The living organism is a system of interwoven and overlapping hierarchies of organization. Starting from atom, each level makes the base for the next higher level and thus forms different levels of organization. The properties of those levels of organization are unique and are different from any levels of organization, e.g. water is a molecule of hydrogen and oxygen. It has its own properties which are different from hydrogen and oxygen, both. Each level of organization is more complex and has fewer units than the previous one i.e. there are lesser tissues than cells. The molecules of life are packed into highly organized, self enclosed units called cells. Each cell has a complex structure that gives it the capability of self-reproduction a unique characteristic of living things. Cell is the lowest level among the levels of organization that is considered to be alive. The structural hierarchy represents how matter has become more and more organized with each levels of organization. At every shift of organizational level, energy is required. This new level of organization has its own properties. Energy is necessary to maintain that level of organization. There is an intimate relationship between organism and environment in every levels of organization. Levels of organization are the basis of evolution. Levels of organization has determined the kind of life that existed in past and is existing today however, the activities of an organism also produce changes in environment so the relationship is in both ways.
A living organism – the most observable unit can be studied at various levels as given:
Genes → Cells → Tissues → Organs → Organisms → Populations → Biotic Community → Ecosystem
The Individual Organism
A living organism is able to carry out its life processes within its body independently. The individuals can be counted, measured and studied as they are the most concrete objects. The individual organism is always derived from the pre – existing ones through vegetative, asexual or sexual reproduction. An individual always has a life span with a definite beginning and a definite end. An individual is able to transmit its characters to the offspring. The aggregation of individuals of one kind forms the population.
Population is a geographically isolated group of the same kind (so called species). It is a group of organisms of same species that occupy a specific area. The population may be closer or widely dispersed geographically. The number of population distributed over a large geographical area may form one species.
Species is supposed to be the basis unit of taxonomy. A species may be defined in number of ways in various biological aspects.
Genetically – A species having group of organisms that show a distinct similarity in the genetic karyotype.
Morphologically – A species is a distinct and recognizable group of organisms.
Ecological – A species having group of organisms that share the same ecological niche. (No two species can have the same niche)
Interbreeding – A species having group of organisms that can interbreed and produce a fertile offspring in nature. The criteria is too simple to understand a species. There are number of species where the organisms may look different and yet they belong to the same species.