HISTORY OF CELL THEORY




History of cell theory:

If we go back to the scientific discoveries, the study of cell has continued for the last more than three and a half centuries. For such a study, basic requirement was that of anmiceoscope or instruments with good resolving power and magnification. Other needs included preservation, sectioning, staining and mounting techniques to study and distinguish various cellular components. Continuous improvement in the tools and techniques in all these period has enhanced our knowledge about cell biology. 

The term cell was introduced by an English scientist Robert Hooke in his book Micrographia published in London in 1665.With this, he launched the study of microscopic anatomy. He examined thin slices of cork under a primitive microscope he had assembled. He saw tiny,empty compartments in the slices .He called them cellulae now termed  cells;and through them to act as passages for conducting fluids. Since cork is the dead bark of a Spanish oak,Quercus,Hooke saw only the dead walls of plant cells.These cells had lost their living contents and looked like small rooms.Hence,he gave each compartment an appropriate name, the cell ( L.Cella= chamber, cellular = dim of cella). Hooke also observed many other plants under the microscope and found the same structure. We now know that the really important parts of a cell are its contents, and it may or may not have a well.Thus, the know that the really important parts of a cell are its contents, and it may or may not have a well.Thus,  the term cell is a misnomer. However, the term is still used because it is simple and a suitable substitute has not been found.After Hooke, the early microscopists started studying the fine structure of the organisms. The greatest among these was Anton Von Leeuwenhoek He was a Dutch merchant and pursued microscope as a hobby.He designed remarkable simple microscopes, and with these he first described free cells such as human  sperm cells, bacteria, protozoan and red blood corpuscles from 1675 to 1680.He celled the swimming creatures "animalcules".Alfonso Corti,in 1772,noted living material in the cells.Nehemia Grew,in his two volumes on microscopic plant anatomy published in  1682,laid the foundation for the cell concept,i.e., a cell is the unit of structure in organisms. 





Cell study developed rapidly in the nineteenth century, aided by great improvement in the microscope lenses.In 1809,Lamarch  stated that " no body can have life if its constituent parts are not cellular tissue or are not formed by cellular tissue." A French biologist H.J.Dutrochet  clearly stated in 1824 that " all organic tissues, are actually globular cells of exceeding smallness, which appear to be United only by simple adhesive forces: thus all tissues, all animal and plant organs are actually only a cellular tissue variously modified." In 1831, a Scotch botanist* Robert Brown  discovered the nucleus in an orchid root cell.By this time, it had also been found that the cells were surrounded by some sort of limiting structure, the cell membrane. A French zoologist Dujardin discovered in 1835 a semi fluid living material in certain protozoan.He called this material sarcode.In 1840, a similar material was noted in the plant cells by a Bohemian biologist Johannes Purkinje.He proposed for it the name  protoplasm,' the first substance ' ( G.protos = first; plasma = substance).A German biologist, Hugo Von Mohl, in 1946, used the same term for the slime_ like substance, which he found circulating in some plant cells.In 1838, a German botanist,  M.j. Schleiden ( 1804__ 1881)announced that all plants were composed of cells; and a year later, in 1839, a German zoologist, Theodore Schwann ( 1810__ 1882)  stated that all animals were also formed of cells.Schwan found that the animal cells had nuclei and were enclosed by thin cell membranes, now cells.Schewam found that the animal cells had nuclei and were enclosed by thin cell membranes, now called plasma membranes,instead of thick cell walls.He noted that, except for the cell wall, the plant and animals cells were more or less similar, having a nucleus located in a clear substance. He defined the cell as a membrane _ bound, nucleus _ containing structure. He proposed Cell Hypothesis stating that the bodies of plants and animals are composed of cells.Schleiden and Schwann exchanged  their investigation and applied the cell concept to all organisms and this formed the basis for the cell theory.



Cell Theory

This theory was jointly put forward by Schleiden and Schwann whi h states that the bodies of a organisms are made up of cells and their products so that cells are units of both structure  and function of living organisms. 

