Chapter 27 Introduction

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CHAPTER 27

Eukaryotic Cells

Origins and Diversity

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Core Concepts

  1. Eukaryotic cells are defined by the presence of a nucleus, but features like a dynamic cytoskeleton and membrane system explain their success in diversifying.
  2. The endosymbiotic hypothesis proposes that the chloroplasts and mitochondria of eukaryotic cells were originally free-living bacteria.
  3. Eukaryotes were formerly divided into four kingdoms, but are now divided into at least seven superkingdoms.
  4. The fossil record extends our understanding of eukaryotic diversity by providing perspectives on the timing and environmental context of eukaryotic evolution.

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Eukaryotic cells have a nucleus—that is their defining characteristic. As we will see, however, it is quite another set of features that distinguishes eukaryotes in terms of function and the capacity for evolutionary innovation. Eukaryotic diversity—the animals, plants, fungi, and protists we are familiar with—does not reflect exceptional versatility in energy metabolism. In fact, compared with Bacteria and Archaea, the Eukarya obtain carbon and energy in only a limited number of ways. For example, oxygenic photosynthesis is widespread among eukaryotes, but anoxygenic photosynthesis is unknown. Furthermore, aerobic respiration is present almost everywhere, but anaerobic respiration has been reported in only a handful of species. Chemoautotrophy has not been documented in eukaryotes at all. Many eukaryotes can ferment organic substrates—yeasts are particularly important in this regard—but most use fermentation only as a supplementary metabolism and lack the diversity of reactions known among prokaryotic organisms.

Why then, have eukaryotes been so successful? Eukaryotic success lies in their remarkable capacity to form cells of diverse shape and size and to produce multicellular structures in which multiple cell types act in concert. Together, these innovations opened up a range of functional possibilities not found among Bacteria and Archaea.