Polymers from shelled creatures
Most of the creatures that have a shell of some
sort are usually found in or near a body of water. This includes clams,
oysters, crabs, and lobsters, and shrimp just to name a few of the main
players. There are many others, such as squids,
which aren't as well known for their shells, and
insects, most of which are not found in the sea. So we will stick with
the ones we know the most about and come from the ocean.
An abundance of natural polymers
are found in these creatures that wear shells. Oddly enough it is
these shells which contain the most used polymers. Chitin is a polysaccharide
that is found in a
variety of creatures, from insects to fungi, but most abundantly in the
shells of crustaceans such as crabs, lobsters, and shrimp.
Chitin looks like this:
One of the things we do with chitin is to cleave some of the amide
groups attached to the polysaccharide backbone to leave free amines. The product of this
reaction is called chitosan.
Because it now has the basic amine groups attached, this polymer
is used in applications from health care to agriculture to dyes
for fabrics. What's so neat about making and using chitosan is that it takes care
of two problems at once: How do we get rid of all those shrimp and crawfish
shells after we've eaten the insides? And how can we make a renewable polymer
that can be composted and converted back to useful nutrients after we're
done using it? You're right: chitosan!
Just so you know, chitin is not the only substance found in these
shells, but like I
said, it is the most used, as well as the most abundant.
Protein is probably the second most used natural
polymer that is found in shells and the creatures themselves, not to
mention your own bones and hair.
Slow shelled creatures
Why call them that? Well, in a race between a lobster and a clam, I'd
put my money on the lobster to win. Clams, oysters and mussels just don't
move around much but they do make up another important family of the shelled
creatures. Their pretty shells are one reason kids love to collect them
when walking along the beach.
These organisms feed on plankton and other microorganisms.
In turn, they feed larger creatures such as turtles,
starfish, squid, and shell-cracking fish of various
types. In addition, smart aquatic mammals such as the sea otter have learned
how to use rocks to break open oysters to get at the meat inside. And you thought
humans were the only creatures on earth to use tools?
Some slow shelled creatures have only one shell (like abalone) while others have two shells that are
hinged together. All of these shells contain chitin,
but the most of the shell is a combination of calcium carbonate and a
tanned protein. By this time in your
exploration of the Polyquarium, I don't need to remind you what a protein
looks like, right?
These creatures have the ability to secrete this protective polymer layer,
which means they are able to produce it themselves and put it where it's needed.
The shells these animals
secrete is used in conjunction with materials they take from their
environments and they then use that to make a
material. A composite is simply a combination of two or more materials
that work better together than the individual components. Got it?
Of course, they aren't the only creatures which secrete
protective layers. Many animals which live on and around coral reefs also have layers they secrete.
Nature has its own recycling system. What is the most
humbling about it is that we are not able to reproduce what nature has been
practicing for millennia. But that is part of
what science is all about, trying to find ways to replicate
natural processes and materials so we enjoy the benefits without
ill effects on nature's creatures and the environment.
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