The Cell theory  is one of the most fundamental generalizations of biology. It was put forward by Schleiden and Schewann in 1839 in their paper" Microscopic Investigations on the Similarities of Structure and Growth in Animals and Plants".




Formulation of Theory :

Schleiden found that  all part tissues were made of one or the other kind of cells and thus concluded that cells constitute the ultimate units of all plant tissues. Schwann found that animal cell lack a cell wall. He proposed a phothesis _ bodies of animal and plants are made of cells and their products. 

    Schleiden  and Schwann compared their findings , discussed  Schwann's hypothesis and formulated the cell theory in their joint paper. The cell theory  proposed by Schland Schwan states that cells are the units of both structure  and function of organisms  and new cells developed either spontaneously or by budding of the nucleus. Rudolf Virchow observed  that new cells develop  by division  of the preexisting  cells_ Omnis cellula a cellula ( theory of cell lineage). Soon Haeckel established that nucleus stores and transmits hereditary  traits. Louis Pasteur successfully  established  that life originates from pre_ existing  life.


Fundamental  Features:

Five fundamental observations of the cell theory are

1): Organisms are composed of different  types of cells and their  products. 


2): Each cell is made of a small mass of protoplasm having a nucleus in its inside and is surrounded by a plasma membrane  with or without a cell wa on its outside

3): New cells arise from pre__ existing  cells.

4): All cells are basically  alike  in their chemistry and physiology. 

5): Activities  of an organism are the sum total of activities and interactions of its constituent cells.

Modern Cell Theory:

1): The bodies of all living beings are made up of cells and their  products. 

2): Cells are units of structure  in the body of living  organisms. 

3): Each cell is made up of a small mass of protoplasm having a nucleus, a number  of organelles  and a covering  membrane. 

4): While a cell can survive independently , the ce organelles  cannot do so.

5): The cells belonging  to different  organisms  and different  regions of the same organism  are fundamentally  similar  in their structure,  chemical  composition  and metabolism. 

6): Life exist only in cells because all the activities of life are performed  by cells.

7): Cells are units of function in living organisms,  that is, the activities  of an organism are the sum total of the activities of its cells.

8): The cells get modified, depending  upon specific requirement, e.g., elongated in muscle and nerve cells, loss of nucleus  in RBCs or cytoplasm  in outer skin cells.

9): Each cells maintains its individuality having a specific internal environment and homeostasis. 

10): Growth of an organism involves the growth and multiplication of its cells.

11): Genetic information  is stored and expressed inside cells.

12): Life passes from one generation to the next in the form of a living  cell.

13): New cells arise from pre__ existing  cells  through  division and all new cells contain the same amount  and degree of genetic  information  as contained in the parent cell.

14): All the present day cell ( and hence organisms) have a common ancestry because they are derived from the first cell that evolved on the planet through  continuous  line of cell generations. 

15): Basically  the cells are totipotent ( i.e, a single cell can give rise to the whole organism) unless and until they have become extremely  specialized .

16): Though a unit of structure  and function, each cell  of an organism is able to act independently in its growth,  division,  metabolism  and even death. 

17): No organisms,  organ or tissue  can have  activity that is absent in its cells.

Objections :

(i)Though virus do not have a cellular machinery  yet they are considered  to be organisms. 
 

(ii) In some organisms  the body is not differentiated into cells though  it may have numerous nuclei ( coenocytes,e.g., Rhizopus).


(iii)Protozoans and many thallophytes have a uninucleate differentiated body ( e.g. Acetabularia) which cannot  be divided into cells. They are acellular. 


(iv) Bacteria  and cyanobacteria do not possess  a nucleus and  many  cell organelles are also absent. 

(v) RBCs and sieve tube cells continue to live without  nucleus and other vital organelles.


(vi)Protoplasm is replaced by nonliving  materials in the surface  cells of skin and cork.


(vii) Connective  tissue contains  a lot of intercellular  material  or matrix in which the living  cells are embedded. 


(viii)Schleiden and Schwan did not know  the mechanism of ce formation .Schwann believed cells to develop spontaneously like a crystal. Schleiden though new cells to develop  from cytoblast or nucleus.

