The big idea: A typical eukaryotic cell has one nucleus. Atypical cells have none, or many.
The atypical cells
- Red blood cells are anucleate — they lose their nucleus as they mature, leaving more room to carry oxygen.
- Skeletal muscle fibres and fungal hyphae are multinucleate — one long cell contains many nuclei sharing the same cytoplasm.
- Phloem sieve tube elements have no nucleus either; a companion cell beside each one keeps its nucleus.
Atypical cells — the exceptions: Most cells have one nucleus. A few do not, and the IB calls these atypical. The shared 'atypical' feature is simply that the number of nuclei is not one.
Real examples: your blood and your leg: A drop of your blood holds millions of red blood cells, none with a nucleus. A single muscle fibre in your thigh can be several centimetres long and holds hundreds of nuclei.
Atypical cells are still eukaryotic: they have membrane-bound organelles, or come from cells that did.
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The big idea: Each atypical cell's number of nuclei suits its job.
The points to remember
- A red blood cell loses its nucleus, leaving more room for haemoglobin.
- A sieve tube element has no nucleus, so it is an open tube for sap; its companion cell does the work.
- A skeletal muscle fibre forms when many cells fuse, so it has many nuclei.
- An aseptate hypha has no cross-walls, so many nuclei share one cytoplasm.
Remember it as: None for room or flow; many for length.
Red blood cell
No nucleus: more room for haemoglobin to carry oxygen.
Sieve tube element
No nucleus: an open tube, so sap flows easily; the companion cell keeps it alive.
Skeletal muscle fibre
Many nuclei: cells fuse into one long fibre that contracts as one.
Aseptate hypha
Many nuclei: no cross-walls, so nuclei and cytoplasm move freely along the hypha.
Atypical is not prokaryotic: A red blood cell has no nucleus, but it is not a prokaryote: it is a eukaryotic cell that lost its nucleus.
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How it is tested: Paper 1A shows two kinds of cell and asks what makes them both atypical: count the nuclei. Paper 2 can ask what is unusual about a fungus: no cross-walls, many nuclei.
Skeletal muscle fibres and red blood cells are both described as atypical. Outline why each is atypical.
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The shared feature: Two multinucleate cells share 'several nuclei'; two anucleate cells share 'no nucleus'. They still have cytoplasm and membranes.