Cell Structure and Functions PDF

CELL — STRUCTURE AND FUNCTIONS Y ou have already learnt that things These boxes appeared like a honey- d around us are either living or comb. non-living. Further, you may...

CELL — STRUCTURE AND FUNCTIONS Y ou have already learnt that things These boxes appeared like a honey- d around us are either living or comb. non-living. Further, you may He also noticed that one box was e recall that all living organisms carry out separated from the other by a wall or h certain basic functions. Can you list partition. Hooke coined the term ‘cell’ these functions? for each box. What Hooke observed as T s Different sets of organs perform the boxes or cells in the cork were actually l i various functions you have listed. In this dead cells. R chapter, you shall learn about the basic Cells of living organisms could be b structural unit of an organ, which is the observed only after the discovery of E cell. Cells may be compared to bricks. improved microscopes. Very little was u Bricks are assembled to make a building. known about the cell for the next 150 C Similarly, cells are assembled to make years after Robert Hooke’s observations. p the body of every organism. Today, we know a lot about cell N re structure and its functions because of 8.1 Discovery of the Cell improved microscopes having high magnification. © e Robert Hooke in 1665 observed slices of cork under a simple magnifying device. Cork is a part of the bark of a 8.2 The Cell b tree. He took thin slices of cork and Both, bricks in a building and cells in observed them under a microscope. He the living organisms, are basic o noticed partitioned boxes or structural units [Fig. 8.2(a), (b)]. The t compartments in the cork slice (Fig. 8.1). buildings, though built of similar bricks, have different designs, shapes and sizes. t Similarly, in the living world, organisms differ from one another but all are made o up of cells. Cells in the living organisms n 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: Cork cells as observed by group of cells? Robert Hooke  e d h (a) Brick wall (b) Onion peel T s Fig. 8.2 : Brick wall and onion peel i The egg of a hen represents a single Organisms made of more than one R l cell and is big enough to be seen by the cell are called multicellular (multi : b unaided eye. many; cellular : cell) organisms. The E number of cells being less in smaller 8.3 Organisms show Variety u organisms does not, in any way, affect C in Cell Number, Shape and the functioning of the organisms. You p Size will be surprised to know that an N re 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 egg. The fertilised egg cell multiplies and © e magnify objects. Stains (dyes) are used to colour parts of the cell to study the the number of cells increase as detailed structure. development proceeds. b 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. The single-celled organisms are called o and sizes. Their organs also vary in unicellular (uni : one; cellular : cell) t shape, size and number of cells. Let us study about some of them. t Number of Cells o Can you guess the number of cells in a tall tree or in a huge animal like the n 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 93 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 amoeba, captures and digests food, in capturing 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 d by groups of specialised cells forming which can change its shape. But while WBC is a cell, amoeba is a full fledged e different tissues. Tissues, in turn, form organs. organism capable of independent h existence. Activity 8.1 What shape would you expect in T s organisms with millions of cells? Fig. i The teacher may show a permanent l 8.4 (a, b, c) shows different cells such R slide of Amoeba and Paramecium as blood, muscle and nerve cells of b under a microscope. Alternatively, human beings. The different shapes are E the teacher can collect pond water related to their specific functions. u and show these organisms by Generally, cells are round, spherical C preparing the slides. or elongated [Fig. 8.4(a)]. Some cells are p long and pointed at both ends. They N re Shape of Cells exhibit spindle shape [Fig. 8.4(b)]. Cells sometimes are quite long. Some are Refer to Fig, 8.3 (a). How do you define branched like the nerve cell or a neuron © e the shape of Amoeba in the figure? You [Fig. 8.4(c)]. The nerve cell receives and may say that the shape appears transfers messages, thereby helping to b irregular. Infact, Amoeba has no control and coordinate the working of definite shape, unlike other organisms. different parts of the body. It keeps on changing its shape. o Observe the projections of varying t lengths protruding out of its body. t These are called pseudopodia (pseudo : false; podia : feet), as you learnt o in Class VII. These projections appear (a) and disappear as Amoeba moves or n feeds. (c) (b) What advantage does Fig. 8.4 :(a) Spherical red blood cells Amoeba derive by of humans, (b) Spindle shaped muscle changing shape? cells, (c) Long branched nerve cell 94 SCIENCE  Can you guess, which part of the cell in the elephant be much bigger than gives it shape? Components of the cell those in a rat. The size of the cell is are enclosed in a membrane. This related to its function. For example, membrane provides shape to the cells nerve cells, both in the elephant and of plants and animals. Cell wall is an rat, are long and branched. They additional covering over the cell perform the same function, that of membrane in plant cells. It gives shape transferring messages. and rigidity to these cells (Fig. 8.7). 