Every cell in your body carries the same DNA. Yet a neuron looks and behaves nothing like a red blood cell. How does one genome produce 200 different cell types — and what happens when that process goes wrong?
Before reading on, make a prediction:
A red blood cell and a muscle cell both come from the same fertilised egg, carrying identical DNA. Predict: how can two cells with identical DNA end up looking and functioning so differently? What mechanism do you think controls this?
Come back to this at the end of the lesson.
Core Content
A human body contains approximately 37 trillion cells and around 200 distinct cell types. Every single one of those cells — from the cone cell in your retina detecting colour, to the osteoblast laying down bone — carries exactly the same DNA. The genome doesn't change. What changes is which genes are switched on.
Cell specialisation is the process by which cells with identical DNA develop distinct structures and functions. It happens through differential gene expression — different genes are activated or silenced in different cells during development. A red blood cell activates haemoglobin genes and silences almost everything else. A pancreatic beta cell activates insulin genes. A muscle cell activates actin and myosin genes.
For each cell type, focus on the link between structural adaptation and function — this is what the HSC tests.
No nucleus: Maximises space for haemoglobin — each cell can carry more O₂.
Biconcave disc shape: Large surface area relative to volume — faster O₂ diffusion. Flexible enough to squeeze through capillaries narrower than the cell itself.
Packed with haemoglobin: ~270 million molecules per cell — the oxygen-binding protein.
No mitochondria: Relies on anaerobic respiration — doesn't consume the O₂ it carries.
Long axon: Can extend over 1 metre — carries electrical signals over long distances without signal loss.
Myelin sheath: Insulating lipid layer formed by Schwann cells — speeds up signal conduction (saltatory conduction).
Dendrites: Multiple branching extensions — receive signals from many other neurons simultaneously.
Many mitochondria: Neurons are metabolically expensive — require constant ATP for ion pumps that restore resting potential.
Flagellum (tail): Propels sperm toward egg — powered by ATP from mitochondria in the midpiece.
Acrosome (head): Cap containing hydrolytic enzymes — digests zona pellucida of egg to enable fertilisation.
Streamlined shape: Minimises drag in fluid — maximises swimming efficiency.
Haploid nucleus: Carries half the genetic information — restores diploid number at fertilisation.
Packed with chloroplasts: 40–50 per cell — maximise light absorption for photosynthesis.
Columnar shape + tight packing: Maximise surface area exposed to incoming light; cells arranged perpendicular to leaf surface.
Large central vacuole: Maintains turgor, keeps cell rigid and leaf flat to intercept sunlight.
Thin cell walls: Minimise resistance to CO₂ diffusion into cell.
Long thin extension (root hair): Dramatically increases surface area for water and mineral ion absorption from soil.
No chloroplasts: Root cells are underground — no light available for photosynthesis.
Large vacuole: Maintains osmotic gradient that draws water in from soil.
Thin cell wall: Reduces diffusion distance for water uptake.
Actin and myosin filaments: Contractile proteins arranged in sarcomeres — slide past each other to shorten the cell and generate force.
Many mitochondria: Muscle contraction requires enormous amounts of ATP — mitochondria densely packed between myofibrils.
Multinucleate: Skeletal muscle fibres form by cell fusion — multiple nuclei coordinate protein synthesis across the long cell.
Extensive sarcoplasmic reticulum: Stores and releases Ca²⁺ ions that trigger contraction.
Specialised cells don't work in isolation. They are organised into increasingly complex structures:
| Level | Definition | Example |
|---|---|---|
| Cell | Basic structural and functional unit of life | Cardiac muscle cell |
| Tissue | Group of similar cells working together to perform a specific function | Cardiac muscle tissue |
| Organ | Structure made of two or more tissue types working together | Heart (muscle, connective, epithelial, nervous tissue) |
| Organ system | Group of organs working together toward a common function | Circulatory system |
| Organism | All organ systems integrated into a functional living entity | Human |
Not all stem cells are equally flexible. Potency describes how many cell types a stem cell can differentiate into:
Can become any cell type including placental cells. Only the fertilised egg (zygote) and first few divisions. Most flexible.
Can become any cell in the body but not placental cells. Embryonic stem cells (ESCs). Source of most stem cell therapy research.
Can differentiate into a limited range of related cell types. Adult stem cells (e.g. bone marrow stem cells → blood cell types). Less flexible, fewer ethical concerns.
Misconception: Specialised cells have different DNA from each other.
