The energyinvesting reactions of glycolysis require ATP

Using Figure 7.6, let us work our way through the glycolytic pathway. The first five reactions of glycolysis are endergonic; that is, the cell is investing free energy in the glucose molecule, rather than releasing energy from it. In two separate reactions (reactions 1 and 3 in Figure 7.6), the energy of two molecules of ATP is invested in attaching two phosphate groups to the glucose molecule to form fructose 1,6-bisphos-phate,* which has a free energy substantially higher than that of glucose. Later, these phosphate groups will be transferred to ADP to make new molecules of ATP.

Although both of these first steps of glycolysis use ATP as one of their substrates, each is catalyzed by a different, spe

*The root bis- means "two." A sugar bisphosphate has two phosphate groups attached to two different carbons. In contrast, the prefix di-implies the serial attachment of two phosphate groups to one carbon, as in ADP (adenosine diphosphate).

Phosphate Group

cific enzyme. The enzyme hexokinase catalyzes reaction 1, in which a phosphate group from ATP is attached to the six-carbon glucose molecule, forming glucose 6-phosphate. (A kinase is any enzyme that catalyzes the transfer of a phosphate group from ATP to another substrate.) In reaction 2, the six-membered glucose ring is rearranged into a five-membered fructose ring. In reaction 3, the enzyme phos-phofructokinase adds a second phosphate (taken from another ATP) to the fructose ring, forming a six-carbon sugar, fructose 1,6-bisphosphate.

Reaction 4 opens up and cleaves the six-carbon sugar ring to give two different three-carbon sugar phosphates: dihy-droxyacetone phosphate and glyceraldehyde 3-phosphate. In reaction 5, one of those products, dihydroxyacetone phosphate, is converted into a second molecule of the other one, glyceraldehyde 3-phosphate (G3P).

By this time—the halfway point of the glycolytic pathway—the following things have happened:

► Two molecules of ATP have been invested.

► The six-carbon glucose molecule has been converted into two molecules of a three-carbon sugar phosphate, glyc-eraldehyde 3-phosphate (G3P, a triose phosphate).

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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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