The big idea: Both muscle types contract with myofibrils, but a striated muscle fibre is long, unbranched and has many nuclei, while cardiac cells are short, branched and have one nucleus.
The points to remember
- Both contain myofibrils (light and dark bands, sarcomeres) and many mitochondria for ATP.
- Striated muscle fibres are very long and unbranched, with many nuclei.
- Cardiac muscle cells are short and branched, usually with one nucleus, joined at intercalated discs.
- Hypothesis: branching lets an impulse spread through a network, so the heart wall contracts together.
- Hypothesis: a long fibre can shorten along much of the muscle, and many nuclei can make enough proteins for so much cytoplasm.
- Is a fibre a cell? One membrane and shared cytoplasm say yes; many nuclei and huge size do not fit a typical cell.
Remember it as: Skeletal: long, straight, many nuclei. Cardiac: short, branched, one nucleus.
Real example: a thigh muscle: A single fibre in a human thigh muscle can be several centimetres long and contain hundreds of nuclei, formed when many embryonic muscle cells fused.
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How it is asked: Expect Paper 1A items on what the two types share and how they differ, and Paper 2 parts on the function of their mitochondria or on whether a fibre is a cell.
IB-style questionDiscuss[3 marks]
Discuss whether a striated muscle fibre should be called a cell.
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See the answer broken into clear points, with guidance on how each point earns marks.