CELL:THE UNIT OF LIFE1.Cells were first discovered by Robert Hooke--- Dead cells.2.Unicellular organisms--The organisms composed of only a single cell. eg- ameoba,bacteria etc.3.Multicellular organisms--organisms made of more than one cell.eg-plants and animals.* Features of unicellular organisms- unicellular organisms are capable of-1. independent existence.2.performing the essential functions of their life. CELL AS THE STUCTURAL N FUNCTIONAL UNIT OF LIFE1.All living organisms are made up of cells.Thus, cell is the structural unit of life.Every type of cell organelle performs a special function,such as making new material in the cell,cleaning up the waste material & so on.A cell is able to live & perform all its functions because of these cell organelles.The organelles constitute the basic unit called cell.2.features of unicellular organisms + anything less than a complete structure of cell does not ensure independent living.Hence.cell is the fundamental structural & functional unit of all living organisms.4.Anton Von Leeuwenhoek first saw & described a live cell.5.Robert Brown discovered the nucleus.CELL THEORY In 1838 ,Mathias Schleiden,a german botanist,examined a large no. of plants & observed that all plants are composed of different kinds of cells which form the tissues of the plant. At the same time,Theodore Schwann(1839),a british zoologist,studied diff. types of animal cells & reported that cells had a thin outer layer known as plasma membrane. He concluded on the basis of his study on plant tissues that the presence of cell wall is the unique feature of the plant cells. On the basis of this Schwann & Schleiden together formulated the cell theory:The bodies of plants and animals are composed of cells and products of cells.Drawback:they couldnt explain as to how new cells were formed. Rudolf Virchow countered this drawback & first explained that cells divided and new cells are formed from the pre-existing cells.He modified the cell theory .Cell theory as understood today is:1.All living organisms are composed of cells and products of cells.2,all cells arise from pre-existing cells. AN OVERVIEW OF CELL1.Prokaryotic cells-cells that lack a membrane bound nucleus and cell organelles are called prokaryotic. Eg- a bacterial cell2.eukaryotic- cells that have a membrane bound nucleus and cell organelles are called eukaryotic.eg-animal & plant cells.3.The human cheek cells have an outer membrane as the delimiting structure of the cell.inside each cell is a nucleus.the nucleus contains the chromosomes which inturn contain the genetis material ,DNA.PARTS OF CELL:1.NUCLEUS:it is a dense membrane bound structure present in the center of the cell.2.PLASMA MEMBRANE: each cell is bound by a thin,delicate and elastic living membrane, called the plasma membrane.it is a semipermeable membrane,provides a definite shape to the cell & allows the entry n exit of required substances.3.CYTOPLASM: A jelly-like matrix occupying the volume of the cell.The cytoplasm is the main arena of cellular activities in both plant n animal cells.Varios chemical reactions occur in it to keep it in the living state.*these are the main parts of a cellSIZE OF A CELL,SHAPE & FUNCTION:Cells differ greatly in size,shape & activities.1.Mycoplasma(pleuro pneumonia like organisms) is the smallest cell-0.3 micrometer in length ,while a bacterial cell could be 3 to 5 microm.Acetabularia is the largest cell of plants.2.The largest isolated single cell is the egg of an ostrich(170x135 mm).among multicellular organisms,human RBCs are about 7.0 microm in diameter.Nerve cells are the longest cells.3.cells may be disc-like,polygonal,columnar,cuboid.thread-like,or irregular.*relationship b/e size & function.:the size of the cell is independent of the size of the organism.i.e.,the elephants or whales do not possess large cells and not even the cells of rats are small.The only thing to be kept in mind is that the size of the cell only depends upon the rate of duration of growth or the function it performs.PROKARYOTIC CELLSCharacteristics of prokaryotic cells1.Pokaryotic cells-bacteria,blue-green algae(cyanobacteria),mycoplasma & PPLO.2They are generally smaller & multiply more rapidly than the eukaryotic cells.3.Bacterial cells vary greatly in shape & size.They may be-bacillus(Irod-shaped),coccus(spherical),vibrio(comma-shaped)& spirullum(spiral).4.All prokaryotes have a cell wall surrounding the cell membrane.The fluid matrix filling the cell is the cytoplasm.There is no well defined nucleus.5.The genetic material is basically naked,not enveloped by a nuclear membrane.6.The bacterial cells have genomic DNA(the single chromosome/circular DNA) along with small circular DNA outside the genomic DNA.These smaller DNA are called plasmids. Functions of plasmids:1.the plasmid DNA confers certain unique phenotypic characters to the bacteria.One such character is the resistance to antibiotics. 