CELL: THE UNIT OF LIFE POINTS TO REMEMBER : CELL THEORY : • Schieiden and Schwann together formulated the cell theory. • They proposed that the body of animal and plant are composed of cells and product of cells. • Rudolf Virchow (1855) he modified and gives the final shape to the cell theory. o All living organisms are composed of cells and products of cells. o All cells arise from pre-existing cells. PROKARYOTIC CELLS : • Represented by bacteria, blue-green algae, mycoplasma and PPLO. • Smaller and multiply more rapidly than eukaryotic cells. • Four basic shape of bacteria are o Bacillus (rod like) o Coccus (spherical) o Vibrio (comma shaped) o Sprillum (spiral) • All prokaryotes have a cell wall surrounds the plasma membrane. • There is no well defined nucleus. • Chromosome is single and circular, not enveloped by nuclear membrane. • Presence of many small, circular, extra chromosomal and self replicable DNA called plasmid. • Plasmid provides unique characteristic to the bacteria. • One of the characteristic is antibiotic resistant. • No membrane bound cell organelles. • A specialized differentiated form of cell membrane called mesosome is present. Cell envelope and its Modifications : • Most prokaryotic cell have characteristic complex cell envelope. • The cell envelops consists of tightly bound three layer structure; o The outermost glycocalyx. o Middle cell wall o Inner plasma membrane. • According to the nature of the cell wall and behavior towards the stain developed by Gram, bacteria are Gram positive or Gram negative. • Glycocalyx may for a loose sheath called slime layer. • Glycocalyx may be thick and tough called the capsule. • The cell wall determines the shape of the cell, strong structural support to prevent the bacterium from bursting and collapsing. • Plasma membrane is semi-permeable in nature and interacts with outside world. • A special membranous structure is the mesosome which is formed by extensions of plasma membrane into the cell. o These extensions are in the form of vesicles, tubules and lamellae. o They help in cell wall formation, DNA replication and distribution to the daughter cell. o They also help in respiration and secretion. • Some bacteria have filamentous extensions called flagella. • Bacterial flagellum has three parts – filament, hook and basal body. • Pili are elongated tubular structures made of a special protein. • Fimbriae are small bristle like fibres sprouting out of the cell. Play role in adhesion. Ribosome and inclusion bodies : • Ribosomes are associated with the plasma membrane of the cell. • Each ribosome (70 S) has two sub units; smaller (30 S ) and large (50 S). • Ribosomes are the site of protein synthesis. • Several ribosomes attached to a single mRNA to form a chain called polyribosome or polysome. • Reserve materials are stored in the form of inclusion bodies. • Phosphate granules, cyanophycean granules and glycogen granules are the inclusion bodies. EUKARYOTIC CELL : • Eukaryotes include all the Protists, plants, animals and fungi. • Extensive compartmentalization due to membrane bound cell organelles. • Possess an organized nucleus with nuclear envelope. • Have a variety of complex locomotory and cytoskeletal structure. • Genetic materials are organized into chromosomes. • The plant cell possess cell walls, plastids and a large central vacuole, which are absent in animal cell. • Animal cells have centrioles which absent in plant cells. Cell membrane : • The cell membrane composed of lipids that arranged in bilayer. • Lipids are arranged within the membrane with the hydrophilic polar head towards the outer sides and the hydrophobic tails towards the inner part. • Non polar tail of saturated hydrocarbons is protected from the aqueous environment. • The lipid component of the membrane mainly consists of phophoglycerides. • Cell membrane also possesses protein and carbohydrates. • Ratio of protein and lipids varies from cell to cell. • Human erythrocyte plasma membrane contains 52 % protein and 40 % lipids. • Membrane protein may be integral or peripheral. • Peripheral protein lie on the surface and integral proteins are partially or totally buried in the membrane. • The improved model of the structure of plasma membrane was proposed by singer and Nicolson (1972) widely accepted as fluid mosaic model. • According to this the quasi fluid nature of lipid enables the lateral movement of proteins within the overall bilayer. Function : • Passive transport. o Simple diffusion o Facilitated diffusion. • Active transport. • Phagocytosis • Exocytosis. • Pinocytosis. Cell wall : • A non-living rigid structure called cell wall present outside the plasma membrane of plant and fungal cell. • Algae have a cell wall made of cellulose, galactans, mannans and minerals like calcium carbonate. • Plant cell wall consists of cellulose, hemicelluloses, pectins and proteins. • The cell wall of young plant is called primary cell wall. • On maturity secondary cell wall formed inner to it. • The middle lamella is a layer of calcium pectate which holds or glues the neighboring cells. • The cell wall and middle lamella may traversed by plasmodesmata; the cytoplasmic connection between two adjacent cell. The endomembrane system : Endoplasmic reticulum : • Network or reticulum of tiny tubular structures scattered in the cytoplasm, called endoplasmic reticulum. • It divides the intracellular space into two distinct compartments: o Luminal (inside the ER) o Extra luminal (cytoplasm). • The ER shows ribosomes attached to their outer surface called Rough Endoplasmic reticulum (RER). • Endoplasmic reticulum without ribosome called SER (smooth endoplasmic reticulum) Function: • RER present in the cell actively involved in protein synthesis. • SER is the site for synthesis of lipid, glycogen and steroid hormones. Golgi apparatus: • Camillo Golgi (1898) first observed this organelle, and named after him. • It consists of many flat, disc shaped sacs or cisternae. • These are staked parallel to each other. • The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cisor the forming face and concave transor the maturing face. Function: • Principally responsible for packing of materials to be delivered intra-cellular target or intercellular target. • Materials are packed in the form of vesicles, from the ER fuse with the cis face of the Golgi apparatus and move towards the trans face. • Important site for the formation of glycoprotein and glycolipids. Lysosomes: • Membrane bound vesicular structure. • Formed by Golgi body. • Rich in all type hydrolytic