Chapter 5 The Fundamental Unit of Life PDF

C hapter 5 THE FUNDAMENTAL UNIT OF LIFE While examining a thin slice of cork, Robert avoid air bubbles while putting the Hooke saw that the cork resembled the cover s...

C hapter 5 THE FUNDAMENTAL UNIT OF LIFE While examining a thin slice of cork, Robert avoid air bubbles while putting the Hooke saw that the cork resembled the cover slip with the help of a mounting structure of a honeycomb consisting of many needle. Ask your teacher for help. We have prepared a temporary mount of little compartments. Cork is a substance onion peel. We can observe this slide which comes from the bark of a tree. This under low power followed by high was in the year 1665 when Hooke made this powers of a compound microscope. chance observation through a self-designed microscope. Robert Hooke called these boxes Eyepiece cells. Cell is a Latin word for ‘a little room’. This may seem to be a very small and insignificant incident but it is very important in the history of science. This was the very first Coarse adjustment Body tube time that someone had observed that living Fine adjustment things appear to consist of separate units. The Arm use of the word ‘cell’ to describe these units is Clip Objective lens being used till this day in biology. Microscope slide Let us find out about cells. Stage Swivel Condenser 5.1 What are Living Organisms Mirror Made Up of? Base Activity ______________ 5.1 Fig. 5.1: Compound microscope Let us take a small piece from an onion What do we observe as we look through bulb. With the help of a pair of forceps, the lens? Can we draw the structures that we can peel of f the skin (called epidermis) from the concave side (inner we are able to see through the microscope, layer) of the onion. This layer can be on an observation sheet? Does it look like put immediately in a watch-glass Fig. 5.2? containing water. This will prevent the peel from getting folded or getting dry. What do we do with this peel? Let us take a glass slide, put a drop of Nucleus water on it and transfer a small piece of the peel from the watch glass to the Cells slide. Make sure that the peel is perfectly flat on the slide. A thin camel hair paintbrush might be necessary to help transfer the peel. Now we put a drop of safranin solution on this piece followed by a cover slip. Take care to Fig. 5.2: Cells of an onion peel 2024-25  We can try preparing temporary mounts Chlamydomonas, Paramoecium and bacteria. of peels of onions of different sizes. What do These organisms are called unicellular we observe? Do we see similar structures or organisms (uni = single). On the other hand, different structures? many cells group together in a single body and assume different functions in it to form What are these structures? various body parts in multicellular organisms These structures look similar to each other. (multi = many) such as some fungi, plants Together they form a big structure like an and animals. Can we find out names of some onion bulb! We find from this activity that more unicellular organisms? onion bulbs of different sizes have similar Every multi-cellular organism has come small structures visible under a microscope. from a single cell. How? Cells divide to The cells of the onion peel will all look the produce cells of their own kind. All cells thus same, regardless of the size of the onion they come from pre-existing cells. came from. These small structures that we see are Activity ______________ 5.2 the basic building units of the onion bulb. We can try preparing temporary These structures are called cells. Not only mounts of leaf peels, tip of roots of onions, but all organisms that we observe onion or even peels of onions of different around are made up of cells. However, there sizes. are also single cells that live on their own. After performing the above activity, let us see what the answers to the following Cells wer e first discovered by questions would be: Robert Hooke in 1665. He observed (a) Do all cells look alike in terms of the cells in a cork slice with the help shape and size? (b) Do all cells look alike in structure? of a primitive micr oscope. (c) Could we find differences among Leeuwenhoek (1674), with the cells from different parts of a improved microscope, discovered the plant body? free living cells in pond water for the (d) What similarities could we find? first time. It was Robert Brown in 1831 who discovered the nucleus in Some organisms can also have cells of More to know the cell. Purkinje in 1839 coined the different kinds. Look at the following picture. ter m ‘protoplasm’ for the fluid It depicts some cells from the human body. substance of the cell. The cell theory, that all the plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839). The cell theory was further expanded by Virchow Blood (1855) by suggesting that all cells cells Nerve Cell Smooth arise from pre-existing cells. With the muscle discovery of the electron microscope cell in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles. Bone Fat cell cell The invention of magnifying lenses led to the discovery of the microscopic world. It is Ovum Sperm now known that a single cell may constitute a whole organism as in Amoeba, Fig. 5.3: Various