H-Evolution-005Eukaryotic

Eukayotes

First Eukayotes: 1.7 billion years ago Eukayotic cells came into being. These big cells by comparison seem to have captured the small cells with special capability and enslaved them, and eventually encompass them as an integral part of the bacterial cell colony encased in a membrane. This cleaver membrane is a double layer of molecule soldiers, one layer opens and closes before the other layer opens and closes, thus letting waste out and bringing food in.

Prior to the electron microscope, humans were oblivious to the existence of small things.

Perimeter “storm door” cell walls

Protists

“No more pleasant sight has met my eye than this of so many thousands of creatures in one small drop of water” wrote Anton van Leeuwenhoek after his discovery of the microbial world more than three centuries ago. It a world every biology student should have the opportunity to rediscover by peering through a microscope into a droplet of pond water filled with diverse creatures we call protists. Protists are eukaryotic, and thus even the simplest are much more complex than the prokaryotes. The first eukaryotes to evolve from prokaryotic ancestors were protists. The very word implies great antiquity (from the Greek protos, first) .The primal eukaryotes were not only the predecessors of the great variety of modern protists, but were also ancestral to all other eukaryotes-plants, fungi, and animals. Two of the most significant chapters in the history of life-the origin of the eukaryotic cell and the subsequent emergence of multicellular eukaryotes-unfolded during the evolution of protists.

The Origin of Eukaryotic Cells .

The many differences between prokaryotic and eukaryotic cells far outnumber the differences between plant and animal cells. The fossil record indicates that eukaryotes evolved from prokaryotes more than 1.7 billion years ago. One of biology's most engaging questions is how this happened-in particular, how the membrane-enclosed organelles of eukaryotic cells arose. A widely accepted theory is that eukaryotic cells evolved through a combination of two processes. In one process, the eukaryotic cell's endomembrane system-all the membrane-enclosed organelles except mitochondria and chloroplasts evolved from inward folds of the plasma membrane of a prokaryotic cell (1>2) A second, very different process, called endosymbiosis, generated mitochondria and chloroplasts.(3>4 & 5>6)

Figure 15.18 How did eukaryote cells evolve?

Symbiosis is a close association between organisms of two or more species. The word symbiosis is from the Greek for "living together” and endosymbiosis refers to one species living within another, called the host. chloroplasts and mitochondria evolved from small symbiotic prokaryotes that established residence within other, larger host prokaryotes (Figure 15.18b ). The ancestors of mitochondria may have been aerobic bacteria that were able to use oxygen to release large amounts of energy from organic molecules by cellular respiration. At some point, such a prokaryote might have been an internal parasite of a larger heterotroph, or an ancestral host cell may have ingested some of these aerobic cells for food. If some of the smaller cells were indigestible, they might have remained alive and continued to perform respiration in the host cell. In a similar way, photosynthetic bacteria ancestral to chloroplasts may have come to live inside a larger host cell. Because almost all eukaryotes have mitochondria but only some have chloroplasts, it is likely that mitochondria evolved first.

By whatever means the relationships began, it is not hard to imagine the symbiosis eventually becoming mutually beneficial. In a world that was becoming increasingly aerobic, a cell that was itself an anaerobe would have benefited from aerobic endosymbionts that turned the oxygen to advantage. And a heterotrophic host could derive nourishment from photosynthetic endosymbionts In the process of becoming more interdependent, the host and endosymbionts would have become a single organism, its parts inseparable.

Developed most extensively by Lynn Margulis, the endosymbiosis theory is supported by extensive evidence. Present-day mitochondria and chloroplasts are similar to prokaryotic cells in a number of ways. For example, both types of organelles contain small amounts of DNA, RNA, and ribosomes that resemble prokaryotic versions more than eukaryotic ones. These components enable cWoroplasts and mitochondria to exhibit some autonomy in their activities. The organelles transcribe and translate their DNA into polypeptides, contributing to some of their own enzymes. They also replicate their own DNA and reproduce within the cell by a process resembling the binary fission of prokaryotes.

The origin of the eukaryotic cell made more complex organisms possible, and a vast variety of protists evolved.

