Cell: The Unit of Life Full Chapter Audiobook | NCERT Class 11 Biology #ncertbiology #beatdneet

Added: 2026-05-28

[00:00:01]Chapter 8. Cell, the unit of life. When you look around, you see both living and non-living things. You must have wondered and asked yourself, what is it that makes an organism living? Or what is it that an inanimate things does not have which a living thing has? The answer to this is the presence of the basic unit of life, the cell in all living organisms.

[00:00:30]All organisms are composed of cells.

[00:00:34]Some are composed of single cell and are called unisellular organisms while others like us composed of many cells are called multisellular organisms.

[00:00:50]8.1 What is a cell?

[00:00:53]Unicellular organisms are capable of number one independent existence and number two performing the essential functions of life. Anything less than a complete structure of cell does not ensure independent living. Hence, cell is the fundamental structural and functional unit of all living organisms.

[00:01:17]Anton von Luvenhog first saw and described a life cell. Robert Brown later discovered the nucleus. The inventions of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.

[00:01:37]8.2 Cell theory. In 1838, Matthew Ceden, a German botonist, examined a large number of plants and observed that all plants are composed of different kinds of cell which form the tissues of the plant. At about the same time, Theodor Sha 1839, a German zoologologist, studied different types of animal cells and reported that cells had a thin outer layer which is today known as the plasma membrane. He also concluded based on his studies on plant tissues that the presence of cell wall is a unique character of the plant cells. On the basis of this, Sha proposed the hypothesis that the bodies of animals and plants are composed of cells and product of cells.

[00:02:33]Sliden and Sha together formulated the cell theory. This theory however did not explain as to how new cells were formed.

[00:02:42]Rudolfph Vero 1855 were first explained that cells divided and new cells are formed from pre-existing cells. Omniscellular e cellular heified the hypothesis of scen and sha to give the cell theory a final shape. Cell theory as understood today is number one all living organisms are composed of cells and product of cells.

[00:03:10]Number two, all cells arise from pre-existing cells.

[00:03:15]8.3 An overview of cell. You have earlier observed cell in an onion peel and or human cheek cells under the microscope. Let us recollect their structure. The onion cell which is a typical plant cell has a distinct cell wall as its outer boundary and just within it is the cell membrane. The cells of the human cheek have an outer membrane as the delimiting structure of the cell. Inside each cell is a dense membrane bound structure called nucleus.

[00:03:48]This nucleus contains the chromosomes which in turn contain the genetic material DNA. Cells that have membrane bound nuclei are called ukareotic whereas cells that lack a membrane bound nucleus are proarotic.

[00:04:05]In both proarotic and ukareotic cells, a semifluid matrix called cytoplasm occupies the volume of the cell. The cytoplasm is the main arena of cellular activities in both the plant and animal cells.

[00:04:21]Various chemical reactions occur in it to keep the cell in the living state.

[00:04:27]Besides the nucleus, the ukareotic cells have other membrane bound distinct structures called organalies like the endopplasmic reticulum, the Golgi complex, lizoomes, mitochondria, microbodies and vacules. The proarotic cells lack such membranebound organal.

[00:04:47]Ribosomes are non-membranebound organal found in all cells both ukareotic as well as proarotic.

[00:04:54]Within the cell ribosomes are found not only in the cytoplasm but also within the two organal chloroplast in case of plants and mitochondria and on rough endopplasmic reticulum.

[00:05:08]Animal cells contain another non-membrane bound organally called centrosome which helps in cell division.

[00:05:16]Cells differ greatly in size, shape and activities. Figure 8.1. For example, microlopplasmas the smallest cells are only.3 micrometer in length while bacteria could be to 3 to 5 micrometer. The largest isolated single cell is the egg of an ostrich. Among multisellular organisms, human red blood cells are about 7 micrometer in diameter. Nerve cells are some of the longest cells.

[00:05:48]Cells also vary greatly in their shape.

