CHAPTER SUMMARY
27.1 EUKARYOTIC CELLS ARE DEFINED BY THE PRESENCE OF A NUCLEUS, BUT OTHER FEATURES, PARTICULARLY A DYNAMIC CYTOSKELETON AND MEMBRANES, 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.
- Eukaryotic cells have dynamic membranes that facilitate movement and feeding.
- Eukaryotic cells compartmentalize their machinery for energy metabolism into mitochondria and chloroplasts, freeing the rest of the cell to interact with the environment in novel ways not available to prokaryotes.
- The eukaryotic genome is larger than that of prokaryotes, allowing new mechanisms of gene regulation.
- Eukaryotes reproduce sexually, which promotes genetic diversity, but cannot on its own help us understand their great diversity.
27.2 THE ENDOSYMBIOTIC HYPOTHESIS PROPOSES THAT THE CHLOROPLASTS AND MITOCHONDRIA OF EUKARYOTIC CELLS WERE ORIGINALLY FREE-LIVING BACTERIA THAT WERE INCORPORATED INTO A HOST CELL.
- The endosymbiotic hypothesis is based on physical, biochemical, and genetic similarities between chloroplasts and cyanobacteria, and between mitochondria and proteobacteria.
- Chloroplasts and mitochondria have their own genomes, but their genomes are small relative to free-living bacteria, to which they are closely related, mostly because of gene migration to the nuclear genome.
- Symbiosis between a heterotrophic host and photosynthetic partner is common throughout the eukaryotic domain; reef-forming corals are an example.
- Photosynthesis spread through the Eukarya by means of multiple independent events involving a protozoan host and a eukaryotic endosymbiont.
- Most eukaryotic cells have mitochondria, but a few do not. Evidence suggests that cells lacking mitochondria once had them but lost them.
- The eukaryotic nuclear genome contains genes unique to Eukarya, but also genes related to Bacteria and Archaea, suggesting either that the ancestor of the modern eukaryotic cell was a primitive eukaryote or an archaeon that engulfed a bacterium.
27.3 EUKARYOTES HAVE HISTORICALLY BEEN DIVIDED INTO FOUR KINGDOMS—ANIMALS, PLANTS, FUNGI, AND PROTISTS—BUT ARE NOW DIVIDED INTO AT LEAST SEVEN SUPERKINGDOMS.
- The terms “protist,” “algae,” and “protozoa” are useful, but do not describe monophyletic groups.
- Protists are eukaryotes that do not have features of animals, plants, and fungi; algae are photosynthetic protists; and protozoa are heterotrophic protists.
- The seven superkingdoms of eukaryotes are Opisthokonta, Amoebozoa, Archaeplastida, Stramenopila, Alveolata, Rhizaria, and Excavata.
- Opisthokonts are the most diverse eukaryotic superkingdom. They include animals and fungi, as well as choanoflagellates (our closest protistan relatives).
- Amoebozoans produce multicellular structures by the aggregation of amoeba-like cells, and include organisms that cause human disease and those important in biological research.
- The archaeplastids are photosynthetic, include land plants, and are divided into three major groups.
- Algae occur within the stramenopiles and several other groups.
- The earliest branch of the eukaryotic tree may separate Opisthokonta and Amoebozoa from the other superkingdoms, although this is still debated.
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.
- The earliest fossil eukaryotes that can be placed into one of the present-day superkingdoms are 1200 million years old and belong to the red algae.
- Eukaryotic fossils diversified around 800 million years ago, coinciding with an increase in oxygen and a decline in sulfide in the atmosphere and oceans.
- 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.
- Protists have evolved to take advantage of the environments provided by animal and plants; these include many protists that cause disease in humans.
Self-Assessment Question 1
List key features that distinguish a eukaryotic cell from a prokaryotic cell.
Model Answer:
Key features that distinguish a eukaryotic cell from a prokaryotic cell are: a membrane-bound nucleus that houses DNA, creating separate cellular compartments for transcription and translation; membrane-bound organelles that further organize the cell interior and compartmentalize different cellular processes; and dynamic membranes and cytoskeleton that can be remodeled quickly, allowing cells to change shape and transport materials throughout the cell.
Self-Assessment Question 2
Describe the forms of energy metabolism found in eukaryotes.
Model Answer:
Forms of energy metabolism found in eukaryotes are aerobic respiration in the mitochondrion and photosynthesis in the chloroplast. A few single-celled eukaryotic organisms lack mitochondria and contain small organelles called hydrogenosomes that generate ATP by anaerobic processes.
Self-Assessment Question 3
Describe the origin and evolution of the chloroplast and mitochondrion.
Model Answer:
The origin and evolution of the chloroplast and mitochondrion is thought to be 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.
Self-Assessment Question 4
Present two hypotheses for the origin of the eukaryotic cell.
Model Answer:
One hypothesis for the origin of eukaryotic cells is that the host for mitochondrion-producing endosymbiosis was itself a true eukaryotic cell with a nucleus, cytoskeleton, and endomembrane system; subsequent engulfment of a proetobacterium led to the evolution of mitochondria. A second hypothesis for the origin of eukaryotic cells argues that the eukaryotic cell as a whole began as a symbiotic association between a proteobacterium and an archaeon, and subsequently evolved a nucleus and endomembrane system through the infolding of plasma membrane.
Self-Assessment Question 5
Name the seven superkingdoms of eukaryotes and an organism in each one.
Model Answer:
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., trypanosomes).
Self-Assessment Question 6
State when the eukaryotic cell is first thought to have evolved and the evidence that supports this date.
Model Answer:
Through evidence in the fossil record of coastal marine environments, the first eukaryotic cell is thought to have evolved 1800 million years ago. These microfossils 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.