In simple terms
A friendly intro before the formal notes — no formulas yet.
One Genome, Many Cells
All the cells in your body descend from a single fertilised egg and carry an identical set of genes, yet a neuron, a muscle fibre and a white blood cell could hardly look more different. The difference is not in which genes a cell has, but in which genes it switches on.
Picture a huge library in which every reading room holds an identical copy of the same collection of books. To turn one room into a 'kitchen' you leave only the cookbooks open; to turn another into a 'workshop' you leave only the tool manuals open. No books are ever removed — each room specialises purely by choosing which volumes to read. In the same way, a specialised cell keeps the whole genome but expresses only the subset of genes its job requires. That selective reading is cell differentiation through differential gene expression.
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Every somatic cell in an organism contains the same complete genome — the same genes.
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During development a cell receives chemical signals from its position and its neighbours.
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Those signals switch certain genes on (expressed) and keep others off (silenced) — differential gene expression.
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The expressed genes are transcribed and translated into a specific set of proteins.
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Those proteins build the cell's specialised structures and carry out its specialised functions — the cell has differentiated.
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Full topic notes
Formal explanation with the rigour you need for the exam.
Differentiation: one genome, different genes expressed
All of the diploid somatic cells in an organism contain the same complete set of genes — the genome. So if the instructions are identical, how do cells become so different? The answer is differential gene expression. At any moment only a subset of a cell's genes is active ('expressed'); the rest are switched off ('silenced'). Cell differentiation is the process in which a cell becomes specialised by expressing a particular subset of its genes. The genes that are switched on are transcribed and translated into a specific set of proteins, and those proteins determine the cell's structure, its metabolism and therefore its function. Crucially, silenced genes are still physically present — nothing is deleted. A liver cell and a neuron differ because they read different parts of the same instruction book, not because they own different books.
All somatic cells in an organism share one identical genome — the same genes.
Differentiation is a cell becoming specialised through differential gene expression.
Only a subset of genes is expressed; the rest are silenced but NOT lost.
The expressed genes are translated into specific proteins, which give the cell its structure and function.
Structure matched to function in specialised cells
Because a specialised cell expresses only the genes its job requires, its structure ends up finely matched to its function. This structure–function relationship is a recurring exam theme, and the reliable way to answer it is to link a specific feature to a specific job. A few standard examples: a red blood cell is biconcave (large surface-area-to-volume ratio for gas exchange) and loses its nucleus and mitochondria (more room for haemoglobin and no oxygen consumed); a muscle fibre is packed with contractile protein filaments and many mitochondria to supply ATP for contraction; a sperm cell carries a flagellum for swimming, many mitochondria in the midpiece to power it, and an acrosome of enzymes to penetrate the egg; a palisade mesophyll cell is column-shaped and full of chloroplasts near the leaf's upper surface to absorb light for photosynthesis; and a root hair cell has a long thin projection that greatly increases surface area for absorbing water and mineral ions.
Red blood cell: biconcave shape (high surface area for O₂ diffusion); no nucleus/mitochondria (maximum haemoglobin, no O₂ used).
Muscle fibre: many contractile filaments and mitochondria — force and ATP for contraction.
Sperm cell: flagellum for movement, midpiece mitochondria for ATP, acrosome enzymes to penetrate the egg.
Palisade mesophyll: column shape and many chloroplasts near the top of the leaf — maximum light absorption for photosynthesis.
Root hair cell: long thin extension — large surface area for absorbing water and mineral ions.
Stem cells: potency and sources
Stem cells are unspecialised cells with two defining properties: self-renewal (they divide to make more stem cells) and potency (they can differentiate into specialised cells). Potency describes HOW MANY types of cell a stem cell can become, and it narrows as development proceeds. Totipotent cells can form every cell type including extra-embryonic tissue such as the placenta — only the zygote and the very early embryo are totipotent. Pluripotent cells can form any cell of the body proper but not the placenta; embryonic stem cells from the inner cell mass of a blastocyst are pluripotent. Multipotent cells form only a limited family of related cell types within one tissue; adult (tissue) stem cells such as the haematopoietic stem cells of bone marrow, which make the blood cells, are multipotent. Stem cells come from three main sources: embryonic stem cells (from early embryos), adult/tissue stem cells (from tissues such as bone marrow), and induced pluripotent stem cells (iPSCs), made by reprogramming a patient's own adult cells back to a pluripotent state.
Totipotent: any cell type INCLUDING extra-embryonic tissue (placenta). Source: zygote / very early embryo.
Pluripotent: any cell of the body proper but NOT extra-embryonic tissue. Source: embryonic stem cells (inner cell mass of blastocyst); also iPSCs.
Multipotent: a limited range of related cell types. Source: adult/tissue stem cells, e.g. bone-marrow haematopoietic stem cells.
Sources overall: embryonic, adult/tissue, and induced pluripotent (reprogrammed adult cells).
Potency narrows as development proceeds — totipotent → pluripotent → multipotent.
Therapeutic use of stem cells and the ethics
Because stem cells can both self-renew and differentiate, they can be used to replace cells lost to injury or disease — the basis of regenerative medicine. Two IB examples: in Stargardt's disease, an inherited condition in which retinal cells of the macula degenerate and vision is lost, stem cells can be directed to become healthy retinal pigment epithelium cells and injected into the retina to slow or reverse the loss. In leukaemia, a cancer of the blood-forming tissue, chemotherapy destroys the cancerous cells in the bone marrow and multipotent haematopoietic stem cells (from a matched donor or the patient's own stored cells) are transplanted to re-establish a healthy population of blood cells. Set against this potential are real ethical considerations. Embryonic stem cells are the most versatile but obtaining them destroys a blastocyst, which some regard as a potential human life; there are also risks of tumour formation and of immune rejection of donor cells. iPSCs and adult stem cells reduce these concerns — iPSCs avoid destroying embryos and, being patient-derived, lower rejection risk — which is a large part of why they are so actively researched.