[00:05:51]They may be discklike, polygonal, columnar, cuboid, threadlike or even irregular. The shape of the cell may vary with the function they perform.

[00:06:02]Figure 8.1 diagram showing different shapes of the cells. Red blood cells round and by concave white blood cells amibboid.

[00:06:13]Columnar epithelial cells long and narrow.

[00:06:18]nerve cells branched and long a tracheid elongated and misophil cells round and oval 8.4 4 proarotic cells. The proaryotic cells are represented by bacteria, blue green algae, micopplasma and pplo pluronneimonialike organisms.

[00:06:38]They are generally smaller and multiply more rapidly than the ukareotic cells.

[00:06:43]Figure 8.2. They may vary greatly in shape and size. The four basic shapes of bacteria are basilus, rodlike, caucus, spherical, vibrio, commash shaped and spiralum spiral. The organization of proaryotic cell is fundamentally similar even though proariots exhibit a wide variety of shapes and functions.

[00:07:07]All proariots have cell walls surrounding the cell membrane except in micopplasma. The semifluid matrix filling the cell is the cytoplasm. There is no well- definfined nucleus. The genetic material is basically naked not enveloped by a nuclear membrane. In addition to the genomic DNA, the single chromosome circular DNA, many bacteria have smaller circular DNA outside the genomic DNA. These smaller DNA are called plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to antibiotics. In higher classes you will learn that this plasmid DNA is used to monitor bacterial transformation with foreign DNA. Nuclear membrane is found in ukariots. No organal like the ones in ukariots are found in proarotic cells except for ribosomes.

[00:08:06]Procariots have something unique in the form of inclusions. A specialized differentiated form of cell membrane called misosome is the characteristic of proariots. They are essentially infoldings of cell membrane. Figure 8.2 diagram showing comparison of ukarotic cell with other organism. A typical ukarotic cell 10 to 20 micrometer size and this is typical bacteria 1 to 2 micrometer PLLO about 0.1 micrometer and viruses 0.02 to 0.2 micrometer 8.4.1 cell envelope and its modifications.

[00:08:51]Most proarotic cells particularly the bacterial cells have a chemically complex cell envelope. The cell envelope consists of a tightly bound three-layer structure that is the outermost glycoalix followed by the cell wall and then the plasma membrane. Although each layer of the envelope performs distinct function, they act together as a single protective unit. Bacteria can be classified into two groups on the basis of the differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Graham. Those that take up the gram stain are gram positive and the others that do not are called gram negative bacteria.

[00:09:33]Glycalix differs in composition and thickness among different bacteria. It could be a loose sheath called the slime layer in some while in others it may be a thick and tough called the capsule.

[00:09:46]The cell wall determines the shape of the cell and provides a strong structural support to prevent the bacterium from bursting or collapsing.

[00:09:56]The plasma membrane is selectively permeable in nature and interacts with the outside world. This membrane is similar structurally to that of the ukariots.

[00:10:06]A special membranous structure is the misosome which is formed by the extensions of the plasma membrane into the cell. These extensions are in the form of vicyles, tubules and lamaly.

[00:10:20]They help in cell wall formation, DNA replication and distribution to daughter cells. They also help in respiration, secretion processes to increase the surface area of the plasma membrane and enzyatic content. In some procariots like cyobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.

[00:10:46]Bacterian cells may be motile or non-motile. If motile, they have thin filamentous extensions from their cell wall called flagagula. Bacteria show a range in the number and arrangement of flagagula. Bacterial flagum is composed of three parts. Filament, hook and basal body. The filament is the longest portion and extends from the cell surface to the outside. Besides flagula, pilli and fimbri are also surface structures of the bacteria but do not play a role in motility. The pili are elongated tubular structures made of a special protein. The fimri are a small bristle-like fibers sprouting out of the cell. In some bacteria, they are known to help attach the bacteria to rocks in streams and also to the host tissues.

