21. Lipids and Carbohydrates   Previous PageNext Page

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.

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