Surface: Volume Ratio

The factors which set the limit of cell size or volume are

(i) Nucleo _ cytoplasmic  or kern__ plasma ration  determines the range of control of metabolic activities  by nucleus. 

(ii) Ability of oxygen  and other materials  to reach every part of the cell.

(iii) Ability  of waste products to pass to the outside. 

(iv) Rate of metabolic  activity. 

(v) Ratio of surface area  to  the volume  of the cell.

Metabolically active cells are usually  smaller due  to higher nucleocytoplasmic ratio and allow the nucleus to have better control of metabolic activities and higher surface volume ratio, which allow quicker exchange  of materials  between  the cell and its outside environment .Surface volume ratio decreases  with the increase in cell size or volume. 

Large cells tend to become less efficient. All passive cells like eggs are larger on size, while all active cells are smaller.If larger cells are to remain active, they are either cylindrical  in shape or possess several extensions  of the cell membrane, e.g., microvilli which are found in all those cells which are active in  absorption . Membrane  infoldings also occur in transfer cells found in plants in the region of absorption or secretion  of nutrients. 

Tools & Techniques Employed in Cell Biology 

Microscopy :

Many cells  are very small to be seen clearly by the naked eye. So, many instruments have been invented, designed and made use of in observing cellular details.Our eye cannot resolve substances smaller than 0.1 mm. So we require the aid of magnifying lenses. A series of lenses are made use of in order to magnify minute objects. This is in principle called microscope. The microscopic measurements are made by an instrument called micrometer. This  consists of 2 parts, a stage micrometre and an ocular micrometre.

Resolving Power of a Microscope 

The power to discern two  separate points as distinct, when they are close together is called the resolving power. 

A good microscope lens has the resolving power of about 1.4. Even powerful microscope using glass lenses can have only magnification of about 2000x. Resolution is improved when source emitting rays that have shorter wavelengths are employed. eg., the resolving power of the ultraviolet microscope ( which requires quartz optics because glass does not transmit ultraviolet light) is approximately double  that of the light mmicroscope. The magnification of the ordinary laboratory  microscope is equal to the product of the magnification of the eye piece system.

A compound microscope is made of 3 units.

1)  Considenser lens system is together the light rays and make them pass through the object on the stage. 

2) The objective lens system is found near the object and magnifies the image. 

3) Eye piece lens system is near the receiver's  eye and magnifies the image formed by the objective system.

Variations of Compound Microscope 

Ultraviolet Microscope 

Ultraviolet rays and quartz lenses are used. The image cannot be viewed with eyes but photographed. Resolving  power is approximately double that of light microscope. 


Fluorescence Microscope 

Ultraviolet rays are used, the cellular components having the fluorescence property is used to distinguish between them. Non fluorescing structure  can be made to fluoresce by staining them with fluorescent dyes (fluorochromes).One of the most popular contemporary uses of fluorescence microscopy involves the preparation of antibodies that will bind to specific cellular proteins. The antibodies are first complexed with fluorescein ( a fluorescent dye) and the fluorescein labelled antibody is then applied  to  the cells. Cell structure  containing the specific proteins capable of binding  the  fluorescein labelled antibody are caused to fluoresce when examined with fluorescence microscope. 

Phase Contrasts microscope:

This microscope was discovered by Zernike ( 1955). It works on the principle of difference between, the refractive index of different  cellular components due to different  densities.  The technique is especially useful for examining living  materials without fixation or staining. In the phase contrast microscope, the phases of light rays entering  the object are shifted by annular diaphragm below the condenser. The phase of the rays  passing through and around the object  are shifted again by a phase plate in the objective lens. The result in increase in the contrast of the object as certain regions appear much brighter,  owing to additive effects of rays brought into phase, while other regions appear  much darker, owing to the cancelling effects of rays shifted further out of phase i.e., In a bright contrast phase system two sets of light rays are added onto make a brighter image and in the dark contrast phase system they partially  cancel each other providing  a darker image.