[00:11:38]8.4.2 ribosomes and inclusion bodies. In proariots, ribosomes are associated with the plasma membrane of the cell. They are about 15 mm by 20 nm in in size and are made of two subunits 50S and 30S units which when represented together form 70S proarotic ribosomes.

[00:12:01]Ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called poly ribosomes or polyome. The ribosomes of a polyme translate the mRNA into proteins.

[00:12:16]Inclusion bodies. Reserve material in proarotic cells are stored in the cytoplasm in the form of inclusion bodies. These are not bound by any membrane system and lie free in the cytoplasm. Example phosphate granules, syopician granules and glycogen granules. Gas fuels are found in blue, green and purple and green photosynthetic bacteria.

[00:12:41]8.5 ukarotic cells. The ukareots include all the protests, plants, animals and fungi. In ukareotic cells, there is an extensive compartmentalization of cytoplasm through the presence of membranebound organal. Ukarotic cells possess an organized nucleus with a nuclear envelope. In addition, ukarotic cells have a variety of complex locomotal structures. Their genetic material is organized into chromosomes. All ukarotic cells are not identical. Plant and animal cells are different as the former possesses cell walls, plastids and a large central vacule which are absent in animal cells. On the other hand, animal cells have centrios which are absent in almost all plant cells. Figure 8.3 diagram showing a plant cell and animal cell.

[00:13:39]This is a plant cell and here is the animal cell. Let us now look at individual cell organalies to understand their structure and functions. 8.5.1 cell membrane. The detailed structure of the membrane was studied only after the advent of the electron microscope in 1950s.

[00:14:02]Meanwhile, chemical studies on the cell membrane, especially a human red blood cells, enabled the scientists to deduce the possible structure of plasma membrane.

[00:14:12]These studies showed that the cell membrane is mainly composed of lipids and proteins. The major lipids are phospholippids that are arranged in a billayer. Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tail towards the inner part.

[00:14:31]This ensures that the non-polar tail of saturated hydrocarbons is protected from the aquis environment. Figure 8.4.

[00:14:40]In addition to phosphoipids membrane also contains cholesterol. Later biochemical investigation clearly revealed that the cell membranes also possess protein and carbohydrate. The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the irthroite has approximately 52% protein and 40% lipids. Depending on the case of depending on the ease of extraction, membrane proteins can be classified as integral and peripheral. Peripheral proteins lie on the surface of membrane while the integral proteins are partially or totally buried in the membrane. Figure 8.4. Fluid mosaic model of plasma membrane.

[00:15:30]These are sugar molecules. These are peripheral protein. And this is the phospholipid billayer.

[00:15:37]This is integral protein.

[00:15:40]This is cholesterol. An improved model of the structure of cell membrane was proposed by Singer and Nicholson in 1972.

[00:15:49]Widely accepted as fluid mosaic model.

[00:15:52]Figure 8.4. According to this the quasifluid nature of lipid enables lateral movement of proteins with the overall billayer. This ability to move within the membrane is measured as its fluidity. The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretions, endoccytosis, cell division etc. One of the most important function of the plasma membrane is the transport of the molecules across it. The membrane is selectively permeable to some molecules present on either side of it. Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport.

[00:16:40]Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient that is from higher concentration to the lower. Water may also move across the membrane from higher to lower concentration. Movement of water by diffusion is called osmosis.

[00:16:58]As the polar molecules cannot pass through the normal polar lipid billayer, they require a carrier protein of the membrane to facilitate their transport across the membrane. A few ions or molecules are transported across the membrane against their concentration gradient that is from lower to the higher concentration. Such a transport is an energy dependent process in which ATP is utilized is called active transport. Example sodium potassium pump 8.5.2 into cell wall. As you may recall, a non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants.

[00:17:41]Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection. It also helps in cellto cell interaction and provides barrier to undesirable macroolelecules.