2.This plasmid DNA is used to monitor bacterial transformation with foreign DNA..7.A specialized differentiated form of cell membrane called Mesosome is the characteristic of prokaryotes.They are essentially infoldings of cell membrane.Structure of bacterial cells./cell envelope1.The bacterial cells have a complex cell envelope. The cell envelope consists of a tightly bound three layered structure: i.e., the outermost glycocalyx followed by the cell wall and then the plasma membrane.Each layer of the envelope performs a distinct function ,they act together as a single protective unit..2.bacterial cells can be divided into 2 groups on the basis of the differences in the cell envelope & the manner in which they respond to the Gram stain,as-Gram positive & gram negative.Those who take up the gram stain are called gram positive bacteria.eg-coccus shaped bacteria & those that do not are called gram negative.eg-Bacillus.3.Glycocalyx differs in composition n thickness among diff bacteria.In some , it could be a loose sheath called the slime layer and in the others it may be thick and tough,called the capsule.4.In the bacterial cells the cell wall determines the shape of the cell & provides a strong structural support to prevent the bacterium from bursting or collapsing.5.The plasma membrane is semi permeable in nature & interacts with the outside world.6.Mesosomes are the infoldings of the plasma membrane.Its a special membranous structure which is formed by the extensions of plasma membrane into the cell.These extensions are in the form of vesicles(round),tubules and lamellae(II to d cell wall).*Functions of Mesosome:1.help in cell wall formation.2.DNA replication & its distribution to the daughter cells.4respiration 5.secretion processes 6.increase the surface area of the plasma membrane & enzymatic content.[In some prokaryotes like the Cyanobacteria,there are other membranous extensions intothe cytoplasm called cromatophores which contain pigments.]Based on motility. Bacterial cells may be motile or non-motile.If motile,they have thin filamentous extensions from their cell wall called flagella. Bacterial flagellum is composed of 3 parts-filament,hook & basal body.The filament is the longest portion & extends from the cell surfsce to the outside. Besides flagella,Pilli & Fimbriae are also the surface structures of the bacteria but do not play any role in motility.The pilli are elongated tubular structures made of a special protein.The fimbriae are small bristle like fibres sprouting out of the cell. They help the bacteria to attatch to rocks in streams & also to host tissues. RIBOSOMES N INCLUSION BODIES: Ribosomes are the site of protein synthesis.In prokaryotes,the ribosomes are associated with plasma membrane of the cell. They are about 15nm x 20nm in size & are made of two subunits-50S & 30S units which together form the 70S prokaryotic ribosomes. Several ribosomes may attatch to a single mRna & form a chain called polyribosomes or polysome.The ribosomes of a polysome translate the mRna into proteins. Inclusion bodies ::Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies(these are the temporary stuctures).These are not bound by any membrane system & lie free in the cytoplasm.eg-phosphate granules,cyanophycean granules & Glycogen granules. P.S.:Gas vacuoles are found in blue green & purple & green photosynthetic bacteria.EUKARYOTIC CELLS The eukaryotic cells possess an organized nucleus with a nuclear envelope and membrane bounded cell organelles.In addition ,they have a variety of complex locomotory & cytoskeletal structures.Their genetic material is organized into chromosomes. The eukaryotes include all the protists,plants ,animals,n fungi. All eukaryotic cells are not identical.Plant & animal cells are diff from each other. Now well discuss the 2 main membranes of these cells:CELL MEMBRANE:FUNCTIONS: It provides a definite shape to the cell. Provides protection of the internal contents of the cel. Is semi-permeable i.e.,allows the entry & exit of required subst.1.chemical studies on the cell membrane in human RBCs enamled the scientists to deduce the structure of Plasma membrane.