The Diversity of Protists

All protists are eukaryotes, but they are so diverse that few other general characteristics can be cited. In fact, protists vary in structure and function more than any other group of organisms. Most protists are unicellular, but there are some colonial and multicellular species. Because most protists are unicellular, they are justifiably considered the simplest eukaryotic organisms. But at the cellular level, many protists are exceedingly complex-the most elaborate of all cells. We should expect this of organisms that must carry out within the boundaries of a single cell, all the basic functions performed by the collective of specialized cells that make up the bodies of plants and animals. Each unicellular protist is not at all analogous to a single cell from a human, but is itself an organism as complete as any whole animal or plant.

For our survey of these diverse organisms, we'll look at four major categories of protists, grouped-more by lifestyle than by their evolutionary relationships: protozoans, slime molds, unicellular algae, and seaweeds.

Protozoans Protists that live primarily by ingesting food, a mode of nutrition that is animal-Iike, are called protozoans ("first animal"). Protozoans thrive in all types of aquatic environments, including wet soil and the watery environment inside animals. Most species eat bacteria or other protozoans, but some can absorb nutrients dissolved in the water. Protozoans that live as parasites in animals, though in the minority, cause some of the world's most harmful human diseases. We'll examine five groups of protozoans: flagellates, amoebas, forams, apicomplexans, and ciliates.

Flagellates are protozoans that move by means of one or more flagella. Most species are free-Iiving (nonparasitic). However, there are also some nasty parasites that make humans sick. An example is Giardia, a flagellate that infects the human intestine and can cause abdominal cramps and severe diarrhea. People become infected mainly by drinking water contaminated with feces from infected animals. Giardia can ruin a camping trip. Another group of dangerous flagellates are the trypanosomes, including a species that causes sleeping sickness, a serious illness prevalent in tropical Africa and transmitted by the tsetse fly (Figure 15.19a)

Figure 15.19 Examples of protozoans.(a) Trypanosomes are flagellates that live as parasites in the bloodstream of vertebrate animals. The squiggles among these human red blood cells are trypanosomes that cause sleeping sickness, a debilitating disease common in parts of Africa. Trypanosomes escape being killed by their host's defenses by being quick-change artists. They alter the molecular structure of their coats frequently, thus preventing immunity from developing in the host. (b) This amoeba is ingesting a smaller protozoan as food. The amoeba's pseuilopodia arch around the prey and engulf it into a food vacuole (also see Chapter 5). (c) Forams are almost all marine. The foram cell secretes a porous, multichambered shell made of organic material hardened with calcium carbonate, the same mineral that makes up limestone. Thin strands of cytoplasm (pseudopodia) extend through the pores, functioning in swimming, shell formation, and feeding. The shells of fossilized forams are major components of the limestone rocks that are now land formations. (d) Plasmodium, the apicomplexan that causes malaria, uses its apical complex to enter red blood cells of its human host. The parasite feeds on the host cell from within, eventually destroying it. (e) The ciliate Paramecium uses its cilia to move through pond water. Cilia also line an indentation called the oral groove, and their beating keeps a current of water containing bacteria and small protists moving toward the cell "mouth" at the base of the groove.

Amoebas are characterized by great flexibility and the absence of permanent locomotor organelles. Most species move and feed by means of pseudopodia ( singular, pseudopodium )', temporary extensions of the cell (Figure 15.19b) .Amoebas can assume virtually any shape as they creep over rocks, sticks, or mud at the bottoII1 of a pond or ocean. Other protoroans with pseudopodia include the forams (Figure 15.19c).

Apicomplexans are all parasitic, and some cause serious human diseases. Theyare named for an apparatus at their apex that is specialized for penetrating host cells and tissues. This protozoan group includes Plasmodium, dIe parasite that causes malaria (Figure 15.19d). Spread by mosquitoes, malaria is one of the most debilitating and widespread human diseases. Each year in the tropics, more than 200 million people become infected, and at least a million die in Africa alone. As part of the effort to combat malaria, scientists determined the complete sequence of the Plasmodium genome in 2002.

Ciliates are protozoans that use locomotor structures called cilia to move and feed. Nearlyall ciliates are free-living (nonparasitic). The best known example is the freshwater ciliate Paramecium (Figure 15.19e) .

Slime Molds These protists are more attractive than their name. Slime molds resemble fungi in appearance and lifestyle, but the similarities are due to convergent evolution; slime molds and fungi are not at all closely related. The filamentous body of a slime mold, like that of a fungus, is an adaptation that increases exposure to the environment. This suits the role of these organisms as decomposers. The two main groups of these protists are plasmodial slime molds and cellular slime molds.

