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Cell Size Is Itself an Adaptation to a Cell's Job
- Specialised cells differ enormously in size, from a red blood cell around 7 micrometres across to a motor neuron that can stretch nearly a metre.
- A cell's size and shape are tuned to its function, so size is a genuine part of specialisation.
- Cell sizes are measured in micrometres (µm), where one micrometre is a thousandth of a millimetre.
Analogy
- A courier van and a bicycle both move goods, but each suits a different job.
- The bicycle wins in tight lanes while the van carries far more cargo.
- Cell size works the same way, matched to the task rather than chosen at random.
Size Is Set by the Job, Then Limited by Geometry
- Small cells such as red blood cells squeeze through narrow capillaries to deliver oxygen.
- Long cells such as motor neurons carry signals over long distances.
- Large cells such as the egg cell (ovum) hold stored nutrients for a developing embryo.
- How large a cell can get is limited by surface area-to-volume considerations, which are covered separately in B2.3.6.
Note
- This topic is about how size fits function.
- The full theory of why volume outgrows surface area belongs to B2.3.6.
Examples: Real Cells Span a Huge Size Range
Red Blood Cell: Small for Fast Gas Exchange
- Size: about 7 µm in diameter.
- The red blood cell (erythrocyte) transports oxygen around the body.
- Its small biconcave shape gives a short path for oxygen to diffuse in and out.
- Being small and flexible lets it bend through the narrowest capillaries.
