Q: What does CORE IDEAS, Topic 1 - The Cell and Biomolecules of Life cover?
A: Build cell theory mastery, compare ultrastructure across kingdoms, and connect biomolecular architecture to membrane dynamics, enzyme kinetics, and stem cell regulation for the 2026 H2 Biology Papers 2–4.
Why this core idea anchors the syllabus
Assessment reach: Paper 2 short-structured items probe microscopy, organelle recognition, and transport calculations; Paper 3 essays often cross-link enzyme regulation with metabolism; Paper 4 practicals grade MMO/PDO/ACE using cytology and enzyme setups.
Conceptual spine: Every later unit (respiration, signalling, inheritance) depends on understanding how biomolecules build organised compartments and how those compartments exchange matter and information.
Bridging O- to A-Level: You are expected to move beyond naming parts-explain why mitochondria have cristae, how phospholipid structure controls membrane fluidity, and what denaturation does to catalytic geometry.
Syllabus map at a glance
Cell theory, microscopy workflow, and structural recognition (light vs electron resolution)
Prokaryotic and viral organisation; challenges to defining life
Membrane structure and selective transport (diffusion to vesicle trafficking)
Protein folding, denaturation, and structural exemplars (haemoglobin, collagen)
Enzyme kinetics, inhibition, and data treatment
Stem cell potency and clinical ethics
Concept 1: Cell theory, microscopy, and ultrastructure
Cell theory evolution: State the three tenets, then enrich with modern revisions-endosymbiosis explains the dual-membrane, 70S ribosome signature of mitochondria/chloroplasts, while viruses test the boundary by lacking autonomous metabolism. Quote microscopy evidence (Schleiden/Schwann, Pasteur) to justify “cells arise from existing cells.”
Light vs electron tools: Light microscopes resolve ≈200nm
; transmission electron microscopes (TEM) reach
≈0.2nm
but require fixed specimens. Calibrate before measuring:
Align the stage micrometer and eyepiece graticule.
Count coincidences to compute the size of one graticule division.
Use M=eyepiece magnification×objective magnification to check total magnification on drawings.
Sketches must feature continuous lines, proportional scaling, and labelled structures (e.g. double membrane of the nuclear envelope, 9+2 microtubule arrangement in flagella).
Organelle recognition: Prepare a comparison table for rough ER vs smooth ER, Golgi cisternae, lysosomes, peroxisomes, chloroplast thylakoids/stroma, mitochondrial cristae, centrioles, nucleolus granules. Practise describing form-function links (e.g. “Cristae increase membrane surface area for embedded ETC protein complexes, accelerating oxidative phosphorylation.”).
Bacteria and viruses: Highlight peptidoglycan walls, nucleoid regions, plasmids, 70S ribosomes, and absence of organelles. For viruses, contrast naked vs enveloped capsids, note host-derived envelopes, and use this to discuss why “replication only inside host cells” challenges the classic cell theory.
Practice prompt
Describe and explain how you would differentiate a mitochondrion from a chloroplast in an electron micrograph. Include magnification checks and evidence for endosymbiont ancestry (Paper 2 style).
Carbohydrates: contrast α- and β-glucose; show how 1,4- and 1,6-glycosidic linkages generate amylose vs amylopectin vs glycogen.
Lipids: glycerol esterified to fatty acids; mention cis double bonds introducing kinks that reduce packing and increase membrane fluidity.
Proteins: general amino acid structure HX2N−CHR−COOH; peptide bonds via condensation.
Use concise structure–function narratives:
Starch vs glycogen: Branched glycogen with many termini accelerates glucose mobilisation for hepatocytes; amylose coils suit plant starch granules.
Cellulose microfibrils:β-1,4 linkages allow hydrogen bonding between chains, creating tensile strength in the plant cell wall.
Triglycerides vs phospholipids: Hydrophobic storage droplets vs amphipathic bilayers.
Haemoglobin: Globular quaternary structure with haem prosthetic groups enables cooperative O2 loading; mention Bohr shift for Paper 3 essays.
Collagen: Triple-helix, glycine-rich motif confers tensile strength; post-translational hydroxylation needs vitamin C (clinical link to scurvy).
Worked example
Explain why a phospholipid with two saturated fatty acid chains solidifies faster than one with unsaturated chains at 4∘C. Tie in van der Waals interactions and cholesterol’s role as a fluidity buffer.
Concept 3: Membranes and transport logistics
Describe the fluid mosaic model: phospholipid bilayer with integral and peripheral proteins, cholesterol, glycolipids, and glycoproteins. Clarify each transport mode with energetics and examples:
Simple diffusion: Non-polar molecules (e.g. OX2) down gradient.
Osmosis: Water movement; link to plasmolysis vs turgidity in plant cells.
Facilitated diffusion: Carrier and channel proteins; GLUT transporters.
Active transport: ATP-powered pumps (Na+/K+ ATPase) establishing electrochemical gradients.
Discuss why kinetic curves for facilitated diffusion plateau (limited carrier sites) while simple diffusion remains linear relative to gradient strength.
Calculation drill
Given external sucrose is 0.20moldm−3 and cytosol is 0.15moldm−3, predict water flow direction and justify using water potential. Extend by asking how aquaporin abundance modulates equilibration time.
Concept 4: Enzyme kinetics, regulation, and inhibition
Explain the catalytic cycle using the induced-fit hypothesis. Show how lowering activation energy Ea changes reaction rates via the Arrhenius relationship k=Aexp(−RTEa).
Temperature effects: Increased kinetic energy until denaturation disrupts hydrophobic cores and disulfide bonds; relate to optimum curves.
pH effects: Ionisation of active site residues alters substrate binding (e.g. histidine in catalytic triads).
Substrate vs enzyme concentration: Use Michaelis–Menten-style reasoning (Paper 4 data analysis). Distinguish between Vmax and Km conceptually.
Inhibitors: Competitive inhibitors resemble substrates (malonate vs succinate); non-competitive inhibitors bind allosteric sites (cyanide on cytochrome oxidase). Mention allosteric regulation as a basis for metabolic control.
Include experimental advice: pre-equilibrate reactants, identify control variables (temperature, pH, enzyme source), and justify data treatment (time-course graphs, initial rate calculations).
Data handling exercise
Interpret a set of absorbance vs time traces for catalase with and without a competitive inhibitor. Deduce which curve represents the inhibitor and explain the change in apparent Km.
Pluripotent: Embryonic stem cells (inner cell mass) differentiating into all somatic tissues.
Multipotent: Adult stem cells (haematopoietic progenitors) restricted to specific lineages.
Discuss self-renewal, niche signals (Notch, Wnt), and applications: bone marrow transplantation, organoids for drug screening. Address ethical considerations-balance therapeutic promises with concerns over embryo-derived lines.
Ethics discussion starter
“Should induced pluripotent stem cells replace embryonic sources entirely?” Require students to weigh reprogramming efficiency, genomic stability, and disease modelling accuracy.
Exam application checkpoints
Paper 2: Practise annotating electron micrographs with precise terminology and scale bars.
Paper 3: Prepare essay outlines linking membrane transport to enzyme pathways (e.g. how lysosomal acid hydrolases depend on membrane trafficking).
Paper 4: Run catalase or amylase investigations, logging MMO, PDO, ACE evidence explicitly. Use error discussion frameworks (systematic vs random, improvement suggestions).
