The Cell Notes

THE CELL CONTENTS    Histroy Unicellular & Multicellular Organism  Cell Size :  The size of different cells ranges between broad limits.  Some plants and animals cells are visible to the naked eye. Ultra Structure of Cell   Most cells are visible only with microscope. The prokaryotic cells usually range between 1 to 10 m.  In 1665, Robert Hooke observed the cell in a cork The eukaryotic cells usually range between 10 to 100 m. slice with the help of a primitive microscope. He saw that cork resembled the structure of a honey comb consisting of many little compartments.   Robert Hooke called these boxes cells. Cell is a Latin word for 'little room.'  Amoeba proteus may reach a diameter of 0.5 mm. The smallest cells are those of Mycoplasma laidlawiil (0.1µ in dimeter) or PPLO (pleuro pneumonia like organism). The largest cell is egg of an Ostrich.  Cell Shape : All living organisms are made up of one or more cells. On the basis of cell number, organism are grouped into 2 categories -  Unicellular organisms :  Organism in which a single cell make the whole body. For example, Amoeba, Paramecium, bacteria & Chlamydomonas.  Multicellular organism :  Organism in which body contains more than one cell. For example - Plante, animals & fungi. Figure : VARIOUS CELLS FROM THE HUMAN BODY    The shape of cell may be variable or fixed.  Cells may be diverse shapes such as polyhedral  Plasma Membrane : Introduction : Variable shape occur in Amoeba, WBC etc. Fixed shape occur in most plant and animals. (8, 12 or 14 sides) spherical (e.g. eggs of mainly animals), spindle shaped (Smooth muscle fibres), elongated (e.g. Nerves cells) so on.  Cell surface in all the cells is enclosed by a living membrane which is called cell membrane by C. Nageli and C. Kramer (1855). Historical Account :  J.Q. Plower (1931) coined the term Plasmalemma for cell membrane. Ultrastructure :  Plasma membrane forms outer covering of each cell.  It is present in both plant and animal cells.  Plasma membrane is a living, thin, delicate elastic, selectively permeable membrane.  It separates contents of a cell from the surrounding medium.  Fluid Mosaic Model :  In 1972, Singer Nicolson proposed this model. According to this, cell membrane consists-two layers of phospholipid molecules, phospholipid & protein molecules are arranged as a mosaic.  Phospholipid molecules have their polar heads directed outward non polar tail pointing inward.   The proteins are of two types : Peripheral and integral. Peripheral proteins are located superficially while integral proteins are embeded in the phospholipid matrix. The protein monolayer have elasticity & mechanical support to the lipid matrix.   Functions of Plasma Membrane : The main function of plasma membrane is to regulate the movement of molecules inside and outside the cell.    It allow the movement of gaseous substance from high concentration to low concentration which known as diffusion. Water also obeys the low of diffusion . The movement of water molecule through a selectively permeable membrane is called osmosis. In plants, one another rigid is called 'Cell wall'. It is made up of cellulose which provide structural strength to plant.   It maintains the shape of cell. It protect the cells from mechanical injury & prevents their desication. It provide mechanical support against gravity. It is due to rigid cell walls that the aerial part of plant are able to keep erect & expose their leaves to sunlight. Cell walls permit the cells to with stand very dilute external media without bursting.   It is the most important part of cell which control all the activities of cell.    It play a important role in cellular reproduction. DNA contain the information necessary for Nucleoid - In some organisms nuclear region of cell may be poorly defined due to the absence of nuclear membrane. Such an undefined nuclear region called nucleoid. , Note :  Prokaryotic cell - Cell which do not have well defined nuclear region . called prokaryotic cells. Pro - Primitive Karyon - nucleus  Eukaryotic cells - Cells which have well defined nuclear region, called eukaryotic cells.  Along with nucleus membrane, prokaryotic cells lack most of cell organells.  Cytoplasm :  The fluid & semifluid matrix of a cell between the nucleus & the plasma membrane, containing various organelles is called cytoplasm.  Cell organelles :   Structure of Nucleus :  In a cell which is not dividing, this DNA is constructing & organizing cells. Nucleus :  Functional segments of DNA are called genes.  Function of nucleus :  Function of cell wall :   Chromosomes are composed of DNA and protein. present as part of chromatin material. The flexibility of cell membrane also enables the cell to engulf in food, which is also known as endocytosis. For example – in Amoeba  Cell wall :     Small membrane bound structures, which perform a lot of chemical activities to support the function & structure of a cell, called cell organelles - The nucleus has a double layered covering called nuclear membrane.  The nuclear membrane has pores inside the nucleus to its outside, that is, to th1e cytoplasm.  Endoplasmic Reticulum : The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.  Some important cell organelles are following - The endoplasmic reticulum (ER) is noticeable only with an electron microscope.  Structure :   Localization of organelles. It keeps the cell organelles properly stationed and distributed in relation synthesis.  Surface for protein synthesis. The RER offers extensive surface on which ribosomes carry protein synthesis.  Surface for synthesis of other substances. The SER provides surface for the synthesis of lipids including phospholipids, cholesterol and steroid hormones.  Packaging. The proteins formed on ribosomes pass into ER lumen where they are modified. Then, the modified proteins move into the transitional area where the ER buds off transport vesicles carrying the proteins to the Golgi apparatus. Here, they are further processed and packaged into secretory vesicle for export by exocytosis at the plasma membrane. Examples of secretory proteins include, mucus, digestive enzymes and hormones.  