describe the behaviour of chromosomes in plant and animal cells during meiosis and the associated behaviour of the nuclear envelope, the cell surface membrane and the spindle (names of the main stages of meiosis, but not the sub-divisions of prophase

Cambridge International AS & A Level Biology – Complete Syllabus Notes (9700)

Key Concepts (Syllabus‑wide)

• Cell structure & function
• Biochemistry & metabolism
• DNA, genes & inheritance
• Natural selection & evolution
• Environment & homeostasis
• Observation, experiment & evaluation

Learning Objectives & Assessment Objectives

1. Cell Structure (AS 1)

1.1 Eukaryotic vs Prokaryotic Cells

• Eukaryotes – nucleus, membrane‑bound organelles (mitochondria, chloroplasts, ER, Golgi, lysosomes, vacuoles, peroxisomes).
  AO1, AO2
• Prokaryotes – no nucleus, DNA in nucleoid, no membrane‑bound organelles.
  AO1
• Viruses – acellular, nucleic acid + protein coat; replicate only inside host cells.
  AO1

1.2 Microscopy

• Light microscope: up to ~2000×, useful for whole cells and large organelles.
• Electron microscope: TEM (ultrastructure) & SEM (surface).
• Resolution limits and staining techniques (e.g., iodine for starch, methylene blue for nuclei).

1.3 Practical Skills (AO3)

• Preparing wet mounts, staining, measuring cell dimensions (micrometer).
• Calculating cell volume (V = πr²h for cylindrical cells) and surface‑area‑to‑volume ratio (SA:V) – essential for diffusion & transport questions.

2. Biological Molecules (AS 2)

2.1 Carbohydrates

2.2 Lipids

• Fatty acids, triglycerides – energy‑dense (≈9 kJ g⁻¹).
• Phospholipids – form bilayer of all membranes; amphipathic nature creates selective barrier.
• Steroids – cholesterol (animals) and phytosterols (plants) modulate membrane fluidity.

2.3 Proteins

• Four levels of structure (primary → quaternary).
• Functions: enzymes, transport, structural, signalling, immune defence.

2.4 Water

• Universal solvent; high specific heat, cohesion, adhesion, surface tension.
• Water potential (Ψ = Ψ_s + Ψ_p) – quantitative calculations for plant uptake.

2.5 Quantitative Skills (AO3)

• Energy calculations: 1 mol glucose → 30 mol ATP (cellular respiration).
• Water‑potential examples: Ψ_s = –iCRT; Ψ_p = ρgh.

3. Enzymes (AS 3)

• Biological catalysts – lower activation energy, increase rate.
• Active site–substrate complex; induced‑fit model.
• Factors affecting activity:

  • Temperature (optimum ≈ 37 °C for human enzymes)
  • pH (e.g., pepsin ≈ 2, alkaline phosphatase ≈ 9)
  • Substrate concentration – Michaelis–Menten kinetics (V_max, K_m)
  • Enzyme concentration
  • Inhibitors – competitive, non‑competitive, irreversible.

• Regulation: allosteric control, covalent modification, synthesis/degradation.

Enzyme Kinetics (AO2, AO3)

Practical (AO3)

• Colourimetric assay (e.g., catalase activity – O₂ bubbles counted).
• Plotting rate vs. substrate concentration; calculating K_m and V_max.

4. Cell Membranes & Transport (AS 4)

4.1 Structure – Fluid‑Mosaic Model

• Phospholipid bilayer with embedded proteins, cholesterol (animals) / phytosterols (plants).
• Integral proteins (channels, carriers, receptors) & peripheral proteins.

4.2 Transport Mechanisms

4.3 Quantitative Applications (AO3)

• Fick’s law of diffusion: Rate = (D·A·ΔC)/d.
• Calculate water potential differences to predict direction of water movement in plants.
• Determine surface‑area‑to‑volume ratios for cells of different shapes (sphere, cylinder, prism).

4.4 Practical Skills (AO3)

• Diffusion chambers – measuring rate of starch breakdown (amylase) or O₂ diffusion.
• Osmosis experiments – plasmolysis in onion cells, calculating Ψ.

5. The Mitotic Cell Cycle (AS 5)

5.1 Interphase

• G₁ – growth, protein synthesis.
• S – DNA replication (semiconservative).
• G₂ – preparation for mitosis, organelle duplication.

