There are following levels of biological organization.
Subatomic and atomic level: All types of matter are made up of elements and each element contains a single kind of atoms. The atoms are actually made up of many subatomic particles. The most stable subatomic particles are electrons, protons and neutrons. Protons and neutrons are located inside nucleus of atom while electrons orbit in energy levels(electron shell) around the nucleus. Out of 92 kinds of elements that occur in nature, 16 are called bioelements. These take part in making the body mass of a living organism. Only six bioelements(O, C, H, N, Ca, and P) make 99% of the total mass of a living organism and other ten(K, S, Cl, Na, Mg, Fe, Cu, Mn, Zn and I) collectively make 1% of total mass.
Molecular level: In organisms elements usually do not occur in isolated forms. The atoms of different elements combine with each other through ionic or covalent bonding to produce compounds. This stable form is called a molecule. Hydrogen, carbon, oxygen, nitrogen, phosphorous and sulfur are most common atoms found in biological molecules. The different types of bonding arrangement permit biological molecules to be constructed in great variety and complexity. These may be micromolecules with low molecular weight like carbon dioxide and water etc. or macromolecules with high molecular weight for example, starch and proteins etc. Biological world has two types of molecules: organic and inorganic. An organic molecule is any molecule containing both carbon and hydrogen. Inorganic molecules do not include carbon and hydrogen together in a molecule. An organism is usually formed by enormous number of micro and macromolecules of hundreds of different types. Some most important and abundant organic molecules in organisms are glucose, amino acids, fatty acids, glycerol and nucleotides like ATP, ADP, AMP etc.
Organelles and cell level: Different and enormous number of micromolecules and macromolecules arrange themselves in a particular way to form cells and their organelles. In case of simple organisms like bacteria and most protists, the entire organism consists of a single cell. In most fungi, plants and animals the organism may consist of up to trillions of cells. Numerous sub cellular structures like mitochondria, Golgi complex, endoplasmic reticulum and ribosomes etc. have been studied for their structure and function. It has become clear that functions of the cells are accomplished by these specialized structures. These structures are called organelles. The prokaryotes have only limited number and type of organelles in their cytoplasm. Eukaryotes are rich in number and kinds of membranous organelles. Cell membrane is however present in all cells whether prokaryotic or eukaryotic.
Tissue level: In multicellular animals and plants, groups of similar cells are organized into loose sheets or bundles performing similar functions, these are called tissues. Each tissue has a particular function in the life of the organism for example muscle tissue, glandular tissue, xylem tissue and phloem tissue etc. They are specialized for contraction, secretion, conducting water and for translocation of sugar and proteins etc.
Organ and organ system level: Different tissues having related functions assemble together in a structure to carry out its function with great efficiency. Such structures are called organs and they are specialized to perform particular functions. For example stomach which is an organ has a function of food digestion, has secretory epithelium which secretes the gastric juice and muscular tissue for contracting the walls of stomach and mixing the food with enzyme thoroughly and moving the food to posterior end. In animals organ formation is far more complex and defined. Organs are the part of organ system where total functions involved in one process or phenomenon are carried out. The organ level of organization is much less definite in plants than it is in animals. At the most, we might distinguish roots, stems, leaves and reproductive structures. Clear cut functions, the distinguishing features can be assigned to each of these structures. Roots are involved in anchoring the plant, storage of food and procuring water and minerals. The shoot supports the entire plant while the leaves are primary organs for food manufacture. Flowers or other reproductive structures are involved in producing the next generation. The complexity of organ systems of animals is associated with far greater range of functions and activities than is found in plants.
Individual level: Various organs in plants and various organ systems in animals are assembled together to form an individual(the whole organism). In organism the functions, processes and activities of various organs and organ systems are coordinated. For example when a man is engaged in continuous and hard exercise, not only his muscles are working but also there is an increase in the rate of respiration and heart beat. This accelerated rate of respiration and heart beat supplies more oxygen and food to the muscles which they need for continuous work.
Population level: A population is a group of living organisms of the same species located in the same place at the same time. Examples are number of rats in the field of rice, the number of students in your biology class or human population in a city. Population is a higher level of biological organization than individual level because here a group of organisms of the same species is involved. This level has its own attributes which come into being by living together of a group of organisms of the same species. Some of these attributes are gene frequency, gene flow, age distribution, population density and population pressure etc. All these are new parameters which have appeared due to population of an organism.
Community level: A community is an assemblage of different populations, interacting with one another within the same environment. A forest may be considered as a community. It includes different plants, microorganisms, fungi and animals species. Communities are collection of organisms in which one population may increase and others may decrease. Some communities are complex e.g. a forest community and a pond community etc. Other communities may be simple e.g. a fallen log with various populations under it. In a simple community number and size of populations is limited. So any change in biotic and abiotic factors may have drastic and long lasting effects.
Biosphere level: The part of earth inhabited by organisms and communities is known as biosphere. It constitute all ecosystems(areas where living organisms interact with the nonliving components of environment) and is also called the zone of life on earth.
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