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8.3: Protist Characteristics - Biology

8.3: Protist Characteristics - Biology



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Sexual or asexual reproduction for protists?

Notice how the Paramecium is dividing into two cells. This, obviously, is a form of asexual reproduction. But, remember that protists are an extremely diverse kingdom, and some protists can also reproduce sexually.

Characteristics of Protists

Like all other eukaryotes, protists have a nucleus containing their DNA. They also have other membrane-bound organelles, such as mitochondria and the endoplasmic reticulum. Most protists are single-celled. Some are multicellular. Because the protist kingdom is so diverse, their ways of getting food and reproducing vary widely.

Protist Habitats

Most protists are aquatic organisms. They need a moist environment to survive. They are found mainly in damp soil, marshes, puddles, lakes, and the ocean. Some protists are free-living organisms. Others are involved in symbiotic relationships. They live in or on other organisms, including humans.

Motility of Protists

Most protists have motility. This is the ability to move. Protists have three types of appendages for movement. As shown in Figure below, they may have flagella, cilia, or pseudopods (“false feet”). There may be one or more whip-like flagella. Cilia are similar to flagella, except they are shorter and there are more of them. They may completely cover the surface of the protist cell. Pseudopods are temporary, foot-like extensions of the cytoplasm.

Protists use cilia, pseudopods, or flagella to move.

Protist Reproduction

Protists have complex life cycles. Many have both asexual and sexual reproduction. An example is a protist called Spirogyra, a type of algae, shown Figure below. It usually exists as haploid cells that reproduce by binary fission. In a stressful environment, such as one that is very dry, Spirogyra may produce tough spores that can withstand harsh conditions. Spores are reproductive cells produced by protists and various other organisms. If two protist spores are close together, they can fuse to form a diploid zygote. This is a type of sexual reproduction. The zygote then undergoes meiosis, producing haploid cells that repeat the cycle.

Spirogyra is a genus of algae with a complex life cycle. Each organism consists of rectangular cells connected end-to-end in long filaments.

Protist Nutrition

Protists get food in one of three ways. They may ingest, absorb, or make their own organic molecules.

  • Ingestive protists ingest, or engulf, bacteria and other small particles. They extend their cell wall and cell membrane around the food item, forming a food vacuole. Then enzymesdigest the food in the vacuole.
  • Absorptive protists absorb food molecules across their cell membranes. This occurs bydiffusion. These protists are important decomposers.
  • Photosynthetic protists use light energy to make food. They are major producers in aquaticecosystems.

Summary

  • Protists have nuclear membranes around their DNA. They also have other membrane-bound organelles.
  • Many protists live in aquatic habitats, and most are motile, or able to move.
  • Protists have complex life cycles that may include both sexual and asexual reproduction.
  • Protists get food through ingestion, absorption, or photosynthesis.

Review

  1. Identify three structures that protists use to move.
  2. Describe three ways that protists get food.
  3. Describe asexual and sexual reproduction in protists.

8.3: Protist Characteristics - Biology

There are over 100,000 described living species of protists, and it is unclear how many undescribed species may exist. Since many protists live as commensals or parasites in other organisms and these relationships are often species-specific, there is a huge potential for protist diversity that matches the diversity of hosts. As the catchall term for eukaryotic organisms that are not animal, plant, or fungi, it is not surprising that very few characteristics are common to all protists.

Learning Objectives

  • Describe the cell structure of protists
  • Describe the motility of protists
  • Describe the metabolic diversity of protists
  • Describe the life cycle and habitat diversity of protists

There are 3 main groups of protista, each has its own characteristics and function. These are algae, slime molds, and protozoa.

Algae – Protists that perform photosynthesis

Algae are a group of protists that comes closer to plants because they can perform photosynthesis. But they different than the plants due to lack of some structure which are leaves, roots, and stems. Algae have a wide variety of colors brown, red, and green.

Slime Molds – Similar to fungus

Slime molds are the group of protists that have some characteristics similar to fungi. These organisms are mostly found in compost and rotten wood. They rely on decaying materials for their survival and spread slowly for the search of food.

When the food scarcity increases, the cells of slime molds come closer to form a dense group. When acellular slimes molds form a dense group, they may fuse together to form a single cell that contains a large number of nuclei. On the other hand, cellular slime molds don’t merge together to form a single cell. Instead, they remain as individual cells in a dense group.

