Adenosine Triphosphate

Adenosine triphosphate (ATP) is the body's most important energy-transfer molecule. It briefly stores energy gained from exergonic reactions such as glucose oxidation and releases it within seconds for physiological work such as polymerization reactions, muscle contraction, and pumping ions through cell membranes. The second and third phosphate groups of ATP are attached to the rest of the molecule by high-energy covalent bonds traditionally indicated by a wavy line in the molecular formula. Since phosphate groups are negatively charged, they repel each other. It requires a high-energy bond to overcome that repulsive force and hold them together—especially to add the third phosphate group to a chain that already has two negatively charged phosphates. Most energy transfers to and from ATP involve adding or removing that third phosphate.

Enzymes called adenosine triphosphatases (ATPases) are specialized to hydrolyze the third high-energy phos-

Chapter 2 The Chemistry of Life 85

phate bond, producing adenosine diphosphate (ADP) and an inorganic phosphate group (PJ. This reaction releases 7.3 kilocalories of energy for every mole (505 g) of ATP. Most of this energy escapes as heat, but we live on the portion of it that does useful work. We can summarize this as follows:

Heat

Work

The free phosphate groups released by ATP hydrolysis are often added to enzymes or other molecules to activate them. This addition of Pi, called phosphorylation, is carried out by enzymes called kinases (phosphokinases). The phosphorylation of an enzyme is sometimes the "switch" that turns a metabolic pathway on or off.

ATP is a short-lived molecule, usually consumed within 60 seconds of its formation. The entire amount in the body would support life for less than 1 minute if it were not continually replenished. At a moderate rate of physical activity, a full day's supply of ATP would weigh twice as much as you do. Even if you never got out of bed, you would need about 45 kg (99 lb) of ATP to stay alive for a day. The reason cyanide is so lethal is that it halts ATP synthesis.

ATP synthesis is explained in detail in chapter 26, but you will find it necessary to understand the general idea of it before you reach that chapter—especially in understanding muscle physiology (chapter 11). Much of the energy for ATP synthesis comes from glucose oxidation (fig. 2.30). The

Ribose

Triphosphate O O

NH2 C

Figure 2.29 Adenosine Triphosphate (ATP) and Cyclic Adenosine Monophosphate (cAMP). (a) ATP. The last two P~O bonds in ATP, indicated by wavy lines, are high-energy bonds. (b) cAMP.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

2. The Chemistry of Life I Text

86 Part One Organization of the Body

which releases energy which is used for

ADP + Pi which is used for

which is then available for

Muscle contraction Ciliary beating Active transport Synthesis reactions etc.

Figure 2.30 The Source and Uses of ATP.

first stage in glucose oxidation (fig. 2.31) is the reaction pathway known as glycolysis (gly-COLL-ih-sis). This literally means "sugar splitting," and indeed its major effect is to split the six-carbon glucose molecule into two three-carbon molecules of pyruvic acid. A little ATP is produced in this stage (a net yield of two ATPs per glucose), but most of the chemical energy of the glucose is still in the pyruvic acid.

What happens to pyruvic acid depends on whether or not oxygen is available. If not, pyruvic acid is converted to lactic acid by a pathway called anaerobic29 (AN-err-OH-bic) fermentation. This pathway has two noteworthy disadvantages: First, it does not extract any more energy from pyruvic acid; second, the lactic acid it produces is toxic, so most cells can use anaerobic fermentation only as a temporary measure. The only advantage to this pathway is that it enables glycolysis to continue (for reasons explained in chapter 26) and thus enables a cell to continue producing a small amount of ATP.

If oxygen is available, a more efficient pathway called aerobic respiration occurs. This breaks pyruvic acid down to carbon dioxide and water and generates up to 36 more molecules of ATP for each of the original glucose molecules. The reactions of aerobic respiration are carried out in the cell's mitochondria (described in chapter 3), so mitochondria are regarded as a cell's principal "ATP factories."

an = without + aer = air + obic = pertaining to life

Glycolysis

Anaerobic fermentation

Glucose f

2 ATP

Pyruvic acid

No oxygen available

Lactic acid

Aerobic respiration

Oxygen available

2 ATP

Pyruvic acid

Oxygen available

No oxygen available

Mitochondrion

What Produces Atp

36 ATP

Lactic acid

Figure 2.31 ATP Production. Glycolysis produces pyruvic acid and a net gain of two ATPs. In the absence of oxygen, anaerobic fermentation is necessary to keep glycolysis running and producing a small amount of ATP. In the presence of oxygen, aerobic respiration occurs in the mitochondria and produces a much greater amount of ATP.

Mitochondrion

36 ATP

Figure 2.31 ATP Production. Glycolysis produces pyruvic acid and a net gain of two ATPs. In the absence of oxygen, anaerobic fermentation is necessary to keep glycolysis running and producing a small amount of ATP. In the presence of oxygen, aerobic respiration occurs in the mitochondria and produces a much greater amount of ATP.

Was this article helpful?

0 0
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.

Get My Free Ebook


Responses

  • JESSE
    Does cAMP hydrolyze the third highenergy phosphate bond of ATP?
    7 years ago

Post a comment