CELL — S TRUCTURE AND FUNCTIONS Y

1 cell structure AND FUNCTIONSCELL structure AND FUNCTIONSYou have already learnt that thingsaround us are either living ornon-living. Further, you mayrecall that all living organisms carry outcertain basic FUNCTIONS . Can you listthese FUNCTIONS ?Different sets of organs perform thevarious FUNCTIONS you have listed. In thischapter, you shall learn about the basicstructural unit of an organ, which is thecell. cells may be compared to are assembled to make a , cells are assembled to makethe body of every of the CellRobert Hooke in 1665 observed slices ofcork under a simple magnifying is a part of the bark of atree.

2 He took thin slices of cork andobserved them under a microscope. Henoticed partitioned boxes orcompartments in the cork slice (Fig. ).These boxes appeared like a honey-comb. He also noticed that one box wasseparated from the other by a wall orpartition. Hooke coined the term cell for each box. What Hooke observed asboxes or cells in the cork were actuallydead of living organisms could beobserved only after the discovery ofimproved microscopes. Very little wasknown about the cell for the next 150years after Robert Hooke s , we know a lot about cellstructure and its FUNCTIONS because ofimproved microscopes having The CellBoth, bricks in a building and cells inthe living organisms, are basicstructural units [Fig.]

3 (a), (b)]. Thebuildings, though built of similar bricks,have different designs, shapes and , in the living world, organismsdiffer from one another but all are madeup of cells . cells in the living organismsare complex living structures unlikenon-living :Fig. :Fig. :Fig. :Fig. :Cork cells as observed byRobert HookeA hen s egg can be seeneasily. Is it a cell or agroup of cells ?2021 22 The egg of a hen represents a singlecell and is big enough to be seen by theunaided show Varietyin Cell Number, Shape andSizeHow do scientists observe and study theliving cells ?

4 They use microscopes whichmagnify objects. Stains (dyes) are usedto colour parts of the cell to study thedetailed are millions of livingorganisms. They are of different shapesand sizes. Their organs also vary inshape, size and number of cells . Let usstudy about some of of CellsCan you guess the number of cells in atall tree or in a huge animal like theelephant? The number runs intobillions and trillions. Human body hastrillions of cells which vary in shapesand sizes. Different groups of cellsperform a variety of made of more than onecell are called multicellular (multi :many; cellular : cell) organisms.

5 Thenumber of cells being less in smallerorganisms does not, in any way, affectthe functioning of the organisms. Youwill be surprised to know that anorganism with billions of cells beginslife as a single cell which is the fertilisedegg. The fertilised egg cell multiplies andthe number of cells increase asdevelopment at Fig (a) and (b). Bothorganisms are made up of a single single-celled organisms are calledunicellular (uni : one; cellular : cell)Fig. : Brick wall and onion peel(a) Brick wall(b) Onion peelA billion is a thousand million.

6 Atrillion is a thousand : (a) Amoeba (b) Paramecium(a)(b) cell structure AND FUNCTIONS912021 22 SCIENCE92organisms. A single-celled organismperforms all the necessary functionsthat multicellular organisms single-celled organism, likeamoeba, captures and digests food,respires, excretes, grows andreproduces. Similar FUNCTIONS inmulticellular organisms are carried outby groups of specialised cells formingdifferent tissues. Tissues, in turn, teacher may show a permanentslide of amoeba and parameciumunder a microscope.

7 Alternatively,the teacher can collect pond waterand show these organisms bypreparing the of CellsRefer to Fig. (a). How do you definethe shape of amoeba in the figure? Youmay say that the shape appearsirregular. Infact, amoeba has nodefinite shape, unlike other keeps on changing its the projections of varyinglengths protruding out of itsbody. These are called pseudopodia(pseudo : false; podia : feet), as youlearnt in Class VII. These projectionsappear and disappear as amoebamoves or white blood cell (WBC) in humanblood is another example of a single cellwhich can change its shape.

8 But whileWBC is a cell, amoeba is a full fledgedorganism capable of shape would you expect inorganisms with millions of cells ? (a, b, c) shows different cells suchas blood, muscle and nerve of humanbeings. The different shapes are relatedto their specific , cells are round, sphericalor elongated [Fig. (a)]. Some cells arelong and pointed at both ends. Theyexhibit a spindle shape [Fig. (b)]. cells sometimes are quite long. Someare branched like the nerve cell or aneuron [Fig.]

9 (c)]. The nerve cellreceives and transfers messages, therebyThe change in shape is due toformation of pseudopodia whichfacilitates movement and helpin capturing :(a) Spherical red blood cellsof humans, (b) Spindleshaped muscle cells ,(c) Long branched nerve cell(a)(b)(c)What advantage doesamoeba derive bychanging shape?2021 22 cell structure AND FUNCTIONS93the elephant be much bigger than thosein a rat. The size of the cell is related toits function. For example, nerve cells ,both in the elephant and rat, are longand branched.

10 They perform the samefunction, that of transferring structure andFunctionYou have learnt that each livingorganism has many organs. You havestudied in Class VII about the digestiveorgans which together constitute thedigestive system. Each organ in thesystem performs different functionssuch as digestion, assimilation andabsorption. Similarly, different organsof a plant perform specific/specialisedfunctions. For example, roots help in theabsorption of water and , as you have learnt in Class VII,are responsible for synthesis of organ is further made up ofsmaller parts called tissues.