AQA A-Level Biology: Cells and Immunity — Complete Revision Guide (7402)
AQA A-Level Biology: Cells and Immunity
Cells and immunity is the second course on the AQA A-Level Biology (7402) path and the first to ask the student to think at two scales simultaneously: the sub-cellular architecture of the organelles, and the population-level behaviour of immune cells responding to an infection. The course bridges the biochemistry built in biological molecules and the whole-organism physiology developed in exchange and transport. Why does the surface-area-to-volume constraint force eukaryotes to internalise transport through endomembrane systems? Why does the lipid bilayer self-assemble and remain semipermeable? How does a population of millions of distinct lymphocytes converge on a clonal response to a single pathogen within days? Each answer routes back through the cellular and immunological vocabulary built here.
Course 2 of 11 on the LearningBro AQA A-Level Biology learning path sits between the molecular foundations and the system-level physiology. It owns four of the twelve required practicals — RP1 (microscopy and graticule calibration), RP2 (the mitotic index), RP3 (membrane permeability) and RP4 (water potential of plant tissue) — making it the practical-densest course on the path. In Phase 2 of the rationalisation, the immunity portion was deliberately split into innate and adaptive sub-treatments and a dedicated lesson on vaccination, monoclonal antibodies and ELISA was added — three lessons rather than one, so that B and T cell biology, vaccination strategy and quantitative serology each receive the depth Paper 3 routinely demands.
Guide Overview
The Cells and Immunity course is structured as ten lessons that move from cell architecture through membrane biology and the cell cycle into the two arms of the immune response and the technological applications that flow from antibody biochemistry.
- Prokaryotic and eukaryotic cells
- Organelle structure and function
- Cell fractionation and microscopy
- The cell membrane
- Transport across membranes
- Endocytosis and exocytosis
- Cell cycle and mitosis
- Non-specific and innate immunity
- Adaptive immunity: B and T cells
- Vaccination, monoclonal antibodies and ELISA
AQA 7402 Specification Coverage
This course covers AQA 7402 Section 3.2 in full. The specification organises cells into five sub-sections — cell structure, all cells, transport across membranes, cell recognition and the immune system, and use of antibodies in medicine — each mapped here to one or more lessons (refer to the official AQA specification document for exact wording).
| Sub-topic | Spec area | Primary lesson(s) |
|---|---|---|
| Cell structure (eukaryotic) | 3.2.1.1 | Organelle structure and function |
| All cells: methods of study | 3.2.1.2 | Cell fractionation and microscopy |
| Prokaryotic vs eukaryotic | 3.2.1.3 | Prokaryotic and eukaryotic cells |
| Cell cycle and mitosis | 3.2.2 | Cell cycle and mitosis |
| Cell membrane structure | 3.2.3 | The cell membrane |
| Transport across membranes | 3.2.3 | Transport across membranes; endocytosis and exocytosis |
| Cell recognition and innate response | 3.2.4 | Non-specific and innate immunity |
| Adaptive immune response | 3.2.4 | Adaptive immunity: B and T cells |
| Vaccination and antibody applications | 3.2.4 | Vaccination, monoclonal antibodies and ELISA |
Section 3.2 is examined on all three AQA 7402 papers but appears most heavily on Paper 1 (cells, transport, immunity) and on Paper 3 in synoptic combination with biochemistry and respiration content. Quantitative items on magnification, actual size, mitotic index and percentage water-potential change are reliable Paper 3 staples.
Prokaryotic and Eukaryotic Cells
The prokaryotic and eukaryotic cells lesson establishes the foundational comparative framework: prokaryotes have no nuclear envelope, no membrane-bound organelles, 70S ribosomes and a peptidoglycan-containing cell wall; eukaryotes have a nuclear envelope, an extensive endomembrane system, 80S ribosomes and (in plants and fungi) a cellulose- or chitin-containing cell wall. Viruses, although not cells, are included for completeness as nucleic-acid-and-capsid particles dependent on host machinery for replication. The comparative table feeds directly into the antibiotic-targeting reasoning developed in non-specific and innate immunity and the pathogen biology returned to in populations and ecosystems.