8.4 Cell Structure and d Bacterial cell also has a cell wall. Function e Size of Cells You have lear nt that each living h The size of cells in living organisms may organism has many organs. You have be as small as a millionth of a metre T s studied in Class VII about the digestive i (micrometre or micron) or may be as organs which together constitute the l large as a few centimetres. However, most R digestive system. Each organ in the of the cells are microscopic in size and system performs different functions b are not visible to the unaided eye. They E such as digestion, assimilation and need to be enlarged or magnified by a absorption. Similarly, different organs u microscope. The smallest cell is 0.1 to C of a plant perform specific/specialised p 0.5 micrometre in bacteria. The largest functions. For example, roots help in the cell measuring 170 mm ×130 mm, is N re absorption of water and minerals. the egg of an ostrich. Leaves, as you have learnt in Class VII, are responsible for synthesis of food. Activity 8.2 © e Each organ is further made up of smaller parts called tissues. A tissue is Boil a hen’s egg. Remove the shell. a group of similar cells performing a b What do you observe? A white specific function. material surrounds the yellow part. Paheli realised that an organ is made White material is albumin which o up of tissues which in turn, are made solidifies on boiling. The yellow part t up of cells. The cell in a living organism is yolk. It is part of the single cell. is the basic structural unit. You can observe this single cell t without any magnifying device. 8.5 Parts of the Cell o Cell Membrane n The basic components of a cell are cell Are the cells in an membrane, cytoplasm and nucleus elephant larger than the (Fig. 8.7). The cytoplasm and nucleus cells in a rat? are enclosed within the cell membrane, also called the plasma membrane. The The size of the cells has no relation membrane separates cells from one with the size of the body of the animal another and also the cell from the or plant. It is not necessary that the cells surrounding medium. The plasma CELL — STRUCTURE AND FUNCTIONS 95 membrane is porous and allows the between the nucleus and the cell movement of substances or materials membrane is called cytoplasm. both inward and outward. Activity 8.3 I want to know why plant In order to observe the basic cells need cell walls? components of the cell, take an onion bulb. Remove the dry pink You have learnt earlier that the cell d coverings (peels). You can easily membrane gives shape to the cell. In e separate these from the fleshy white addition to the cell membrane, there is layers of the bulb with the help of an outer thick layer in cells of plants h forceps or even with your hand. You called cell wall. This additional layer T s can also break the onion bulb and surrounding the cell membrane is i separate out thin layers. Place a required by the plants for protection. R l small piece of the thin onion peel Plant cells need protection against in a drop of water on a glass slide. b variations in temperature, high wind E The thin layer can be cut into speed, atmospheric moisture, etc. They smaller pieces with the help of a u are exposed to these variations because C blade or forceps. Add a drop of they cannot move. Cells can be p methylene blue solution to the layer observed in the leaf peel of N re and place a coverslip on it. While Tradescantia, Elodea or Rhoeo. You placing the coverslip ensure that can prepare a slide as in the case there are no air bubbles under the of onion. © e coverslip. Observe the slide under Paheli asks Boojho if he can also the microscope. Draw and label. You observe animal cells. may compare it with Fig. 8.5. b Activity 8.4 The boundary of the onion cell is the o cell membrane covered by another thick Take a clean tooth pick, or a t covering called the cell wall. The central matchstick with the tip broken. dense round body in the centre is called Scrape inside of your cheek without t the nucleus. The jelly-like substance hurting it. Place it in a drop of water on a glass slide. Add a drop of iodine o and place a coverslip over it. nucleus n Alternatively, add 1-2 drops of methylene blue solution. Observe it under the microscope. You may notice several cells in the scraped cytoplasm material (Fig. 8.6). You can identify the cell membrane, the cytoplasm and nucleus. A cell wall is absent in animal cells. Fig. 8.5 : Cells observed in an onion peel 96 SCIENCE  Gene nuclear cytoplasm membrane 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 nucleus has brown eyes, you may also have cell d membrane brown eyes. If your mother has curly hair, you might also end up having e curly hair. However, the different Fig. 8.6 : Human cheek cells combination of genes from parents h result in different characteristics. T s Cytoplasm i Nucleus, in addition to its role in l It is the jelly-like substance present R inheritance, acts as control centre of the between the cell membrane and the activities of the cell. The entire content b nucleus. Various other components, or E of a livinhg cell is known as protoplasm. organelles, of cells are present in the It includes the cytoplasm and the u cytoplasm. These are mitochondria, golgi C nucleus. Protoplasm is called the living bodies, ribosomes, etc. You will learn p substance of the cell. about them in later classes. Nucleus N re Paheli wants to know if the © e It is an important component of the structure of the nucleus is living cell. It is generally spherical and the same in cells of plants, located in the centre of the cell. It can animals and bacteria. b be stained and seen easily with the help of a microscope. Nucleus is separated The nucleus of the bacterial cell is from the cytoplasm by a membrane o not well organised like the cells of called the nuclear membrane. This multicellular organisms. There is no t membrane is also porous and allows the