11th Class

Cell Division / Cell Reproduction / Cell Cycle

It is the process by which a mature cell divides and forms two nearly equal daughter cells which resemble the parental cell in a number of characters. In unicellular organisms, cell division is the means of reproduction by which the mother cell produces two or more new cells. In multicellular organism also, new individual develop from a single cell. Cell division is central to life of all cell and is essential for the perpetuation of the species. Discovery : Prevost and Dumans (1824) first to study cell division during the cleavage of zygote of frog. Nagelli (1846) first to propose that new cells are formed by the division of pre-existing cells. Rudolf Virchow (1859) proposed “omnis cellula e cellula” and “cell lineage theory”. A cell divides when it has grown to a certain maximum size which disturb the karyoplasmic index (KI)/Nucleoplasmic ratio (NP)/Kernplasm connection. Cell cycle : Howard and Pelc more...

Catgeory: 11th Class

Metabolically Inactive Cell Inclusions

Within the cytoplasm of a cell there occur many different kinds of non-living structures which are called inclusions or ergastic / Deutoplasmic substances. (1) Vacuoles : The vacuole in plants was discovered by Spallanzani. It is a non-living reservoir, bounded by a differentially or selectively permeable membrane, the tonoplast. The vacuole is filled with cell sap or tonoplasm. They contain water, minerals and anthocyanin pigments. Some protozoans have contractile vacuoles which enlarge by accumulation of fluid or collapse by expelling them from the cell. The vacuoles may be sap vacuoles, contractile vacuoles or gas vacuoles (pseudo vacuoles). Function of vacuoles : Vacuole maintains osmotic relation of cell which is helpful in absorption of water. Turgidity and flaccid stages of a cell are due to the concentrations of sap in the vacuole. (2) Reserve food material The reserve food material may be classified as follows : (i) Carbohydrates : Non-nitrogenous, soluble more...

Catgeory: 11th Class

Nucleus

The nucleus also called director of the cell. It is the most important part of the cell which directs and controls all the cellular function. Discovery : The nucleus was first observed by Robert Brown (1831), in orchid root cells. Nucleus plays determinative (in heredity) role in cell and organism, that was experimentally demonstrated by Hammerling (1934) in surgical experiments with green marine unicelled algae Acetabularia. Occurence : A true nucleus with definite nuclear membrane and linear chromosome, is present in all the eukaryotes except mature mammalian RBCs, sieve tube cell of phloem, tracheids and vessels of xylem. The prokaryotes have an incipient nucleus, called nucleoid or prokaryon or genophore or false nucleus. Number : Usually there is a single nucleus per cell i.e., mononucleate condition, e.g., Acetabularia. (1) Anucleate (without nucleus) : RBCs of mammals, phloem sieve tube, trachids and vessels of xylam. (2) Binucleate : e.g., Ciliate, Protozoans more...

Catgeory: 11th Class

Chromosome (Gr. chroma = colour soma = body)

Chromosome were discover by Hofmeister (1848) in filament of pollen mother cells of tradescantia (Rhoeodiscolour) studied by strasburger (1875) and given the persent name by Waldeyer (1888). During interphase, chromatin threads are present in the form of a network called chromatin reticulum. At the time of cell division, these thread like structures of chromatin become visible as independent structures, called chromosomes. The haploid set of chromosomes is define as genome. Structure : Each chromosome consists of two coiled filaments throughout its length called chromonemata by Vejdovsky. These have bead like structures called chromomeres which bear genes. Chromatid is a half chromosome or daughter chromosome. The two chromatids are connected at the centromere or primary constriction. Primary constriction (centromere) and secondary constriction gives rise to satellite. The secondary constriction consists of genes which code for ribosomal RNA and nucleolus hence it is called as “nucleolar organizer region”. Chromosomes having satellite are more...

Catgeory: 11th Class

Types Of Cells

Chatton gave the term prokaryote and eukaryote. Depending upon the nature of nucleus cells are classified. Incipient nucleus is present in prokaryotes, where as in eukaryotes well organised nucleus is present. Differences between prokaryotic and eukaryotic cell

Prokaryotic cell

Eukaryotic cell

It is a single membrane system.

more...

