The big idea: A multicellular organism is built as a hierarchy: cells form tissues, tissues form organs, organs form organ systems, and the systems make up the organism. Each level is a subsystem of the one above. When the systems are integrated, the organism gains emergent properties that no single part has.
The points to remember
- Cells of one type working together form a tissue.
- Several tissues form an organ.
- Organs working together form an organ system.
- Integrated systems make up the organism.
- Integration gives emergent properties: more than the sum of the parts.
Remember it as: Cell, tissue, organ, system, organism.
Real example: the cheetah. A cheetah can sprint at around 100 km/h for a few seconds. That only works because its muscular, skeletal, nervous, circulatory and respiratory systems act together: integrated, they make it an effective predator, which no single system could be on its own.
Trap: An organ is made of several kinds of tissue (a muscle also has nerve, blood and connective tissue), not just one.
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How it is asked: Expect a question that asks you to put the levels in order, or to explain how integration of systems gives an organism new properties.
IB-style questionOutline[3 marks]
Outline how a multicellular organism is organized as a hierarchy of subsystems.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.