The big idea: Most cells fit a 'typical' description, but some real cells break these expectations. These are called atypical cells.
A 'typical' cell
- Has one nucleus
- Is microscopic in size
- Is sealed off by its own membrane (and wall, in plants and fungi)
Atypical cells may have
- No nucleus
- Many nuclei
- A surprisingly large size
- Shared cytoplasm
Not 'wrong': Atypical cells are not 'wrong' — each unusual feature is an adaptation that helps the cell do a particular job.
- Typical cell
- A cell that fits the standard description: one nucleus, microscopic size, and its own sealed membrane (and wall in plants/fungi).
- Atypical cell
- A cell that does NOT fit the typical description in one or more ways — for example it lacks a nucleus, has many nuclei, is unusually large, or shares cytoplasm with neighbours.
- Anucleate
- Having no nucleus. (The prefix 'a-' means 'without', so anucleate = without a nucleus.)
- Multinucleate
- Having many nuclei inside one cell or fibre. ('Multi-' means 'many'.)
- Aseptate
- Having no cross-walls (no septa) between sections, so the cytoplasm is continuous — seen in some fungal hyphae.
What 'atypical' really means: Atypical does not mean broken or dead.
It means the cell differs from the typical picture — usually because that difference makes it better at a specific function (like carrying oxygen, or contracting along a long muscle).
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Four examples come up again and again. Each one breaks a different rule about what a 'typical' cell looks like.
Learn what is unusual about each — and, just as importantly, why that unusual feature is useful.
1 · Anucleate — no nucleus at all: A mature mammalian red blood cell loses its nucleus as it develops.
Removing the nucleus frees up internal space for haemoglobin, so the cell can carry more oxygen.
A plant example is the phloem sieve tube element: it also loses most of its contents (including the nucleus) so that sugary sap can flow through it freely.
2 · Multinucleate — many nuclei in one cell: A skeletal (striated) muscle fibre is one long cell containing many nuclei.
It forms when many smaller cells fuse together into a single large fibre.
Having many nuclei lets the long fibre make enough proteins along its whole length to power a strong, coordinated contraction.
3 · Unusually large — a single giant cell: Some single-celled algae are surprisingly large — a single cell can be several centimetres long, big enough to see without a microscope.
This breaks the usual rule that cells are microscopic, yet the whole organism is still just one cell.
4 · Aseptate — cytoplasm with no cross-walls: Some fungal hyphae are aseptate: they have no cross-walls (septa) dividing the thread into separate cells.
The cytoplasm is continuous along the hypha and contains many nuclei that are not separated from one another.
This lets materials and nuclei move freely along the growing fungus.
| Typical-cell expectation | Atypical cell that breaks it | What is unusual |
|---|---|---|
| A cell has exactly one nucleus | Red blood cell (mature, mammalian); phloem sieve tube element | Anucleate — it has NO nucleus once mature |
| A cell has exactly one nucleus | Skeletal (striated) muscle fibre | Multinucleate — it has MANY nuclei in one long fibre |
| A cell is microscopic (a few µm to tens of µm) | Some single-celled algae (e.g. a giant alga) | Very large — a single cell several centimetres long |
| Each cell is sealed off by its own walls/membrane | Aseptate fungal hypha | No cross-walls (septa) — cytoplasm is continuous and shares many nuclei |
A memory hook: Count the nuclei to spot the exception:
Zero nuclei = anucleate (red blood cell, sieve tube element).
Many nuclei = multinucleate (striated muscle fibre; aseptate fungal hypha).
And remember the odd one out on size: a giant single-celled alga is one cell, yet centimetres long.
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How this is tested: Atypical cells are tested in three common formats. Most ask what shared feature makes two cells atypical.
| Format | What you are asked | The answer |
|---|---|---|
| 1-mark Paper 1A | Two named cells — for example a red blood cell and a phloem sieve tube element: identify why both are regarded as atypical | Both lack a nucleus (anucleate) |
| Two micrographs | The shared feature that makes both cells atypical | For instance, both are multinucleate (have many nuclei) |
| Paper 2 or 3 | Explain the structure-to-function payoff of one atypical feature | For example, why a red blood cell loses its nucleus |
IB-style question — identify why two cells are atypical
A mature red blood cell and a phloem sieve tube element are both described as atypical cells. Identify the structural feature that both cells share that makes them atypical. [1]
How to score the mark
- Look for the rule both cells break. A typical cell has one nucleus — so check the nucleus first.
- State the shared feature. Both the mature red blood cell and the sieve tube element have lost their nucleus as they matured.
- Name it precisely. They are both anucleate (they have no nucleus), which is why both are regarded as atypical. (1 mark for: both lack / have no nucleus.)
Final answer
Both cells lack a nucleus (they are anucleate) once mature — that is the shared feature that makes them atypical.
Why this scores the mark: The question asks for one shared feature, so name the single thing both cells have in common: no nucleus.
Listing features of only one of the cells, or naming a feature they do not share, would not score.
| Atypical feature of a red blood cell | Why it is unusual | Function payoff |
|---|---|---|
| No nucleus (anucleate) | Almost every other body cell keeps its nucleus | More room inside for haemoglobin → carries more oxygen |
| No mitochondria | Most active cells are packed with mitochondria | Cannot 'use up' the oxygen it carries, so it delivers more to tissues |
| Biconcave disc shape | Most cells are not dimpled discs | Larger surface area for faster oxygen loading and unloading |