CHAPTER-5
CLASS-IX THE FUNDAMENTAL UNIT OF LIFE-CELL
Cell is the structural and functional unit of an organism because a cell in itself is the smallest part of an organism which is capable of functioning independently and can carry out the fundamental duties of life like reproduction, metabolism (photosynthesis and/or respiration). • Organisms which are made up of one cell are called unicellular organisms, eg Amoeba, aramecium • Organisms which are made up of many cells are called !ulticellular organisms, eg "ydra, #arthworm !an etc. • Cell was disco$ered by %obert "ooke in &'', in a cork slide bark of tree* using self de$eloped primiti$e microscope. Cells were obser$ed for the first time in a slice of cork. •+ Cell means a compartment. • -i$ing cell was first obser$ed by -eeuwenhoek in a drop of pond water. • Cell theory was proposed by chleiden and chwann Postulates of the cell theor &) All li$ing beings are made up of cells ) 0he function of an organism as a whole is the result of the functions of the constituent cells. 1) All cells arise from pre e2isting cells. 3) .All cells are alike in structure and metabolic acti$ities. ! Cells are of t"o t#es &) rokaryotic cells eg 4acteria, 4lue 5reen Algae ) #ukaryotic cells eg lant cells and Animal cells Pro$arot%c a&' Eu$arot%c Cells • 0he main difference between these two cell types is that rokaryotic (primiti$e nucleus) cells do not ha$e a nuclear membrane • 0he nuclear material of rokaryotes consists of a single chromosome and lies in the cytoplasm • 6n prokaryotes, the nuclear region in the cytoplasm is called nucleoid • !embrane7bound organelles are absent in all prokaryotes rokaryotic cells are found in bacteria and cyanobacteria (blue 8green algae) Structure of Eu$arot%c cell
• Cells $ary in shape and si9e. 0hey may be o$al or drop like ( Chlamydomonas), spherical (egg) , rectangular (#pidermal cells), polygonal ( #pithelial cells), spindle shaped ( mooth muscle fibre), irregular (:4C), rod7shaped ( 4acilli )or totally irregular like the ner$e cell. • 0he $ariation in the shape of the cells is due to the function they ha$e to perform as part of the tissue or an organ system.
• !ycoplasmas, a bacteria has the smallest cell of the si9e of ;.& micron i.e. ten thousandth part of a millimetre Cells ha$e different shapes to perform different functions. #2ample< =er$e cells are long and branched to carry signals from one part of the body to other and white blood cells are irregular in shape in order to engulf the pathogens in the body.
Co(#o&e&ts of the cell Cell "all • 0he cell wall is present only in plant cells. 6t is made up of cellulose. • 6ts function is to gi$e strength and rigidity to the cell.
• 6t is li$ing and ermeable and allow e$ery substance to enter and e2it. • Cell wall between two ad>acent cells is >oined by middle lamella. • !iddle lamella is made up of Calcium pectate. • Cell wall may get depositions of substances like -ignin , uberin etc, which make it impermeable. %ole of cell wall< Cell wall pro$ides protection to plant cell against $ariations in temperature, high wind speed, atmospheric moisture, etc. Cell Me()ra&e *Plas(a (e()ra&e+
• 6t is present in both plant and animal cells.
• 6t is li$ing, elastic and made of proteins and phosphor lipids (fats). • 6ts function is to pro$ide a mechanical barrier for the protection of the inner cell contents and to regulate the e2change or transport of substances in and out of the cell • 6t is called selecti$ely permeable as it allows only some substance to enter and e2it in and out the cell. • 0he most accepted model of lasma membrane is the ?luid !osaic !odel+ gi$en by inger and =icholson. Tra&s#ort across Plas(a (e()ra&e ,Pass%e tra&s#ort. Osmosis 8 it is mo$ement of sol$ent (water)particles from the region of its higher concentration to the region of its lower concentration across a semi permeable membrane till it reaches e@uilibrium.