Protozoa – Similar to Animals

Protozoa are one of the major groups of protists that have some characteristics that are somewhat similar to animal cells. Protozoa are mobile, they can move freely, and can’t produce food for themselves similar to animals. Protozoa can move from one point to another by one of three methods pseudopodia, cilia, and flagella. Moreover, there are some protozoa that seize to move when they reach adulthood.

One of the examples of protozoa is an amoeba, which is a single cell organism. Amoeba is able to move with pseudopodia (false feet) and can take external sources of food. It is also considered as a shapeless organism because it changes the shape all the time and doesn’t follow a single shape.


Metabolism

Protists exhibit many forms of nutrition and may be aerobic or anaerobic. Those that store energy by photosynthesis belong to a group of photoautotrophs and are characterized by the presence of chloroplasts. Other protists are heterotrophic and consume organic materials (such as other organisms) to obtain nutrition. Amoebas and some other heterotrophic protist species ingest particles by a process called phagocytosis, in which the cell membrane engulfs a food particle and brings it inward, pinching off an intracellular membranous sac, or vesicle, called a food vacuole (Figure 1). In some protists, food vacuoles can be formed anywhere on the body surface, whereas in others, they may be restricted to the base of a specialized feeding structure. The vesicle containing the ingested particle, the phagosome, then fuses with a lysosome containing hydrolytic enzymes to produce a phagolysosome , and the food particle is broken down into small molecules that can diffuse into the cytoplasm and be used in cellular metabolism. Undigested remains ultimately are expelled from the cell via exocytosis.

Figure 1: Phagocytosis. The stages of phagocytosis include the engulfment of a food particle, the digestion of the particle using hydrolytic enzymes contained within a lysosome, and the expulsion of undigested materials from the cell.

Subtypes of heterotrophs, called saprobes, absorb nutrients from dead organisms or their organic wastes. Some protists can function as mixotrophs , obtaining nutrition by photoautotrophic or heterotrophic routes, depending on whether sunlight or organic nutrients are available.


Motility

The majority of protists are motile, but different types of protists have evolved varied modes of movement (Figure). Some protists have one or more flagella, which they rotate or whip. Others are covered in rows or tufts of tiny cilia that they coordinately beat to swim. Still others form cytoplasmic extensions called pseudopodia anywhere on the cell, anchor the pseudopodia to a substrate, and pull themselves forward. Some protists can move toward or away from a stimulus, a movement referred to as taxis. Movement toward light, termed phototaxis, is accomplished by coupling their locomotion strategy with a light-sensing organ.

Protists use various methods for transportation. (a) Paramecium waves hair-like appendages called cilia to propel itself. (b) Amoeba uses lobe-like pseudopodia to anchor itself to a solid surface and pull itself forward. (c) Euglena uses a whip-like tail called a flagellum to propel itself.


Metabolism

Figure 1: The stages of phagocytosis include the engulfment of a food particle, the digestion of the particle using hydrolytic enzymes contained within a lysosome, and the expulsion of undigested materials from the cell.

Protists exhibit many forms of nutrition and may be aerobic or anaerobic. Protists that store energy by photosynthesis belong to a group of photoautotrophs and are characterized by the presence of chloroplasts. Other protists are heterotrophic and consume organic materials (such as other organisms) to obtain nutrition. Amoebas and some other heterotrophic protist species ingest particles by a process called phagocytosis, in which the cell membrane engulfs a food particle and brings it inward, pinching off an intracellular membranous sac, or vesicle, called a food vacuole ([Figure 1]). The vesicle containing the ingested particle, the phagosome, then fuses with a lysosome containing hydrolytic enzymes to produce a phagolysosome , and the food particle is broken down into small molecules that can diffuse into the cytoplasm and be used in cellular metabolism. Undigested remains ultimately are expelled from the cell via exocytosis.

Subtypes of heterotrophs, called saprobes, absorb nutrients from dead organisms or their organic wastes. Some protists can function as mixotrophs , obtaining nutrition by photoautotrophic or heterotrophic routes, depending on whether sunlight or organic nutrients are available.


