Viruses are non-cellular structures ranging from 20 to 300 nm. They consist of a self-replicating molecule of DNA or RNA enclosed within a protective protein coat (capsid) made of capsomeres. All viruses are obligate internal parasites.
Chapter Summary & Self-Assessment
- Microscopy & Resolution: Resolution is limited to half the wavelength of radiation used. Light microscopes resolve down to 200 nm, while electron microscopes resolve down to 0.5 nm due to shorter electron wavelengths.
- Magnification Formula: $A = \frac{I}{M}$ where $I$ is image size, $A$ is actual size, and $M$ is magnification.
- Organelle Functions: Compartmentalization allows specialization. Mitochondria produce ATP via aerobic respiration; chloroplasts carry out photosynthesis; Golgi modifies and packages proteins.
- Prokaryotes vs. Eukaryotes: Prokaryotes lack nuclei and membrane-bound organelles and contain 70S ribosomes and circular DNA. Eukaryotes contain 80S ribosomes, linear DNA inside a nucleus, and an extensive endomembrane system.
Self-Assessment Knowledge Check
Q1: Which structure is present in animal cells but absent from higher plant cells?
Q2: Why can ribosomes (~25 nm) never be seen using a light microscope?
Building Blocks of Life
Living organisms are primarily composed of four elements: Hydrogen, Carbon, Oxygen, and Nitrogen (accounting for >99% of all atoms).
Key Definitions
- Macromolecule: A giant biological molecule (e.g., proteins, polysaccharides, nucleic acids).
- Monomer: A relatively simple repeating subunit used as a building block for larger molecules (e.g., monosaccharides, amino acids, nucleotides).
- Polymer: A giant molecule constructed from many similar repeating monomers linked together in a chain.
Carbohydrates
Carbohydrates contain Carbon, Hydrogen, and Oxygen with a H:O ratio of 2:1 (general formula Cx(H2O)y). They are divided into monosaccharides, disaccharides, and polysaccharides.
Core Definitions
- Monosaccharide: A molecule consisting of a single sugar unit with the general formula (CH2O)n.
- Disaccharide: A sugar molecule consisting of two monosaccharides joined together by a glycosidic bond.
- Polysaccharide: A polymer whose subunits are monosaccharides joined together by glycosidic bonds.
Roles of Monosaccharides in Living Organisms
- Energy Source: Monosaccharides (primarily glucose) are broken down during respiration. Their numerous carbon-hydrogen (C–H) bonds yield significant energy to synthesize ATP from ADP and phosphate.
- Building Blocks: Glucose acts as the precursor for starch, glycogen, and cellulose. Pentoses like ribose form RNA and ATP, while deoxyribose forms DNA.
Glucose & Ring Structures
Monosaccharides like glucose (C6H12O6) form ring structures in solution. In α-glucose, the hydroxyl group (-OH) on Carbon 1 points below the ring; in β-glucose, it points above the ring.
Disaccharides & Glycosidic Bond Formation
Disaccharides form when two monosaccharides combine via a condensation reaction, releasing a water molecule (H2O) and creating a covalent glycosidic bond:
- Maltose: α-glucose + α-glucose (1,4-glycosidic bond). Can repeat continuously to form polysaccharides.
- Sucrose: α-glucose + β-fructose (the primary transport sugar in plants and common household sugar). Fructose features a different ring shape than glucose.
- Lactose: glucose + galactose (the primary sugar in milk, serving as a key nutrient for young mammals).
Testing for Sugars (Box 2.1 Simulation)
Reducing Sugars: Monosaccharides (e.g., glucose) and some disaccharides (e.g., maltose) reduce soluble blue Cu2+ ions in Benedict's reagent to insoluble red-brown Cu+ (copper(I) oxide) precipitate upon heating.
Non-Reducing Sugars: Sucrose gives a negative Benedict's result initially. It must first be hydrolysed with hydrochloric acid (HCl) into monosaccharides, neutralised with sodium hydroxide (NaOH), and then heated with Benedict's reagent.
Lipids
It is difficult to define precisely what we mean by a ‘lipid’ because lipids are a very varied group of chemicals. They are all organic molecules which are insoluble in water. The most familiar lipids are fats and oils. Fats are solid at room temperature and oils are liquid at room temperature – chemically they are very similar. We could say that true lipids are esters formed by fatty acids combining with an alcohol.
Fatty Acids & Saturation
Fatty acids contain an acidic carboxyl group (-COOH) forming the acid 'head', attached to a long hydrocarbon tail made of a chain of carbon atoms combined with hydrogen (often 15 or 17 carbon atoms long).
- Saturated Fatty Acids: Contain no double bonds between carbon atoms in the tail (-C-C-), holding the maximum possible amount of hydrogen. Animal lipids are often saturated and occur as solid fats.
- Unsaturated Fatty Acids: Contain double bonds between carbon atoms (-C=C-). Each double bond creates a kink in the hydrocarbon chain, preventing tight packing and causing lipids to melt more easily. If there is one double bond, it is monounsaturated; if there are multiple, it is polyunsaturated. Plant lipids are often unsaturated and occur as oils (e.g., olive oil, sunflower oil).
Alcohols, Esters & Triglycerides
Alcohols contain a hydroxyl group (-OH) attached to a carbon atom. Glycerol is an alcohol with three hydroxyl groups. When the -COOH group of a fatty acid reacts with an -OH group of an alcohol, an ester bond (-COO-) is formed via a condensation reaction, releasing a water molecule (H2O). This ester can be converted back to an acid and alcohol by hydrolysis.
Triglycerides are formed when glycerol undergoes condensation with three fatty acids, yielding 3 ester bonds and 3 hydrocarbon tails. They are non-polar, hydrophobic, and insoluble in water, but soluble in organic solvents such as ether, chloroform, and ethanol.
Roles of Triglycerides
- Energy Reserves: Richer in C-H bonds than carbohydrates, yielding more energy per unit mass upon oxidation (higher calorific value).
- Thermal Insulation & Buoyancy: Stored below the dermis of the skin and around organs (e.g., kidneys). Blubber in marine mammals provides heat conservation and buoyancy.
- Metabolic Water Source: Respiration oxidises triglycerides to CO2 and H2O. Desert animals (e.g., the kangaroo rat) survive on metabolic water without drinking liquid water.
Phospholipids
Phospholipids are a special type of lipid where one of the three fatty acid tails is replaced by a polar, charged phosphate group. This makes the phosphate head hydrophilic (water-loving) and soluble in water, while the two remaining fatty acid tails are hydrophobic (water-hating).
In cell membranes, phospholipids arrange into a bilayer with hydrophilic heads facing the external/internal watery solutions and hydrophobic tails forming a central layer impermeable to hydrophilic substances.
Proteins
Proteins are an extremely important class of macromolecule in living organisms. More than 50% of the dry mass of most cells is protein. They serve diverse functional and structural roles:
- Enzymes: All enzymes are proteins.
- Cell Membranes: Essential membrane components including receptors and signaling proteins.
- Hormones: Key chemical messengers such as insulin and glucagon.
- Transport Pigments: Oxygen-carrying molecules like haemoglobin and myoglobin.
- Defense: Antibodies that attack and destroy invading microorganisms.
- Structural Components: Collagen in animal tissues (bones, cartilage, blood vessels), keratin in hair, nails, and skin, and actin/myosin for muscle contraction.
- Storage Products: Casein in milk and ovalbumin in egg white.
Structure of Amino Acids
Despite their vast functional diversity, all proteins are built from 20 naturally occurring monomers called amino acids. Each amino acid features a central carbon atom bonded to:
- An amine group (-NH2).
- A carboxylic acid group (-COOH).
- A hydrogen atom.
- A variable R group (side-chain) that determines the unique chemical identity of the amino acid (e.g., glycine has -H as its R group).
Peptide Bond Formation
Two amino acids join together via a condensation reaction. One amino acid loses a hydroxyl group (-OH) from its carboxyl group, while the other loses a hydrogen atom (-H) from its amine group, yielding a water molecule (H2O). The resulting covalent bond between the carbon of the first amino acid and the nitrogen of the second is a peptide bond, forming a dipeptide. Repeating this process forms long chains called polypeptides synthesized at ribosomes. Polypeptides can be broken down back into amino acids by hydrolysis (+H2O) during digestion.
