NEET

First Law of Thermodynamics   First law of thermodynamics was proposed by Helmholtz and Robert Mayer. This law is also known as law of conservation of energy. It states that, “Energy can neither be created nor destroyed although it can be converted from one form into another.” (1) Justification for the law : The first law of thermodynamics has no theoretical proof. This law is based on human experience and has not yet been violated. The following observations justify the validity of this law (i) The total energy of an isolated system remains constant although it can undergo a change from one form to another. (ii) It is not possible to construct a perpetual machine which can do work without the expenditure of energy, If the law were not true, it would have been possible to construct such a machine.           (iii) James Joule (1850) more...

  Asexual Reproduction   Asexual reproduction. The methods of reproduction which do not involve meiosis and fertilization are known as apomixis or asexual reproduction. Only mitotic divisions are involved in these methods, resulting into the formation of offsprings which are genetically similar to the parent plant.   Asexual reproduction is of following two types: (1) Agamospermy: Agamospermy is a kind of plant apomixis in which the embryos and seeds are formed by asexual reproductive methods without involving meiotic gametogenesis and sexual fusion of gametes. It occurs widely in ferns and the flowering plants. There are three different types of agamospermy:         (i) Adventive embryony : Formation of embryo directly from the diploid sporophytic cells (nucellus or integument) of ovule is called adventive embryony. Such embryos are formed more...

  Microsporogenesis   Microsporogenesis The process of the formation and differentiation of microspores (pollen grains) from microspore mother cells (MMC) by reductional division is called microsporogenesis. Microsporogenesis is well studied under following heads:       (1) Structure of anther: The fertile portion of stamens is called anther. Each anther is usually made up of two lobes connected by a connective. In turn each anther lobe contains two pollen chambers placed longitudinally. Each pollen chamber represents a microsporangium and is filled with a large number of pollen grains or microspores. A typical anther consist of four microsporangia (tetrasporangiate) and such anthers is called dithecous e.g. mostly plants. In members of Malvaceae anthers are reniform or kidney shaped and consist of two microspoangia (bisporangiate), such anthers is called monothecous. more...

  Megasporogenesis   Megasporogenesis The process of formation of megaspore from megaspore mother cell by meiotic division is known as megasporogenesis. This process takes place in ovule. Megasporogenesis can be studied under following heads:                                                                                                                                                                                                                                             (1) Structure of ovule: Ovule is considered to be an integumented megasporangium. The ovule consists of the stalk and the body. The stalk is called funicle. One end of the funicle is attached to placenta and the other end to the body of the ovule. The point of attachment of funicle with the body is called hilum. Sometimes funicle gets fused with the body of more...

  Pollination   Pollination The process of transfer of pollen grains, from an anther to the stigma of the same flower or of different flower. It is of two types: (1) Self pollination                       (2) Cross pollination     (1) Self-pollination: This process involves the transfer of pollen grains from the anthers to the stigma of the same flower or of another flower borne by the same plant. It is of two types: (i) Autogamy : It is a kind of pollination in which the pollen from the anthers of a flower are transferred to the stigma of the same flower. (ii) Geitonogamy : It is an kind of pollination in which the pollen from the anthers of one flower are transferred to the stigma of another flower borne more...

  Fertilization   Fertilization The fusion of two dissimilar sexual reproductive units (gametes) is called fertilization. This process was discovered by Strasburger (1884). (1) Germination of pollen grain on stigma and growth of pollen tube: Pollen grains reach the receptive stigma of the carpel by the act of pollination. Pollen grains, after getting attached to the stigma, absorb water and swell. Subsequent to mutual recognition and acceptance of pollen grains, the pollen grain germinates (in vivo) to produce a pollen tube which grows into stigma towards the ovarian cavity. G.B. Amici (1824) discovered the pollen tube in Portulaca oleracea. Generally, only one pollen tube is produced by a pollen grain (monosiphonous). But in some plants like members of Cucurbitaceae produce many pollen tubes (polysiphonous). The pollen tube contains a vegetative nucleus or tube nucleus and two more...

  Embryo   Embryo (1) Development of embryo (Embryogeny): The zygote after a period of rest develops into embryo. The process of development of mature embryo from diploid zygote is called embryogenesis. (i) In dicotyledons : The normal type of dicot embryo development has been studied in Shephered purse  (Capsella bursapastoris) family Cruciferae. This is called as crucifer or onagrad type of embryo development. This development of embryo is endoscopic i.e. apex is downward or towards inside. The first division of zygote is transverse which produces a basal cell (cb) towards the micropyle and a terminal cell (ca) towards chalaza. The basal cell divides by transverse division and the terminal cell by a longitudinal division, so 4 celled T-shaped proembryo is produced. The two basal cells divide by transverse division and form 6-10 celled suspensor. The more...

  Parthenocarpy       Parthenocarpy The formation of fruits without fertilization is called parthenocarpy. Such fruits are either seedless or non-viable seeds. Parthenocarpy is of two types:   (1) Natural parthenocarpy: When seedless fruits are produced without any special treatment from the ovaries in the absence of pollination and fertilization, the phenomenon is called natural parthenocarpy. e.g., Grapes, Banana, Pineapple and Noval oranges.   (2) Induced parthenocarpy: When seedless fruits are produced by spraying the flowers with either water extract of pollen grains or growth promoting hormones such as Indole acetic acid (IAA), Naphthalene acetic acid (NAA), Gibberellic acid (GA), etc. the phenomenon is called induced parthenocarpy. e.g., Tomato, Black berry, Fig, Lemon, Apple, Orange, Pear. etc.

  Morphology of Frog     Introduction Frogs are carnivorous tailless amphibians which are widely found in India. A diverse variety of frogs can be found all over the world; among them, the Indian frogs are called Rana tigirna. They are vertebrates, coming under the class Amphibia (phylum Chordata). Frogs are cold-blooded animals (poikilotherms) whose body temperature varies according to their environment. Hence, they need to protect themselves from extreme heat and cold for maintaining optimum body temperature. Thus, they follow aestivation and hibernation during summer and winter seasons. Another characteristic feature of frogs is that they are camouflage i.e., they can change their skin color according to their surroundings. Morphology of Frogs more...

  Smoking   Smoking of tobacco dried and cured leaves of plant ‘Nicotiana tobacum’ and N. rustica in the form of cigars, cigarettes, bides etc. is very toxic to the body. Smoke of tobacco contains about 300 compounds. The main compounds are nicotine, CO, HCN, polycyclic aromatic hydrocarbons, certain other stimulating products etc. Nicotine in the blood stimulates the nervous system, relax the muscles, release adrenalin hormone and increase the rate of heart beats. In pregnant ladies growth of fetus decreases and loss of weight may takes place.   (i) Tobacco: The tobacco was first smoked by Red Indians in America. It then spread to European countries in the early 1600’s, and today a large part of the world population smoke tobacco, while some others chew it.   (ii) Effect of Nicotine: Smoking was reported to more...