To understand how water moves in our world, attention must be directed to the hydrologic cycle. The hydrologic cycle is essentially a water continuum, representing the different paths through which water circulates and is transformed in the natural environment (Fig. 1). Being a cycle, it has no specific beginning or ending. Rather, liquid water from the Earth s surface, particularly the oceans, is evaporated into a gaseous form and enters the atmosphere as water vapor (clouds). The atmospheric moisture is eventually returned to the Earth s surface in the form of rain or snow.
Cell differentiation. In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Different tissues have different kinds of cells.
Cell differentiation refers to the normal process by which a less specialized cell goes through development and maturation in order to become more distinct in terms of form and function. It is the process in which a cell changes into another cell type. During cell differentiation, the changes may include cell shape, cell size, membrane potential, metabolic activities, and responsiveness. These changes are brought about by modifications in gene expressions. An example of cell differentiation is the development of a single-celled zygote into a multicellular embryo that further develops into a more complex multisystem of distinct cell types of a fetus. The cell size, shape, polarity, metabolism and responsiveness to signals change dramatically such that a less specialized cell becomes more specialized and acquires a more specific role.