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The Fundamental Unit of Life, Cell, Cells: An Overview, Diverse forms of living organisms are present in our surroundings. Like ourselves, all of, them are made up of tiny structures called cells. Cells are the building blocks of life. They, are the basic structural and functional units from which life takes shape. A cell is the, smallest living entity in a living organism., , How cells are formed, In 1838 & 1839, the two German scientists Matthias Schleiden (1838) and Theodor, Schwann (1839) proposed the cell theory and formulated that all plant and animal tissues, are made up of cells. They, however, were unsuccessful in explaining how new cells are, formed. Later, in 1855, Rudolf Virchow further expanded the cell theory by suggesting that, all cells arise from pre-existing cells. The cell theory states that:, , , All living organisms are composed of cells and products of cells., , , , Cells are the basic units of structure and function in an organism., , , , All cells arise from pre-existing cells., Know Your Scientist

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Robert Hooke (1635-1703), The term ‘cell’ was introduced by Robert Hooke in 1665 after observing the cellular, structure of cork (a substance obtained from the bark of a tree). While examining a thin, slice of cork under a compound microscope, Hooke observed many small compartments, resembling honeycombs. These he termed as cells., , Robert Brown (1773-1858), In 1831, Robert Brown discovered the presence of nucleus in the centre of a plant cell., , Theodor Schwann (1810-1882) and Matthias Schleiden (1804-1881), In 1838, Matthias Schleiden, a German physiologist, discovered that all plant tissues are, made up of cells, i.e., cells are the fundamental units of all plants. In the next year(1839),, Theodor Schwann, a German physiologist, discovered that all animal tissues are made up of, cells, i.e., cells are the fundamental units of all animals.

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Invention of Microscope, Cells are very small living entities that are not visible to the naked eyes. The invention of, microscopes hence played the key role in the discovery of cells., Simple Microscopes, , , , , First simple microscopes were constructed by Antony van Leeuwenhoek (1632-1723)., They consisted of single biconvex lens., Their magnifying power was up to 200 times., Compound Microscopes, , , , , , These were first constructed by Robert Hooke (1635-1703)., He developed the compound microscope using two lenses for increasing the magnifying, power., He examined a thin slice of cork under it and observed tiny, box-like compartments, that he, named 'cells'., , The modern ordinary compound microscope has greatly improved in design and, magnification power (up to 2,000 times).

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Electron Microscopes, , , , , The invention of electron microscope has led to great advancements in the study of cells., Electron microscopes use beams of electrons which are bent by magnets to magnify, the images., They can magnify an object up to 200,000 times., Properties of Living Cells, Some important properties of living cells are as follows:, , , , Generally, a cell is so small that it is not visible to the naked eye., , , , Cell shape and size vary both within an organism and between different organisms. The, shape and size of a cell is related to the specific function it performs., , , , All living cells exhibit certain basic properties like respiration, growth and metabolism., , , , Nerve cells are some of the longest cells., Examples of cells with different shapes and sizes, , Did You Know?, , , The smallest unicellular organism we know is the Mycoplasma, a type of bacteria. Its, diameter is 0.1 µm., , , , There are more red blood cells in our body than any other type of cell., Solved Examples Medium, Example 1: Illustrate how the shape and size of a cell is related to the specific function it, performs.

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Solution: Different types of cells with different shapes and sizes are present in our body. A, cell’s shape and size are relevant to the specific function it performs. The irregularly, shaped white blood cell is a case in point. A white blood cell protects the body by killing, harmful foreign bodies. Whenever it encounters any antigen, it changes its shape, accordingly and engulfs the antigen. Thus, the shape of the white blood cell is directly, related to the function it performs., Classification of Cells, Based on the number of cells: Unicellular and multicellular, , As you now know, a cell is the, smallest living entity capable of independent existence. There are certain organisms that, are made up of only a single cell; such organisms are known as unicellular organisms., Examples of unicellular organisms include Amoeba and yeast. All other organisms (i.e., those made up of more than one cell) are known as multicellular organisms. Examples, of multicellular organisms include humans, plants and animals., , , Multicellular organisms can perform a variety of tasks efficiently due to division of labour., This gives the organisms a wide range of adaptabilities to survive., , , , In multicellular organisms, dead cells play an important role. For example, the dead, epidermal cells in animal skin protect the underlying cells., Division of labour, Divisio12701n of labour refers to the specialized roles of the different organs present in a, multicellular organism. All organs, tissues or cells of a multicellular organism cannot carry, out all the functions. Each of them is evolved to carry out a specific set of tasks. Each organ, system coordinates with the others to perform the activities required for life. This division, of labour minimizes the load of carrying out all the functions and, consequently, it allows, the organs to operate efficiently., Concept Builder, Let us understand this concept of division of labour using the example of a cricket team. As, you know, in a cricket team, some members specialize in batting while some specialize in, bowling. Each member is assigned a specific set of functions in the team. More often than

