 All animals need to exchange substances with the environment  Diffusion › Surface area › Difference in concentration › Distance  SURFACE AREA : VOLUME › Bacteria – /m › Whale – 0.06/m  Maximum size limit of single cell  All organisms larger than size limit are MULTICELLULAR

As the cell gets larger, surface area to volume ratio gets smaller.

Avoidance Avoidance Geometric solutions Geometric solutions Increase surface area Increase surface area Decrease effective volume Decrease effective volume Increase rate of supply Increase rate of supply High concentration of nutrients High concentration of nutrients Improve nutrient transport within Improve nutrient transport within Improve efficiency to reduce demand Improve efficiency to reduce demand Division of labor within the cell Division of labor within the cell Division of labor between cells Division of labor between cells

 Evolved many times in eukaryotes  Three theories › Symbiotic Theory  Like the endosymbiotic theory  Different species are involved › Syncytial Theory  Ciliates and slime molds  Commonly occur in multinucleated cells › Colonial Theory (Haeckel, 1874)  Same species are involved  Green algae (Chlorophyta) > 7000 species  Model: Volvocine series – Order Volvocales

 Unicellular flagellate  Isogamy

Small colony (4, 8,16, or 32 cells) Small colony (4, 8,16, or 32 cells) Flat plane, mucilage Flat plane, mucilage No differentiation No differentiation Isogamy Isogamy Intercellular communication Intercellular communication

 Colony (8, 16, or 32 cells) in 1 layer  Spherical  Isogamy  Anterior cells  larger eyespots  Coordinate flagellar movement  Colony dies when disrupted

 16 or 32 cells  16 cells – no specialization  32 – 4 for motility, the rest for reproduction  Heterogamy – female gametes not released  Halves are more pronounced

 32 to 128 cells  Heterogamy – female gametes not released, in some cases becoming truly non- motile  Division of labor › Anterior vegetative cells › Larger posterior reproductive cells

 Spherical colonies ( cells)  Hollow sphere – coenobium  Cell differentiation: somatic/vegetative cells and gonidia  2-50 scattered in the posterior  reproductive  Female reproductive cells  daughter colonies  Intercellular communication possible

 Unicellular  colonial life  Increase in # of cells in colonies  Change in shape of colony  Increase in interdependence among vegetative cells  Increase in division of labor: vegetative and reproductive cells  Isogamy  anisogamy  oogamy  Fewer female gametes are produced

 Increase in size of the organism  Permits cell specialization  Increase in surface area to volume ratio

 Interdependence  Complexity