The problem of size All animals need to

The problem of size All animals need to exchange substances with the environment Diffusion › Surface area › Concentration gradient › Distance SURFACE AREA : VOLUME › Bacteria – 6 000/m › Whale – 0. 06/m Maximum size limit of single cell All organisms larger than size limit are MULTICELLULAR

Surface area to volume ratio gets smaller as the cell gets larger!

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

Evolution of multicellularity 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

Chlamydomonas Unicellular flagellate Isogamy

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

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

Eudorina 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

Pleodorina 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

Volvox Spherical colonies (500 -50000 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

Reproduction in the Volvox

Summary of Evolutionary Changes Shown 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

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

Problems of multicellularity Interdependence Complexity

Images http: //protist. i. hosei. ac. jp/pdb/images/Chlorophyt a/Gonium/pectorale/sp_2 b. jpg http: //www. rbgsyd. nsw. gov. au/__data/assets/im age/48212/Gonium 2. gif http: //www. acrennes. fr/pedagogie/svt/photo/microalg/pandori n. jpg http: //protist. i. hosei. ac. jp/PDB/images/Chloroph yta/Eudorina/elegans/sp_5. jpg http: //www. fytoplankton. cz/Fyto. Atlas/thm/0078. j pg