Primary active transport occurs when the hydrolysis of ATP is directly required for the function of the carriers. These carriers are composed of proteins that span the thickness of the membrane. The following sequence of events is believed to occur: (1) the molecule or ion to be transported binds to a specific "recognition site" on one side of the carrier protein; (2) this bonding stimulates the breakdown of ATP, which in turn results in phosphorylation of the carrier protein; (3) as a result of phosphorylation, the carrier protein undergoes a conformational (shape) change; and (4) a hingelike motion of the carrier protein releases the transported molecule or ion on the opposite side of the membrane. This model of active transport is illustrated in figure 6.16.
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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.