Four Levels of Protein Structure
- Primary Structure: The specific linear sequence of amino acids in a polypeptide chain joined by peptide bonds. Even a single amino acid substitution can completely alter protein properties.
- Secondary Structure: Regular local coiling or folding of the polypeptide chain held by hydrogen bonds between the -CO- and -NH- groups. Common structures include the α-helix (corkscrew shape) and β-pleated sheet.
- Tertiary Structure: The compact, precise 3D shape formed by the complex folding of the already-coiled chain. Held together by four types of bonds between R groups:
- Hydrogen bonds: Form between strongly polar R groups.
- Disulfide bonds: Strong covalent bonds forming between cysteine molecules containing sulfur. Broken by reducing agents.
- Ionic bonds: Form between ionized amine (NH3+) and carboxyl (COO-) groups. Broken by pH changes.
- Hydrophobic interactions: Weak interactions between non-polar R groups forced together away from water.
- Quaternary Structure: The 3D arrangement of two or more polypeptide chains (or non-protein prosthetic groups) working together as a functional protein complex.
Testing for Proteins (Box 2.4 Simulation)
All proteins contain peptide bonds, which feature nitrogen atoms[cite: 1]. These nitrogen atoms form a purple complex with copper(II) ions in alkaline conditions, forming the basis of the Biuret test[cite: 1].
- Two-step Reagent Method: Add dilute potassium hydroxide (KOH) or sodium hydroxide (NaOH), then add dilute copper(II) sulfate solution ($\text{CuSO}_4$)[cite: 1].
- Ready-mixed Biuret Reagent: Contains both copper(II) sulfate and hydroxide pre-mixed, alongside sodium potassium tartrate or sodium citrate to prevent copper hydroxide precipitation[cite: 1].
- Key Procedure Feature: No heating is required[cite: 1]. Color changes slowly from blue to purple over several minutes when protein is present[cite: 1].
Globular vs. Fibrous Proteins
| Property | Globular Proteins (e.g., Haemoglobin) | Fibrous Proteins (e.g., Collagen) |
|---|---|---|
| Shape & Structure | Curl up into a compact, spherical 'ball' shape. Hydrophobic R groups face inward; hydrophilic R groups face outward. | Form long, unbranched strands or triple-stranded helices ('rope-like'). |
| Solubility | Usually soluble in water due to outward-pointing hydrophilic R groups. | Insoluble in water. |
| Biological Roles | Metabolic and physiological roles (enzymes, transport pigments, antibodies). | Structural support and tensile strength (collagen in skin/tendons, keratin in hair). |
Key Case Studies: Haemoglobin & Collagen
- Haemoglobin (Globular Protein): Consists of 4 polypeptide chains (2 α-globin and 2 β-globin chains). Each chain contains an iron-containing haem group (prosthetic group) that reversibly binds one O2 molecule (carrying up to 8 oxygen atoms per haemoglobin). In sickle cell anaemia, a single amino acid substitution (polar glutamic acid replaced by non-polar valine) on the surface of the β chain reduces solubility, making red blood cells sickle-shaped.
- Collagen (Fibrous Protein): Makes up 25% of total mammal protein. Formed of 3 helical polypeptide chains wound into a 'triple helix' held by hydrogen and covalent bonds. Every third amino acid is glycine (the smallest amino acid), allowing tight coiling. Molecules lie side-by-side with staggered ends, cross-linked by covalent bonds to form strong fibrils and fibres capable of withstanding high tensile stress.
Water & Dipolar Properties
Water (H2O) is a polar molecule due to unequal electron sharing between oxygen (δ-) and hydrogen (δ+). This creates hydrogen bonds between adjacent molecules.
Key Biological Properties of Water
- Solvent: Polar/ionic substances dissolve easily as water molecules cluster around them.
- High Specific Heat Capacity: Stores heat energy well, buffering cells against sharp temperature fluctuations.
- High Latent Heat of Vaporisation: Provides an effective cooling mechanism via evaporation (sweating/transpiration).
- Cohesion & Surface Tension: Allows continuous water column transport in plant xylem and allows organisms to stand on water.
- Density Anomaly: Ice is less dense than liquid water below 4°C, forming an insulating surface layer for aquatic life.
Lesson Summary & Biochemical Tests
A complete overview of the core biological molecules, structural subunits, bond types, and identification assays.
| Molecule | Monomer / Subunits | Chemical Linkage | Biochemical Test |
|---|---|---|---|
| Reducing Sugars (e.g., Glucose) | Monosaccharide | N/A | Heat with Benedict's Reagent → Blue to Brick-Red Precipitate |
| Non-Reducing Sugars (e.g., Sucrose) | Disaccharide | Glycosidic Bond | Hydrolyse with HCl, neutralise with NaOH, heat with Benedict's → Brick-Red |
| Starch | α-Glucose | α-1,4 & α-1,6 Glycosidic | Add Iodine Solution → Orange-Brown to Blue-Black |
| Lipids | Glycerol + Fatty Acids | Ester Bond | Emulsion Test: Dissolve in Ethanol, pour into Water → Cloudy White Emulsion |
| Proteins | Amino Acids | Peptide Bond | Add Biuret Reagent → Blue to Purple (No Heat)[cite: 1] |
Mode of Action & Activation Energy
Enzymes are biological catalysts—globular proteins that speed up metabolic reactions without being consumed in the process . They operate both intracellularly (inside cells) and extracellularly (outside cells, like digestive enzymes) .
The bombardier beetle produces an explosive defense spray by using catalase and peroxidase enzymes to decompose hydrogen peroxide and oxidize hydroquinone instantaneously, generating boiling, high-pressure fluid .
Lock & Key vs. Induced Fit Model
While the initial Lock and Key hypothesis suggested a completely rigid fit between the substrate and active site , modern evidence supports the Induced Fit hypothesis . The active site and substrate adjust their shape slightly upon binding to optimize catalytic interaction .
Interactive Activation Energy Diagram
Enzymes lower the activation energy (Ea) required to reach the transition state, allowing reactions to occur rapidly at normal body temperatures .
Enzyme Kinetics (Vmax and Km)
As substrate concentration [S] increases, reaction velocity (v) rises because active sites are occupied more frequently . When all active sites are saturated, the reaction reaches its maximum theoretical velocity (Vmax) .
Michaelis-Menten Constant (Km)
The Michaelis-Menten constant (Km) is the substrate concentration at which the reaction velocity is half of Vmax .
- Low Km: High affinity for substrate; reaches Vmax at lower substrate concentrations .
- High Km: Low affinity for substrate; requires higher substrate concentrations to reach saturation .
Factors Affecting Activity & Inhibition
Types of Reversible Inhibition
- Competitive Inhibition: Inhibitor resembles the substrate shape and competes directly for binding at the active site . Overcome by increasing [S] .
- Non-Competitive Inhibition: Inhibitor binds to an allosteric site elsewhere on the enzyme, distorting active site tertiary structure . Cannot be overcome by adding more substrate .
Immobilised Enzymes
Immobilisation involves trapping enzymes in inert matrices (such as calcium alginate beads) . This industrial technique allows enzymes to be reused easily and prevents product contamination .
Lactase enzyme is immobilized inside sodium alginate beads packed into a column . Milk passing through the column has its lactose hydrolyzed into glucose and galactose, yielding lactose-free milk without enzyme contamination .
Advantages of Enzyme Immobilisation
- Reusability: Enzymes are easily recovered and used in repeated industrial cycles, cutting operational costs .
- Product Purity: Products do not contain enzyme residue, eliminating downstream filtration steps .
- Enhanced Thermal & pH Stability: Matrix entrapment limits structural movement, making enzymes resistant to denaturation from heat and pH shifts .
Chapter Summary
1. Mechanism & Catalysis
Enzymes lower reaction activation energy . Substrates bind to active sites forming temporary enzyme-substrate complexes according to the induced fit model .
2. Kinetic Metrics
Vmax is maximum reaction rate at full active site saturation . Km is the substrate concentration at ½ Vmax; lower Km indicates higher enzyme affinity .
