compare the structure of a prokaryotic cell as found in a typical bacterium with the structures of typical eukaryotic cells in plants and animals

Objective

To compare the structure of a typical prokaryotic cell (bacterium) with typical eukaryotic plant and animal cells, and to link each structural feature to the key biological concepts required by the Cambridge International AS & A Level Biology (9700) syllabus.

1. Cell Structure (Syllabus 1 – LO 1.1 & 1.2)

1.1 Practical Microscopy Skills

• Preparing a temporary mount: place a drop of distilled water on a clean slide, add a single bacterial colony or a thin fragment of plant/animal tissue, lower a cover‑slip gently to avoid air bubbles.
• Calculating total magnification: total = ocular power × objective power (e.g., 10 × 40 = 400 ×).
• Using an eyepiece graticule: count the number of squares that span a known distance on a stage micrometer (usually 0.1 mm) and derive the scale bar for photomicrographs.
• Photomicrographing: adjust illumination, focus and camera settings; annotate key structures directly on the image.

1.2 Viruses – non‑cellular entities

Viruses consist of nucleic acid (DNA or RNA) surrounded by a protein capsid; some have a lipid envelope. They lack a membrane‑bound cytoplasm and cannot carry out metabolism independently, so they are not cells. Their existence reinforces that the cell is the fundamental unit of life while showing that acellular agents can mediate biological functions.

1.3 Comparative Overview of Cell Types

Prokaryotic Cell (Typical Bacterium)

• Size: 0.5–5 µm diameter.
• Genetic material: single circular chromosome in a nucleoid; may carry plasmids.
• Cell envelope:

  • Cell wall – peptidoglycan (Gram‑positive) or thin peptidoglycan + outer membrane (Gram‑negative).
  • Plasma membrane just inside the wall.

• Ribosomes: 70 S (≈30 nm), free in the cytoplasm.
• External structures: simple flagella (motility), pili/fimbriae (attachment, conjugation).
• Organelles: none (no nucleus, mitochondria, chloroplasts, ER, Golgi, etc.).
• Energy metabolism: aerobic respiration, anaerobic fermentation, photosynthesis (cyanobacteria) – all occur at the plasma membrane.

Eukaryotic Plant Cell

• Size: 10–100 µm diameter.
• Genetic material: multiple linear chromosomes within a double‑membrane nucleus (nuclear envelope) containing a nucleolus.
• Cell wall: cellulose, hemicellulose and pectin layers external to the plasma membrane.
• Plasma membrane: phospholipid bilayer with embedded proteins.
• Membrane‑bound organelles:

  • Mitochondria – cellular respiration.
  • Chloroplasts – photosynthesis (thylakoid stacks, stroma).
  • Endoplasmic reticulum (rough & smooth).
  • Golgi apparatus – modification & sorting of proteins.
  • Large central vacuole – storage, turgor, waste sequestration.

• Ribosomes: 80 S (20–30 nm), free or bound to rough ER.
• Specialised structures: plasmodesmata (cytoplasmic channels) for intercellular communication.

Eukaryotic Animal Cell

• Size: 10–30 µm diameter.
• Genetic material: multiple linear chromosomes within a nucleus.
• Cell envelope: no rigid cell wall; only a flexible plasma membrane.
• Membrane‑bound organelles:

  • Mitochondria.
  • Endoplasmic reticulum (rough & smooth).
  • Golgi apparatus.
  • Lysosomes – hydrolytic digestion.
  • Centrosome (with centrioles) – microtubule‑organising centre.
  • Numerous small vacuoles (storage, transport).

• Ribosomes: 80 S, free or attached to rough ER.
• Surface specialisations:

  • Cilia or flagella (e.g., sperm).
  • Microvilli (intestinal epithelium).

1.4 Comparative Table

2. Biological Molecules (Syllabus 2 – LO 2.1‑2.3)

Key Concepts Box

3. Enzymes (Syllabus 3 – LO 3.1‑3.4)

3.1 Kinetic Parameters

• Michaelis–Menten equation: v = (V_max · [S]) / (K_m + [S])
• V_max: maximum rate when all enzyme molecules are saturated with substrate.
• K_m: substrate concentration at which the reaction rate is half of V_max; a measure of affinity.
• Typical classroom graph: plot reaction velocity against substrate concentration; draw a hyperbola or use a Lineweaver‑Burk plot for linearisation.

