[bio] 02 - Origin Of Multicellularity Crisologo

All animals need to exchange substances with the environment Diffusion › Surface area › Difference in concentration › Distance SURFACE AREA : VOLUME › Bacteria – 6 000 000/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!

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

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 (Chlorophyta)) > 7000 species  Model: Volvocine series – Order Volvocales

Unicellular flagellate
Isogamy


    

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




Division of labor › Anterior

vegetative cells › Larger posterior reproductive cells









Spherical colonies (500(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

Anisogamy Anisogamy/ Heterogamy

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











http://protist.i.hosei.ac.jp/pdb/images/Chlo rophyta/Gonium/pectorale/sp_2b.jpg http://www.rbgsyd.nsw.gov.au/__data/asse ts/image/48212/Gonium2.gif http://www.acrennes.fr/pedagogie/svt/photo/microalg/p andorin.jpg http://protist.i.hosei.ac.jp/PDB/images/Chlo rophyta/Eudorina/elegans/sp_5.jpg http://www.fytoplankton.cz/FytoAtlas/thm/ 0078.jpg