cells from the human body 50 SCIENCE 2024-25  The shape and size of cells are related to every cell; plasma membrane, nucleus and the specific function they perform. Some cells cytoplasm. All activities inside the cell and like Amoeba have changing shapes. In some interactions of the cell with its environment cases the cell shape could be more or less are possible due to these features. Let us see fixed and peculiar for a particular type of cell; how. for example, nerve cells have a typical shape. Each living cell has the capacity to 5.2.1 P LASMA MEMBRANE OR CELL perform certain basic functions that are MEMBRANE characteristic of all living forms. How does a living cell perform these basic functions? This is the outermost covering of the cell that We know that there is a division of labour in separates the contents of the cell from its multicellular organisms such as human external environment. The plasma membrane beings. This means that different parts of allows or permits the entry and exit of some the human body perform different functions. materials in and out of the cell. It also The human body has a heart to pump blood, prevents movement of some other materials. a stomach to digest food and so on. Similarly, The cell membrane, therefore, is called a division of labour is also seen within a single selectively permeable membrane. cell. In fact, each such cell has got certain How does the movement of substances specific components within it known as cell take place into the cell? How do substances organelles. Each kind of cell organelle move out of the cell? performs a special function, such as making Some substances like carbon dioxide or new material in the cell, clearing up the oxygen can move across the cell membrane waste material from the cell and so on. A by a process called diffusion. We have studied cell is able to live and perform all its the process of diffusion in earlier chapters. functions because of these organelles. These We saw that there is spontaneous movement organelles together constitute the basic unit of a substance from a region of high called the cell. It is interesting that all cells concentration to a region where its are found to have the same organelles, no concentration is low. matter what their function is or what Something similar to this happens in cells organism they are found in. when, for example, some substance like CO2 (which is cellular waste and requires to be uestions Q excreted out by the cell) accumulates in high concentrations inside the cell. In the cell’s 1. Who discovered cells, and how? 2. Why is the cell called the external environment, the concentration of structural and functional unit of CO2 is low as compared to that inside the life? cell. As soon as there is a difference of concentration of CO2 inside and outside a cell, CO2 moves out of the cell, from a region of high concentration, to a region of low concentration outside the cell by the process 5.2 What is a Cell Made Up of? of diffusion. Similarly, O2 enters the cell by What is the Structural the process of diffusion when the level or concentration of O2 inside the cell decreases. Organisation of a Cell? Thus, diffusion plays an important role in We saw above that the cell has special gaseous exchange between the cells as well components called organelles. How is a cell as the cell and its external environment. organised? Water also obeys the law of diffusion. The If we study a cell under a microscope, we movement of water molecules through such a would come across three features in almost selectively permeable membrane is called osmosis. THE FUNDAMENTAL UNIT OF LIFE 51 2024-25 The movement of water across the plasma Activity ______________ 5.3 membrane is also affected by the amount of substance dissolved in water. Thus, Osmosis with an egg osmosis is the net diffusion of water across a (a) Remove the shell of an egg by dissolving it in dilute hydrochloric acid. The shell selectively permeable membrane toward a is mostly calcium carbonate. A thin higher solute concentration. outer skin now encloses the egg. Put What will happen if we put an animal cell the egg in pure water and observe after or a plant cell into a solution of sugar or salt 5 minutes. What do we observe? in water? The egg swells because water passes One of the following three things could into it by osmosis. happen: (b) Place a similar de-shelled egg in a concentrated salt solution and observe 1. If the medium surrounding the cell has for 5 minutes. The egg shrinks. Why? a higher water concentration than the Water passes out of the egg solution cell, meaning that the outside solution into the salt solution because the salt is very dilute, the cell will gain water by solution is more concentrated. osmosis. Such a solution is known as We can also try a similar activity with dried a hypotonic solution. raisins or apricots. Water molecules are free to pass across the cell membrane in both Activity ______________ 5.4 directions, but more water will come Put dried raisins or apricots in plain into the cell than will leave. The net water and leave them for some time. Then place them into a concentrated (overall) result is that water enters the solution of sugar or salt. You will