Biology is the study of living things and their vital processes.

The field deals with all the physicochemical aspects of life.

As a result of the modern tendency to unify scientific knowledge and investigation, however, there has been an overlapping of the field of biology with other scientific disciplines.

Modern principles of other sciences--chemistry and physics, for example--are integrated with those of biology in such areas as biochemistry and biophysics.

Because biology is such a broad subject, it is subdivided into separate branches for convenience of study.

Despite apparent differences, all the subdivisions are interrelated by basic principles.

Thus, though it was once the custom to separate the study of plants (botany) from that of animals (zoology), and the study of the structure of organisms (morphology) from that of function (physiology), the current practice is to investigate those biological phenomena that all living things have in common.

Biology is often approached today on the basis of levels that deal with fundamental units of life.

At the level of molecular biology, for example, life is regarded as a manifestation of chemical and energy transformations that occur among the many chemical constituents that comprise an organism.

As a result of the development of more powerful and precise laboratory instruments and techniques, it is now possible to understand and define more exactly not only the invisible ultimate physiochemical organization (ultrastructure) of the molecules in living matter but also how living matter reproduces at the molecular level.

Cell biology, the study of the fundamental unit of structure and function in a living organism, may be said to have begun in the 17th century, with the invention of the compound microscope.

Before that, the individual organism was studied as a whole (organismic biology), an area of research still regarded as an important level of biological organization.

Population biology deals with groups or populations of organisms that inhabit a given area or region.

Included at this level are studies of the roles that specific kinds of plants and animals play in the complex and self-perpetuating interrelationships that exist between the living and nonliving world, as well as studies of the built-in controls that maintain these relationships naturally.

These broadly based levels may be further subdivided into such specializations as morphology, taxonomy, biophysics, biochemistry, genetics, eugenics, and ecology.

In another way of classification, a field of biology may be especially concerned with the investigation of one kind of living thing--e.g., botany, the study of plants; zoology, the study of animals; ornithology, the study of birds; ichthyology, the study of fishes; mycology, the study of fungi; microbiology, the study of microorganisms; protozoology, the study of one-celled animals; herpetology, the study of amphibians and reptiles; entomology, the study of insects; and physical anthropology, the study of man.

Basic concepts of biology

Biological principles


The concept of homeostasis--i.e., that all living things maintain a constant internal environment--was first suggested by Claude Bernard, a 19th-century French physiologist, who stated that "all the vital mechanisms, varied as they are, have only one object: that of preserving constant the conditions of life."

As originally conceived by Bernard, homeostasis applied to the struggle of a single organism to survive. The concept was later extended to include any biological system from the cell to the entire biosphere, all the areas of the Earth inhabited by living things.


All living organisms, regardless of their uniqueness, have certain biological, chemical, and physical characteristics in common. All, for example, are composed of the same basic units, or cells, and the same chemical substances, which, when analyzed, exhibit noteworthy similarities, even in such disparate organisms as bacteria and man.

Furthermore, since the action of any organism is determined by the manner in which its cells interact and since all cells interact in much the same way, the basic functioning of all organisms is also similar.

There is not only unity of basic living substance and functioning but also unity of origin of all living things. According to

a theory proposed in 1855 by Rudolf Virchow, a German pathologist, "all living cells arise from pre-existing living cells." This theory appears to be true for all living things at the present time under existing environmental conditions. If, however, li

fe originated more than once in the past, the fact that all organisms have a sameness of basic structure, composition, and function would seem to indicate that only one original type succeeded.

A common origin of life would explain why in man or slime mold--and in all forms of life in between--the same chemical substance, deoxyribonucleic acid (DNA), in the form of genes accounts for the ability of all living matter to replicate itself exactly and to transmit genetic information from parent to offspring. Furthermore, the mechanisms for this transmittal follow a pattern that is the same in all organisms.

Whenever a change in a gene (a mutation) occurs, there is a change of some kind in the organism that contains the gene.

It is this universal phenomenon that gives rise to the differences (variations) in populations of organisms from which nature selects for survival those that are best able to cope with changing conditions in the environment.


In his theory of natural selection, which is discussed in greater detail later, Charles Darwin suggested that "survival of the fittest" was the basis for organic evolution (the modification of living things with time). Evolution itself is a biological phenomenon common to all living things, even though it has led to their differences. Evidence to support the theory of evolution has come primarily from the fossil record, from comparative studies of structure and function, and from studies of embryological development.


Despite the basic biological, chemical, and physical similarities found in all living things, a diversity of life exists not only among and between species but also within every natural population. The phenomenon of diversity has had a long history of study because so many of the variations that exist in nature are visible to the eye.

The fact that organisms changed during prehistoric times and that new variations are constantly evolving can be verified by paleontological records as well as by breeding experiments in the laboratory. Long after Darwin had assumed that variations existed, biologists discovered that they are caused by a change in the genetic material (DNA). This change can be a slight alteration in the sequence of the constituents of DNA (nucleotides), a larger change such as a structural alteration of a chromosome, or a complete change in the number of chromosomes.

In any case, a change in the genetic material in the reproductive cells manifests itself as some kind of structural or chemical change in the offspring.

The consequence of such a mutation depends upon the interaction of the mutant offspring with its environment.

It has been suggested that sexual reproduction became the dominant type of reproduction among organisms because of its inherent advantage of variability, which is the mechanism that enables a species to adjust to changing conditions.