Organelle Structure and Function
The organelle lesson develops the structure-function relationships for the nucleus, mitochondria, chloroplasts, ribosomes, rough and smooth endoplasmic reticulum, the Golgi apparatus, lysosomes, the cell-surface membrane, the cell wall, vacuoles and the cytoskeleton. Each organelle's adaptations are tied to the metabolic process it performs: cristae increase the inner mitochondrial membrane area available for electron-transport-chain complexes, ribosomes on the rough endoplasmic reticulum thread nascent polypeptides into the lumen for secretion, and the Golgi modifies, sorts and packages those polypeptides into transport vesicles that exocytose at the plasma membrane.
This lesson contains the early microscopy anchor for Required Practical 1: use of optical microscopy to view cells, with eyepiece-graticule calibration against a stage micrometer. The same microscopy technique returns explicitly in the cell-cycle and mitotic-index practical anchored in cell cycle and mitosis.
Cell Fractionation and Microscopy
The cell fractionation lesson covers the three-step preparative sequence (cold, isotonic, buffered homogenisation; homogenate filtration; differential centrifugation) used to isolate organelles for downstream analysis, and contrasts the resolution and magnification of light, transmission electron and scanning electron microscopy. Resolution is the minimum separation at which two points can be distinguished; magnification is the ratio of image size to actual size. Electron microscopes achieve nanometre resolution because the de Broglie wavelength of accelerated electrons is several orders of magnitude shorter than visible light. Calculations of magnification, actual size and scale-bar conversion are examined every series.
The Cell Membrane
The cell membrane lesson develops the fluid-mosaic model originally proposed by Singer and Nicolson: a bilayer of phospholipids with their hydrophobic fatty-acid tails interior-facing and their hydrophilic phosphate heads facing the aqueous cytosol and extracellular environment, with integral and peripheral proteins, glycoproteins, glycolipids and cholesterol distributed across the bilayer. Cholesterol intercalates between phospholipid tails and modulates membrane fluidity: it stiffens membranes at body temperature and prevents excessive rigidity at lower temperatures.
This lesson owns Required Practical 3: investigation into the effect of a named variable on the permeability of cell-surface membranes, conventionally using beetroot discs and a temperature or ethanol gradient with the leakage of betalain pigment quantified by colorimetry. The pigment leakage above critical temperatures or solvent concentrations is the direct experimental evidence for the protein-and-lipid-bilayer model — denaturation of integral proteins and disruption of bilayer packing both increase permeability.
Transport Across Membranes
The transport lesson develops simple diffusion (non-polar, small molecules direct through the bilayer), facilitated diffusion (polar and charged species through channel and carrier proteins, down their concentration gradient, passive), osmosis (the net movement of water from a region of higher to lower water potential through a partially permeable membrane), and active transport (ATP-dependent transport against the concentration gradient, often as primary active transport through pumps or as secondary active transport through co-transporters).
Water potential is examined with care: candidates must use the convention that pure water has a water potential of zero, with all solutions having a negative water potential, and that water moves down its potential gradient (from less negative to more negative). This lesson owns Required Practical 4: investigation into the water potential of plant tissue via the standard potato-cylinder mass-change method across a sucrose gradient. The water potential of the tissue is read off the intercept of the percentage mass change plot at zero net change.
Co-transport of glucose and amino acids into intestinal epithelial cells is the canonical example of secondary active transport, returned to explicitly in digestion and absorption.
Endocytosis and Exocytosis
The endocytosis and exocytosis lesson covers the bulk-transport mechanisms used for material too large to cross through channels or pumps: receptor-mediated endocytosis, phagocytosis (the engulfment of solid particles, central to the innate immune response in the next lesson) and exocytosis of vesicles containing secreted proteins. Bulk transport is ATP-dependent and depends on the dynamic membrane behaviour established in the cell membrane lesson.
