Specialised cells: structure fits function sim

What the sim shows

Differentiation is how a cell becomes specialised for one job. Structure fits function, at every level. Whenever you meet something new, ask what job it has to do, and then look at how it is built to do exactly that. Seven prepared slides hold seven specialised cells; the close-up shows the one on the stage as a 3D model at its true shape, with a scale bar (each cell has its own scale). The red blood cell contains haemoglobin to carry oxygen. It has no nucleus, so it can hold more haemoglobin. It is biconcave, and that shape increases its surface area to volume ratio: the Evans–Fung shape gives 134 µm² of surface around 94 µm³, while a sphere of the same volume (radius 2.82 µm) has only 100 µm², so the disc has 34 % more. The meter multiplies loading speed (shape) by oxygen carried (haemoglobin). It is a teaching model. The muscle cell has a nucleus, protein fibres and mitochondria that provide the energy for contraction. The root hair cell is elongated to raise its surface area to volume ratio: with its hair (10 µm × 250 µm) a 40 × 20 × 20 µm cell has 11775 µm² of surface instead of 4000 µm². Sperm cell: haploid nucleus: half the chromosomes, so the fertilised egg has the full number. A myelinated 10 µm nerve fibre carries impulses at about 60 m/s (6 m/s per µm, Hursh 1939); without myelin it would manage about 3.5 m/s (a thought experiment: no 10 µm unmyelinated vertebrate axon exists). The meter is a stated teaching model: the intact cell scores 100 %, and each feature removed multiplies the score by a factor, calculated from a real ratio where one exists (surface areas, oxygen carried, anaerobic respiration releasing 2/32 of the energy per glucose, impulse speeds) and otherwise illustrative and marked ≈ (illustrative) on the meter. Idealised on purpose: the root hair is drawn 125 µm long (real 250 µm, up to about 1 mm); the muscle fibre and the axon run off the card (they are millimetres to a metre long), and the myelin segments are drawn 30 µm long (really about 1 mm); carrier proteins and mitochondria are drawn a little large so they can be seen and dragged; the sperm head is drawn round in section (it is flattened). Each removal is one step, and nothing changes on its own.

• Drag a slide from the tray onto the stage to choose a cell
• In the close-up, drag a feature off the cell; drag it back from the shelf
• The meter: how well the cell can still do its job (a stated model)

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