[00:17:54]Elgie have cell wall made of cellulose, galactins, manins and minerals like calcium carbonate. While in other plant it consists of cellulose, hemiselulose, pectins and proteins. The cell wall of a young plant cell the primary cell wall is capable of growth which gradually diminishes as the cell matures and the secondary wall is formed on the inner towards membrane side of the cell. The middle lamula is a layer mainly of calcium pactate which holds or glues the different neighboring cells together.

[00:18:30]The cell wall and middle lamina may be traversed by plasmoda which connect the cytoplasm of neighboring cells.

[00:18:40]8.5.3 endommembrane system. While each of the membranous organalies is distinct in terms of its structure and function, many of these are considered together as an endommembrane system because their functions are coordinated. The endommembrane system include endopplasmic reticulum, GGI complex, lizoomes and vacules. Since the functions of the mitochondria, chloroplast and peroxyomes are not coordinated with the above components, these are not considered as part of the endommembrane system. 8.5.3.1 The endopplasmic reticulum, ER.

[00:19:21]Electron microscopic studies of ukarotic cells reveal the presence of a network or reticulum of tiny tubular structures scattered in the cytoplas that is called the endopplasmic reticulum. Figure 8.5.

[00:19:36]Hence the ER divides the intracellular space into two distinct compartments that is luminal inside ER and extra luminal cytoplasmic compartments. The ER often shows ribosomes attached to their outer surface. The endopplasmic reticulum bearing ribosomes on their surface is called rough endopplasmic reticulum reer. In the absence of ribosome, they appear smooth and are called smooth endopplasmic reticulum.

[00:20:05]Reer is frequently observed in the cells actively involved in protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus. The smooth endopplasmic reticulum is the major site of synthesis of lipid in animal cells. Lipidlike steroidal hormones are synthesized in STR.

[00:20:26]8.5 Endopplasmic reticulum. 8.5.3.2 GI apparatus.

[00:20:34]Cameo GI 1898 first observed densely stained reticular structures near the nucleus. These were later named GGI bodies after him. They consist of many flat discshaped sacks or of 0.5 micrometer to 1 micrometer diameter.

[00:20:53]Figure 8.6. These are stacked parall to each other. Varied number of sistni are present in a gi complex. The golgi sistnia are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face. The cis and trans faces of the organally are entirely different but interconnected.

[00:21:19]The gg apparatus principally performs the function of packaging materials to be delivered either to the intracellular targets or secreted outside the cell.

[00:21:29]Materials to be packaged in the form of vycles from the ER fused with the cyst face of the Golgi apparatus and move towards the maturing phase. This explains why the Golgi apparatus remain in close association with the endopplasmic reticulum. A number of proteins synthesized by ribosomes on the endopplasmic reticulum are modified in the sony of the Golgi apparatus before they are released from its transface.

[00:21:56]Golgi apparatus is the important site of formation of glyoproteins and glyolippids.

[00:22:03]8.5.3 lizoomes.

[00:22:06]These are membrane bound vasicular structures formed by the process of packaging in the Golgi apparatus. The isolated lizooal vicycles have been found to be very rich in almost all types of hydraytic enzyme hydrolases that is lipases proteasis carbohydrases.

[00:22:24]Optimally active at the acidic pH, these enzymes are capable of digesting carbohydrates, proteins, lipids, and nucleic acids.

[00:22:34]8.5.3.4 vacuoles. The vacule is a membrane bound space found in the cytoplas. It contains water, sap, excrety products, and other materials not useful for the cell. The vacule is bound by a single membrane called tonoplast. In plant cells, the vacules can occupy up to 90% of the volume of the cell. In plants, the tonoplast facilitates the transport of number of ions and other materials against concentration gradient into the vacules.

[00:23:08]Hence, their concentration is significantly higher in the vacule than in the cytoplasm.

[00:23:14]In amoeba, the contractile vacule is important for osmogulation and excretion. In many cells as in protest food vacules are formed by engulfing the food particle.

