A large number of communicable diseases are caused by germs or microbes. Of these, the main infecting agents are viruses and bacteria. These germs enter the human body through cuts in the skin, the air that is i
nhaled, the food and water that is taken in and through the bites of animals, especially insects.
Germs cause diseases directly by attacking and destroying body cells and indirectly by producing poisonous substances called toxins. The virus of poliomyelitis destroys certain cells of the spinal cord while the diphtheria causing bacterium (namely, corynebacterium diphtheriae) produces a powerful toxin.
The human body's natural defence against such invaders are the white corpuscles in the blood which engulf bacteria, and certain cells in the lymphatic glands which produce antibodies. Antibodies can destroy the invading organism or make it's toxin harmless. The type of antibody produced depends on the bacterium or virus which invades the body. Sometimes when a particular organism invades the body, enough antibodies are formed and retained in the body for a few months to a few years, in which case the person is said to have acquired an immunity against the disease.
Most bacterial and viral diseases can be controlled by good health habits, sterilization of food and water, extermination of animals which carry disease germs, proper sanitary conditions, maintenance of bodily health and artificial immunity pronounced by vaccinations and inoculations.
Edward Jenner and Louis Pasteur have contributed greatly towards the use of artificial immunity as a means of controlling diseases. Certain disease causing bacteria or viruses can be artificially altered so that their disease causing abilities are reduced. When these are injected into the human body, they cause the production of antibodies in the body but not the disease. The antibodies produced confer immunity against the disease for a period of This method of immunization is used for controlling disease such as poliomyelitis, whooping cough and rabies.
Sometimes, instead of inoculating a modified disease causing agent, a modified toxin called a toxoid produced by the bacterium or virus is inoculated into the body. This also results in the formation of antibodies. Immunization against diphtheria and tetanus is brought about by this method since the serious effects of these diseases are due to the powerful toxins produced by the bacteria concerned. In most countries infants, pre school and school children are immunized against small pox, diphtheria, tetanus, whooping cough and poliomyelitis as a routine.
Artificial immunity against disease like cholera and typhoid fever lasts only for a short period (six months). Therefore, these inoculations are given when epidemics of these disease occur.
Many diseases, caused by germs against which there are no vaccines, can be treated by the use of certain drugs. These drugs called antibiotics, e.g penicillin, tetracyclins, streptomycin and aureomycin, inhibit the growth of disease causing germs. Antibiotics are produced by certain moulds, actinomycetes and bacteria.
UNICELLULAR PLANTS
Two common free living unicellular plants are Chlamydomonas rivalis and Euglena virdis. These simple plants are examples of unicellular algae.
CHLAMYDOMONAS RIVALIS
Chlamydomonas rivalis is a unicellular plant found in fresh water. It is considered to be primitive because it is flagellated, a feature usually associated with ancentral forms. It is found abundantly in very slow moving waters or in ponds and ditches.
STRUCTURE: The chlamydomonas cell is round or ovoid in shape and green in colour. Each cell which can live independently has a thin outer covering, the cell wall, which is made of cellulose. The upper or anterior end has a small conical projection known as the anterior From this anterior end arise two elongated flagella which the cell uses for locomotion.
Inside the cell wall is the clear cytoplasm and a large ovoid central nucleus. In front of the latter, in the clear region of the cytoplasm, are two small contractile vacuoles which contracts at intervals, the main function of these vacuoles being to eliminate excess water. The lower, or posterior, part of the cytoplasm is occupied by a single, large cup shaped chloroplast which gives the green colour to the entire cell. Embedded in this chloroplast is the round body known as pyrenoid. Though the function of the pyrenoid is not quite known yet, some biologists assume that it stores carbohydrates. At the anterior end, lying on one side of the cell, is a dark red spot known as the pigment spot, or eye spot, which is very sensitive to light.
MODE OF LIFE: Chlamydomonas is considered to be a plant cell because of the presence of the green chloroplast. It is also known as a green algae. It lives in water and manufactures it's own food. For this purpose, it absorbs water and dissolved carbon dioxide which enter through it's cell wall. In the presence of sunlight, the green chloroplast utilizes the water and carbon dioxide to photosynthesize it's carbohydrates. During the preparation of food, which is known as photosynthesis, oxygen is liberated.
The chlamydomonas respires by absorbing oxygen from the surrounding water and giving out carbon dioxide. This exchange of gases takes place by direct diffusion between the cell wall and the surrounding water.
Plants generally do not move about but unucellular plants such as these do show movement in their environment. Chlamydomonas uses its whip like flagella to beat against the water and in doing so, pushes its body forward and rotates itself in the direction in which it wants to move. Although it is a simple unicellular organism, chlamydomonas is sensitive to changes in environmental conditions and respond to these changWhen there is excess light, the pigment spot senses it and the entire cell moves away from the bright light.
REPRODUCTION: Young chlamydomonas cells grow until they reach a certain maximum size. They are then considered to be mature and begin to reproduce new young ones.
Reproduction in chlamydomonas is of two types— asexual and sexual.
Asexual reproduction begins when an adult cell comes to rest, loses its flagella and becomes rounded. The nucleus, cytoplasm and other contents of the cell divides into eight or sixteen parts. Each part, consequently, gets a small portion of the nucleus and cytoplasm and proceeds to form a new cell wall and flagella. The new cells soon resemble the parent cell. After some time the cell wall of the parent cell bursts open and these eight or sixteen young chlamydomonas cells emerge and swim around in the water.
Each obe then lives independently.es.papilla.time.
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