Lesson Overview

1. Lesson Overview 21.1 Protist Classification —The Saga Continues

2. THINK ABOUT IT • Some of the organisms we call “protists” live quietly on the bottom of shallow ponds, soaking up the energy of sunlight. Others swim vigorously in search of tiny prey. Some, such as diatoms, sparkle in coastal waters. Still others drift in the human bloodstream, destroying blood cells and killing nearly a million people a year. • What kind of life is this, capable of such beauty and such destruction?

3. The First Eukaryotes • What are protists?

4. The First Eukaryotes • What are protists? • Protists are eukaryotes that are not members of the plant, animal, or fungi kingdoms.

5. The First Eukaryotes • More than a billion years ago, the first eukaryotes appeared on Earth. • Single-celled eukaryotes are still with us today and are often called “protists”—a name that means “first.” Traditionally, protists are classified as members of the kingdom Protista. • Protists are eukaryotes that are not members of the plant, animal, or fungi kingdoms.

6. The First Eukaryotes • Although most protists are unicellular, quite a few are not. Brown algae called kelp are the largest protists. They contain millions of cells arranged in differentiated tissues. • Kelp are considered protists because they are related more closely to certain unicellular protists than to members of any other kingdom. • Otters wrap themselves in giant kelp to keep from drifting out to sea while they sleep.

7. The “Protist” Dilemma • Biologists have discovered that “protists” display a far greater degree of diversity than any other eukaryotic kingdom. • Euglena, brown algae, diatoms, and slime molds are examples of protists.

8. The “Protist” Dilemma • In addition to their diversity, biologists also found that many “protists” are far more closely related to members of other eukaryotic kingdoms than they are to other “protists.” • By definition, the members of a living kingdom should be more like one another than like members of other kingdoms. This is not true of protists, which means that reclassification is necessary. • In the past, scientists sorted protists into three groups: plantlike protists, animal-like protists, and funguslike protists. However, this solution began to fail as biologists learned that many protists do not fit into any of these groups. • Biologists also discovered that many of the animal-like and funguslike protists are so similar that they belong in a single group, not two.

9. Multiple Kingdoms? • The most recent studies of protists divide them into six major clades, each of which could be considered a kingdom.

10. Multiple Kingdoms? • This cladogram represents an understanding of protist relationships supported by current research.

11. Multiple Kingdoms? • Surprisingly, the plant, animal, and fungi kingdoms fit right into these six clades. Animals and fungi actually emerge from the same protist ancestors. • Protists were the first eukaryotes, and evolution has had far more time to develop differences among protists than among more recently evolved eukaryotes like plants and animals. • By finding the fundamental divisions among protists, we also identify the most basic differences among all eukaryotes.

12. What “Protist” Means Today • Biologists assembling the Tree of Life favor the classification shown in the cladogram.

13. What “Protist” Means Today • Even though the biologist building the Tree of Life prefer a different classification, the word “protist” remains in common usage, even among scientists. • Bear in mind that “protists” are not a single kingdom but a collection of organisms that includes several distinct clades.

14. Protists—Ancestors and Descendants • How are protists related to other eukaryotes?

15. Protists—Ancestors and Descendants • How are protists related to other eukaryotes? • Today’s protists include groups whose ancestors were among the very last to split from the organisms that gave rise to plants, animals, and fungi.

16. Protists—Ancestors and Descendants • Microscopic fossils of eukaryotic cells, like Tappania plana shown, have been found in rocks as old as 1.5 billion years. • Genetic and fossil evidence indicates that eukaryotes evolved from prokaryotes and are more closely related to present-day Archaea than to Bacteria. • The split between Archaea and Eukarya may have come as early as 2.5 billion years ago. Since that time, protists have diversified into as many as 300,000 species.

17. Protists—Ancestors and Descendants • Most of the major protist groups have remained unicellular, but two have produced multicellular organisms. Plants, animals, and fungi arose from the ancestors of these multicellular groups.

18. Protists—Ancestors and Descendants • The roots of all eukaryotic diversity, from plants to animals, are found among the ancestors of protists.

19. Lesson Overview 21.2 Protist Structure and Function

20. THINK ABOUT IT • Protists move, sense the environment, digest food, and even reproduce—all within the confines of a single cell. • Imagine what such cells would have to be like to succeed in the never-ending struggle for life on Earth. Protists are winners in that struggle.

