The Stuff of Life: Profiles of the Molecules That Make Us Tick - Softcover

Widmaier, Eric P.

Softcover

ISBN 10: 0805074376 ISBN 13: 9780805074376

Verlag: Henry Holt & Co, 2003

Inhaltsangabe

An accessible handbook seeks to deconstruct the fundamental processes and biological substances that make up the human body, offering more rounded coverage of such topics as fat varieties, the role of cholesterol, how the stomach digests food, and the sources of energy. Reprint. 15,000 first printing.

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Über die Autorinnen und Autoren

Eric Widmaier is the author of Why Geese Don't Get Obese and is a professor of biology at Boston University. He has written numerous articles for scientific and nonscientific publications. He lives in Boston, Massachusettes.

Eric Widmaier is the author of Why Geese Don't Get Obese and is a professor of biology at Boston University. He has written numerous articles for scientific and nonscientific publications. He lives in Boston, Massachusettes.

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The Stuff of Life

[1]

The Blueprints

The human body is built in layers of complexity. Viewed in its entirety, the body's different parts can be observed to work together in an integrative way. The brain controls the function of the heart, which in turn controls the function of the muscles and other organs. Likewise, the pituitary gland controls several hormone glands, which in turn determine electrolyte balance, blood pressure, and sugar metabolism. On a less global scale, each of the organs in the body (the liver, kidneys, skin, heart, and so on) are themselves composed of smaller units. Within the kidney, for example, specialized cells regulate the balance of salt and water in our bodies. Other groups of specialized kidney cells secrete hormones, or filter blood to remove waste products.

On an atomic scale, however, even a single cell is an entire universe. Within cells are smaller structures that make proteins, package chromosomes, and generate energy. To understand how these tasks are accomplished, we must look to the molecular level. A large protein molecule, for example, is composed of many smaller molecules called amino acids. But even an amino acid is not the smallest functional unit in a protein. Within any amino acid (or sugar molecule, or oxygen molecule, or any other molecule) are a number of atoms.

Just what is an atom, anyway? Every element, like hydrogen, nitrogen, and oxygen, is composed of atoms. Atoms are the smallest functional unit of matter. That is, an atom can be broken into subatomic (smaller than an atom) particles if sufficient (enormous) amounts of energy are provided, but those smaller bits of matter do not by themselves participate in biologically meaningful reactions. An atom can be crudely envisioned as a sort of mini-solar system, although in reality an atom's structure is much more complex and less orderly than a solar system. At the center of the system is the nucleus, composed of subatomic particles called protons and neutrons. Protons carry a positive electric charge, while neutrons have no charge (they are electrically neutral). Surrounding this densely packed nucleus are anywhere from 1 to more than 100 electrons, arranged in increasingly wide and complex orbits around the nucleus. The electrons are only a tiny fraction of the size of the nucleus and carry a negative electric charge.

Each atom has the same number of electrons as protons, which allows the two opposite electrical charges within the atom to cancel each other. Let's imagine a relatively small atom such as carbon, which contains a nucleus with six protons. It will, therefore, have six electrons orbiting around it. Although that gives an atom a neutral electrical charge, it turns out that there is "room" for two additional electrons to spin around a carbon nucleus. That's because electrons are arranged around the nucleus in poorly defined, mutually exclusive orbits, each of which has a predetermined capacity to permit extra electrons to buzz around within that orbital sphere. Having as close as possible to a full complement of electrons in the outermost shells of an atom increases a molecule's stability. So if two atoms--say, carbon and oxygen--come together under the right circumstances, they may "share" some of each other's outermost electrons. In that way, both atoms will appear to have filled up their empty "electron slots." This is true because the speed at which electrons zip around their orbital shells is so fantastic that it makes little difference if the shell is enlarged a bit by the merging of two or more atoms. When this happens, we say that the two atoms have formed a chemical bond and have joined to make a molecule. One atom of carbon and one of oxygen, by the way, would yield the poisonous molecule carbon monoxide.

It is common in nature to find two or more different kinds of atoms sharing electrons and combining to createa new, larger substance, a molecule. Some molecules are quite simple. Water, for example, is composed of one oxygen atom combined with two hydrogen atoms. Others are extremely complex; a protein may be composed of hundreds of amino acids, and each amino acid may be composed of several atoms of nitrogen, carbon, oxygen, hydrogen, and sulfur. Thus, molecules can be broken down into atoms, but atoms cannot be broken down into any other functional unit.

In the world of the molecule, anything heavier than about 0.00000000000000000000001 (one ten-billionth of one-trillionth) grams is considered large. If that number seems meaningless, then consider that there are about as many molecules of water in a drop of blood as stars in the known universe!

It may be a cliché, but proteins are truly the building blocks of all life. They are the cinder blocks and the 2x4's of our cells. While we hear about DNA, it only exists to direct the making of proteins. But proteins don't just provide the body with physical structure, they also catalyze chemical reactions, taxi gases like oxygen through the blood, and produce energy. Enzymes are also proteins. Enzymes are molecules with very precise three-dimensional structures, which allow them to interact with other molecules. In some cases, this interaction produces the destruction of another molecule. In other cases, enzymes help fuse two simple molecules to produce one complex molecule.

Different species of the same phylogenetic class (for example, mammals) share much of the same DNA, or at least DNA that is recognizably similar. And even species that on the surface bear little or no resemblance to each other have much of their DNA in common. The lowly nematode worm shares roughly 40 percent of its DNA sequence with that of humans. As you move up the scale of animal complexity, the similarity increases, of course, so that by the time you reach the other primates, such as chimps, the similarity to human DNA approaches 98 percent. Unrelated people share 99 percent similarity, and related people 99.5 percent. We are not as different from one another as we may think.

Despite this similarity, a relatively small amount of different DNA can make vast differences in the appearance and behavior of an organism. A single molecule of DNA may contain hundreds or thousands of separate functional units, called genes. Each gene is a strip of DNA distinguished from the next strip by telltale regions that signal the beginning of a new gene. The enzyme responsible for converting genes into RNA recognizes these starting positions. Every cell in our bodies has the exact same DNA, and thus the exact same set of genes. But cells in our skin, for example, have an active gene for a fibrous protein called keratin, which is the basis of skin. This same gene is inactive in most other cells, and this prevents keratin, and therefore skin, from showing up in, say, the liver or bone marrow. The waysin which a particular cell is able to activate only a specific subset of genes and not others is obviously of enormous importance to scientists, who are only beginning to find the answers. This question holds the key to understanding how an organism develops from an immature, undifferentiated embryo of just a few cells into a fully functional adult animal with, in the case of humans, trillions of cells. On a more practical level, it holds the key to regenerating lost or damaged tissues and having them function and appear like the original.

The discovery that the DNA molecule exists in the form of a twisted helix and contains only four major chemical elements, repeated in various arrays, was the landmark event that ushered in the field of science known today as molecular biology. That discovery has allowed us to begin understanding how genes can be active at one time and silent at others; how simple changes (mutations) in any of the four chemical elements of DNA can lead to the formation of...

Details zur Ausgabe

• Verlag: Henry Holt & Co
• Erscheinungsdatum: 2003
• Sprache: Englisch
• ISBN 10: 0805074376
• ISBN 13: 9780805074376
• Einband: Taschenbuch
• Anzahl der Seiten: 160
• Kontakt zum Hersteller: Nicht verfügbar
• Verantwortliche Person: Nicht verfügbar