Energies driving contraction

Without ATP, muscle is in a constant state of tension. Its actin and myosin proteins are cross-linked to one another and contracted (Pollack, 2001). ATP breaks actin-myosin crosslinks, and restores the proteins to their extended state. ATP does this despite the fact that its binding sites are located remotely from the sites of cross-linking and contraction. ATP drives the system to its high energy state. 

FIGURE 17.23 The mechanism of skeletal muscle contraction. The free energy of ATP hydrolysis drives a conformational change in the myosin head, resulting in net movement of the myosin heads along the actin filament. Inset) A ribbon and space-filling representation of the actin—myosin interaction. (SI myosin image courtesy of Ivan Rayment and Hazel M. Holden, University of Wiseonsin, Madison.)... 

The energy released is captured in the form of a molecule known as adenosine triphosphate (ATP) and is used to drive all our energy-requiring processes synthesis of complex molecules, movement, nerve conduction, muscle contraction, and so on. 

Only ADP is phosphorylated to form ATP in glycolysis, oxidative phosphorylation, and photophosphorylation. ATP provides the energy, directly or indirectly, to drive most biosynthetic reactions. The functions of membranes such as active transport and osmotic relations, which regulate the volume of cells, are energy dependent. The structural organization, contraction, and orientation of chromosomes and microtubules of the spindle apparatus during mitosis depend on ATP energy. The intracellular concentrations and stoichiometric relations of ATP, ADP, and AMP also modulate cellular metabolism. 

Reduced redox cofactors channel their electrons to 02 via the process of oxidative phosphorylation. The products of this very complex pathway are H20 and ATP. ATP is generated from ADP and P,. The nature of high-energy phosphate bonds was discussed in some detail in Chapter 2, where the role of ATP in human biochemistry was introduced. ATP and related triphosphonucleosides may be used to drive various processes, such as muscle contraction, maintenance of ion gradients across membranes, or biosynthesis of macromolecules. 

The free energy liberated in the hydrolysis of ATP is harnessed to drive reactions that require an input of free energy, such as muscle contraction. In turn, ATP is formed from ADP and P when fuel molecules are oxidized in chemotrophs or when light is trapped by phototrophs. This ATP—ADP cycle is the fundamental mode of energy exchange in biological systems. 

Adenosine triphosphate, better known as ATP, is able to store energy for cellular processes. When ATP reacts, the energy stored in its bonds is used to drive various processes such as the transport of materials in the cell, the synthesis of compounds needed by the cell, and, in animals, muscular contraction. To generate enough ATP to do all this work, however, there have to be other ATP-generating mechanisms, which begin with the dark phase of photosynthesis. 

The life of a cell is maintained by the continuous activity of a myriad of biochemical reactions that provide metabolic energy, synthesize (and degrade) structural and functional molecules such as proteins, nucleic acids and lipids, and drive cellular dynamic functions such as contraction, locomotion, and cytokinesis. These reactions are organized into networks or modules that have specific functions such as protein synthesis or production of energy by oxidative phosphorylation. Then, when a cell is in a stable state, these reaction networks must also operate stably i.e., energy is continuously generated and... 

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