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not, the team that wins a game is one whose members perform their specific tasks, efficiently., In the same way, the different organ systems in the human body are assigned different, functions. For example, the digestive system is assigned to carry out digestion, while the, excretory system is assigned to carry out excretion. This is division of labour. Ultimately, a, healthy body is one in which the different organ systems perform their respective functions, properly., Classification of Cells, Based on the cellular complexity: prokaryotes and eukaryotes, This type of classification is based on the sub-cellular organization of a cell., The given table lists the characteristic features of prokaryotes and eukaryotes., Characteristics, , Prokaryotes, , Size of the cell, , Cells are small in size., , Nucleus, , No nucleus with a nuclear, membrane is present., , Membrane-enclosed, organelles, Cell wall, , Genetic material, , Eukaryotes, Cells vary in size and are, generally larger than those in, prokaryotes., There is a well-defined, nucleus with a nuclear, membrane., , Organelles having a, membrane around them (e.g., Membrane-enclosed, mitochondria, plastids) are organelles are present., absent., Cell wall is usually present. It Cell wall is usually present in, is composed, plant cells. It is composed, of peptidoglycan., of cellulose., The genetic material is, The genetic material is, present as nucleoid, i.e., a, present inside the wellproperly defined nucleus is, defined nucleus., absent., , Solved Examples Medium, Example 2: Distinguish between bacteria and yeast., Solution:, Bacterium, It is a unicellular prokaryote., It lacks a well-defined nucleus., , Yeast, It is a unicellular eukaryote., It has a well-defined nucleus

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It has no cellular organelles such as, mitochondria and endoplasmic, reticulum., , It has cellular organelles such as, mitochondria and endoplasmic, reticulum., , Structure of Eukaryotic and Prokaryotic Cell, , Cell Division, Cells undergo division to form new cells. These new cells are used to grow, replace old,, dead and injured cells, and to form gametes required for reproduction. There are two types, of division a cell undergoes Mitosis - Each cell divides to form two daughter cells. The daughter cells have the same, chromosome number as the mother cell., Meiosis - This type of division is shown by specific cells of the reproductive organs or, tissues in animals and plants. These cells divide to form gametes, which after fertilisation, give rise to new offsprings. In meiosis, four cells are produced from a single cell and the, new cells have half the chromosome number than the mother cell., Cell Wall, Plasma Membrane and Cytoplasm, Basic Components of a Cell, The two basic components of a cell are the protoplasm and plasma/cell membrane. The, protoplasm consists of the cytoplasm and nucleus. The cell membrane is the outermost, covering in animal cells, and is next to the cell wall in plant cells.

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The term ‘protoplasm’ refers to the living contents of a cell, i.e., the nucleus and cytoplasm, enclosed by a membrane. The cytoplasm is a jelly-like matrix surrounding the nucleus., Plasma Membrane, In animal cells (which lack the cell wall), the cell membrane is the outermost covering. It, acts as a barrier between the internal cell machinery and the harsh external environment., Hence, it functions as a protecting agent., Basic structure of the plasma membrane, Conceived by Singer and Nicolson in 1972, the fluid mosaic model is used for describing, the structure of the plasma membrane. According to this model, the major components of, the plasma membrane are lipids and proteins. A small amount of carbohydrates can also, be found in it. The plasma membrane is flexible in nature and allows the entry and exit of, selective molecules., , To understand the cell membrane better, imagine it to be like a big, plastic bag with many pores on its surface. This bag contains all the cellular contents,, including the organelles, and keeps them separate from the outside environment. The, pores present on the surface allow the entry and exit of only some, molecules/substances.The cell membrane is extremely delicate, thin and elastic. It is the, living membrane of the cell. It surrounds the cytoplasm and regulates the movement of, substances into and out of the cell. This means that the cell membrane allows only certain, substances to enter and exit. For this reason it is known as a selectively permeable, membrane., Functions of the plasma membrane, , , , , , It protects the cellular organelles from the outside environment., It selectively allows molecules to move into and out of the cell., It can engulf substances within a cell through endocytosis. It can also expel substances out, of the cell through exocytosis., It establishes communication between cells., Whiz KidWhat makes the plasma membrane selectively permeable?

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The plasma membrane is made up of several organic molecules, but the major component, is phospholipid. A phospholipid is a lipid molecule containing phosphorus. It has two, parts—the head region and the tail region. The former is hydrophilic in nature, i.e., it has a, strong affinity for water. The latter is hydrophobic in nature, i.e., it lacks the affinity for, water., The phospholipids arrange themselves in such a way that the polar heads face toward the, outside (i.e., toward water) and the fatty-acid tails face toward the inner side of the bilayer., Consequently, the hydrophobic region remains protected from the aqueous environment, (as shown in the figure)., Due to this special arrangement, all molecules cannot pass through the membrane. Polar, molecules like glucose and water can easily pass through the membrane, but non-polar, molecules like oils cannot pass through. This structural arrangement that allows only, selective molecules to pass through the plasma membrane is what makes it a selectively, permeable membrane., , Diffusion

