2-45 The implications of eukaryotic structures on their growth

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• Eukaryotic cells are generally bigger than prokaryotic cells. This allows eukaryotes to have organelles, have slower turnover rates of macromolecules, and requires the presence of mechanisms to move things around the cell.
• The division of the chromosome from the rest of the cell and the larger size of eukaryotes, slows the rate at which they can replicate.
• The presence of organelles, which are sensitive to harsh conditions, might limit the extreme environments in which eukaryotes can grow.

Eukaryotic cells are generally much larger than prokaryotic ones and this difference in volume has several implications. First bigger cells can afford to have more things stored in the cytoplasm. This means it is not as costly to a eukaryotic cell to have structures taking up space. In a prokaryote, space is at a premium and anything not being used is pretty rapidly degraded. This may be one reason that organelles are possible. Second, larger cells have a lower surface-to-volume ratio than do smaller cells and therefore prokaryotes effectively have more contact with their environment. This greater exposure can mean a more rapid response to changing environmental conditions. Finally bigger cells have more of a challenge moving molecules within themselves. Prokaryotes can often depend on simple diffusion to move molecules around the cell, but this process might be too slow and inefficient in much larger cells. Eukaryotes overcome this by having specific transport mechanisms (i.e. microtubules) inside the cell.

Size constrains eukaryotes in two important ways: how fast they can grow and what environments they can tolerate. The compartmentalization of the genome inside the nucleus limits the rate at which eukaryotic cells can divide. The complete cell division cycle in a multicellular eukaryotic organism depends upon the cell type, but even in rapidly dividing skin cells it takes at least 8 hours. In unicellular yeast cultures, the shortest cell cycle is about 1.7 hours under ideal conditions. Due to their smaller genomes, lack of a nucleus and the ability to couple transcription and translation, bacteria can grow much faster. Clostridium perfringens has been shown to go through a complete division cycle in at little as 6.6 minutes at 43 °C in beef cubes!

There are environments that have one or more physical characteristics that prevent the growth of most organisms. These extreme environments might be too hot, cold, acidic or alkaline for typical organisms to grow. However, a small subset of prokaryotes has evolved to take advantage of these environments and thrive, and prokaryotes always define the extremes of where life can exist. In part this is probably due to the fact that simpler cells have fewer "body parts" that must be changed in order for growth under very different conditions. In eukaryotes, they would need to modify not only their cytoplasmic contents to tolerate the extreme environment, but also the makeup of their organelles.

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