Electron Microscope 

Developed in 1930s by Knoll and Ruska, with which magnifications of several hundred thousands are a practical  possibility. In this, electrons are made to pass through the object instead of light rays. Electromagnet lenses are used to focus the electrons instead of glass lenses. The object is mounted on a grid  and living objects cannot be used. The image of the object  is made to fall or may be viewed on Zinc__ sulphide screen ( fluorescent screen) or photographic plate.TEM and SEM are  distinct types of electron microscopy that employ different  methods of specimen preparation, irradiation  and image formation to study the details  of cell structures and components. 

TEM uses thinly sliced, plastic embedded sections that are stained with heavy metal salts. TEM is used to study  the fine details of cell structure. Such as the morphology  of cell surfaces ( in cross sections) and the internal elements of cells; it can resolve features as small as 0.5 nm. 

SEM uses whole specimens that are subjected to critical  point drying ( dehydration of specimen without passing it through an air_ water interface, thus minimizes specimen distortion due to tension) and then coated with a thin layer of gold and palladium. It is used to study 3_ D features of cell surfaces; it can resolve features as small as 5nm.

Phase Interference Mucroscope 

It is used for quantitative studies of various macromolecules of the cells.

Differential Interference Contrast Mucroscopy 

It uses special condensers and objective lenses to transform differences in refractive index into an image that appear  to have 3_ D character. In this type of microscopy, the nucleus and various  cytoplasmic Inclusions appear in low relief.

Cytological Techniques 

The preparation  of biological material for examination with either  the light microscope or the transmission electron microscope involves a series of physical and chemical manipulations that include 

(a)  Slection (b) Fixation (c) embedding  (d) Sectioning (e) Staining  (f) mounting 

Selection 

The material  to be examined is first selected because for certain studies only certain tissues give best results. These are selected and fixed at particular times. 

Fixation 

Fixation kills the cells but preserves the molecules in that state. Fixation can be done by many chemicals like formaldehyde, acetic acid, osmium tetraoxide, potassium permanganate, glutaraldehyde etc. After fixation for the required length of time, the samples are dehydrated by successive  exposures to increasing concentrations of alcohol or acetone.This is followed by embedding. 

Embedding 

If the tissues are very small they are embedded   in paraffin or celloidin in order to take sections. The wax, which both surrounds the tissue and infiltrates it, hardens on cooling, thereby supporting the tissue externally and internally. The resulting paraffin block in then trimmed to the appropriate shape before being sectioned. 

Sectioning 

Sectioning or cutting tissues is done by instrument called microtome.This is followed by staining. 

Staining 

There are many chemical dyes to stain particular  components of cells. The stains which do not kill the living tissues are called vital stains which improves contrast but do not interfere with  normal cell activity. 

Mounting 

Sections prepared after staining  are mounted on glass slides ( light mucroscopy). However, sections to be examined with electron microscope are generally  not stained ( no colours are seen with the electron micro), although contrast may be improved by ' post _ staining ' with electron_ dense materials such as  uranyl acetate, uranyl nitrate and  lead citrate. The sections are mounted on copper ' grids' ( small disks perforated with numerous opening) that have been coated with a thin film of carbon. The grid supports the film, which in turn supports the thin section. Thus, the sections are ready for observation under a light or electron microscope. 

Cell Fractionation 

Organelles of cells can be separated and examined  using technique. First the cells are broken in order to release  the contents. This is called homogenization. This centrifuges of different rpm ( revolutions per minute) are used to separate particular components. During centrifugation, particles sediment through the medium  in which they are suspended at rates related  to their size, shape and density.  Differential centrifugation, introduced in 1940s by Albert Claude, is one of the procedures for isolating subcellular  particles and involves  the stepwise removal of classes of particles by successive  centrifugation. 

Chromatography 

This is the technique  used to separate the molecules of different  substances present together  in a solution. There are different  types of chromatography such as paper chromatography, Ion_ exchange chromatography, thin large chromatography ( TLC), affinity chromatography, gel filtration, high _ performance liquid chromatography ( HPLC), gas ch chroma__ tography. 


