HERITAGE

By: Phoenix

Biology is the science of life. The term was introduced in Germany in 1800 and popularized by the French naturalist Jean-Baptiste de Lamarck as a means of encompassing the growing number of disciplines involved with the study of living forms. The unifying concept of biology received its greatest stimulus from the English zoologist Thomas Henry Huxley, who was also an important educator. Huxley insisted that the conventional segregation of zoology and botany was intellectually meaningless and that all living things should be studied in an integrated way. Huxley’s approach to the study of biology is even more cogent today, because scientists now realize that many lower organisms are neither plants nor animals. The limits of the science, however, have always been difficult to determine, and as the scope of biology has shifted over the years, its subject areas have been changed and reorganized. Today biology is subdivided into hierarchies based on the molecule, the cell, the organism, and the population.
Molecular biology, which spans biophysics and biochemistry, has made the most fundamental contributions to modern biology. Much is now known about the structure and action of nucleic acids and protein, the key molecules of all living matter. The discovery of the mechanism of heredity was a major breakthrough in modern science. Another important advance was in understanding how molecules conduct metabolism, that is, how they process the energy needed to sustain life.
Cellular biology is closely linked with molecular biology. To understand the functions of the cell—the basic structural unit of living matter—cell biologists study its components on the molecular level. Organismal biology, in turn, is related to cellular biology, because the life functions of multicellular organisms are governed by the activities and interactions of their cellular components. The study of organisms includes their growth and development (developmental biology) and how they function (physiology). Particularly important are investigations of the brain and nervous system (neurophysiology) and animal behavior (ethology).
Population biology became firmly established as a major subdivision of biological studies in the 1970s. Central to this field is evolutionary biology, in which the contributions of Charles Darwin have been fully appreciated after a long period of neglect. Population genetics, the study of gene changes in populations, and ecology, the study of populations in their natural habitats, have been established subject areas since the 1930s. These two fields were combined in the 1960s to form a rapidly developing new discipline often called, simply, population biology. Closely associated is a new development in animal-behavior studies called sociobiology, which focuses on the genetic contribution to social interactions among animal populations.
Biology also includes the study of humans at the molecular, cellular, and organismal levels. If the focus of investigation is the application of biological knowledge to human health, the study is often termed biomedicine. Human populations are by convention not considered within the province of biology; instead, they are the subject of anthropology and the various social sciences. The boundaries and subdivisions of biology, however, are as fluid today as they have always been, and further shifts may be expected.
Life as the main topic of study in biology is a term used to summarize the activities characteristic of all organisms—ranging from such primitive forms as cyanobacteria (formerly known as blue-green algae) to plants and animals. These activities fall into two major categories: reproduction and metabolism. The mechanism of reproduction is now known to be controlled by the properties of certain large molecules called nucleic acids. Deoxyribonucleic acid (DNA) constitutes the hereditary material that can be passed from one cell or organism to another, because DNA molecules can make copies of themselves by means of a process known as template replication. The DNA molecule consists of a long series of coded messages capable of directing the synthesis of specific proteins at any time in the cell or life cycle. In turn, these proteins are responsible for the synthesis of many other substances within the living organism. Reproduction therefore involves making copies of the molecules constituting an organism and ultimately results in copies of the organism itself.
The other major activity of living organisms is metabolism, the physical and chemical processes by which energy from the outside world is used in such activities as reproduction (including growth), locomotion, and responsiveness to the environment (which constitutes the activities of the nervous system in animals). The energy source can be either the radiant energy of the sun, converted to a usable form by photosynthesis, or the chemical energy of ingested food. A living organism thus resembles a motor in that both convert one kind of energy into another. A precise definition of life is difficult, but, in a rough sense, an organism is considered alive if both metabolism and reproduction are active.

Both metabolism and reproduction are carried on in cells. In eukaryotic cells the DNA lies within the nucleus, a central structure bounded by a membrane; in prokaryotic cells (such as bacteria), which do not have distinct nuclei, the DNA is not enclosed. The proteins coded for in the DNA are synthesized in the cytoplasm, the fluid material lying outside the nucleus (in eukaryotic cells) and bounded by the cell membrane. All of the structures required for metabolism are contained there; thus, the cell is the unit of both reproduction and metabolism.
The only exceptions to the above description of life are viruses. They are only partly living organisms: They possess the replicating nucleic acids but lack the ability to convert energy. In order to obtain enough energy to reproduce, viruses act as parasites; they invade a host cell and cause it to follow the instructions of the viral genetic material. In this way the virus takes over the genetic apparatus of the host to create more virus particles, a process that prevents the host cell from reproducing normally. Virus particles consist only of nucleic acid wrapped in a protein coat. In some groups of viruses, the nucleic acid is ribonucleic acid (RNA) instead of DNA.
One of the central questions about life is how it originated. The generally accepted theory is that early in the history of the earth some system of replication powered by external sources of energy must have been formed. A further assumption is that the Darwinian principle of natural selection soon began to play an important role in this process, favoring those replicating molecules that could find energy most readily. Such an assumption is reasonable because evolutionary success through natural selection is measured in terms of the ability of a living system to perpetuate its replicating molecules, or genes. Thus, primitive systems capable of carrying out the metabolic processes necessary to perpetuate their genes had a competitive advantage and eventually evolved into cells. The changes that have taken place since the origin of the cell—the rise of prokaryotes, nucleated cells, multicellular organisms, and, ultimately, higher plants and animals—are also thought to have occurred as a result of natural selection. Given such an evolutionary progression, it is possible that parallel evolution could have occurred on other planets in the universe.

SOURCE: John Tyler Bonner
Microsoft Encarta 2004