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In order to maintain a proper balance in the internal environment of the body, cells need to, take in useful substances and release harmful materials., Do you know how food, waste matter and water are transported across a cell?, Such transportation occurs mainly because of two processes—diffusion and osmosis., DIFFUSION, Diffusion is the movement of individual molecules from a region of higher concentration to, a region of lower concentration., , , The larger the difference in concentration, the more rapid is the flow of molecules., , , , In diffusion, the movement of molecules is random and independent of each other., , , , The most common example of diffusion is the transportation of gases and absorption of, nutrients in the human body., , , , The inhaled air contains oxygen. In the lungs, oxygen diffuses through the blood capillaries, and reaches the red blood cells. There, it binds with the haemoglobin to form oxyhaemoglobin., , , , Oxy-haemoglobin is then circulated throughout the body., , , , Oxygen concentration is lesser in tissues than in the blood or the red blood cells. Thus,, oxygen diffuses out of the blood and enters the tissues and, later, the cells., , , , Similarly, in the digestive tract, food is broken down into simpler products such as glucose., Nutrients are then absorbed by the intestinal cells through diffusion., Osmosis: A Special Case of Diffusion, Osmosis describes a special case of diffusion in which two solutions having different, concentrations are separated by a semi-permeable membrane. Some features of osmosis, are as follows:, , , , The movement of water (i.e., the solvent) takes place from the region of lower, concentration of the solute to the region where there is a higher concentration of the, solute., , , , In other words, the movement of the solvent takes place from the region where its, concentration is higher to the region where its concentration is lower.

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Significance of osmosis in living organisms, , , , Plant roots absorb soil water through osmosis. Water concentration is higher in soil than in, root cells. Therefore, water moves from the soil to the cells, Water is re-absorbed in the tubules of nephrons of the kidneys through osmosis.., Solved Examples, Easy, Example 1: Distinguish between diffusion and osmosis, The basic differences between diffusion and osmosis are given in the table., Diffusion, , Osmosis, , Solute molecules move across a concentration Water molecules move across a concentration, gradient., gradient, It does not require a semi-permeable, It requires a semi-permeable membrane., membrane., Experiment on Osmosis, Take three beakers with a few raisins in them. Pour pure water hypotonic solution in the, first beaker, an isotonic solution in the second beaker and a highly concentrated solution, in the third beaker. Let the raisins soak in the liquids for five minutes., Observation, After five minutes, you will observe that the raisins in the first beaker have swollen up. In, the second beaker, you will see no change in the raisins. In the third beaker, you will, observe that the raisins have shrunk and broken down.

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Explanation, Pure water does not contain salt. Thus, to maintain equilibrium, water enters the raisins in, the first beaker. This swells up the raisins.The salt concentration in an isotonic solution is, the same as that in raisins. This results in osmotic balance. Hence, no change can be, observed in the raisins in the second beaker.The third beaker, contains hypertonic solution. To maintain equilibrium, water moves out of the raisins., This shrinks the raisins. Excessive shrinkage results in the breakdown of the membrane., This phenomenon is called plasmolysis., , Solved Examples, Medium, Example 1: Osmosis is a special kind of diffusion. Explain., Solution:, Diffusion refers to the movement of the particles of a substance from a region where their, concentration is higher to a region where their concentration is lower. This happens so that, the particles spread uniformly in the given space. Diffusion can take place in all three, states—solid, liquid and gaseous. Osmosis is a special case of diffusion wherein the, medium is water. This phenomenon occurs through a semi-permeable membrane., In diffusion, the moving particles can be those of the solute or of the solvent. Also, these, particles can move in both directions. However, in osmosis, only the unidirectional flow of, water takes place. Therefore, osmosis is a special kind of diffusion., Easy, Example 2: There are two bowls—one containing hot rice pudding and the other, containing dry raisins. What will you observe if you put dry raisins in the rice, pudding?

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Solution:, When transferred to the rice pudding, the dry raisins will swell up. The reason for this is, the difference between the concentration of water in the pudding and in the raisins. The, concentration of water in the former is higher than it is in the latter. To maintain osmotic, balance, water will enter the raisins from the pudding, according to the concentration, gradient; consequently, the dry raisins will swell., Cell Wall, As mentioned before, in animal cells, the plasma membrane is the outermost covering. In, plant cells, however, the outermost covering is the cell wall., Now, you might wonder why the cell wall is present only in plant cells. Let us first, understand what the cell wall is and then we will ascertain as to why it is found solely in, plant cells., , , The cell wall is the hard, protective outermost covering of plants, fungi and bacteria., , , , It is a rigid structure that gives support to a plant cell., , , , It allows a plant to stand upright and maintain the shape of the cell when placed in a, hypotonic or hypertonic solution., , , , Its constituent compounds are different in case of different organisms. The cell wall of, plants, fungi and bacteria is made up of cellulose, chitin and peptidoglycan respectively., , , , The cell wall connects the internal environment of a cell to the external environment., , , , It can withstand dilute hypotonic media and prevent bursting of cells. For this reason plant, cells can withstand changes in environmental concentration better than animal cells., Solved Examples, Medium, Example 1: Why is the cell wall not necessary in animal cells?, Solution:, The cell wall is the tough and rigid layer around the cells of plants and bacteria. It provides, both structural support and protection to the cells. Animal cells do not require the cell wall, for the following reasons., 1) Animal cells have other forms of support such as the exoskeleton and endoskeleton.

