Page 1 :
CELL — STRUCTURE AND FUNCTIONS, , Y, , ou have already learnt that things, around us are either living or, non-living. Further, you may, recall that all living organisms carry out, certain basic functions. Can you list, these functions?, Different sets of organs perform the, various functions you have listed. In this, chapter, you shall learn about the basic, structural unit of an organ, which is the, cell. Cells may be compared to bricks., Bricks are assembled to make a building., Similarly, cells are assembled to make, the body of every organism., , 8.1 Discovery of the Cell, Robert Hooke in 1665 observed slices of, cork under a simple magnifying device., Cork is a part of the bark of a, tree. He took thin slices of cork and, observed them under a microscope. He, noticed, partitioned, boxes, or, compartments in the cork slice (Fig. 8.1)., , These boxes appeared like a honeycomb. He also noticed that one box was, separated from the other by a wall or, partition. Hooke coined the term ‘cell’, for each box. What Hooke observed as, boxes or cells in the cork were actually, dead cells., Cells of living organisms could be, observed only after the discovery of, improved microscopes. Very little was, known about the cell for the next 150, years after Robert Hooke’s observations., Today, we know a lot about cell, structure and its functions because of, improved microscopes having high, magnification., , 8.2 The Cell, Both, bricks in a building and cells in, the living organisms, are b a s i c, structural units [Fig. 8.2(a), (b)]. The, buildings, though built of similar bricks,, have different designs, shapes and sizes., Similarly, in the living world, organisms, differ from one another but all are made, up of cells. Cells in the living organisms, are complex living structures unlike, non-living bricks., , A hen’s egg can be seen, easily. Is it a cell or a, group of cells?, , Fig. 8.1: Cork cells as observed by, Robert Hooke, , 2019-2020

Page 2 :
(a) Brick wall, , (b) Onion peel, , Fig. 8.2 : Brick wall and onion peel, , The egg of a hen represents a single, cell and is big enough to be seen by the, unaided eye., , 8.3 Organisms show Variety, in Cell Number, Shape and, Size, How do scientists observe and study the, living cells? They use microscopes which, magnify objects. Stains (dyes) are used, to colour parts of the cell to study the, detailed structure., There are millions of living, organisms. They are of different shapes, and sizes. Their organs also vary in, shape, size and number of cells. Let us, study about some of them., , Organisms made of more than one, cell are called multicellular (multi :, many; cellular : cell) organisms. The, number of cells being less in smaller, organisms does not, in any way, affect, the functioning of the organisms. You, will be surprised to know that an, organism with billions of cells begins, life as a single cell which is the fertilised, egg. The fertilised egg cell multiplies and, the number of cells increase as, development proceeds., Look at Fig 8.3 (a) and (b). Both, organisms are made up of a single cell., The single-celled organisms are called, unicellular (uni : one; cellular : cell), , Number of Cells, Can you guess the number of cells in a, tall tree or in a huge animal like the, elephant? The number runs into, billions and trillions. Human body has, trillions of cells which vary in shapes, and sizes. Different groups of cells, perform a variety of functions., A billion is a thousand million. A, trillion is a thousand billion., , (a), , (b), , Fig. 8.3 : (a) Amoeba (b) Paramecium, 91, , CELL — STRUCTURE AND FUNCTIONS, , 2019-2020

Page 3 :
organisms. A single-celled organism, performs all the necessary functions, that multicellular organisms perform., A single-celled organism, like, amoeba, captures and digests food,, respires, excretes, grows and, reproduces. Similar functions in, multicellular organisms are carried out, by groups of specialised cells forming, different tissues. Tissues, in turn, form, organs., , Activity 8.1, The teacher may show a permanent, slide of amoeba and paramecium, under a microscope. Alternatively,, the teacher can collect pond water, and show these organisms by, preparing the slides., Shape of Cells, Refer to Fig. 8.3 (a). How do you define, the shape of amoeba in the figure? You, may say that the shape appears, irr egular. Infact, amoeba has no, definite shape, unlike other organisms., It keeps on changing its shape., Observe the projections of varying, lengths protruding out of its, body. These are called pseudopodia, (pseudo : false; podia : feet), as you, learnt in Class VII. These projections, appear and disappear as amoeba, moves or feeds., , The change in shape is due to, formation of pseudopodia which, facilitates movement and help, in capturing food., , A white blood cell (WBC) in human, blood is another example of a single cell, which can change its shape. But while, WBC is a cell, amoeba is a full fledged, organism capable of independent, existence., What shape would you expect in, organisms with millions of cells? Fig., 8.4 (a, b, c) shows different cells such, as blood, muscle and nerve of human, beings. The different shapes are related, to their specific functions., Generally, cells are round, spherical, or elongated [Fig. 8.4(a)]. Some cells are, long and pointed at both ends. They, exhibit a spindle shape [Fig. 8.4(b)]., Cells sometimes are quite long. Some, are branched like the nerve cell or a, neuron [Fig. 8.4(c)]. The nerve cell, receives and transfers messages, thereby, , (a), , (b), , What advantage does, amoeba derive by, changing shape?, , (c), , Fig. 8.4 : (a) Spherical red blood cells, of humans, (b) Spindle, shaped muscle cells,, (c) Long branched nerve cell, , 92, , SCIENCE, , 2019-2020

