CELL BIOLOGY
ROBERT HOOKE GREGOR JOHANN MENDEL
(Father of cytology) (Father of genetics)
Born in 28 July 1635 Born in 20 July 1822
Cell is the basic unit that organizes a body. It is defined as “the structural, functional and hereditary unit of life.”
Cytology (cell biology) is the specialized branch of biological science that deals with the the study of different types of cells, its structure and its functions.
The concept that cell is basic (structural and functional) unit of life is known as cell theory. Credit goes to Robert Hook for the discovery of cell in 1665 in cork tissue.
HISTORICAL APPROACH IN CYTOLOGY
Aristotle (384-322B.C) put forward a concept that all animals and plants consist of certain structural units, termed as cell (by Robert Hook). R.Hook observed the cell in the form of cellula(empty space) in cork in 1665. Antony Van Leeuwenhoek discover the modern compound micro scope in 1674.He also discovered the bacteria in the form of wild animalcules(1675). In 1772 Corti purposed that cells is field by a dense proteinous substance. Felix Dujardin(1836), gives the name sarcode for dense substance of the cell. M.J Shleiden and Theodor formulated cell theory in 1838-1839. This theory states that, the animals and plants differs from each other superficially, but the pattern of organization and construction is similar. According to this theory animals and plants body composed of cell. Each cell acts independently and also functions as an integral part of complete organism. Schwann and Schielden considered the cell as functional and biological units.
Protoplasm theory:-
The term protoplasm (juicy living substance) was coined by Purkinje in 1840.It is derived from Greek words i.e. Protos = First and plasm = organization.
According to protoplasm theory, all living matters out of which animals and plants are formed is protoplasm. Also cell is the accumulation of protoplasm possesses a nucleus. The protoplasm occurs every where in the cell i.e. in the plasma membrane, nucleous and the portion in between the plasma membrane and nucleous.
The portion of protoplasm which occurs between the plasma membrane and nucleous is called cytoplasm. The term cytoplasm was given by E. Strasburger in 1882.
TYPES OF CELLS
On the basis specific function of cell, it is of three type’s i.e.
(a) Undifferentiated or, stem cells.
(b) Differentiated or, post mitotic cells.
(c) Dedifferentiated cells.
Undifferentiated or, stem cells
They are unspecialized and most important types of the cells which usually possess the power of division. For example zygot cell, stem apical meristem cells, Root apical merisem cells, bone marrow cells, etc.
Differentiated or, post mitotic cells
These are specialized cells and well known for their function. Cells are diffretiated in shape, size, structure and function. Cell activity is associated with some particular genes i.e. inducers and repressors.
Dedifferentiated cells
They are differentiated cells which take over the function of division. Such types of cells help in healing of wounds, regeneration in animals and vegetative propagation in plants.
Shape, Size and Number of cells
Cell shape:-
Measurable variability is found in the shape of different types of cells. The plants and the animal cells exhibit various forms and shape. Cells may be circular, oven, irregular, triangular, polygonal, cylindrical, rounded and elongated. Some cells like Amoebae and Leucocytes change their shape and some others have a typical shape. Some bacteria’s are rod shaped, some are spiral and even comas shaped. One important fact is the correlation of cell shape with its function. For example: - Epithelial cells are flat and macular cells are elongated. Cell shape also depends upon the external and internal environment.
Cell size:-
Cells are widely varies in the size. Mostly eukaryotic cells are microscopic in size but comparatively larger than bacterial cells.
The size of cells varies from 1 micron to 175 mm. The smallest cells are my co plasma having a size of 0.1 to 0.5 micron meter. The ostrich egg cell is usually considered largest cell with a diameter of 175 mm. Muscles and nerve cells are comparatively very large. Certain nerve cells are 3 – 3.5 feet in length.
Cell Number:-
Unicellular organisms consist of a single cell but in case of multicellular organisms, the number of cell depend upon the size and biomass of an organisms. Small sized organisms have less number of cells in comparison to large sized organisms.
Cell Structure:-
A plant cell consist of cell wall (outer protective wall) which is absent in animal cell. The protoplasmic concentration of a cell is denoted by protoplast. Plasma membrane, cytoplasm, nucleolus and vacuoles are the major components of the cell. Cytoplasm consists of Cytoplasmic matrix (hyaloplasm) and endoplasmic reticulum. Cytoplasm divisible into ectoplasm(outer cytoplasm) and endoplasm (cytoplasm which lies near nucleous). Cytoplasmic matrix are embedded in a large number of cell organelles i.e., mitochondria, plastic, ribosome’s, Golgi bodies, centrioles, lysosome, peroxisomes, microfilaments, microtubules, etc. These cell organelles are regarded as protoplasmic subunits.