cell. The cell is likely to swell up. observe the following: 2. If the medium has exactly the same (a) Each gains water and swells water concentration as the cell, there when placed in water. (b) However, when placed in the will be no net movement of water across concentrated solution it loses the cell membrane. Such a solution is water, and consequently shrinks. known as an isotonic solution. Unicellular freshwater organisms and Water crosses the cell membrane in most plant cells tend to gain water through both directions, but the amount going osmosis. Absorption of water by plant roots in is the same as the amount going out, is also an example of osmosis. so there is no overall movement of Thus, diffusion is important in exhange of gases and water in the life of a cell. In water. The cell will stay the same size. additions to this, the cell also obtains 3. If the medium has a lower nutrition from its environment. Different concentration of water than the cell, molecules move in and out of the cell meaning that it is a very concentrated through a type of transport requiring use solution, the cell will lose water by of energy. osmosis. Such a solution is known as The plasma membrane is flexible and is a hypertonic solution. made up of organic molecules called lipids and proteins. However, we can observe the Again, water crosses the cell structure of the plasma membrane only membrane in both directions, but this through an electron microscope. time more water leaves the cell than The flexibility of the cell membrane also enters it. Therefore the cell will shrink. enables the cell to engulf in food and other Thus, osmosis is a special case of diffusion material from its external environment. Such through a selectively permeable membrane. processes are known as endocytosis. Amoeba Now let us try out the following activity: acquires its food through such processes. 52 SCIENCE 2024-25  Activity ______________ 5.5 What do we infer from this activity? It appears that only living cells, and not dead Find out about electron microscopes cells, are able to absorb water by osmosis. from resources in the school library or Cell walls permit the cells of plants, fungi through the internet. Discuss it with your teacher. and bacteria to withstand very dilute (hypotonic) external media without bursting. In such media the cells tend to take up water uestions Q by osmosis. The cell swells, building up 1. How do substances like CO2 and pressure against the cell wall. The wall exerts water move in and out of the cell? an equal pressure against the swollen cell. Discuss. Because of their walls, such cells can 2. Why is the plasma membrane withstand much greater changes in the called a selectively permeable surrounding medium than animal cells. membrane? 5.2.3 NUCLEUS 5.2.2 CELL WALL Remember the temporary mount of onion peel Plant cells, in addition to the plasma we prepared? We had put iodine solution on membrane, have another rigid outer covering the peel. Why? What would we see if we tried called the cell wall. The cell wall lies outside observing the peel without putting the iodine the plasma membrane. The plant cell wall is solution? Try it and see what the difference mainly composed of cellulose. Cellulose is a is. Further, when we put iodine solution on complex substance and provides structural strength to plants. the peel, did each cell get evenly coloured? When a living plant cell loses water According to their chemical composition through osmosis there is shrinkage or dif ferent regions of cells get coloured contraction of the contents of the cell away differentially. Some regions appear darker from the cell wall. This phenomenon is known than other regions. Apart from iodine solution as plasmolysis. We can observe this we could also use safranin solution or phenomenon by performing the following methylene blue solution to stain the cells. activity: We have observed cells from an onion; let us now observe cells from our own body. Activity ______________ 5.6 Mount the peel of a Rhoeo leaf in water Activity ______________ 5.7 on a slide and examine cells under the high power of a microscope. Note Let us take a glass slide with a drop of the small green granules, called water on it. Using an ice-cream spoon chloroplasts. They contain a green gently scrape the inside surface of the substance called chlorophyll. Put a cheek. Does any material get stuck on strong solution of sugar or salt on the the spoon? With the help of a needle mounted leaf on the slide. Wait for a we can transfer this material and minute and observe under a spread it evenly on the glass slide kept microscope. What do we see? ready for this. To colour the material Now place some Rhoeo leaves in boiling we can put a drop of methylene blue water for a few minutes. This kills the solution on it. Now the material is ready cells. Then mount one leaf on a slide and observe it under a microscope. Put for observation under microscope. Do a strong solution of sugar or salt on not forget to put a cover-slip on it! the mounted leaf on the slide. Wait for What do we observe? What is the a minute and observe it again. What shape of the cells we see? Draw it on do we find? Did plasmolysis occur now? the observation