21.1 Protist Classification —The Saga Continues

21.1 Protist Classification —The Saga Continues
Lesson Overview 21.1 Protist Classification —The Saga Continues

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?

The First Eukaryotes What are protists?

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

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.

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.

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.

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.

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

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

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.

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

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.

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

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.

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.

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.

The roots of all eukaryotic diversity, from plants to animals, are found among the ancestors of protists.

21.2 Protist Structure and Function
Lesson Overview 21.2 Protist Structure and Function

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.

How Protists Move How do protists move in the environment?

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.

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.

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.

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

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.

Protist Reproduction How do protists reproduce?

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

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.

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.

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.

Conjugation

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.

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.

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

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

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

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

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

21.3 The Ecology of Protists
Lesson Overview 21.3 The Ecology of Protists

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.

Autotrophic Protists What is the ecological significance of photosynthetic protists?

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.

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.

Diversity Not all photosynthetic protists are closely related to plants. In fact, the red algae are the most closely related to plants. Many other photosynthetic protists, however, are more closely related to nonphotosynthetic protists. In some cases certain species within a group have lost chloroplasts. In other cases endosymbiosis added a chloroplast to some species but not to their relatives.

Ecological Roles Photosynthetic protists play major ecological roles on Earth. The position of photosynthetic protists at the base of the food chain makes much of the diversity of aquatic life possible.

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.

Feeding Fish and Whales
Phytoplankton provide a direct source of nourishment for organisms as diverse as shrimp and baleen whales. Phytoplankton are an indirect source of nourishment for humans. When you eat tuna fish, you are eating fish that fed on smaller fish that fed on still smaller animals that fed on photosynthetic protists.

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.

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.

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 Euglena can grow to enormous numbers and create an algal bloom, which can disrupt ecosystem homeostasis.

Recycling Wastes 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.

Heterotrophic Protists
How do heterotrophic protists obtain food?

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.

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. Once inside the cell, the material is digested and the nutrients are passed along to the rest of the cell. Indigestible waste materials remain inside the vacuole until the vacuole releases them outside the cell.

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.

Ciliates The food vacuoles pinch off into the cytoplasm and fuse with lysosomes, which contain digestive enzymes. Waste materials are emptied into the environment when the food vacuole fuses with a region of the cell membrane called the anal pore.

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.

Slime Molds At one stage in their life cycle, slime molds exist as a collection of individual amoebalike cells.

Slime Molds Eventually these aggregate to form a large structure known as a plasmodium, which may continue to move.

Slime Molds The plasmodium eventually develops sporangia, in which meiosis produces haploid spores to continue the cycle.

Protists That Absorb Some protists survive by absorbing molecules that other organisms have released to the environment. Water molds, for example, grow on dead or decaying plants and animals, absorbing food molecules through their cellulose cell walls and cell membranes. This dead goldfish is covered with the common water mold Saprolegnia.

Symbiotic Protists—Mutualists and Parasites
What types of symbiotic relationships involve protists?

What types of symbiotic relationships involve protists? Many protists are involved in mutualistic symbioses, in which they and their hosts both benefit. 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.

Many protists are involved in symbiotic relationships with other organisms. Symbiosis is a relationship in which two species live closely together. Many of these symbiotic relationships are mutualistic: Both organisms benefit. However, some are parasitic relationships, in which the protist benefits at the expense of its host.

Mutualists Many protists are involved in mutualistic symbioses, in which they and their hosts both benefit. For example, red algae maintain a mutualistic relationship with the animals of the coral reef, which could not survive without the protists’ help.

Mutualists Trichonympha is another example of a mutualistic protist. Trichonympha is a flagellated protist that lives within the digestive system of termites and makes it possible for the insects to digest wood. Termites themselves do not have enzymes to break down the cellulose in wood. Trichonympha and other organisms in the termite’s gut manufacture an enzyme called cellulose that breaks the chemical bonds in cellulose, making it possible for termites to digest wood.

Parasites and Disease 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.

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.

Intestinal Diseases For example, the flagellated protist Giardia causes severe diarrhea and digestive-system problems. Even crystal-clear streams may be contaminated with Giardia, which produces tough cysts that can be killed only by boiling water thoroughly or by adding iodine to the water.

Intestinal Diseases Entamoeba causes a disease known as amebic dysentery. The amoebas live in the intestines, where they absorb food from the host. They also attack the wall of the intestine itself, destroying parts of it and causing severe bleeding.

