Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
The ATP Cycle and Cellular Bioenergetics
ATP is the primary chemical mediator in the cell, linking energy-yielding and energy-consuming processes

Having reviewed some of the fundamental laws governing energy exchange in chemical systems, we can now turn to the energy cycle within Cells. For heterotrophic cells, the source of Free energy in chemical form is the degradation, or Catabolism, of nutrient molecules (primarily CARBOHYDRATES and fats). Cells utilize this energy for the following purposes: 1) the synthesis of Biomolecules from small precursor molecules; 2) the performance of mechanical work, such as Muscle contraction; 3) the Transport of substances across membranes against a concentration gradient; and 4) the accurate transmission of information. The primary link between energy-yielding and energy-consuming cellular reactions is adenosine triphosphate (ATP; Fig. 14-2). When high-energy cellular fuel is broken down, a portion of the free energy contained within it is captured, meaning it is used to synthesize ATP from adenosine diphosphate (ADP) and inorganic phosphate (P_i)—a process that requires an input of free energy. Later, by breaking down into ADP and phosphate (Fig. 14-3), ATP releases a significant amount of its chemical energy to drive processes that require energy. Thus, ATP acts as a chemical energy carrier, bridging energy-yielding cellular processes with the primary forms of energy-consuming cellular activity. ATP also supplies energy for vital processes such as Bioluminescence, which in fireflies serves as a signal to attract mates.

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Fig. 14-2. A. Structures of ATP, ADP, and AMP. The phosphate groups of ATP are designated by the Greek letters α (alpha), β (beta), and γ (gamma).

The terminal phosphate group of ATP can be enzymatically transferred to various phosphate acceptors. At pH 7, the phosphate groups are fully ionized. B. Molecular model of ATP

ATP was first discovered in Skeletal Muscle extracts by Karl Lohmann in Germany, and almost simultaneously in 1929, two American researchers—Cyrus Fiske and Yellapragada SubbaRow—isolated the compound. Initially, ATP was thought to play a vital role solely in muscle contraction; however, it was subsequently found to be present in all Cell types—animal, plant, and bacterial. It was also discovered that ATP participates in A wide variety of cellular processes. In 1941, recognizing the universal significance of these observations, Fritz Lipmann advanced the unifying concept that ATP acts as the primary and universal chemical energy carrier in cells. He was the first to propose the existence of the ATP cycle in cells (Fig. 14-3).