Atomic & Subatomic Levels
All living and nonliving matter is formed of simple units called atoms and sub atomic particles such as protons, electrons & neutrons.
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 sculpture are the 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 macromolecules with low molecular weight like CO2, H2O etc or macromolecules with high molecular weights e.g. starch, 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 macro molecules of hundreds of different types. Some most important and abundant organic molecules in organisms are glucose, amino acids, fatty acids, glycerol, nucleotides like ATP, ADP, AMP etc.
Organelles & cell
Different and enormous number of macromolecules and macromolecules arrange themselves in a particular way to form cells and their organelles. In case of simple organisms like bacteria and most protests, the entire organisms consist of a single cell. In most fungi, plants and animals, the organism may consist of up to trillions of cells.
The arrangement of the organelles speaks of the division of labour within the cell. The prokaryotes have only a limited number and type of organelles in their cytoplasm. Eukaryotes are rich in number and kinds of membranous organelles. A 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 e.g. muscle tissue glandular tissue, xylem tissue, phloem tissue etc. they are specialized for contraction (movement), secretion, conducting water and for translocation of sugar, proteins etc.
Organ & system
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 (protein part) has a secretary epithelium which secretes the gastric juice, and a muscular tissue (smooth) for contracting the walls of the stomach and mixing the food with the enzyme thoroughly and moving the food to the posterior end. The formation of organs also has a selective value because this leads to an efficient accomplishment of their functions both qualitatively and quantitatively. N animal’s organ formation is far more complex and defined. Organs re part of organ systems 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 while the leaves are primary organs for food manufacture. Flowers of other reproductive structures are involved in producing the next generation (reproduction). The complexity of the organ systems of animals is associated with a far greater range of functions and activities than is found in plants.