Every cell ever observed fits a small set of plans. Inside that plan: organelles, membranes, surface area, and division of labour.
Every sub-topic below feeds at least one of these questions.
What are the features common to all cells and the features that differ?
How is microscopy used to investigate cell structure?
The required syllabus content for A2.2, in order. Each card is one lesson-sized checkpoint.
Nature of Science: Students should be aware that deductive reason can be used to generate predictions from theories.
Nature of Science: Students should appreciate that measurement using instruments is a form of quantitative observation.
Developments in microscopy
Structures common to cells in all living organisms
Prokaryote cell structure
Eukaryote cell structure
Processes of life in unicellular organisms
Differences in eukaryotic cell structure between animals, fungi and plants
Atypical cell structure in eukaryotes
Cell types and cell structures viewed in light and electron micrographs
Drawing and annotation based on electron micrographs
The three-part cell theory is biology's oldest big idea, and still its most universal.
Cells are the basic units of structure and function in all living organisms.
All living organisms are composed of one or more cells. The cell is the universal unit.
All cells come from pre-existing cells — never from non-living matter (today). Spontaneous generation was disproved by Pasteur in 1861.
From the general (all known organisms are made of cells), we deduce the specific (any newly discovered organism will be made of cells). Cell theory is a hypothesis-generator, not just a description.
A microscope by itself doesn't tell you how big anything is. To extract size from a micrograph you need the magnification formula, a scale bar, and clean unit conversions.
Or 1,000,000 nm. The full conversion chain.
Or 10⁻⁶ m. Typical eukaryotic cells: 10–100 µm.
The scale of large molecules. Hydrogen bond ~3 nm; ribosome ~25 nm.
Image size ÷ actual size. Units must match.
A scale bar on a micrograph measures 20 mm with a ruler, and the caption says it represents 0.4 µm. Convert: 20 mm = 20,000 µm. Magnification = 20,000 ÷ 0.4 = ×50,000. Now any other length on the micrograph can be converted: measured length × (1 / magnification) = real length.
Four key techniques the IB names. Each opened up a new scale or a new kind of question.
Uses a beam of electrons (much shorter wavelength than light → much better resolution). Reveals organelle structure, viruses, individual proteins. But: samples must be dead, dried, and coated; can't image live cells.
Samples flash-frozen in vitreous (non-crystalline) ice, then imaged with electrons. Lets us view proteins and biomolecules in near-native state — and biomolecules that don't crystallise are now imageable at atomic resolution.
Sample frozen rapidly, then fractured along lines of weakness — often through the middle of a phospholipid bilayer. Allowed identification of integral membrane proteins; led directly to the Singer–Nicolson fluid mosaic model of membranes.
Fluorescent dyes attached to specific antibodies bind to specific proteins. Different colours can label different molecules in the same sample. Works on living tissue → enables studies of dynamic processes like cell division.
Whatever the kingdom, whatever the domain, every cell on Earth shares these four features.
Phospholipid bilayer enclosing the cell. Controls what enters and leaves.
Mostly water. The location of most of the cell's chemistry.
The genetic material — circular in prokaryotes, linear and bound to histones in eukaryotes.
The molecular machines that synthesise proteins. 70S in prokaryotes, 80S in eukaryotes.
Prokaryotes have no membrane-bound organelles and no nucleus. The IB expects you to know Gram-positive eubacteria — Bacillus and Staphylococcus — as standard examples.
| Structure | Function |
|---|---|
| Cell wall (peptidoglycan) | Provides strength and shape; prevents bursting under osmotic pressure. |
| Plasma membrane | Phospholipid bilayer; controls movement of substances into and out of the cell. |
| Cytoplasm | Site of most metabolism — including respiration (no mitochondria in prokaryotes). |
| 70S ribosomes | Smaller than eukaryote ribosomes; site of protein synthesis. |
| Nucleoid region | Lighter-staining region of cytoplasm containing the single circular chromosome (naked DNA — no histones). |
| Flagellum | Long, rotating appendage used for movement. Bacillus has flagella; Staphylococcus does not. |
| Pilus | Shorter protein hair. For adhesion to surfaces and for transferring DNA between cells (conjugation). |
Eukaryotic cells contain membrane-bound organelles, each compartmentalising a different chemistry. The cell becomes a tiny factory with specialised departments.