Intestinal Diseases Cryptosporidium is resistant to the chlorine compounds often used to sanitize drinking water and therefore poses a special threat to public water systems. In 2008, an outbreak in Utah sickened more than 2,000 people.

African Sleeping Sickness
The flagellated protists Trypanosoma cause African sleeping sickness. Trypanosomes are spread from person to person by the bite of the tsetse fly. Trypanosomes destroy blood cells and infect other tissues, including nerve cells. Severe damage to the nervous system causes some individuals to lose consciousness and lapse into a deep and sometimes fatal sleep. Control of the tsetse fly and the protist pathogens that it spreads is a major goal of health workers in Africa.

Malaria Malaria is one of the world’s most serious infectious diseases. Malaria is caused by Plasmodium, a spore-forming protist carried by the female Anopheles mosquito. Plasmodium requires two hosts to complete its life cycle: an Anopheles mosquito and a human.

Malaria

Lesson Overview 21.4 Fungi

THINK ABOUT IT What is the largest organism in this photo? You might think it’s the tree, but in fact it’s a fungus. The only trace of the fungus is the ring of mushrooms, but the mushrooms are just the reproductive structures of a much larger organism. Most of the mass of the fungus is underground, spanning at least the width of the ring of mushrooms, and extending more than 2 meters into the ground! Hundreds of years ago, some cultures believed these rings of mushrooms marked spots where fairies danced. Today people still call them fairy rings.

What Are Fungi? What are the basic characteristics of fungi?

What Are Fungi? What are the basic characteristics of fungi?
Fungi are heterotrophic eukaryotes with cell walls that contain chitin.

What Are Fungi? Many fungi grow from the ground, but fungi aren’t plants. Instead of carrying out photosynthesis, fungi produce enzymes that digest food outside their bodies. Then they absorb the small molecules released by the enzymes. Many fungi feed by absorbing nutrients from decaying matter in the soil. Others live as parasites, absorbing nutrients from their hosts.

What Are Fungi? The cell walls of fungi are composed of chitin, a polymer made of modified sugars that is also found in the external skeletons of insects. The presence of chitin is one of several features that show fungi are more closely related to animals than to plants.

Structure and Function
Yeasts are tiny fungi that live most of their lives as single cells. Mushrooms and other fungi, on the other hand, grow much larger. Their bodies are made up of cells that form long, slender branching filaments called hyphae.

In most fungi, cross walls divide the hyphae into compartments resembling cells, each containing one or two nuclei. In the cross walls, there are openings through which cytoplasm and organelles can move.

The body of a mushroom is actually the fruiting body, the reproductive structure of the fungus. The fruiting body grows from the mycelium, the mass of branching hyphae below the soil. Clusters of mushrooms are often part of the same mycelium, which means they are part of the same organism.

The mycelium of the soil fungus in a fairy ring has grown so large that it has used up all of the nutrients near its center. It grows and produces fruiting bodies—the mushrooms—only at its edges, where it comes in contact with fresh soil and abundant nutrients.

Reproduction Fungi can reproduce asexually, primarily by releasing spores that are adapted to travel through air and water. Breaking off a hypha or budding off a cell can also serve as asexual reproduction.

Reproduction Most fungi can also reproduce sexually. The life cycle of the bread mold Rhizopus stolonifer is shown.

Reproduction Sexual reproduction in fungi often involves two different mating types. One mating type is called “+” (plus) and the other “–” (minus). Hyphae of opposite mating types fuse together, bringing + and – nuclei together in the same cell.

Reproduction The + and – nuclei form pairs that divide as the mycelium grows. Many of the paired nuclei fuse to form diploid zygotes within a zygospore.

Reproduction The zygospore germinates and a sporangium emerges.
The sporangium reproduces asexually, releasing haploid spores produced by meiosis. Each spore has a different combination of parental genes, and each can make a new mycelium.

Diversity of Fungi More than 100,000 species of fungi are known. Biologists have placed fungi into several distinct groups. The major groups of fungi differ from one another in their reproductive structures.

Diversity of Fungi

The Ecology of Fungi How do fungi affect homeostasis in other organisms and the environment?

The Ecology of Fungi How do fungi affect homeostasis in other organisms and the environment? Fungi are champions of decomposition. Many species help ecosystems maintain homeostasis by breaking down dead organisms and recycling essential elements and nutrients.