3.2 Types of Inhibition

3.3 Practical Enzyme Activities

1. Catalase assay – measure O₂ evolution from H₂O₂ in onion epidermal strips (demonstrates mitochondrial respiration).
2. Amylase digestion – add saliva to starch solution, monitor iodine colour change (cytoplasmic enzyme).
3. Effect of temperature & pH – repeat either assay at different temperatures or pH values to generate activity curves.

4. Cell Membranes & Transport (Syllabus 4 – LO 4.1‑4.4)

4.1 Fluid‑Mosaic Model

The plasma membrane is a fluid phospholipid bilayer in which proteins move laterally. Major components:

• Phospholipids – amphipathic molecules forming the bilayer.
• Integral (intrinsic) proteins – span the membrane; include channels, carriers, receptors.
• Peripheral (extrinsic) proteins – attached to the inner or outer leaflets.
• Carbohydrate chains – glycolipids or glycoproteins; involved in cell‑cell recognition.

4.2 Transport Mechanisms

4.3 Osmotic Potential & Surface‑Area‑to‑Volume (SA:V) Calculations

Osmotic potential (Ψ_s) = –iCRT (where i = ionisation factor, C = molar concentration, R = 0.0831 L·bar mol⁻¹ K⁻¹, T = temperature in K). Example:

C = 0.150 M (NaCl), i = 2, T = 298 K  
Ψs = –2 × 0.150 × 0.0831 × 298 ≈ –7.4 bar

SA:V ratio influences diffusion distance. For a sphere:

Surface area = 4πr²  
Volume = (4/3)πr³  
SA:V = 3/r

Thus, halving the radius doubles the SA:V, enhancing diffusion efficiency – a key reason prokaryotes remain small.

4.4 Practical Activities

1. Diffusion of coloured dye in agar plates – visualise concentration gradients.
2. Potato core osmosis – measure mass change in hypo‑ and hyper‑tonic solutions.
3. Red blood cell swelling assay – compare glucose‑free vs. glucose‑rich media (active transport).
4. Fluorescent bead endocytosis in cultured animal cells – observe uptake under a fluorescence microscope.

5. Transport in Plants (Syllabus 5 – LO 5.1‑5.3)

5.1 Xylem – Water & Mineral Transport

• Transpiration pull: water evaporates from stomata, creating a negative pressure that pulls water upward (cohesion‑tension theory).
• Cohesion & adhesion: hydrogen bonding between water molecules (cohesion) and between water and xylem walls (adhesion) maintain a continuous column.
• Root pressure: osmotic uptake of ions in root cells generates a positive pressure that can push water upward, especially at night.

5.2 Phloem – Photoassimilate Transport (Pressure‑Flow Hypothesis)

• Source (e.g., mature leaf) loads sucrose into sieve‑tube elements → osmotic water entry → increased turgor pressure.
• Sink (e.g., growing root) unloads sucrose → water exits → lower pressure.
• Bulk flow from high to low pressure transports sugars throughout the plant.

5.3 Supporting Structures

• Plasmodesmata: cytoplasmic channels linking adjacent cells, allowing symplastic movement of small molecules.
• Tonoplast: vacuolar membrane that regulates ion and solute storage, contributing to osmotic balance.

5.4 Practical Work

1. Capillary action of dyed water in celery stalks – demonstrate xylem transport.
2. Starch test in leaves after covering with aluminium foil – confirm photosynthate accumulation in source leaves.
3. Measurement of phloem exudate flow using aphid stylet technique (advanced).

6. Transport in Mammals (Syllabus 5 – LO 5.4‑5.6)

6.1 Blood Plasma & Capillary Exchange

• Diffusion of gases (O₂, CO₂) and small solutes across thin respiratory and systemic capillary walls.
• Bulk flow (filtration & reabsorption) driven by hydrostatic and osmotic pressures (Starling forces).
• Endothelial fenestrations in specialised capillaries (renal glomeruli, endocrine glands).

