At some later stage of evolution, unicellular organisms found it advantageous to cluster together, thereby acquiring greater motility, efficiency, or reproductive success than their free-living single-celled competitors. Further evolution of such clustered organisms led to permanent associations among individual cells and eventually to specialization within the colony – to cellular differentiation.
The advantages of
cellular specialization led to the evolution of ever more complex and
highly differentiated organisms, in which some cells carried out the
sensory functions, others the digestive, photosynthetic, or reproductive
functions. Many modern multicellular organisms contain hundreds of
different cell types, each specialized for some function that supports
the entire organism. Fundamental mechanisms that evolved early have been
further refined and embellished through evolution. The simple mechanism
responsible for the motion of myosin along actin filaments in slime
molds has been conserved and elaborated in vertebrate muscle cells,
which are literally filled with actin, myosin, and associated proteins
that regulate muscle contraction. The same basic structure and mechanism
that underlie the beating motion of cilia in Paramecium and flagella in Chlamydomonas
are employed by the highly differentiated vertebrate sperm cell. Figure
2–25 illustrates the range of cellular specializations encountered in
Figure 2–26 Three types of junctions between cells.
(a) Tight junctions produce a seal between adjacent cells. (b) Desmosomes, typical of plant cells, weld adjacent cells together and are reinforced by various cytoskeletal elements. (c) Gap junctions allow ions and electric currents to flow between adjacent cells.
Lehninger-Nelson-Cox: Principles of Biochemistry, 49.o.
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