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When a fertilised egg divides by mitosis, it first makes a ball of cells that all look much the same. Yet a finished organism contains hundreds of different cell types — nerve cells that stretch for great distances, flat disc-shaped red blood cells, root hair cells reaching out into the soil. How does one kind of cell give rise to so many? The answer is differentiation. And a small number of cells keep themselves unspecialised, holding on to the ability to divide and to turn into other cell types — these are stem cells, one of the most important and most debated ideas in modern biology. This lesson, part of Topic B2 of your OCR Gateway Combined Science course, first explains how cells become specialised for particular jobs, then turns to stem cells: what they are, where they come from, how they are used, and the risks and ethical questions they raise.
By the end of this lesson you should be able to explain what differentiation is, describe how named specialised cells are adapted to their functions, define a stem cell, compare embryonic, adult and plant (meristem) stem cells, outline their uses, and discuss the risks and ethical issues involved.
This lesson builds AO1 (knowledge of differentiation and the types of stem cell) and AO3 (evaluating the benefits, risks and ethical issues of using stem cells).
Differentiation is the process by which a cell becomes specialised for a particular job, taking on a specific shape and the right set of sub-cellular structures. The crucial point — and a favourite of examiners — is how this happens:
Every cell in an organism contains the same DNA (the same genes), but a specialised cell switches on only the genes it needs. Switching different genes on or off makes different proteins, and it is those proteins that give each cell type its particular structure and function. That is why a nerve cell and a red blood cell can look completely different even though they carry an identical set of genes.
flowchart TD
A["Unspecialised cell<br/>(same genome in every cell)"] --> B["Differentiation<br/>(specific genes switched on or off)"]
B --> C["Nerve cell<br/>long, with branches"]
B --> D["Red blood cell<br/>biconcave, no nucleus"]
B --> E["Sperm cell<br/>tail and many mitochondria"]
B --> F["Root hair cell<br/>long extension into soil"]
B --> G["Palisade cell<br/>packed with chloroplasts"]
In animals, most cells differentiate at an early stage and then stay specialised — once a cell has become, say, a muscle cell, it cannot turn back into something else. In plants, by contrast, many cells keep the ability to differentiate throughout the plant's life. The unspecialised cells that hold on to the ability to divide and differentiate are called stem cells, which we come to shortly.
Exam Tip: The most common differentiation question asks how cells become different even though they contain the same DNA. The idea that scores is that different genes are switched on (and others off) in different cells, so different proteins are made. Avoid vague answers such as "the cells just change".
A specialised cell has features that suit its function. Learn two or three adaptations for each cell below, and — this is where the marks are — be ready to say how each feature helps.
| Cell | Adaptation | How it helps the function |
|---|---|---|
| Sperm cell | Long tail (flagellum) | Allows it to swim to the egg |
| Many mitochondria in the mid-piece | Release energy (aerobic respiration) for swimming | |
| Acrosome (enzyme cap on the head) | Enzymes digest the egg membrane so the sperm can enter | |
| Egg cell (ovum) | Large store of nutrients in the cytoplasm | Feeds the developing embryo after fertilisation |
| Membrane changes after fertilisation | Stops more than one sperm entering | |
| Ciliated epithelial cell | Tiny hair-like cilia on the surface | Beat to sweep mucus (with trapped dust and microbes) along, e.g. out of the airways |
| Red blood cell | Biconcave disc shape | Large surface area for absorbing oxygen |
| No nucleus | More room for haemoglobin to carry oxygen | |
| Nerve cell (neurone) | Very long | Carries electrical impulses over large distances |
| Branched ends | Connect to many other cells | |
| Muscle cell | Protein fibres that can shorten | The cell contracts to produce movement |
| Many mitochondria | Release the energy needed for contraction |
The red blood cell is the highest-frequency exam example, so it is worth dwelling on. It is one of the few animal cells with no nucleus — losing the nucleus frees up space, so the cell can be crammed with haemoglobin, the red pigment that binds oxygen. Its biconcave disc shape (like a doughnut without the hole going all the way through) gives it a large surface area compared with its volume, so oxygen can diffuse in and out quickly. It is also small and flexible, so it can squeeze through the narrowest blood vessels.
Exam Tip: For the red blood cell, two adaptations score most reliably: the biconcave disc shape (large surface area for gas exchange) and the absence of a nucleus (more space for haemoglobin). Always pair the feature with the reason.
Plant cells specialise too, and several of them connect directly to later parts of B2 on transport and exchange.
| Cell | Adaptation | How it helps the function |
|---|---|---|
| Root hair cell | Long hair-like extension | Large surface area to absorb water and mineral ions from the soil |
| Many mitochondria | Release energy for active transport of mineral ions | |
| Xylem vessel | Hollow tubes with no end walls and no living contents (dead) | Form a continuous pipe so water and minerals can flow up the plant |
| Walls strengthened with lignin | Provide support and keep the tube open | |
| Phloem cell | Sieve plates with pores between cells; living | Allow dissolved sugars to flow up and down the plant (translocation) |
| Palisade mesophyll cell | Packed with chloroplasts | Absorb the most light for photosynthesis |
The root hair cell is a perfect example of structure matching function, and it links forward to osmosis and active transport. Its long, thin extension dramatically increases the surface area in contact with the soil water, so it can absorb water (by osmosis) and dissolved mineral ions far faster than a flat cell could. Crucially, it contains many mitochondria: minerals are often more concentrated inside the cell than in the soil, so they have to be taken in by active transport, which needs energy released by respiration.
Exam Tip: A common misconception is that a root hair cell has many mitochondria so it can move or swim. It does not move at all — the mitochondria supply energy for the active transport of mineral ions into the cell. Always link the feature to the correct job.
Most cells in your body have already differentiated — a muscle cell will stay a muscle cell. But a small number of cells stay unspecialised. A stem cell is an unspecialised cell that can:
Because they can become other cells, stem cells are the source of new cells for growth, repair and replacement. The two ideas — unspecialised and able to become many cell types — sit at the heart of every definition question on this topic.
Exam Tip: The definition that scores is "an unspecialised cell that can divide and differentiate into different (specialised) cell types". Saying only "a cell that can turn into other cells" usually misses the "unspecialised" and "divide" marks.
In animals there are two main sources of stem cells, and OCR expects you to compare them.
| Embryonic stem cells | Adult stem cells | |
|---|---|---|
| Found in | Early embryos (a few days old) | Certain adult tissues, especially bone marrow |
| What they can become | Almost any type of cell in the body | A more limited range of cell types |
| Ethical concern | High — the embryo is destroyed | Lower — taken from a consenting adult |
Exam Tip: The key difference is how many cell types each can become: embryonic stem cells can become almost any cell type, whereas adult stem cells can become only a limited range. This difference is also why embryonic stem cells sit at the centre of the ethical debate.
Plants keep stem cells throughout their lives, in regions called meristems. Meristems are found at the tips of roots and shoots (and in other growing regions). The cells there stay unspecialised and can divide and differentiate for as long as the plant lives, which is why plants can keep growing taller and produce new roots, leaves and flowers.
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