ICell Structure

1.1 Microscopy

• Magnification — how many times larger the image is than the actual object.
• Resolution — minimum distance between two points that can still be distinguished.
• Light microscope: resolution ~200 nm (limited by wavelength of light).
• Electron microscope (TEM, SEM): resolution ~0.5 nm; uses electron beams.

1.2 Eukaryotic organelles

1.3 Prokaryote vs eukaryote

IIBiological Molecules

2.1 Water

• Polar — O is δ⁻, H is δ⁺ → hydrogen bonding between molecules.
• High specific heat capacity — stabilises temperature in aquatic habitats and inside organisms.
• Cohesion + adhesion — drives the transpiration stream in xylem.
• Universal solvent — most metabolic reactions occur in aqueous solution.

2.2 Carbohydrates

Starch = amylose (helix, α-1,4) + amylopectin (branched, α-1,4 + α-1,6). Insoluble, compact — perfect plant storage.

Glycogen — more branched than amylopectin; rapid mobilisation in animals.

Cellulose — β-1,4 linkages; chains run anti-parallel; cross-linked by hydrogen bonds into microfibrils; gives plant walls tensile strength.

2.3 Lipids

Triglycerides = glycerol + 3 fatty acids, joined by ester bonds via condensation. Saturated FAs (no C=C) = solid at room T; unsaturated (≥1 C=C) = liquid (oils).

Phospholipids — one fatty acid replaced by phosphate; the resulting amphipathic molecule forms the bilayer of membranes.

2.4 Proteins

• Primary — amino acid sequence joined by peptide bonds.
• Secondary — α-helix or β-pleated sheet, stabilised by H-bonds.
• Tertiary — 3D folding: H-bonds, ionic, disulfide bridges, hydrophobic interactions.
• Quaternary — multiple polypeptides (e.g. haemoglobin: 4 subunits).

2.5 Food tests

IIIEnzymes

3.1 Mechanism

Lock-and-key (Fischer, 1894): substrate fits a rigid active site. Induced fit (Koshland, 1958): the active site moulds around the substrate, straining bonds — accepted as more accurate.

3.2 Factors affecting rate

• Temperature — rate doubles per 10 °C until optimum, then denaturation collapses tertiary structure.
• pH — narrow optimum; extreme pH disrupts ionic bonds → denaturation.
• Enzyme concentration — rate increases linearly until substrate becomes limiting.
• Substrate concentration — rate increases until V_max (all active sites occupied).

3.3 Inhibition

IVCell Membranes & Transport

4.1 Fluid-mosaic model (Singer–Nicolson, 1972)

• Phospholipid bilayer — hydrophilic heads outward, hydrophobic tails inward.
• Proteins float laterally — integral (channel, carrier) or peripheral.
• Cholesterol acts as a fluidity buffer — at high temperatures it restricts phospholipid movement (decreases fluidity); at low temperatures it prevents tight packing of tails (increases fluidity). It also reduces permeability to small polar molecules and ions.
• Glycoproteins and glycolipids — cell recognition, signalling.

4.2 Transport types

4.3 Water potential (Ψ)

Ψ_s = solute potential (always ≤ 0); Ψ_p = pressure potential. Water moves from high Ψ to low Ψ.

VThe Mitotic Cell Cycle

5.1 Cell cycle

1. G1 — growth, organelle synthesis.
2. S — DNA replication; each chromosome now 2 sister chromatids.
3. G2 — growth, prep for division.
4. M — mitosis (P-M-A-T) + cytokinesis.

5.2 Mitosis stages

• Prophase — chromosomes condense, nuclear envelope breaks down, spindle forms.
• Metaphase — chromosomes align on the equator, attached at centromere.
• Anaphase — centromeres divide, sister chromatids pulled to poles.
• Telophase — nuclear envelopes reform, chromosomes decondense.

