Immediate Energy

In a short, intense exercise such as a 100 m dash, the respiratory and cardiovascular systems cannot deliver oxygen to the muscles quickly enough for aerobic respiration to meet the increased ATP demand. The myoglobin in a muscle fiber supplies oxygen for a limited amount of aerobic respiration, but in brief exercises a muscle meets most of its ATP demand by borrowing phosphate (Pi) groups from other molecules and transferring them to ADP. Two enzyme systems control these phosphate transfers (fig. 11.19):

  1. Myokinase (MY-oh-KY-nase) transfers Pj groups from one ADP to another, converting the latter to ATP.
  2. Creatine kinase (CREE-uh-tin KY-nase) obtains Pj groups from an energy-storage molecule, creatine phosphate (CP), and donates them to ADP to make ATP. This is a fast-acting system that helps to maintain the ATP level while other ATP-generating mechanisms are being activated.

ATP and CP, collectively called the phosphagen system, provide nearly all the energy used for short bursts of intense activity. Muscle contains about 5 millimoles of ATP and 15 millimoles of CP per kilogram of tissue, which is enough to power about 1 minute of brisk walking or 6 seconds of sprinting or fast swimming. The phosphagen system is especially important in activities requiring brief but maximal effort, such as football, baseball, and weight lifting.

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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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