Atoms combine to form molecules. The latter are variously combined to form compounds. Such compounds can be found in both living and nonliving worlds. But in living matter the compounds are organized to form sub microscopically or microscopically visible structures called organelles (such as mitochondria, Golgi bodies, etc.). But these organelles by themselves cannot be qualified as living units. When different organelles are put together t form higher organizational level, a cell is formed. The cell becomes the smallest unit which remains alive because it can carry out functions necessary to be called living as opposed to a piece of rock, such is nonliving.
A cell is bounded by a membrane. Inside the cell there is a tiny droplet of semi liquid substance called protoplasm. This substance, which is highly complex, forms the physical basis of life. All the molecules which constitute the protoplasm are arranged and organized in such a way that they are able to perform characteristic functions of living beings. Their organizational complexity enables them to not only metabolize but also to recreate themselves. This is the reason why you cannot form living protoplasm simply by adding all those molecules in a test tube.
There are some organisms whose entire body consists of just a single cell like Amoeba, Paramoecium, chlamydomonas, etc. such single-celled organisms are capable of performing all the functions of life. On the other hand, majority of plants and animals are multicellular. Their bodies are formed of a large number of cells. The level of organization exhibited by all living forms is the cellular level of organization.
Take on onion peel. Keep it on a drop of water over a glass slide. Stain it with a drop of sofranin. Examine it under low and high magnification of microscope after covering the peel by a cover slip. Do you see bricklike structure? Are they joined to each other? These comportments are cells and this organization is cellular organization
Cells assume definite shapes to perform a particular function. Cells of the same shape combine into a group for discharging a common function. Each group of such cells constitutes a tissue.
A group of cells of similar structure and function is known as tissue.
In multicellular organisms, cells play specialized roles. For example, intestine is made up of four types of tissues. These are epithelial tissue, muscular tissue, connective tissue and nervous tissue. The cells comprising our heart continuously contract and relax to supply blood to different parts of the body. The cells constituting our brain enable us to think. The cells composing our kidney help us to get rid of waste materials from the body. Thus, the cells of different parts of the body not only perform different functions but also differ in shape, size and appearance. The process whereby descendants of generalized cells become specialized to perform specific function is called cell differentiation. Various parts of plants also show differentiation of cells. The leaf of a plant is made up of different kinds of tissues such as epidermal tissue, vascular tissues (xylem and phloem) and ground tissues like parenchyma and collenchyma.
Group of tissues are further organized to form organs. It is the next higher level of organization. For example, the leaf of a plant is an organ which consists of many tissues. The group of green-coloured cells i.e., mesophyll tissue, synthesizes food by photosynthesis. Epidermis protects the internal tissues, vascular tissues help in conduction of water and translocation of food. All these tissues constitute a leaf, an organ of a plant. Likewise the branches, flowers, fruits and seeds are the other organs of the plant. The organ of an animal with the greatest number of jobs is the liver, Kidney, heart, lungs, brain, etc., are different organs which perform definite functions.
The next level of organization above the organ level is organ system. Here the different organs of an organism work in a co-ordinate manner and form a system. For example, the shoot system of a plant is formed by organs such as levels, branches, flowers and fruits. Likewise, the root system is formed. These two systems together constitute a whole plant i.e. and organism. Thus the organ system is the level of organization just below the organism and performs a specific work. In a human body, the digestive system, for example, comprises the organs like mouth, stomach, intestine, liver, pancreas and salivary glands. These different organs are involved in different functions, i.e., mouth in intake of food; stomach and intestine in digestion of food; pancreas and salivary glands in providing the enzymes for the digestion of food; liver in providing the proper medium for digestion of food.
Organismic Level individual Level
An individual organism is a separate and real biological unit. This level is known as organismic level of organization. The individual may be an animal or a plant irrespective of its kind, structure and habitat. On certain general principles, levels of organization can be constructed from the simple to the most complex. The individual is the relevant unit as regards physiological organization, total metabolism, growth, development and reproduction.
This constitutes the lower levels of organization. In unicellular conditions, the cell constitutes the organism itself. Although there may be difference in the complexities but all multicellular organisms exhibit similar pattern of organization.