One property which arises at each new higher level of organisation is united, integrated function. Disunited structure means independent function and by extension, competition; while united structure means joint function and by extension, cooperation. For example, the cells can remain structurally independent and can compete for space and raw materials. But if they form a multicellular unit, they surrender their independence and become a cooperative, integrated system.
The fundamental advantage of cooperation is operational efficiency: the cooperating whole is more efficient in performing the functions of life than its lower-level components separately and competitively. For example, separate cells must expend more energy and materials for survival than if that same number of cells were integrated as a tissue.
One underlying reason for this difference is that, in the integrated unit, duplication of effort can be avoided. In a set of separate cells, for example, every cell is exposed to the environment on all sides and must, therefore, expend energy and materials on all sides to cope with the impact of the environment.
However, if the same cells are grouped together as a compact tissue only the outermost cells are in direct contact with the environment, and inner cells then need not to channel their resources into protective activities.
Besides avoiding duplication of efforts, cooperative groupings make possible continuity of efforts. For example, an unicellular form must necessarily carry out all survival functions in its one cell. Sometimes, the performance of even one of these functions requires most or all of the capacities of the entire cell.
Thus, the entire cell surface must often serve as gateway for entering nutrients and departing wastes. And all parts of the cell may have to participate directly in locomotion or in feeding, for example.
By contrast, in multicellular organisms continuity of a given effort becomes possible through division of labour. The total task of survival can be divided up into several subtasks, and each of these can become the continuous responsibility of particular cells only. Some cells might function in feeding, continuously so, and others in locomotion, again continuously.
The division of labour becomes so pronounced that many or most cells are permanently limited in functional capacity: they can perform only certain jobs and no others. For example, mature nerve cells can conduct nerve impulses only and are quite unable to reproduce or move. Thus, different cells of multicellular organisms exhibit specialization for particular set of functions.