�� All animals need to All animals need to exchange substances with exchange substances with the environmentthe environment

�� DiffusionDiffusion

›› Surface areaSurface area

›› Difference in Difference in concentrationconcentration

›› DistanceDistance›› DistanceDistance

�� SURFACE AREA : VOLUMESURFACE AREA : VOLUME

›› Bacteria Bacteria –– 6 000 000/m6 000 000/m

›› Whale Whale –– 0.06/m0.06/m

�� Maximum size limit of Maximum size limit of single cellsingle cell

�� All organisms larger than All organisms larger than size limit are size limit are MULTICELLULARMULTICELLULAR

Surface area to Surface area to volume ratio volume ratio

gets gets as the as the gets gets smallersmaller as the as the cell gets cell gets largerlarger!!

�� AvoidanceAvoidance

�� Geometric solutionsGeometric solutions�� Increase surface areaIncrease surface area

�� Decrease effective volumeDecrease effective volume

Increase rate of supplyIncrease rate of supply�� Increase rate of supplyIncrease rate of supply�� High concentration of nutrientsHigh concentration of nutrients

�� Improve nutrient transport withinImprove nutrient transport within

�� Improve efficiency to reduce Improve efficiency to reduce demanddemand�� Division of labor within the cellDivision of labor within the cell

�� Division of labor between cellsDivision of labor between cells

�� Evolved many times in eukaryotesEvolved many times in eukaryotes

�� Three theoriesThree theories

›› Symbiotic TheorySymbiotic Theory�� Like the Like the endosymbioticendosymbiotic theorytheory

�� Different species are involvedDifferent species are involved�� Different species are involvedDifferent species are involved

›› SyncytialSyncytial TheoryTheory�� Ciliates and slime moldsCiliates and slime molds

�� Commonly occur in multinucleated cellsCommonly occur in multinucleated cells

›› Colonial Theory (Haeckel, 1874)Colonial Theory (Haeckel, 1874)�� Same species are involvedSame species are involved

�� Green algae (Green algae (ChlorophytaChlorophyta) > 7000 species) > 7000 species

�� Model: Model: VolvocineVolvocine series series –– Order Order VolvocalesVolvocales

�� Unicellular Unicellular

flagellateflagellate

�� IsogamyIsogamy

�� Small colony (4, 8,16, or 32 cells)Small colony (4, 8,16, or 32 cells)

�� Flat plane, mucilageFlat plane, mucilage

�� No differentiationNo differentiation

�� IsogamyIsogamy

�� Intercellular communicationIntercellular communication

�� Colony (8, 16, or 32 Colony (8, 16, or 32

cells) in 1 layercells) in 1 layer

�� SphericalSpherical

�� IsogamyIsogamy

�� Anterior cells Anterior cells ��������

larger eyespotslarger eyespotslarger eyespotslarger eyespots

�� Coordinate Coordinate flagellarflagellar

movementmovement

�� Colony dies when Colony dies when

disrupteddisrupted

�� 16 or 32 cells16 or 32 cells

�� 16 cells 16 cells –– no specializationno specialization

�� 32 32 –– 4 for motility, the rest for reproduction4 for motility, the rest for reproduction

�� HeterogamyHeterogamy –– female gametes not releasedfemale gametes not released

�� Halves are more pronouncedHalves are more pronounced

�� 32 to 128 cells32 to 128 cells

�� HeterogamyHeterogamy ––female gametes not female gametes not released, in some cases released, in some cases becoming truly nonbecoming truly non--motilemotile

Division of laborDivision of labor�� Division of laborDivision of labor

›› Anterior Anterior vegetative cellsvegetative cells

›› Larger posterior Larger posterior reproductive reproductive cellscells

�� Spherical colonies (500Spherical colonies (500--50000 cells)50000 cells)

�� Hollow sphere Hollow sphere –– coenobiumcoenobium

�� Cell differentiation: Cell differentiation: somatic/vegetative cells and somatic/vegetative cells and gonidiagonidia

�� 22--50 scattered in the posterior 50 scattered in the posterior ��������reproductivereproductive

�� Female reproductive cells Female reproductive cells �������� daughter daughter coloniescolonies

�� Intercellular communication possibleIntercellular communication possible

Reproduction in the Volvox

Anisogamy

Anisogamy/Heterogamy

�� Unicellular Unicellular �������� colonial lifecolonial life

�� Increase in # of cells in coloniesIncrease in # of cells in colonies

�� Change in shape of colonyChange in shape of colony

Increase in interdependence among Increase in interdependence among �� Increase in interdependence among Increase in interdependence among vegetative cellsvegetative cells

�� Increase in division of labor: vegetative Increase in division of labor: vegetative and reproductive cellsand reproductive cells

�� IsogamyIsogamy �������� anisogamyanisogamy �������� oogamyoogamy

�� Fewer female gametes are producedFewer female gametes are produced

��Increase in size of the Increase in size of the

organismorganism

Permits cell specializationPermits cell specialization��Permits cell specializationPermits cell specialization

��Increase in surface area to Increase in surface area to

volume ratiovolume ratio

�� InterdependenceInterdependence

�� ComplexityComplexity

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