**FLUID MOSAIC MODEL OF PLASMA MEMBRANE The cell membrane is composed of lipids that are arranged in a bilayer.The lipids are arranged within the membrane with the polar head(hydrophilic) towards the outer sides & the non-polar tails(hydrophobic) towards the inner part. This ensures that the non-polar tail of saturated hydrocarbons is protected from the aqueous environment.The lipid component of the membrane mainly consists of phosphoglycerides. The ratio of protein & lipids varies in diff cell types.Depending on the ease of extraction,membrane proteins can be classified as integral or peripheral.Peripheral proteins lie on the surface of membrane while the integral proteins are partially or totally burried in the membrane. According to this fluid mosaic model(proposed by Singer & Nicholson), the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer.This ability to move within the membrane is measured as its Fluidity. The fluid nature of the membrane is also imp from the points of view of functions like cell growth,formation of intercellular junctions,secretion,endocytosis,cell division,etc.2.Passive transport : Many molecules can move briefly across the membranewithou any requirement of energy & this is called passive transport. Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient i.e.,from higher conc to lower conc. Water may also move across this membrane from higher to lower conc. Movement of water by diffusion is called Osmosis.3.ACTIVE TRANSPORT: As the polar molecules cannot pass through the non-polar lipid bilayer,they require a carrier protein of the membrane to facilitate their transport across the membrane. A few ions or molecules are transported across the membrane against their concentration gradient ie., from lower to higher conc..Such transport is an energy dependent process,in which ATP is utilized & this is called active transport.eg:Na+/K+.CELL WALL: In plant and fungal cells ,there is a rigid, fully permeable cellulose containing non-living layer called the cell wall. It provides shape to the cell& protects the cell from damage & infection,it also helps in cell to cell interaction & provides barrier to undesirable macromolecules. Algae have cell wall made up of cellulose,galactans,mannans & minerals like calcium carbonate. In plants it consists of cellulose,hemicellulose,pectins & proteins. The cell wall of a young plant cell,the primary wall is capable of growth, which gradually diminishes as the cell matures & the secondary wall formed on the inner(towards membrane) side of the cell. The middle lamella is a layer mainly of calcium pectate(chemical nature of cell wall) which holds or glues the diff neighbouring cells together.The cell wall and the lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells.ENDOMEMBRANE SYSTEMEach of the membranous organelles is distinct in terms of its structure & function, many of these are considered together as an endomembrane system because their functions are are coordinated.It comprises of: the endoplasmic reticulum,golgi apparatus,lysosomes & vacuoles. Since the functions of the mitochondria ,chloroplast & peroxisomes are not coordinated with ER,GA n vacuoles, these are not considered as a part of the endomembrane system.ENDOPLASMIC RETICULUM

A network or reticulum of tiny tubular stuctures scattered in the cytoplasm is called the ER. The ER divides the intercellular space into 2 distinct compartments:luminal(inside ER)& extra luminal (cytoplasm) compartments. Its of 2 types:1.RER:The ER bearing ribosomes on their surface is called RER. RER is frequently observed in cells actively involved in protein synthesis & secretion.They are extensive & continuous with the outer membrane of the nucleus.2.SER:The ER in the absence of ribosomes appear smooth & are called SER.The SER is the major site for synthesis of lipids..In animal cells lipid-like steroidal hormones are synthesized in SER. The main function of ER is to provide channels for the transport of materials(especially proteins) between various regions of the cytoplasm & the nucleus.GOLGI APPARATUS They are the membrane-bound,fluid filled vesicles,vacuoles & stacks of many flattened cisternae.They are usually seen near the nucleus. Camillo Golgi( 1898)first observed densely stained reticular stuctures near the nucleus.These were later named Golgi bodies after him. They consist of many flat,disc-shaped sacs or cisternae of 0.5 microm-1.0microm diameter.These are stacked II to each other.Varied no of cisternae are present in a Golgi complex. The golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face & concave trans or the maturing face. The cis & trans faces of the organelle are entirely different but interconnected.FUNCTIONS OF GA::1.It principally performs the function of packaging of materials,to be deliverd either to the intra-cellular targets or secreted outside the cell.2.Golgi apparatus is an imp site for the synthesis of glycoproteins & glycolipids.3.Matrix of connective tissue is formed in GA.4.Transfer of the packaged material to the body parts.5.they are involved in the formation of lysosomes.Q.Why the golgi apparatus remains in close association with the ER?Ans.Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the golgi apparatus & move towards the maturing face.A no. of proteins synthesized by the ribosomes on the ER are modified in the cisternae of the golgi apparatus before they are released from its trans face.LYSOSOMES These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus. They are rich in almost all types of hydrolytic enzymes(hydrolases-lipases,proteases,carbohydrases) optimically active at the acidic Ph. These enzymes are capable of digesting carbohydrates,proteins,lipids & nucleic acids.VACUOLES The vacuole is the membrane bound space found in the cytoplasm.It contains water,sap,excerory product & other materials not useful for the cell. The vacuole is bounded by a single membrane called tonoplast. In plant cells,vacuoles can occupy up to 90%of the volume of the cell.In animal cells they are smaller in size & less in in no as compared to the plant cells. Function of tonoplast:In plants the tonoplast facilitates the transport of a number of ions & other materials against the concentration gradients into the vacuole,hence their conc. Is significantly higher in the vacuole than in the cytoplasm. In amoeba the contractile vacuole is imp for excretion. In protists,food vacuoles are formed by engulfing the food particles.MITOCHONDRIA They are small rod-shaped or spherical organelles present in large number in the cell. The no. of mitochondria is variable depending upon the physiological activity of the cells. Typically it is sausage-shaped or cylindrical having a diameter of about 0.2microm-1.0microm & length 1.0micm-4.1microm.STRUCTURE OF MITOCHONDRIA: Each mitochondrion is a double membrane bound structure with the outer membrane & the inner membrane dividing its lumen distinctly into two aqeous compartments.i.e., the outer compartment & the inner compartment. The inner compartment is called thr matrix .The outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a no. of infoldings called the cristae towards the matrix. The cristae increase the surface area.The 2 membranes have their own specific enzymes associated with the mitochondrial function. The matrix also possesses single circular DNA molecule,a few RNA molecules,ribosomes(70S) & the components required for theFUNCTIONS OF MITOCHONDRIA:1.they are the sites of aerobic respiration.2.they produce cellular energy in the form of ATP,hence they are called thepower houses of the cell.3. The matrix also possesses single circular DNA molecule,a few RNA molecules,ribosomes(70S) & the components required for the synthesis of proteins.*the mitochondria divide by fission.Do practice the diagram given in ncert(pg135)PLASTIDS1.These are the coloured pigments present in plant cells.they are found in all plant cells & euglenoids.They bear some specific pigments thus imparting specific colours to the plants.2.Based on the type of pigments plastids can be classified into : Chloroplasts:these chloroplasts contain chlorophyll & carotenoid pigments which are responsible for trapping light energy essential for photosynthesis. Chromoplasts:They are of diff colours.They are mostly present in flowers & fruits..Their function is to provide diff colours to flowering parts of a plant. In the chromoplasts fat soluble carotenoid pigments like carotene,xanthophylls & others are present. This gives a part of the plant yellow,orange or red colour. Leucoplasts: These are the colourless plastids of diff shapes and sizes with stored nutrients.:Amyloplasts store carbohydrates(starch) eg:potato;Elaioplasts store oils & fats & the Aleuroplasts stores proteins.MORE INFO. ABOUT THE CHLOROPLASTS:1.Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves.The chloroplasts are lens-shaped,oval,spherical,discoid or even ribbon-like organelles having variable length(5-10mm) & width(2-4mm).Their no. varies from 1 per cellof the chlamydomonas,a green algae to 20-40 per cell in the mesophyll.2.Like mitochondria,the chloroplasts are also double-membrane bound..Of the two,the inner membrane of the chloroplast is relatively less permeable.The space limited by the inner membrane of the chloroplast is called the stroma.3.A number of organized flattened membranous sacs called the thylakoids are presnt in the stroma.The thylakoids are arranged in stacks like the piles of coins called grana or the intergranal thylakoids.In addition there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.The membrane of the thylakoids enclose a space called a lumen.4.The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates & proteins.It also contains small,double-stranded circular DNA molecules & ribosomes.Chlorophyll pigments are present in the thylakoids.5.The ribosomes of the chloroplasts are smaller(70S)than the cytoplasmic ribosomes(80S).RIBOSOMES Ribosomes are extremely small,round bodies present either in free state in the cytoplasm or attatched to the surface of endoplasmic reticulum.They are composed of RNA & proteins. They were first observed under the electron microscope as dense particles by George Palade(1953).They are composed of RNA & proteins & are not surrounded