Introduction:
Cell are to survive in varying conditions; therefore, they should be able to get stimuli and respond to differences caused in the environment. Such responses are based on the integration of several components or systems mediated by its membranes, receptors, chemicals, effectors, pH differences, etc. They perform appropriate activities that enhance the chances of survival and purposefulness. Cell create their own environment by secreting a thin layer around them. In favorable and unfavorable conditions, cells cannot be free of their environment.
In the course of evolution living cells searched new environments above the aquatic habitat and adapted themselves to the new terrestrial habitat locations. Their adaptive ability depended on the susceptibility of their genome to change. During the course of adaption, acquisition of multicellular nature was proved advantageous over single cell structure; thus, the cellular specialization is the natural outcome.
Cell not only interacted with the environment but also with neighboring cells in their immediate proximity. Like, this cell created their own environment to become dependent on each other.
Fig: Structure of Plant Cell Wall
Cell Surfaces:
Cell is bound by plasma membranes. Some regions of this are specialized to perform, absorption, secretion, transport, adhesion, communication and other physiological activities, according to theses functions the membrane develops specialization. In plant cell cellulose wall, us present, whereas, in animal cell external coating exits.
Cell Junctions:
Plant cells are held together side by side by middle lamella. These cells are further interconnected by five cytoplasmic strands passing through the cell wall and middle lamella; they are called plasmodesmata. In the several animal tissues, cell membranes of adjacent cells are held together at the several points, forming specific cell junctions, which are responsible for cell-to-cell attachment. These attachments may occur throughout the plasma membrane, but at certain places special junctions are formed to provide extra strength. There are three types of cell junctions:
Adhering Junctions (Separate Junctions)
They are also called as septate junctions. Adhering junctions are desmosomes which are common in epithelial cells; they provide anchorage to adjoining cells by string points of attachments. Adhering junctions are fine cytoplasmic tone filaments, which run across two parallel thickening of cells, separated by intercellular spaces. At the basal surfaces of some cells, only desmosomes are present, hence they are considered hemi-desmosomes, which prevent the cell from pooling off the underlying surface.
 Fig: Types of Cell Junction
Tight Junctions
They are formed by fusion of cell membranes, at the point of contact, without possessing any intercellular space. They may be band like to belt like in form totally sealing off the portion of contact. Physiologically, tight junctions are important, as they prevent flow of materials across the epithelium in either direction, i.e., Form lumen to basal region or vice versa. Second functions of these junctions are maintaining cell polarity by functioning as a physical barrier to the mobility of integral proteins laterally.
Gap Junctions
They are also called as nexuses and very simple junctions common in animal cells. Nexuses are tools for cellular interaction among themselves. Besides forming cellular adhesion, they also help in intercellular communications. The latter may take place in many ways. In a majority of cells, gap junctions possess intercellular channels, which serves a transport pathway for small ions and molecules. Therefore, low molecular weight, ions sugars, vitamins, hormones and other molecules can be easily distributed from cell to cell. This is called as metabolic coupling. Metabolic cooperation is regulated by calcium-ion concentration. With low calcium concentration, permeability of channels is high; however, with increased calcium concentration, permeability is reduced.
 Fig: Desmosomes, Hemidesmosomes, and Intermediate Filament Network
Gap junctions helps as electrical pathways for communications. Gap junctions normally have low resistance but the impulse conduction is very quick. Electrical coupling is found in embryonic cells, cardiac cells and liver cells.
Proper functioning of call is possible only by proper interaction and integration of the cell. Plasmalemma in may cells is surrounded by cell coats, which serve several functions. Basically, cell coats are protective in function; added to this cell coats contain enzymes. That possess sites for interaction of signals from other cells.
- Cell-surface carbohydrates serve for interaction
- Every cell carries its own complement of receptions, but the good example is lymphocyte. Receptors is the site that communicate with the surrounding cells and often triggers a metabolic reaction.
- Lectins are cell agglutinating proteins; they possess affinity for specific sugars and bind with them, producing agglutination reactions. Lections may contain two or more sugar binding sites and may bind with free sugars as well as sugars attached to the plasma membrane.
Cell Integration:
Integration results between cells to form cell aggregates depends upon an enzyme substate binding between adjacent cells. Cell integration property was describing in sponges by H.V. Wilson in 1908. He practically proved that if sponges are pressed and forced through a fine cheese cloth, the cells disperse in the solution and reassociate to form aggregates.
Cell integrations appears to be due the three factors:
- Aggregating cell should be motile.
- Some integration factors are involved.
- Divalent cations (calcium) are essential to s stabilize the aggregation factors.
In multicellular organisms’ cells of tissues have an inherent tendency to remain in permanent association with each other. If these cells lose their adhesive property, they gain the capacity of uncontrolled growth, develop malignancy and may also distribute themselves in other tissues.
Summary:
It is universal phenomena of different types of cells, tissues, organs and systems with specialized role to perform in biological organisms. Proper functioning is possible only by proper interaction and integration of the cell which is possible only through proper recognitions of the cell by the cell.