Lesson Overview

1. Lesson Overview 21.3 The Ecology of Protists

2. 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.

3. 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.

4. Feeding Fish and Whales • Photosynthetic protists make up a large portion of phytoplankton, the small, free-floating photosynthetic organisms found near the surface of oceans and lakes. • About half of Earth’s photosynthesis is carried out by phytoplankton.

5. Supporting Coral Reefs • Coral reefs, which are found in warm ocean waters throughout the world, provide food and shelter to large numbers of fish and other organisms. • Protist algae known as zooxanthellae provide most of the coral's energy needs by photosynthesis. By nourishing coral animals, these algae help maintain the equilibrium of the coral ecosystem. • Coralline red algae also help to provide calcium carbonate to stabilize growing coral reefs.

6. Providing Shelter • The largest known protist is giant kelp, a brown alga that can grow to more than 60 meters in length. • Kelp forests provide shelter for many marine species. Kelp is also a source of food for sea urchins.

7. Recycling Wastes • Many protists grow rapidly in regions where sewage is discharged, where they play a vital role in recycling waste materials. • When the amount of waste is excessive, however, populations of protists like Euglenacan grow to enormous numbers and create an algal bloom, which can disrupt ecosystem homeostasis. • An algal bloom in a pond or lake can deplete nutrients from the water, and the decomposition of the dead protists can rob water of its oxygen, causing fish and invertebrates to die. • Algal blooms of marine protists called dinoflagellates create what is known as a red tide. The buildup of toxins produced by these protists can poison fish and shellfish.

8. Heterotrophic Protists • How do heterotrophic protists obtain food? • Some heterotrophic protists engulf and digest their food, while others live by absorbing molecules from the environment.

9. Amoebas • Amoebas can capture and digest their food, surrounding a cell or particle and then taking it inside themselves to form a food vacuole. A food vacuole is a small cavity in the cytoplasm that temporarily stores food.

10. Ciliates • Paramecium and other ciliates use their cilia to sweep food particles into the gullet, an indentation in one side of the organism. • The particles are trapped in the gullet and forced into food vacuoles that form at its base.

11. Slime Molds • A slime mold is a heterotrophic protist that thrives on decaying organic matter. • Slime molds are found in places that are damp and rich in organic matter—on the floor of a forest or a backyard compost pile, for example. • Slime molds play key roles in recycling nutrients in an ecosystem.

12. Symbiotic Protists—Mutualists and Parasites • What types of symbiotic relationships involve protists? • Many protists are involved in mutualistic symbioses, in which they and their hosts both benefit. Red algae • Parasitic protists are responsible for some of the world’s most deadly diseases, including several kinds of debilitating intestinal diseases, African sleeping sickness, and malaria.

13. Intestinal Diseases • Water-borne protists are found in streams, lakes, and oceans. • Water supplies contaminated by animal or human feces can spread protist parasites, causing serious and sometimes deadly outbreaks of intestinal disease. • flagellated protist Giardia causes severe diarrhea and digestive-system problems. • Entamoeba causes a disease known as amebic dysentery.

14. Lesson Overview 20.2 Prokaryotes

15. Classifying Prokaryotes • How are prokaryotes classified? • Prokaryotes are classified as Bacteria or Archaea—two of the three • domains of life.

16. Classifying Prokaryotes • The smallest and most abundant microorganisms on Earth are prokaryotes—unicellular organisms that lack a nucleus. • Prokaryotes have DNA, like all other cells, but their DNA is not found in a membrane-bound nuclear envelope as it is in eukaryotes. Prokaryote DNA is located in the cytoplasm. • A bacterium such as E. coli has the basic structure typical of most prokaryotes.

17. Bacteria • Bacteria are usually surrounded by a cell wall that protects the cell from injury and determines its shape. • The cell walls of bacteria contain peptidoglycan—a polymer of sugars and amino acids that surrounds the cell membrane. • Some bacteria, such as E. coli, have a second membrane outside the peptidoglycan wall that makes the cell especially resistant to damage.

18. Archaea • Under a microscope, archaea look very similar to bacteria. Both are equally small, lack nuclei, and have cell walls, but there are important differences. • The walls of archaea lack peptidoglycan, and their membranes contain different lipids. • The DNA sequences of key archaea genes are more like those of eukaryotes than those of bacteria.

