CELL — STRUCTURE AND FUNCTIONS ou have already learnt that things These boxes appeared like a honey- around us are either living or comb. He also noticed that one box was Ynon-living. Further, you may separated from the other by a wall or recall that all living organisms carry out partition. Hooke coined the term ‘cell’ certain basic functions. Can you list for each box. What Hooke observed as these functions? boxes or cells in the cork were actually Different sets of organs perform the dead cells. various functions you have listed. In this Cells of living organisms could be chapter, you shall learn about the basic observed only after the discovery of structural unit of an organ, which is the improved microscopes. Very little was cell. Cells may be compared to bricks. known about the cell for the next 150 Bricks are assembled to make a building. years after Robert Hooke’s observations. Similarly, cells are assembled to make Today, we know a lot about cell the body of every organism. structure and its functions because of improved microscopes having high 8.1 Discovery of the Cell magnification. Robert Hooke in 1665 observed slices of 8.2 The Cell cork under a simple magnifying device. Cork is a part of the bark of a Both, bricks in a building and cells in tree. He took thin slices of cork and the living organisms, are basic observed them under a microscope. He structural units [Fig. 8.2(a), (b)]. The noticed partitioned boxes or buildings, though built of similar bricks, compartments in the cork slice (Fig. 8.1). 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 Fig. 8.1:8.1:Fig. Cork cells as observed by group of cells? Robert Hooke 2021–22 (a) Brick wall (b) Onion peel Fig. 8.2 : Brick wall and onion peel The egg of a hen represents a single Organisms made of more than one cell and is big enough to be seen by the cell are called multicellular (multi : unaided eye. many; cellular : cell) organisms. The number of cells being less in smaller 8.3 Organisms show Variety organisms does not, in any way, affect in Cell Number, Shape and the functioning of the organisms. You Size will be surprised to know that an How do scientists observe and study the organism with billions of cells begins living cells? They use microscopes which life as a single cell which is the fertilised magnify objects. Stains (dyes) are used egg. The fertilised egg cell multiplies and to colour parts of the cell to study the the number of cells increase as detailed structure. development proceeds. There are millions of living Look at Fig 8.3 (a) and (b). Both organisms. They are of different shapes organisms are made up of a single cell. and sizes. Their organs also vary in The single-celled organisms are called shape, size and number of cells. Let us unicellular (uni : one; cellular : cell) study about some of them. 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 (a) (b) trillion is a thousand billion. Fig. 8.3 : (a) Amoeba (b) Paramecium CELL — STRUCTURE AND FUNCTIONS 91 2021–22 organisms. A single-celled organism performs all the necessary functions The change in shape is due to that multicellular organisms perform. formation of pseudopodia which A single-celled organism, like facilitates movement and help in capturing food. amoeba, captures and digests food, respires, excretes, grows and reproduces. Similar functions in A white blood cell (WBC) in human multicellular organisms are carried out blood is another example of a single cell by groups of specialised cells forming which can change its shape. But while different tissues. Tissues, in turn, form WBC is a cell, amoeba is a full fledged organs. organism capable of independent existence. Activity 8.1 What shape would you expect in organisms with millions of cells? Fig. The teacher may show a permanent 8.4 (a, b, c) shows different cells such slide of amoeba and paramecium as blood, muscle and nerve of human under a microscope. Alternatively, beings. The different shapes are related the teacher can collect pond water to their specific functions. and show these organisms by Generally, cells are round, spherical preparing the slides. or elongated [Fig. 8.4(a)]. Some cells are long and pointed at both ends. They Shape of Cells exhibit a spindle shape [Fig. 8.4(b)]. Cells sometimes are quite long. Some Refer to Fig. 8.3 (a). How do you define are branched like the nerve cell or a the shape of amoeba in the figure? You neuron [Fig. 8.4(c)]. The nerve cell may say that the shape appears receives and transfers messages, thereby irregular. 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 (a) learnt in Class VII. These projections appear and disappear as amoeba moves or feeds. (c) (b) What advantage does Fig. 8.4 : (a) Spherical red blood cells amoeba derive by of humans, (b) Spindle changing shape? shaped muscle cells, (c) Long branched nerve cell 92 SCIENCE 2021–22 helping to control and coordinate the the elephant be much bigger than those working of different parts of the body. in a rat. The size of the cell is related to Can you guess, which part of the cell its function. For example, nerve cells, gives it shape? Components of the cell both in the elephant and rat, are long are enclosed in a membrane. This and branched. They perform the same membrane provides shape to the cells function, that of transferring messages. of plants and animals. Cell wall is an additional covering over the cell 8.4 Cell Structure and membrane in plant cells. It gives shape Function and rigidity to these cells (Fig. 8.7). You have learnt that each living Bacterial cell also has a cell wall. organism has many organs. You have Size of Cells studied in Class VII about the digestive The size of cells in living organisms may organs which together constitute the be as small as a millionth of a metre digestive system. Each organ in the (micrometre or micron) or may be as large system performs different functions as a few centimetres. However, most of the such as digestion, assimilation and cells are microscopic in size and are not absorption. Similarly, different organs of a plant perform specific/specialised visible to the unaided eye. They need to functions. For example, roots help in the be enlarged or magnified by a microscope. absorption of water and minerals. The smallest cell is 0.1 to 0.5 micrometre Leaves, as you have learnt in Class VII, in bacteria. The largest cell measuring are responsible for synthesis of food. 170 mm ×130 mm, is the egg of an ostrich. Each organ is further made up of Activity 8.2 smaller parts called tissues. A tissue is a group of similar cells performing a Boil a hen’s egg. Remove the shell. specific function. What do you observe? A white Paheli realised that an organ is made material surrounds the yellow part. up of tissues which in turn, are made White material is albumin which up of cells. The cell in a living organism solidifies on boiling. The yellow part is the basic structural unit. is yolk. It is part of the single cell. You can observe this single cell 8.5 Parts of the Cell without any magnifying device. Cell Membrane The basic components of a cell are cell membrane, cytoplasm and nucleus (Fig. 8.7). The cytoplasm and nucleus Are the cells in an are enclosed within the cell membrane, elephant larger than the also called the plasma membrane. The cells in a rat? membrane separates cells from one The size of the cells has no relation another and also the cell from the with the size of the body of the animal or surrounding medium. The plasma plant. It is not necessary that the cells in membrane is porous and allows the CELL — STRUCTURE AND FUNCTIONS 93 2021–22 movement of substances or materials between the nucleus and the cell both inward and outward. membrane is called cytoplasm. Activity 8.3 In order to observe the basic I want to know why plant components of the cell, take an cells need cell walls? onion bulb. Remove the dry pink coverings (peels). You can easily You have learnt earlier that the cell separate these from the fleshy white membrane gives shape to the cell. In layers of the bulb with the help of addition to the cell membrane, there is forceps or even with your hand. You an outer thick layer in cells of plants can also break the onion bulb and called cell wall. This additional layer separate out thin layers. Place a surrounding the cell membrane is small piece of the thin onion peel required by plants for protection.