amazon.co.ukA-Level Biology revision on cell structure: eukaryotic and prokaryotic ultrastructure, the fluid mosaic membrane, transport across membranes, mitosis and meiosis, with required practicals (AQA RP1/RP2, OCR PAG1) and multi-board exam practice.
Exams routinely present an unlabelled electron micrograph and ask you to identify organelles and link each to a function. The safest answers connect structure to process, not just location:
| Organelle | Structural features to quote | Function link that earns marks |
|---|---|---|
| Nucleus | Double membrane (nuclear envelope) with pores; chromatin; nucleolus | Pores allow mRNA out; nucleolus makes ribosomes; contains genetic material |
| Mitochondrion | Double membrane; inner folded into cristae; matrix | Cristae give large surface area for the electron transport chain; matrix holds Krebs-cycle enzymes; own circular DNA and ribosomes |
| Rough ER | Membrane sheets studded with ribosomes | Folds and transports proteins made on its ribosomes |
| Smooth ER | Membrane tubes, no ribosomes | Lipid and phospholipid synthesis and transport |
| Golgi apparatus | Stack of flattened membrane sacs; vesicles pinch off | Modifies (e.g. adds sugars), sorts and packages proteins; makes lysosomes |
| Lysosome | Single membrane, hydrolytic enzymes (lysozyme) | Digest worn organelles and engulfed pathogens; can release enzymes for cell death |
| Chloroplast | Double membrane; thylakoid membranes stacked into grana; stroma; starch grains | Thylakoids hold photosystems for the light-dependent reaction; stroma contains Calvin-cycle enzymes |
| Cell-surface membrane | Phospholipid bilayer + proteins (fluid mosaic) | Controls entry and exit; cell recognition |
| Ribosome | 80S, no membrane | Site of translation (protein synthesis) |
| Centrioles | Two bundles of microtubules (animals only) | Organise the spindle in cell division |
| Vacuole (plants) | Tonoplast membrane; cell sap | Stores solutes; water entry makes cells turgid for support |
The classic six-marker is mitochondrial structure enabling efficient aerobic respiration: double membrane isolates reactions; cristae maximise surface area for respiratory enzymes and the electron transport chain; the narrow intermembrane space builds a steep proton gradient for chemiosmosis; the matrix contains the Krebs-cycle enzymes, DNA and ribosomes so the organelle can make some of its own proteins quickly. Practise writing that as linked structure-function pairs because the mark scheme awards each pair, not description alone.
Compare prokaryotes with eukaryotes in the terms the mark schemes use: prokaryotes have no membrane-bound organelles and no true nucleus - their single circular DNA molecule (not associated with histones) lies free in the cytoplasm along with plasmids, which carry non-essential genes such as antibiotic resistance; ribosomes are 70S (smaller than the 80S of eukaryotes); the cell wall is peptidoglycan (murein), not cellulose; many have a capsule of slime, pili and flagella without the 9+2 microtubule arrangement. Reproduction is by binary fission: DNA replication followed by division roughly every 20 minutes in ideal conditions. Viruses sit outside both groups: a nucleic acid core (DNA or RNA) inside a protein capsid, sometimes with a lipid envelope stolen from the host membrane and attachment proteins (e.g. HIV has reverse transcriptase). Viruses are acellular and can only reproduce inside host cells.
Microscopy maths is guaranteed easy marks. Optical microscopes resolve to about 0.2 μm and magnify to about ×1500; transmission electron microscopes (TEM) pass electrons through ultrathin sections for ~0.1 nm resolution and 2D internal detail; scanning electron microscopes (SEM) scan surfaces for 3D topography at ~20 nm. Always convert to the same units before dividing: magnification = measured image size ÷ actual size, and actual size = image size ÷ magnification. For graticule calibration: at ×100 the eyepiece graticule may be 100 divisions; if each stage-micrometer division is 10 μm and 10 graticule divisions span one stage division, one eyepiece division = 10 μm at that objective - recalculate for each objective lens. In drawings, a clean single line, no shading, and a correctly calculated scale bar or ×magnification gain the practical marks (AQA RP1; OCR PAG1).