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2) Animal cells can regulate osmotic pressure by pumping ions and salts across the cell, membrane. So, they do not require the cell wall to protect themselves from bursting due to, endosmosis., 3) Animal cells require flexibility for function and movement, which would not have been, possible if the cells were surrounded by the cell wall. This is because the cell wall restricts, flexibility., Cytoplasm, Cytoplasm is the inner content of the cell membrane which separates the cell membrane, from the nucleus. Some important features of cytoplasm are as follows:, , , It is composed of cytosol, organelles and inclusions., , , , Cytosol is the soft, sticky and semi-transparent fluid in which various cell organelles are, suspended., , , , Cytoplasm is not a simple clear fluid. Rather, it is a complex viscous fluid that contains 70%, water. The remaining portion is made up of proteins, carbohydrates and lipids., , , , Cytoplasm is one of the most active parts of a cell. While it does not take part in the cellular, processes, it does host most of the metabolic reactions., , , , It helps a cell to perform several vital functions by transporting essential nutrients to the, required destinations., , , , One of the important components of the cytoplasm is the cytoskeleton. Cytoskeleton is a, network of proteins (microtubules and microfilaments) which together form the skeleton, of the cytoplasm. The cytoskeleton is responsible for the shape and movement of a cell., Solved Examples, Medium, Example 1: Differentiate between cytoplasm, cytosol, protoplasm and protoplast., Solution:, Cytoplasm is the inner content of the cell membrane. It separates the cell membrane from, the nucleus. It comprises cytosol, organelles and inclusions., ∴ Cytoplasm = Cytosol + Cell organelles + Inclusions

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A constituent of cytoplasm, cytosol is a soft, sticky and semi-transparent fluid. In it the, various cell organelles remain suspended., Along with the nucleus, the cytoplasm makes up the protoplasm—the living part of a cell. It, does not include the cell membrane or the cell wall, ∴ Protoplasm = Cytoplasm + Nucleus, Protoplast is the protoplasm of a living plant or bacterial cell whose cell wall has been, removed but has its cell membrane intact., , Know More, Have you ever heard of a cybrid?, The word ‘cybrid’ is the combination of the words ‘cytoplasmic’ and ‘hybrid’. The plasma, membranes of cells (having different origins) are broken down to obtain cytoplasms. These, naked cells are then fused to obtain a hybrid cell called a cytoplasmic hybrid or cybrid., Cybrids are often known as heterokaryons as they contain multiple and geneticallydifferent nuclei. Cybrids are important for research purposes., Cytoplasmic Streaming, Cytoplasmic streaming refers to the movement of the viscous fluid of a cell (i.e., cytoplasm), in order to perform a particular function. This phenomenon can be easily understood with, the help of the unicellular eukaryotic organism Amoeba., Amoeba can constantly change its shape. It forms finger-like projections, called pseudopodia (meaning ‘false feet’) as shown in figures 1 and 2. These pseudopodia, are nothing but the flowing projections of cytoplasm., , When Amoeba senses its prey, it uses its pseudopodia to engulf the prey. These cytoplasmic, projections not only help Amoeba in engulfing food but also in locomotion. Thus, they, exemplify the phenomenon of cytoplasmic streaming.

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Did You Know?, , , Cells can be grown outside the body through a process known as cell culture., , , , Cell culture media is always isotonic in nature. The salt concentration of an isotonic, solution is 0.9% of NaCl, which is equivalent to the intracellular salt concentration., , , , Viruses do not have the plasma membrane and cytoplasm., Nucleus: Structure and Function, Nucleus: General Introduction, In the centre of all eukaryotic cells, there is a large prominent membrane-bound organelle, known as the nucleus. Some features of nucleus are as follows:, , , , The nucleus is visible under a light microscope. It is acidic in nature. Any basic stain can be, used to see the nucleus., , , , It is also present in prokaryotes, but it lacks a well-defined membrane., , , , Generally, one nucleus is present in a cell; however, there are certain eukaryotic cells which, do not contain even a single nucleus (e.g., red blood cell) and there are certain others that, contain more than one nucleus i.e. they are multinucleate (e.g., slime mould)., , , , In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to, the absence of a nuclear membrane. Such an undefined nuclear region containing only, nucleic acids is called the nucleoid. It is found in prokaryotes., , Solved Examples Medium, Example 1: Can you give a reason as to why a red blood cell lacks the nucleus?

