NCERT Class 8 Science Cell — Structure And Functions
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CELL — STRUCTURE AND FUNCTIIONS
You have already learnt that things around us are either living or non-living. Further, you may recall that all living organisms carry outcertain 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 atree. 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 cellstructure 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 basic 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.
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 inshape, size and number of cells. Let us 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. organisms. A single-celled organismperforms 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 outby groups of specialised cells forming different tissues. Tissues, in turn, form organs.
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 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 learntin Class VII. These projections appear and disappear as Amoeba moves orfeeds.
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:
(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?
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. You may visit www.usc.ernet.in/currsci/ sep252001/655.pdf
4. Find out about Bt cotton from an agriculture expert (or from envfor.nic.in/divisions/csnrv/btcotton/bgnote.pdf). Prepare a short note on its advantages/disadvantages.
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