1.5Unicellular and Multicellular Organisms
Many eukaryotes including humans are multicellular. In general, cells that comprise a multicellular organism are not identical; they differ in size, morphology and function (this is called differentiation). Complex organisms such as animals and plants are composed of groups of analogous cells called tissues and organs that perform specific functions. An individual is composed of an aggregation of organs (multicellular organism). In contrast, there are many examples that a individual is composed of one cell (unicellular organism). Eukaryotes, excluding plants and animals, and most prokaryotes fall under this category. Although eukaryotic fungi are generally multicellular, a type of fungi called yeast spends all or part of its life cycle as a unicellular organism. Some unicellular organisms can also assume forms in which many cells occur in union. However, in most cases, cellular differentiation is absent and minimal interconnection is present, indicating that these forms are not multicellular bodies such as those present in humans.
Molecular aggregates are composed of several molecules, and cells are composed of several molecular aggregates. Furthermore, multicellular organisms are composed of several cells, and biospheres are composed of several unicellular and multicellular organisms. This is called hierarchy of life (Fig. 1-3) with escalations in the phenomena at each level. Furthermore, there is a gap between phenomena at respective levels; higher order phenomena that cannot be observed at lower levels can be observed at higher levels. Although biological phenomena can be fundamentally understood by considering the components involved, an important viewpoint to consider is that different logics exist at each level.