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Biology

Five-Kingdom System

Two kingdoms of living forms, Plantae and Animalia, have been recog- nized since Aristotle, in the 4th century BC, established the first taxonomy. In their way of life and evolutionary path, rooted plants are so distinct from mobile, food-ingesting animals that the concept of the two kingdoms remained intact until recently. Only in the 19th century, long after it was revealed that one-celled organisms could not fit comfortably into either of the two categories, was it proposed that unicellular forms be placed in a third kingdom, the Protista. Fur- thermore, long after photosynthesis was discovered to be the basic nutritional mode of plants, the fungi, which feed by absorption, continued to be classified as plants because of their apparently rooted manner of growth.

As techniques for examining the cell have improved dramatically, it has become clear that the major division in the living world is not that between plants and animals but between organisms whose cells have no enclosed nucleus and organisms whose cells have nuclei bound by membranes. The former are called procaryotes ("before kernels") and the latter eucaryotes ("true kernels"). Pro- caryotic cells are also lacking in a number of organelles-mitochondria, chloro- plasts, advanced flagella, and other special cell structures-at least some of which occur in all eucaryotic cells. The bacteria and blue-green algae are procaryotic cells, and they have been recognized in modern taxonomies as a fourth kingdom, Procaryotae. Fungi form the fifth kingdom.

Eucaryotic cells arose much later and may have evolved as symbiotic associations of procaryotes. The kingdom Protista is composed of diverse one- celled organisms, either free-living or colony-forming. Each of the multicellular kingdoms is believed to have arisen more than once from protist ancestors. The kingdom Animalia comprises organisms that are multicellular, have their cells organized into different tissues, are mobile or partly mobile by means of contrac- tile tissues, and digest food internally. The kingdom Plantae is made up of multi- cellular organisms that usually have walled cells and that contain chloroplasts in which they produce their own food by photosynthesis. The fifth kingdom, Fungi, comprises multicellular or multinucleate organisms that digest food externally and absorb it through the surfaces of protoplasmic tubes called hyphae (of which

their bodies are composed). This five-kingdom classification of the living world is thus based on three levels of organization: the primitive procaryotic; the relatively simple and primarly unicellular eucaryotic; and the complex, multicellular eucaryotic. Within the last of these levels, the three major directions of evolution are each based on a different kind of nutrition and are expressed in the different kinds of tissue orga- nization that are characteristic of animals, plants, and fungi.

Computer Science

Future Developments

One ongoing trend in computer development is microminiaturization, the effort to compress more circuit elements into smaller and smaller chip space. Researchers are also trying to speed up circuitry functions through the use of superconductivity, the phenomenon of decreased electrical resistance observed as objects exposed to very low temperatures become increasingly colder.

Computer networks have become increasingly important in the develop- ment of computer technology. Networks are groups of computers that are inter- connected by communications facilities. The public Internet is an example of a global network of computers. Networks enable connected computers to rapidly exchange information and in some cases, to share a workload, so that many computers may cooperate in performing a task. New software and hardware tech- nology is being developed that will accelerate both of these processes.

The "fifth-generation" computer effort to develop computers that can solve complex problems in what might eventually be called creative ways is another trend in computer development, the ideal goal being true artificial intelligence. One path actively being explored is the parallel-processing computer, which uses many chips to perform several different tasks at the same time. Parallel processing may eventually be able to duplicate to some degree the complex feedback, ap- proximating, and assessing functions of human thought. Another form of parallel processing that is being investigated is the use of molecular computers. In these computers, logical symbols are expressed by chemical units of DNA instead of by the flow of electrons in regular computers. Molecular computers could potential- ly solve complicated problems much faster than current supercomputers and would use much less energy.