Photosynthetic protists photoautotrophs are characterized by the presence of chloroplasts. Other protists are heterotrophs 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 2].
This vesicle then fuses with a lysosome, 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 through exocytosis. Some heterotrophs absorb nutrients from dead organisms or their organic wastes, and others are able to use photosynthesis or feed on organic matter, depending on conditions. Protists reproduce by a variety of mechanisms. Most are capable some form of asexual reproduction, such as binary fission to produce two daughter cells, or multiple fission to divide simultaneously 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 that may be better suited to surviving in the new environment.
However, sexual reproduction is also often associated with cysts that are a protective, resting stage. Depending on their habitat, the cysts may be particularly resistant to temperature extremes, desiccation, or low pH. With the advent of DNA sequencing, the relationships among protist groups and between protist groups and other eukaryotes are beginning to become clearer.
Many relationships that were based on morphological similarities are being replaced by new relationships based on genetic similarities. Protists that exhibit similar morphological features may have evolved analogous structures because of similar selective pressures—rather than because of recent common ancestry. This phenomenon is called convergent evolution.
It is one reason why protist classification is so challenging. The supergroups are believed to be monophyletic; all organisms within each supergroup are believed to have evolved from a single common ancestor, and thus all members are most closely related to each other than to organisms outside that group. There is still evidence lacking for the monophyly of some groups.
Many protists are pathogenic parasites that must infect other organisms to survive and propagate. Protist parasites include the causative agents of malaria, African sleeping sickness, and waterborne gastroenteritis in humans.
Other protist pathogens prey on plants, effecting massive destruction of food crops. Members of the genus Plasmodium must infect a mosquito and a vertebrate to complete their life cycle. In vertebrates, the parasite develops in liver cells and goes on to infect red blood cells, bursting from and destroying the blood cells with each asexual replication cycle [Figure 4].
Of the four Plasmodium species known to infect humans, P. In , it was estimated that malaria caused between 0. During the course of malaria, P. There are over , described living species of protists. Nearly all protists exist in some type of aquatic environment, including freshwater and marine environments, damp soil, and even snow. Paramecia are a common example of aquatic protists.
Due to their abundance and ease of use as research organisms, they are often subjects of study in classrooms and laboratories. In addition to aquatic protists, several protist species are parasites that infect animals or plants and, therefore, live in their hosts.
Amoebas can be human parasites and can cause dysentery while inhabiting the small intestine. Other protist species live on dead organisms or their wastes and contribute to their decay. Approximately species of slime mold thrive on bacteria and fungi within rotting trees and other plants in forests around the world, contributing to the life cycle of these ecosystems.
Privacy Policy. Skip to main content. Search for:. Characteristics of Protists. Cell Structure, Metabolism, and Motility Protists are an incredibly diverse set of eukaryotes of various sizes, cell structures, metabolisms, and methods of motility. Learning Objectives Describe the metabolism and structure of protists, explaining the structures that provide their motility. Key Takeaways Key Points Protist cells may contain a single nucleus or many nuclei; they range in size from microscopic to thousands of meters in area.
Protists may have animal-like cell membranes, plant-like cell walls, or may be covered by a pellicle. Some protists are heterotrophs and ingest food by phagocytosis, while other types of protists are photoautotrophs and store energy via photosynthesis. Most protists are motile and generate movement with cilia, flagella, or pseudopodia. Key Terms amorphous : lacking a definite form or clear shape multinucleate : having more than one nucleus pellicle : cuticle, the hard protective outer layer of certain life forms taxis : the movement of an organism in response to a stimulus; similar to kinesis, but more direct phagocytosis : the process where a cell incorporates a particle by extending pseudopodia and drawing the particle into a vacuole of its cytoplasm phagosome : a membrane-bound vacuole within a cell containing foreign material captured by phagocytosis.
Most protists have mitochondria , the organelle which generates energy for cells to use. The exceptions are some protists that live in anoxic conditions, or environments lacking in oxygen, according to an online resource published by University of California, Los Angeles. They use an organelle called the hydrogenosome which is a greatly modified version of mitochondria for some of their energy production.
For example, the sexually transmitted parasite Trichomonas vaginalis , which infects the human vagina and causes trichomoniasis , contains hydrogenosomes. Protists gain nutrition in a number of ways. According to Simpson, protists can be photosynthetic or heterotrophs organisms that seek outside sources of food in the form of organic material. In turn, heterotrophic protists fall into two categories: phagotrophs and osmotrophs.
Phagotrophs use their cell body to surround and swallow up food, often other cells, while osmotrophs absorb nutrients from the surrounding environment. They have their own plastids, but will also happily eat other organisms. Most protists reproduce primarily through asexual mechanisms according to Simpson.
This can include binary fission, where a parent cell splits into two identical cells or multiple fission, where the parent cell gives rise to multiple identical cells. Simpson added that most protists probably also have some kind of sexual cycle, however, this is only well documented in some groups. The classification history of protists traces our understanding of these diverse organisms.
Often complex, the long history of protist classification introduced two terms, still used today, into the scientific lexicon: protozoa and protists. However, the meaning of these terms has also evolved over time. The observable living world was once neatly divided between plants and animals. But the discovery of various microscopic organisms including what we now know as protists and bacteria brought forth the need to understand what they were, and where they fit taxonomically.
The first instinct of scientists was to relate these organisms to plants and animals by relying on morphological characteristics. The term protozoan plural: protozoa or protozoans , meaning "early animals," was introduced in by naturalist Georg A. Goldfuss, according to a article published in the journal International Microbiology. These traditional subdivisions, which were largely based on non-scientific characteristics, have been replaced by classifications based on phylogenetics evolutionary relatedness among organisms.
However, the older terms are still used as informal names to describe the general characteristics of various protists. Protists range from single-celled amoebas to multicellular seaweed. Protists may be similar to animals, plants, or fungi. Prokaryote or eukaryote? Kingdom Protista Protists are a group of all the eukaryotes that are not fungi, animals, or plants.
Summary Kingdom Protista includes all eukaryotes that are not animals, plants, or fungi. Kingdom Protista is very diverse.
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