Prokaryotic cell
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Ekaryotic cells
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Primitive or, false nucleus carrying cell
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Good or, true nucleus carrying cell
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Prokaryotic cells are generally present in uni-cellular organisms
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Eukaryotic cells are generally present in multi-cellular organisms
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DNA is circular and lies in the cytoplasm
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DNA is linear and bonded in a nucleus
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RNA and proteins synthesis occurs in same compartments
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RNA synthesized in the nucleus and protein synthesized in the cytoplasm
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Cell division occurs by amitosis
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Cell division occurs by mitosis and meiosis
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In prokaryotic cells nucleolus and spindle fibers are absent
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In eukaryotic cells nucleolus and spindle fibers are present
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Cell wall is made up of polysaccharides and muramic acid
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Cell wall is made up of cellulose and pectin
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Metabolic activity occurs by both aerobic and anaerobic activity
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Metabolic activity is occurs by only aerobic activity
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Eg:- Bacteria, cyanobacteria, E.coli, etc
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Eg:- protista, fungi, plants and animals.
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Difference between plant cell and animal cell.
Plant cell
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Animal cell
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Plant cell has a regide cell wall of cellulose
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animal cell having lack of cell wall
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Cell wall is the outer most boundary of plant cell
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plasma membrane is the outer most of animal cell
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Usually large in size
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Comparatively smaller than plant cell
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Having fix shape
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can change their shape
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A plant cell in general is autotrophic as it Contains chloroplast
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An animal cell is heterotrophic in general as it lack of chloroplast
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Nucleus lies towards the peripheral region of cytoplasm
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Nucleus lies in the centre
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Centriole, basal body and flagella are absent In the plant cell.
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Cntriole(except invertebrate), basal Body and flagella are present in the animal cell
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Mitochondria are comparatively fewer more
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Mitochondria are generally numerous
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Mitochondrial cristae are tubular
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Mitochondrial cristae are plate like
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A plant cell capable to synthesized all the Amino acids, coenzymes and vitamins
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An animal cell can not able to synthesized the amino acid coenzyme and vitamins
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CELL WALL
(Discovered by Robert Hook in 1865)
Cell wall is a nonliving, thick and rigid wall of cellulose. It is present in plant cell (including bacterial cell) and absent in animal cells. In case of plant cell, cell wall is the outer most layer but in animal cell, the outer most layer is plasma lemma which is also called plasma membrane or, cell membrane. Cell wall is secreted by cytoplasm (living protoplasm) of the cell.
The main function of cell wall is to protect the cell from mechanical injury and maintenance of cell shape.
Cell wall is mainly consisting of cellulose. However it may contain certain amounts of hemicelluloses, pectin, lignin, chitin, and chutin. Cell wall is composed of the following four layers.
(i) Middle lamella:-
It is mainly composed of calcium and magnesium pectate and having lack of cellulose. It is the outer most layer of the plant cell wall and connects the two adjacent cells. There is a minute pore like structure possessed by the middle lamella and adjacent primary cell wall which extend fine Cytoplasmic strands that link the content of another cells. These are called plasmodesmata that help in the exchange of materials between the cells.
(ii) Primary cell wall:-
Primary cell wall is mainly composed of cellulose and it is a thin elastic membrane lies between middle lamella and secondary cell wall. It develops after middle lamella by the deposition of hemicellulose, cellulose and pectin. Cells of the most leaves, fruits, and pits have only primary cell wall.
Secondary cell wall:-
The composition of secondary cell wall is basically similar to the primary cell wall i.e. it is primarily composed of cellulose microfibrils. In some tissues, besides cellulose, lignin and suberin are also found in the secondary cell wall. Secondary cell wall is multilayered in it self and relatively thick (5-10micron meter) and it is the inner most layer of the cell wall which lies between primary cell wall and plasma membrane.
(iv) Tertiary cell wall:-
It is also known as the lamella. Comparatively it is thin and formed of cellulose and xylan (polymer of xylose).
Function of cell wall:-
(i) # Protect the protoplasm from mechanical injury.
(ii) # Provide the shape and rigidity to the cell.
(iii) # Gives strength to the cells.
(iv) # Protect the cell from attack of pathogens.
(V) # It helps in the transport of substances between two cells.
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