[00:23:27]8.5.4 mitochondria. Mitochondria unless specifically stained are not easily visible under the microscope. The number of mitochondria per cell is variable depending on the physiological activity of the cells. In terms of shape and size also considerable degree of variability is observed. Typically it is a sausage shaped or cylindrical having a diameter of 0.2 to 1 micrometer. Average is 0.5 micrometer and length 1 to 4.1 micrometer. Each mitochondria is a double membrane bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aquous compartments that is the outer compartment and the inner compartment.

[00:24:17]The inner compartment is filled with a dense homogeneous substance called the matrix. The outer compartment forms the continuous limiting boundary of the organal. The inner membrane forms a number of infoldings called the christi.

[00:24:31]Singular is christa towards the matrix.

[00:24:35]Figure 8.7. The christi increase the surface area. The two membranes have their own specific enzymes associated with the mitochondrial function.

[00:24:45]Mitochondria are the sides of aerobic respiration. They produce cellular energy in the form of ATP. Hence they are called powerhouses of the cell. The matrix also possess a single circular DNA molecule. A few RNA molecules ribosomes 7S and the components required for the synthesis of proteins. The mitochondria divide by fision. Figure 8.7. Structure of mitochondria. This is a longitudinal section. This is outer membrane. This is inner membrane.

[00:25:18]Intermembrane is a space between these two membrane. The fluid is matrix and these are the crystal 8.5 plastids. Plastids are found in all plant cells and in nucleoids. These are easily observed under the microscope as they are large. They bear some specific pigments thus imparting specific colors to the plants. Based on the types of pigments, plastids can be classified into chloroplast, chromoplast and lucoplast. The chloroplast contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis. In the chromoplast fats soluble keratenoid pigments like kerotin, zenthophils and other are present. This gives a part of the plant a yellow, orange or red color.

[00:26:09]The lucoplast are the colorless plastids of varied shapes and sizes with stored nutrients. Amoplast store carbohydrates that is starch example potato. Elioplast store oils and fats whereas the eluroplast store proteins.

[00:26:29]Figure 8.8 sectional view of chloroplast. This is outer membrane inner membrane granomide and estro lamine.

[00:26:42]Majority of the chloroplast of the green plants are found in the misoil cells of the leaves. These are lens shaped, oval, spherical, discoid or even ribble-l like organales having variable length 5 to 10 micrometer and width 2 to 4 micrometer.

[00:26:58]Their number varies from one per cell of the cloidonas a green elga to 20 to 40 per cell in the misophil.

[00:27:06]Like mitochondria, the chloroplast are also double membrane bound. Of the two inner chloroplast membrane is relatively less permeable. The space limited by the inner membrane of the chloroplast is called the stroma. A number of organized flattened membranous sacks called the philyloids are present in the stroma.

[00:27:26]Figure 8.8. Thyoids are arranged in stacks like the piles of coins called granana. Singular is granom or the intergranal philyloids. In addition, there are flat membranous tubules called the stroma lamaly connecting the philyloids of the different granom. The membrane of the philyloids enclose a space called a lumen. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins. It also contains a small double stranded circular DNA molecules and ribosomes. Chlorophyll pigments are present in the philyloids. The ribosomes of the chloroplast are smaller 70s than the cytoplasmic ribosomes 80s.

[00:28:13]8.5.6 ribosomes.

[00:28:16]Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palad 1953.

[00:28:26]They are composed of ribbouclelic acid RNA and proteins and are not surrounded by any membrane.

[00:28:33]The ukareotic ribosomes are 80s while the proaryotic ribosomes are 70s. Each ribosome has two subunits larger and smaller subunits. Figure 8.9. The two subunits of 80s ribosomes are 60s and 40s. While that of 70s ribosomes are 50s and 30s. Here S is Sidberg's unit. It stands for the steadymentation coefficient. It is indirectly a measure of density and size. Both 70S and ATS ribosomes are composed of two subunits.