by any membrane. The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.Here S(Svedbergs Unit) stands for the sedimentation coefficient.;it indirectly is a measure of density & size.CYTOSKELETON An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively reffered to as the cytoskeleton. FUNCTION:The cytoskeleton in a cell are involved in many functions such as mechanical support,motility,maintenance of the shape of the cell. Read the notes on types of cytoskeleton from ur bio ntbk.CILIA & FLAGELLACilia & flagella are hair like outgrowths of the cell-membrane. Cilia are small structures which work like oars,causing the movement of either the cell or the surrounding fluid. Flagella are the thin long filamentous extensions responsible for cell movement.The flagella in prokaryotic cells is structurally diff from that of eukaryotic cells.STRUCTURES A slender cylindrical stalk extension of P.M. containing cytoplasmic matrix. Parallel groups of peripheral microtubules embedded in the matrix,9 of them are in pairs & arranged in a circle surrounding two single centre ones.This arrangement is known as 9+2 structure. Microtubules(9+2) are derived from a basal body situated just below the point at which the flagellum protrudes from the surface of PM. In prosection the basal body shows a ring of 9 triplets of microtubules but lacks 2 central single microtubules(9+10 arrangement). Both the cilium & flagellum emerge from centriole like structure called the basal bodies.CENTROSOME Centrosome is an organelle usually containing two cylindrical structures called centrioles.They are surrounded by amorphous pericentriolar materials.CENTRIOLES These are non-membranous organelles found in animal cells,also found in flagellated plant cells,algae & motile cells of some primitive land plants.STRUCTURE & NUMBER: Both the centrioles in a centrosome lie perpendicular to each other in which each has an organization like the cartwheel. They aye basically cylindrical in shape & occur in pairs at right angles to each other near on pole of the nucleus. It seems to be made of 9 triplets of microtubules arranged in a circle just like a basal body. The central tubules of basal bodies are absent. The space b/w & around triplets is filled with amorphous electron dense material. FUNCTIONS: At interface,centrioles duplicate & each daughter cell receives one pair of centrioles. During cell division,one pair of centrioles separate & migrate to the opposite pole of the cell.There they may function as centres for the organization of spindle. Involved in the organization & development of cilia & flagella. When these centrioles move towards the peripheral microtubules,they turn into basal bodies which give rise to cilia & flagella.NUCLEUS All the components of cell including plasma membrane are called as protoplasm.It contains a major control structure called nucleus.Nucleus as a cell organelle was first described by Robert Brown in 1831.PARTS OF A NUCLEUS/STRU.: Inside nucleus the mass of thread like tangled structures is called chromatin.It is composed of chromosomes which are having genetic material DNA & proteins. The Interphase nucleus(nucleus of cell when it is not dividing) has highly extended & elaborate nucleoprotein fibres called chromatin,nuclear matrix & one or more spherical bodies called nucleoli(sing:nucleolus).Nucleolus is a highly dense & rounded organelle inside the nucleus.It acts as a platform for protein synthesis. The nuclear envelope consists of parallel membranes with a space b/w them called perinuclear space, forms a barrier b/w the materials present inside the nucleus &that of the cytoplasm. The outer membrane remains continuous with the ER & also bears ribosomes on it.At a no. of places the nucleur envelope is interrupted by minute pores called the nuclear pores which are formed by the fusion of its two membranes.These nuclear pores are the passages through which movement of RNA & protein molecules takes place in both directions b/w the nucleus & the cytoplasm. Inside nuclear membrane a colourless dense sap is present,it is called nucleoplasm. The nuclear matrix or the nucleoplasm contains nucleolus &chromatin. The content of nucleolus is continuous with rest of the nucleoplasm as it is not a membrane bound structure.It is a site for active ribosomal RNA synthesis.Larger & more numerous nucleoli are present in cells actively carrying out protein synthesis.Functions of nucleus: Many vital roles are performed as well as controlled by the nucleus of the cell,so it is sometimes called as the control centre of the cell/director of cell. It controls all the activities of the cell. It has a direct role in sexual as well as asexual reproduction. Nucleus takes part in cell division.

Read about chromosomes frm ncert n MICROBODIES ALSO FROM NTBK N BOOK.N DO PRACTISE ALL THE DIAGRAMS.