Stargardt's disease: stem cells directed into retinal pigment epithelium cells, injected to replace degenerated retinal cells and preserve vision.
Leukaemia: chemotherapy kills cancerous marrow cells; transplanted haematopoietic (multipotent) stem cells rebuild healthy blood.
Benefit: potential to repair or replace tissues that the body cannot regenerate on its own.
Ethical/practical concerns: destruction of embryos for embryonic stem cells; risk of tumour formation; immune rejection of donor cells.
iPSCs/adult stem cells: avoid destroying embryos and (for iPSCs) lower rejection risk, but iPSCs carry their own reprogramming and tumour-risk questions.
The hierarchy of organisation
Specialisation only pays off if specialised cells are organised together. Cells of the same type that carry out a shared task form a tissue (for example, muscle tissue or the palisade mesophyll of a leaf). Different tissues combine into an organ that performs a defined function (the heart, the leaf, the stomach). Organs that co-operate in a common process make up an organ system (the circulatory system, the digestive system). Together the systems form the whole organism. Each level up the hierarchy — cell → tissue → organ → system — represents an increasing division of labour, and it is this layered organisation that lets a large multicellular organism keep every part efficient.
Cells: specialised units, each expressing the genes for one role.
Tissues: groups of similar cells doing a shared task (e.g. muscle, palisade mesophyll).
Organs: several tissues combined to perform a function (e.g. heart, leaf).
Organ systems: organs working together on one process (e.g. circulatory, digestive).
Organism: all systems together — the sequence is cell → tissue → organ → system → organism.
Common mistakes examiners penalise
Saying specialised cells 'lose' unused genes — differentiation is differential gene EXPRESSION; every somatic cell keeps the whole genome, and unused genes are silenced, not deleted.
Calling embryonic stem cells 'totipotent' — they are PLURIPOTENT. Only the zygote and very early embryo, which can also form extra-embryonic tissue, are totipotent.
Listing features without linking them to function — 'it is biconcave' scores nothing; you must connect the feature to the job ('…giving a large surface area for oxygen diffusion').
Confusing pluripotent and multipotent — pluripotent forms any body cell; multipotent forms only a limited related family within one tissue.
Treating stem cells as a finished 'cure' — you must describe the mechanism: differentiate into a named specialised cell, then transplant it to replace the damaged one.
Ignoring the ethics or giving only one side — 'evaluate' expects a benefit AND an ethical/practical drawback (e.g. destruction of the embryo, tumour risk, rejection).
Jumbling the hierarchy — the order is cell → tissue → organ → organ system → organism; a tissue is not an organ.
Model answer — marked the way our engine marks it
B2.3 explain-type questions are marked analytically: each distinct, valid biological point is worth one mark, up to the total available. Answer marks (A) credit a correct step in the reasoning, and error-carried-forward (ECF) means one weak line does not sink the rest, provided each idea is written down separately. Equivalent wording is accepted. Study how each mark below is tied to a specific, named idea rather than to loose phrasing.
Where this leads
Differentiation by differential gene expression is the foundation for much of what follows in the course: the control of gene expression, the way tissues and organs develop, and the promise and problems of stem-cell medicine. Carry two habits forward — first, that a specialised cell's structure is always readable as a set of adaptations to its function, and second, that 'explain' answers are built from distinct, named points. Both will keep earning marks well beyond this topic.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Explain how the structure of a red blood cell is adapted to its function of transporting oxygen. [4]
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Model answer — each line links a feature to the job it does.
Outline the use of stem cells in the treatment of Stargardt's disease. [3]
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Model answer. Stargardt's disease is caused by the degeneration of retinal cells (the retinal pigment epithelium / photoreceptors) in the macula, leading to progressive vision loss. Stem cells are directed to differentiate into healthy retinal pigment epithelium cells in the laboratory. These cells are then injected into the patient's retina, where they replace the damaged cells with the aim of halting or reversing the loss of vision.
Explain how cells with identical genomes become specialised to carry out different functions. [4]
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Model answer. Every somatic cell in the organism contains the same complete genome, so all cells have the same genes. During differentiation only certain of these genes are expressed in a given cell while the others are switched off. The genes that are expressed are transcribed and translated to produce a specific set of proteins in that cell. These proteins build the cell's specialised structures and carry out its particular functions, so cells that express different genes develop into different, specialised cell types even though their DNA is identical.
How it all connects
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Glossary
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Revision flashcards
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Cell differentiation
The process by which an unspecialised cell develops a specialised structure and function. It occurs through the expression of some of the cell's genes but not others — no genes are lost.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
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All somatic cells in an organism share one identical genome — the same genes.
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Differentiation is a cell becoming specialised through differential gene expression.
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Only a subset of genes is expressed; the rest are silenced but NOT lost.
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The expressed genes are translated into specific proteins, which give the cell its structure and function.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Get a Paper 2 question marked: explain how identical genomes give rise to specialised cells, and link a named cell's structure to its function
Get a Paper 2 question marked: explain how identical genomes give rise to specialised cells, and link a named cell's structure to its function
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