Plasmodial slime molds are named for the feeding stage in their life cycle, an amoeboid mass called a plasmodium (not to be confused with Plasmodium, the parasite that causes malaria). You can find plasmodial slime molds among the leaf littler and other decaying material on a forest floor, and you won't need a microscope to see them. A plasmodium can measure several centimeters across, with its network of fine filaments taking in bacteria and bits of dead organic matter amoeboid style. Large as it is, the plasmodium is actually a single cell with many nuclei (Figure 15.20).

Cellular slime molds pose a semantic question about what it means to be an individual organism. The feeding stage in the life cycle of a cellular slime mold consists of solitary amoeboid cells. They function individually, using their pseudopodia to feed on decaying organic matter. But when food is depleted, the cells aggregate to form a slug-like colony that moves and functions as a single unit (Figure 15.21 ).

Unicellular Algae Photosynthetic protists are called algae (singular, alga). Their chloroplasts support food chains in freshwater and marine ecosystems. Many unicellular algae are components of plankton (from the Greek planktos, wandering), the communities of organisms, mostly microscopic, that drift or swim weakly near the surfaces of ponds, lakes, and oceans. More specifically, planktonic algae are referred to as phytoplankton. We'll look at three groups of unicellular algae: dinoflagellates, diatoms, and green algae (a group that also includes colonial and truly multicellular species).

Left: Figure 15.20 A plasmodial slime mold. Pseudopodia of the huge cell engulf small food particles in mulch or moist soil. The web-like form is an adaptation that enlarges the organism's surface area, increasing its contact with food, water, and oxygen. Within the fine channels of the plasmodium, cytoplasm streams first one way and then the other, in pulses that are beautiful to watch with a microscope. The cytoplasmic streaming helps distribute nutrients and oxygen within the giant cell.

Right: Figure 15.21 Life cycle of a cellular slime mold. Most of the time, cellular slime molds live as solitary amoeboid cells, using their pseudopodia to creep through compost and engulf bacteria. When food is in short supply, the amoeboid cells swarm together, forming a colony that looks and moves like a slug. After wandering around for a short time, the colony extends a stalk and develops into a multicellular reproductive structure.

Figure 15.22 Unicellular and colonial algae. (a) A dinoflagellate, with its wall of protective plates. (b) A sample of diverse diatoms, which have glassy walls. (c) Chlamydomonas, a unicellular green alga with a pair of flagella. (d) Volvox, a colonial green alga.

Dinoflagellates are abundant in the vast aquatic pastures of phytoplankton. Each dinoflagellate species has a characteristic shape reinforced by external plates made of cellulose (Figure 15.22a). The beating of two flagella in perpendicular grooves produces the spinning movement for which these organisms are named (from the Greek dinos, whirling). Dinoflagellate blooms-population explosions-sometimes cause warm coastal waters to turn pinkish orange, a phenomenon known as a red tide. Toxins produced by some red-tide dinoflagellates have caused massive fish kills, especially in the tropics, and are poisonous to humans as well.

Diatoms have glassy cell walls containing silica, the mineral used to make glass (Figure 15.22b) .The cell wall consists of two halves that fit together like the bottom and lid of a shoe box. Diatoms store their food , reserves in the form of an oil that provides buoyancy, keeping diatoms floating as phytoplankton near the sunlit surface. Massive accumulations of fossilized diatoms make up thick sediments known as diatomaceous earth, which is mined for its use as both a filtering material and an abrasive. Green algae are named for their grass-green chloroplasts. Unicellular green algae flourish in most freshwater lakes and ponds. Some species are flagellated (Figure 15.11c). The green algal group also includes colonial forms, such as the Volvox in Figure 15.11d. Each Volvox colony is a ball of flagellated cells ( the small green dots in the photo) that are very similar to certain unicellular green algae. The balls within the balls in Figure 15.22d are "daughter" colonies that will be released when the parent colonies rupture. Of all photosynthetic protists, green algae are the most closely related to true plants.