3. Rate Factors
Activity is influenced by enzyme concentration, substrate concentration, pH, and temperature . High temperatures or extreme pH denature tertiary structure .
4. Inhibition & Applications
Competitive inhibitors compete for the active site . Non-competitive inhibitors alter active site shape from afar . Immobilised enzymes in alginate beads increase stability and reusability .
Structure of Membranes
The phospholipid bilayer is visible using an electron microscope at high magnifications (≥ ×100,000). The double black line represents the hydrophilic heads of the two phospholipid layers, while the pale middle zone shows the non-polar hydrophobic interior. The total thickness of the membrane is about 7 nm on average.
Features of the Fluid Mosaic Model (Singer & Nicolson, 1972)
Key Components & Properties
- Phospholipids: Hydrophilic heads face outward toward aqueous media; non-polar hydrophobic tails point inward. Fluidity increases with unsaturated fatty acids and shorter tail lengths.
- Intrinsic (Integral) Proteins: Embedded within the membrane. Transmembrane proteins span the entire bilayer with hydrophobic regions aligning with fatty acid tails.
- Extrinsic (Peripheral) Proteins: Bound to inner/outer surfaces.
- Glycoproteins & Glycolipids: Branching carbohydrate chains acting as cell markers and receptors.
- Cholesterol: Regulates fluidity and mechanical stability.
Interactive Phospholipid Behavior
Select an environmental configuration to render structural orientation:
Figure 4.2: Phospholipids in water: monolayer, micelle, bilayer, and compartment.
Cell Signalling
Cell signalling is the essential biological process by which living organisms control and coordinate their bodies across cells, tissues, and systems.
Core Principles & Transduction
- Definition: Signalling is getting a message from one location to another, enabling appropriate response mechanisms.
- Transduction: Converting an original stimulus or signal into a transmitted message crossing cell surface membranes.
- Transport Distances: Short distances via local diffusion or long distances through vascular systems.
- Stimulus Diversity: Internal signals (hormones) or external factors (light, drugs).
→
1. Hydrophobic Signalling Molecules
Hydrophobic molecules, such as lipid-soluble steroid hormones (e.g., oestrogen), diffuse directly across the non-polar hydrophobic core.
- They bind directly to intracellular receptors in the cytoplasm or nucleus.
- Example: Oestrogen receptors reside within the nucleus and directly regulate gene expression.
2. Water-Soluble Signalling Molecules
Water-soluble signals cannot cross the lipid bilayer. They bind to external protein receptors embedded in the cell surface membrane.
- Specific Recognition: Receptors feature precise binding sites matching specific signalling molecules.
- Conformational Change: Alters receptor shape, transmitting the signal across the membrane.
G Proteins, Second Messengers & Signalling Cascades
A conformational change in a membrane-spanning receptor activates a membrane-bound G protein, which acts as a molecular switch.
- G Protein Mechanism: Switch mechanism involves binding to guanosine triphosphate (GTP).
- Second Messengers: Stimulate enzymes to release small, soluble molecules rapidly diffusing through cytoplasm.
- Signal Amplification: A single activated receptor produces many second messenger molecules, magnifying the signal exponentially.
Alternative Receptor Mechanisms
Besides second messengers, receptors alter cell activity through three primary mechanisms:
- Ion Channel Gating: Opening ion channels to alter membrane potential.
- Membrane-Bound Enzymes: Acting directly as an enzyme upon ligand binding.
- Intracellular Receptors: Direct control of nuclear gene expression.
Direct Cell-to-Cell Contact
Apart from secreted chemicals, direct contact between adjacent cell surface membranes serves as a vital signalling mechanism (Figure 4.8).
- Embryonic Development: Guides cell differentiation and tissue organization.
- Immune Recognition: Lymphocytes detect foreign antigens directly on target cells.
Movement of Substances Into and Out of Cells
The phospholipid bilayer forms an effective barrier against water-soluble molecules and ions. Exchange occurs via five mechanisms: diffusion, facilitated diffusion, osmosis, active transport, and bulk transport.
1. Simple Diffusion
Diffusion is the net movement of molecules or ions down a concentration gradient resulting from random motion until equilibrium is reached.
Steeper gradients result in a higher difference in molecular passing rates, increasing diffusion speed.
Higher temperatures increase kinetic energy, accelerating molecular movement.
Larger exchange surfaces allow more simultaneous crossings (e.g., microvilli, cristae).
Non-polar molecules dissolve in non-polar tails and cross rapidly. Small size helps water diffuse quickly.
Practical Investigations & Interactive Simulations
Visking tubing allows small molecules like glucose to pass through while blocking large macromolecules like starch.
Environmental stress damages membranes, causing pigment leakage via diffusion.
Tests how surface-area-to-volume ratios limit transport rates in larger structures.
2. Facilitated Diffusion
Diffusion through transport proteins providing hydrophilic pathways for large polar molecules or charged ions.
- Channel Proteins: Water-filled gated pores for charged ions.
- Carrier Proteins: Change shape to alternate binding sites between membrane sides.
Clinical Application: Cystic Fibrosis
Caused by a defect in channel proteins moving chloride ions (Cl-) out of lung cells, impairing fluid balance.
3. Osmosis
Osmosis is diffusion of water from a dilute solution to a concentrated solution across a partially permeable membrane.
- Initial State: Solution A is dilute; Solution B is concentrated. Large solute molecules bounce back.
- Net Movement: Water moves randomly, with net movement from A to B.
- Equilibrium: Solution A concentrates while B dilutes, increasing fluid volume in B.
Water Potential, Active Transport & Bulk Transport
1. Water Potential (Ψ) Fundamentals
Water potential (Ψ) measures the tendency of water to move out of a solution, dependent on solute concentration and applied pressure.
| Component | Symbol | Definition |
|---|---|---|
| Pure Water | Ψ | Pure water at atmospheric pressure has a value of 0. |
| Solute Potential | Ψs | Decreases water potential. Always 0 or negative. |
| Pressure Potential | Ψp | Physical pressure. Increases water potential. |
2. Osmotic Effects on Living Cells
- Hypotonic: Water enters, cell bursts (haemolysis).
- Isotonic: No net movement; shape constant.
- Hypertonic: Water leaves, cell shrinks (crenated).
- Pure Water: Protoplast expands against wall, becoming turgid.
- Incipient Plasmolysis: Ψp = 0, protoplast just about to separate.
- Plasmolysed: Protoplast shrinks away from the cell wall.
3. Active Transport Mechanism
Active transport moves substances against a concentration gradient using energy from ATP.
Pumps 3 Na+ OUT while moving 2 K+ IN per ATP hydrolysed, generating membrane potential.
4. Bulk Transport
Bulk uptake of solids (phagocytosis) or liquids (pinocytosis) by infolding of the cell membrane using ATP.
Bulk removal of materials via vesicle fusion with the cell surface membrane, requiring ATP.
Lesson Summary
- Fluid Mosaic Structure: Membranes consist of a phospholipid bilayer with mobile hydrophobic tails and hydrophilic heads, integrated with proteins and cholesterol.
- Component Roles: Phospholipids block polar molecules; cholesterol regulates stability; transport proteins handle passage.
- Cell Signalling: Signal transduction via receptors, G-proteins, and second messenger cascades.
- Passive Transport: Simple and facilitated diffusion move solutes down concentration gradients without ATP.
- Active & Bulk Transport: Active transport and bulk transport handle movement against gradients or in bulk using ATP.
- Osmosis & Water Potential: Water moves down a water potential (Ψ) gradient governed by solute and pressure potentials.
1. Chromosomes & Chromatin Structure
Chromosomes are thread-like structures composed of chromatin—a complex of DNA and basic histone proteins. Human cells contain 46 chromosomes. Before nuclear division occurs (during Interphase), each DNA molecule replicates. The resulting chromosome consists of two identical sister chromatids joined at a narrow region called the centromere.
Euchromatin vs. Heterochromatin
- Euchromatin: Loosely coiled, transcriptionally active DNA dominant during Interphase.
- Heterochromatin: Tightly coiled, densely staining, transcriptionally inactive DNA.