Page 4 :
helping to control and coordinate the, working of different parts of the body., Can you guess, which part of the cell, gives it shape? Components of the cell, are enclosed in a membrane. This, membrane provides shape to the cells, of plants and animals. Cell wall is an, additional covering over the cell, membrane in plant cells. It gives shape, and rigidity to these cells (Fig. 8.7)., Bacterial cell also has a cell wall., Size of Cells, The size of cells in living organisms may, be as small as a millionth of a metre, (micrometre or micron) or may be as large, as a few centimetres. However, most of the, cells are microscopic in size and are not, visible to the unaided eye. They need to, be enlarged or magnified by a microscope., The smallest cell is 0.1 to 0.5 micrometre, in bacteria. The largest cell measuring, 170 mm ×130 mm, is the egg of an ostrich., , Activity 8.2, Boil a hen’s egg. Remove the shell., What do you observe? A white, material surrounds the yellow part., White material is albumin which, solidifies on boiling. The yellow part, is yolk. It is part of the single cell., You can observe this single cell, without any magnifying device., , the elephant be much bigger than those, in a rat. The size of the cell is related to, its function. For example, nerve cells,, both in the elephant and rat, are long, and branched. They perform the same, function, that of transferring messages., , 8.4 Cell Structure and, Function, You have lear nt that each living, organism has many organs. You have, studied in Class VII about the digestive, organs which together constitute the, digestive system. Each organ in the, system performs different functions, such as digestion, assimilation and, absorption. Similarly, different organs, of a plant perform specific/specialised, functions. For example, roots help in the, absorption of water and minerals., Leaves, as you have learnt in Class VII,, are responsible for synthesis of food., Each organ is further made up of, smaller parts called tissues. A tissue is, a group of similar cells performing a, specific function., Paheli realised that an organ is made, up of tissues which in turn, are made, up of cells. The cell in a living organism, is the basic structural unit., , 8.5 Parts of the Cell, Cell Membrane, , Are the cells in an, elephant larger than the, cells in a rat?, , The size of the cells has no relation, with the size of the body of the animal or, plant. It is not necessary that the cells in, , The basic components of a cell are cell, membrane, cytoplasm and nucleus, (Fig. 8.7). The cytoplasm and nucleus, are enclosed within the cell membrane,, also called the plasma membrane. The, membrane separates cells from one, another and also the cell from the, surrounding medium. The plasma, membrane is porous and allows the, 93, , CELL — STRUCTURE AND FUNCTIONS, , 2019-2020

Page 5 :
movement of substances or materials, both inward and outward., , between the nucleus and the cell, membrane is called cytoplasm., , Activity 8.3, In order to observe the basic, components of the cell, take an, onion bulb. Remove the dry pink, coverings (peels). You can easily, separate these from the fleshy white, layers of the bulb with the help of, forceps or even with your hand. You, can also break the onion bulb and, separate out thin layers. Place a, small piece of the thin onion peel, in a drop of water on a glass slide., The thin layer can be cut into, smaller pieces with the help of a, blade or forceps. Add a drop of, methylene blue solution to the layer, and place a coverslip on it. While, placing the coverslip ensure that, there are no air bubbles under the, coverslip. Observe the slide under, the microscope. Draw and label. You, may compare it with Fig. 8.5., The boundary of the onion cell is the, cell membrane covered by another thick, covering called the cell wall. The central, dense round body in the centre is called, the nucleus. The jelly-like substance, , I want to know why plant, cells need cell walls?, , You have learnt earlier that the cell, membrane gives shape to the cell. In, addition to the cell membrane, there is, an outer thick layer in cells of plants, called cell wall. This additional layer, surrounding the cell membrane is, required by plants for protection. Plant, cells need protection against variations, in temperature, high wind speed,, atmospheric moisture etc. They, are exposed to these variations, because they cannot move. Cells can, be observed in the leaf peel of, Tradescantia, Elodea or Rhoeo. You, can prepare a slide as in the case, of onion., Paheli asks Boojho if he can also, observe animal cells., , Activity 8.4, , nucleus, , cytoplasm, , Fig. 8.5 : Cells observed in an onion peel, 94, , Take a clean tooth pick, or a, matchstick with the tip broken., Scrape inside of your cheek without, hurting it. Place it in a drop of water, on a glass slide. Add a drop of iodine, and place a coverslip over it., Alternatively, add 1-2 drops of, methylene blue solution. Observe it, under the microscope. You may, notice several cells in the scraped, material (Fig. 8.6). You can identify, the cell membrane, the cytoplasm, and nucleus. A cell wall is absent, in animal cells., SCIENCE, , 2019-2020