5.3 Cancer

VINucleic Acids & Protein Synthesis

6.1 DNA structure

Two antiparallel strands forming a right-handed double helix. Backbone = alternating deoxyribose + phosphate. Bases inside: A–T (2 H-bonds), G–C (3 H-bonds).

6.2 Semi-conservative replication

• Helicase unwinds the helix.
• DNA polymerase adds nucleotides 5'→3' to the template.
• Leading strand synthesised continuously; lagging strand in Okazaki fragments joined by ligase.
• Meselson–Stahl (¹⁵N → ¹⁴N) confirmed semi-conservative model.

6.3 Transcription & translation

Transcription — RNA polymerase reads template DNA, builds mRNA (U replaces T). Occurs in nucleus.

Translation — ribosome reads mRNA codons; tRNA brings amino acids matched by anticodon; peptide bonds form between adjacent amino acids.

VIITransport in Plants

7.1 Xylem & transpiration

Xylem vessels — dead, lignified, hollow tubes. Water rises by the cohesion–tension theory: transpiration at leaves pulls a continuous column of water up by cohesion + adhesion.

• Factors increasing transpiration: ↑ temperature, ↑ wind, ↓ humidity, ↑ light intensity.
• Measured with a potometer (records water uptake, not transpiration directly).

7.2 Phloem & translocation

Phloem sieve tube elements — alive but mostly empty, supported by companion cells. Mass-flow hypothesis: sucrose actively loaded at sources lowers Ψ → water enters by osmosis → hydrostatic pressure drives sap to sinks where sucrose is unloaded.

VIIITransport in Mammals

8.1 Heart structure & cardiac cycle

• Four chambers: 2 atria (thin walls), 2 ventricles (thick muscular walls; left thicker — systemic circuit).
• Valves: atrioventricular (tricuspid + bicuspid) and semilunar (aortic + pulmonary).
• Cycle: atrial systole → ventricular systole → diastole. Driven by SAN → AVN → bundle of His → Purkyne fibres.

8.2 Haemoglobin & oxygen dissociation

Each Hb molecule has 4 haem groups → carries up to 4 O₂. Sigmoid dissociation curve = cooperative binding.

• Bohr shift — high CO₂ / low pH → curve shifts right → Hb releases O₂ more readily at respiring tissues.
• Foetal haemoglobin — curve shifts left → higher O₂ affinity to take O₂ from mother's blood.

IXGas Exchange

9.1 Alveolar features that aid exchange

• Huge surface area (~70 m² in adults).
• Thin walls — single squamous epithelium + capillary endothelium → diffusion distance ~0.5 μm.
• Steep diffusion gradient maintained by ventilation and blood flow.
• Surfactant prevents alveolar collapse on expiration.

9.2 Smoking

Tar — paralyses cilia, irritates epithelium, contains carcinogens. Nicotine — addictive, raises heart rate and BP. Carbon monoxide — binds Hb 240× more tightly than O₂. Diseases: chronic bronchitis, emphysema, lung cancer, cardiovascular disease.

XInfectious Disease

XIImmunity

11.1 Defence overview

Two layers of defence: innate (non-specific) — present from birth, fast, no memory; adaptive (specific) — slow on first exposure, has memory, distinguishes self from non-self via antigens.

• Innate: skin barrier, mucus, stomach HCl, lysozyme in tears/saliva, inflammation, phagocytosis.
• Adaptive: humoral (B cells → antibodies) + cell-mediated (T cells).

11.2 Phagocytosis

Carried out by neutrophils (short-lived, abundant, first responders) and macrophages (derived from monocytes; long-lived; also present antigens).

1. Pathogen chemicals (and opsonin-tagged antibodies) attract the phagocyte by chemotaxis.
2. The phagocyte's plasma membrane engulfs the pathogen → forms a phagosome.
3. A lysosome fuses with the phagosome → phagolysosome.
4. Lysosomal enzymes (e.g. lysozyme) hydrolyse the pathogen.
5. Macrophage displays antigens on its surface bound to MHC class II → becomes an antigen-presenting cell (APC).