Metabolism

Protists exhibit many forms of nutrition and may be aerobic or anaerobic. Those that store energy by photosynthesis belong to a group of photoautotrophs and are characterized by the presence of chloroplasts. Other protists are heterotrophic and consume organic materials (such as other organisms) to obtain nutrition. Amoebas and some other heterotrophic protist species ingest particles by a process called phagocytosis, in which the cell membrane engulfs a food particle and brings it inward, pinching off an intracellular membranous sac, or vesicle, called a food vacuole (Figure). In some protists, food vacuoles can be formed anywhere on the body surface, whereas in others, they may be restricted to the base of a specialized feeding structure. The vesicle containing the ingested particle, the phagosome, then fuses with a lysosome containing hydrolytic enzymes to produce a phagolysosome , and the food particle is broken down into small molecules that can diffuse into the cytoplasm and be used in cellular metabolism. Undigested remains ultimately are expelled from the cell via exocytosis.

Phagocytosis. The stages of phagocytosis include the engulfment of a food particle, the digestion of the particle using hydrolytic enzymes contained within a lysosome, and the expulsion of undigested materials from the cell.

Subtypes of heterotrophs, called saprobes, absorb nutrients from dead organisms or their organic wastes. Some protists can function as mixotrophs, obtaining nutrition by photoautotrophic or heterotrophic routes, depending on whether sunlight or organic nutrients are available.


117 Characteristics of Protists

By the end of this section, you will be able to do the following:

  • Describe the cell structure characteristics of protists
  • Describe the metabolic diversity of protists
  • Describe the life cycle diversity of protists

There are over 100,000 described living species of protists, and it is unclear how many undescribed species may exist. Since many protists live as commensals or parasites in other organisms and these relationships are often species-specific, there is a huge potential for protist diversity that matches the diversity of their hosts. Because the name “protist” serves as a catchall term for eukaryotic organisms that are not animal, plant, or fungi, it is not surprising that very few characteristics are common to all protists. On the other hand, familiar characteristics of plants and animals are foreshadowed in various protists.

Cell Structure

The cells of protists are among the most elaborate of all cells. Multicellular plants, animals, and fungi are embedded among the protists in eukaryotic phylogeny. In most plants and animals and some fungi, complexity arises out of multicellularity, tissue specialization, and subsequent interaction because of these features. Although a rudimentary form of multicellularity exists among some of the organisms labelled as “protists,” those that have remained unicellular show how complexity can evolve in the absence of true multicellularity, with the differentiation of cellular morphology and function. A few protists live as colonies that behave in some ways as a group of free-living cells and in other ways as a multicellular organism. Some protists are composed of enormous, multinucleate, single cells that look like amorphous blobs of slime, or in other cases, like ferns. In some species of protists, the nuclei are different sizes and have distinct roles in protist cell function.

Single protist cells range in size from less than a micrometer to three meters in length to hectares! Protist cells may be enveloped by animal-like cell membranes or plant-like cell walls. Others are encased in glassy silica-based shells or wound with pellicles of interlocking protein strips. The pellicle functions like a flexible coat of armor, preventing the protist from being torn or pierced without compromising its range of motion.

Metabolism

Protists exhibit many forms of nutrition and may be aerobic or anaerobic. Those that store energy by photosynthesis belong to a group of photoautotrophs and are characterized by the presence of chloroplasts. Other protists are heterotrophic and consume organic materials (such as other organisms) to obtain nutrition. Amoebas and some other heterotrophic protist species ingest particles by a process called phagocytosis, in which the cell membrane engulfs a food particle and brings it inward, pinching off an intracellular membranous sac, or vesicle, called a food vacuole ((Figure)). In some protists, food vacuoles can be formed anywhere on the body surface, whereas in others, they may be restricted to the base of a specialized feeding structure. The vesicle containing the ingested particle, the phagosome, then fuses with a lysosome containing hydrolytic enzymes to produce a phagolysosome , and the food particle is broken down into small molecules that can diffuse into the cytoplasm and be used in cellular metabolism. Undigested remains ultimately are expelled from the cell via exocytosis.


Subtypes of heterotrophs, called saprobes, absorb nutrients from dead organisms or their organic wastes. Some protists can function as mixotrophs , obtaining nutrition by photoautotrophic or heterotrophic routes, depending on whether sunlight or organic nutrients are available.