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A red blood cell (RBC) loses its nucleus at maturity. The main function of RBCs is to, transport oxygen to all body parts. For meeting the oxygen requirements of the body, they, need to carry large amounts of oxygen. So, an RBC loses its nucleus to attain the extra, space. Thus, it is part of the cell’s adaptation to lose the nucleus at maturity so that the, oxygen requirements of the body can be fulfilled., , Structure of the Nucleus, The following points list the various parts of the nucleus with their respective functions., , , , Nuclear envelope: The nucleus is enclosed by a double-layered cellular membrane called, the nuclear envelope. The space between the two layers is known as the perinuclear space, and is filled with fluid. The nuclear envelope separates the contents of the nucleus from the, cytoplasm. The nuclear membrane is pierced with holes known as the nuclear pores. These, pores allow the nucleus to communicate with the rest of the cell as they control the passage, of substances into and out of the nucleus. The outer nuclear membrane (ONM) is connected, to a network of tubules and sacs i.e., the endoplasmic reticulum ., , , , Nucleolus: When observed under a microscope, the centre of the nucleus has a dark dense, spherical area known as the nucleolus. The nucleolus is the production factory, of ribosomes, which in turn manufacture proteins. Thus, it can be said that the nucleolus is, indirectly involved in protein and RNA synthesis., , , , Nucleoplasm: The nucleus contains nuclear sap or a semi-fluid matrix enclosed by the, nuclear envelope. This is known as the nucleoplasm or karyoplasm. It holds the nucleolus, and the suspended chromatin., , , , Chromatin network: The nucleus contains the genetic material of an organism in the form, of a network of chromatin. DNA gets associated with protein molecules to form chromatin., This chromatin gets folded and coiled to form chromosomes.

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Solved Examples Medium, Example 1: Distinguish between the nucleus and nucleolus., Solution: The following table illustrates the differences between nucleus and nucleolus., Nucleus, The nucleus is the main organelle of, a cell., It is surrounded by the nuclear, envelope., It contains DNA, proteins and the, nucleolus., It is the control centre of a cell,, controlling important functions such, as DNA replication and protein, synthesis., , Nucleolus, The nucleolus is present inside the, nucleus., It is not covered by any membrane., It contains RNA and proteins., It performs the function of ribosome, synthesis., , Nucleus: The Director of a Cell, , Inside the nucleus, rod-shaped, structures called chromosomes are present. These chromosomes bear the hereditary units, known as genes. Genes are the segments of DNA which provide information for carrying, out the regular metabolic activity of a cell. For example, DNA contains the information on, how to make the proteins of the cells. The proteins, in turn, control the chemical reactions, of the cells and provide structural support to both the cells and tissues. The nucleus also, plays a crucial part, along with the environment, in determining the way a cell will develop, and the form that it will exhibit at maturity by directing the chemical activities of the cell., Thus, also known as the director of a cell., Functions of the nucleus:, , , The nucleus controls all metabolic activities of a cell., , , , It is concerned with the transmission of hereditary traits from the parent to the offspring.

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, , It plays an important role in cellular reproduction—the process by which a single cell, divides to form two new cells., , , , It also plays a crucial part in determining the way a cell will develop and the form it will, exhibit at maturity. It does this by directing the chemical activities of the cell., Cloning, We often hear of scientists producing clones. Do you know what clones are and how they, are created? Let us understand cloning and the role of the nucleus in it., Cloning refers to the technique of producing an identical copy of an organism. In biology,, clones signify genetically identical organisms.A clone of an animal is produced by, transferring the nucleus from a donor adult cell (somatic cell) to an egg cell from which the, nucleus has been removed in the laboratory. When provided with all the required, conditions, the egg begins to divide normally to form a zygote. This zygote is then placed, inside the uterus of a female recipient, who then gives birth to the baby. The baby thus, born is identical to the donor organism (whose nucleus was used) and is said to be a clone, of the donor., , Did You Know?, , , If the entire DNA from a single cell is stretched out, it would be two metres long!, , , , The nucleus is the largest organelle in a cell., Solved Examples Medium, Example 1: What is the chemical nature of the nucleus—acidic or basic? Give reason, for your answer.

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Solution: The nucleus consists of genetic material—DNA (deoxyribonucleic acid), which is, acidic in nature. This makes the entire nucleus acidic., Vacuoles, Vacuoles: An Overview, A vacuole is a storage sac (for solid or liquid contents) found in a cell. Its enclosing, membrane is known as tonoplast which has a large number of pores on its surface. These, pores control the entry and exit of different molecules. Vacuoles are found in both plant, and animal cells, but are much larger in plant cells. The central vacuole of some plant cells, may occupy 50%−90% of the cell volume.In plant cells, vacuoles are full of cell sap. They, provide turgidity and rigidity to the cells. A vacuole stores food and nutrients that a cell, needs to survive. These include amino acids, sugars, various organic acids and some, proteins. It may store the waste products of a cell. In single-celled organisms, like Amoeba, the food vacuole contains the food items that Amoeba consumes. In some, unicellular organisms, a specialized vacuole plays an important role in expelling excess, water and some wastes from the cell., , Vacuole in a plant cell, Functions of Vacuoles, Some important functions of vacuoles are as follows:, , , They help in the removal of unwanted structural debris., , , , They maintain the turgor pressure within cells., , , , In Amoeba, the food vacuole stores food., , , , They help in maintaining the proper pH of cells.