Cell Cycle and Mitosis
The cell cycle lesson develops the interphase-mitosis-cytokinesis sequence, with interphase subdivided into G1, S (DNA replication) and G2, and mitosis into prophase, metaphase, anaphase and telophase. Each stage's chromosomal appearance — condensed chromatids, equatorial alignment on the metaphase plate, sister-chromatid separation toward the poles, decondensation in two daughter nuclei — is examined in microscopy form.
This lesson owns Required Practical 2: preparation of stained squashes of cells from plant root tips; set-up of microscopy to identify the stages of mitosis in these stained squashes and calculation of a mitotic index. The mitotic index — the percentage of cells in any phase of mitosis — is a routine quantitative output and is examined as a measure of tissue proliferation rate. Uncontrolled cell-cycle progression (loss of checkpoint regulation) underwrites the cancer biology threaded through DNA, genes and inheritance and gene expression and biotechnology. Meiosis, which generates haploid gametes with genetic variation, is treated in detail in meiosis and genetic variation, and the mitotic-index practical here is the natural cross-reference.
Non-Specific and Innate Immunity
The non-specific and innate immunity lesson develops the first line of defence: physical and chemical barriers (the skin, the mucous membranes of the respiratory and gastrointestinal tracts, stomach acid, lysozyme in tears and saliva) and the cellular response of phagocytosis. The phagocyte recognises foreign antigen, engulfs the pathogen into a phagosome, fuses the phagosome with a lysosome to form a phagolysosome, and digests the pathogen with hydrolytic enzymes. Antigen fragments are then presented on MHC class II molecules at the phagocyte surface — the bridge into adaptive immunity in the next lesson.
The non-specific arm is fast (minutes to hours), broad-spectrum and does not generate memory. Its principal cell types — neutrophils and macrophages — are the workhorses of acute infection control. The complement cascade is included in outline as a humoral component that opsonises pathogens for phagocytosis and can directly lyse some bacteria.
Adaptive Immunity: B and T Cells
The adaptive immunity lesson — a Phase 2 dedicated treatment — develops the lymphocyte response in full. T cells with receptors complementary to the presented antigen-MHC complex are activated by clonal selection. Helper T cells (CD4+) release cytokines that stimulate B cell proliferation, cytotoxic T cell activation and macrophage activation. Cytotoxic T cells (CD8+) directly kill infected cells displaying antigen on MHC class I. Activated B cells with surface immunoglobulin complementary to the antigen are also clonally selected, and divide by mitosis into plasma cells (which secrete soluble antibody) and memory B cells (which persist for years and confer long-lasting immunity).
The adaptive arm is slow on first exposure (days to weeks for full activation), highly specific, and generates immunological memory. The secondary response on re-exposure is faster, larger in magnitude and longer-lasting because of the memory B and T cell populations — the basis of vaccination, covered in the next lesson. Antibody structure (Y-shaped immunoglobulin with two heavy and two light chains, variable regions forming the antigen-binding sites, constant region driving effector function) is developed here with explicit reference to the protein structure-function principle introduced in protein structure and function. The variable region's complementarity to a specific epitope is the structural analogue of the enzyme-substrate complementarity covered in enzyme kinetics.
Phase 2 separated this content from non-specific immunity because the adaptive response requires a depth — clonal selection, MHC class I vs II, helper vs cytotoxic T cell roles, plasma vs memory B cell fates — that was crowded out in the legacy single-lesson treatment. A dedicated lesson now allows full coverage of antibody structure, the primary-vs-secondary response graph (a Paper 3 staple) and the active-vs-passive immunity distinction.
Vaccination, Monoclonal Antibodies and ELISA
The vaccination and antibody applications lesson — also a Phase 2 addition — develops three high-yield applied topics that previously sat in a thin condensed treatment. Vaccination uses an attenuated, killed, subunit or mRNA pathogen preparation to provoke a primary adaptive response and generate memory cells, without the morbidity of natural infection. Herd immunity (the protection of unvaccinated individuals when a sufficient fraction of the population is immune) is the population-level consequence and is examined as an applied AO2 item. Active vs passive, natural vs artificial immunity is a four-cell matrix candidates must be able to populate.