Nucleosomes
The structural unit of chromatin. Consists of 147 base pairs of DNA wrapped 1.5 times around an octamer core of 8 histone proteins. Linker DNA (53 bp) connects adjacent nucleosomes.
2. The Mitotic Cell Cycle
The cell cycle consists of three main phases: Interphase, Mitosis (M phase), and Cytokinesis.
- G₁ Phase (Gap 1): Cell grows, synthesizes RNA, enzymes, and structural proteins required for growth.
- S Phase (Synthesis): DNA replicates so each chromosome consists of two chromatids. Kinetochores begin forming.
- G₂ Phase (Gap 2): Continued growth, checking/repairing replicated DNA, and increased tubulin synthesis for spindle fibers.
- M Phase (Mitosis): Nuclear division dividing genetic material into two identical nuclei.
- Cytokinesis: Division of the cytoplasm. Constriction from edges in animal cells; cell plate formation in plant cells.
3. Telomeres & Stem Cells
Telomeres: Repeated guanine/cytosine-rich non-coding DNA sequences at chromosome ends. They prevent gene loss during DNA replication because DNA polymerase cannot copy to the extreme end of the strand. The enzyme telomerase extends telomeres in stem cells and cancer cells. Fully differentiated cells lack active telomerase, causing gradual telomere shortening and eventual cell death.
Stem Cell Potency
| Potency | Description | Examples |
|---|---|---|
| Totipotent | Can form any cell type including extraembryonic tissues (placenta). | Zygote, early blastomeres (≤16-cell stage) |
| Pluripotent | Can form all body cell types, but not extraembryonic tissues. | Embryonic stem cells |
| Multipotent | Can form a limited range of specialized cell types. | Adult bone marrow stem cells |
4. Carcinogenesis & Cancer Development
Uncontrolled mitosis caused by mutated genes (oncogenes) leads to an irregular mass of cells called a tumour. Mutagenic agents are known as carcinogens (e.g., UV, tobacco tar, asbestos, X-rays).
Metastasis: Malignant tumour cells break away, travel via blood/lymphatic vessels, and establish secondary tumours in other body tissues.
Mitosis and cytokinesis in an animal cell
Interactive Diagram Viewer
Select a diagram below to examine detailed structural models generated using responsive vector graphics:
Interactive Mitosis & Spindle Dynamics Canvas
Click the buttons below to switch stages or toggle continuous playback:
Summary & Self-Check
Comprehensive Chapter Summary
- Growth of a multicellular organism is a result of parent cells dividing to produce genetically identical daughter cells.
- During cell division, the nucleus divides first, followed by division of the whole cell. Division of a nucleus to produce two genetically identical nuclei is achieved by the process of mitosis. Mitosis is used in growth, repair, asexual reproduction and cloning of cells during an immune response.
- Although a continuous process, for convenience, mitosis can be divided into four phases: prophase, metaphase, anaphase, telophase. The period between successive nuclear and cell divisions is called interphase. During interphase chromosomes are visible only as loosely coiled material called chromatin. Chromatin consists of DNA held in position by basic proteins called histones in subunits known as nucleosomes.
- During prophase of mitosis, chromatin condenses (coils up more tightly) to form chromosomes which are easily visible in the light microscope when stained. Each chromosome consists of two identical chromatids, held together by a centromere. Each chromatid contains a single DNA molecule, formed when DNA replicates during interphase.
- The period from one cell division to the next is called the cell cycle. This has four phases: G₁ is the first growth phase after cell division; S phase is when the DNA replicates (S for synthesis); G₂ is a second growth phase; M phase (M for mitosis) is when nuclear division takes place (followed by cell division). G stands for 'gap' (between the S and M phases).
- The ends of chromosomes are capped with special regions of DNA known as telomeres. Telomeres are needed to prevent the loss of genes from the ends of chromosomes during replication of DNA.
- Many specialised cells lose the ability to divide, but certain cells known as stem cells retain this ability. Stem cells are essential for growth from zygote to adult and for cell replacement and tissue repair in the adult.
- The behaviour of chromosomes during mitosis can be observed in stained preparations of root tips, either in section or in squashes of whole root tips.
- Cancers are tumours resulting from repeated and uncontrolled mitosis. They are thought to start as the result of mutation.
Self-Assessment Challenge
Test your knowledge based on the core learning outcomes:
Q: A human cell enters mitosis. How many chromatids and kinetochores are present at metaphase?
Nucleotide & Nucleic Acid Structure
Nucleic acids (DNA and RNA) are macromolecules made of repeating units called nucleotides. Each nucleotide consists of three fundamental components:
1. Pentose Sugar
A 5-carbon sugar: Deoxyribose in DNA (lacks one oxygen atom) or Ribose in RNA.
2. Phosphate Group
An acidic, negatively charged group (PO43−) that links adjacent sugars via phosphodiester bonds.
3. Nitrogenous Base
Nitrogen-containing rings:
- Purines (double ring): Adenine (A), Guanine (G)
- Pyrimidines (single ring): Cytosine (C), Thymine (T) [DNA only], Uracil (U) [RNA only]
Key Structural Differences
DNA consists of two antiparallel polynucleotide strands running in 5′ → 3′ and 3′ → 5′ directions held together by hydrogen bonds between complementary base pairs (A = T via 2 bonds, C ≡ G via 3 bonds). RNA is single-stranded, contains ribose, and uses Uracil instead of Thymine.
Semi-Conservative DNA Replication
During S-phase of interphase, DNA replicates semi-conservatively: each original strand serves as a template to form a new daughter strand.
Mechanism Steps
- Unwinding: DNA Helicase breaks hydrogen bonds between bases, unzipping the double helix.
- Activation: Free nucleoside triphosphates (activated nucleotides) pair up with exposed bases via complementary base pairing.
- Polymerization: DNA Polymerase links adjacent nucleotides via phosphodiester bonds, releasing two inorganic phosphate groups.
- Directionality: DNA Polymerase moves in the 5′ → 3′ direction along the new strand.
BOX 6.1: Experimental Evidence for Semi-Conservative Replication
In the 1950s, three main hypotheses were proposed for how DNA replicates[cite: 2]:
1. Conservative
One completely new double helix is synthesized from the intact original molecule[cite: 2].
2. Semi-Conservative
Each newly formed DNA molecule contains one original parent strand and one newly synthesized strand.
3. Dispersive
Each new strand consists of random fragments of old and new DNA interspersed throughout the molecules[cite: 2].
The Meselson and Stahl Experiment (1958)
Matthew Meselson and Franklin Stahl tested these hypotheses using Escherichia coli (E. coli) grown in media containing isotopes of nitrogen[cite: 2]:
- Bacteria were initially cultured in a medium containing the heavy isotope nitrogen-15 (15N) until all their DNA contained heavy nitrogen[cite: 2].
- The bacteria were then transferred to a light medium containing nitrogen-14 (14N) and allowed to replicate over several generations[cite: 2].
- DNA was extracted at each generation, placed in a solution of caesium chloride, and separated by density using high-speed centrifugation[cite: 2].
1: DNA grown entirely in 15N forms a single heavy band near the bottom of the tube[cite: 2].
2: Generation 1 in 14N produces a single band of intermediate density (15N-14N hybrid DNA), ruling out conservative replication[cite: 2].
3: Generation 2 in 14N produces two equal bands: one intermediate band and one light band (14N-14N), ruling out dispersive replication and proving semi-conservative replication[cite: 2].
4: Generation 3 in 14N shows an increased proportion of light DNA relative to intermediate DNA[cite: 2].
The Triplet Code & Gene Mutations
The genetic sequence is read as a continuous series of 3-base units called codons (triplets).
Triplet Code Features
- Universal: Same codons specify identical amino acids across virtually all organisms[cite: 2].
- Degenerate: 64 possible triplets (43) code for 20 amino acids; multiple triplets code for the same amino acid[cite: 2].
- Non-overlapping: Each base is part of only one triplet[cite: 2].
Sickle Cell Anaemia Case Study
A single base substitution mutation alters the β-globin polypeptide chain:
Normal (HbA): DNA triplet CTT → Glutamic acid.
Sickle (HbS): DNA triplet CAT → Valine.
Effect: Non-polar valine alters hemoglobin solubility, causing cell sickling and low oxygen transport capabilities.
