12th Class

(1) Population density : Population density is the number of individuals present per unit area or volume at a given time. For instance, number of animal per square kilometer, number of trees per area in a forest, or number of plank tonic organism per cubic meter of water. If the total number of individuals is represents by letter N and the number of units of space by Letter S, the population density D can be obtained as \[D=N/S.\] Space is indicated in two dimensions \[({{m}^{2}})\] for land organisms, and in three dimensions \[({{m}^{3}})\] for aquatic organisms and for the organisms suspended in space. (2) Birth rate or Natality : The birth rate of a population refers to the average number of young ones produced  by birth, hatching or germination per unit time (usually per year). In the case of humans, it is commonly expressed as the number of births per more...

Meaning : The regulation of conception by preventive methods or devices to limit the number of offspring  is called birth control. Methods : A variety of methods are known for birth control. The birth control methods which deliberately prevent fertilization are referred to as contraception. These methods are of 2 main types : temporary and permanent. (1) Temporary  Methods : These  are  further  of  many types : (i) Safe Period (Rhythm Methods) : A week before and a week after menses is considered the safe period for sexual intercourse. The idea is based on the following facts- (a) Ovulation occurs on about the 14th day (may be 13 th to 16th day) of menstruation. (b) Ovum survives for about 1-2 days. (c) Sperms remain alive for about 3 days. This method may reduce the chances of pregnancy by about 80 percent. However, a great care is needed in its use. more...

Population growth refers to the increase in its size. It is determined by the number of individuals added to the population and the number of individuals lost from the population. Addition occurs by births and immigration. Loss results from deaths and emigration. If more individuals are added than are lost i.e., the vital index is more than 100, the population will increase or show positive growth. If more individuals are lost than are added i.e., the vital index is less than 100, the population will decrease or show negative growth.  If addition and loss are balanced, i.e., the vital index is 100, the population will become stationary or show zero growth. Malthus Theory of Human Population Growth : Thomas Malthus, a British political economist, put forward a theory of human population growth in 1778. Malthus in his "Essay on the principle of population" pointed out that population tends to increase more...

Aim : It is a technique to determine : (1) Sex of the developing baby. (2) Genetically controlled congenital diseases. (3) Metabolic disorders in foetus. So amniocentesis is a pre-natal diagnostic technique. Procedure : It involves following steps : (1) Location of the foetus is determined by a technique called sonography (using high frequency ultrasound waves) to prevent accidental damage to the foetus. (2) A fine hollow needle is passed through the abdominal and uterine wall of a pregnant female (about 14th to 15th week after conception) into the amniotic cavity.     (3) A small amount of amniotic fluid is withdrawn. It contains foetal skin cells and a number of proteins, especially enzymes. The cells can be cultured in vitro for further examination.   Significance (1) Sex determination : The somatic cells of foetal skin drawn with the amniotic fluid are more...

The environment is the aggregate of all those things and set of conditions which directly or indirectly influence not only the life of organisms but also the communities at a particular place. Any external force or influence, which surrounds and affects the life of a plant in any way, becomes a factor of its environment. These factors are called environmental factors and may be living (biotic) as well as non-living (abiotic). The abiotic factors affect the structure, life history, physiology and behaviour of organisms. The biotic factors mostly influence growth and reproduction. The environmental conditions which influence the life and development of plants, each part of the environment is called ecological factors. Ecological factors are grouped into four main classes (ecological factors) which are as follows : Climatic factors : The study of climatic factor is known as climatology. The chief climatic factors are : (1) Water : Rainfall is more...

Out of about 3,50,000 known plants at this time, a few i.e., about 100. Scientists are in search of less known and underutilized crop plants, which can be used for food and other purposes and thus exploitation of traditional plants can be reduced. Such under-utilized and under-exploited plants are known as new crops. Some of these new and underutilized crops are as follows : (1) Triticale : Triticale is the first man made cereal or crop,  which has been produced by intergeneric hybridization between common wheat (Triticum aestivum) and European rye (Secale cereale) with a view to combine characters of these two parent plants. Triticale is hexaploid, i.e, \[2n=6x=42\] (tetraploid Durum wheat × diploid Rye) or octaploid, i.e., \[2n=8x=56\] (hexaploid Bread wheat × diploid Rye). Triticale is the first new man-made plant to join the rank of cereals which have long evolutionary plants are being used for fulfilling man's daily more...