5.2 Mitosis (Main Stages)

5.3 Chromosome Behaviour & Associated Structures (AO2)

• Telomeres – protect chromosome ends; shorten with each division; telomerase restores length in germ cells.
• Spindle checkpoints – ensure all chromosomes are correctly attached before anaphase onset.
• Tumour formation – loss of checkpoint control → uncontrolled mitosis (e.g., p53 mutation).

5.4 Practical (AO3)

• Preparing and staining onion root tip squashes; identifying mitotic phases.
• Measuring mitotic index (% of cells in mitosis) under different treatments (e.g., colchicine).

6. Meiosis – Chromosome Behaviour & Associated Structures (AS 6)

6.1 Overview

Two successive nuclear divisions (Meiosis I & II) reduce chromosome number from diploid (2n) to haploid (n). Four genetically distinct gametophytes are produced. The process differs in plant and animal cells in spindle origin and cytokinesis.

6.2 Main Stages (Sub‑stages of prophase are not listed)

6.3 Plant vs Animal Differences (AO2)

• Spindle origin: Animal cells – centriolar centrosomes; Plant cells – diffuse MTOCs, no centrioles.
• Cytokinesis: Animal – contractile actin‑myosin ring → cleavage furrow; Plant – vesicle‑laden Golgi vesicles coalesce at phragmoplast to form a cell plate, later becomes a new cell wall.
• Cell wall formation: Only in plants, occurs after cytokinesis.

6.4 Quantitative/Analytical Skills (AO3)

• Calculate chromosome numbers at each stage (e.g., 2n → n after Meiosis I).
• Predict outcomes of meiotic errors (non‑disjunction) – e.g., trisomy 21 frequency.
• Interpret karyotype diagrams showing tetrad formation and chiasmata.

6.5 Practical (AO3)

• Squash preparations of onion root tips or mouse spermatocytes; identify stages of meiosis.
• Use fluorescent staining (DAPI) to visualise chiasmata.
• Quantify crossing‑over frequency by scoring recombination in test‑cross progeny (e.g., Drosophila eye colour).

7. Nucleic Acids & Protein Synthesis (AS 7)

7.1 DNA Structure & Replication (AO1, AO2)

• Double helix; antiparallel strands; deoxyribose sugar; phosphate backbone; bases A‑T, G‑C.
• Semiconservative replication: each parental strand serves as template.
• Key enzymes: helicase (unwinds), DNA polymerase (adds nucleotides 5'→3'), primase (RNA primer), ligase (joins Okazaki fragments), topoisomerase (relieves supercoiling).
• Proofreading (3'→5' exonuclease) reduces mutation rate.

7.2 Transcription (AO1, AO2)

• RNA polymerase synthesises pre‑mRNA (5'→3') using DNA template.
• Processing in eukaryotes: 5' cap, poly‑A tail, intron splicing (spliceosome).

7.3 Translation (AO1, AO2)

• Ribosome (large + small subunit) reads mRNA codons.
• tRNA delivers specific amino acids; anticodon pairs with codon.
• Peptidyl‑transferase forms peptide bonds; elongation, termination (release factors).

7.4 Quantitative Example (AO3)

Calculate number of nucleotides required to synthesize a protein of 300 amino acids:

• Each amino acid → 3 nucleotides (codon) → 300 × 3 = 900 nt of mRNA.
• Including a 5' cap and poly‑A tail (≈30 nt) → total ≈ 930 nt.

7.5 Practical Skills (AO3)

• Gel electrophoresis of DNA fragments – calculate fragment size using a DNA ladder.
• PCR amplification – design primers, calculate melting temperature (Tm = 2°C × (A+T) + 4°C × (G+C)).
• Western blot – detect specific proteins using antibodies.

8. Plant Transport (AS 8)

8.1 Xylem

• Conducts water & minerals from roots to shoots.
• Driving forces: transpiration pull, root pressure, capillary action.
• Structure: dead, lignified vessels & tracheids with pits.

8.2 Phloem

• Transports organic nutrients (mainly sucrose) from sources to sinks.
• Pressure‑flow hypothesis – loading at source creates high turgor, unloading at sink creates low turgor.
• Living elements: sieve‑tube elements + companion cells.