nuclear membrane. The cells having nuclear material without nuclear t movement of materials between the cytoplasm and the inside of the nucleus. membrane are termed prokaryotic o cells. The organisms with these kinds With a microscope of higher of cells are called prokaryotes (pro : magnification, we can see a smaller n primitive; karyon : nucleus). Examples spherical body in the nucleus. It is called are bacteria and blue green algae. The the nucleolus. In addition, nucleus cells, like onion cells and cheek cells contains thread-like structures called having well organised nucleus with chromosomes. These carry genes and a nuclear membrane are designated help in inheritance or transfer of as eukaryotic cells. All organisms characters from the parents to the other than bacteria and blue green algae are called eukaryotes. (eu : offspring. The chromosomes can be seen true; karyon: nucleus). only when the cell divides. CELL — STRUCTURE AND FUNCTIONS 97  While observing the onion cells called chloroplasts. They provide green under the microscope, did you notice colour to the leaves. You may recall that any blank-looking structures in the chlorophyll in the chloroplasts of leaves, cytoplasm? It is called vacuole. It could is essential for photosynthesis. be single and big as in an onion cell. Cheek cells have smaller vacuoles. Large 8.6 Comparison of Plant and vacuoles are common in plant cells. Animal Cells Vacuoles in animal cells are much smaller. If you recall Activities 8.3 and 8.4, you d You might have noticed several small should be able to compare plant and e coloured bodies in the cytoplasm of the animal cells. Observe the plant and cells of Tradescantia leaf. They are animal cell carefully in Fig. 8.7 (a), (b). h scattered in the cytoplasm of the leaf Let us tabulate the similarities and T s cells. These are called plastids. They are disinguishing features of plant and i of different colours. Some of them animal cells. Only a few features are R l contain green pigment called mentioned. You may add more in chlorophyll. Green coloured plastids are Table 8.1. cell wall E u b cell membrane C N re p vacuole nucleus © e cell membrane b chloroplast o cytoplasm (b) t (a) Fig. 8.7 : (a) Plant cell (b) Animal cell t Table 8.1 : Comparison of Plant Cell and Animal Cell o Sl. No. Part Plant Cell Animal Cell n 1. Cell membrane Present Present 2. Cell wall Present Absent 3. Nucleus 4. Nuclear membrane 5. Cytoplasm 6. Plastids 7. Vacuole 98 SCIENCE KEYWORDS WHAT YOU HAVE LEARNT CELL
All organisms are made of smaller parts called CELL MEMBRANE organs.
Organs are made of still smaller parts. The CELL WALL smallest living part of an organism is a ‘cell’. CHLOROPLAST
Cells were first observed in cork by Robert Hooke in 1665. d CHROMOSOME
Cells exhibit variety of shapes and sizes. e CYTOPLASM
Number of cells also varies from organism to organism. h EUKARYOTES
Some cells are big enough to be seen with the T s GENE unaided eye. Hen’s egg is an example. i
Some organisms are single-celled, while others R l MULTICELLULAR contain large number of cells. b NUCLEAR MEMBRANE
The single cell of unicellular organisms E performs all the basic functions performed by u NUCLEOLUS C a variety of cells in multicellular organisms. p NUCLEUS
The cell has three main parts, (i) the cell N re membrane, (ii) cytoplasm which contains ORGAN smaller components called organelles, and (iii) ORGANELLES the nucleus. © e
Nucleus is separated from cytoplasm by a PLASMA MEMBRANE nuclear membrane. b PLASTID
Cells without well organised nucleus, i.e. lacking nuclear membrane, are called PROKARYOTES o prokaryotic cells. t PSEUDOPODIA
Plant cells differ from animal cells in having an additional layer around the cell membrane t TISSUE termed cell wall. o UNICELLULAR
Coloured bodies called plastids are found in the plant cells only. Green plastids containing n VACUOLE chlorophyll are called chloroplasts. WHITE BLOOD CELL
Plant cell has a big central vacuole unlike a (WBC) number of small vacuoles in animal cells. CELL — STRUCTURE AND FUNCTIONS 99  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) d 2. Make a sketch of the human nerve cell. What function do nerve cells e perform?. 3. Write short notes on the following: h (a) Cytoplasm T i s (b) Nucleus of a cell R l 4. Which part of the cell contains organelles? 5. Make sketches of animal and plant cells. State three differences between E b them. u 6. State the difference between eukaryotes and prokaryotes. C 7. Where are chromosomes found in a cell? State their function. p 8. ‘Cells are the basic structural units of living organisms’. Explain. N re 9. Explain why chloroplasts are found only in plant cells? 10. Complete the crossword with the help of clues given below: © e Across 1. This is necessary for b photosynthesis. 3. Term for component present in the cytoplasm. o 6. The living substance in the t cell. t 8. Units of inheritance present on the chromosomes. o Down n 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. 100 SCIENCE  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 d 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. e 3. Visit an agriculture extension centre in your area. Find out about h genetically modified (GM) crops. Prepare a short speech for your s class on this topic. You may visit www.usc.ernet.in/currsci/ T i sep252001/655.pdf R l 4. Find out about Bt cotton from an agriculture expert (or from b envfor.nic.in/divisions/csnrv/btcotton/bgnote.pdf). Prepare a short E note on its advantages/disadvantages. u For more on cell, visit : C p www.enchatedbearning.com/subjects/plants/cell/ N re Did You Know? © e 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 b finger on a dusty table, you shed a lot of old skin. t o o t n CELL — STRUCTURE AND FUNCTIONS 101