575
27.1 Eukaryotic cells are defined by the presence of a nucleus, but features like a dynamic cytoskeleton and membrane system explain their success in diversifying.
Eukaryotic cells have a network of proteins inside the cell that allow them to change shape, move, and transfer substances in and out of the cell. page 554
Eukaryotic cells have dynamic membranes that facilitate movement and feeding. page 554
Eukaryotic cells compartmentalize their machinery for energy metabolism into mitochondria and chloroplasts, freeing the cell to interact with the environment in novel ways not available to prokaryotes. page 555
The eukaryotic genome is larger than that of prokaryotes, allowing new mechanisms of gene regulation. page 556
Sexual reproduction promotes genetic diversity in eukaryotic populations. page 556
27.2 The endosymbiotic hypothesis proposes that the chloroplasts and mitochondria of eukaryotic cells were originally free-
The endosymbiotic hypothesis is based on physical, biochemical, and genetic similarities between chloroplasts and cyanobacteria, and between mitochondria and proteobacteria. page 557
Chloroplasts and mitochondria have their own genomes, but their genomes are small relative to free-
Symbiosis between a heterotrophic host and photosynthetic partner is common throughout the eukaryotic domain; reef-
Photosynthesis spread through Eukarya by means of multiple independent events involving a protozoan host and a eukaryotic endosymbiont. page 559
Most eukaryotic cells have mitochondria, but a few do not. Evidence suggests that cells lacking mitochondria once had them but lost them. page 559
The eukaryotic nuclear genome contains genes unique to Eukarya, but also genes related to Bacteria and Archaea, suggesting that the ancestor of the modern eukaryotic cell was either a primitive eukaryote descended from archaeal ancestors or an archaeon that engulfed a bacterium. page 560
27.3 Eukaryotes were formerly divided into four kingdoms, but are now divided into at least seven superkingdoms.
The terms “protist,” “algae,” and “protozoa” are useful, but do not describe monophyletic groups. page 562
Protists are eukaryotes that do not have features of animals, plants, and fungi; algae are photosynthetic protists; and protozoa are heterotrophic protists. page 562
The seven superkingdoms of eukaryotes are Opisthokonta, Amoebozoa, Archaeplastida, Stramenopila, Alveolata, Rhizaria, and Excavata. page 563
Opisthokonts are the most diverse eukaryotic superkingdom. They include animals and fungi, as well as choanoflagellates (our closest protistan relatives). page 563
Amoebozoans produce multicellular structures by the aggregation of amoeba-
The archaeplastids, which include land plants, are photosynthetic, and are divided into three major groups. page 566
The related superkingdoms called the stramenopiles, alveolates, and rhizarians dominate eukaryotic diversity in the oceans. page 568
27.4 The fossil record extends our understanding of eukaryotic diversity by providing perspectives on the timing and environmental context of eukaryotic evolution.
Fossils in sedimentary rocks as old as 1800 million years have unmistakable signs of eukaryotic cells, including complicated wall structures. page 573
The earliest fossil eukaryotes that can be placed into one of the present-
Eukaryotic fossils diversified greatly about 800 million years ago, reflecting a new capacity for predation on other protists and, perhaps, a modest increase in oxygen. page 573
Protists continue to diversify to the present. Green algae and cyanobacteria dominated primary production in earlier oceans, but, since about 200 million years ago, dinoflagellates, coccolithophorids, and diatoms have become the primary photosynthetic organisms in the oceans. page 574
Protists have evolved to take advantage of the environments provided by animals and plants; these protists include many that cause disease in humans. page 574
576
List key features that distinguish a eukaryotic cell from a prokaryotic cell.
Key features that distinguish a eukaryotic cell from a prokaryotic cell are: a membrane-
Describe the forms of energy metabolism found in eukaryotes.
Forms of energy metabolism found in eukaryotes are aerobic respiration in the mitochondrion and photosynthesis in the chloroplast. A few single-
Describe the origin and evolution of the chloroplast and mitochondrion.
The chloroplast and mitochondrion are thought to have originated through endosymbiosis (a symbiosis where one partner lives within the other). Chloroplasts closely resemble cyanobacteria and are thought to be descendants of symbiotic cyanobacteria that lived within eukaryotic cells. Mitochondria closely resemble proteobacteria and are also thought to have evolved as endosymbionts.
Present two hypotheses for the origin of the eukaryotic cell.
One hypothesis for the origin of eukaryotic cells is that the host for mitochondrion-
Name the seven superkingdoms of eukaryotes and an organism in each one.
The seven superkingdoms of eukaryotes are: (1) Opisthokonta (e.g., animals); (2) Amoebozoa (e.g., slime molds); (3) Archaeplastida (e.g., red algae); (4) Stramenopila (e.g., brown algae); (5) Alveolata (e.g., ciliates); (6) Rhizaria (e.g., cercozoans); and (7) Excavata (e.g., euglenid algae).
State when the eukaryotic cell is first thought to have evolved and the evidence that supports this date.
Based on evidence in the fossil record of coastal marine environments, the first eukaryotic cell is thought to have evolved 1,800 million years ago. Microfossils found in these environments have complicated wall structures such as interlocking plates, long and branching arms, and complex internal layering. Comparison with living organisms suggests that such fossils could only be formed by organisms with a cytoskeleton and endomembrane system, the hallmarks of eukaryotic biology.