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Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope. Leeuwenhoek (1674), with the improved microscope, discovered the free-living cells in pond water for the first time. It was Robert Brown in 1831 who discovered the nucleus in the cell.
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ravi kumar sharma,India,Student
Published Date:17-06-2017
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FUNDAMENTAL UNIT OF LIFE DISCOVERY OF CELL  While, examining a thin slice of cork, Robert Hooke saw that the cork resembled the structure of a honeycomb consisting of many little compartments. Cork is a substance which comes from the bark of a tree. This was in the year 1665 when Hooke made this chance observation through a self-designed microscope. Robert Hooke called these boxes cells. Cell is a Latin word for ‘a little room’.  This may seem to be a very small and insignificant incident but it is very important in the history of science. This was the very first time that someone had observed that living things appear to consist of separate units. The use of the word ‘cell’ to describe these units is used till this day in biology.• COMPOUND MICROSCOPE & CELLS OF ONION PEEL Nucleus Cell MORE TO KNOW ABOUT CELLS Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope. Leeuwenhoek (1674), with the improved microscope, discovered the free living cells in pond water for the first time. It was Robert Brown in 1831 who discovered the nucleus in the cell.Invention of magnifying lenses & discovery of microscopic world  The invention of magnifying lenses led to the discovery of the microscopic world. It is now known that a single cell may constitute a whole organism as in Amoeba, Chlamydomonas, Paramoecium and bacteria.  These organisms are called unicellular organisms (uni = single). On the other hand, many cells group together in a single body and assume different functions in it to form various body parts in multicellular organisms (multi = many) such as some fungi, plants and animals.  Every multi-cellular organism has come from a single cell. Cells divide to produce cells of their own kind. All cells thus come from pre-existing cells.The shape and size of cells are related to the specific function they perform. Some cells like Amoeba have changing shapes. In some cases the cell shape could be more or less fixed and peculiar for a particular type of cell; for example, nerve cells have a typical shape.• • •• • •CELL WALL  Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants.  When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.NUCLEUS,CHROMOSOMES & DNA DEOXYRIBO OR DEFINED NUCLIEC ACID The nucleus has a double layered covering called nuclear membrane. The nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside, that is, to the cytoplasm. The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo\Defined Nucleic Acid) molecules. Chromosomes are composed of DNA and protein. DNA molecules contain the information necessary for constructing and organizing cells. Functional segments of DNA are called genes. In a cell which is not dividing, this DNA is present as part of chromatin material. Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organized into chromosomes.NucleusPROKARYOTIC CELLS In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a nucleoid. Such organisms, whose cells lack a nuclear membrane, are called prokaryotes (Pro = primitive or primary; karyote ≈ karyon = nucleus). For ex. Bacteria , Algae etc. Prokaryotic cells also lack most of the other cytoplasmic organelles present in eukaryotic cells. Many of the functions of such organelles are also performed by poorly organized parts of the cytoplasm (see section 5.2.4). The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles (bag like structures). Cell organelles are enclosed by membranes. In prokaryotes, beside the absence of a defined nuclear region, the membrane-bound cell organelles are also absent.EUKARYOTIC CELLS  The Cells having nuclear membrane and well defined nuclear region are known as eukaryotic = true & karyote = nucleus means good or true nuclei. Eukaryotic cells can be easily distinguished through a membrane-bound nucleus. For ex. Animal cell , Plant cell.  Eukaryotic cells are membrane-bound organelles to carry out specific cell tasks. They have different internal membranes, which are known as organelles. These organelles play a vital role in cell maintenance and other functions. These organelles generally consist of cell wall, plasma membrane, nucleus, mitochondria, chloroplasts (plastids), endoplasmic reticulum, ribosome, Golgi apparatus, lysosomes, vacuoles, cytoplasm and chromosomes.  The eukaryotic cells have nuclear membrane as well as membrane- enclosed organelles.Fill the spaces with correct answers about prokaryotic and eukaryotic cells.CELL ORGANELLES ANIMAL CELL PLANT CELL Cell wall Absent Present Usually round or spherical. Rectangular Cell shape (irregular in shape - absence of cell wall ) (fixed shape - presence of cell wall) Nucleus Present Present Plasma membrane Present Present Nucleolus Present Present Endoplasmic reticulum Present Present Golgi apparatus Present Present Lysosomes Present Present Vacuoles Present (Small and more in number) Present (Large and few in number) Mitochondria Present Present Plastids Absent Present Chromosomes Present Present Ribosome Present Present Centrosomes Present Absent Chloroplast Absent Present Cytoplasm Present PresentPLANT & ANIMAL CELLS  As we know that plant cells are found in plants ,and animal cells in animals & human being & they have some characteristics some of them are :-  Endoplasmic reticulum (ER)  Golgi Apparatus  Lysosomes  Mitochondria  Plastids  Vacuoles