Transcription and Translation
1. Transcription (Nucleus)
RNA Polymerase binds to the promoter, unwinds DNA, and aligns free RNA nucleotides along the template strand (3′ → 5′). It links nucleotides to synthesize a complementary single-stranded mRNA molecule, stopping at a terminator sequence.
2. Translation (Ribosome in Cytoplasm)
mRNA binds to the small ribosomal subunit, exposing two codons at a time. tRNA molecules with matching anticodons deliver specific amino acids. Peptidyl transferase forms peptide bonds between neighboring amino acids until a STOP codon is reached.
Interactive Transcription & Translation Simulator
Select a DNA template strand to transcribe into mRNA and translate into an amino acid chain.
Summary
- DNA and RNA are polynucleotides, made up of long chains of nucleotides . A nucleotide contains a pentose sugar, a phosphate group and a nitrogen-containing base . In RNA, the sugar is ribose; in DNA, it is deoxyribose .
- A DNA molecule consists of two polynucleotide chains, linked by hydrogen bonds between bases . In DNA there are four bases – adenine always pairs with thymine, and cytosine always pairs with guanine .
- RNA, which comes in several different forms, has only one polynucleotide chain, although this may be twisted back on itself, as in tRNA . In RNA, the base thymine is replaced by uracil .
- DNA molecules replicate during interphase by semi-conservative replication . In DNA replication, the hydrogen bonds between the bases break, allowing free nucleotides to fall into position opposite their complementary ones on each strand of the original DNA molecule . Adjacent nucleotides are then linked, through their phosphates and sugars, to form new strands . Two complete new molecules are thus formed from one old one, each new molecule containing one old strand and one new strand .
- The sequence of nucleotide bases on a DNA molecule codes for the sequence of amino acids in a polypeptide . Each amino acid is coded for by three bases . A length of DNA coding for just one polypeptide is a gene . A change in the nucleotide sequence of DNA is a mutation, producing a new allele of the gene .
- The DNA sequences for the HbA (normal) and HbS (sickle cell) alleles of the gene for the β-globin polypeptide differ by only one base . The triplet CTT in HbA is replaced by CAT in HbS, swapping the amino acid from glutamic acid to valine . This single difference in the polypeptide results in sickle cell anaemia in individuals with two HbS alleles .
- During protein synthesis, a complementary copy of the base sequence on a gene is made, by building a molecule of messenger RNA (mRNA) against one DNA strand . This stage is called transcription . After transcription, the next stage is called translation . During translation the mRNA moves to a ribosome in the cytoplasm .
- Transfer RNA (tRNA) molecules with complementary triplets of bases temporarily pair with base triplets on the mRNA, bringing appropriate amino acids . When two amino acids are held side by side, a peptide bond forms between them . The ribosome moves along the mRNA molecule, so that appropriate amino acids are gradually linked together, following the sequence laid down by the base sequence on the mRNA .
Plant Transport Needs & Tissue Anatomy
Unlike animals, plants do not possess a central mechanical pump or a rapid gaseous transport circulation system. Photosynthetic gases (CO₂ and O₂) move independently via passive diffusion through connected intercellular air spaces.
- Absorbed to Required: Water and inorganic mineral ions absorbed by roots must move upward to photosynthetic stems and leaves.
- Produced to Required: Assimilates (e.g., sucrose) manufactured in leaves during photosynthesis must be transported to metabolic sinks (roots, buds, tubers).
- Storage Movements: Sugars are transported to storage organs (e.g., roots/tubers) to be synthesized into insoluble starch.
Overall Water Movement Through a Plant
Water moves down a continuous water potential gradient from regions of higher water potential ($\Psi$) in the soil through the root tips and xylem tissue to the lower water potential of ambient air spaces in leaves.
Organ Cross-Sections (Dicotyledonous Structures)
Vascular tissues (xylem and phloem) are arranged distinctly in dicot stems, roots, and leaves to handle specialized structural stresses:
Tissue Structure Summary
| Tissue Type | Cell Types / Structure | Primary Function |
|---|---|---|
| Epidermis | Single layer, waxy cuticle, contains stomata/guard cells | Protects against water loss and pathogen entry |
| Parenchyma | Thin primary cellulose walls, unspecialized, air spaces | Packing tissue, starch storage, turgor support |
| Collenchyma | Living cells with extra cellulose thickening at corners | Flexible mechanical support in stems & leaf midribs |
| Endodermis | Inner cell boundary surrounding vascular cylinder; Casparian strip | Selectively controls ion entry into root xylem |
| Sclerenchyma | Dead cells with thick, lignified secondary walls | Rigid mechanical support (fibres in vascular bundles) |
Detailed Dicotyledonous Leaf Anatomy
Xylem Vessel Structure & Pathway of Water
Water moves passively from regions of higher water potential ($\Psi$) in the soil to lower water potential in the air. Xylem vessels consist of dead, hollow, elongated vessel elements linked end-to-end. End walls break down completely to form continuous drainpipe-like tubes. Secondary cell walls are reinforced with lignin to withstand negative hydrostatic pressure (tension) without collapsing. Unlignified gaps called pits permit lateral water movement between adjacent vessels.
Root Tissue Pathways (Apoplast vs. Symplast)
Water absorbed by root hairs crosses the root cortex via two primary pathways:
- Apoplast Pathway: Water movement through non-living cellulose cell walls and intercellular spaces by capillary movement. Blocked at the endodermis by the waterproof Casparian strip (suberin).
- Symplast Pathway: Water moves through cytoplasm and living cell interiors linked by plasmodesmata via osmosis.
Transpiration & Potometer Simulation
Transpiration is the loss of water vapour from a plant to its environment by diffusion down a water potential gradient, predominantly taking place through leaf stomata.
- Solar radiation causes liquid water to evaporate from wet mesophyll cell walls into internal sub-stomatal air spaces.
- Water vapour diffuses out through open stomata into drier surrounding air down a water potential gradient.
- Transpiration lowers hydrostatic pressure at the top of xylem vessels, creating tension.
- Water is pulled up xylem in a continuous mass-flow column, facilitated by cohesion (H-bonding between water molecules) and adhesion (attraction to lignified walls).
Interactive Potometer Simulation
A potometer estimates transpiration rate by measuring the rate of water uptake by a cut plant shoot. Adjust environmental variables to observe capillary meniscus movement.
Xerophytic Leaf Adaptations
Xerophytes are specialized plants adapted to habitats with limited water supply or high potential transpiration demand. They possess anatomical structural modifications to restrict transpiration loss.
Xerophytic Structural Features
| Adaptation | Mechanism for Water Conservation | Example Plant |
|---|---|---|
| Rolled Leaves | Traps humid air inside roll; reduces exposure of stomata to dry air currents. | Marram grass (Ammophila) |
| Sunken Stomata | Pits trap a boundary layer of moist air, reducing water potential gradient. | Sitka spruce, Marram grass |
| Epidermal Hairs (Trichomes) | Physical barrier trapping humid air layer, reflecting light. | Phlomis italica, Marram grass |
| Thick Waxy Cuticle | Impenetrable hydrophobic cutin layer minimizes cuticular evaporation. | Cactus (Opuntia), Conifers |
| Needle/Spine Leaves | Drastically reduces surface area-to-volume ratio. | Spruce, Cacti (Opuntia) |
Phloem Structure & Mass Flow Translocation
Translocation is the energy-requiring transport of soluble organic assimilates (predominantly sucrose and amino acids) through phloem tissue from sources (e.g. photosynthesizing mature leaves) to sinks (e.g. growing root tips, developing seeds, tubers).
Phloem Sieve Tube & Companion Cell Structure
- Sieve Tube Elements: Living cells joined end-to-end. Lacks nuclei, microfilaments, or ribosomes to reduce resistance to fluid flow. End walls form perforated sieve plates.
- Companion Cells: Metabolically active helper cells connected to sieve tube elements via high-density plasmodesmata. Contain abundant mitochondria and rough ER to generate ATP required for active loading.
Active Sucrose Loading Mechanism
- H⁺ Ion Pumping: Companion cells use ATP to actively pump $H^+$ ions across their membrane into the apoplastic cell wall space.
