Cellular Pathways that Harvest Chemical Energy

. ^ Agriculture was a key step in the development of human civilizations. The harvesting, planting, and cultivation of seeds began about 10,000 years ago. One of the earliest plants to be domesticated and turned l ■ into a reliable crop was barley, and one of the first uses of barley was to brew beer. Living in what is now Iraq, the ancient Sumerians learned that partly germinated and then mashed up barley seeds, stored under the right conditions, could produce a potent and pleasant alcoholic beverage. Beer making spread to Egypt, and was so important in that ancient civilization that the hieroglyphic symbol for food was a pitcher of beer and a loaf of bread.

Fermented beverages such as beer and wine were important to ancient civilizations because pure water, without infectious disease-causing organisms, was hard to obtain. In the nineteenth century, scientists were able to demonstrate that the conversion of seed mash into alcohol is carried out by living cells—in this case, yeast. By the middle of the twentieth century, biochemists had identified the intermediate substances in the metabolic pathway that converts the starch in seeds—a polysaccha-ride—into alcohol. In addition, they showed that each intermediate step in the pathway is catalyzed by a specific enzyme.

In this chapter, we will describe some aspects of this and related pathways for the breakdown of sugars. The metabolism of sugars is important not only in making alcoholic beverages, but in providing the energy that organisms store in ATP—the energy you use all the time to fuel both conscious actions such as turning the pages of this book and automatic ones such as the beating of your heart.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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