iffusion 8 6t is the mo$ement of substance from the region of their higher concentration to the region of their lower concentration till it reaches e@uilibrium. Effect o& cell %& '%ffere&t $%&'s of solut%o&s
:hen an animal cell is placed in a hypertonic solution, it will undergo e2o osmosis and will shrink. 7 6f an animal cell is placed in a hypotonic solution it will it will undergo endo osmosis and will swell (become turgid) and will burst. 7 lant cell when placed in hypertonic solution undergoes e2o osmosis, and only the cytoplasmic content of the cell shrinks, the shape does not change due to the rigid cell wall. uch a cell is called Plas(olse' cell/ 7 A plasmolysed cell if placed in water or hypotonic solution regains its turgidity and gets deplasmolysed. 7 lant cell in hypotonic solution undergoes endo osmosis, becomes turgid but does not burst due to the rigid cell wall. S%0&%f%ca&ce of Os(os%s a&' '%ffus%o&
• 5uard cells undergo endo osmosis resulting in to opening of stomata. • %oot hair absorb soil water by endo osmosis where as 5aseous e2change takes place by diffusion E&'octos%s 8 taking in of big particles which cannot cross the plasma membrane. 7 Pha0octos%s 7 the process by which cells engulf solid food particles, also known as cell eating.( Amoeba takes in food by this process) P%&octos%s 8 0he process by which cells take in li@uid substances, commonly known as cell drinking.
E1octos%s 8 it is the process of throwing out of the undigested food by the cell , commonly known as cell $omiting. Cto#las(
• 0he fluid part of the cell between the cell membrane and the nuclear membrane is called the cytoplasm. • 0he cytoplasm consists of the matri2 and speciali9ed li$ing structures called the organelles. • 0he matri2 is a transparent semi fluid substance. • Cytoplasm act as a seat of many metabolic acti$ities.6t stores raw material. • 6t also helps in distribution of nutrients and metabolites. Cell or0a&elles mall components of the cell present in the protoplasm are called cell organelles e.g. mitochondrion,5olgibodies and ribosomes etc. Nucleus • 6t is found in all eukaryotic cells e2cept mature mammalian %4C, ie$e tubes and sie$e cells.
• 0his is a prominent, spherical or o$al structure found at the centre of the cell. • =umber of nucleus per cell is generally one but in Paramoecium (4inucleate), many in keletal muscle fibres (multinucleate) • 6t is the controlling centre of all cell acti$ities and has been called the brain of the cell. • 6t regulates all metabolic and hereditary acti$ities of the cell The &ucleus %s co(#ose' of the follo"%&0 structures2
Nuclear (e()ra&e
• 0his is a double7layered membrane which separates the nucleoplasm from the cytoplasm. • 0he nuclear membrane has minute pores which allow the selecti$e transfer of material between the nucleoplasm and the cytoplasm. Nucleo#las(
• :ithin the nuclear membrane, completely filling up the space, • 6t is a clear, semi7solid, granular substance or matri2 called the nucleoplasm. • 0he nucleolus and the chromatin network lie suspended in the nucleoplasm. Nucleolus
• 0his dense, spherical granule found in the nucleus contains %=A (ribonucleic acid) which is responsible for protein synthesis in the cytoplasm Chro(at%& &et"or$
• 0hese are $ery fine thread7like, coiled filaments uniformly distributed in the nucleoplasm. • At the time of cell di$ision, the chromatin becomes thick and ribbon like and are known as chromosomes. • 0he chromosomes contain genes, which are composed of =A (deo2yribonucleic acid). • 5enes are responsible for storing and transmitting hereditary characteristics from one generation to another Cell Or0a&elles
Cell7organelles are membrane bound structures found in cytoplasm, performing special functions • #ndoplasmic %eticulum (#%) • 5olgi Apparatus • !itochondria • lastids • -ysosomes • %ibosomes • Centrosome E&'o#las(%c Ret%culu( *ER+ 0his is a comple2 network of interconnected cisternae, tubules and $esicles, the lumen of which is filled with fluid.
