IB Biology · Theme A: Unity and diversity · SL and HL
A2.2 Cell structure
A one-page summary of A2.2 Cell structure, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What are the features common to all cells and the features that differ?
How is microscopy used to investigate cell structure?
What A2.2 covers
The cell as the unit of life
- A2.2.1Every organism is built from cells
- A2.2.4Structures every cell shares
Seeing the invisible: microscopy
- A2.2.2 · Practical skillUsing a light microscope
- A2.2.2 · Practical skillCalculating magnification
- A2.2.2 · Practical skillMeasuring with a graticule
- A2.2.3Resolution and electron microscopes
- A2.2.3Freeze fracture: splitting the membrane
- A2.2.3Cryo-EM: freezing molecules mid-shape
- A2.2.3Fluorescent stains and immunofluorescence
Prokaryotes, eukaryotes, and single-celled life
- A2.2.5Prokaryote cell structure
- A2.2.6Eukaryote cell structure
- A2.2.7One cell doing everything
Diversity, exceptions, and reading cells
- A2.2.8Animal, fungal and plant cells
- A2.2.9Red blood cells lose their nucleus
- A2.2.9 continuedSieve tubes lose their nucleus but stay alive
- A2.2.9 continuedOne giant cell, many nuclei
- A2.2.9 continuedNo walls between nuclei
- A2.2.10Reading an electron micrograph
- A2.2.10Prokaryote, plant or animal?
- A2.2.11 · Practical skillDrawing what you actually see
Origins and specialisation
- A2.2.12 · HLMitochondria were once free-living bacteria
- A2.2.13 · HLSame genome, different jobs
- A2.2.14 · HLMulticellularity evolved again and again
A2.2 Cell structure: summary
Seeing cells
- Magnification: image size ÷ actual size.
- Resolution — not magnification — sets the detail limit (LM ~200 nm; EM ~0.1–2 nm).
Universal toolkit
- Every cell: plasma membrane, cytoplasm, ribosomes, DNA.
- Everything else is added on top of this shared plan.
Prokaryote vs eukaryote
- Prokaryote: no nucleus, 70S ribosomes, nucleoid DNA.
- Eukaryote: nucleus + organelles, 80S ribosomes.
Diversity & exceptions
- Animal/fungal/plant cells differ in wall, vacuole, chloroplasts.
- RBCs, muscle fibres, hyphae all break the generalised model.
HL · Origins
- Mitochondria evolved by endosymbiosis.
- Evidence: double membrane, own DNA, 70S ribosomes, binary fission.
HL · Specialisation
- Differentiation: same genome, different genes switched on.
- Multicellularity evolved independently, many times over.
Key terms
- Magnification
- How many times larger an image is than the real, actual object.
- Resolution
- The smallest distance between two points at which they can still be seen as separate.
- Prokaryotic cell
- A cell with no nucleus and no membrane-bound organelles; DNA lies free in the nucleoid.
- Eukaryotic cell
- A cell with a membrane-bound nucleus and membrane-bound organelles.
- Nucleoid
- The region of a prokaryotic cell where the circular DNA is located; not membrane-bound.
- Compartmentalisation
- The division of a eukaryotic cell into separate membrane-bound regions, each with its own environment.
- Endosymbiosis HL
- A permanent, mutually beneficial relationship in which one organism lives inside the cell of another.
- Cell differentiation HL
- A cell becoming structurally and functionally specialised by expressing a specific subset of its genome.
- Multicellularity HL
- An organism built from many cells that cooperate and divide labour between them.
- Convergent evolution HL
- The independent evolution of similar features in separate lineages, such as multicellularity arising repeatedly.
Sample exam questions
Three of the 43 multiple-choice questions for A2.2. Try each one before opening the answer.
Question 1. According to cell theory, cells are best described as:
- the smallest particles from which atoms are built
- the basic structural and functional unit of life
- structures found only in animals and plants
- non-living building blocks that cannot reproduce
Show the answer
Answer: B. Cell theory states that all living organisms are composed of one or more cells, and that the cell is the smallest unit that carries out the functions of life.
Question 2. A cell is found to contain 80S ribosomes, chloroplasts and a cell wall made of cellulose. What type of cell is it most likely to be?
- An animal cell
- A prokaryotic cell
- A plant cell
- A fungal cell
Show the answer
Answer: C. 80S ribosomes show the cell is eukaryotic, and chloroplasts together with a cellulose cell wall are characteristic of plant cells in particular.
Question 3. Nearly every cell in your body carries the same complete genome, yet a muscle cell and a pancreatic cell are very different. Cell differentiation occurs because:
- each cell type contains a completely different set of genes
- cells randomly lose the genes they do not need
- differentiated cells stop containing any DNA
- different genes are expressed in different cells
Show the answer
Answer: D. Differentiation happens because only some of the genes present are switched on in any given cell type. A muscle cell expresses the actin and myosin genes; a pancreatic beta cell expresses the insulin gene. The genome itself stays essentially complete.
Linking questions
Questions that connect A2.2 to other parts of the course, the kind that come up in Paper 2.
- How does the structure of the plasma membrane control the exchange of substances that a cell's surface area allows? (see B2.1)
- How do genetically identical cells become specialised into different cell types in a multicellular organism? (see B2.3)
- How do mitochondria use their internal structure to release energy in aerobic cell respiration? (see C1.2)
- How do viruses differ from cells, given that they lack cytoplasm and cannot carry out the functions of life alone? (see A2.3)
Practise A2.2
Study notes, every question and full markschemes for A2.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.