Chemical compounds are variously joined together as even more elaborate units or complexes of compounds (macromolecules). These units are representing successively higher levels of organisation of matter.
They form a pyramid or hierarchy, in which any given level contains all lower levels as components and is itself a component of all higher levels. For example, atoms contain subatomic particles as components and atoms are themselves components of molecules.
All structural levels upto and including those of macromolecules are encountered both in the non-living and the living world. For example, water and table salt are two chemical compounds which are found in non-living and living matter.
Examples of complexes of compounds in the non-living word are rocks which are composed of several types of compounds. In living world, complexes of compounds often occur as microscopic and submicroscopic bodies called organelles.
But even in their most elaborated and complicated forms, complexes of compounds (macromolecules like proteins, lipids, carbohydrates, nucleic acids, etc.) or organelles (i.e., mitochondria, endoplasmic reticulum, chloroplasts, etc.) cannot qualify as living units. To reach the level of life, we must go to the next higher structural level, that of cells.
A cell is a specific combination of organelles. It is usually a microscopic bit of matter organised just complexly enough to contain all the necessary apparatus for the performance of metabolism and self-perpetuation. A cell in effect represents the least-elaborate known structure that can be fully alive. It follows that a living organism must consist of at least one cell.
In fact, unicellular (single celled) organisms like Monera (Bacteria and blue-green algae) and Protista (unicellular eukaryotic organisms like Protozoa and algae) constitute the majority of living creatures on earth. All other organisms are multicellular, each composed of up to hundred of trillions of joined cells.
Further, within a multicellular organism, several distinct levels of organisations can be distinguished. The simplest multicellular types contain comparatively small number of cells. If all such cells are more or less alike, the organism is often referred to as a cellular colony. If two or more different groups of cells are present each such group usually forms a tissue.
Thus, there occur cellular colony in bacteria, two-tissue colony in slime-molds and three- tissue colony in Volvox. In structurally more complex organisms, there are not only several tissues, but some of the tissues also joined- further as one or more units called organs. The most complex organisms contain not only many tissues and organs, but also groups of organs united as one or more organ systems.
Thus, living organisms exhibit at least five levels of structural complexity: the single-celled form, the colonial form the organism with the tissues the type with organs and type of organ systems.
Thus, whether of one cell or of trillions cells, the individual is the relevant unit as regards physiological organisation, total metabolism, responsiveness, reproduction, growth and development, and certain other essential phenomena of living things. The; individual is the separate and objectively concrete biological unit. However, several still higher forms of life can be distinguished beyond the level of the individual organism.
An individual cannot live in isolaton in nature. For its survival and for perpetuation of its race, an individual often associates with other individuals of its own kind. All the similar individuals of a given locality which potentially form a single sexually interbreeding group form a next higher level of organisation called population.
Since Mendelian laws of heredity apply to the transmission of genes among the individuals of a population, so, it is also termed as Mendelian population by the geneticists. The size of a population may vary, but it is usually considered to be a local group which is called local population or deme.
Each individual member of a deme has an equal chance of mating with any other member of the opposite sex. For example, all individuals of Paramecium in a fresh-water pond form a deme or local population or population.
A group of similar demes or of actually interbreeding natural populations is called species. Thus, a species is a group of like individuals that live together and interbreed to produce fertile offspring. Thus all corn plants on earth, all bull frogs, and all the human beings, each represent a species.
Many groups of populations belonging to different species form the next higher level of organisation, the biotic community. For example, different species of micro-organisms, plants and animals inhabiting a fresh water pond along the population of Paramecium form a biotic community.
A biotic community together with the physical space in which it exists represents an ecological unit, the ecosystem. For example, a fresh water pond with its biotic community represents an ecosystem.
The sum of all ecosystems existing on earth form the biggest ecological organisation called biosphere. Biosphere is a huge ecosystem which contains living components in the form of biological world (living matter) and non-living or abiotic components in the form of earth or soil (Lithosphere), water (hydrosphere) and air (atmosphere).