Motility

The majority of protists are motile, but different types of protists have evolved varied modes of movement ((Figure)). Some protists have one or more flagella, which they rotate or whip. Others are covered in rows or tufts of tiny cilia that they beat in a coordinated manner to swim. Still others form cytoplasmic extensions called pseudopodia anywhere on the cell, anchor the pseudopodia to a substrate, and pull themselves forward. Some protists can move toward or away from a stimulus, a movement referred to as taxis. For example, movement toward light, termed phototaxis, is accomplished by coupling their locomotion strategy with a light-sensing organ.


Life Cycles

Protists reproduce by a variety of mechanisms. Most undergo some form of asexual reproduction, such as binary fission, to produce two daughter cells. In protists, binary fission can be divided into transverse or longitudinal, depending on the axis of orientation sometimes Paramecium exhibits this method. Some protists such as the true slime molds exhibit multiple fission and simultaneously divide into many daughter cells. Others produce tiny buds that go on to divide and grow to the size of the parental protist.

Sexual reproduction, involving meiosis and fertilization, is common among protists, and many protist species can switch from asexual to sexual reproduction when necessary. Sexual reproduction is often associated with periods when nutrients are depleted or environmental changes occur. Sexual reproduction may allow the protist to recombine genes and produce new variations of progeny, some of which may be better suited to surviving changes in a new or changing environment. However, sexual reproduction is often associated with resistant cysts that are a protective, resting stage. Depending on habitat of the species, the cysts may be particularly resistant to temperature extremes, desiccation, or low pH. This strategy allows certain protists to “wait out” stressors until their environment becomes more favorable for survival or until they are carried (such as by wind, water, or transport on a larger organism) to a different environment, because cysts exhibit virtually no cellular metabolism.

Protist life cycles range from simple to extremely elaborate. Certain parasitic protists have complicated life cycles and must infect different host species at different developmental stages to complete their life cycle. Some protists are unicellular in the haploid form and multicellular in the diploid form, a strategy employed by animals. Other protists have multicellular stages in both haploid and diploid forms, a strategy called alternation of generations, analogous to that used by plants.

Habitats

Nearly all protists exist in some type of aquatic environment, including freshwater and marine environments, damp soil, and even snow. Several protist species are parasites that infect animals or plants. A few protist species live on dead organisms or their wastes, and contribute to their decay.

Section Summary

Protists are extremely diverse in terms of their biological and ecological characteristics, partly because they are an artificial assemblage of phylogenetically unrelated groups. Protists display highly varied cell structures, several types of reproductive strategies, virtually every possible type of nutrition, and varied habitats. Most single-celled protists are motile, but these organisms use diverse structures for transportation.

Review Questions

Protists that have a pellicle are surrounded by ______________.

Protists with the capabilities to perform photosynthesis and to absorb nutrients from dead organisms are called ______________.

Which of these locomotor organs would likely be the shortest?

Alternation of generations describes which of the following?

  1. The haploid form can be multicellular the diploid form is unicellular.
  2. The haploid form is unicellular the diploid form can be multicellular.
  3. Both the haploid and diploid forms can be multicellular.
  4. Neither the haploid nor the diploid forms can be multicellular.

The amoeba E. histolytica is a pathogen that forms liver abscesses in infected individuals. Its metabolic classification is most likely ______.

  1. Anaerobic heterotroph
  2. Mixotroph
  3. Aerobic phototroph
  4. Phagocytic autotroph

Critical Thinking Questions

Explain in your own words why sexual reproduction can be useful if a protist’s environment changes.

The ability to perform sexual reproduction allows protists to recombine their genes and produce new variations of progeny that may be better suited to the new environment. In contrast, asexual reproduction generates progeny that are clones of the parent.

Giardia lamblia is a cyst-forming protist parasite that causes diarrhea if ingested. Given this information, against what type(s) of environments might G. lamblia cysts be particularly resistant?

As an intestinal parasite, Giardia cysts would be exposed to low pH in the stomach acids of its host. To survive this environment and reach the intestine, the cysts would have to be resistant to acidic conditions.

Explain how the definition of protists ensures that the kingdom Protista includes a wide diversity of cellular structures. Provide an example of two different structures that perform the same function for their respective protist.