All cells in an organism carry identical DNA (with rare exceptions like mature red blood cells which lose their nucleus). Specialisation is controlled by which genes are expressed, not by changes to the DNA sequence.
Misconception: Stem cells only come from embryos.
Adult stem cells exist in many tissues — bone marrow, skin, gut lining, and brain. They are multipotent rather than pluripotent, meaning they can only differentiate into a limited range of cell types, but they are ethically uncontroversial to use.
Misconception: A tissue is just a large group of identical cells.
Tissues often contain multiple cell types working together. Blood, for example, is a connective tissue containing red blood cells, white blood cells, platelets, and plasma — four distinct components with different functions.
Specialisation = differential gene expression. Same DNA, different genes switched on. Structure always reflects function.
Activities
In your book, construct a comparison table for the following three pairs of cells. For each pair, identify: (a) one structural feature they share, (b) two structural features that differ, and (c) how each difference relates to their different functions.
After completing the table, write one sentence that applies to all six cells: what is the underlying principle that explains why all their structural features exist?
Write your unifying principle sentence here.
A cell biologist describes a newly characterised cell type with the following features:
Write your responses here or in your book.
Assessment
1. A red blood cell has no nucleus and no mitochondria. Which of the following best explains the advantage of having no nucleus?
2. Two cells in the same organism have identical DNA but completely different structures and functions. This is best explained by:
3. Which of the following correctly describes the potency of embryonic stem cells?
4. A palisade mesophyll cell has approximately 40–50 chloroplasts, while a root hair cell has none. The best explanation for this difference is:
5. A heart consists of cardiac muscle tissue, connective tissue, epithelial tissue lining blood vessels, and nervous tissue controlling rhythm. This makes the heart best described as:
1. For TWO specialised cell types of your choice, explain how each cell's structural features are adapted to its function. (3 marks)
1.5 marks per cell: 1 mark for structural feature correctly described, 0.5 mark for linking to function
2. Explain the difference between a tissue and an organ. Use the human heart as an example in your answer. (3 marks)
1 mark tissue definition; 1 mark organ definition; 1 mark correct heart example with tissue types named
3. Stem cell therapies offer significant medical potential but also raise ethical concerns. Evaluate the use of embryonic stem cells in medicine, referring to both their scientific advantages and ethical issues. (3 marks)
1 mark scientific advantage; 1 mark ethical concern; 1 mark for evaluation (balanced conclusion or reference to iPSCs as alternative)
Answers
SA1 (example answer): Red blood cell: the biconcave disc shape increases surface area relative to volume, maximising the rate of oxygen diffusion into and out of the cell. The absence of a nucleus removes a physical barrier to oxygen loading and frees space for haemoglobin molecules. Neuron: the long axon allows electrical signals to be transmitted over long distances (up to 1 metre) without loss of signal strength. The myelin sheath insulates the axon and enables saltatory conduction, dramatically increasing signal speed.
SA2: A tissue is a group of similar cells that work together to perform a specific function. An organ is a structure composed of two or more tissue types working together to carry out a more complex function. The human heart is an organ because it contains multiple tissue types: cardiac muscle tissue provides the contractile force; connective tissue forms the valves and outer wall; epithelial tissue lines the internal chambers and blood vessels; and nervous tissue (including the sinoatrial node) coordinates the rhythmic contraction.
SA3: Embryonic stem cells (ESCs) are scientifically valuable because they are pluripotent — they can differentiate into any cell type in the human body, making them potentially useful for treating degenerative diseases such as Parkinson's disease, type 1 diabetes, and spinal cord injuries where specific cell types are lost. However, obtaining ESCs requires the destruction of a human embryo at the blastocyst stage, raising significant ethical concerns about the moral status of embryos and whether their destruction for research or therapy is justifiable. This debate is partially resolved by induced pluripotent stem cells (iPSCs), which reprogram adult cells to a pluripotent state without embryo destruction — though questions remain about their safety and equivalence to ESCs.
You predicted how two cells with identical DNA could end up so different. The answer is differential gene expression — the selective activation and silencing of genes in different cell types during development, controlled by transcription factors and epigenetic modifications.
The DNA is the same. The instruction manual is the same. But which pages are open differs from cell to cell — and that's everything.
If you predicted "different genes are turned on" — you were exactly right, even if you didn't know the term. If you predicted environmental signals or chemical signals during development — also correct. Those signals are precisely what controls which genes get switched on.