The fluid mosaic model explains everything the examiners ask about membranes: phospholipids move laterally (fluid) while proteins are scattered through the bilayer (mosaic). The hydrophobic fatty-acid core makes the membrane selectively permeable: small non-polar molecules (O₂, CO₂, lipid-soluble vitamins) diffuse straight through, whereas polar molecules and ions need protein pathways. Cholesterol slots between phospholipids, regulating fluidity and stability; glycoproteins and glycolipids on the outer face act as antigens and receptors for cell recognition and signalling. Temperature questions: at low temperatures the membrane loses fluidity; above ~40 °C proteins denature and the bilayer disrupts, increasing permeability - the basis of the beetroot practical (AQA RP4; OCR PAG3) where you measure pigment leakage with a colorimeter and explain increased permeability with heat/solvent/ethanol via disruption of the bilayer and denaturation of membrane proteins.
Know the five transport mechanisms as definition + example + energy source:
Large-scale transport uses endocytosis (membrane engulfs material into vesicles; phagocytosis is a special case) and exocytosis (vesicles fuse with the membrane, e.g. neurotransmitter release) - both active processes requiring ATP. When asked to distinguish mechanisms, always name the energy source and the direction relative to the gradient; that pairing earns the second mark in most schemes.
The cell cycle is interphase (G₁ growth, S - DNA replicates, G₂ growth and organelle synthesis) followed by M phase (mitosis + cytokinesis). In mitosis: prophase - chromosomes condense into two sister chromatids joined at centromeres, the nuclear envelope breaks down and the spindle forms; metaphase - chromosomes line up on the equator, attached to spindle fibres by their centromeres; anaphase - spindle fibres shorten, pulling sister chromatids to opposite poles (centromeres divide); telophase - nuclear envelopes reform and chromosomes decondense; cytokinesis then divides the cytoplasm to give two genetically identical diploid cells. Prokaryotes divide by binary fission instead; viruses are not cells and replicate inside hosts. Mitotic index calculations from a stained root-tip squash (AQA RP2; OCR PAG4) are common: count only cells with visible condensed chromosomes in the numerator, and state safety points (wear gloves/goggles, iodine or toluidine blue stain, macerate the tip to spread cells). Interphase cells vastly outnumber mitotic cells, so expect a small index; a high index in a tumour sample underlies cancer chemotherapy that targets rapidly dividing cells.
Meiosis produces four genetically different haploid cells from one diploid cell via two divisions. After interphase I, meiosis I separates homologous chromosome pairs (prophase I: crossing over between homologous chromatids at chiasmata creates new allele combinations; metaphase I: independent assortment of homologous pairs at the equator), halving the chromosome number; meiosis II separates sister chromatids like mitosis. The exam loves the two variation sources: crossing over (recombination of alleles between chromatids) and independent assortment (2ⁿ combinations - for humans 2²³ ≈ 8 million per gamete), plus random fusion of gametes at fertilisation. Non-disjunction (failure of homologs or chromatids to separate) produces gametes with an extra or missing chromosome (e.g. trisomy 21). Compare with mitosis in a table if asked for differences: number of divisions, daughter cells produced, ploidy, genetic identity, role in life cycle.
Specialised cells are adapted so their structure matches function - quote adaptations, not just names. Erythrocytes: biconcave disc (large surface area to volume for gas exchange, flexible to squeeze through capillaries), no nucleus or mitochondria (more room for haemoglobin), packed with haemoglobin that binds oxygen. Sperm: acrosome with hydrolytic enzymes to penetrate the egg, many mitochondria in the mid-piece for ATP to power the flagellum, haploid nucleus, minimal cytoplasm for streamlining. Ciliated epithelium: cilia beat to move mucus in the airways; goblet cells secrete mucus trapping pathogens. Palisade cells: many chloroplasts, thin walls, large vacuole. Root hair cells: projections increase surface area for water and mineral uptake.
Levels of organisation are always examinable: specialised cells form tissues (similar cells with the same function), tissues form organs (several tissues working together, e.g. the stomach: muscle, epithelium, secretory tissue), organs form systems (digestive, circulatory). Stem cells supply new specialised cells: totipotent cells (early embryo/zygote) can become any cell type including extra-embryonic tissue; pluripotent cells (inner cell mass of the blastocyst) form all somatic cell types; multipotent cells (e.g. bone marrow) form a limited range of related cells. In mature animals most cells are permanently in G₀; plants retain meristems. Stem-cell uses (bone marrow transplants, regenerative medicine, iPS cells made by reprogramming adult cells with transcription factors) and their ethics (embryo destruction vs. medical benefit) link this sub-topic to gene-expression content - expect a discussion question for 3+ marks on why iPS cells avoid the ethical objections of embryo use.