- Co-Transport: $H^+$ ions diffuse back into companion cells down their concentration gradient through $H^+$/sucrose co-transporter carrier proteins, moving sucrose against its concentration gradient.
- Symplastic Entry: Sucrose moves from companion cells into sieve tube elements through interconnecting plasmodesmata.
- Osmotic Hydrostatic Pressure: Accumulation of sucrose dramatically lowers $\Psi$ inside the sieve element. Water enters from xylem by osmosis, generating a high hydrostatic pressure that drives mass flow toward sinks.
Chapter Summary
1. Transport Requirements
- Multicellular organisms with small surface area : volume ratios need specialized transport systems .
- Flowering plants do not have compact bodies like animals; they spread and branch above and below ground to obtain carbon dioxide, light energy, water, and mineral ions .
- Plants do not need systems for transporting carbon dioxide or oxygen—diffusion is sufficient .
2. Water Movement & Xylem
- Water and mineral salts are transported through a plant in xylem vessels .
- Movement through the plant is a passive process down a water potential gradient from soil to air .
- Energy comes from the Sun, causing water evaporation from wet mesophyll cell walls in leaves .
- Water vapour diffuses out through stomata via transpiration, setting up the water potential gradient throughout the plant .
- Water enters root hairs by osmosis, crosses the root via cytoplasm (symplastic pathway) or cell walls (apoplastic pathway), and enters dead, empty xylem vessels .
- Water moves across the leaf by both symplast and apoplast pathways .
- Xylem sap moves up vessels by mass flow due to pressure differences from leaf transpiration; root pressure can also contribute .
- Xylem transport is strictly unidirectional (from roots to the rest of the plant) .
3. Transpiration & Potometer
- Transpiration is an inevitable consequence of gaseous exchange needed for CO₂ and O₂ exchange through stomata .
- Transpiration rate is affected by environmental factors: temperature, light intensity, wind speed, and humidity .
- Measuring transpiration rate directly is difficult, but water uptake can be measured using a potometer .
4. Xerophytes
- Xerophytes are adapted to places where conditions cause high transpiration rates and soil water is in short supply .
- They have evolved structural adaptations that help reduce the rate of water vapour loss from leaves .
5. Phloem & Translocation
- Translocation of organic solutes (e.g., sucrose) occurs through living phloem sieve tubes .
- Phloem sap moves by mass flow from source to sink .
- Sucrose is produced at the source (e.g., photosynthesising leaves) and used at the sink (e.g., a flower or storage organ) .
- Active loading of sucrose at the source causes water entry by osmosis, creating high hydrostatic pressure in sieve tubes .
- Phloem sap can move in different directions in different sieve tubes .
6. Structural Adaptations
- Both xylem vessels and phloem sieve tubes show unique structural features and distributions adapted specifically to their transport roles .
Comparative Overview: Xylem vs. Phloem
| Feature | Xylem Vessels | Phloem Sieve Tubes |
|---|---|---|
| Living Status | Dead at maturity (empty hollow tubes) | Living cells |
| Substances Carried | Water and mineral salts / ions | Organic solutes (e.g. sucrose) |
| Direction of Flow | Unidirectional (roots to rest of plant) | Multidirectional (different directions in different tubes) |
| Transport Mechanism | Passive mass flow driven by transpiration & root pressure | Mass flow driven by hydrostatic pressure from active sucrose loading |
1. Mammalian Circulatory System & Blood Vessels
Mammals possess a closed double circulatory system. Blood remains inside blood vessels and passes through the heart twice during one complete circuit around the body:
- Pulmonary Circulation: Deoxygenated blood is pumped from the right ventricle to the lungs via pulmonary arteries and returned oxygenated to the left atrium via pulmonary veins.
- Systemic Circulation: Oxygenated blood is pumped from the left ventricle through the aorta to the rest of the body tissues, returning deoxygenated to the right atrium via the vena cava.
Figure 8.2: The general plan of the mammalian transport system, viewed as though looking at someone facing you. It is a closed double circulatory system.
Comparison of Blood Vessels
Blood vessel walls (except capillaries) consist of three main layers (tunicae): tunica intima (endothelium), tunica media (smooth muscle and elastic tissue), and tunica externa (collagen fibers).
| Vessel Type | Lumen Size | Wall Thickness & Composition | Blood Pressure | Function & Key Adaptations |
|---|---|---|---|---|
| Artery | Narrow | Thick; heavy tunica media rich in elastic fibers & smooth muscle. | High (~120 to 80 mmHg) | Withstands high pressure; elastic recoil evens out pulsed blood flow. |
| Arteriole | Variable | Contains higher proportion of smooth muscle than elastic tissue. | Drops (85 to 35 mmHg) | Vasoconstriction/vasodilation controls local blood distribution & lowers pressure. |
| Capillary | Extremely narrow (~7 μm) | Single layer of endothelial cells (~1 μm thick). | Low (35 to 10 mmHg) | Exchanges nutrients/gases; gaps allow tissue fluid formation. |
| Vein | Wide | Thin; small amount of muscle/elastic tissue; presence of semilunar valves. | Very Low (<5 mmHg) | Returns blood to heart; valves prevent backflow under low pressure. |
Figure 8.3: The positions of some of the main blood vessels in the human body.
2. Blood Plasma, Tissue Fluid, and Lymph
As blood flows through capillaries, hydrostatic pressure forces fluid out into surrounding intercellular spaces.
- Arterial End: Hydrostatic pressure (~4.7 kPa) exceeds osmotic pressure pulling water back in (~3.3 kPa). Fluid, glucose, amino acids, and dissolved ions are filtered out through endothelial gaps.
- Venous End: Hydrostatic pressure drops (~1.3 kPa) below osmotic pressure. Net movement of water occurs back into capillaries by osmosis.
- Large plasma proteins (e.g., albumin) and blood cells remain inside capillaries as they cannot pass through capillary pores.
The Lymphatic System
About 10% of leaked fluid does not return to capillaries and is drained by blind-ending lymphatic vessels as lymph.
- Composition: Virtually identical to tissue fluid, but in specific areas (e.g., small intestine lacteals) it contains digested lipids.
- Drainage Path: Lymphatic vessels contain valves to enforce one-way flow, passing through lymph nodes (where white blood cells remove pathogens) before emptying back into the blood via the subclavian veins.
Figure 8.12: Outline of the human lymphatic system.
3. Blood Components & Gas Transport Dynamics
Cellular Components of Blood
- Erythrocytes (Red Blood Cells): Biconcave discs (~7 μm diameter) lacking a nucleus, ER, or mitochondria. Packed with haemoglobin for maximum oxygen-carrying capacity.
- Leucocytes (White Blood Cells): Possess nuclei; involved in defense. Divided into Phagocytes (Neutrophils with lobed nuclei and Monocytes) and Lymphocytes (large round nuclei producing antibodies or cellular immunity).
- Platelets: Cell fragments involved in blood clotting.
Haemoglobin (Hb) is a tetrameric protein that binds reversibly with up to 4 oxygen molecules (Hb + 4O2 ⇌ HbO8). Binding of the first O2 alters protein tertiary structure, making subsequent bindings easier (cooperative binding), producing a sigmoidal (S-shaped) dissociation curve.
The Bohr Shift: In actively respiring tissues, high CO2 production lowers blood pH via carbonic acid dissociation (CO2 + H2O carbonic anhydrase⟶ H2CO3 → H+ + HCO3-). H+ ions bind to haemoglobin forming haemoglobinic acid (HHb), reducing Hb affinity for oxygen and shifting the curve to the right, causing increased oxygen release.
Figure 3.1: Oxygen Dissociation Curve and the Bohr Shift.
4. Heart Structure, Cardiac Cycle, & Myogenic Control
Structure of the Heart
The mammalian heart has 4 chambers: two thin-walled atria (receive blood) and two thick-walled ventricles (pump blood out). The muscular septum completely separates left and right sides. The left ventricle wall is substantially thicker than the right ventricle wall because it must pump blood through high resistance in the systemic circulation.
Figure 8.22: Diagram of the external structure of a human heart, seen from the front.