Page 6 :
nuclear, membrane, , Gene, , cytoplasm, , cell, membrane, , nucleus, , Fig. 8.6 : Human cheek cells, , Cytoplasm, It is the jelly-like substance present, between the cell membrane and the, nucleus. Various other components, or, organelles, of cells are present in the, cytoplasm. These are mitochondria, golgi, bodies, ribosomes, etc. You will learn, about them in later classes., , Gene is a unit of inheritance in living, organisms. It controls the transfer of, a hereditary characteristic from, parents to offspring. This means that, your parents pass some of their, characteristics on to you. If your father, has brown eyes, you may also have, brown eyes. If your mother has curly, hair, you might also end up having, curly hair. However, the different, combination of genes from parents, result in different characteristics., , Nucleus, in addition to its role in, inheritance, acts as control centre of the, activities of the cell. The entire content, of a living cell is known as protoplasm., It includes the cytoplasm and the, nucleus. Protoplasm is called the living, substance of the cell., , Nucleus, It is an important component of the, living cell. It is generally spherical and, located in the centre of the cell. It can, be stained and seen easily with the help, of a microscope. Nucleus is separated, from the cytoplasm by a membrane, called the nuclear membrane. This, membrane is also porous and allows the, movement of materials between the, cytoplasm and the inside of the nucleus., With a microscope of higher, magnification, we can see a smaller, spherical body in the nucleus. It is called, the nucleolus. In addition, nucleus, contains thread-like structures called, chromosomes. These carry genes and, help in inheritance or transfer of, characters from the parents to the, offspring. The chromosomes can be seen, only when the cell divides., , Paheli wants to know if the, structure of the nucleus is, the same in cells of plants,, animals and bacteria., The nucleus of the bacterial cell is, not well-organised like the cells of, multicellular organisms. There is no, nuclear membrane. The cells having, nuclear material without nuclear, membrane are termed prokaryotic, cells. The organisms with these kinds, of cells are called prokaryotes (pro :, primitive; karyon : nucleus). Examples, are bacteria and blue green algae. The, cells, like onion cells and cheek cells, having well-organised nucleus with, a nuclear membrane are designated, as eukaryotic cells. All organisms, other than bacteria and blue, green algae are called eukaryotes., (eu : true; karyon: nucleus)., 95, , CELL — STRUCTURE AND FUNCTIONS, , 2019-2020

Page 7 :
While observing the onion cells, under the microscope, did you notice, any blank-looking structures in the, cytoplasm? It is called vacuole. It could, be single and big as in an onion cell., Cheek cells have smaller vacuoles. Large, vacuoles are common in plant cells., Vacuoles in animal cells are much, smaller., You might have noticed several small, coloured bodies in the cytoplasm of the, cells of T radescantia leaf. They are, scattered in the cytoplasm of the leaf, cells. These are called plastids. They are, of different colours. Some of them, contain green pigment called, chlorophyll. Green coloured plastids are, , called chloroplasts. They provide green, colour to the leaves. You may recall that, chlorophyll in the chloroplasts of leaves,, is essential for photosynthesis., , 8.6 Comparison of Plant and, Animal Cells, If you recall Activities 8.3 and 8.4, you, should be able to compare plant and, animal cells. Observe the plant and, animal cell carefully in Fig. 8.7 (a), (b)., Let us tabulate the similarities and, disinguishing features of plant and, animal cells. Only a few features are, mentioned. You may add mor e in, Table 8.1., cell membrane, , cell wall, , vacuole, , nucleus, , cell membrane, chloroplast, cytoplasm, (a), , (b), , Fig. 8.7 : (a) Plant cell (b) Animal cell, , Table 8.1 : Comparison of Plant Cell and Animal Cell, Sl. No., 1., 2., 3., 4., 5., 6., 7., , Part, , Plant Cell, , Animal Cell, , Cell membrane, Cell wall, Nucleus, Nuclear membrane, Cytoplasm, Plastids, Vacuole, , Present, Present, , Present, Absent, , 96, , SCIENCE, , 2019-2020