| Organelle | Function |
|---|---|
| Nucleus | Contains chromosomes (DNA + histones). Double membrane (the nuclear envelope) with pores that allow mRNA out. |
| Mitochondrion | Aerobic respiration → ATP. Double membrane; inner membrane folded into cristae for surface area. |
| 80S ribosomes | Protein synthesis. Free in cytoplasm or attached to rough ER. |
| Rough endoplasmic reticulum | Membrane network with ribosomes attached. Synthesises proteins for export and transports them onward. |
| Smooth endoplasmic reticulum | Membrane network without ribosomes. Synthesises lipids; detoxifies drugs and toxins. |
| Golgi apparatus | Stack of flattened membrane sacs. Modifies, sorts and packages proteins for secretion. |
| Vesicles & vacuoles | Membrane-bound sacs for transport and storage. Lysosomes are vesicles full of digestive enzymes. |
| Cytoskeleton | Network of microtubules and microfilaments. Maintains shape, anchors organelles, drives transport and cell division. |
Even a unicellular organism — Paramecium, Amoeba, Chlamydomonas — has to perform all eight functions inside a single cell.
Maintaining stable internal conditions despite a changing environment.
The interconnected network of chemical reactions that keep the cell alive.
Obtaining and using food. Autotrophs (e.g. plants, cyanobacteria) make their own; heterotrophs (animals, fungi) consume others'.
Changes in position — flagella, cilia, pseudopodia, or muscle contraction in animals.
Removal of metabolic waste — CO₂, urea, ammonia.
Increase in size or mass over time.
Reacting to stimuli, both internal and external — chemotaxis, phototaxis, etc.
Producing offspring — asexual (binary fission, mitosis) or sexual (meiosis + fertilisation).
All three are eukaryotic — same general plan. But their differences are syllabus-named and exam-frequent.
| Feature | Animal | Fungal | Plant |
|---|---|---|---|
| Cell wall | None | Chitin | Cellulose |
| Vacuoles | Small, scattered | Variable | One large central sap vacuole |
| Plastids | None | None | Chloroplasts, chromoplasts, amyloplasts |
| Centrioles | Present | Absent | Absent |
| Cilia/flagella | Some cells (sperm, airway) | Absent | Absent (rare in some sperm) |
Most eukaryotic cells have a single nucleus. The IB names four exceptions you should know — and each one's atypia is a clue to its function.
Fungal bodies are made of long thread-like hyphae. In some species, hyphae lack septa (internal cell walls) and form one continuous multinucleate "cell" — many nuclei in shared cytoplasm.
Muscle fibres are formed by fusion of many embryonic precursor cells. The result is one giant cell with many nuclei distributed along its length — better able to coordinate gene expression across a long fibre.
Mammalian RBCs lose their nucleus during maturation. The space gained makes room for more haemoglobin and improves oxygen-carrying capacity. The trade-off: they can't divide and only live ~120 days.
Sieve tube elements lose their nucleus and most organelles at maturity. They become hollow conduits for transport, with companion cells (which keep their nuclei) supporting them.
In any micrograph you should be able to identify the cell type (prokaryote, plant, or animal) and a list of structures. In any annotation, name and function are required.
Prokaryote: small (1–5 µm), no nucleus, clear nucleoid region. Plant: regular shape, cell wall, often a large central vacuole, possibly chloroplasts. Animal: irregular shape, no cell wall.
Drawings in dark pencil; labels in pen. Use a ruler for label lines. Include scale bar where appropriate. For nuclei: double membrane with pores. For mitochondria: smooth outer membrane, folded inner membrane (cristae). For chloroplasts: double membrane plus internal thylakoids.
Nucleoid region (prokaryote) · prokaryotic cell wall · nucleus · mitochondrion · chloroplast · sap vacuole · Golgi apparatus · rough & smooth endoplasmic reticulum · chromosomes · ribosomes · cell wall · plasma membrane · microvilli.
An extra 3 sub-topics for HL — same syllabus, deeper mechanism.
Origin of eukaryotic cells by endosymbiosis
Cell differentiation as the process for developing specialized tissues in multicellular organisms
Evolution of multicellularity
If you can't define one of these in a sentence, that's where to revise next.
“What explains the use of certain molecular building blocks in all living cells?”
“What are the features of a compelling theory?”