ELLELL TRUCTURETRUCTURE ANDAND UNCTIONSUNCTIONS Y · 2010. 8. 26. · 94 SCIENCE organisms. A single-celled organism performs all the necessary functions that multicellular organisms
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CELL — STRUCTURE AND FUNCTIONSCELL — STRUCTURE AND FUNCTIONS
You have already learnt that thingsaround us are either living ornon-living. Further, you may
recall 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 bricks.Bricks are assembled to make a building.Similarly, cells are assembled to makethe body of every organism.
8.1 Discovery of the Cell
Robert Hooke in 1665 observed slices ofcork under a simple magnifying device.Cork is a part of the bark of atree. He took thin slices of cork andobserved them under a microscope. Henoticed partitioned boxes orcompartments in the cork slice (Fig. 8.1).
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 cells.
Cells 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 observations.Today, we know a lot about cellstructure and its functions because ofimproved microscopes having highmagnification.
8.2 The Cell
Both, bricks in a building and cells inthe living organisms, are basicstructural units [Fig. 8.2(a), (b)]. Thebuildings, though built of similar bricks,have different designs, shapes and sizes.Similarly, in the living world, organismsdiffer from one another but all are madeup of cells. Cells in the living organismsare complex living structures unlikenon-living bricks.
The egg of a hen represents a singlecell and is big enough to be seen by theunaided eye.
8.3 Organisms show Varietyin Cell Number, Shape andSize
How do scientists observe and study theliving cells? They use microscopes whichmagnify objects. Stains (dyes) are usedto colour parts of the cell to study thedetailed structure.
There 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 them.
Number of Cells
Can 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 functions.
Organisms made of more than onecell are called multicellular (multi :many; cellular : cell) organisms. 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 proceeds.
Look at Fig 8.3 (a) and (b). Bothorganisms are made up of a single cell.The single-celled organisms are calledunicellular (uni : one; cellular : cell)
Fig. 8.2 : Brick wall and onion peel
(a) Brick wall (b) Onion peel
A billion is a thousand million. Atrillion is a thousand billion.
organisms. A single-celled organismperforms all the necessary functionsthat multicellular organisms perform.
A 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, formorgans.
Activity 8.1
The teacher may show a permanentslide of Amoeba and Parameciumunder a microscope. Alternatively,the teacher can collect pond waterand show these organisms bypreparing the slides.
Shape of Cells
Refer to Fig, 8.3 (a). How do you definethe shape of Amoeba in the figure? Youmay say that the shape appearsirregular. Infact, Amoeba has nodefinite shape, unlike other organisms.It keeps on changing its shape.Observe the projections of varyinglengths protruding out of its body.These are called pseudopodia (pseudo: false; podia : feet), as you learntin Class VII. These projections appearand disappear as Amoeba moves orfeeds.
A white blood cell (WBC) in humanblood is another example of a single cellwhich can change its shape. But whileWBC is a cell, amoeba is a full fledgedorganism capable of independentexistence.
What shape would you expect inorganisms with millions of cells? Fig.8.4 (a, b, c) shows different cells suchas blood, muscle and nerve cells ofhuman beings. The different shapes arerelated to their specific functions.
Generally, cells are round, sphericalor elongated [Fig. 8.4(a)]. Some cells arelong and pointed at both ends. Theyexhibit spindle shape [Fig. 8.4(b)]. Cellssometimes are quite long. Some arebranched like the nerve cell or a neuron[Fig. 8.4(c)]. The nerve cell receives andtransfers messages, thereby helping tocontrol and coordinate the working ofdifferent parts of the body.
The change in shape is due toformation of pseudopodia whichfacilitates movement and help
in capturing food.
Fig. 8.4 :(a) Spherical red blood cellsof humans, (b) Spindle shaped musclecells, (c) Long branched nerve cell
(a)
(b)(c)
What advantage doesAmoeba derive bychanging shape?
in the elephant be much bigger thanthose in a rat. The size of the cell isrelated to its function. For example,nerve cells, both in the elephant andrat, are long and branched. Theyperform the same function, that oftransferring messages.
