As per evidences, life originated on the planet Earth billions of years ago. Since then, new species have evolved from old ones. Simultaneously, many species had become extinct during the process of evolution. As of now, scientists have identified about 1.8 million species, which are classified into several groups based on the similarities and dissimilarities between the organisms. Among the many proposals of classification, the Carl Linnaeus system is the most accepted one. According to him, living things are classified into 5 kingdoms, based on the mode of nutrition and cellular organization.
5 Kingdoms of Living Things
In the past, all living things comprised of two categories, namely, the plants and animals. Organisms that remained stationary were categorized under plants. On the contrary, animals encompassed all living things that had the ability to move. In due course of time, scientists discovered more living organisms that could neither be included in plants nor animals. This was how the Linnaean system of taxonomic classification originated. Let's discuss in brief each of the five kingdoms of living things.
Living things included in the kingdom Monera are minute and single-celled prokaryotes (organisms that lack membrane-bound nuclei). Members of this kingdom are bacteria, cyanobacteria or blue-green algae (BGA) and spirochetes. Some members of the same organism join together to form chains. Cyanobacteria is a type of organism, which is intermediate between algae (it possesses chlorophyll) and bacteria (it is a prokaryote). Their mode of nutrition is by absorbing food through the cell wall.
Protista includes single-celled eukaryotic organisms, which contain membrane-bound cell organelles. It includes organisms that are neither plants nor animals. In simpler terms, the living things classified under Protista are unusual and diverse forms, which cannot be grouped in any of the four remaining kingdoms. For example, the simplest organisms on Earth, amoeba (a protozoan) and giant sea kelp (an algae) belong to this kingdom. The members of Protista obtain nutrition by absorption, ingestion and photosynthesis.
Fungi are group of multicellular, eukaryotic, non-motile organisms that form hyphae and mycelium. Members belonging to this kingdom lack chlorophyll, hence they are differentiated from plants. The type of organisms classified under Fungi include molds, yeasts, mildews, smuts and mushrooms. Their size may range from small microscopic yeasts to large mushrooms. Fungi derive their nutrients by absorption from dead and decaying organic materials.
Kingdom Plantae encompasses multicellular, eukaryotic, non-motile living things. The type of organisms included in this kingdom are algae, mosses, ferns, flowering and non-flowering plants. These organisms contain the photosynthetic pigment, called chlorophyll. Hence, they synthesize their own food by means of photosynthesis, which takes place in the presence of carbon dioxide, water and sunlight.
Animalia are group of multicellular, eukaryotic and motile living things. Members belonging to Animalia are insects, worms, fish, reptiles, amphibians, birds and mammals. They cannot synthesize food and their mode of nutrition is by ingesting food. They can feed either on plants or other living things.
It is to be noted that viruses and other non-cellular entities are not included in the classification of living things. In recent times, scientists have further divided the kingdom Monera into Eubacteria and Archaebacteria. The former refers to true bacteria, whereas the latter encompasses bacteria-like organisms that are adapted to extreme environmental conditions like hot springs and volcanic vents.