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, , In some fungal cells, vacuoles act like lysosomes and play an important role in autophagy, , Solved Examples, Medium, Example 1: Why do plant cells have large-sized vacuoles as compared to animal cells?, Solution:, Plants form the base of the food chain. They are capable of converting inorganic matter into, organic matter. To store these inorganic ions, they are provided with large vacuoles., Moreover, plants cannot move from one place to another. So, it is very essential for plants, to be able to store extra nutrients for future use, when the nutrients may not be available., Unlike plants, animals can take their food from their surroundings according to their, requirement. Therefore, they do not need organelles for food storage., Hard, Example 2: How does vacuole maintain turgor pressure inside a cell?, Solution:, Turgor pressure is the pressure buildup in cell cytoplasm against the cell wall. This, pressure pushes the wall outward. In a normal situation, an equal and opposite force called, wall pressure is exerted by the cell wall to maintain the shape and structure of the cell., When a cell is placed in a hypotonic solution, the water tends to move from the solution to

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the cell along the concentration gradient. This causes the volume of the cell to increase, considerably. In this situation, the size of the vacuole increases. It stores the excess water in, order to avoid the cell from bursting and dying due to the continuous supply of water into, the cell. Consequently, the concentration of cell cytoplasm is maintained and it exerts an, optimum turgor pressure., Endoplasmic Reticulum and Golgi Apparatus, Endoplasmic Reticulum: Introduction, In ‘endoplasmic reticulum’, ‘endo’ means ‘inside’, ‘plasmic’ means ‘plasm’ and ‘reticulum’, means ‘network’. The endoplasmic reticulum (ER) is a system or network of folded, membranes—tubules, vesicles and cisternae—that form channels. Cisternae are, flattened disc-like membranous structures. Tubules are tubular in shape, while vesicles are, sac-like structures.ER is found fused with the nuclear membrane inside eukaryotic cells.It, is the site where many important chemical reactions take place, including the synthesis of, important proteins, lipids and many other materials.It functions as a packaging system., However, it does not work alone; it works closely with the Golgi apparatus and, ribosomes.ER may look different from cell to cell, but it always forms a network system., , Types of Endoplasmic Reticulum, When seen under an electron microscope, two types of endoplasmic reticulum can be, observed—the rough endoplasmic reticulum (RER) and the smooth endoplasmic, reticulum (SER)., , , Rough endoplasmic reticulum: When ribosomes get attached to the surface of ER, it, becomes RER.

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, , Smooth endoplasmic reticulum: When ribosomes are absent from the surface of ER, it is, called SER. This type of ER is found in liver cells, interstitial cells, adipose cells and muscle, cells., Difference between RER and SER, , Smooth endoplasmic reticulum (SER), , Rough endoplasmic reticulum (RER), , It is the endoplasmic reticulum that does, not bear ribosomes and appears smooth., , It is the endoplasmic reticulum that bears, ribosomes on the surface., , It is the major site for the synthesis of lipids. It is actively involved in protein synthesis., It maintains the calcium ion concentration, in cytosol., , It transports proteins to different, destinations like the plasma membrane., , Functions of the Endoplasmic Reticulum, Some important functions performed by endoplasmic reticulum are as follows, , ER serves as a channel for the transport of materials (especially proteins) between various, regions of the cytoplasm or between the cytoplasm and the nucleus., , , , It functions as a cytoplasmic framework, providing a surface for some of the biochemical, activities of the cell., , , , ER gives mechanical support by forming a network in the cytoplasm., Smooth endoplasmic reticulum, , , , SER acts as a storage organelle., , , , It stores those steroids and ions in solution which the cell may need at a later time., , , , It helps in manufacturing fat molecules or lipids that are important for cell functioning., , , , Some of these proteins and lipids help in building the cell membrane. This process is, known as membrane biogenesis., , , , It actively participates in drug detoxification.

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, , It maintains the calcium ion concentration in cytosol., Rough endoplasmic reticulum, , , , RER is important for the synthesis and packaging of proteins. Most of the lysosomal, proteins are produced in RER. These proteins might be used in the cell or sent out to, various destinations depending upon the needs of the other processes., , , , RER is the major site of glycosylation (addition of carbohydrates in proteins)., Ribosomes, Ribosomes are the organelles found in all living cells. These are found attached to the, membrane of rough endoplasmic reticulum, nuclear membrane and even in the nucleolus., They are the sites of protein synthesis. Hence, are also known as the protein factories of the, cell., , Golgi Apparatus: In Depth, , The Golgi apparatus is another packaging organelle like the endoplasmic reticulum. Let's, study the golgi apparatus in detail., , , It consists of a system of membrane-bound vesicles arranged approximately parallel to, each other in stacks of cisternae., , , , The Golgi apparatus was discovered by Camillo Golgi in 1898., , , , A Golgi body is usually composed of five−eight stacks of cisternae., , , , Vesicles leave the Golgi body from the end known as the cis face. The other end is known as, the trans face.

