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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!
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
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) > 7000 species
 Model: Volvocine series – Order Volvocales
Unicellular
flagellate
 Isogamy
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Small colony (4, 8,16, or 32 cells)
Flat plane, mucilage
No differentiation
Isogamy
Intercellular communication
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Colony (8, 16, or 32
cells) in 1 layer
Spherical
Isogamy
Anterior cells 
larger eyespots
Coordinate flagellar
movement
Colony dies when
disrupted
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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 nonmotile

Division of labor
› Anterior
vegetative cells
› Larger posterior
reproductive
cells

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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
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
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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