Our environment is full of microorganisms. These organisms are not visible to our naked eyes but can be well studied under the microscope. Amoeba is one such microorganism belonging to the phylum Protozoa. The most commonly studies species is the Amoeba proteus. Amoeba was first discovered by Rosenhof in 1757. Earlier naturalists classified this organism under Protista as the animal is able to change its shape. This animal has derived its name from a Greek word meaning change. The name was given by Bory de Saint-Vincent. In 1918 it was found that this animal may be harmful to humans also. The cell organelles as well as the cytoplasm are enclosed within the well developed cell membrane and food is engulfed by phagocytosis. The anterior end is characterized by the presence of a single large pseudopod and small secondary ones are present along the sides. The body length may vary from 220-740 µm with the pseudopodia stretched. Few species can attain much larger sizes also.
The most recognizable feature of these protozoans is the presence of one or more nuclei and a simple contractile vacuole to maintain osmotic equilibrium. The food engulfed by phagocytosis is stored and digested inside the contractile vacuole. They like other eukaryotes reproduce asexually by mitosis and cytokinesis which may not be confused with the binary fission occurring in the bacteria. If they are allowed to divide forcefully then the portion which contains the nuclei divides and will produce a new cell along with cytoplasm and the other portion will eventually die. They lack a definite shape. They bear a remarkable and large genome. The genome of Amoeba proteus is known to contain about 290 billion base pairs much larger as compared to the human genome.
The contractile vacuole is a membrane bound organelle. It is filled up with water from the cytoplasm and then fuses with the cell membrane and releases its content to outside by exocytosis. By this process the amount of water is regulated within the body of this protozoan and the whole phenomenon is called as osmoregulation. As the contents of the contractile vacuole are expelled to outside its membrane crumples and several small vesicles or vacuoles appear around it. It is believed that these vesicles are formed by the membrane of the contractile vacuole itself. These small vesicles also take in water and increase in size and then combine together to join with the contractile vacuole and then expel water to outside. So it can be suggested that the function of these vesicles is to collect excess water and the passing it to the contractile vacuole. The whole cycle is repeated again and again. The membrane bounding the contractile vacuole and the vesicles contain aquaporin proteins. These proteins facilitate water passage. The vesicles participate in rapid water passage by their larger surface area.
The membrane of the vesicle also contains another type of proteins known as Vacuolar-type H+-ATPase or V-ATPase. These proteins pump hydrogen ions into the lumen of the vesicle and lower the pH as compared to that of the cytosol. The pH inside the contractile vacuole is mildly acidic which focuses that hydrogen ions are being removed from them. The electrochemical gradient developed by V-ATPase proteins help in the transport of ions into the vesicles. An osmotic gradient is developed because of this electrochemical gradient and this is facilitated by the aquaporins. The removal of ions during osmoregulation is compensated by an unidentified mechanism. Amoebae are adversely affected by the osmotic pressure caused by extremely saline or dilute water. When the conditions surrounding them become unfavourable they become converted into microbial cysts. These cysts are covered by well protected membranes. When conditions become favourable the cyst wall ruptures and they become active again. They do not divide in the cyst stage and may die if fail to change into active stage again. The marine amoebae remain unaffected by the changes in salinity of water although they are microscopic but some may be as large as grapes.
Navodita Maurice
The most recognizable feature of these protozoans is the presence of one or more nuclei and a simple contractile vacuole to maintain osmotic equilibrium. The food engulfed by phagocytosis is stored and digested inside the contractile vacuole. They like other eukaryotes reproduce asexually by mitosis and cytokinesis which may not be confused with the binary fission occurring in the bacteria. If they are allowed to divide forcefully then the portion which contains the nuclei divides and will produce a new cell along with cytoplasm and the other portion will eventually die. They lack a definite shape. They bear a remarkable and large genome. The genome of Amoeba proteus is known to contain about 290 billion base pairs much larger as compared to the human genome.
The contractile vacuole is a membrane bound organelle. It is filled up with water from the cytoplasm and then fuses with the cell membrane and releases its content to outside by exocytosis. By this process the amount of water is regulated within the body of this protozoan and the whole phenomenon is called as osmoregulation. As the contents of the contractile vacuole are expelled to outside its membrane crumples and several small vesicles or vacuoles appear around it. It is believed that these vesicles are formed by the membrane of the contractile vacuole itself. These small vesicles also take in water and increase in size and then combine together to join with the contractile vacuole and then expel water to outside. So it can be suggested that the function of these vesicles is to collect excess water and the passing it to the contractile vacuole. The whole cycle is repeated again and again. The membrane bounding the contractile vacuole and the vesicles contain aquaporin proteins. These proteins facilitate water passage. The vesicles participate in rapid water passage by their larger surface area.
The membrane of the vesicle also contains another type of proteins known as Vacuolar-type H+-ATPase or V-ATPase. These proteins pump hydrogen ions into the lumen of the vesicle and lower the pH as compared to that of the cytosol. The pH inside the contractile vacuole is mildly acidic which focuses that hydrogen ions are being removed from them. The electrochemical gradient developed by V-ATPase proteins help in the transport of ions into the vesicles. An osmotic gradient is developed because of this electrochemical gradient and this is facilitated by the aquaporins. The removal of ions during osmoregulation is compensated by an unidentified mechanism. Amoebae are adversely affected by the osmotic pressure caused by extremely saline or dilute water. When the conditions surrounding them become unfavourable they become converted into microbial cysts. These cysts are covered by well protected membranes. When conditions become favourable the cyst wall ruptures and they become active again. They do not divide in the cyst stage and may die if fail to change into active stage again. The marine amoebae remain unaffected by the changes in salinity of water although they are microscopic but some may be as large as grapes.
Navodita Maurice