Study of use of living organisms and the substances produced due to their activities is called biotechnology. Mostly micro-organisms are used in many industries as alcohol, enzymes, vaccines, vitamins, antibiotics, organic acids, etc. So biotechnology is controlled use of microbes for benefit of human beings. In this science, principles of biochemistry, molecular biology and microbiology are mostly used. Some important examples of biotechnology products are : Alcohol : It is the result of yeast fermentation, which is the incomplete oxidation of complex organic compounds with the help of enzymes produced by yeast (Invertase and Zymase) \[\underset{(\text{Sucrose})}{\mathop{{{C}_{12}}{{H}_{22}}{{O}_{11}}}}\,+{{H}_{2}}O\underset{\text{Invertase}}{\mathop{\xrightarrow{\text{Yeast}}}}\,\underset{(\text{Glucose})}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,+\underset{(\text{Fructose})}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}}}\,\] \[{{C}_{6}}{{H}_{12}}{{O}_{6}}\underset{\text{Zymase}}{\mathop{\xrightarrow{\text{Yeast}}}}\,\underset{(\text{Ethyl}\,\text{alcohol})}{\mathop{2{{C}_{2}}{{H}_{5}}OH}}\,+2C{{O}_{2}}\] In the middle of the 19th century, Louis Pasteur reported that alcohol, beer and butter milk are the result of yeast fermentation. Yeast fermentation is the basis of baking as well as brewing industries, besides preparation of fermented foods like idli, dosa etc. Types of yeasts (1) Baker’s yeast : These include the more...

Pacemakers (1) Pacemaker is a device that supplies electrical impulses to the heart to maintain the heartbeat at a regular rate. (2) The artificial pacemaker was introduced by Chardack in 1960. (3) Pacemakers are life saving when the normal heart rate 72-80 drops to abnormally low levels like \[30-40\] due to diseases.             (4) Pacemaker consists of a pulse generator and an electrode. (5) The pulse generator is hermetically sealed box; it contain lithium halide cells to provide power for over 10 years. (6) The electrode is a fine metallic spring ensheathed in a thin layer of biocompatible plastic; the special tip remains in contact with the interior of right ventricle. (7) An artificial pacemaker is implanted when a person's sinu-atrial node is not functioning properly. (8) Two basic types of pacemakers are fixed-rate and demand type. (9) Fixed-rate pacemaker discharges impulses at a steady rate, irrespective of the heart's activity. more...

Major advancements in the medical sciences have been the development of new imaging techniques that provide detailed pictures of internal organs.        (1) X-rays (i) Following their discovery by Wilhelm Roentgen, a German physicist in 1895, X-ray became an important tool for medical diagnosis. (ii) X-ray are a form of electromagnetic radiation of extremely short wavelength. (iii)  When a beam of X-rays is directed at a part of the body such as chest, the rays are absorbed more by dense structures such as the ribs or heart muscles than by less dense structures such as the skin or lungs. (iv) This causes shadows of variable intensity to be cast on a photographic film. (v) X-rays cause no sensation when passed through body tissues. (vi) Large or frequent radiation doses may damage the skin and internal organs and may cause cancer in later life. (vii) The study of X-rays for detection and more...

(1) Sphygmomanometer   (i) Sphygmomanometer, commonly called B.P. Apparatus, is an instrument for measuring blood pressure. (ii) This instrument consists of a rubber cuff attached by a rubber tube to a compressible hand pump or bulb. (iii)  Another tube attaches to the cuff and to a column of mercury or pressure dial marked off in millimeters. (iv) Blood pressure is usually taken in the left brachial artery. (v) Blood pressure is recorded by giving the systolic pressure and diastolic pressure expressed as millimeters of mercury (mm Hg). (vi) A healthy young adult male has blood pressure reading of about 120/80 (i.e. 120 mm Hg systolic and 80 mm Hg diastolic). (vii) The difference between systolic and diastolic pressure is called Pulse pressure. (viii) Blood pressure often rises normally with age to about 130/90 at age 60. (ix) Abnormally high blood pressure is known medically as hypertension; abnormally low blood pressure is more...