(a) Paper Chromatography : It is a technique in which a mixture of solute is separated into discrete zone on a sheet of filter paper on the basis of differences in solute partition between  the stationary aqueous phase bound to the cellulose fibres of the paper and s mobile organic liquid phase passing through the sheet by capillary action ( liquid_ liquid partition) and differences in solute adsorption to the cellulose fibres and dissolution in the mobile liquid ( solid_ liquid partition).


(b)  Thin layer chromatograph ( TLC) : It is modification of paper chromatography in which sheets of filter paper are replaced by glass or plastic plates covered with thin, uniform layer of absorbent. 


(c) Ion__ exchange chromatography ( column) : Proteins and other macromolecules may also be separated by this technique. The separation is done in tall glass columns packed with resins such as diethylami noethylcellulose ( DEAE_ cellulose) and carboxymethyl_ cellulose ( CM  _ cellulose). The rate of separation depends on the affinity for cation_ anion exchanges  sites. 

(d) Affinity chromatography : It is novel form of column chromatography in which molecules  ( proteins and nucleic acids) to be isolated  are retarded in their through the column by their biological reactions with the column matrix. 

(e) Gel filtration : A chromatographic method for separating  molecules on the basis of size ( molecular weight) differences; technique is also called gel permeation chromatography,  exclusion chromatography and molecular  sieving. The glass column are packed with non_ ionic, porous gel particles ( acrylamide).

 (f)  High__ performance liquid chromatography  ( HPLC) : It is highly specialized and versatile form of chromatography that, depending on the particular combination of packing material and solvent can incorporate the principles of partition, ion_ exchange, exclusion or affinity  chromatography.

(g) Gas chromatography : It is a special  form of column chromatography in which a gas is used as the mobile phase ( instead of  a liquid) and either a liquid  or a solid is used as the stationary phase. When liquid is used as the stationary phase, the technique is called gas_ liquid chromatography ( GLC).When stationary phase is solid, it is called gas_ solid chromatography. The gas generally used are nitrogen, carbondioxide, helium or argon.

Electrophoresis 

Molecules are separated from one another in electrical potential gradients on the basis of differences in their net charges, sizes and shapes. The  method is employed most often for the separation of different  proteins. 

Spectrophotometry 

This technique is used to find the qualitative and quantitative analysis of mixtures of molecules in solution. The  solution containing  the  dissolved  chemicals is exposed to selected wavelength and the absorption spectrum recorded. It is compared to standard absorption spectra of different molecules to know composition of solution. 

Autoradiography 

Cells use many atoms of substances to synthesize their molecules. Some of these can be made radioactive and these radioactive substances emit rays which can form images in photographic emulsion. By studying successive stages of the cells by autoradiography one can follow the fate of molecules. This helps one to understand  many chemical events in succession. Various  radio isotopes used are_ H³, C¹⁴, P³², S³⁵.

X_ ray crystallography ( Bragg, 1913):

It is the process of determination of molecular structure by passing  x_ ray through the substance present in a crystalline state. It can give important information about the arrangement of atoms in the molecule of substances such as enzymes.  X_ ray microscope was developed by Kirkpatric. Watson and Crick ( 1953) confirmed the structure of DNA from x_ ray diffraction  obtained  by Astbury and Franklin ( 1953).

Reagents used 

(a)Normal saline _ 0.9 % solution of sodium chloride. 

(b) Ringer solution _ It is used in cytological studies. It is prepared by dissolving NaCI ( 0.85gms), KCI ( 0.025gms.) and CaCI ( 0.03gm) in 100 c.c. of distilled water. 

Stains Used in Cytochemistry 

(a) Acid fuchsin _ for chitinous tissue 

(b) Acetocarmine_ for chromosome 

(c) Aceto _ orecin _ for chromosome 

(d) Borax _ carmine _ for cytoplasm 

(e) Eosin_ for cytoplasm 

(f) Fast green _ for  cytoplasm 

( g) Janus green _for Mitochondria 














































































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