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, , Plant cells contain many freely distributed subunits of the Golgi apparatus,, called dictyosomes., Functions of the Golgi apparatus:, , , , It is involved in the transport of lipids in cells., , , , It is involved in the formation of lysosomes., , , , It also takes part in glycosylation and phosphorylation of certain proteins., Solved Examples, Easy, Example:1 How is the Golgi apparatus linked to the endoplasmic reticulum?, Solution: Mainly, the Golgi apparatus performs the function of packing the material to be, delivered. The membranes of the Golgi apparatus are often connected to the endoplasmic, reticulum membranes. A number of proteins synthesized by the ribosomes on the, endoplasmic reticulum are modified in the cisternae of the Golgi apparatus before they are, released. The Golgi apparatus also helps endoplasmic reticulum in the making of another, organelle called lysosome., Mitochondria and Lysosomes, Mitochondria: General Introduction, We know that our body needs energy to perform various activities. We get this energy from, the food we eat. But have you ever given thought to how this energy is produced from the, food inside our body? This energy is produced by the organelle called mitochondrion., Features of mitochondria, , , , In most eukaryotic cells, double-membrane-enclosed organelles known as mitochondria, are present in the cytoplasm., , , , Mitochondria are responsible for the production of most of the energy in cells. For this, reason they are also known as the powerhouse of cells., , , , Energy is produced as a result of the breaking of sugar. This process occurs in, mitochondria., , , , The energy released by mitochondria is stored in the form of ATP molecules. ATP is known, as the energy currency of a cell. The body uses the energy stored in ATP for making new

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chemical compounds and for mechanical work. The number of mitochondria in a cell is, dependent upon the nature of the physiological activity carried out by it. For example, in, humans, the red blood cells do not contain any mitochondria, while liver and muscle cells, may contain hundreds or even thousands of them., Structure of Mitochondria, Mitochondria are present in both plant and animal cells. Mitochondria are typically round, to oval in shape. Their sizes range from 0.5 μm to10 μm. A mitochondrion is enclosed by, two membranes—the outer membrane and the inner membrane, which are composed, of phospholipids and proteins. Let us learn more about the different components of, mitochondria., , 1. Outer membrane: The entire, mitochondrion is enclosed by a smooth outer membrane. Like the plasma membrane, it is, made up of phospholipids and proteins. It contains many pores on its surface known as, porins. These enable the movement of molecules of a specific size., 2. Inner membrane: Unlike the outer membrane, the inner membrane is more complex. It, contains a high amount of proteins. Except for oxygen, carbon dioxide and water, it does, not allow any molecules to pass through it. This membrane is the main site for the, production of ATP(energy currency of the cell), 3. Perimitochondrial space:The space between the inner and outer membranes is known, as the perimitochondrial space. It contains several molecules that enter the organelle by, crossing the outer membrane but are not able to pass through the inner membrane., Structure of Mitochondria, 4. Cristae: These are the structures formed by the folding of the inner membrane (as, shown in the figure). They contain several enzymes and provide enough area for chemical, reactions to occur.

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5. Matrix: Just like the cytoplasm inside a cell, a mitochondrion contains a viscous fluid, enclosed by its inner membrane. This fluid is a mixture of dissolved carbon dioxide, oxygen,, proteins, water, enzymes and many other important compounds required for various, chemical reactions that take place in the organelle. This matrix contains mitochondrial DNA, and ribosomes. The presence of DNA and ribosomes allows the mitochondria to make some, of their own proteins., Functions of Mitochondria, , , They produce the energy (in the form of ATP) required for the functioning of cells., , , , They regulate the free calcium ion concentration in cytosol., , , , They participate in apoptosis or programmed cell death., , , , They provide intermediates for the synthesis of various chemicals such as fatty acids,, steroids and amino acids., , Lysosomes, Lysosomes are membrane-bound vesicles that are usually found in animal cells. Some, important features of lysosomes are as follows:, , , They are formed by budding off from the endoplasmic reticulum and Golgi bodies., , , , They contain digestive hydrolytic enzymes such as acid hydrolase. With the help of these, enzymes, they are able to digest a variety of substances including worn out organelles,, proteins, nucleic acids, lipids and carbohydrates., , , , They are also known as the ‘suicide bags’ of cells. This is because they burst out and, release hydrolytic enzymes into cytosol when a cell is damaged., , , , Lysosomes provide energy during starvation by controlled breakdown of stored food., How lysosomes are produced?, The enzyme proteins are created in the Rough ER. These proteins are packaged in small, vesicles and sent to golgi apparatus. The golgi apparatus does the modification work on the, vesicles by preparing digestive enzymes and shoots off very small vesicle in the cytoplasm., This vesicle is lysosome and it floats in the cytoplasm until it is needed by the cell.