The Cardiac Cycle Sequence
- Atrial Systole: Atria contract; blood is forced through open atrioventricular (AV) valves (tricuspid/bicuspid) into ventricles.
- Ventricular Systole: Ventricles contract; ventricular pressure exceeds atrial pressure, snapping AV valves shut (first heart sound). Semilunar (SL) valves open, forcing blood into aorta and pulmonary artery.
- Ventricular Diastole: Muscle relaxes; ventricular pressure drops below arterial pressure, causing SL valves to snap shut (second heart sound). Blood flows passively into atria from veins.
Figure 8.25: Diagrammatic section through a heart.
The heart beat originates internally without external nervous impulses:
SAN (Sinoatrial Node / Pacemaker) → Wave across Atria → AVN (Atrioventricular Node - 0.1s delay) → Bundle of His / Purkyne Tissue → Ventricles contract from Apex upwards.
5. Interactive Cardiac Conduction & Pressure Simulator
Observe real-time cardiac muscle contraction, electrical impulse conduction, and associated pressure variations during the cardiac cycle.
6. Lesson Summary
Key Takeaways
- Double Circulation: Separate pulmonary and systemic loops maintain high metabolic blood pressure to body tissues.
- Vessel Specialization: Arteries absorb high pressure pulses via elastic recoil; Arterioles regulate flow; Capillaries enable rapid diffusion; Veins store low-pressure blood with valves preventing backflow.
- Fluid Exchange: Hydrostatic pressure pushes fluid out at arterial ends; osmotic gradients pull water back at venous ends. Excess tissue fluid drains into lymphatics.
- Gas Transport: Haemoglobin displays cooperative binding. High CO2 concentrations lower blood pH, driving the Bohr shift to release O2 at active tissues.
- Myogenic Heart Cycle: The SAN initiates contraction. A brief delay at the AVN ensures full atrial emptying before Purkyne fibers trigger ventricular contraction from the apex upward.
Chapter 9: Gas Exchange and Smoking
The human gas exchange system links the circulatory system with the atmosphere. It is adapted to clean and warm incoming air, maximize surface area for diffusion, minimize diffusion distance, and maintain steep concentration gradients.
The Airway System & Structure
Air passes through a branching system of airways starting from the trachea, moving into the bronchi, bronchioles, and finally terminating in the alveoli.
| Airway | Number | Approx. Diameter | Cartilage | Goblet Cells | Smooth Muscle | Cilia |
|---|---|---|---|---|---|---|
| Trachea | 1 | 1.8 cm | Yes | Yes | Yes | Yes |
| Bronchus | 2 | 1.2 cm | Yes | Yes | Yes | Yes |
| Terminal Bronchiole | 48,000 | 1.0 mm | No | No | Yes | Yes |
| Respiratory Bronchiole | 300,000 | 0.5 mm | No | No | No | A few |
| Alveolar Duct | 9 × 10⁶ | 400 µm | No | No | No | No |
| Alveoli | 3 × 10⁹ | 250 µm | No | No | No | No |
Key Tissues and Defence Mechanisms
- Cartilage: C-shaped rings in the trachea and irregular blocks in the bronchi keep airways open and reduce air resistance.
- Goblet Cells & Mucus: Produce sticky mucus containing mucin glycoproteins to trap dust, pollen, bacteria, and viruses.
- Ciliated Epithelium: Beats continually to move trapped particles upward towards the larynx at ~1 cm/min to be swallowed and destroyed by stomach acid.
Structure of the Human Lungs
Figure 9.2 outlines the structural organization of the human respiratory system, showing how air is channeled from the upper airways down to the microscopic gas exchange surfaces.
Key Anatomical Features
- Thoracic Cavity & Pleural Membranes: The lungs sit in an airtight space enclosed by pleural membranes containing fluid for frictionless movement during ventilation.
- Branching Airways (a & b): Trachea splits into two bronchi, branching extensively into terminal and respiratory bronchioles.
- Alveolar Clusters (c): Surrounded by dense capillary networks where deoxygenated blood is converted to oxygenated blood.
Mechanisms of Gas Exchange
The alveoli provide a massive surface area of around $70\text{ m}^2$ in an adult human packed inside a thoracic cavity of about $5\text{ dm}^3$.
Adaptations of Alveoli
- Thin Walls: Single layer of squamous epithelial cells (~0.5 µm thick).
- Capillary Network: Closely pressed microvessels maintain short diffusion pathways.
- Concentration Gradients: Maintained continuously by constant blood flow and ventilation.
- Elastic Fibres: Stretch during inspiration and recoil during expiration to force air out effectively.
Diffusion Dynamics
Oxygen moves down its concentration gradient from alveolar air into the deoxygenated blood capillaries. Conversely, carbon dioxide diffuses from the blood into the alveolar air space to be exhaled.
Interactive Lung Ventilation & Gas Flow Simulation
Use the controls below to simulate normal breathing versus restricted airways caused by smoking or chronic bronchitis.
Smoking, COPD, and Cardiovascular Impacts
Tobacco smoke contains over 4,000 chemicals, many of which are toxic. The primary harmful agents are Tar, Carbon Monoxide, and Nicotine.
Chronic Obstructive Pulmonary Disease (COPD)
- Chronic Bronchitis: Tar enlarges goblet cells, overproduces mucus, and destroys cilia. Bacteria accumulate, leading to smoker's cough and scarred, narrowed airways.
- Emphysema: Phagocytes release elastase to break down alveolar walls. Elastin is destroyed, preventing proper recoil. Alveoli burst, drastically reducing surface area.
Cardiovascular Effects
- Nicotine: Stimulates nervous system release of adrenaline, increasing heart rate and blood pressure while restricting peripheral blood flow. Highly addictive via dopamine pathways.
- Carbon Monoxide: Binds irreversibly with hemoglobin to form carboxyhaemoglobin, reducing oxygen-carrying capacity by 5–10%.
Chapter Summary
- Multicellular organisms often have surfaces that are specialised to allow exchange of gases to take place between their bodies and the environment. Alveoli in the lungs form the gas exchange surface in mammals.
- In the human lungs, air passes down the trachea and through a branching system of airways to reach the alveoli. The airways are lined by a ciliated epithelium with mucus-secreting goblet cells. The epithelium protects the alveoli by moving a carpet of mucus towards the throat, where it can be swallowed. There are C-shaped rings of cartilage in the trachea and irregularly shaped blocks of cartilage in the bronchi to keep the large airways open and so reduce resistance to the flow of air. Smooth muscle in the airways contracts and relaxes to adjust the diameter of the airways.
- The alveoli are lined by a squamous epithelium that gives a short diffusion distance for the exchange of oxygen and carbon dioxide. The alveoli are well supplied with blood by the many capillaries surrounding the gas exchange surface. The constant flow of blood and the continuous ventilation of the lungs maintain concentration gradients between blood and air for oxygen and carbon dioxide. Recoil of the elastic fibres surrounding the alveoli helps to move air out during expiration.
- Damage to the airways and alveoli occurs in chronic obstructive pulmonary disease (COPD). In chronic bronchitis, the airways are obstructed by mucus and infection; in emphysema, alveoli are destroyed, reducing the surface area for gas exchange. Some of the signs and symptoms of COPD are breathlessness, wheezing and constant coughing.
- Tobacco smoke contains tar, carbon monoxide and nicotine. Tar settles on the epithelium lining the bronchi and bronchioles and stimulates inflammation, an increase in the secretion of mucus and an accumulation of phagocytes from the blood. Tar contains carcinogens, which cause changes in DNA in bronchial epithelial cells, often leading to the development of a bronchial carcinoma. This is lung cancer. Two of the symptoms of lung cancer are coughing up blood and chest pains.
- Carbon monoxide combines irreversibly with haemoglobin, reducing the oxygen-carrying capacity of the blood. Nicotine stimulates the nervous system, increasing heart rate and blood pressure, and stimulating vasoconstriction, which reduces blood flow to the extremities.
- Smoking damages the cardiovascular system, increasing and increasing the risk of coronary heart disease and stroke.
Infectious vs. Non-Infectious Diseases
A disease is an illness or disorder of the body or mind that leads to poor health, associated with a set of specific signs and symptoms.
