The preceding two chapters emphasized the variety and diversity
of carbon compounds. It may seem paradoxical, therefore, to say
that over half of the organic carbon on our planet is found in only
one compound: cellulose. The runner-up, with a clear lead over any
third substance, is starch. Both starch and cellulose are polymers
of a simple six-carbon sugar, glucose (C6H12O6);
thus glucose by long odds must be considered the most outstandingly
successful organic molecule on the face of the planet. As we shall
see in Chapter 23, the central energy-extraction and energy-storage
mechanisms common to all life also are based on this molecule. Life
on Earth revolves around glucose.
Cellulose is plentiful because it is the universal building material
for cell walls of plants of all kinds, from green algae to California
redwoods. The scaffolding of plants is polymerized glucose. Plants
also store energy in glucose polymers of a slightly different kind:
starch. With the same molecule used for both support and energy
storage, a plant is faced with the dilemma of Hansel and Gretel
- how to tell the food from the furniture. The answer is an ingenious
bit of chemical trickery, the a versus
b connections shown to the right, which
will be discussed later in the chapter.