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 parasite is an organism that lives on or in another organism and feeds on it, often without killing it. A few protist species live on dead organisms or their wastes, and contribute to their decay. 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 the 3-meter lengths of the multinucleate cells of the seaweed Caulerpa. 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.
The majority of protists are motile, but different types of protists have evolved varied modes of movement. 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 coordination to swim. Still others send out lobe-like pseudopodia from anywhere on the cell, anchor the pseudopodium to a substrate, and pull the rest of the cell toward the anchor point.
Some protists can move toward light by coupling their locomotion strategy with a light-sensing organ. Protists exhibit many forms of nutrition and may be aerobic or anaerobic. 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. Can grow up to meters underwater. Grow to about a maximum of 1 meter. Brown Algae. The algae that live in colder climates are mostly brown algae. They attach to rocks or form big floating beds in the ocean.
Have chlorophyll and yellowish- brownish pigment. Are very large and can grow up to 60 meters in one season. Since only the top gets sunlight, they make food chlorophyll and send it to the roots. Green Algae. They are green because the chlorophyll is the main pigment they have. Live in water and moist soil, but can be found in melting snow and inside other organisms.
Some are singlecellular and some are multicellular. Multicelled grows up to 8 meters. Some singlecelled live in colonies.
Live in salt and fresh water and sometimes hold on to plants, shellfish, sea turtles, and whales. Use photosynthesis. Are a large portion of phytoplankton.
Cell walls have silica and cellulose. The cells are inside a shell that fits together almost perfectly. Piles of these shells over millions of years form a substance that is in toothpaste, silver polish, filters, insulation, and more. Most are single cell. Live mostly in salt water, but some are found in fresh water and snow. Have 2 flagella that make the protists spin. Most use photosynthesis, but some get food as consumers, decomposers, or parasites.
They are sometimes red and produce a strong poison. When they produce very fast, they cause red tide, where the water turns red and the shellfish eat the algae, making the shellfish poisonous to all vertebrates. Single cell. Live mostly in fresh water. On land, terrestrial plants serve as primary producers. Protists do not only create food sources for sea-dwelling organisms. Certain anaerobic parabasalid species exist in the digestive tracts of termites and wood-eating cockroaches where they contribute an essential step in the digestion of cellulose ingested by these insects as they bore through wood.
Learning Objectives Give examples of how protists act as primary producers. Key Points Photosynthetic protists serve as producers of nutrition for other organisms. Protists like zooxanthellae have a symbiotic relationship with coral reefs; the protists act as a food source for coral and the coral provides shelter and compounds for photosynthesis for the protists.
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