Biol. 2320 – Fall 2000 - First “Thought” Question - Possible correct answers

The slime mold shown in Figure 1.17 and discussed on pages 14-16 is a eucaryote. A) [6 points] What eucaryotic characteristics might be important to the life cycle of this organism?  Use the information in table 1.1 plus readings on the extracellular matrix to identify the characteristics requested.  Describe 3 eucaryotic characteristics and explain why each is relevant to the slime mold life cycle.  (this should take 3 compound sentences, or 6 short ones)  B) [2 points] What problem is presented by the slime mold life cycle in the context of surface area to volume ratios?  (1-2 sentences) C) [2 points] Explain why the problem you describe in part B would be worse for the procaryote Myxococcus, which has a similar life cycle.(1-2 sentences)

The answers below are more extensive than was required for full credit.

A)                1) Internal organelles would be useful for regulation of different aspects of biosynthesis required as cells differentiate.  One example would be that endomembranes would mediate slime secretion during slug motility.  2) Cytoskeleton would be helpful for amoeboid movement and for shape changes carried out as fruiting body differentiates. (Note: amoebae don’t have cilia or flagella)  3) Mitochondria could make more efficient use of energy stores during starvation.  4) Large genome with DNA organized in chromosomes could allow more precise regulation of genes involved in cell differentiation.  5) Nucleus with nuclear envelope and pores could allow more precise regulation of genes involved in cell differentiation. 6) Complex extracellular matrix could be modified both to facilitate specific structures and cell-cell communication (but these cells could not be said to have a true “basement membrane”).  7) Mitotic-spindle-based cell division could place daughter cells in correct orientation for forming fruiting body (although it was not made clear whether any cell divisions occurred during this part of life cycle).

Some off-the-mark answers: I) Differentiation (procaryote cells can do this).  II) Sexual reproduction or meiosis (not involved here – cells start and stay diploid).  III) Phagocytosis is only important when cell eating not starving.  IV) Several answers indicated that cytoskeleton and extracellular matrix were the same thing.

B)                When the amoebae aggregate to become a slug the volume increases more than the surface area.  Diffusion driven processes across membranes will not happen as efficiently.

C)                Procaryotes have no internal membranes, so when Myxococcus forms an aggregate, any functions carried out by surface area is going to be more challenged by the loss of surface area to cell-cell contacts.