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!

AvoidanceGeometric solutions

Increase surface areaDecrease effective volume

Increase rate of supplyHigh concentration of nutrientsImprove nutrient transport within

Improve efficiency to reduce demandDivision of labor within the cellDivision 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) 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 non-motile

Division of labor› Anterior

vegetative cells› Larger posterior

reproductive cells

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

http://protist.i.hosei.ac.jp/pdb/images/Chlorophyta/Gonium/pectorale/sp_2b.jpg

http://www.rbgsyd.nsw.gov.au/__data/assets/image/48212/Gonium2.gif

http://www.ac-rennes.fr/pedagogie/svt/photo/microalg/pandorin.jpg

http://protist.i.hosei.ac.jp/PDB/images/Chlorophyta/Eudorina/elegans/sp_5.jpg

http://www.fytoplankton.cz/FytoAtlas/thm/0078.jpg