[00:29:07]Figure 8.9 ribosome small subunit and this is the large subunit.

[00:29:13]8.5.7 cytokeleton.

[00:29:17]An elaborate network of filamentous proteinacious structures consisting of microtubules, microfilaments and intermediate filaments present in the cytoplasm is collectively referred to as the cytokeleton.

[00:29:30]The cytokeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell.

[00:29:43]Celia and flagagula.

[00:29:46]Celia singularis celium and flagagula singularis flagagulum are hairike outgrowths of the cell membrane. Celia is smaller structures which work like oes causing the movement of either the cell or the surrounding fluid. Flagula are comparatively longer and responsible for cell movement. The proarotic bacteria also possess flagagula but these are structurally different from that of the ukareotic flagagula.

[00:30:14]The electron microscopic study of a celium or the flagulum show that they are covered with plasma membrane. Their core called the exonym possesses a number of microtubules running parallel to the long axis. The exonym usually has nine doulets of radially arranged peripheral microtubules and a pair of centrallylo microtubules.

[00:30:38]Such an arrangement of exonal microtubules is referred to as the 9 plus2 array. Figure 8.10. 10. The central tubules are connected by bridges and is also enclosed by a central sheath which is connected to one of the tubules of each peripheral dublets by a radial spoke. Thus there are nine radial spokes. The peripheral dublets are also interconnected by linkers. Both the celium and flagulum emerge from centriol like a structure called the basal bodies.

[00:31:10]Figure 8.10. section of celia flag showing different parts. A is the electron microraph and B is the diagrammatic representation of internal structure.

[00:31:22]Central sheath plasma membrane peripheral microtubules that is dlets inter bridge and this is the central microtubule. These are the radial spoke and that is in red that is the central sheath.

[00:31:39]8.5.9 Centrosome and centrioles.

[00:31:42]Centrosome is an organally usually containing two cylindrical structures called centrioles. They are surrounded by amorphous perentolar materials. Both the centrioles and centrosomes lie perpendicular to each other in which each has an organization like the cardfield. They are made up of nine evenly spaced peripherals fibbrals of tubulin protein. Each of the peripheral fibbral is a triplet. The adjacent triplets are also linked. The central part of the proximal region of the centle is also proteinious and called the hub which is connected with the tubules of the peripheral triplets of biradial spokes made of protein. The centules form the basal body of celia or flagagula and spindle fibers that give rise to spindle apparatus during cell division in animal cells. diagram we have already seen 8.5.10 10 nucleus nucleus as a cellogal leaf was first described by Robert Brown as early as 1831.

[00:32:49]Later the material of the nucleus sustained by the basic dice was given the name chromatin by flamming. The interphase nucleus nucleus of a cell when it is not dividing has highly extended and elaborate nucleoproin fibers called chromatin nuclear matrix and one or more spherical bodies called nucleoli. Singular is nucleolus. Figure 8.11.

[00:33:14]Electron microscopy has revealed that the nuclear envelope which consists of two parallel membranes with a space between 10 to 15 nanome called the perinuclear space forms a barrier between the materials present inside the nucleus and that of the cytolas. The outer membrane usually remains continuous with the endopplasmic reticulum and also beers ribosomes on it.

[00:33:40]At a number of places, the nuclear envelope is interrupted by minute pores which are formed by the fusion of its two membranes. These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both direction between the nucleus and the cytoplas.

[00:34:00]Normally there is only one nucleus per cell. Variations in the number of nuclei are also frequently observed. Can you recollect names of organisms that have more than one nucleus per cell? Some mature cells even lack nucleus. Example of many mammals and safe tube cells of vascular plants. Would you consider these cells as living?

[00:34:25]The nuclear matrix or the nucleoplasm contains nucleolus and chromatin. The nuclei are spherical structures present in the nucleoplast. The content of nucleolus is continuous with the rest of the nucleoplast as it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis. Large and more numerous nuclei are present in cells actively carrying out protein synthesis.

