The big idea: Being made of many cells is not a one-off invention. It evolved again and again, in separate groups.
The points to remember
- Multicellularity has evolved repeatedly: separately in different groups of eukaryotes.
- All plants and all animals are multicellular.
- Many fungi (mushrooms, moulds) and many eukaryotic algae (seaweeds) are too; yeasts are single cells.
- Each group started from single-celled ancestors.
Remember it as: Not once, but many times.
It has even been watched happening:
Real example: snowflake yeast: In 2012 scientists grew single-celled yeast and kept only the cells that sank fastest. Within weeks, the yeast formed clusters of cells that stayed stuck together: 'snowflake yeast'.
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The big idea: Many cells working together can build a bigger body, and the cells can share out the jobs.
The points to remember
- Larger body size: a body of many cells can grow far bigger than one cell can.
- Cell specialization: different cells take on different jobs, such as support, feeding or reproduction.
- Specialized cells do their one job more efficiently than one cell doing everything.
Single-celled alga
- One cell does every job
- Microscopic in size
Multicellular seaweed
- Some cells anchor it, others photosynthesize
- Can grow to many metres long
Real example: Volvox: Volvox is a green alga made of hundreds of cells in a hollow ball. Most cells swim and photosynthesize; a few specialized cells inside make new colonies.
Watch the wording:
Not mutualism: Cells of one multicellular organism are one species working together. Mutualism is between two different species.
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How it is tested: Paper 1A shows a single-celled organism and the group it forms, and asks which statements about multicellularity are correct.
IB-style questionOutline[2 marks]
Giant kelp is a multicellular brown alga that grows over 30 metres long. Outline two advantages of multicellularity that kelp shows.
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The two advantages: The two advantages to name are a larger body size and cell specialization.