6.2 Respiratory Gas Exchange

• Alveolar walls: Type I pneumocytes provide a thin diffusion barrier; Type II pneumocytes secrete surfactant.
• Partial pressure gradients (P_O₂ ≈ 100 mm Hg in alveoli, ≈ 40 mm Hg in peripheral blood) drive O₂ diffusion; opposite gradient for CO₂.
• Fick’s law: Rate ∝ (D × A × ΔP) / d (where D = diffusion coefficient, A = surface area, ΔP = pressure difference, d = membrane thickness).

6.3 Kidney Filtration & Reabsorption

• Glomerular filtration barrier: fenestrated endothelium, basement membrane, podocyte slit diaphragms.
• Active reabsorption of glucose, amino acids, Na⁺ via secondary active transport (Na⁺/glucose cotransporter).
• Urea recycling and counter‑current multiplication create the osmotic gradient for water reabsorption.

6.4 Practical Activities

1. Dialysis tubing experiment – model glomerular filtration of glucose vs. albumin.
2. Oxygen‑hemoglobin dissociation curve using a spectrophotometer – illustrate cooperative binding.
3. Measurement of lung diffusion capacity (DL) using the single‑breath CO method (advanced).

7. Gas Exchange (Syllabus 6 – LO 6.1‑6.3)

7.1 Structures Involved

• Alveoli – thin walls, extensive capillary network, moist surface.
• Plant stomata – guard cells regulate opening, controlling CO₂ uptake and water loss.

7.2 Diffusion Principles

• Fick’s law (see 6.2) governs the rate of gas diffusion.
• Key variables: surface area (A), membrane thickness (d), diffusion coefficient (D), and partial pressure difference (ΔP).

7.3 Factors Affecting Rate

7.4 Practical Demonstration

• Diffusion of bromothymol blue in agar at different temperatures – measure colour change rate as a proxy for gas diffusion.
• Stomatal aperture measurement under varying light intensities – link to CO₂ uptake.

8. Mitotic Cell Cycle (Syllabus 7 – LO 7.1‑7.4)

8.1 Phases & Checkpoints

• Interphase – G₁ (growth), S (DNA synthesis), G₂ (pre‑mitotic checks).
• Prophase – chromatin condenses, nuclear envelope breaks down, spindle fibres form from centrosomes.
• Metaphase – chromosomes align at the metaphase plate; kinetochores attach to spindle microtubules.
• Anaphase – sister chromatids separate, pulled to opposite poles.
• Telophase – nuclear envelopes re‑form, chromosomes de‑condense.
• Cytokinesis – division of cytoplasm; plant cells form a cell plate, animal cells develop a cleavage furrow.

8.2 Molecular Control

• Cyclins bind cyclin‑dependent kinases (CDKs) → phosphorylation of target proteins.
• Spindle‑assembly checkpoint ensures all kinetochores are attached before anaphase onset.
• p53 tumour suppressor monitors DNA integrity; triggers cell‑cycle arrest or apoptosis if damage is detected.

8.3 Practical Observation

1. Root tip squash – stain with Feulgen or acetocarmine to visualise chromosomes at different stages.
2. Time‑lapse video of cultured animal cells undergoing mitosis (phase‑contrast microscopy).

9. Nucleic Acids & Protein Synthesis (Syllabus 8 – LO 8.1‑8.5)

9.1 DNA Structure & Replication

• Double helix: deoxyribose‑phosphate backbone + nitrogenous bases (A, T, G, C).
• In eukaryotes DNA is wrapped around histone octamers → nucleosomes → chromatin.
• Key enzymes:

  • DNA helicase – unwinds the double helix.
  • DNA primase – synthesises short RNA primers.
  • DNA polymerase III (prokaryotes) / DNA polymerase δ & ε (eukaryotes) – adds nucleotides (5’→3’).
  • DNA ligase – joins Okazaki fragments on the lagging strand.
  • Topoisomerase (gyrase) – relieves supercoiling.

• Semiconservative replication: each daughter DNA contains one parental and one newly synthesised strand.

9.2 Transcription & RNA Processing

• RNA polymerase II synthesises a primary transcript (pre‑mRNA) using one DNA strand as template.
• 5’‑capping – addition of a modified guanine nucleotide protects mRNA from degradation.
• Splicing – removal of introns by the spliceosome; exons are ligated to form mature mRNA.
• Poly‑A tail – addition of a string of adenine residues at the 3’ end enhances stability and export.

