IB Biology · Theme B: Form and function · SL and HL
B2.3 Cell specialization
A one-page summary of B2.3 Cell specialization, 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 roles of stem cells in multicellular organisms?
How are differentiated cells adapted to their specialized functions?
What B2.3 covers
From one cell to many kinds of cell
- B2.3.1One genome, many fates
- B2.3.2What actually makes a cell a stem cell
- B2.3.3Bone marrow: the blood-cell niche
- B2.3.3Hair follicles: a second real niche
- B2.3.4Potency narrows as development proceeds
- B2.3.5Size itself is a specialisation
- B2.3.6Surface area cannot keep up with volume
Structure built for one job
- B2.3.7 · HLFlattening buys the erythrocyte surface area
- B2.3.7 · HLMicrovilli buy the tubule cell surface area
- B2.3.8 · HLType I pneumocytes: built to be thin
- B2.3.8 · HLType II pneumocytes: built to secrete
- B2.3.9 · HLCardiac muscle: built to coordinate
- B2.3.9 · HLSkeletal muscle: built by fusion
- B2.3.10 · HLThe sperm is built to travel
- B2.3.10 · HLThe egg is built to provide
B2.3 Cell specialization: summary
Differentiation
- Same genome, different gene expression.
- Morphogen gradients give positional information.
Stem cells & niches
- Defined by self-renewal + potency, both required.
- Niches: bone marrow, hair follicles.
Potency & size
- Totipotent → pluripotent → multipotent.
- Cell size fits the job: egg, sperm, RBC, neuron.
SA:V constraint
- Exchange needs area; demand needs volume.
- Volume outgrows area as cells get bigger.
SA:V fixes & lungs HL
- Erythrocyte flattens; PCT cell grows microvilli.
- Type I/II pneumocytes: diffusion vs. surfactant.
Muscle & gametes HL
- Cardiac branched; skeletal multinucleate.
- Sperm built to move; egg built to provide.
Key terms
- Differentiation
- The process by which an unspecialised cell becomes specialised for a function.
- Stem cell niche
- The local microenvironment that maintains stem cells and regulates their fate.
- Totipotent
- Able to form any cell type, including extra-embryonic tissue, e.g. the zygote.
- Pluripotent
- Able to form any embryonic cell type, but not extra-embryonic tissue.
- Multipotent
- Restricted to differentiating into cell types within one lineage.
- Surfactant HL
- Lipoprotein secreted by type II pneumocytes that reduces alveolar surface tension.
- Acrosome HL
- Vesicle at the sperm head holding enzymes that help penetrate the egg.
Sample exam questions
Three of the 31 multiple-choice questions for B2.3. Try each one before opening the answer.
Question 1. A stem cell is defined as a cell that can:
- Only divide to produce identical copies of itself
- Function without a nucleus
- Only differentiate into a single predetermined cell type
- Divide repeatedly to produce both more stem cells (self-renewal) and daughter cells that can differentiate into specialised cell types
Show the answer
Answer: D. Stem cells have two key properties: self-renewal (ability to divide and maintain the stem cell pool) and potency (ability to give rise to differentiated daughter cells). The range of cell types they can produce defines their potency — totipotent, pluripotent, multipotent, or unipotent.
Question 2. All cells in a multicellular organism contain the same genome, yet they can develop into neurons, muscle cells, or skin cells. This is possible because:
- All genes are expressed equally in all cells, but proteins are modified differently after translation
- Different cell types express different sets of genes — some genes are switched on while others are switched off
- Different cell types contain different subsets of the genome — some genes are permanently deleted during development
- The genome changes over an organism's lifetime to suit the cell's function
Show the answer
Answer: B. Cell specialization (differentiation) is achieved through differential gene expression — all cells retain the full genome, but each cell type activates only the genes relevant to its function while silencing others. For example, the insulin gene is active in pancreatic β-cells but silent in neurons.
Question 3. Therapeutic use of stem cells to treat type 1 diabetes would require:
- Removing all existing pancreatic cells and replacing them with embryonic stem cells
- Injecting any type of stem cell into the bloodstream where they will automatically become any needed cell type
- Using antibiotics to stimulate the existing β-cells to regenerate
- Generating functional insulin-secreting pancreatic β-cells from stem cells and transplanting them into the patient
Show the answer
Answer: D. In type 1 diabetes, the patient's β-cells are destroyed by an autoimmune response. A stem-cell-based therapy would involve directing stem cells (embryonic or iPSC-derived) to differentiate specifically into insulin-producing β-cells, then transplanting them into the patient — ideally with immune protection to prevent recurrence of autoimmunity.
Linking questions
Questions that connect B2.3 to other parts of the course, the kind that come up in Paper 2.
- Specialised cells express different sets of membrane transport proteins. How does differential expression of aquaporins and ion channels (B2.1) between cell types relate to their distinct physiological roles? (see B2.1)
- Cells that secrete large amounts of protein (e.g. plasma cells producing antibodies) have abundant RER and Golgi (B2.2). How does the differentiation of B-lymphocytes into plasma cells involve coordinated upregulation of genes encoding the entire secretory machinery? (see B2.2)
- During the cell cycle (D2.1), stem cells must balance self-renewal division with differentiation. Explain how asymmetric cell division enables a stem cell to produce one stem cell and one differentiating daughter cell. (see D2.1)
- Mutations in genes encoding transcription factors can cause developmental disorders. Explain how a loss-of-function mutation in the PAX6 gene (a master regulator of eye development) relates to the concept of cell specialization, and why mutations in developmental genes often have pleiotropic effects. (see D1.3)
Practise B2.3
Study notes, every question and full markschemes for B2.3 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.