19. Archaea • Many archaea live in extremely harsh environments. • One group of archaea produce methane gas and live in environments with little or no oxygen, such as thick mud and the digestive tracts of animals. • Other archaea live in extremely salty environments, such as Utah’s Great Salt Lake, or in hot springs where temperatures approach the boiling point of water.

20. Structure and Function • How do prokaryotes vary in their structure and function? • Prokaryotes vary in their size and shape, in the way they move, and in the • way they obtain and release energy.

21. Size, Shape, and Movement • Prokaryotes range in size from 1 to 5 micrometers, making them much smaller than most eukaryotic cells. Prokaryotes come in a variety of shapes. • Rod-shaped prokaryotes are called bacilli. • Spherical prokaryotes are called cocci. • Spiral and corkscrew-shaped prokaryotes are called spirilla.

22. Growth, Reproduction, and Recombination • When a prokaryote has grown so that it has nearly doubled in size, it replicates its DNA and divides in half, producing two identical cells. This type of reproduction is known as binary fission. • binary fission does not involve the exchange or recombination of genetic information, it is an asexual form of reproduction.

23. Growth, Reproduction, and Recombination • When growth conditions become unfavorable, many prokaryotic cells form an endospore—a thick internal wall that encloses the DNA and a portion of the cytoplasm. • Endospores can remain dormant for months or even years.

24. Conjugation • Many prokaryotes exchange genetic information by a process called conjugation. • During conjugation, a hollow bridge forms between two bacterial cells, and genetic material, usually in the form of a plasmid, moves from one cell to the other.

25. The Importance of Prokaryotes • What roles do prokaryotes play in the living world? • Prokaryotes are essential in maintaining every aspect of the ecological • balance of the living world. In addition, some species have specific uses in • human industry.

26. Decomposers • Bacteria called actinomycetes are present in soil and in rotting plant material such as fallen logs, where they decompose complex organic molecules into simpler molecules.

27. Decomposers • By decomposing dead organisms, prokaryotes, supply raw materials and thus help to maintain equilibrium in the environment. • Bacterial decomposers are also essential to industrial sewage treatment, helping to produce purified water and chemicals that can be used as fertilizers.

28. Producers • Photosynthetic prokaryotes are among the most important producers on the planet. • Food chains everywhere are dependent upon prokaryotes as producers of food and biomass.

29. Lesson Overview 20.1 Viruses

30. The Discovery of Viruses • How do viruses reproduce? • Viruses can reproduce only by infecting living cells.

31. Discovery of Viruses • A virus is a nonliving particle made of proteins, nucleic acids, and sometimes lipids. • Viruses can reproduce only by infecting living cells.

32. Structure and Composition • Viruses differ widely in terms of size and structure. • Most viruses are so small they can be seen only with the aid of a powerful electron microscope.

33. Structure and Composition • The protein coat surrounding a virus is called a capsid. • Some viruses, such as the influenza virus, have an additional membrane that surrounds the capsid. • The simplest viruses contain only a few genes, whereas the most complex may have more than a hundred genes.

34. Structure and Composition • Most viruses infect only a very specific kind of cell. • Plant viruses infect plant cells; most animal viruses infect only certain related species of animals; viruses that infect bacteria are called bacteriophages.

35. Viral Infections • What happens after a virus infects a cell? • Inside living cells, viruses use their genetic information to make multiple • copies of themselves. Some viruses replicate immediately, while others • initially persist in an inactive state within the host.

36. Lytic Infections • In a lytic infection, a virus enters a bacterial cell, makes copies of itself, and causes the cell to burst, or lyse. • Bacteriophage T4 is an example of a bacteriophage that causes such an infection.

37. Lysogenic Infection • Bacteriophage DNA that becomes embedded in the bacterial host’s DNA is called a prophage.

38. HIV • The deadly disease called acquired immune deficiency syndrome (AIDS) is caused by an RNA virus called human immunodeficiency virus (HIV). • HIV belongs to a group of RNA viruses that are called retroviruses. • The genetic information of a retrovirus is copied from RNA to DNA instead of from DNA to RNA.

39. Viruses and Cells • All viruses are parasites. Parasites depend entirely upon other living organisms for their existence, harming these organisms in the process. • Viruses must infect living cells in order to grow and reproduce, taking advantage of the nutrients and cellular machinery of their hosts.

40. Viruses and Cells • Some of the main differences between cells and viruses are summarized in this chart.