Monoclonal antibodies are antibodies of a single specificity produced by fusing a B cell with a myeloma to create an immortal hybridoma cell line. Applications include diagnostic assays (pregnancy testing detecting human chorionic gonadotropin), therapeutic targeting (rituximab against CD20+ lymphoma cells, trastuzumab against HER2+ breast cancer cells) and immunohistochemistry. The ethical issues around mouse-derived antibodies (sensitisation, immunogenicity) and the modern humanisation strategies are included.
The enzyme-linked immunosorbent assay (ELISA) is developed in mechanistic detail: target antigen immobilised in a microtiter plate well, primary antibody binding, washing, enzyme-conjugated secondary antibody binding, washing again, substrate addition and colorimetric quantification. ELISA's quantitative output (optical density vs concentration in a standard curve) connects directly to the calibration-curve technique introduced in biochemical tests and developed statistically in statistics and practical skills.
Synoptic Links Across the Specification
Cells and immunity is the conceptual hub of AQA 7402. Membrane biology developed here underwrites every gas-exchange and transport-system lesson in exchange and transport — alveolar surfactant, capillary endothelium, the proximal tubule and the absorptive epithelium of the small intestine all depend on the fluid-mosaic and co-transport vocabulary built in this course. The cell cycle anchors the cancer biology revisited in gene expression and biotechnology, where checkpoint failure and oncogene activation are the molecular causes of uncontrolled proliferation. Antibody-antigen complementarity is the structural cousin of enzyme-substrate complementarity from biological molecules, and the immune-response graphs feed directly into the disease ecology and epidemiology of populations and ecosystems.
Required Practical Anchors
This course owns four of the twelve AQA required practicals — the highest density on the path:
- RP1 (optical microscopy and graticule calibration) is anchored in the organelle structure and function lesson.
- RP2 (mitotic index from root-tip squashes) is anchored in the cell cycle and mitosis lesson.
- RP3 (membrane permeability of beetroot) is anchored in the cell membrane lesson.
- RP4 (water potential of plant tissue) is anchored in the transport across membranes lesson.
All four appear on Paper 3 in their quantitative analysis form (magnification, mitotic index, percentage transmittance, percentage mass change vs molarity) and on Paper 1 as the practical context for short structured items. Drilling the calculations to automaticity is a high-yield revision target.
Revision Strategy
Cells and immunity rewards a two-track revision habit. The cellular content (organelles, membranes, transport, cell cycle) is highly visual and benefits from blank-page recall: sketch a labelled eukaryotic cell, a fluid-mosaic membrane and an interphase-to-cytokinesis cell-cycle diagram from memory each week. The immunology content is process-driven and benefits from flow-diagram practice: the phagocytosis sequence, the T-helper / B-cell / plasma-cell / memory-cell branching, the primary-vs-secondary response graph. Convert both into question-and-answer flashcards on a spaced-repetition schedule.
Drill the four required practical calculations until they are automatic. A magnification question with a 1.5 cm image and a stated actual size of 30 micrometres should produce the answer of x500 in under fifteen seconds. A mitotic index question with 17 mitotic cells out of 200 should produce 8.5 percent immediately. Interleave cells questions with biochemistry questions, because Paper 3 will reliably combine the two.
Closing
Cells and immunity is the practical-densest course on the AQA 7402 path and the structural bridge between molecular biochemistry and whole-organism physiology. Phase 2's split into a dedicated adaptive-immunity lesson and a standalone vaccination-monoclonal-antibodies-ELISA lesson gives B and T cell biology and the applied serology each the depth Paper 3 routinely demands. Start with the Cells and Immunity course and work through all ten lessons in sequence; treat the membrane and transport content as essential foundations for exchange and transport, and the adaptive immunity content as the canonical worked example of structure-function in the immune system. The full AQA A-Level Biology learning path walks the whole sequence end-to-end.