The Ecology of Fungi How do fungi affect homeostasis in other organisms and the environment? Parasitic fungi can cause serious diseases in plants and animals by disrupting homeostasis. Some fungi form mutualistic associations with photosynthetic organisms in which both partners benefit.

Decomposition Many fungi feed by releasing digestive enzymes that break down leaves, fruit, and other organic material into simple molecules. These molecules then diffuse into the fungus. Many organisms remove important trace elements and nutrients from the soil. Fungi recycle these essential elements and nutrients. If these materials were not returned, the soil would quickly be depleted.

Parasitism Parasitic fungi can cause serious diseases in plants and animals by disrupting homeostasis.

Plant Diseases A number of parasitic fungi cause diseases that threaten food crops. Corn smut, for example, destroys corn kernels. Some mildews, which infect a wide variety of plants, are also fungi.

Animal Diseases Fungal diseases also affect insects, frogs, and mammals. For example, the Cordyceps fungus infects grasshoppers in rain forests in Costa Rica. Microscopic spores become lodged in the grasshopper, where they germinate and produce enzymes that slowly penetrate the insect’s external skeleton. The spores multiply in the insect’s body, digesting all its cells and tissues until the insect dies. Hyphae develop, cloaking the decaying exoskeleton in a web of fungal material. Reproductive structures, which will produce more spores and spread the infection, then emerge from the grasshopper’s remains.

Animal Diseases Parasitic fungi can also infect humans.
The fungus that causes athlete’s foot forms a mycelium in the outer layers of the skin, which produces a red, inflamed sore from which the spores can easily spread from person to person. The yeast Candida albicans is often responsible for vaginal yeast infections and for infections of the mouth called thrush.

Lichens A lichen is a symbiotic association between a fungus and a photosynthetic organism. The photosynthetic organism is either a green alga or a cyanobacterium, or both. The protective upper surface of a lichen is made up of densely packed fungal hyphae. Below this are layers of green algae or cyanobacteria and loosely woven hyphae. The bottom layer contains small projections that attach the lichen to a rock or tree.

Lichens Lichens are extremely resistant to drought and cold. Therefore, they can grow in places where few other organisms can survive—on dry bare rock in deserts and on the tops of mountains. Lichens are able to survive in these harsh environments because the green algae or cyanobacteria carry out photosynthesis, providing the fungus with a source of energy, while the fungus provides the green algae or cyanobacteria with water and minerals. The densely packed hyphae protect the delicate green cells from intense sunlight.

Lichens Lichens are often the first organisms to enter barren environments, gradually breaking down the rocks on which they grow. In this way, lichens help in the early stages of soil formation. Lichens are also remarkably sensitive to air pollution: They are among the first organisms to be affected when air quality deteriorates.

Mycorrhizae Fungi also form mutualistic relationships with plant roots. These symbiotic associations of plant roots and fungi are called mycorrhizae. Researchers estimate that 80 to 90 percent of all plant species form mycorrhizae with fungi. The hyphae collect water and minerals and bring them to the roots, greatly increasing the effective surface area of the root system. In addition, the fungi release enzymes that free nutrients in the soil. The plants, in turn, provide the fungi with the products of photosynthesis.

Mycorrhizae The presence of mycorrhizae is essential for the growth of many plants. The seeds of orchids, for example, cannot germinate in the absence of mycorrhizal fungi. Many trees are unable to survive without fungal symbionts.

Mycorrhizae This graph illustrates the growth rates of three species of trees with and without mycorrhizae.

Mycorrhizae The roots of plants are plugged into mycorrhizal networks that connect many plants. In an experiment using isotopes to trace the movement of carbon, ecologist Suzanne Simard found that mycorrhizal fungi transferred carbon from paper birch trees growing in the sun to Douglas fir trees growing in the shade nearby. As a result, the sun-starved fir trees thrived, basically by being “fed” carbon from the birches. Simard’s findings suggest that plants—and their associated fungi—may be evolving as part of an ecological partnership.

21.1 Protist Classification —The Saga Continues

Similar presentations

Presentation on theme: "21.1 Protist Classification —The Saga Continues"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

21.1 Protist Classification —The Saga Continues

Similar presentations

Presentation on theme: "21.1 Protist Classification —The Saga Continues"— Presentation transcript:

Similar presentations

About project

Feedback