Energy and Electrons from Glucose

We are all familiar with fuels and their uses. Petroleum fuels contain stored energy that is harvested to move cars and heat homes. Wood burning in a stove or campfire releases energy as light and heat. Living organisms also need fuels, which must be obtained from foods. This is true whether we are speaking of organisms that make their own foods through photosynthesis or organisms that obtain foods by eating other organisms. The most common fuel for living cells is the sugar glucose (C6H12O6). Many other compounds serve as foods, but almost all of them are converted to glucose, or to intermediate compounds in the step-by-step metabolism of glucose.

As you will see in this section, cells obtain energy from glucose by the chemical process of oxidation, which is carried out through a series of metabolic pathways. Before we examine that process, let's take a brief

An Ancient Brewer In the civilizations of ancient Sumeria and Egypt, the important task of brewing beer was usually the domain of women such as the one depicted in this Egyptian statue.The figurine dates from the period known as the "Old Kingdom" and is almost 4,500 years old— about 100 years younger than the Great Pyramid of Giza.

How Metabolic Pathways Are Regulated

look at how metabolic pathways operate in the cell. Several principles govern metabolic pathways:

► Complex chemical transformations in the cell do not occur in a single reaction, but in a number of separate reactions that form a metabolic pathway (see Chapter 6).

► Each reaction in a pathway is catalyzed by a specific enzyme.

► Metabolic pathways are similar in all organisms, from bacteria to humans.

► Many metabolic pathways are compartmentalized in eukaryotes, with certain reactions occurring inside an organelle.

► The operation of each metabolic pathway can be regulated by the activities of key enzymes.

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