3

key concepts

3.1

Macromolecules Characterize Living Things

3.2

The Function of a Protein Depends on Its Three-Dimensional Structure

3.3

Simple Sugars Are the Basic Structural Unit of Carbohydrates

3.4

Lipids Are Defined by Their Solubility Rather Than by Chemical Structure

Weaving a Web

A spider web is an amazing structure. It is not only beautiful to look at, but an architectural wonder serving as the spider’s home, its mating place, and its way to capture food. Consider a fly that chances to interact with a spider web. The fibers of the web must slow down the fly, but they cannot break, so they need to stretch to dissipate the energy of the fly’s movement. The fibers holding the fly cannot stretch too much; they must be strong enough to hold the web in place. Web fibers are far thinner than a human hair, yet they are very strong. The fibers can also be long; for example, the Darwin’s bark spider makes strands up to 25 meters long.

Chemical investigations reveal that spider silk is made of variations on a single type of large molecule called a protein. Proteins are polymers: long chains of individual smaller units called amino acids. The proteins in spider silks have characteristic structures and amino acid compositions that determine particular functions. Proteins in the stretchy web fibers have amino acids that allow them to curl into spirals, and these spirals can slip along one another to change the fiber’s length. Another kind of spider silk is the dragline silk, which is less stretchy and used to construct the outline of the web, its spokes, and the lifeline of the spider. The proteins in these strong fibers are made up of amino acids that cause the proteins to fold into flat sheets with ratchets, so that parallel sheets can fit together like Lego blocks. This arrangement makes these fibers hard to pull apart.

Just how strong is spider silk? A popular series of films features the fictional character Spider-Man. He often uses his web to ensnare evildoers and protect the innocent. In one scene, he stops a train by using ten ropes made of spider silk, each about 1 centimeter thick. Scientists who study spider silk have estimated the number of silk fibers in each rope and calculated that this scene would indeed be possible. All of this from covalent bonds holding long chains of amino acids together and weaker forces between the chains! Given the amazing strength shown by these proteins, it is not surprising that there is a lot of interest in using spider silk for human activities.

What are practical uses for spider silk?