sheet. THE FUNDAMENTAL UNIT OF LIFE 53 2024-25  Was there a darkly coloured, spherical present in eukaryotic cells. Many of the or oval, dot-like structure near the functions of such organelles are also centre of each cell? This structure is performed by poorly organised parts of the called nucleus. Were there similar cytoplasm (see section 5.2.4). The chlorophyll structures in onion peel cells? in photosynthetic prokaryotic bacteria is The nucleus has a double layered covering associated with membranous vesicles (bag called nuclear membrane. The nuclear like structures) but not with plastids as in membrane has pores which allow the transfer eukaryotic cells (see section 5.2.5). of material from inside the nucleus to its outside, that is, to the cytoplasm (which we will talk about in section 5.2.4). Ribosomes Plasma The nucleus contains chromosomes, membrane which are visible as rod-shaped structures only when the cell is about to divide. Cell wall Chromosomes contain infor mation for inheritance of characters from parents to next Nucleoid generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. Chromosomes are composed of DNA and protein. DNA molecules contain the infor mation necessary for constructing and organising cells. Functional Fig. 5.4: Prokaryotic cell segments of DNA are called genes. In a cell which is not dividing, this DNA is present as part of chromatin material. Chromatin 5.2.4 CYTOPLASM material is visible as entangled mass of thread like structures. Whenever the cell is about to When we look at the temporary mounts of divide, the chromatin material gets organised onion peel as well as human cheek cells, we into chromosomes. can see a large region of each cell enclosed The nucleus plays a central role in cellular by the cell membrane. This region takes up reproduction, the process by which a single very little stain. It is called the cytoplasm. cell divides and forms two new cells. It also The cytoplasm is the fluid content inside the plays a crucial part, along with the plasma membrane. It also contains many environment, in determining the way the cell specialised cell organelles. Each of these will develop and what form it will exhibit at organelles performs a specific function for the maturity, by directing the chemical activities cell. of the cell. Cell organelles are enclosed by In some organisms like bacteria, the membranes. In prokaryotes, beside the nuclear region of the cell may be poorly absence of a defined nuclear region, the defined due to the absence of a nuclear membrane-bound cell organelles are also membrane. Such an undefined nuclear region absent. On the other hand, the eukaryotic containing only nucleic acids is called a cells have nuclear membrane as well as nucleoid. Such organisms, whose cells lack membrane-enclosed organelles. a nuclear membrane, are called prokaryotes The significance of membranes can be (Pro = primitive or primary; karyote ≈ karyon illustrated with the example of viruses. = nucleus). Organisms with cells having a Viruses lack any membranes and hence do nuclear membrane are called eukaryotes. not show characteristics of life until they enter Prokaryotic cells (see Fig. 5.4) also lack a living body and use its cell machinery to most of the other cytoplasmic organelles multiply. 54 SCIENCE 2024-25  uestion 5.2.5 (i) ENDOPLASMIC RETICULUM (ER) Q 1. Fill in the gaps in the following table illustrating differences between prokaryotic and eukaryotic cells. Prokaryotic Cell Eukaryotic Cell The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane. There are two types of ER– rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). RER looks rough under a 1. Size : generally 1. Size: generally microscope because it has particles called small ( 1-10 µm) large ( 5-100 µm) ribosomes attached to its surface. The 1 µm = 10–6 m ribosomes, which are present in all active cells, are the sites of protein manufacture. 2. Nuclear region: 2. Nuclear region: The manufactured proteins are then sent to _______________ well defined and various places in the cell depending on need, _______________ surrounded by a using the ER. The SER helps in the and known as__ nuclear membrane manufacture of fat molecules, or lipids, important for cell function. Some of these 3. Chromosome: 3. More than one proteins and lipids help in building the cell single chromosome membrane. This process is known as membrane biogenesis. Some other proteins 4. Membrane-bound 4. _______________ and lipids function as enzymes and cell organelles _______________ hormones. Although the ER varies greatly in absent _______________ appearance in different cells, it always forms a network system. 5.2.5 CELL ORGANELLES Every cell has a membrane around it to keep its own contents separate from the external environment. Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function. To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures (or ‘organelles’) within themselves. This is one of the features of the eukaryotic cells that distinguish them from prokaryotic cells. Some of these organelles are visible only with an electron microscope. Fig. 5.5: Animal cell We have talked about the nucleus in a previous section. Some important examples Thus, one function of the ER is to serve as of cell organelles which we will discuss now channels for the transport of materials are: endoplasmic reticulum, Golgi apparatus, (especially proteins) between various regions lysosomes, mitochondria and plastids. They of the cytoplasm or between the cytoplasm are important because they carry out some and the nucleus. The ER also functions as a very crucial functions in cells. cytoplasmic framework providing a surface THE FUNDAMENTAL UNIT OF LIFE 55 2024-25 for some of the biochemical activities of the Camillo Golgi was born at cell. In the liver cells of the group of animals Corteno near Brescia in called vertebrates (see Chapter 7), SER plays 1843. He studied a crucial role in detoxifying many poisons medicine at the University and drugs. of Pavia. After graduating in 1865, he continued to work in Pavia at the Hospital of St. Matteo. At that time most of his investigations were concerned with the nervous system, In 1872 he accepted the post of Chief Medical Officer at the Hospital for the Chronically Sick at Abbiategrasso. He first started his investigations into the nervous system in a little kitchen of this hospital, which he had converted into a laboratory. However, the work of greatest importance, which Golgi carried out was a revolutionary method of staining individual nerve and cell structures. This method is referred to as the ‘black reaction’. This method uses a weak solution of silver nitrate and is particularly valuable in tracing the processes and most delicate ramifications of cells. All through his life, he continued to work on these lines, modifying and improving this technique. Fig. 5.6: Plant cell Golgi received the highest honours and awards in recognition of his work. He shared the Nobel prize in 1906 with Santiago 5.2.5 (ii) GOLGI APPARATUS Ramony Cajal for their work on the structure of the nervous system. The Golgi apparatus, first described by Camillo Golgi, consists of a system of membrane-bound vesicles (flattened sacs) 5.2.5 (iii) LYSOSOMES arranged approximately parallel to each other in stacks called cisterns. These membranes Structurally, lysosomes are membrane-bound often have connections with the membranes sacs filled with digestive enzymes. These of ER and therefore constitute another portion enzymes are made by RER. Lysosomes are a of a complex cellular membrane system. kind of waste disposal system of the cell. These The material synthesised near the ER is help to keep the cell clean by digesting any packaged and dispatched to various targets foreign material as well as worn-out cell organelles. Foreign materials entering the cell, inside and outside the cell through the Golgi such as bacteria or food, as well as old apparatus. Its functions include the storage, organelles end up in the lysosomes, which modification and packaging of products in break complex substances into simpler vesicles. In some cases, complex sugars may substances. Lysosomes are able to do this be made from simple sugars in the Golgi because they contain powerful digestive apparatus. The Golgi apparatus is also enzymes capable of breaking down all organic involved in the formation of lysosomes [see material. During the disturbance in cellular 5.2.5 (iii)]. metabolism, for example, when the cell gets 56 SCIENCE 2024-25 damaged, lysosomes may burst and the In plant cells vacuoles are full of cell sap enzymes digest their own cell. Therefore, and provide turgidity and rigidity to the cell. lysosomes are also known as the ‘suicide bags’ Many substances of importance in the life of of a cell. the plant cell are stored in vacuoles. These include amino acids, sugars, various organic 5.2.5 (iv) MITOCHONDRIA acids and some proteins. In single-celled organisms like Amoeba, the food vacuole Mitochondria are known as the powerhouses contains the food items that the Amoeba has of the cell. Mitochondria have two membrane consumed. In some unicellular organisms, coverings. The outer membrane is porous specialised vacuoles also play important roles while the inner membrane is deeply folded. in expelling excess water and some wastes These folds increase surface area for ATP- from the cell. generating chemical reactions. The energy required for various chemical activities needed uestions Q for life is released by mitochondria in the form of ATP (Adenosine triphopshate) molecules. ATP is known as the energy currency of the 1. Can you name the two organelles cell. The body uses energy stored in ATP for we have studied that contain making new chemical compounds and for their own genetic material? mechanical work. 2. If the organisation of a cell is Mitochondria are strange organelles in the destroyed due to some physical sense that they have their own DNA and or chemical influence, what will ribosomes. Therefore, mitochondria are able happen? to make some of their own proteins. 