The Respiratory System and the Lungs - Respiration at the one-cell level

Oxygen molecules, then, are carried to the body's cells by the hemoglobin of the arterial blood pumped by the heart's left ventricle. But how does a cell take oxygen from the blood and use it?

Exchange of Gases

The transfer of oxygen from blood to cell is accomplished in much the same way as the exchanges that take place in the lungs. The circulating arterial blood has a surplus of oxygen; the cells have a surplus of carbon dioxide. When the oxygen-rich blood reaches the finest capillaries, only the very thinnest membranous walls (of cell and capillary) separate it from the carbon-dioxide-rich cells. As in the lungs, both these gases (dissolved in water) diffuse through these thinnest of membranes: the oxygen into the cell, the carbon dioxide into the blood for eventual deposit in the alveoli, and exhalation.

Within the cell, the oxygen is needed so that food, the body's fuel, can be burned to produce energy. At the cellular level, the most convenient and common food is a fairly simple carbohydrate molecule called glucose.

Conversion of Carbohydrates into Energy

Energy is locked into a carbohydrate molecule such as glucose in the form of chemical bonds between its atoms. If one of these bonds is broken—say, a bond holding together a carbon and a hydrogen atom—a bit of pent-up energy is released as if, in a stalemated tug of war, the rope suddenly broke and both teams went hurtling off a few feet in opposite directions. This is precisely the effect of respiration within a cell: the cell “breaks the ropes” holding together a carbohydrate molecule. The result is the release of energy—either as body heat or to power other activities within the cell.

It is useful—but a somewhat misleading oversimplification—to consider cellular respiration as a type of burning, or combustion. When a typical cell burns food, a carbohydrate molecule (glucose) together with molecules of oxygen are changed into carbon dioxide and water. During this change, chemical energy is released—energy that has been trapped, as we have seen, in the carbohydrate molecule. That complex, energy-rich molecule has been dismantled into the simpler molecules of carbon dioxide (CO 2 ) and water (H 2 O). Oxygen is necessary here just as it is in fiery combustion. But the energy released here, instead of rushing out as heat and flame, is used to power the living activities of the cell.

This description of cellular respiration is all right in principle, but the trouble is this: if it all happened at once—if carbohydrate was so abruptly dismantled, split up at one stroke to water and carbon dioxide—such a great amount of energy would be released that the cell would simply burn itself up. As one biologist has said, the cell would be in exactly the same position as a wood furnace built of wood.

What protects the cell is its army of enzymes. These remarkable protein molecules combine briefly with energy-containing food molecules, causing them to break down bit by bit, so that energy is released gradually rather than all at once.

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