11.3 Lymphocytes — origin & maturation

11.4 The adaptive immune response — clonal selection

1. An APC presents the antigen to a complementary T-helper (T_H) cell.
2. The T_H cell is activated, proliferates by mitosis (clonal expansion), and releases cytokines.
3. Cytokines activate the matching B cell — which has bound the same antigen via its BCR. The B cell proliferates and differentiates into:
   • Plasma cells — short-lived, secrete ~2000 antibodies s⁻¹.
   • B memory cells — long-lived, no antibody secretion in resting state.
4. Cytokines also activate T-killer (cytotoxic) cells — which release perforin to lyse virus-infected or cancer cells, plus form T memory cells.

11.5 Antibody structure & function

• Quaternary structure — 4 polypeptide chains: 2 heavy + 2 light, held by disulfide bonds (Y-shaped immunoglobulin).
• Variable region — at the tips of the Y; sequence varies between antibodies; the antigen-binding site. Two binding sites per antibody.
• Constant region — same within an isotype (IgG, IgM, IgA, IgE, IgD); the hinge region gives flexibility.
• Specificity — antigen-binding site is complementary in shape to a specific antigen epitope (one antibody → one antigen).

How antibodies work:

• Neutralisation — blocking pathogen attachment to host cells / neutralising toxins.
• Agglutination — two binding sites cross-link multiple pathogens into clumps, easier to phagocytose.
• Opsonisation — antibody tags pathogen for phagocyte recognition.
• Activation of complement — triggers lysis of cell-walled pathogens.

11.6 Primary vs secondary response

The fast secondary response is driven by memory B and T cells from the primary response.

11.7 Active vs passive immunity

11.8 Vaccination & herd immunity

A vaccine introduces antigen in a safe form so the immune system mounts a primary response and forms memory cells. Vaccine types:

• Live attenuated — weakened pathogen (MMR, BCG, oral polio).
• Inactivated / killed — heat- or chemically-killed pathogen (rabies, hepatitis A).
• Subunit — purified antigen only (HepB surface antigen).
• Toxoid — inactivated toxin (tetanus, diphtheria).
• mRNA / DNA — host cells make the antigen from the nucleic acid (COVID-19 mRNA vaccines).

Vaccination programmes can eradicate diseases when: (i) human-only host, (ii) stable antigen, (iii) effective vaccine, (iv) cold-chain logistics. Smallpox eradicated in 1980; polio close.

11.9 Why some diseases are hard to vaccinate against

• Antigenic variation — pathogen changes its surface antigens (influenza drift & shift; HIV; Plasmodium).
• Hides inside host cells — TB inside macrophages; HIV inside T_H cells.
• Animal reservoirs — re-introduction after human eradication (TB in cattle, flu in birds).
• Multiple species/strains — malaria has four species (see §10).

11.10 Monoclonal antibodies

Monoclonal antibodies (mAbs): identical antibodies produced by clones of a single B cell — all specific to one antigen epitope.

Production (Köhler & Milstein, 1975):

1. Inject antigen into a mouse → B cells make specific antibody.
2. Harvest the mouse's spleen B cells (specific, but die in culture).
3. Fuse them with myeloma (cancer) cells → hybridoma cells (specific AND immortal).
4. Screen hybridomas for the desired antibody, clone the chosen one.
5. Culture indefinitely → continuous supply of identical mAb.

Uses:

• Diagnosis — pregnancy test (hCG), HIV ELISA, COVID lateral-flow, blood typing.
• Treatment — trastuzumab (HER2 breast cancer), rituximab (B-cell lymphoma), infliximab (TNF-α, autoimmune disease).
• Targeted drug delivery — mAb carries a cytotoxic drug to cancer-cell antigen, sparing healthy tissue.
• Research / purification — affinity chromatography to isolate a single protein from a mixture.