Catgeory: 11th Class

Ribosome

The ribosomes are smallest known electron microscopic without membrane, ribonucleo–protein particles attached either on RER or floating freely in the cytoplasm and are the sites of protein synthesis. Discovery : In 1943 Claude observed some basophilic bodies and named them as microsome. Palade (1955) coined the term ribosome (form animal cell). Ribosomes in nucleoplasm were observed by Tsao and Sato (1959). First isolated by Tissieres and Watson (1958) from E. coli. Ribosomes found in groups are termed as polyribosomes or ergosomes (Rich and Warner 1963 observed first time polyribosomes). Occurrence : In prokaryotes ribosomes are found only in free form in the cytoplasm. While in the eukaryotes the ribosomes are found in two forms in the cytoplasm, free form and bind form (bound on RER and outer nuclear membrane). These are also reported inside some cell organelles like mitochondria and plastids respectively called mitoribosomes and plastidoribosomes. Types of ribosomes (1) more...

Catgeory: 11th Class

Protoplasm (Proto = First, Plasm = Fluid)

Protoplasm is a complex, granular, elastic, viscous and colourless substance. It is selectively or differentially permeable. It is considered as “Polyphasic colloidal system”. Discoveries (1) J. Huxley defined it as “physical basis of life”. (2) Dujardin (1835) discovered it and called them “sarcode”. (3) Purkinje (1837) renamed it as “Protoplasm”. (4) Hugo Von Mohl (1844) gave the significance of it. (5) Max Schultz (1861) gave the protoplasmic theory for plants. (6) Fischer (1894) and Hardy (1899) showed its colloidal nature. (7) Altman (1893) suggested protoplasm as granular. Chemically Composition

Water

more...

Catgeory: 11th Class

Plastids

Plastids are semiautonomous organelles having DNA, RNA, Ribosomes and double membrane envelope. These are largest cell organelles in plant cell. History (1) Haeckel (1865) discovered plastid, but the term was first time used by Schimper (1883). (2) A well organised system of grana and stroma in plastid of normal barley plant was reported by de Von Wettstein. (3) Park and Biggins (1964) gave the concept of quantasomes. (4) The term chlorophyll was given by Pelletier and Caventou, and structural details were given by Willstatter and Stall. (5) The term thylakoid was given by Menke (1962). (6) Fine structure was given by Mayer. (7) Ris and Plaut (1962) reported DNA in chloroplast and was called plastidome. Types of plastids : According to Schimper, Plastids are of 3 types: Leucoplasts, Chromoplasts and Chloroplasts. Leucoplasts : They are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae. more...

Catgeory: 11th Class

Plasma Membrane

Every living cell is externally covered by a thin transparent electron microscopic, elastic regenerative and selective permeable membrane called plasma membrane. It is quasifluid in nature. Membranes also occur inside the cells. They are collectively called biomembranes. The term cell membrane was given by C. Nageli and C. Cramer (1855) for outer membrane covering of the portoplast. It was replaced by the term plasmalemma by Plower (1931). Chemical composition : Proteins lipoprotein (Lipid +Protein) are the major component forming 60% of the plasma membrane. Proteins provide mechanical strength and responsible for transportation of different substances. Proteins also act as enzyme. Lipids account may 28%-79% depending upon the type of cell and organism involved (in humans, myelin 79%). The lipids of plasma membrane are of three types namely phospholipids, glycolipids and sterols. The sterol found in the membrane may be cholesterol (Animals), phytosterol (Plants) or ergosterol (Microorganisms). Carbohydrates form 2%–10%. Oligosaccharides more...

Catgeory: 11th Class

Microbodies

(1) Sphaerosomes Discovery : These were first observed by Hanstein (1880) but discovered by Perner (1953). Term sphaerosomes was given by Dangeard. Occurrence : These are found in all the plant cells which involves in the synthesis and storage of lipids i.e., endosperm and cotyledon of oil seeds. Shape, size and structure : These are spherical or oval in shape about \[0.5-2.5\,\mu \,m\]in diameter. They contain hydrolytic enzymes like protease, ribonuclease, phosphatase, esterase etc. They are bounded by a single unit membrane. Function : The main function of sphaerosomes is to help in lipid metabolism. These are also known as plant lysosomes. (2) Peroxisomes (Uricosomes) Discovery : These were first discovered by J. Rhodin (1954) in the cells of mouse kidney and were called microbodies. De Duve (1965) isolated certain sac like organelles from various types of animals and plants. These were called peroxisomes because these contain peroxide producing enzymes more...

Catgeory: 11th Class

11th Class

Articles Categories

Archive

Trending Articles