Unit Four. The Evolution and Diversity of Life
17. Protists: Advent of the Eukaryotes
17.3. General Biology of Protists, the Most Ancient Eukaryotes
Protists are the most ancient eukaryotes and are united on the basis of a single negative characteristic: They are not fungi, plants, or animals. In all other respects, they are highly variable with no uniting features. Many are unicellular, like the Vorticella you see in figure 17.5 with its contractible stalk, but there are numerous colonial and multicellular groups. Most are microscopic, but some are as large as trees. We will start our discussion of the protists with an overview of some of their important features.
Figure 17.5. A unicellular protist.
The protist kingdom is a catch-all kingdom for many different groups of unicellular organisms, such as this Vorticella (phylum Ciliophora), which is heterotrophic, feeds on bacteria, and has a contractible stalk.
The Cell Surface
Protists possess varied types of cell surfaces. All protists have plasma membranes. But some protists, like algae and molds, are additionally encased within strong cell walls. Still others, like diatoms and radiolarians, secrete glassy shells of silica.
Movement in protists is also accomplished by diverse mechanisms. Protists move by cilia, flagella, pseudopods, or gliding mechanisms. Many protists wave one or more flagella to propel themselves through the water, whereas others use banks of short, flagella-like structures called cilia to create water currents for their feeding or propulsion. Pseudopodia are the chief means of locomotion among amoebas, whose pseudopods are large, blunt extensions of the cell body called lobopodia. Other related protists extend thin, branching protrusions called filopodia. Still other protists extend long, thin pseudopodia called axopodia supported by axial rods of microtubules. Axopodia can be extended or retracted. Because the tips can adhere to adjacent surfaces, the cell can move by a rolling motion, shortening the axopodia in front and extending those in the rear.
Many protists with delicate surfaces are successful in quite harsh habitats. How do they manage to survive so well? They survive inhospitable conditions by forming cysts. A cyst is a dormant form of a cell with a resistant outer covering in which cell metabolism is more or less completely shut down. Amoebic parasites in vertebrates, for example, form cysts that are quite resistant to gastric acidity (although they will not tolerate desiccation or high temperature).
Protists employ every form of nutritional acquisition except chemoautotrophy, which has so far been observed only in prokaryotes. Some protists are photosynthetic autotrophs and are called phototrophs. Others are heterotrophs that obtain energy from organic molecules synthesized by other organisms. Among heterotrophic protists, those that ingest visible particles of food are called phagotrophs, or holozoic feeders. Those ingesting food in soluble form are called osmotrophs, or saprozoic feeders.
Phagotrophs ingest food particles into intracellular vesicles called food vacuoles, or phagosomes. Lysosomes fuse with the food vacuoles, introducing enzymes that digest the food particles within. As the digested molecules are absorbed across the vacuolar membrane, the food vacuole becomes progressively smaller.
Protists typically reproduce asexually, most reproducing sexually only in times of stress. Asexual reproduction involves mitosis, but the process is often somewhat different from the mitosis that occurs in multicellular animals. The nuclear membrane, for example, often persists throughout mitosis, with the microtubular spindle forming within it. In some groups, asexual reproduction involves spore formation, in others fission. The most common type of fission is binary, in which a cell simply splits into nearly equal halves. When the progeny cell is considerably smaller than its parent, and then grows to adult size, the fission is called budding. In multiple fission, or schizogony, common among some protists, fission is preceded by several nuclear divisions, so that fission produces several individuals almost simultaneously.
Sexual reproduction also takes place in many forms among the protists. In ciliates, gametic meiosis occurs just before gamete formation, as it does in most animals. In the sporozoans, zygotic meiosis occurs directly after fertilization, and all the individuals that are produced are haploid until the next zygote is formed. In algae, there is sporic meiosis, producing an alternation of generations similar to that seen in plants, with significant portions of the life cycle spent as haploid as well as diploid.
A single cell has limits. It can only be so big without encountering serious surface-to-volume problems. Said simply, as a cell becomes larger, there is too little surface area for so much volume. The evolution of multicellular individuals composed of many cells solved this problem. Multicellularity is a condition in which an organism is composed of many cells, permanently associated with one another, that integrate their activities. The key advantage of multicellularity is that it allows specialization—distinct types of cells, tissues, and organs can be differentiated within an individual’s body, each with a different function. With such functional “division of labor” within its body, a multicellular organism can possess cells devoted specifically to protecting the body, others to moving it about, still others to seeking mates and prey, and yet others to carry on a host of other activities. This allows the organism to function on a scale and with a complexity that would have been impossible for its unicellular ancestors. In just this way, a small city of 50,000 inhabitants is vastly more complex and capable than a crowd of 50,000 people in a football stadium— each city dweller is specialized in a particular activity that is interrelated to everyone else’s, rather than just being another body in a crowd.
Colonies. A colonial organism is a collection of cells that are permanently associated but in which little or no integration of cell activities occurs. Many protists form colonial assemblies, consisting of many cells with little differentiation or integration. In some protists, the distinction between colonial and multicellular is blurred. For example, in the green algae Volvox shown in figure 17.6, individual motile cells aggregate into a hollow ball of cells that moves by a coordinated beating of the flagella of the individual cells—like scores of rowers all pulling their oars in concert. A few cells near the rear of the moving colony are reproductive cells, but most are relatively undifferentiated.
Figure 17.6. A colonial protist.
Individual, motile, unicellular green algae are united in the protist Volvox as a hollow colony of cells that moves by the beating of the flagella of its individual cells. Some species of Volvox have cytoplasmic connections between the cells that help coordinate colony activities. The Volvox colony is a highly complex form that has many of the properties of multicellular life.
Aggregates. An aggregation is a more transient collection of cells that come together for a period of time and then separate. Cellular slime molds, for example, are unicellular organisms that spend most of their lives moving about and feeding as single-celled amoebas. They are common in damp soil and on rotting logs, where they move around and ingest bacteria and other small organisms. When the individual amoebas exhaust the supply of bacteria in a given area and are near starvation, all of the individual organisms in that immediate area aggregate into a large moving mass of cells called a slug. By moving to a different location, the aggregation increases the chance that food will be found.
Multicellular Individuals. True multicellularity, in which the activities of the individual cells are coordinated and the cells themselves are in contact, occurs only in eukaryotes and is one of their major characteristics. Three groups of protists have independently attained true but simple multicellularity—the brown algae (phylum Phaeophyta), green algae (phylum Chlorophyta), and red algae (phylum Rhodophyta). In multicellular organisms, individuals are composed of many cells that interact with one another and coordinate their activities.
Simple multicellularity does not imply small size or limited adaptability. Some marine algae grow to be enormous. An individual kelp, one of the brown algae, may grow to tens of meters in length—some taller than a redwood! Red algae grow at great depths in the sea, far below where kelp or other algae are found. But not all algae are multicellular. Green algae, for example, include many kinds of multicellular organisms but an even larger number of unicellular ones.
Key Learning Outcome 17.3. Protists exhibit a wide range of forms, locomotion, nutrition, and reproduction. Their cells form clusters with varying degrees of specialization, from transient aggregations to more persistent colonies to permanently multicellular organisms.