Page 76 - The Miracle in the Spider
P. 76

4 MICRONS
                                     100 NANOMETRES       20 NANOMETRES
                 Enlargement of spider thread (a) shows it to be a composite material
                 made of strands of disordered amino acid chains and ordered
                 crystals (b and c). Each crystal is made up of different-size amino
                 acid groups pressed into an accordionlike formation, called beta-
                 pleated sheets (d). The surrounding strands are called alpha helixes;
                 their contracted disarray provides silk with its elasticity. When silk is
                 emitted, shearing forces like those shown at the right (e) are applied
                 to some alpha helixes. Consequently, their hydrogen bonds break
                 and they become beta-pleated sheets. (f), as the similarity of the
                 highlighted molecular strands shows. (1 nanometre= 0.000000001
                 meter).



              apparently reinforce the frame and dragline silks; cocoon silk; a silk to
              wrap captured prey; and a silk to attach the frame and dragline silks to
              the substrate. 24
                   These silks, in the same way as they have different qualities from the
              point of view of strength and elasticity, also exhibit different thicknesses
              and levels of stickiness. For example, although the dragline, which plays
              such a large part in the spider's life, does not possess the quality of
              stickiness, it is nevertheless strong and elastic. It can easily bear weights
              up to two or three times the weight of the spider. It is thanks to this silk
              that the spider, carrying the prey it has caught, can move safely up and
              down.
                   As we have seen, in order to live, the spider needs to be able to
              produce different types of silk and also to know where to use each one.
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