9.3 Translation

• Occurs on 80 S ribosomes (40 S small subunit + 60 S large subunit) in the cytoplasm or on the rough ER.
• mRNA codons are read sequentially; tRNA anticodons deliver the appropriate amino acid.
• Peptidyl‑transferase centre of the large subunit forms peptide bonds.
• Post‑translational modifications (phosphorylation, glycosylation, cleavage) diversify protein function.

9.4 Mutations & DNA Repair

• Point mutations: substitution, insertion, deletion – may be silent, missense, or nonsense.
• Frameshift mutations – shift the reading frame, usually producing non‑functional proteins.
• Repair mechanisms:

  • Base‑excision repair (BER) – removes damaged bases.
  • Nucleotide‑excision repair (NER) – removes bulky lesions (e.g., UV‑induced thymine dimers).
  • Mismatch repair – corrects replication errors.
  • Homologous recombination – repairs double‑strand breaks.

9.5 Practical Skills

1. Gel electrophoresis of PCR‑amplified DNA fragments – demonstrate size separation.
2. Restriction digest mapping – identify cut sites of EcoRI, HindIII, etc.
3. Western blot – detect a specific protein using antibodies.

10. Infectious Diseases (Syllabus 9 – LO 9.1‑9.4)

10.1 Bacterial Pathogens

• Structure: cell wall (peptidoglycan), plasma membrane, sometimes capsule, flagella, pili.
• Examples: Mycobacterium tuberculosis (acid‑fast cell wall), Streptococcus pneumoniae (capsule aids evasion).
• Virulence mechanisms: toxin production (exotoxins, endotoxins), adhesion factors, immune evasion.
• Control: antibiotics (β‑lactams inhibit cell‑wall synthesis; quinolones inhibit DNA gyrase).

10.2 Viral Pathogens

• Structure: nucleic acid (DNA or RNA), protein capsid, optional lipid envelope.
• Replication strategies:

  • DNA viruses (e.g., Herpesvirus) – enter nucleus, use host DNA polymerase.
  • RNA viruses (e.g., Influenza) – replicate in cytoplasm using viral RNA‑dependent RNA polymerase.
  • Retroviruses (e.g., HIV) – reverse‑transcribe RNA into DNA, integrate into host genome.

• Control: antivirals (e.g., oseltamivir inhibits neuraminidase), vaccination (inactivated, live‑attenuated, subunit).

10.3 Practical Techniques

1. Gram staining – differentiate Gram‑positive and Gram‑negative bacteria.
2. Plaque assay – quantify lytic virus particles on a lawn of host cells.
3. Antibiotic susceptibility (Kirby‑Bauer disc diffusion) – assess resistance patterns.

11. Immunity (Syllabus 10 – LO 10.1‑10.5)

11.1 Innate (Non‑Specific) Immunity

• Physical barriers: skin, mucous membranes, cilia.
• Chemical barriers: lysozyme, low pH, antimicrobial peptides.
• Cellular defences: phagocytes (macrophages, neutrophils), natural killer (NK) cells, complement cascade.
• Inflammatory response – vasodilation, increased permeability, chemotaxis.

11.2 Adaptive (Specific) Immunity

• B‑cells – produce antibodies (IgM, IgG, IgA, IgE, IgD); undergo class switching and somatic hypermutation.
• T‑cells – CD4⁺ helper T‑cells (Th1, Th2) coordinate response; CD8⁺ cytotoxic T‑cells kill infected cells.
• MHC molecules – present antigenic peptides (MHC I to CD8⁺ T‑cells, MHC II to CD4⁺ T‑cells).
• Immunological memory – memory B‑ and T‑cells enable a faster secondary response.

11.3 Vaccination

• Live‑attenuated, inactivated, subunit, toxoid, and conjugate vaccines.
• Goal: generate protective antibodies and memory cells without causing disease.

11.4 Hypersensitivity & Autoimmunity

• Type I – IgE‑mediated allergy.
• Type II – Antibody‑mediated cytotoxicity (e.g., haemolytic disease).
• Type III – Immune‑complex deposition (e.g., serum sickness).

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