|MadSci Network: Cell Biology|
Cell size and membranes What a fascinating question, Paul! Superficially, it should be easy to answer, but, once we begin to look into the factors involved, we realise that it is anything but simple! The strength of a membrane is low and like a balloon, if it expands too much, it will burst. If you fill a balloon with water, you can immediately see that the shape of the balloon depends on the surfaces around it – it has little mechanical strength and so deforms against a flat surface (e.g. a table), elongates when held from the neck and bursts when thrown! Thus, a cell must have a maximum size based on the mechanical strength of its membrane. But, in the real world, unicellular organisms are all aquatic, so they are supported by the surrounding medium. An Amoeba, Stentor and Paramecium are all unicellular, active free-living and mobile Protists. They are amongst the biggest such organisms, and they are about 0.5 – 2.0 mm long. They are thus clearly visible to the naked eye. Multicellular organisms, such as Jellyfish and Sea Anemones are much larger in size, since each cell relies on its neighbours for support. Plants and Fungi have cell walls around their cells, which provide great mechanical strength and give a definite shape to each cell. They allow the cell to generate turgor, by using osmosis to inflate each cell with water under pressure. A few animals use a hydroskeleton too (e.g. earthworms), but the pressure they can generate is much less. You can compare this by holding a thin plant stem in the middle and an earthworm in the middle; the plant bends a little, the worm collapses on either side of your finger. The result of this cell wall is that each cell can become much, much larger. A single fungal cell (or hypha) can be several metres in length – some plant cells can also be very long too. The longest I am aware of being found in grape vines, with phloem cells 5.0 m long! Animal cells which are more than about 1.00 mm in diameter are surrounded by supporting material. The best example would be an egg, which may have jelly (e.g. fish, frog) or a hard shell (e.g. bird) to support it. The largest single cell known is the Ostrich egg, which can be 150mm long and weigh1.0 kg or more. (I’m told they make very good - and very large - omelettes!). The question of diffusion is harder to answer, since the requirements of any cell will depend on how active it is. Clearly, an ostrich egg is not very active, but the developing chick certainly is and the early stages of its development rely on diffusion of gases through not only the shell, and several membranes but also through much of the rest of the egg, before the soon-to-be-an-embryo is reached. Therefore the maximum size limit for diffusion will, depend on the metabolic activity of the cell. It will also depend on the shape of the cell too. A long, thin cell will have a greater surface/volume ratio than a spherical cell of the same bulk. Neurones (nerve cells) can be several metres long, yet each has a very narrow diameter to allow diffusion along much of its length. This is a fascinating question, as I said at the beginning. It is often the case in biology that the simplest questions are the hardest to answer and that the more we understand, the more we realise that there is so much more left that we do NOT understand! Perhaps you will be the one to fill in some of the gaps – the trick is to ask the RIGHT questions!
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