21. How Protists Move • How do protists move in the environment?

22. How Protists Move • How do protists move in the environment? • Some protists move by changing their cell shape, and some move by means of specialized organelles. Other protists do not move actively but are carried by wind, water, or other organisms.

23. Amoeboid Movement • Many unicellular protists move by changing their shape, a process that makes use of cytoplasmic projections known as pseudopods. The cytoplasm of the amoeba, for example, streams into the pseudopod and the rest of the cell follows. • This type of locomotion is called amoeboid movement and is found in many protists. • Amoeboid movement is powered by a cytoskeletal protein called actin. Actin also plays a role in the muscle contractions of animals.

24. Cilia and Flagella • Many protists move by means of cilia and flagella, structures supported by microtubules. Cilia are short and numerous, and they move somewhat like oars on a boat. • Flagella are relatively long and usually number only one or two per cell. Some flagella spin like tiny propellers, but most produce a wavelike motion from base to tip.

25. Cilia and Flagella • Protists that move using cilia are known as ciliates, and those that move with flagella are called flagellates.

26. Passive Movement • Some protists are nonmotile—they depend on air or water currents and other organisms to carry them around. • These protists form reproductive cells called spores that can enter the cells of other organisms and live as parasites. • Spore-forming protists include Plasmodium, which is carried by mosquitoes and causes malaria, and Cryptosporidium, which spreads through contaminated drinking water and causes severe intestinal disease.

27. Protist Reproduction • How do protists reproduce?

28. Protist Reproduction • How do protists reproduce? • Some protists reproduce asexually by mitosis. Others have life cycles that combine asexual and sexual forms of reproduction.

29. Cell Division • Amoebas, and many other protists, reproduce by mitosis: They duplicate their genetic material and then divide into two genetically identical cells. • Mitosis enables protists to reproduce rapidly, especially under ideal conditions, but it produces cells that are genetically identical to the parent cell, and thus limits the development of genetic diversity.

30. Conjugation • Paramecia and most ciliates reproduce asexually by mitotic cell division. • However, under stress, paramecia can remake themselves through conjugation—a process in which two organisms exchange genetic material. • After conjugating, the cells then reproduce by mitosis.

31. Conjugation • Paramecium has two types of nuclei: a macronucleus and one or more smaller micronuclei. • The micronucleus holds a “reserve copy” of every gene in the cell. • The macronucleus has multiple copies of the genes the cell uses in its day-to-day activities.

32. Conjugation

33. Conjugation

34. Conjugation

35. Conjugation

36. Conjugation

37. Conjugation

38. Conjugation

39. Conjugation • Conjugation is not a type of reproduction because no new individuals are formed. • Conjugation is, however, a sexual process because new combinations of genetic information are produced. • In a large population, conjugation helps produce and maintain genetic diversity.

40. Sexual Reproduction • Many protists have complex sexual life cycles in which they alternate between a diploid and a haploid phase, a process known as alternation of generations.

41. Sexual Reproduction • A water mold is an example of a protist that undergoes alternation of generations.

42. Sexual Reproduction • Water molds grow into long branching filaments consisting of many cells formed by mitotic cell division.

43. Sexual Reproduction • Water molds reproduce asexually by producing spores in a structure called a sporangium. • In water molds the spores are flagellated.

44. Sexual Reproduction • Water molds also reproduce sexually by undergoing meiosis and forming male and female structures.

45. Sexual Reproduction • The male and female structures produce haploid nuclei that fuse during fertilization, forming a zygote that begins a new life cycle.

46. Lesson Overview 21.3 The Ecology of Protists

47. THINK ABOUT IT • After a few days of rain, you notice a small spot of yellow slime at the base of a stand of tall grass. You mark its position. A few days later, you come back, and it has grown and moved away from the mark. • Is it an animal? A fungus? A strange plant? The correct answer is none of the above. It’s a protist called a slime mold.

48. Autotrophic Protists • What is the ecological significance of photosynthetic protists?

49. Autotrophic Protists • What is the ecological significance of photosynthetic protists? • The position of photosynthetic protists at the base of the food chain makes much of the diversity of aquatic life possible.

50. Diversity • Organisms commonly called “algae” actually belong to many different groups. Some (the cyanobacteria) are prokaryotes, some (like green algae) belong to the plant kingdom, and some are protists. • Photosynthetic protists include many phytoplankton species and the red and brown algae, as well as euglenas and dinoflagellates. • These organisms share an autotrophic lifestyle, marked by the ability to use the energy from light to make a carbohydrate food source.