Figure 15.23 The three major groups of seaweeds. (a) Green algae. This sea lettuce is an edible species that inhabits the intertidal lone. In addition to seaweeds, the green algal group includes unicellular and colonial species, such as those in Figures 15.22c and d. (b) Red algae. These seaweeds are most abundant in the warm coastal waters of the tropics. Of all the seaweeds, red algae can generally live in the deepest water. Their chloroplasts have special pigments that absorb the blue and green light that penetrates best through

water. The species in this photo is an example of corraline algae, which contribute to the architecture of some coral reefs. The cell walls are hardened bya mineral. (c) Brown algae. This group includes the largest seaweeds, known as kelp, which grow as marine "forests" in relatively deep water beyond the intertidal lone. Some species grow to a length of over 60 m in a single season, the fastest linear growth of any organism. Kelp is a renewable resource reaped by special boats that cut and collect the tops of the algae. More importantly, kelp forests provide habitat for many animals, including a great diversity of fishes. If you have walked on a beach covered with kelp that has washed ashore after a storm, you may have noticed the organs called floats, which keep the photosynthetic blades of the kelp in the light near the water's surface. Maybe you even picked up and popped some of those floats, the way you do those irresistible packing-material bubbles.

Seaweeds Defined as large, multicellular marine algae, seaweeds grow on rocky shores and just offshore beyond the zone of the pounding surf. Their cell walls have slimy and rubbery substances that cushion their bodies against the agitation of the waves. Some seaweeds are as large and complex as many plants. Even the word seaweed implies plantlike appearance, but the similarities between these algae and true plants are a consequence of convergent evolution. In fact, the closest relatives of seaweeds are certain unicellular algae, which is why many biologists include seaweeds with the protists. Seaweeds are classified into three different groups, based partly on the types of pigments present in their chloroplasts: green algae, red algae, and brown algae (Figure 15.23) .

Coastal people, particularly in Asia, harvest seaweeds for food. For example, in Japan and Korea, some seaweed species are ingredients in soups. Other seaweeds are used to wrap sushi. Marine algae are rich in iodine and other essential minerals. However, much of their organic material consists of unusual polysaccharides that humans cannot digest, which prevents seaweeds from becoming staple food. They are ingested mostly for their rich tastes and unusual textures. The gel-forming substances in the cell walls of seaweeds are widely used as thickeners or such processed foods as puddings, ice cream, and salad dressing. And the seaweed extract called agar provides the gel forming base for the media microbiologists use to culture bacteria in Petri dishes.

Evolution Connection

The Origin of Multicellular Life

An orchestra can playa greater variety of musical compositions than a violin soloist can. Put simply, increased complexity makes more variations possible. Thus, the origin of the eukaryotic cell led to an evolutionary radiation of new forms of life. Unicellular protists, which are organized on the complex eukaryotic plan, are much more diverse in form than the simpler prokaryotes. The evolution of multicellular bodies broke through another threshold in structural organization.

Figure 15.24 A model for the evolution of multicellular organisms from unicellular protists.

(1) An ancestral colony may have formed, as colonial protests do today, when a cell divided and its offspring remained attached to one another. (2) The cells in the colony may have become somewhat specialized and interdependent, with different cell types becoming more and more efficient at performing specific, limited tasks. Cells that retained a flagellum may have become specialized for locomotion, while others that lost their flagellum could have assumed functions such as ingesting or synthesizing food. (3) Additional specialization among the cells in the colony may have led to distinctions between sex cells (gametes) and non-reproductive cells (somatic cells).

Multicellular organisms are fundamentally different from unicellular ones. In a unicellular organism, all of life's activities occur within a single cell. In contrast, a multicellular organism has various specialized cells that perform different functions and are dependent on each other. For example, some cells procure food, while others transport materials or provide movement.

The evolutionary links between unicellular and multicellular life were probably colonial forms, in which unicellular protists stuck together as loose federations of independent cells (Figure 15.2.4). The gradual transition from colonies to truly multicellular organisms involved the cells becoming increasingly interdependent as a division of labor evolved. We can see one level of specialization and cooperation in the colonial green alga Volvox (see Figure 15.22d). Volvox produces gametes (sperm and ova), which depend on nonreproductive cells, or somatic cells, while developing. Cells in truly multicellular organisms are specialized for many more nonreproductive functions, including feeding, waste disposal, gas exchange, and protection, to name a few.

Multicellularity evolved many times among the ancestral stock of protists, leading to new waves of biological diversification. The diverse seaweeds are examples of the descendants, and so are plants, fungi, and animals. In the next chapter, we II trace the long evolutionary movement of plants and fungi onto land.