Atypical cells at Higher Level: The same statement as SL. HL papers pair phloem sieve tubes with red blood cells: both have no nucleus. Know all four examples by their number of nuclei.
Practise this as you read
- Identify what two atypical cells share.
- Outline why each one is atypical.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
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: bread mould and a maple tree: Bread mould (Rhizopus) grows as hyphae with no cross-walls, so hundreds of nuclei share one tube of cytoplasm. In a sugar maple, sap flows through sieve tubes that have no nuclei.
Atypical cells are still eukaryotic: they have membrane-bound organelles, or come from cells that did.
Memorize terms 3x faster
Smart flashcards show you cards right before you forget them. Perfect for definitions and key concepts.
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.
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.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
The shared feature: Two multinucleate cells share 'several nuclei'; two anucleate cells share 'no nucleus'. They still have cytoplasm and membranes.