• 0wo types of endoplasmic reticulum are • SER 7 0ubes with a smooth surface are called smooth endoplasmic reticulum. #% mainly contains tubules . • #% helps in synthesis / secretion of lipids, cholesterol, hormones like testosterone, progesterone etc. • 0he also help in deto2ification ( remo$al of to2ic substances like drugs etc) • RER 7 0ubes with ribosomes attached are known as rough endoplasmic reticulum. • %#% mainly contains cisternae and $esicles • 0he functions of the rough endoplasmic reticulum is to synthesi9e proteins • 0hey also help in transport of these proteins. • #%s pro$ide a pathway for the distribution of nuclear material from one cell to the other • 0hey also form the skeletal framework of the cell. 3ol0% A##aratus • Also known as 5olgi Comple2 or 5olgi 4odies, named after the scientist Camillo 5olgi who disco$ered them.
• 0hey consist of tiny, elongated, flattened sacs called cisternae, which are stacked parallel to one another along with some $acuoles and clusters of $esicles • 0he 5olgi body is usually found close to the nucleus. • 0he function of the 5olgi 4ody is to secrete certain hormones and en9ymes. • 6t also forms lysosomes which contain digesti$e en9ymes.( packaging of en9ymes) • 0hey also help in yolk formation in eggs. • 0hey form the acrosome of the sperm. M%tocho&'r%a
• 0hey are found in all eukaryotes but absent in mature mammalian %4C and prokaryotes. • 0hese may be sausage shaped, cylindrical, rod7shaped or spherical and distributed in the cytoplasm • #ach mitochondrion is bound by a double membrane. • Outer membrane is smooth • 0he inner membrane is folded into finger like structures called cristae, which increase the surface area of the membrane.
• On the crista are found knob like structures called O2ysomes which help in A0 synthesis. • 6n the mitochondria the sugar (glucose) is finally broken down during cellular respiration to release energy.
• 0he energy thus released is stored as high7energy chemicals called A0 • 0he body cells use the energy stored in A0 for synthesis of new chemical compounds and other acti$ities • "ence, mitochondria are termed as the power house+ of the cell. Plast%'s
• 0hese organelles are found only in plant cells. • lastids are of three types A ) Chloro#lasts
• 0hey are green and found in lea$es. • 0he green colour is due to the presence of chlorophyll. . • 0hey are double membrane structures, • !atri2 is called troma. • 6n the stroma are membrane bound structures called thylakoids. • 0hese thylakoids are staked to form a granum. • !any grana are found in stroma and they are interconnected by stroma lamellae. • troma also contains ribosomes and =A so Chloroplasts are also called Se(% auto&o(ous or0a&elle • 0he function of the chloroplast is to trap solar energy for photosynthesis. • 0hey synthesi9e carbohydrate( glucose) by photosynthesis so called the kitchen of the cell.
4) Chro(o#lasts • 0hey contain yellow, orange and red, and found in flowers and fruits. • Chromoplasts impart colour to flowers to attract insects for pollination. C) Leuco#lasts • 0hey are colourless and found in roots, seeds and underground stems. • . -eucoplasts store food in the form of carbohydrates, fats and proteins. Lsoso(es
• 0hese are tiny, spherical, sac7like structures scattered all o$er the cytoplasm. • 0heir main function is digestion. • 0hey contain powerful digesti$e en9ymes capable of digesting all organic material, and hence called digesti$e bags+. • -ysosomes present in white blood cells are capable of digesting bacteria and $iruses. • uring star$ation, lysosomes digest proteins, fats and glycogen in the cytoplasm, and supply energy to the cell. • 0hey are also capable of digesting worn out cell organelles, or e$en digesting the entire damaged cell containing them. "ence, called suicide bag+. R%)oso(es
• 0hese are spherical, granular particles which occur freely in the matri2 or remain attached to the rough endoplasmic reticulum. • %ibosomes contain %=A (%ibosomal %=A) and proteins. • 0heir function is synthesi9ing proteins so called rotein factories or #ngine of the cell+. Ce&tr%ole • 0he centriole is found o&l %& the a&%(al cell/
• 0his is found in the cytoplasm near the outer surface of the nucleus . • 0he centrioles play an important role during cell di$ision. 4acuole
• ?ound in all eukaryotic cells • mall and many in animal cells • 4ig and few in plant cells • 0heir membrane is known as tonoplast • 0hey are filled with cell sap (water, mineral, sugar etc) • ?ood $acuoles store food . Contactile $acuoles help in osmoregulation (water balance) • Bacuole ?unctions 7 storage pro$ide turgidity
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