8.4 Cell Structure andFunction
You 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 minerals.Leaves, as you have learnt in Class VII,are responsible for synthesis of food.
Each organ is further made up ofsmaller parts called tissues. A tissue isa group of similar cells performing aspecific function.
Paheli realised that an organ is madeup of tissues which in turn, are madeup of cells. The cell in a living organismis the basic structural unit.
8.5 Parts of the Cell
Cell Membrane
The basic components of a cell are cellmembrane, cytoplasm and nucleus(Fig. 8.7). The cytoplasm and nucleusare enclosed within the cell membrane,also called the plasma membrane. Themembrane separates cells from oneanother and also the cell from thesurrounding medium. The plasma
Can you guess, which part of the cellgives it shape? Components of the cellare enclosed in a membrane. Thismembrane provides shape to the cellsof plants and animals. Cell wall is anadditional covering over the cellmembrane in plant cells. It gives shapeand rigidity to these cells (Fig. 8.7).Bacterial cell also has a cell wall.
Size of Cells
The size of cells in living organisms maybe as small as a millionth of a metre(micrometre or micron) or may be aslarge as a few centimetres. However, mostof the cells are microscopic in size andare not visible to the unaided eye. Theyneed to be enlarged or magnified by amicroscope. The smallest cell is 0.1 to0.5 micrometre in bacteria. The largestcell measuring 170 mm ×130 mm, isthe egg of an ostrich.
Activity 8.2
Boil a hen’s egg. Remove the shell.What do you observe? A whitematerial surrounds the yellow part.White material is albumin whichsolidifies on boiling. The yellow partis yolk. It is part of the single cell.You can observe this single cellwithout any magnifying device.
Are the cells in anelephant larger than the
cells in a rat?
The size of the cells has no relationwith the size of the body of the animalor plant. It is not necessary that the cells
membrane is porous and allows themovement of substances or materialsboth inward and outward.
Activity 8.3
In order to observe the basiccomponents of the cell, take anonion bulb. Remove the dry pinkcoverings (peels). You can easilyseparate these from the fleshy whitelayers of the bulb with the help offorceps or even with your hand. Youcan also break the onion bulb andseparate out thin layers. Place asmall piece of the thin onion peelin a drop of water on a glass slide.The thin layer can be cut intosmaller pieces with the help of ablade or forceps. Add a drop ofmethylene blue solution to the layerand place a coverslip on it. Whileplacing the coverslip ensure thatthere are no air bubbles under thecoverslip. Observe the slide underthe microscope. Draw and label. Youmay compare it with Fig. 8.5.
The boundary of the onion cell is thecell membrane covered by another thickcovering called the cell wall. The centraldense round body in the centre is calledthe nucleus. The jelly-like substance
between the nucleus and the cellmembrane is called cytoplasm.
Fig. 8.5 : Cells observed in an onion peel
I want to know why plantcells need cell walls?
nucleus
cytoplasm
You have learnt earlier that the cellmembrane gives shape to the cell. Inaddition to the cell membrane, there isan outer thick layer in cells of plantscalled cell wall. This additional layersurrounding the cell membrane isrequired by the plants for protection.Plant cells need protection againstvariations in temperature, high windspeed, atmospheric moisture, etc. Theyare exposed to these variations becausethey cannot move. Cells can beobserved in the leaf peel ofTradescantia, Elodea or Rhoeo. Youcan prepare a slide as in the caseof onion.
Paheli asks Boojho if he can alsoobserve animal cells.
Activity 8.4
Take a clean tooth pick, or amatchstick with the tip broken.Scrape inside of your cheek withouthurting it. Place it in a drop of wateron a glass slide. Add a drop of iodineand place a coverslip over it.Alternatively, add 1-2 drops ofmethylene blue solution. Observe itunder the microscope. You maynotice several cells in the scrapedmaterial (Fig. 8.6). You can identifythe cell membrane, the cytoplasmand nucleus. A cell wall is absentin animal cells.