New variations are potentially present in genetic differences, but how preponderant a variation becomes in a gene pool depends upon the number of offspring the mutants or variants produce (differential reproduction). It is possible for a genetic novelty (new variation) to spread in time to all members of a population, especially if the novelty enhances the population's chances for survival in the environment in which it exists. Thus, when a species is introduced into a new habitat, it either adapts to the change by natural selection or by some other evolutionary mechanism or else it eventually dies off.

Because each new habitat means new adaptations, habitat changes have been responsible for the millions of different kinds of species and for the heterogeneity within each species.

The total number of animal and plant species is estimated at between 2,000,000 and 4,500,000; authoritative estimates of the number of extinct species range from 15,000,000 up to 16,000,000,000.

Although the use of classification as a means of producing some kind of order out of this staggering number of different types of organisms appears as early as the book of Genesis--with references to cattle, beasts, fowl, creeping things, trees, etc.--the first scientific attempt at classification is attributed to the Greek philosopher Aristotle, who tried to establish a system that would indicate the relationship of all things to each other.

He arranged everything along a scale, or "ladder of nature," with nonliving things at the bottom; plants were placed below animals, and man was at the top. Other schemes that have been used for grouping species include large anatomical similarities, such as wings or fins, which indicate a natural relationship, and also similarities in reproductive structures.

At the present time taxonomy is based on two major assumptions: one is that similar body construction can be used as a criterion for a classification grouping; the other that, in addition to structural similarities, evolutionary and molecular relationships between organisms can be used as a means for determining classification.

Behaviour and interrelationships

As was mentioned earlier, the study of the relationships of living things to each other and to their environment is known as ecology. Because these interrelationships are so important to the welfare of Earth and because they can be seriously disrupted by man's activities, ecology is becoming one of the most important branches of biology.


Whether an organism is man or a bacterium, its ability to reproduce is one of the most important characteristics of life.

Because life comes only from preexisting life, it is only through reproduction that successive generations can carry on the properties of a species.

The study of structure

Living things are defined in terms of the activities or functions that are missing in nonliving things. The life processes of every organism are carried out by specific materials assembled in definite structures.

Thus, a living thing can be defined as a system, or structure, that reproduces, changes with its environment over a period of time, and maintains its individuality by constant and continuous metabolism. This pattern of action or function results from and occurs in a pattern of organization.

Cells and their constituents

Knowledge of the structure and function of the cell has resulted from technological developments and methods.

Biologists once depended on the light microscope to study the morphology of cells found in higher plants and animals.

The functioning of cells in unicellular and in multicellular organisms was then postulated from observation of the structure; the discovery of the chloroplastids in the cell, for example, led to the investigation of the process of photosynthesis.

With the invention of the electron microscope, the fine organization of the plastids could be utilized for further quantitative studies of the different parts of this process.

Quantitative studies make use of histochemistry to identify proteins, carbohydrates, and other chemical constituents of cells.

Histochemistry has also been used to identify RNA and DNA in various cell parts.

A valuable method useful in tracing the movement of substances in living matter is radioautography: when radioactive nutrients, which can be incorporated into cells, are injected into animals, they give off detectable rays by which their presence and location can be determined. Thymidine, for example, can be made radioactive and, when injected, becomes part of the DNA being synthesized in the nucleus before cell division; the nuclei then can be identified by their radioactivity and the process of the origin of new DNA studied. Radioautography has been used to locate the site of protein synthesis and enzyme storage in cells.

Advanced technological developments--the microspectrophotometer, the X-ray probe, laser beam, computer, stereoscopic microscope, quartz-fibre microbalance, and television microscopy--are used to study the action of enzymes in living cells.

The elucidation of such processes as lipid synthesis, active transport of large particles from the blood into cells, and continuous formation of taste cells has been dependent on similar instrumentation.

Tissues and organs

Early biologists viewed their work as a study of the organism. The organism, then considered the fundamental unit of life, is still the prime concern of some modern biologists, and the maintenance of organisms is still an important part of biological research.

In 1912 an experiment showed that cells can be kept alive indefinitely if proper conditions are maintained.

Utilizing stringent laboratory techniques, workers have kept bits of chicken heart tissue alive for more than 30 years. Techniques for keeping organs alive in preparation for transplants stem from such experiments.

Modern biological research deals with the study of structure and function at all levels of biological organization from the molecule to the organism. Electronics, mathematics, and computers have become increasingly important in solving problems at all of these levels.

The study of function

To maintain life, an organism not only repairs or replaces (or both) its structures by a constant supply of the materials of which it is composed but also keeps its life processes in operation by a steady supply of energy.

The initial source of this energy is the environment outside of the organism.

The process by which the organism provides the necessary raw materials for the continuation of life is called nutrition. Plants obtain their nutrients from water, from minerals, and from the carbohydrates they manufacture.

Animals, which cannot manufacture their own food, need at least the following kinds of nutrients: water, minerals, organic carbon, organic nitrogen, vitamins, certain amino acids, and fatty acids.

Many experiments have been directed toward solving the problem of biological differentiation. It has been determined that, although all genes of an organism are present in every cell, they do not all act at the same time: some genes act only at certain times during development; others never act in some cells.

Whether a gene is active is sometimes the result of an interaction between cells.

Cells seem to develop differently in different locations. How this is controlled is not definitely known; one possibility is the presence of an electrical communication between cells or of a substance that diffuses out of the cell.

The latter idea is suggested by experiments demonstrating that the formation of the tissues of organs such as the eye, kidney, and liver are directly influenced by the tissues bordering them.

Many of these experiments make use of tissue culture techniques, which permit the growth of cells outside of the body.

It is possible to grow a single embryonic muscle cell into a colony of differentiated muscle. It is through such experiments that the questions about development and its implications may eventually be answered.

- Encyclopedia Britannica