3. Why are lysosomes known as suicide bags? 5.2.5 (V) PLASTIDS 4. Where are proteins synthesised Plastids are present only in plant cells. There inside the cell? are two types of plastids – chromoplasts (coloured plastids) and leucoplasts (white or Each cell thus acquires its structure and colourless plastids). Chromoplasts containing ability to function because of the organisation the pigment chlorophyll are known as of its membrane and organelles in specific chloroplasts. Chloroplasts are important for ways. The cell thus has a basic structural photosynthesis in plants. Chloroplasts also organisation. This helps the cells to perform contain various yellow or orange pigments in functions like respiration, obtaining nutrition, addition to chlorophyll. Leucoplasts are and clearing of waste material, or forming primarily organelles in which materials such new proteins. as starch, oils and protein granules are stored. Thus, the cell is the fundamental structural The internal organisation of the Chloroplast unit of living organisms. It is also the basic consists of numerous membrane layers functional unit of life. embedded in a material called the stroma. These Cell Division are similar to mitochondria in external structure. Like the mitochondria, plastids also New cells are formed in organisms in order to have their own DNA and ribosomes. grow, to replace old, dead and injured cells, and to form gametes required for 5.2.5 (vi) VACUOLES reproduction. The process by which new cells are made is called cell division. There are two Vacuoles are storage sacs for solid or liquid main types of cell division: mitosis contents. Vacuoles are small sized in animal and meiosis. cells while plant cells have very large vacuoles. The process of cell division by which most The central vacuole of some plant cells may of the cells divide for growth is called mitosis. occupy 50-90% of the cell volume. In this process, each cell called mother cell THE FUNDAMENTAL UNIT OF LIFE 57 2024-25  Fig. 5.7: Mitosis Fig. 5.8: Meiosis divides to form two identical daughter cells They divide by a different process called (Fig. 5.7). The daughter cells have the same meiosis which involves two consecutive number of chromosomes as mother cell. It divisions. When a cell divides by meiosis it helps in growth and repair of tissues produces four new cells instead of just two (Fig. in organisms. 5.8). The new cells only have half the number Specific cells of reproductive organs or tissues of chromosomes than that of the mother cells. in animals and plants divide to form gametes, Can you think as to why the chromosome which after fertilisation give rise to offspring. number has reduced to half in daughter cells? What you have learnt The fundamental organisational unit of life is the cell. Cells are enclosed by a plasma membrane composed of lipids and proteins. The cell membrane is an active part of the cell. It regulates the movement of materials between the ordered interior of the cell and the outer environment. In plant cells, a cell wall composed mainly of cellulose is located outside the cell membrane. The presence of the cell wall enables the cells of plants, fungi and bacteria to exist in hypotonic media without bursting. The nucleus in eukaryotes is separated from the cytoplasm by double-layered membrane and it directs the life processes of the cell. The ER functions both as a passageway for intracellular transport and as a manufacturing surface. The Golgi apparatus consists of stacks of membrane-bound vesicles that function in the storage, modification and packaging of substances manufactured in the cell. Most plant cells have large membranous organelles called plastids, which are of two types—chromoplasts and leucoplasts. 58 SCIENCE 2024-25  Chromoplasts that contain chlorophyll are called chloroplasts and they perform photosynthesis. The primary function of leucoplasts is storage. Most mature plant cells have a large central vacuole that helps to maintain the turgidity of the cell and stores important substances including wastes. Prokaryotic cells have no membrane-bound organelles, their chromosomes are composed of only nucleic acid, and they have only very small ribosomes as organelles. Cells in organisms divide for growth of body, for repalcing dead cells, and for forming gametes for reproduction. Exercises 1. Make a comparison and write down ways in which plant cells are different from animal cells. 2. How is a prokaryotic cell different from a eukaryotic cell? 3. What would happen if the plasma membrane ruptures or breaks down? 4. What would happen to the life of a cell if there was no Golgi apparatus? 5. Which organelle is known as the powerhouse of the cell? Why? 6. Where do the lipids and proteins constituting the cell membrane get synthesised? 7. How does an Amoeba obtain its food? 8. What is osmosis? 9. Carry out the following osmosis experiment: Take four peeled potato halves and scoos each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now, (a) Keep cup A empty (b) Put one teaspoon sugar in cup B (c) Put one teaspoon salt in cup C (d) Put one teaspoon sugar in the boiled potato cup D. Keep these for two hours. Then observe the four potato cups and answer the following: (i) Explain why water gathers in the hollowed portion of B and C. (ii) Why is potato A necessary for this experiment? (iii) Explain why water does not gather in the hollowed out portions of A and D. 10. Which type of cell division is required for growth and repair of body and which type is involved in formation of gametes? THE FUNDAMENTAL UNIT OF LIFE 59 2024-25