It is the jelly-like substance presentbetween the cell membrane and thenucleus. Various other components, ororganelles, of cells are present in thecytoplasm. These are mitochondria, golgibodies, ribosomes, etc. You will learnabout them in later classes.
Nucleus
It is an important component of theliving cell. It is generally spherical andlocated in the centre of the cell. It canbe stained and seen easily with the helpof a microscope. Nucleus is separatedfrom the cytoplasm by a membranecalled the nuclear membrane. Thismembrane is also porous and allows themovement of materials between thecytoplasm and the inside of the nucleus.
With a microscope of highermagnification, we can see a smallerspherical body in the nucleus. It is calledthe nucleolus. In addition, nucleuscontains thread-like structures calledchromosomes. These carry genes andhelp in inheritance or transfer ofcharacters from the parents to theoffspring. The chromosomes can be seenonly when the cell divides.
Nucleus, in addition to its role ininheritance, acts as control centre of theactivities of the cell. The entire contentof a livinhg cell is known as protoplasm.It includes the cytoplasm and thenucleus. Protoplasm is called the livingsubstance of the cell.
Gene
Gene is a unit of inheritance in livingorganisms. It controls the transfer ofa hereditary characteristic fromparents to offspring. This means thatyour parents pass some of theircharacteristics on to you. If your fatherhas brown eyes, you may also havebrown eyes. If your mother has curlyhair, you might also end up havingcurly hair. However, the differentcombination of genes from parentsresult in different characteristics.
Paheli wants to know if thestructure of the nucleus isthe same in cells of plants,
animals and bacteria.
The nucleus of the bacterial cell isnot well organised like the cells ofmulticellular organisms. There is nonuclear membrane. The cells havingnuclear material without nuclearmembrane are termed prokaryoticcells. The organisms with these kindsof cells are called prokaryotes (pro :primitive; karyon : nucleus). Examplesare bacteria and blue green algae. Thecells, like onion cells and cheek cellshaving well organised nucleus witha nuclear membrane are designatedas eukaryotic cells. All organismsother than bacteria and blue greenalgae are called eukaryotes. (eu :true; karyon: nucleus).
While observing the onion cellsunder the microscope, did you noticeany blank-looking structures in thecytoplasm? It is called vacuole. It couldbe single and big as in an onion cell.Cheek cells have smaller vacuoles. Largevacuoles are common in plant cells.Vacuoles in animal cells are muchsmaller.
You might have noticed several smallcoloured bodies in the cytoplasm of thecells of Tradescantia leaf. They arescattered in the cytoplasm of the leafcells. These are called plastids. They areof different colours. Some of themcontain green pigment calledchlorophyll. Green coloured plastids are
called chloroplasts. They provide greencolour to the leaves. You may recall thatchlorophyll in the chloroplasts of leaves,is essential for photosynthesis.
8.6 Comparison of Plant andAnimal Cells
If you recall Activities 8.3 and 8.4, youshould be able to compare plant andanimal cells. Observe the plant andanimal cell carefully in Fig. 8.7 (a), (b).
Let us tabulate the similarities anddisinguishing features of plant andanimal cells. Only a few features arementioned. You may add more inTable 8.1.
Table 8.1 : Comparison of Plant Cell and Animal Cell
1. Visit a laboratory for senior secondary students in your school orin a neighbouring school. Learn about the functioning of amicroscope in the laboratory. Also observe how a slide is observedunder the microscope.
2. Talk to the senior biology teacher in your school or a neighbouringschool. Find out if there are diseases which are passed on fromparents to the offspring. Find out how these are carried and also ifthese diseases can be treated. For this you can also visit a doctor.
3. Visit an agriculture extension centre in your area. Find out aboutgenetically modified (GM) crops. Prepare a short speech for yourclass on this topic. You may visit www.usc.ernet.in/currsci/sep252001/655.pdf
4. Find out about Bt cotton from an agriculture expert (or fromenvfor.nic.in/divisions/csnrv/btcotton/bgnote.pdf). Prepare a shortnote on its advantages/disadvantages.