8.3 Quantitative Calculations (AO3)

• Water potential difference (ΔΨ) to predict direction of water movement: Ψ = Ψ_s + Ψ_p (Ψ_s = –iCRT).
• Calculate pressure gradient needed for phloem flow: ΔP = (R·T·ΔC)/V (using ideal gas approximation for osmotic pressure).

8.4 Practical (AO3)

• Transpiration rate measurement using potometer; calculate water loss (ml h⁻¹).
• Starch test in leaves after dark period – evidence of photosynthate transport.

9. Animal Transport (AS 9)

9.1 Circulatory System

• Closed network: heart pumps blood through arteries, veins, capillaries.
• Blood composition – plasma (water, proteins, nutrients, waste) + formed elements (RBCs, WBCs, platelets).
• Gas exchange in pulmonary and systemic capillaries.

9.2 Respiratory Transport

• O₂ bound to haemoglobin (Hb) in red blood cells – cooperative binding (sigmoidal O₂‑Hb curve).
• CO₂ transported as bicarbonate (CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻).
• Bohr effect – low pH or high CO₂ shifts curve right, facilitating O₂ release.

9.3 Quantitative Aspects (AO3)

• Fick’s law for O₂ diffusion across alveolar membrane.
• Calculate cardiac output: CO = stroke volume × heart rate.
• O₂‑binding capacity: 1 g Hb binds ≈ 1.34 ml O₂.

9.4 Practical Skills (AO3)

• Measuring pulse and blood pressure; calculating mean arterial pressure (MAP = DP + 1/3 × SP).
• Blood gas analysis – interpreting pH, pCO₂, HCO₃⁻ values.

10. Infectious Diseases & Immunity (AS 10‑11)

10.1 Pathogens & Disease Mechanisms

• Bacteria (e.g., Mycobacterium tuberculosis – granuloma formation).
• Viruses (e.g., HIV – reverse transcription, integration).
• Protozoa (e.g., Plasmodium falciparum – malaria life cycle).
• Fungi (e.g., Candida albicans – opportunistic infection).

10.2 Host Defence

• Physical barriers: skin, mucous membranes.
• Innate immunity: phagocytosis, inflammation, complement.
• Adaptive immunity:

  • Humoral – B‑cell activation, antibody production (IgM → IgG class switching).
  • Cell‑mediated – T‑cell activation (CD4⁺ helper, CD8⁺ cytotoxic).

• Vaccination – principle of active immunity (e.g., attenuated, inactivated, subunit vaccines).

10.3 Antimicrobial Agents (AO2)

• Antibiotics – target bacterial cell wall (penicillins), protein synthesis (tetracyclines), DNA replication (fluoroquinolones).
• Resistance mechanisms – enzymatic degradation, altered targets, efflux pumps.

10.4 Practical (AO3)

• Disc diffusion assay – measure zone of inhibition, calculate % inhibition.
• ELISA – detect specific antibodies in serum.
• PCR for pathogen identification – design primers for conserved gene regions.

11. Energy & Respiration (A‑Level 12)

11.1 Aerobic Respiration

Overall equation: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ≈ 30 ATP

• Glycolysis (cytoplasm) – 2 ATP net, 2 NADH.
• Link reaction (mitochondrial matrix) – 2 NADH, CO₂ released.
• Krebs cycle – 2 ATP (GTP), 6 NADH, 2 FADH₂, CO₂.
• Electron transport chain (inner mitochondrial membrane) – chemiosmotic ATP synthesis (≈ 28 ATP).

11.2 Anaerobic Respiration & Fermentation

• Alcoholic fermentation (yeast): glucose → ethanol + CO₂ + 2 ATP.
• Lactic acid fermentation (muscle): glucose → lactate + 2 ATP.

11.3 Quantitative Calculations (AO3)

• Calculate ATP yield from a given amount of glucose (e.g., 1 mol glucose → 30 mol ATP).
• Determine ΔG°′ for glycolysis steps using standard free‑energy values.
• O₂ consumption and CO₂ production rates from respirometry data.

11.4 Practical (AO3)

• Respirometer – measure O₂ uptake or CO₂ release in germinating seeds.
• Enzyme assays for lactate dehydrogenase activity.