Protists are defined as any eukaryotes that do not fall into the Plantae, Fungi, or Animal Kingdoms. Since the unifying characteristics describe what they are NOT, rather than what they are, Protista can include almost any cellular/organism organization.
Possible examples of structure variety:

  • Barrier to exterior world: cell wall, plasma membrane, pellicle
  • Locomotion: flagella, cilia, pseudopodia

Glossary


Cell Structure

The cells of protists are among the most elaborate of all cells. Most protists are microscopic and unicellular, but some true multicellular forms exist. A few protists live as colonies that behave in some ways as a group of free-living cells and in other ways as a multicellular organism. Still other protists are composed of enormous, multinucleate, single cells that look like amorphous blobs of slime, or in other cases, like ferns. In fact, many protist cells are multinucleated in some species, the nuclei are different sizes and have distinct roles in protist cell function.

Single protist cells range in size from less than a micrometer to three meters in length to hectares. Protist cells may be enveloped by animal-like cell membranes or plant-like cell walls. Others are encased in glassy silica-based shells or wound with pellicles of interlocking protein strips. The pellicle functions like a flexible coat of armor, preventing the protist from being torn or pierced without compromising its range of motion.


Study the Protist Kingdom Like Never Before

Unicellular protozoa and algae are unicellular eukaryotes. Pluricellular algae are also eukaryotes of simple structure. Protists are believed to be the phylogenetic ancestors of the living organisms of the other eukaryotic kingdoms (fungi, animals and plants).

More Bite-Sized Q&As Below

2. What is the fundamental difference between protozoa and algae?

The basic difference between protozoa and algae is the fact that protozoa are heterotrophs whereas algae are photosynthetic autotrophs.

Protozoa 

3. What characteristics of protozoa make them resemble animals?

Protozoa are unicellular organisms that present some characteristics in common with animal cells.

In comparison to pluricellular organisms, protozoa are closer to the animal kingdom than to plants, as they are heterotrophs, they have a rudimentary locomotion system (amoeboid movements, cilia, flagella), they do not have cell wall, and some species present structures that resemble the structures of a primitive digestive system, with a cytostome (mouth) and cytopyge (anus) specialized in digestion and excretion.

There is strong support for the hypothesis that animal cells evolved from protozoa.

4. What is the basic morphology of the cells of protozoa?

Protozoa are eukaryotic cells and, as a result, have organelles and structures common to this kind of cell: endoplasmic reticula, a Golgi apparatus, digestive vesicles, ribosomes, mitochondria, a nucleus with genetic material, karyotheca, etc. All these elements are found dispersed throughout the cytoplasm. Protozoa do not have cell walls.

Protozoa from the mastigophora group (such as trichomonas) have flagella and others from the ciliated group (like paramecium) have cilia.

5. Do protozoa have a nucleus?

All protozoa, because they are eukaryotes, have a nucleus. Some species, such as paramecium, have two nuclei: the macronucleus and micronucleus.

6. What are the respective functions of the macronucleus and of the micronucleus in  paramecium?

The macronucleus is the cell nucleus in the normal sense, it contains DNA and RNA and acts as the center of cellular control and regulation. The micronucleus has reproductive functions and is related to the conjugation process (sexual reproduction).

7. What do protozoa “eat”? Do they move in search for food?

Protozoa are heterotrophic organisms, meaning that they do not make their own food and therefore they need to search for it in the environment. Protozoa have developed several locomotion mechanisms and actively move towards food.

8. How do amoebae, paramecia and trichomonas move?

Amoebae move through amoeboid movements, which are small projections and invaginations of their plasma membrane (pseudopods) that alter the external shape of the cell, making it move along surfaces. The outer face of the plasma membrane of paramecia is covered by cilia that flap around, helping the cell to move. Trichomonas are flagellated protozoa, meaning that they have relatively long filaments outside the cell that vibrate and make it possible for them to swim in fluid environments.

9. How do protozoa digest?

Digestion in protozoa is intracellular digestion: organic material is internalized and broken down inside the cell.

Protozoa obtain food via phagocytosis. This food is digested when phagosomes fuse with lysosomes within the cell, forming digestive vacuoles. The digestive vacuoles produce residual bodies that are eliminated from the cell by exocytosis.

