Cell specialization — IB Diploma Biology HL
IB Biology B2.3: differentiation, stem cells, niches and potency, cell size and SA:V, and HL adaptations of pneumocytes, muscle cells and gametes.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Stem cells are cells that can divide endlessly and differentiate along different pathways. After fertilization, the zygote divides to form unspecialized cells, which differentiate as gradients of signal molecules switch on different genes. Early embryo cells are totipotent, then pluripotent; adult stem cells in niches such as bone marrow and hair follicles are multipotent.
Cell size is an aspect of specialization: human cells range from small sperm and red blood cells to large egg cells, very long neurons and striated muscle fibres. Size is limited by surface area-to-volume ratio, because exchange across the surface depends on area, while the need for exchange depends on volume, which grows faster as cells enlarge.
Cells increase their surface area-to-volume ratio by flattening, microvilli and invagination. Erythrocytes are flattened biconcave discs, and proximal convoluted tubule cells have microvilli and an infolded basal membrane. In alveoli, type I pneumocytes are extremely thin to shorten diffusion distances, while type II pneumocytes contain many lamellar bodies that discharge surfactant into the alveolus.
Cardiac muscle cells and striated muscle fibres both contain contractile myofibrils. Cardiac muscle cells are short, branched and usually have one nucleus, joined end to end so impulses spread through the heart wall. Striated muscle fibres are unbranched, very long and multinucleate because they form by fusion of many cells, so they are atypical cells.
Human sperm and egg cells are haploid gametes with very different adaptations. The sperm is small and streamlined, with an acrosome of enzymes, a midpiece packed with mitochondria and a flagellum for swimming. The egg is large, with cytoplasm and nutrients for the early embryo, a protective zona pellucida and cortical granules that prevent more than one sperm fertilizing it.
Exam questions
8 exam-style questions (29 marks), each with its mark scheme.
Answer the questionsFlashcards
17 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of cells (form and function) on one page, so you can see where this subtopic fits.
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Free PDFs to print or save.
Quick check questions
State the two properties of stem cells.
Show answer
They can divide endlessly and differentiate along different pathways.
Name two stem cell niches in adult humans.
Show answer
Bone marrow and hair follicles.
What type of stem cell is found in adult bone marrow?
Show answer
Multipotent.
What happens to the SA:V of a cube when its side length doubles?
Show answer
It halves.
HL only What do type II pneumocytes secrete?
Show answer
Surfactant, from lamellar bodies.
Frequently asked questions
Why is the surface area to volume ratio important for cells?
Exchange of materials across a cell surface depends on its area, but the need for exchange depends on cell volume. As a cell grows, volume increases faster than surface area, so the surface area-to-volume ratio falls. Above a certain size the cell cannot exchange oxygen, nutrients and waste fast enough, which limits cell size.
What is the difference between totipotent, pluripotent and multipotent stem cells?
Totipotent cells, such as the zygote and very early embryo cells, can form any cell type including the placenta. Pluripotent cells, found in the embryo a few days later, can form any body cell but not a whole organism. Multipotent cells, such as adult bone marrow stem cells, form only a limited range of related cells.
What is a stem cell niche?
A stem cell niche is a location in the body where stem cells live, whose conditions either maintain the stem cells or promote their proliferation and differentiation when new cells are needed. Bone marrow is a niche for stem cells that make blood cells, and hair follicles contain stem cells that regenerate the follicle and help repair skin.
How do cells in an embryo become specialized?
After fertilization the zygote divides to make unspecialized cells. Signal molecules released from particular cells form concentration gradients across the embryo. Each cell's position in a gradient determines which genes it expresses, so cells in different places differentiate into different specialized cell types while keeping the same genes.
Is a striated muscle fibre a cell?
HL only It is usually treated as an atypical cell. A striated muscle fibre has one continuous plasma membrane like a cell, but it forms by the fusion of many precursor cells and contains many nuclei, so it does not fit the usual idea of one nucleus per cell. Arguments can be made either way.
Written and checked against the IB Biology HL specification · Updated October 2026