12. Photosynthesis (A‑Level 13)

12.1 Overall Equation

6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂

12.2 Light‑dependent Reactions (Thylakoid Membranes)

• Photons excite chlorophyll → electrons transferred through photosystem II → plastoquinone → cytochrome b₆f → plastocyanin → photosystem I → ferredoxin → NADP⁺ reductase → NADPH.
• Water splitting (photolysis) releases O₂, provides electrons, and generates H⁺ gradient.
• ATP synthesis by chemiosmosis (ATP synthase) using H⁺ gradient.

12.3 Light‑independent Reactions (Calvin Cycle, Stroma)

• CO₂ fixation by Rubisco → 3‑phosphoglycerate (3‑PGA).
• Reduction of 3‑PGA to glyceraldehyde‑3‑phosphate (G3P) using ATP and NADPH.
• Regeneration of ribulose‑1,5‑bisphosphate (RuBP) – requires ATP.

12.4 Quantitative Aspects (AO3)

• Calculate theoretical maximum photosynthetic efficiency: (ΔG of glucose formation ÷ energy of absorbed photons) × 100%.
• Determine light‑saturation point from photosynthetic rate vs. irradiance graph.
• Use chlorophyll fluorescence (Fv/Fm) to assess photosystem II efficiency.

12.5 Practical (AO3)

• Measuring O₂ evolution with a gas‑evolution apparatus.
• Using a pulse‑amplitude modulated fluorometer to assess photoinhibition.
• Effect of CO₂ concentration on rate of photosynthesis – calculate % increase.

13. Homeostasis (A‑Level 14)

13.1 Regulation of Blood Glucose

• Insulin (β‑cells) – promotes glucose uptake, glycogen synthesis.
• Glucagon (α‑cells) – stimulates glycogenolysis, gluconeogenesis.
• Negative feedback loop – blood glucose deviation → hormone secretion → correction.

13.2 Thermoregulation

• Endothermy – metabolic heat production, insulation (fur, feathers), evaporative cooling (sweating, panting).
• Ectothermy – behavioural adaptations (basking, burrowing).

13.3 Fluid Balance

• ADH (vasopressin) – increases water re‑absorption in kidney collecting ducts.
• Osmoregulation in plants – stomatal opening/closing, root pressure.

13.4 Quantitative Examples (AO3)

• Calculate osmotic pressure (π = iMRT) for a 0.15 M NaCl solution at 25 °C.
• Determine basal metabolic rate (BMR) using the Harris‑Benedict equation.

13.5 Practical (AO3)

• Blood glucose monitoring using a glucometer – plot response to glucose load.
• Thermal imaging of endothermic animals to assess heat loss.

14. Control & Coordination (A‑Level 15)

14.1 Nervous System

• Neuron structure – dendrite, soma, axon, myelin sheath.
• Action potential: depolarisation (Na⁺ influx), repolarisation (K⁺ efflux), refractory period.
• Synaptic transmission – neurotransmitter release, receptor binding, termination.

14.2 Hormonal (Endocrine) System

• Hormone types – peptide (insulin), steroid (cortisol), amine (adrenaline).
• Signal transduction pathways – G‑protein coupled receptors, second messengers (cAMP, Ca²⁺).

14.3 Quantitative Modelling (AO3)

• Calculate nerve impulse speed: distance ÷ time (e.g., 1 m in 2 ms → 0.5 m s⁻¹).
• Dose‑response curves – EC₅₀ determination for a drug.

14.4 Practical (AO3)

• Record action potentials with an intracellular electrode; analyse amplitude and frequency.
• ELISA for hormone quantification (e.g., cortisol levels in saliva).

15. Inheritance (A‑Level 16)

15.1 Mendelian Genetics

• Monohybrid & dihybrid crosses – phenotypic & genotypic ratios.
• Law of independent assortment, law of segregation.

15.2 Non‑Mendelian Inheritance

• Incomplete dominance, codominance, multiple alleles (e.g., blood groups).
• Polygenic traits – continuous variation, normal distribution.
• Linkage & recombination – map distance (1 cM ≈ 1 % recombination).

15.3 Quantitative Genetics (AO3)

• Calculate heritability (h² = V_G/V_P).
• Predict offspring phenotype frequencies using Punnett squares and probability rules.

15.4 Practical (AO3)

• Test‑cross experiments in Drosophila – analyse wing phenotype ratios.
• DNA fingerprinting – restriction fragment length polymorphism (RFLP) analysis.

16. Selection & Evolution (A‑Level 17)

16.1 Natural Selection

• Variation,