In paramecium, the entrance of food into the cell and the excretion of digestive waste products occur in specialized regions of the plasma membrane, called the cytostome and the cytopyge.

10. Are protozoans presenting contractile vacuoles (also called pulsatile vacuoles) more commonly found in fresh or in salt water?

Fresh water has a lower concentration of solutes than sea water and it (fresh water) tends to be less concentrated than the intracellular environment, causing cells to swell. Sea water, on the other hand, since it is very concentrated, tends to dehydrate the cell.

The vacuoles of protozoans are internal structures specialized in water storage that release water into the cytoplasm when necessary. Therefore, vacuoles can dilute the cytoplasm to put it into osmotic equilibrium with the environment. As a result, fresh water protozoa need vacuoles more, since their intracellular environment is hypertonic in relation to the exterior. Without the dilution mechanism provided by the vacuoles, fresh water protozoa would absorb too much water and would die.

11. Do protozoa use sexual or asexual reproduction?

In protozoa, reproduction can be either sexual or asexual. The most frequent form of sexual reproduction is binary division, or scissiparity, in which the cell divides via mitosis, producing two daughter cells. Some species, such as plasmodium, the agent of malaria, reproduce asexually via schizogony (multiple fission) in this form of reproduction, the cell becomes multinucleate, generally inside a host cell, and each nucleus is expelled together with a portion of the cytoplasm, producing new protozoans.

Sexual reproduction in protozoa can happen via conjugation, with the incorporation of genetic material from one cell into another, or through gametes that fertilize others and form zygotes. In plasmodium, sexual reproduction happens in the mosquito, the definitive host, where the zygote undergoes mitosis (sporogony), creating many sporozoites.

12. Which form of protozoa reproduction generates more variation?

Sexual reproduction always generates more genetic variation than asexual reproduction. That is because, in sexual reproduction, the fusion of genetic material from different specimens occurs and, as a result, the offspring is not genetically identical to the parent cell.

The hypothesis that protozoa are the origin of multicellular animals is further strengthened by the fact that these protozoa were able to reproduce sexually, since only genetic variation can produce biological differentiation to the point of creating new types of living organisms.

13. What are the four groups of protozoa?

The four main groups of protozoa are sarcodines (those that form pseudopods, such as amoebae), mastigophores (flagellated, like trypanosome which causes Chagas’ disease), ciliated (like paramecia) and sporozoans (spore-forming, like plasmodia).

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Importance

Protists are responsible for a variety of human diseases including malaria, sleeping sickness, amoebic dysentery and trichomoniasis. Malaria in humans is a devastating disease. It is caused by five species of the parasite Plasmodium, which are transmitted to humans by female Anopheles mosquitoes, according to the Centers for Disease Control and Prevention (CDC). The species Plasmodium falciparum infects red blood cells, multiplies rapidly and destroys them. Infection can also cause red blood cells to stick to the walls of small blood vessels. This creates a potentially fatal complication called cerebral malaria (according to the CDC). The World Health Organization (WHO) states that Plasmodium falciparum is the most prevalent and lethal to humans. According to their recent malaria fact sheet, in 2015 there were an estimated 438,000 deaths due to malaria in the world, the majority of which (90 percent) occurred in Africa. Certain strides have been made in reducing the rates of incidence (occurrence of new cases) and mortality rates in part by supplying insecticide treated mosquito nets, spraying for mosquitoes and improving diagnostics. Between 2000 and 2015 the rate of incidence fell by 37 percent globally and mortality rates fell by 60 percent globally. The WHO has a goal of eliminating malaria in at least 35 countries by 2030.

Protists also play an important role in the environment. According to a 2009 review article published on the Encyclopedia of Life Sciences (eLS) website, nearly 50 percent of photosynthesis on Earth is carried out by algae. Protists act as decomposers and help in recycling nutrients through ecosystems, according to a 2002 review article published in the journal ACTA Protozoologica. In addition, protists in various aquatic environments, including the open water, waterworks and sewage disposal systems feed upon, and control bacterial populations (ACTA Protozoologica, 2002). "If you took all the protists out of the world, the ecosystem would collapse really quickly," Simpson said.


Watch the video: Bacteria Updated (August 2022).