[00:34:52]8.11 structure of nucleus. This is nucleoplast nucleolus nuclear pore and nuclear membrane.

[00:35:02]You may recall that the interface nucleus has loose and indistinct network of nucleoprotein fibers called chromatin. But during different stages of cell division, cells show structured chromosome in place of the nucleus.

[00:35:17]Chromatin contains DNA and some basic proteins called histones. Some non-histone proteins and also RNA. A single human cell has approximately 2 m long thread of DNA distributed among its 46 or 23 pairs chromosomes. You will study the details of DNA packaging in the form of a chromosome in class 12.

[00:35:40]Every chromosome visible only in dividing cells essentially has a primary constriction or the centromeir on the sides of which discshaped structures called kintoes are present. Figure 8.12.

[00:35:54]Centromeir holds two chromatids of a chromosome. Based on the position of the centromeir, the chromosome can be classified into four types. Figure 8.3.

[00:36:04]Sorry, figure 8.13.

[00:36:06]The metacentric chromosome has middle centromeir forming two equal arms of the chromosome. The subacentric chromosome has centromeir slightly away from the middle of the chromosome resulting into one shorter arm and one longer arm. In case of ecosentric chromosome the centromeia is situated close to its end forming one extremely short and one very long arm whereas the tilocentric chromosome has terminal centromeia.

[00:36:34]This is figure 8.12 chromosome with kinettoore. This is kinetto and this is 8.13 types of chromosome based on the position of centromeia.

[00:36:48]Sometimes a few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite.

[00:36:58]8.5.11 microbodies.

[00:37:01]Many membrane bound minute vicles called microbodies that contain various enzymes are present in both plant and animal cells. Now summary, all organisms are made of cells or aggregate of cells. Cells vary in their shape, size, activities, functions.

[00:37:19]Based on the presence or absence of membrane bound nucleus and other oralies, cells and hence organism can be named as ukareotic or proaryotic.

[00:37:28]A typical ukareotic cell consist of a cell membrane, nucleus and cytoplas.

[00:37:34]Plant cells have a cell wall outside the cell membrane. The plasma membrane is selectively permeable and facilitates transport of several molecules. The endommembrane system includes ER, GGI complex, lizoomes and vacules. All the cell organalies perform different but specific functions. Centrosome and centriole form the basal body of celia and flagagula that facilitate locomotion. In animal cells, centrioles also form spindle apparatus during cell division. Nucleus contains nuclei and chromatin network. It not only controls the activities of organal but also plays a major role in heridity.

[00:38:15]Endopplasmic reticulum contains tubules or cesterni. They are of two types rough and smooth. ER helps in the transport of substances. Synthesis of proteins lipoproteins and glycogen. The GGI body is a membranous organally composed of flattened sacks. The secretion of cells are packed in them and transported from the cell. Lizoomes are single membrane structures containing enzymes for digestion of all types of macroolelecules.

[00:38:44]Ribosomes are involved in protein synthesis. These occur freely in the cytoplasm or are associated with ER.

[00:38:51]Mitochondria help in oxidative phosphorilation and generation of adnosin triphosphate. They are bound by double membrane. The outer membrane is a smooth and inner fold inner one folds into several crysti plastids are pigment containing organal found in plant cells only in plant cells. Chloroplast are responsible for trapping light energy essential for photosynthesis. The gran in the plastid is the site of light reaction and the sto of dark reaction.

[00:39:22]The green colored plastids are chloroplast which contain chlorophyll whereas the other colored plastids are chromoplast which may contain pigments like kerotin and xanthophil.

[00:39:36]The nucleus is enclosed by nuclear envelope a double membrane structure with nuclear pores. The inner membrane encloses the nucleoplas and the chromatin material. The cell is a structural and functional unit of life.