Cell as a Self Contained or Autonomous Body:
Though the activities of cells are integrated for the survival of organisms, each cell is capable of independent existence and acts as an autonomous unit. Each cell is capable to carry out the fundamental biological processes, the characteristics of living beings. Following cell activities show its independence.
(1) Each cell obtains food material often in the form of micromolecules.
(2) Each cell oxidizes food molecules to produce energy which is released in the form of ATP.
(3) Each cell synthesizes macromolecules from nutrient micromolecules and energy from ATP to control and coordinate various body activities.
(4) Each synthesizes new living matter (protoplasm) from these micromolecules to be utilized for the growth and replacement of worn-out parts.
(5) Each cell can digest and dispose of its old, worn-out or dead organelles with the help of lysosomes.
(6) Each cell gets rid of nitrogenous wastes and carbon dioxide.
(7) DNA replication, RNA formation and protein synthesis follow the same basic plan in all cells.
(8) Cells are capable of dividing and producing daughter cells with similar hereditary material.
(9) Cells regulate all essential activities going on within them and maintain internal physicochemical homeostasis.
(10) Each cell has a fixed life span, after which either it dies or is digested or it divides into two and thus follows the cell cycle.
Cellular Autonomy in Unicellular Organisms:
In unicellular organisms, the single body cell illustrates all the above-mentioned activities. The cell depends on its own internal or intrinsic information and responds to the environment with which it is in direct contact. All the life activities are carried out by the same cell. Thus, the question of the division of labour does not arise.
Cellular Autonomy in Multicellular Organisms:
In multicellular organisms, the cells do not lead to a totally independent existence. They interact and cooperate and exhibit division of labour. Due to the division of labour, cells of a multicellular organism become specialized to carry out different functions like support in sclerenchyma, protection in epidermal cells, secretion in gland cells, contraction in muscle cells or conduction of stimulus in a nerve cell. Though they become specialized for a specific function, they still carry out some fundamental biological processes like respiration, the release of energy, synthesis of complex organic molecules, etc.
Due to specialization, the cells of multicellular organisms may lose certain activities essential for autonomy either temporarily or permanently, e.g. erythrocytes do not respire. Because of the loss of the nucleus, they have lost the ability to divide. Other specialized cells are muscle cells and nerve cells. These cells normally do not divide but can be made to do so. This can be demonstrated by cell culture or tissue culture. By this technique, isolated cells of certain types, when grown on an artificial nutrient medium, develop independently, multiply and may even differentiate into a new organism. This is called totipotency.