Infectious (Communicable)
Caused by organisms known as pathogens (bacteria, viruses, protoctists, or fungi). Passed from infected to uninfected individuals directly or indirectly (e.g., cholera, malaria, HIV/AIDS, TB, measles).
Non-Infectious
Not caused by pathogens. Includes long-term degenerative conditions (e.g., lung cancer, COPD), genetic disorders (e.g., cystic fibrosis, sickle cell anaemia), and deficiency diseases.
Key Epidemiological Terms
- Endemic: A disease that is constantly maintained within a region or population (e.g., malaria in tropical regions).
- Incidence: The number of people diagnosed with a disease over a specific period (week, month, year).
- Prevalence: The total number of people who have the disease at any given time.
- Epidemic / Pandemic: A sudden sharp increase locally/regionally (epidemic) or across continents/worldwide (pandemic).
Major Infectious Diseases At-a-Glance
Select a disease to examine its comprehensive features, strains, prevention, and treatment data.
Antibiotics and Bacterial Resistance
Antibiotics are drugs that kill or stop the growth of bacteria without harming human host cells. They target unique bacterial structures such as peptidoglycan cell walls, 70S ribosomes, or specific enzymes.
How Penicillin Works
Penicillin inhibits the bacterial enzymes responsible for building cross-links between peptidoglycan polymers in cell walls. Autolysins continue to make holes, water enters via osmosis, and the bacterial cell bursts due to high internal pressure. Human cells lack cell walls and are unaffected.
Resistance Mechanisms
Vertical Transmission: Spreading mutant resistance genes rapidly through asexual binary fission.
Horizontal Transmission: Transferring resistance plasmids between bacteria via conjugation tubes.
Chapter Summary
- Infectious diseases are caused by transmissible pathogens, whereas non-infectious conditions stem from genetics, lifestyle, or deficiency.
- Global health relies on understanding transmission cycles (water-borne, vector-borne, airborne, or direct contact) to break spread vectors.
- Bacterial infections are treated with targeted antibiotics, but misuse drives vertical and horizontal resistance (e.g., MRSA, MDR-TB).
- Public health measures—including sanitation, clean water, contact tracing, and vaccination—remain vital in controlling outbreaks.
Chapter 11: Immunity – External & Internal Defences
Immunity is the protection against disease provided by the body's internal defence or immune system. Pathogens are disease-causing organisms recognized by foreign molecules on their surfaces called antigens.
External Defences
- Epithelia: Physical barriers covering airways and skin.
- Stomach Acid: Hydrochloric acid kills ingested bacteria.
- Blood Clotting: Stops blood loss and prevents pathogen entry via wounds.
Internal Defences (White Blood Cells)
- Origin: All immune cells originate from bone marrow stem cells.
- Self vs. Non-Self: The immune system distinguishes the body's own molecules (self) from foreign substances (non-self) to prevent attacking host tissue.
Phagocytosis Process
Neutrophils and macrophages destroy foreign invaders through a series of cellular steps:
- Attraction (Chemotaxis): Chemicals released by damaged tissues or pathogens attract neutrophils.
- Recognition & Attachment: Receptors on the neutrophil membrane attach directly to bacteria or antibody "markers" coating the pathogen.
- Endocytosis: The cell surface membrane engulfs the pathogen, trapping it within a phagocytic vacuole.
- Fusion of Lysosomes: Lysosomes fuse with the phagocytic vacuole and release hydrolytic enzymes (such as proteases) to digest the pathogen.
Lymphocytes: B Cells and T Cells
Lymphocytes are smaller white blood cells with large nuclei central to adaptive immunity.
B-Lymphocytes (B Cells)
Originates and matures in the bone marrow. Each matures to carry a specific antibody receptor on its surface. When activated by matching antigens, they undergo clonal selection and expansion, differentiating into:
- Plasma Cells: Short-lived cells that secrete massive quantities of antibodies (up to thousands per second) using extensive rough endoplasmic reticulum.
- Memory Cells: Long-lived cells providing rapid secondary responses upon reinfection.
T-Lymphocytes (T Cells)
Originates in bone marrow but matures in the thymus gland. They carry T cell receptors and perform two main roles:
- Helper T Cells: Secrete cytokines to stimulate B cell division, antibody production, and macrophage activity.
- Killer T Cells (Cytotoxic): Search out and destroy pathogen-invaded body cells by secreting lethal toxins (e.g., hydrogen peroxide) and punching holes in target membranes.
Antibody Structure & Function
Antibodies are globular glycoproteins (immunoglobulins, e.g., IgG) featuring a quaternary structure built from two heavy chains and two light chains held by disulfide bonds.
- Variable Region: Contains unique amino acid sequences forming specialized antigen-binding sites complementary to specific antigens.
- Hinge Region: Provides molecular flexibility for binding.
- Mechanisms of Action: Neutralization of toxins (antitoxins), opsonization (coating bacteria to aid phagocytosis), agglutination (clumping), and lysis (punching cell walls with complement proteins).
Active vs. Passive Immunity & Vaccination
Active Immunity
The body produces its own antibodies and active lymphocytes following exposure to an antigen.
- Natural: Gained through natural infection.
- Artificial: Gained via vaccination (introduction of live, attenuated, dead pathogens, or toxoids). Provides long-term protection via memory cells.
Passive Immunity
Antibodies are introduced from an external source without activating the recipient's immune system. Provides immediate, temporary protection.
- Natural: Maternal antibodies crossing the placenta or passing through breast milk (colostrum containing IgA).
- Artificial: Injection of exogenous antibodies or antitoxins (e.g., tetanus treatment).
Eradication Challenges & Successes
- Smallpox Eradication (1980): Succeeded due to a stable viral genome (no antigenic variation), a cheap stable vaccine (vaccinia), easy identification, and effective ring vaccination strategies.
- Measles & Other Pathogens: Difficult to eradicate due to shifting populations, required high herd immunity (93–95%), malnutrition causing poor vaccine responses, and eukaryotic antigenic variation (e.g., Plasmodium in malaria, Trypanosoma, and Vibrio cholerae concealment).
Autoimmune Diseases & Monoclonal Antibodies
Autoimmune Diseases
Occurs when the immune system mistakenly targets self-antigens. Example:
- Myasthenia Gravis: Antibodies target and destroy acetylcholine receptors at neuromuscular junctions. This blocks nerve impulses, preventing sodium ion influx and causing progressive, activity-dependent muscle weakness and fatigue.
- Other examples include Type 1 diabetes (destruction of insulin-secreting cells) and rheumatoid arthritis (joint destruction).
Monoclonal Antibodies (Mabs)
Identical antibodies produced by fusing a specific plasma cell with a cancer cell to form an immortal hybridoma line.
- Diagnostics: Used to locate deep vein thrombosis (radiolabelled anti-fibrin), identify cancer cells, and perform blood/tissue typing.
- Treatments: Humanized Mabs like Trastuzumab (Herceptin) for breast cancer receptors, Infliximab for rheumatoid arthritis inflammation, and Rituximab to deplete overactive B cells.
Interactive Primary vs. Secondary Immune Response Simulation
Simulate pathogen exposure and observe antibody concentration changes over time (logarithmic scale representation reflecting primary vs. secondary memory response).
Lesson Summary
- Cellular Components: Phagocytes (neutrophils and macrophages) engulf pathogens via phagocytosis; lymphocytes (B and T cells) manage adaptive and cellular immunity originating from bone marrow stem cells.
- Antigens & Antibodies: Foreign non-specific antigens trigger specific B cells to multiply into plasma cells, generating globular glycoprotein antibodies with variable regions tailored for neutralization, opsonization, and agglutination.
- Immunological Memory: Memory cells enable rapid, magnified secondary antibody responses compared to slow primary responses, underpinning lifelong natural immunity and vaccination.
- Medical Applications & Failures: Active immunity provides long-lasting defense through vaccines, while passive immunity offers immediate temporary protection. Antigenic variation, concealment, and mutations hinder vaccine design for complex pathogens like malaria and HIV.
- Pathology: Leukaemias disrupt bone marrow blood cell balance; autoimmune conditions like myasthenia gravis target self-receptors; and monoclonal hybridoma technologies revolutionize clinical diagnostics and targeted treatments.