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The lysosomes are the digestive factories of the cell so their action starts when cell absorbs, or eat some food particle. As soon as the food particle enters the cell, the lysosome releases, the digestive enzymes which break down the complex molecules., Solved Examples Medium, Example 1: Mitochondria are semi-autonomous organelles. Justify this statement., Solution: Mitochondria contain their own genetic material, i.e., DNA (which is known as, mitochondrial genome) and ribosomes. They are considered as semi-autonomous, organelles because they are only partially dependent upon the nucleus for the proteins, required for their functioning. The presence of the DNA and ribosomes allows them to, synthesize some of these proteins. This is in contrast to other organelles which wholly, depend upon the nucleus for the proteins required for their functioning., Easy, Example 2: What would happen if the mitochondria of a cell were to get damaged?, Solution: The most important function of mitochondria is the production of energy in the, form of ATP. Any damage to these organelles would block the production of ATP, thereby, depleting the energy in the cell. This would ultimately lead to the death of the cell., Plastids, Plastids: An Overview, You see different kinds of plants and trees in your surroundings. Most of them are green in, colour. What do you think is the reason behind this green colour?

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, , Green plants and trees appear so because of the presence of the green colour pigment, called chlorophyll. This pigment is actually present inside the organelle, called chloroplast., , , , Chloroplast is a type of plastid which in turn is a major organelle found in plant cells and, algae. The presence of plastids allows plants to prepare their own food through, photosynthesis., , , , Animals cannot make their own food because they lack plastids., , , , Plastids are present in the cytoplasmic matrix of a plant cell. They are round- or ovalshaped bodies., , , , Like mitochondria, chloroplasts have their own DNA and, thus, have the ability to self, replicate., Types of Plastids, The two major types of plastids are Leucoplasts and chromoplasts .They are the major, sites for the production and storage of food and many other useful chemical compounds., Leucoplasts: They are white or colourless plastids; hence, they do not contain any, coloured pigments. They are found in those plant regions wherein photosynthesis is not, carried out, e.g., the roots. Their main function is storage and synthesis of various, important compounds such as fatty acids and amino acids.

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Chromoplasts:They are the coloured plastids containing coloured pigments such as, xanthophylls (yellow) and carotene (orange). These pigments are responsible for the, colour of a plant. Chloroplast is the most common type of coloured plastid., Functions of plastids:, , , They carry out photosynthesis., , , , They contribute to the colours of leaves and flowers., , , , Some plastids perform the function of storage and production of many important, compounds like lipids and proteins., Chloroplasts, , Location: Chloroplasts are the main, sites for photosynthesis. As we know, photosynthesis efficiently takes place in the green, parts of a plant, i.e., in leaves (and to a lesser extent in green stems or floral parts). This is, because leaves have specialized cells that contain chloroplasts. These cells are, called mesophyll cells ., Structure: Chloroplasts are disc-shaped organelles enclosed by a double membrane., A plastid has two distinct regions—grana and stroma., Grana are the stacks of flattened discs containing chlorophyll molecules., These molecules are responsible for the production of food through photosynthesis.

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The inner membrane of the chloroplast encloses the protein-rich fluid-like ground, substance known as stroma. Stroma is the homogeneous mixture in which the grana are, embedded., Thylakoids usually lie in stacks (like coins in piles) called grana., Thylakoids are believed to be the main sites for photosynthesis. The grana are, interconnected by tubular membranes called lamellae., Solved Examples, Medium, Example 1: Is it true that only plants with green leaves can carry out photosynthesis?, Give reason for your answer., Solution: No, it is not true that only plants with green leaves can carry out photosynthesis., Leaves of certain plants are green in colour due to the large presence of the chlorophyll, pigment. This pigment is responsible for carrying out photosynthesis in plants., In some plants, leaves are orange or yellow in colour, but it does not mean that they do not, contain chlorophyll. Instead, it implies that in such leaves other pigments are in greater, amounts than chlorophyll. Nevertheless, such plants are also capable of carrying out, photosynthesis., Easy, Example 2: Distinguish between grana and stroma., Solution:, Grana, , Stroma, , They are the stacks of thylakoids, present within chloroplasts., , It is the fluid material present inside, chloroplasts. It forms the floor of a, chloroplast., , They contain chlorophyll molecules, that are responsible for photosynthesis, , It contains various enzymes that are, required for photosynthesis to occur., , Cell Inclusions, Cell inclusions are the result of various chemical reactions that take place inside the cell,, either in the cytoplasm or in the vacuole.

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Cell inclusions may be the food products like starch or oil globules or the waste materials, like gums, resins, tannins, and latex