Scientists currently recognize four groups of biological entities :
(I) Archaea – are a group of recently discovered organisms that sometimes live in extremely hostile habitats like thermal volcanic vents, saline pools and hot springs. Archaea are single – celled organisms that are similar in appearance to bacteria. However, they are biochemically and genetically very different from bacteria. Many books and other forms of scientific literature refer to them as archaebacteria.
(II) Bacteria – are simple single – celled organisms that generally lack chlorophyll (an exception is cyanobacteria). Bacteria have a prokaryote cell type. They also generally obtain energy for survival through the breakdown of organic matter via fermentation and respiration. Bacteria such as Rhizobium spp. and Cyanobacteria play an important role in the fixing of atmospheric nitrogen. Without these bacteria ecosystems would be severely short of nitrogen for plant and animal growth. The oldest fossils of life on Earth are bacteria – like organisms.
(III) Eukaryota – are all organisms with a eukaryote cell type. This group of life includes the kingdoms Protista, Fungi, Animalia and Plantae.
(IV) Viruses – are fragments of DNA or RNA that depend on host cells that they infect for their reproduction. They are not cells. Viruses are thought to be parts of the genetic code that originated from either eukaryote or prokaryote cells. These code fragments contain enough genetic information for self-existence. At times, viruses are metabolically inert and technically non-living. Viruses cause a variety of diseases in eukaryote organisms. In humans they can cause smallpox, influenza, shingles, herpes, polio, ebola, AIDS, rabies and some types of cancer.
These four main types of biological entities described above share some unique characteristics that can allow us to distinguish them from non-living things.
Living and Non-living :
There are seven characteristics that can be observed in, more or less, all living organisms. These are the observable outcome of the all-important property of a living organism, namely, the extraction, conversion and use of energy from the environment. In addition, a living organism is able to maintain and even increase its own energy content. In contrast, dead organic matter tends to disintegrate as a result of the chemical and physical forces of the environment. In order to maintain themselves and prevent such disintegration, living organisms have an inbuilt self-regulating system to ensure that there is no net energy loss. This control is referred to as homeostasis and operates at all levels of biological organisation, from the molecular level to the community level.