enzymes, optimally active in acidic pH. • These enzymes are capable to digesting carbohydrates, proteins, lipids and nucleic acids. Vacuoles: • The vacuole is the membrane-bound space found in the cytoplasm. • It contains water, sap, excretory product. • Vacuole is bounded by a single membrane called tonoplast. • In plant cells the vacuole can occupy 90% of the volume. • Tonoplast facilitates active transport of material from cytoplasm into the vacuole. • In Amoeba the contractile vacuole is important for excretion and osmoregulation. • In many protists, food vacuoles are formed by engulfing the food particles. Mitochondria: • The number of mitochondria varies according to the physiological activity of the cell. • Each mitochondrion is a double membrane bound structure with outer and inner membrane, dividing its lumen into two aqueous compartments o Outer compartment. o Inner compartment. • The inner compartment is called the matrix. • The inner membrane forms a number of infoldings called the cristae towards the matrix. • The cristae increase the surface area. • Two membranes have their own specific enzymes associated with the mitochondrial function. • Mitochondria are the site of aerobic respiration. • They produce cellular energy in the form of ATP, hence called ‘power house’ of the cell. • The matrix also possess single circular DNA molecule and a few RNA molecules, ribosomes (70S), they synthesize their own protein. • Mitochondria divide by fission. Plastids : • Plastids are found in all plant cells and in Euglenoids. • They bear some specific pigment, impart specific colour to the plants. • Based on the type of pigments plastids can be classified into o Chloroplast. o Chromoplast o Leucoplast. • The chloroplasts contain chlorophyll and carotenoid pigments, traps solar energy for photosynthesis. • In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls are present. • The leucoplasts are colourless plastids of varied shapes and size with stored nutrients. o Amyloplast - store carbohydrates. o Elaioplasts – store oils and fats. o Aleuroplast – store proteins and minerals. • Chloroplasts are found in the mesophyll cells of the leaves. • These are oval, spherical, discoid or even ribbon like organelles. • Chloroplast is a double membrane organelle. • The space limited by inner membrane is called stroma. • A number of organized flattened membranous sacs called thylakoid are present in the stroma. • Thylakoids are arranged like stakes of coins to form grana. • There are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana. • They thylakoids enclose a space called lumen. • Chlorophyll pigments are located in the thylakoids. • Chloroplast contains enzymes required for the synthesis of carbohydrates and proteins. • Stroma contains small circular DNA and ribosomes. Ribosomes : • Ribosomes are granular structure first observed by George Palade (1953). • Composed of ribonucleic acid (RNA) and proteins. • Non-membranous cell organelles. • Eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S. • ‘S’ stands for sedimentation coefficient; measure of density and size. • Both 70S and 80S ribosomes consists of two subunits. • Primary function is protein synthesis hence called protein factory of the cell. Cytoskeleton : • An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively known as cytoskeleton. • Cytoskeleton involved in many function such as mechanical support, motility, maintenance of the shape of the cell. Cilia and Flagella : • Cilia and flagella are hair-like outgrowths of the cell membrane. • Cilia are small help in the movement of cell or surrounding fluid. • Flagella are longer and responsible for cell movement. • Cilia and flagella covered by plasma membrane. • Their core called axoneme, possess a number microtubules running parallel to the long axis. • The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules and a pair of centrally located microtubules. • The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublet by radial spoke. • The peripheral doublets are also interconnected by linkers. • Both cilia and flagella emerge from centrioles-like structure called basal bodies. Centrosome and centrioles : • Centrosome is an organelle usually containing two cylindrical structures called centrioles. • They are surrounded by amorphous pericentriolar materials. • Both centrioles in a Centrosome lie perpendicular to each other. • Each centriole has an organization like the cartwheel. • They are made of nine evenly spaced peripheral fibrils of tubulin. • Each of the peripheral fibril is a triplet. • The adjacent triplets are also linked to each others. • The central part of the centriole is called hub. • The hub connected to peripheral triplets by radial spokes. • The centriole forms the basal body for cilia, flagella and form spindle fibres during cell division. Nucleus : • Nucleus as a cell organelle was first described by Robert Brown in 1831. • Materials inside the nucleus was stained by Flemming and named as chromatin. • The interphase nucleus has highly extended and elaborates nucleoprotein fibres called chromatin. • The nucleus also contains nuclear matrix and one or two spherical bodies called nucleoli. • Nuclear envelope consists of two membranes with perinuclear space (10- 50 nm). • The outer membrane remains continuous with endoplasmic reticulum. • Presence of nuclear pore due to fusion of two membranes. • Nuclear pores allow the movement of RNA and protein in both directions. • The nuclear matrix or nucleoplasm contains nucleolus and chromatin. • Nucleolus is the site for active ribosomal RNA synthesis. • During cell division the chromatins condensed to form chromosomes. • Chromatin contains DNA and some basic proteins called histones, non-histone proteins and some RNA. • A single human cell contains approximately two meter long thread of DNA in 46 chromosomes. • Each chromosome essentially has a primary constriction or the centromere. • On each side of centromere there is disc shaped structures called kinetochores. • Based on the position of the centromere chromosomes are classified into four types:- o Metacentric: centromere at the middle with two equal arms. o Sub-Metacentric: one short arm and one long arm. o Acrocentric: with extremely short arm and a very long arm. o Telocentric: with terminal centromere. • A few chromosomes have non-staining constrictions at a constant location. This gives the appearance of a small fragment called the satellite.
Posted on: Mon, 29 Sep 2014 05:30:12 +0000
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