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Multicellularity and the Volvocine Series

Multicellularity and the Volvocine Series. Limitation of Size. Size poses a lot of problems Exchange of substances Diffusion (surface area, concentration gradient and distance) SA to Volume ratio Maximum size limit of cell Larger than size limit = multicellular. SA to Volume Ratio.

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Multicellularity and the Volvocine Series

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  1. Multicellularity and the Volvocine Series

  2. Limitation of Size • Size poses a lot of problems • Exchange of substances • Diffusion (surface area, concentration gradient and distance) • SA to Volume ratio • Maximum size limit of cell • Larger than size limit = multicellular

  3. SA to Volume Ratio • Inc in cell size leads to decrease in SA: V • Life processes are affected

  4. Solutions to the SA:V Problem • Avoidance • Geometric solutions • Inc SA; Dec V • Increase rate of supply • High conc. of nutrients; efficient transport • Improved efficiency to reduced demand • Specialization of cells (intercellularly and intracellularly)

  5. Movement towards Multicellularity • Occurred many times in eukaryotes • Theories: • Symbiotic Theory (e.g. endosymbiotic theory) • Syncitial Theory (slime molds and ciliates) • Colonial Theory (Haeckel, 1874) • Green algae (Chlorophyta)>7000 species • The Volvocine series

  6. Modes of reproduction • Isogamy- involves reproduction of morphologically the same gametes (+or -) • Anisogamy- (heterogamy) gametes are of different size; sperm is smaller than the egg • Oogamy-(specialized heterogamy) egg is non-motile, sperm is flagellated

  7. Chlamydomonas • Unicellular flagellate • Isogamy

  8. Gonium • Small colony • Flat plane, mucilage • No differentiation • Isogamy • Intercellular communication

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

  10. 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

  11. 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

  12. 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

  13. Evidence of Evolution • 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

  14. Advantages of Multicellularity • Increase in size of the organism • Permits cell specialization • Increase in surface area to volume ratio

  15. Disadvantages of multicellularity • Interdependence • Complexity

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