Start

| After a mosquito ingests blood, male and female gametocytes develop into gametes, which fuse.

The resulting zygote, the only diploid stage in the life cycle, enters the mosquito's gut wall and forms a cyst.

| After a mosquito ingests blood, male and female gametocytes develop into gametes, which fuse.

The resulting zygote, the only diploid stage in the life cycle, enters the mosquito's gut wall and forms a cyst.

Multiplication Sporozo Dans Anophele

|3| Within the cyst, the zygote gives rise to sporozoites.

5) The mosquito injects sporozoites into a human's blood when it feeds.

6) Sporozoites penetrate liver cells and develop into merozoites.

|3| Within the cyst, the zygote gives rise to sporozoites.

A feeding mosquito ingests the gameto-cytes and the cycle begins again.

10 Eventually, some merozoites develop into male and female gametocytes.

5) The mosquito injects sporozoites into a human's blood when it feeds.

A feeding mosquito ingests the gameto-cytes and the cycle begins again.

10 Eventually, some merozoites develop into male and female gametocytes.

Macronuclear Merozoites

6) Sporozoites penetrate liver cells and develop into merozoites.

hour cycles of invasion, lysis, and reinvasion cause the characteristic fevers and chills of the host victim.

Merozoites can reinfect the liver, producing new generations.

Merozoites can reinfect the liver, producing new generations.

They also invade red blood cells, grow and divide, and lyse the cells.

They also invade red blood cells, grow and divide, and lyse the cells.

hour cycles of invasion, lysis, and reinvasion cause the characteristic fevers and chills of the host victim.

tory system when an infected Anopheles mosquito penetrates the human skin in search of blood. The parasites find their way to cells in the liver and the lymphatic system, change their form, multiply, and reenter the bloodstream, attacking red blood cells. The apical complex enables Plasmodium to enter human liver and red blood cells.

The parasites multiply inside red blood cells, which then burst, releasing new swarms of parasites. If another Anopheles bites the victim, the mosquito takes in Plasmodium cells along with blood. Some of these cells develop into gametes, which unite to form zygotes that lodge in the mosquito's gut, divide several times, and move into its salivary glands, from which they can be passed on to another human host. Thus, Plasmodium is an extracellular parasite in the mosquito vector and an intracellular parasite in the human host.

Plasmodium has proved to be a singularly difficult pathogen to attack. The Plasmodium life cycle is best broken by the removal of stagnant water, in which mosquitoes breed. The use of insecticides to reduce the Anopheles population can be effective, but their benefits must be weighed against the possible ecological, economic, and health risks posed by the insecticides themselves.

New hope arises from the publication, in 2002, of the genomes of both Plasmodium falciparum and one of its vectors,

Anopheles gambiae, as well as a partial proteome of P. falciparum. These advances should lead to a better understanding of the biology of malaria and to the possible development of drugs, vaccines, or other means of dealing with this pro-tist pathogen or its insect vectors.

Ciliates have two types of nuclei

The ciliates are so named because they characteristically have hairlike cilia. This group is noteworthy for its diversity and ecological importance (Figure 28.15). Almost all ciliates are heterotrophic (a few contain photosynthetic endosym-bionts), and they are much more complex in body form than are most flagellates and other unicellular protists.

The definitive characteristic of ciliates is the possession of two types of nuclei, commonly a single macronucleus and, within the same cell, from one to several micronuclei. The mi-cronuclei, which are typical eukaryotic nuclei, are essential for genetic recombination. The macronucleus is derived from micronuclei. Each macronucleus contains many copies of the genetic information, packaged in units containing very few genes each. The macronuclear DNA is transcribed and translated to regulate the life of the cell. Although we do not know how this system of macro- and micronuclei came into being,

Binary Fission Paramecium

28.15 Diversity among the Ciliates (a) A free-swimming organism, this paramecium belongs to a ciliate group whose members have many cilia of uniform length. (b) Members of this subgroup have cilia on their mouthparts. (c) In this group, tentacles replace cilia as development proceeds. (d) This ciliate "walks"on fused cilia,called cirri, that project from its body. Other cilia are fused into flat sheets that sweep food particles into the oral cavity; this individual has ingested green algae.

