Medical brain notes 33

  Cell Biology   'All living things are made up of cells'. 'The cell is the basic unit of structure and function'. These generalised statements are known as  Cell   theory . This theory was forwarded by  Mathias Schleiden  and  Theodor Schwann  in 1838 - 39.   Today the cell theory includes four more ideas :   the cells are the building block of structures in living things   the cell is derived from other cells by division   the cell contains information that is used as instructions for growth,   development and functioning   the cell is the functioning unit of life; the chemical reactions of life   take place within cells.   The idea and concept of cell biology evolved during the 19th century as a result of gradual advancement in the field of microscopy and biochemis-try. Today the study of the structure of cells (cytology) is part of a major branch of biology known as  cell biology . Due to its wide a...

  Microscopy   The cells are very minute and complex organisations. The small dimensions and transparent nature of cell and its organelles pose problems to cell biologists trying to understand its organisation and functioning. Various instruments and techniques have been developed to study cell structure, molecular organization and function.   The diameters of majority of cells range from 5-500  Å  m, but most are between 10-150  Å  m. The systeme International (SI) units of length are     1 metre (m) =  1000 millimetres (mm) 1 mm (10-3m) =          1000 micrometres ( Å  m) 1  Å  m (10 -6m) =          1000 nanometres (nm) 1 nm (10-9m) =           1000 picometres (pm)   The Angstrom ( Å  ) is 10  -10  m. It is sometimes used to record the thickness of cell membranes and the sizes of macromol...

  Compound light, Dark field, Phase contrast, Oil - immersion and Electron microscope     Compound light microscope   This microscope uses visible light for illuminating the object. It contains glass lenses that magnify the image of the object and focus the light on the retina of the observer's eye. It has two lenses one at each end of a hollow tube. The lens closer to the object being viewed is called  objective   lens . The lens closer to the eye is called  ocular lens  or  eyepiece . The object   is illuminated by light beneath it. A third lens called  condenser lens  is located between the object and the light source and it serve to focus the light on the object. Dark field microscope   This type of microscope is useful for viewing suspensions of bacteria. It has a special  condenser  that allows only rays of light scattered by struc-tures within specimen. The result is an image that appears bright against a ...

  Cytological Techniques   Cells are transparent and optically homogeneous organisations. They can be observed either directly or after preservation. For direct observation, the specimen needs sufficient contrast. Direct observation is possible by using vital stains.   Vital stains  :- Vital dyes or stains are taken up by living cells without killing   them. They selectively stain intracellular structures without affecting cellular metabolism and function. For example,  Janus green B  selectively stains mitochondria, Golgi apparatus, nuclear chromatin in a dividing cell can be stained by  methylene blue ;  Neutral Red  dye or  Congo Red  dye can be used to stain yeast cells.   Preserved and stained tissues  :- For detailed microscopic study, tissues   containing cells are passed through various stages. The stages of cell prepa-ration on a glass slide involves killing, fixation, dehydration, embedding, sectioning,...

  Plasma Membrane   It is the outer limiting membrane of both prokaryotic and eukaryotic cells. It is an ultra thin, elastic, living membrane. Plasma membrane is a dy-namic and selective transport barrier.   Since the plasma membrane is ultra thin, it could be observed only under electron microscope. Structure of the membrane is studied by isolating the same from the cell and conducting biochemical investigations.   In 1895  Overton  suggested that the membrane is made of fatty sub-stances. Other workers later concluded that two layers of lipid were present in the cell membrane. According to a model proposed by  Danielli  and  Davson  in 1935, the  lipid bilayer  of the membrane was coated on either side with protein.   In 1960,  Robertson  using electronmicrographs proposed a  unit   membrane hypothesis . According to this hypothesis the two outer layers of  protein are about 2 nm thick and appear...

  Mitochondria   The mitochondria are filamentous or granular cytoplasmic organelles of all aerobic cells of higher animals and plants. They are also found in micro organisms including Algae, Protozoa and Fungi.   They were first observed by  Kolliker  in 1850 as granular structures in the striated muscles. The name '  mitochondria ' was given to them by  Benda (1897-98). Various steps of glycolysis in mitochondria was discovered by two German biochemists  Embden  and  Meyerhof . Embden got the Nobel Prize in 1922.  Sir Hans Adolph Krebs , in 1937 found out various reactions of citric acid cycle.  Kennedy  and  Lehninger  (1948-50) showed that  Citric acid   cycle , oxidative phosphorylation  and  fatty acid oxidation  took place in   the mitochondria.   The number of mitochondria in a cell depends on the type and functional state of the cell. Certain cells contain large number ...

  Ribosomes   The ribosomes are small dense, rounded and granular particles. They contain  ribonucleoprotein . They occur either freely in the matrix of the mi-tochondria, chloroplast and cytoplasm or remain attached with the membrane of the endoplasmic reticulum and nucleus.   The ribosomes were described by  G.E.Palade  in 1952. The name '  ribosome ' was coined by  R. B. Roberts  in 1958.   The ribosomes occur in both prokaryotic and eukaryotic cells. In the cells in which active protein synthesis takes place, the ribosomes remain at-tached with the membranes of the endoplasmic reticulum. The cells where such active synthesis happens are pancreatic cells, hepatic cells, osteoblasts, serous cells of submaxillary gland, chief cells of the glandular stomach, thy-roid cells and mammary gland cells.   The ribosomes are spheroid structures with a diameter of 150 to 250 �. Each ribosome is composed of two subunits. One subunit is large ...

  Endoplasmic Reticulum. (ER)   Electron microscopic study of sectioned cells has revealed the pres-ence of a three dimensional network of sac-like and tubular cavities called cisternae  bounded by a unit membrane inside the cell. Since these structures   are concentrated in the endoplasmic portion of the cytoplasm, the entire organisation is called the  endoplasmic reticulum . This name was coined by  Porter  in 1953.   The occurrence of ER varies from cell to cell. They are absent in erythrocytes, egg cells and embryonic cells.   The ER is the site of specific enzyme controlled biochemical reactions. Its outer surface carries numerous ribosomes. The presence of ribosomes gives a granular appearance . In this condition ER is described as  rough endoplasmic reticulum  (RER). RER is the site of syn-thesis of proteins. Ribosomes are absent on  smooth endoplasmic reticulum  (SER). SER is concerned with lipid metabolism. ...

  Golgi apparatus   The Golgi apparatus was discoverd by an Italian neurologist,  Camillo   Golgi  in 1873.   The Golgi apparatus occurs in almost all animal cells except red blood cells. Animal cells usually have a single Golgi apparatus. Some cells have more of Golgi apparatus.   In most of the ectodermal and endodermal cells it occurs in between the nucleus and the periphery. In nerve cells it occupies a circum-nuclear position.   The simplest unit of the Golgi apparatus is the  cisterna . A cisterna is about 1 Am in diameter. It has a membrane bound space. This space accumu-lates secretions. Numerous such cisternae are associated with each other and appear in a lamellar arrangement. In the lamellar arrangement the space between each cisterna is 20-30 nm. A group of these cisternae is called the  dictyosome . A group of dictyosomes constitute the Golgi apparatus.   Typically a Golgi apparatus appears as a complex arrangement of i...