Protozoa can obtain a variety of shapes due to the absence of a cell wall. The free-living vegetative form of protozoa is called the atropohozoite whereas the resting form is called a cyst. The cyst of the protozoa is analogous to the spore of bacteria. Since protozoa are eukaryotic organisms, they contain a membrane-bound nucleus.
Some protozoa may contain more than one micronuclei. The excess water in the cell is removed by the contractile vacuoles. Food vacuoles can also be identified in protozoa. Amoeba , ciliates, flagellates, and sporozoans are examples of protozoa.
A flagellate is shown in figure 1. Protozoa are heterotrophic animals and obtain food by phagocytosis. The digestion occurs inside a food vacuole by the lysosomal enzymes. Flagella , cilia , and pseudopodia are used for the locomotion by protozoans. Algae refer to a small, non-flowering, aquatic plant, which contains chlorophyll but lacks a true stem, root, leaves and a vascular system. However, both large and tiny creatures play an important role in balancing the ecosystem.
Two among them are protozoa and algae. Although, these two sets of organisms can be differentiated easily based on their structure and looks. In this article, we will study how algae differ from protozoa in detail. The difference between algae and protozoa is that algae are non-flowering, small, aquatic plants having a green pigment called chlorophyll in them.
They lack root, true stem, vascular system and leaves. In contrast, protozoa are prokaryotic organisms belonging to the Protista kingdom. The cell wall of algae is made up of a substance called cellulose. In contrast, the cell wall is absent in protozoa. Algae contain a green coloured pigment in them, called chlorophyll. Spore is the resting unit of algae. Some examples of algae are cyanobacteria, brown algae, Seaweeds, red algae, green algae. They also have specialized cellular machinery called organelles that execute defined functions within the cell.
Photosynthetic protists such as the various types of algae contain plastids. These organelles serve as the site of photosynthesis the process of harvesting sunlight to produce nutrients in the form of carbohydrates. The plastids of some protists are similar to those of plants. According to Simpson, others protists have plastids that differ in the color, the repertoire of photosynthetic pigments and even the number of membranes that enclose the organelle, as in the case of diatoms and dinoflagellates , which constitute phytoplankton in the ocean.
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. In fact, many can live in water with very low concentrations of oxygen. Some ciliates have specially adapted green algae living inside them. In higher light conditions, these algae convert the carbon dioxide produced by the ciliate into oxygen, ensuring an abundant internal supply of oxygen for the ciliate.
On the flip side, a few groups are anaerobic and intolerant of oxygenated water. These organisms are often endosymbionts living in the digestive system of multi-celled animals. Protists use contractile vacuoles to remove excess water from their cells. If the contractile function of a cell is compromised, the cell swells until it ruptures. The same will also happen to a marine protozoan when placed in fresh water; marine members have no contractile vacuoles.
Ciliates have permanent contractile vacuole pathways and pores where amoebas will release them from any point along the surface of its body. Many protozoans reproduce both asexually and sexually during their lifetime. The move to sex is often either controlled by an internal clock or by the arrival of harsh environmental conditions.
The majority of protozoans reproduce asexually by binary fission. However, some are endosymbionts species that live within another organism that often engage in multiple fission with many tiny cells produced from a single parent cell released to search out a new host. Sexual reproduction is common in ciliates, but rare in heliozoans and amoebas, and absent in flagellates. The three basic types of sex are gametogamy, autogamy, and conjugation—all of which are explained on the reproduction strategies page.
Ciliates reproduce sexually through conjugation, which involves the exchange of haploid nuclei between two joined protists. Once the genetic information is exchanged, each of the ex-conjugants clones itself. These resulting daughter cells go through a long period of "sexual immaturity," during which they will only reproduce asexually. Flagellates employ their flagella for both swimming and acquiring food.
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