(d) Euplotes sp

28.15 Diversity among the Ciliates (a) A free-swimming organism, this paramecium belongs to a ciliate group whose members have many cilia of uniform length. (b) Members of this subgroup have cilia on their mouthparts. (c) In this group, tentacles replace cilia as development proceeds. (d) This ciliate "walks"on fused cilia,called cirri, that project from its body. Other cilia are fused into flat sheets that sweep food particles into the oral cavity; this individual has ingested green algae.

(d) Euplotes sp

The complex cell surface structure is called the pellicle.

Contractile vacuole

The macronucleus controls the cell's activities.

The complex cell surface structure is called the pellicle.

Contractile vacuole

The macronucleus controls the cell's activities.

Paramecium Diagram

Anal pore

28.16 Anatomy of Paramecium This diagram shows the major structures of a typical paramecium.

Anal pore

28.16 Anatomy of Paramecium This diagram shows the major structures of a typical paramecium.

we do know something about the behavior of these nuclei, which we will discuss after describing the body plan of one important ciliate, Paramecium.

A closer look at one ciliate. Paramecium, a frequently studied ciliate genus, exemplifies the complex structure and behavior of ciliates (Figure 28.16). The slipper-shaped cell is covered by an elaborate pellicle, a structure composed principally of an outer membrane and an inner layer of closely packed, membrane-enclosed sacs (the alveoli) that surround the bases of the cilia. Defensive organelles called trichocysts are also present in the pellicle. In response to a threat, a microscopic explosion expels the trichocysts in a few milliseconds, and they emerge as sharp darts, driven forward at the tip of a long, expanding filament.

The cilia provide a form of locomotion that is generally more precise than locomotion by flagella or pseudopods. A paramecium can direct the beating of its cilia to propel itself

Macronucleus either forward or backward in a spiraling manner. It can also back off swiftly when it encounters a barrier or a negative stimulus. The coordination of ciliary beating is probably the result of a differential distribution of ion channels in the plasma membrane near the two ends of the cell.

REPRODUCTION WITHOUT SEX, AND SEX WITHOUT REPRODUCTION.

Paramecia reproduce asexually by binary fission. The micronuclei divide mitotically. The macronuclei divide by a still unknown mechanism following a round of DNA replication.

Paramecia also have an elaborate sexual behavior called conjugation, in which two paramecia line up tightly against each other and fuse in the oral region of the body. Nuclear material is extensively reorganized and exchanged over the next several hours (Figure 28.17). As a result of this process, each cell ends up with two haploid micronuclei, one of its own and one from the other cell, which fuse to form a new diploid micronucleus. New macronuclei develop from the micronuclei through a series of dramatic chromosomal rearrangements. The exchange of nuclei is fully reciprocal— each of the two paramecia gives and receives an equal amount of DNA. The two organisms then separate and go their own ways, each equipped with new combinations of alleles.

Conjugation in Paramecium is a sexual process of genetic recombination, but it is not a reproductive process. The same two cells that begin the process are there at the end, and no new cells are created. As a rule, each asexual clone of paramecia must periodically conjugate. Experiments have shown that if some species are not permitted to conjugate, the clones can live through no more than approximately 350 cell divisions before they die out.

28.17 Paramecia Achieve Genetic Recombination by Conjugating

The exchange of micronuclei by conjugating Paramecium individuals permits genetic recombination. After conjugation, the cells separate and continue their lives as two individuals.

Macronucleus

28.17 Paramecia Achieve Genetic Recombination by Conjugating

The exchange of micronuclei by conjugating Paramecium individuals permits genetic recombination. After conjugation, the cells separate and continue their lives as two individuals.

Conjugation Paramecium
ij Two paramecia conjugate; all but one micronucleus in each cell disintegrate. The remaining micronucleus undergoes meiosis.
Conjugation Paramecia Steps
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • Christine
    What is a typical paramecium?
    7 years ago
  • tina watkins
    Why is periodic conjugation necessary in some paramecium?
    7 years ago

Post a comment