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
- B2.3.1From zygote to specialized cells
- B2.3.1Gradients and gene expression
- B2.3.2What makes a stem cell?
- B2.3.3Bone marrow: a stem cell niche
- B2.3.3Hair follicles: a second niche
- B2.3.4Totipotent, pluripotent, multipotent
- B2.3.5Cell size as specialization
- B2.3.6Surface area and volume
- B2.3.6 · Nature of scienceCubes as a model
Cells adapted to their functions
- B2.3.7 · HLFlattening: the erythrocyte
- B2.3.7 · HLMicrovilli and invagination
- B2.3.8 · HLType I pneumocytes
- B2.3.8 · HLType II pneumocytes
- B2.3.9 · HLCardiac muscle cells
- B2.3.9 · HLIs a muscle fibre a cell?
- B2.3.10 · HLThe sperm cell
- B2.3.10 · HLThe egg cell
B2.3 Cell specialization: summary
Differentiation
- Zygote → unspecialized cells → specialized cells.
- Same genome; gradients switch on different genes.
Stem cells and niches
- Divide endlessly and differentiate along different pathways.
- Niches maintain them or promote division: bone marrow, hair follicles.
Potency and size
- Totipotent → pluripotent → multipotent.
- Sizes: gametes, blood cells, neurons, muscle fibres.
SA:V
- Exchange depends on area; need depends on volume.
- SA:V falls as cells grow. NOS: cubes as a model.
HL · More surface
- Flattening: erythrocytes. Microvilli and invagination: PCT cells.
- Type I pneumocytes thin; type II secrete surfactant.
HL · Muscle and gametes
- Cardiac: branched, 1–2 nuclei. Striated fibre: long, many nuclei.
- Sperm: small, motile. Egg: large, food reserves.
Key terms
- Differentiation
- development of an unspecialized cell into a specialized cell by expressing particular genes.
- Gradient
- a change in the concentration of a signalling molecule across an embryo.
- Stem cell
- a cell that can divide endlessly and differentiate along different pathways.
- Stem cell niche
- the place where stem cells live, which maintains them or makes them divide and differentiate.
- Totipotent
- able to form any cell type, including the placenta.
- Pluripotent
- able to form any cell type of the body, but not the placenta.
- Multipotent
- able to form a limited range of related cell types.
- SA:V
- surface area divided by volume; it falls as a cell gets larger.
- Model
- a simplified version of a complex system, such as cubes for cells.
- Microvilli
- tiny projections of the plasma membrane that increase its area.
- Invagination
- inward folding of the plasma membrane.
- Type I pneumocyte
- an extremely thin alveolar cell, for diffusion of gases.
- Type II pneumocyte
- a rounded alveolar cell that secretes surfactant.
- Lamellar body
- a secretory vesicle of surfactant in a type II pneumocyte.
- Surfactant
- a secretion that lowers surface tension in the alveoli.
- Myofibril
- a contractile structure in muscle cells and fibres.
- Intercalated disc
- the junction joining cardiac muscle cells end to end.
- Acrosome
- a vesicle of enzymes on the head of a sperm.
Sample exam questions
Three of the 31 multiple-choice questions for B2.3. Try each one before opening the answer.
Question 1. All the cells of a multicellular organism contain the same genome. How do they become specialized?
- They lose the genes they do not use.
- They express different genes.
- They gain new genes from the environment.
- They mutate in different ways.
Show the answer
Answer: B. Differentiation is the expression of different genes in different cells; the genes themselves are not removed or changed.
Question 2. The haemoglobin gene is present in all human cells but is expressed only in developing red blood cells. What does this show?
- Differentiation is due to gene expression.
- Other cells have lost the gene.
- Haemoglobin genes are mutated in other cells.
- Only blood cells have a complete genome.
Show the answer
Answer: A. All cells have the gene, but only some express it: differentiation depends on which genes are switched on.
Question 3. Which two properties together define a stem cell?
- It is large and has many nuclei.
- It can divide only a few times.
- It is fully specialized for one role.
- It self-renews and can differentiate.
Show the answer
Answer: D. A stem cell can divide endlessly, a stem cell remaining each time, and its daughter cells can differentiate along different pathways.
Linking questions
Questions that connect B2.3 to other parts of the course, the kind that come up in Paper 2.
- What are the advantages of small size and large size in biological systems? Use surface area-to-volume ratios in your answer. (see B3.1)
- How do cells become differentiated? How do gradients in an early embryo change which genes are expressed? (see D2.2)
- 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)
- How are the structures of the sperm and the egg involved in fertilization and in preventing polyspermy? (see D3.1)
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.