Detoxification. The SER brings about detoxification in the liver, i.e., it converts harmful materials (drugs, insecticides, pollutants and poisons) into harmless substances for excretion by the cell.  Formation of organelles. The SER produces Golgi apparatus, lysosomes and vacuoles.  Membrane formation. Plasma membrane and other cellular membranes are formed by ER. The ER is an extensive network of intracellular membrane-bound tubes and that occupies most of the cytoplasm in almost all eukaryotic cells. The membranes of this system are lipoproteinic in nature similar in structure to the plasma membrane. The ER is more prominent in young and dividing cells as compared to older cells. It is absent in prokaryotic cells.  Types. The ER is of two types :   Rough endoplasmic reticulum (RER)  Rough Endoplasmic reticulum (RER). These appear rough under a microscope because of the presence of a large number of grain-like ribosomes over their cytoplasmic surface. The ribosomes are the sites of protein synthesis. Thus, RER is engaged in the synthesis and transport of proteins. Generally, RER is more abundant in the deeper part of cytoplasm near the nucleus where it is connected with the outer membrane of the nuclear envelope. RER is well developed in the cells that synthesize and secrete proteins.  Smooth endoplasmic reticulum (SER) Smooth Endoplasmic Reticulum (SER). It consists mainly of tubules and vesicles. It is free of ribosomes and is more abundant near the peripheral part of the cytoplasm where it may be attached to the plasma membrane. The SER helps in the synthesis of fat or lipid molecules. It is, therefore, well developed in the cells that secrete lipids.  Golgi Complex : Introduction :   Functions :  Support. The ER acts as supporting skeletal framework of the cell and also maintains its form.  Transport of materials. The ER facilitates transport of materials from one part of the cell to another.  Exchange of materials. The ER helps in the exchange of materials between the cytoplasm and the nucleus. Golgi bodies are absent in prokaryotic cells. Golgi complex is found in all eukaryotic cells except RBCs. Historical Account :  Camillo Golgi (1898), a zoologist, observed Golgi bodies in the form of a network in nerve cells of barn owl. Ultrastructure :  It is also called Golgi complex or Golgi apparatus or Dictyosome (in plants cell).   It is made up of cisternae.  Functions of Lysosomes : Golgi bodies are interconnected with the tubules.  Lysosomes serve as interacellular digestive system, hence called digestive bags.  Lysosomes also remove the worn out and poorly working cellular organelles by digesting them to make way for their new replacement.  Mitochondria : Introduction :  A single mitochondrion is present in unicellular green alga, Microsterias. Number of mitochondria varies from 50–50,000 per cell. Mitochondria of a cell are collectively known as chondriome. Historical Account :  Functions of Golgi Apparatus :  The main function of Golgi apparatus is secretory.  It produces vacuoles or secretory vesicles which contain cellular secretions like enzymes, proteins, cellulose etc.  Golgi apparatus is also involved in the synthesis of cell wall, plasma membrane and lysosomes.  Lysosomes : Introduction :  Lysosomes are generally found in the cytoplasm of animal cells. Lysosomes exhibit polymorphism. Historical Account :  The term lysosome was introduced by De Duve in 1955. Ultrastructure :  It is also called demolition squads, scavengers, cellular house keepers and suicidal bags.  Lysosome are simple tiny spherical sac like structures evenly distributed in the cytoplasm.  Lysosome is small vesicle surrounded by a single membrane and contains powerful enzymes.  C. Benda (1897) gave the name Mitochondria (Mitos, thread + Chondrion, granules).  Term ‘Bioplast’ for mitochondria was used by Altman. Ultrastructure :  Mitochondria are rod shaped organelles, bounded by a double membrane envelope.  The outer membrane is smooth, the inner membrane surrounds a central cavity of matrix. Central cavity is filled with jel like substances   Inner membranes folds are called cristae, these folding are tubular and called microvilli.  Mitochondria contain electron transport systems aggregated into compact structure. F1 particles or  mitochondria i.e. they have the power to divide. Kreb’s cycle occurs in mitochondria.  Each particle is made up of base, stalk and head.  Functions of Mitochondria :  Mitochodria are called power plants or power houses or cellular furnaces.  Synthesis of ATP (Adenosine Tri-phosphate) in mitochondria is called oxidative phosphorylation.  Mitochondria as place of cellular respiration was first observed by Hogeboom.  Functions of Plastids :  Introduction :  Chromoplast impart various colour of flower to Vacuoles :  Function : Plastids are organelles enclosed by a double Vacuoles perform following functions :  Historical Account :  animal cells. In some inicellular organisms, specialized vacuoles maintain water balance of the body (osmoregulation). Ultrastructure : Plastids occur in most plant cells and are absent in They ar storage sacs of the cell. The stored matieral may be solid or liquid food or toxic metabolic by-products or end products of cells. E.Heckel (1865) gave the term plastid. Plastids are largest cell organelles. Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90% of the cell volume. membrane found in all plants.  Chloroplasts trap solar energy and utilized it to manufacture food for the plant.  Plastids :  Plastids also have double membrane but no cristae. attract insect for pollination.   Schimper divided plastids into three types : (a) Chromoplast - Coloured plastids (except green colour) (b) Chloroplast - Green coloured plastids (c) Leucoplast - Colourless plastid. oxysome, tennis racket like bodies on inner membrane involved in oxidation & phosphorylation.  Plastids are self replicating organelles like  In plants, they provide turgidity and rigidity to the cells.