Earthworm [Part 2]

5) Locomotion:
Locomotion in earthworm is caused by setae and the musculature of the body wall. When as earthworm starts to crawl, the first few segments become thinner and extend forward. This is due to contraction of the circular muscle fibres and relaxation of the longitudinal muscles in that region. The longitudinal muscles of the anterior segments now contract and the circular muscles relaxes due to which anterior end becomes shorter and thicker. The seta of these segments then protrude out and get gripped into the soil. The contraction of anterior segments pull the posterior body of the worm forward. The contraction of longitudinal muscles passes backwards like a wave. Again a wave of contraction of circular muscles starts from the anterior end making it thinner and extended forwards. These alternating series of waves of extension and contraction bring about the locomotion of worm. Setae always protrude during longitudinal contraction and retract during circular contraction. During locomotion, the coelomic fluid serves as a kind of hydraulic skeleton. When compressed due to contraction of circular muscles, it provides stiffness to the body and aids in the relaxation of the longitudinal muscles. The earthworm moves at the rate of 25 cm per minute. The worms can move backward also. When the direction of wave is reversed, the worm crawls backwards.

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Shirish 9:13 PM   2 comments

Earthworm [Part 1]

The name earthworm is given to the annelid worms, because they live and burrow in the ground. Annelids are triploblastic, bilaterally symmetrical and coelomate animals. The presence of true segmentation (metamerically segmented) in their body is a diagnostic feature of these animals. The locomotary organs are setae or chaetae present in each segment. Phylum Annelida, also sometimes known as Annulata, contains elongated, vermiform and true segmented animals.

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Shirish 8:15 PM   4 comments

Human Evolution

Like any other species human also evolved over millions of years. The early stages of human evolution are studied by means of the comparative anatomy of fossils and the evidence of thee comparative biochemistry of present-day humans and other mammalian species. Humans belong to an order of class mammals called Primates. The present day order primates consists of two suborders: Prosimii, includes tree-shrews, lemurs, loris, tarsiers etc. and Anthropoidea, includes monkeys, apes and humans. Humans belong to the family Hominidae.

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Evidences of Evolution

The doctrine of organic evolution is supported by following evidences:

1. Structural evidence: The most extensive evidence in support of evolution comes from the study of structural similarities. Related groups of organisms possess homologous structure because of common ancestry. Among the vertebrates from the frog to man, the vertebral column, eyes, ears, brain and the digestive system are constructed on the same basic plan. Some homologous structures suggest a common ancestry. In plants, the structural similarities of roots, stems, leaves, flowers show the evolutionary tendencies of plants. Morphological similarities in the type of venation, shape of corolla, cohesion or adhesion of stamens among a group of plants suggest a common ancestry.

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Shirish 10:36 PM   2 comments

Origin and Evolution of Life

Origin of Life:

The ability to grow and produce new forms distinguishes living organisms fron non-living objects. The origin of life on Earth is a most complicated problem for thinkers, philosophers and naturalists from ancient time. Different views have been put forward concerning the origin of life by workers of different ages. some of the most important hypothesis and theories regarding the origin of life are as follows:

Origin of Earth and its Atmosphere:

The earth was probably originated 4600 million years ago. The sun and the planets were formed together from a spherical cloud of gases and cosmic dust by gradual condensation. The earth was so hot that everything in it probably existed in gaseous form. It was a molten fire-ball about 5000-6000 centigrade for millions of years. The gases condensed into a molten core and differnt elemnts got stratified according to their density. Heavy metals such as Nickel, Iron formed the solid core of earth whereas lighter ones came towards the surface of earth. Lighter elements like Hydrogen, helium, carbon, oxygen and nitrogen constituted the early atmosphere. The earth had a reducing atmosphere, i.e. the free oxygen was not found in volcanic gases, because the reduced substances immediately combined with oxygen to form oxides.

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Shirish 10:36 PM   1 comment

Structure of Cell [Part 3]

Plastids:

Plastids are cytoplasmic organelles about 5 micron in diameter and 3 micron in thickness, found freely in cytoplasm of the most of plant cell. they are not found in animal cell. the term plastid was first used by Schimper (1885). They are usually disc shaped and arise from small granular bofies, the protoplastids. Based on the types of pigments, three types of plastids are recognized: leucoplast, chloroplast and chromoplast. One form of plastid can change into another form, for example, in tomatoes the ovary contains leucoplasts which change into chloroplasts in young fruits and finally into chromoplasts as the fruit ripens.

1)Leucoplasts: [Gr. leukos: white; plastikos: formed] These are colourless plastids found in non-green parts of the plant such as under-ground stem, root and meristemic cells. They may be rod-like, spherical or oval in shape. Each leucoplast is bound by double layered membrane which encloses granular matrix. They contain few lamellae and on exposure to sun light change into chloroplast by developing thylakoid structure. Leucoplasts are mainly concerned with storage of various kinds of reserve food materials and are named variously such as amyloplasts (involved in storage of starch), elaioplasts or lipoplast (involved in storage of oil), aleuroplasts (involved in storage of proteins).

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Shirish 12:25 AM   No comment

Structure of Cell [Part 2]

Cytoplasmic Organelles:

Ribosome : The ribosomes were first reported in plant cells by Robinson and Brown (1953) and Palage (1995). They observed itin animal cell, usually attached to the outer surface of endoplasmic reticulum. The name ribosome was proposed by Haguenau (1958). It is a minute rounded ribonucleoprotein structure, consisting of RNA and proteins. the ribosome contains two unequal subunits and these subunits remain attahed together by means of binding force of magnesium atom. The larger subunit has dome like shape, while the smaller subunit occurs above the larger subunit as a cap like structure.

Occurence: The ribosomes occur in both prokaryotic and eukaryotic cells. In prokaryotic cell, it is found freely in cytoplasm. In eukaryotic cell, it is found freely in cytoplasm, matrix of chloroplasts, mitochondria or remains attached to the endoplasmic reticulum.

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Shirish 1:51 AM   No comment

Structure of Cell [Part 1]

Before discussing about the structure of cell, lets have a brief introduction of cell in terms of its structure. Cell is a microscopic protoplasm surrounded by the cell membrane. The protoplasm consists of the cytoplasm and the nucleus. In a plant cell, the cell membrane forms the outermost covering of the cytoplasm beneath the cell wall. The cytoplasm contains several living organelles and non-living inclusions. A cell under a microscope shows the following:

Cell Size:

An average cell varies from 0.01mm to 0.1mm in diameter. The cell of Acetabularia, an unicellular algae is about 10 cm long. The Ostrich egg is the largest cell which measures about 6 inches or 170*135mm and the smallest living cells are bacteria (0.1-10 micron). The smallest cell is Mycoplasma gallisepticum (0.1 to 0.2 micron). The nerve cells are the longest cells, that measure upto over a metre. A micron is one-thousandth (10^-3) of a mm or 10^-4 cm. One thousandth of a micron is millimicron, which is equal to 10^-6mm or 10^-7cm. Under the electron microscope, the unit of measurement is angstorm. An angstrom is one-ten-thousandth (10^-4) of a micron or ten-millionth (10^-7) of a mm.

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Shirish 4:29 AM   No comment