A2.2 Cell structureCell structure connects microscopy, universal cellular organization, prokaryotic and eukaryotic diversity, specialized exceptions, differentiation, and evolutionary explanations for complex cells and multicellularity.SyllabusFirst assessment 2025TopicA2.2LevelHL
What you’ll learn14 learning objectivesChoose one objective for a focused lesson, or study the complete topic.172 minStart full topicA2.2.1Cells as basic structural unit• Cells are the basic structural and functional units of living organisms• Cells carry out essential life processes and are usually microscopicLOW frequency · 1 paper10 minView conceptA2.2.2Microscopy skills• Use light microscopes to prepare mounts, stain specimens, and observe cells• Measure cells with eyepiece graticules and stage micrometers• Calculate magnification, actual size, and scale barsLOW frequency · 16 papers15 minView conceptA2.2.3Developments in microscopy• Resolution, not only magnification, determines visible detail• TEM reveals internal ultrastructure; SEM reveals surface detail• Cryogenic EM, fluorescence, and immunofluorescence reveal molecules and specific structuresLOW frequency · 3 papers12 minView conceptA2.2.4Structures common to all cells• All cells have a plasma membrane, cytoplasm, DNA, and ribosomes• These structures control exchange, support metabolism, store information, and make proteinsLOW frequency10 minView conceptA2.2.5Prokaryote cell structure• Prokaryotes lack a nucleus and membrane-bound organelles• Structure includes cell wall, plasma membrane, cytoplasm, 70S ribosomes, naked circular DNA, and plasmids• E. coli, Bacillus, and Staphylococcus are bacterial examplesLOW frequency · 11 papers12 minView conceptA2.2.6Eukaryote cell structure• Eukaryotes have a nucleus, 80S ribosomes, cytoskeleton, and compartmentalized cytoplasm• Organelles include mitochondria, ER, Golgi apparatus, vesicles, lysosomes, and vacuoles• Plant cells may include chloroplasts, large vacuoles, and cellulose cell wallsLOW frequency · 2 papers15 minView conceptA2.2.7Processes of life in unicellular organisms• Unicellular organisms carry out all life processes in one cell• Examples include Amoeba, Chlamydomonas, and Escherichia coli• Processes include nutrition, metabolism, response, excretion, homeostasis, growth, and reproductionLOW frequency · 4 papers10 minView conceptA2.2.8Differences in eukaryotic cells• Animal, fungal, and plant cells differ in cell walls, vacuoles, and storage structures• Plant cells have cellulose walls, chloroplasts, and large permanent vacuoles• Animal cells may have centrioles, cilia, flagella, lysosomes, and temporary vacuolesLOW frequency · 3 papers12 minView conceptA2.2.9Atypical cell structure• Atypical cells show limits of simple cell theory• Examples include multinucleate fungal hyphae and striated muscle fibres• Red blood cells and phloem sieve tubes lack nuclei at maturityLOW frequency · 5 papers12 minView conceptA2.2.10Cell identification in micrographs• Identify prokaryotic, plant, and animal cells in light and electron micrographs• Recognize organelles such as nucleus, mitochondria, chloroplasts, ER, Golgi, vacuoles, and ribosomes• Use visible structures, scale, and tissue context as evidenceLOW frequency · 2 papers15 minView conceptA2.2.11Drawing and annotation• Draw clear biological diagrams from electron micrographs without shading• Label visible organelles and include magnification or scale information• Annotate structures with functions where requiredLOW frequency10 minView conceptA2.2.12(HL)—Endosymbiosis origin of eukaryotes• Larger prokaryotic cells engulfed smaller prokaryotes that became mitochondria or chloroplasts• Evidence includes binary fission, circular naked DNA, 70S ribosomes, double membranes, and similar size• Chloroplast DNA links to cyanobacteria; mitochondrial DNA links to alphaproteobacteria/RickettsialesLOW frequency · 12 papers15 minView conceptA2.2.13(HL)—Cell differentiation• Differentiation produces specialized cells, tissues, and organs in multicellular organisms• Cells share the same genome but express different genes• Proteome differences reflect cell specializationLOW frequency · 1 paper12 minView conceptA2.2.14(HL)—Evolution of multicellularity• Multicellularity evolved repeatedly in fungi, algae, plants, and animals• Required features include cell adhesion, communication, and differentiation• Advantages include larger body size and cell specializationLOW frequency12 minView concept
Exam analysisAn aggregate view of how the complete A2.2 Cell structure appears across past papers.Chance of appearing41%of analysed past papersLatest appearanceNovember 2025Most common paperPaper1Most tested objectivesA2.2.2—Microscopy skills20 questionsA2.2.5—Prokaryote cell structure13 questionsA2.2.12 (HL)—Endosymbiosis origin of eukaryotes12 questionsCommon question formatsDefinition or recallGraph interpretationProcess explanationCalculationEvaluationStructured responseExam question appearancesSessionPaperQuestionMarksFocusNovember 2025Paper1A ["HL"] · TZ312[ 1 ]A2.2.9—Atypical cell structureNovember 2025Paper1B ["HL"] · TZ31(b)(ii)[ 1 ]A2.2.2—Microscopy skillsNovember 2025Paper1B ["HL"] · TZ31(b)(i)[ 1 ]A2.2.2—Microscopy skillsNovember 2025Paper1B ["HL"] · TZ31(a)[ 1 ]A2.2.2—Microscopy skillsMay 2025Paper2 ["HL"] · TZ28(a)[ 3 ]A2.2.8—Differences in eukaryotic cells•••61 more appearancesPractice this topic10 original exam-style questions based on the patterns above. Start practice Coverage 2012–2025 · Updated 15 Jul 2026