Page 8 :
KEYWORDS, CELL, , WHA, T Y, OU HA, VE LEARNT, WHAT, YOU, HAVE, Ü, , organs., , CELL MEMBRANE, Ü, CELL WALL, CHLOROPLAST, , All organisms are made of smaller parts called, Organs are made of still smaller parts. The, smallest living part of an organism is a ‘cell’., , Ü, , Cells were first observed in cork by Robert, Hooke in 1665., , CHROMOSOME, CYTOPLASM, , Ü, , Cells exhibit a variety of shapes and sizes., , Ü, , Number of cells also varies from organism to, organism., , EUKARYOTES, , Ü, , unaided eye. Hen’s egg is an example., , GENE, MULTICELLULAR, NUCLEAR MEMBRANE, , Ü, Ü, , a variety of cells in multicellular organisms., Ü, , smaller components called organelles, and, (iii) the nucleus., Ü, , Nucleus is separated from cytoplasm by a, nuclear membrane., , Ü, , Cells without well-organised nucleus, i.e., lacking nuclear membrane, are called, , PROKARYOTES, PSEUDOPODIA, , The cell has three main parts: (i) the cell, membrane, (ii) cytoplasm which contains, , ORGANELLES, , PLASTID, , The single cell of unicellular organisms, performs all the basic functions performed by, , ORGAN, , PLASMA MEMBRANE, , Some organisms are single-celled, while others, contain large number of cells., , NUCLEOLUS, NUCLEUS, , Some cells are big enough to be seen with the, , prokaryotic cells., Ü, , Plant cells differ from animal cells in having, an additional layer around the cell membrane, , TISSUE, , termed cell wall., UNICELLULAR, , Ü, , Coloured bodies called plastids are found in, the plant cells only. Green plastids containing, , VACUOLE, , chlorophyll are called chloroplasts., WHITE BLOOD CELL, (WBC), , Ü, , Plant cell has a big central vacuole unlike a, number of small vacuoles in animal cells., , 97, , CELL — STRUCTURE AND FUNCTIONS, , 2019-2020

Page 9 :
Exercises, 1. Indicate whether the following statements are True (T) or False (F)., (a), , Unicellular organisms have one-celled body., , (T/F), , (b), , Muscle cells are branched., , (T/F), , (c), , The basic living unit of an organism is an organ., , (T/F), , (d), , Amoeba has irregular shape., , (T/F), , 2. Make a sketch of the human nerve cell. What function do nerve, cells perform?., 3. Write short notes on the following., (a), , Cytoplasm, , (b), , Nucleus of a cell, , 4. Which part of the cell contains organelles?, 5. Make sketches of animal and plant cells. State three differences, between them., 6. State the difference between eukaryotes and prokaryotes., 7. Where are chromosomes found in a cell? State their function., 8. ‘Cells are the basic structural units of living organisms’. Explain., 9. Explain why chloroplasts are found only in plant cells?, 10. Complete the crossword with the help of clues given below., Across, 1., , This is necessary, photosynthesis., , 3., , Term for component present, in the cytoplasm., , 6., , The living substance in the, cell., , 8., , for, , Units of inheritance present, on the chromosomes., , 1, , 3, , C, , 2, , O, , O, 4, 5, , 6, , P, , 7, , M, , V, , T, , Down, 1., , Green plastids., , 2., , Formed by collection of, tissues., , 4., , It separates the contents of, the cell from the surrounding, medium., , 5., , Empty structure in the cytoplasm., , 7., , A group of cells., , 98, , 8, , G, , SCIENCE, , 2019-2020

Page 10 :
Extended Learning — Activities and Projects, 1., , Visit a laboratory for senior secondary students in your school or, in a neighbouring school. Learn about the functioning of a, microscope in the laboratory. Also observe how a slide is observed, under the microscope., , 2., , Talk to the senior biology teacher in your school or a neighbouring, school. Find out if there are diseases which are passed on from, parents to the offspring. Find out how these are carried and also if, these diseases can be treated. For this you can also visit a doctor., , 3., , Visit an agriculture extension centre in your area. Find out about, genetically modified (GM) crops. Prepare a short speech for your, class on this topic., , 4., , Find out about Bt cotton from an agriculture expert. Prepare a, short note on its advantages/disadvantages., , Did You Know?, The cells in the outermost layer of our skin are dead. An average, adult carries around about 2 kg of dead skin. Billions of tiny, fragments of the skin are lost every day. Every time you run your, finger on a dusty table, you shed a lot of old skin., , 99, , CELL — STRUCTURE AND FUNCTIONS, , 2019-2020