THE CELL - THEORY & QUESTION ANSWERS Chapter # 04 Theory & Question Answers

 BIOLOGY XI NOTES

THE CELL - THEORY & QUESTION ANSWERS
Chapter # 04
Theory & Question Answers
Section II - Unity of Life
THE CELL

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DISCOVERY OF CELL:
The discovery of cells and their structure became possible with the development of optical lenses and with the construction of compound microscope, which was invented by David Jansen in 1590 and in 1610 Galileo designed it properly. Cells were first described in 1665 by Robert Hooke. Later, Robert Brown in 1831 discovered a spherical body, the nucleus, in the cells of orchids. In 1839 Theodore Schwann, observed that the nucleus was surrounded by a fluid in the cell.

DEFINITION OF CELL:
According to T. Schwann "cell as a structure which consists of a nucleus surrounded by semi fluid substance enclosed by a membrane". Later it was pointed out that plant cell has a cell wall in addition to the cell membrane. After proposed of cell theory a comprehensive definition of cell established. Now, a cell is defined as the structural and functional unit of living organisms, made up of protoplasm containing nucleus surrounded by cytoplasm and bounded by cell membrane.

CELL THEORY:
In 1938 a German botanist M. J. Schleiden concluded that plants were made of cells. In 1939 a German zoologist T. Schwann published a comprehensive report on the cellular basis of animals. The observations of Schleiden, Schwann and others led to the formation of cell theory. The cell theory has a far reaching effect in all fields of biological researches. The salient feature of cell theory are as follows.

• All organisms are composed of one or more cells.
• The cell is the structural and functional unit of life. According to them cell is a 'de novo' structure could arise from noncellular materials, this idea was not convincing. In 1855 Rudolf Virchow established the fact and added third point to the cell theory.
• Cells can arise only by division of pre-existing cell, it is not a 'de novo' structure.

MICROSCOPE:
It is an instrument which is used to study cells and microorganism.

Kinds Of Microscope:
According to the source of light microscope is of three types.

• Light Microscope:
  It utilizes visible light as the source of illumination. It can magnify an object very high but it resolving power is limited.
• X-Ray Microscope:
  In this type short wave length X — rays are used as source of illumination.

Electron Microscope:
In which electron beam is used as source of illumination. Most of the sub cellular structure are studied by electron microscope. It uses electromagnets instead of glass lenses.

RESOLUTION Vs MAGNIFICATION:
The three attributes of microscopes are magnification, resolution and contrast.

Resolution:
It is minimum resolved distance. The resolving power is the minimum distance by which two adjacent form or object must be separated. Light microscope has limited resolving power which is about 500 times better than unaided human eye, while electron microscope are capable of resolving objects about 10,000 times better than unaided human eye.

Contrast:
Contrast is important in distinguishing one part of cell from another. In light microscope contrast is often obtained by fixing and staining the material.

Magnification:
It is a means of increasing the apparent size of the object. Magnification of microscope = Power of eye piece X magnifying power of objectives

Techniques To Isolate Components Of Cell:
Isolation of cellular components is necessary to study and observe various aspects of cell, isolation of cellular components to determine their chemical composition is called fractionation. It is stepwise process.

• First step is to break or open a large number of similar type of cells in ice cold environment.
• ii. The cells are usually placed in a homogenizer and motor are broken. The freed content of the cells are subjected to spinning action known as centrifugation.
• Initially at a low speed large particles like cell nuclei, settle down are in the sediment smaller particles are still in the supernatant.
• Later on at higher speed smaller particles are separated out.
• The various cell fraction can then be biochemically examined.

EUKARYOTE AND PROKARYOTE:

Prokaryote	Eukaryote
Organisms which do not contain membrane nucleus in their cells are called Prokaryote.	Eukaryote are those organisms having a true nucleus in their cells.
These organisms do not contain other membrane bounded structure like mitochondria, plastids, endoplasmic reticulum etc. Their nuclear material is usually coiled and concentrated in a region of  the cell called the nucleoid.	They contain a variety of membrane bound organelles like mitochondria plastids, endoplasmic reticulum etc. Their nuclear material is bounded by nuclear membrane and form true nucleus.
Ribosomes are of small size and freely scattered in cytoplasm. Ribosomes are of large size and present either on endoplasmic reticulum or free in cytoplasm. Nucleoplasm is absent. Nucleoplasm is present. Mesosomes are simple infoldings of the plasma membrane responsible for respiration, photosynthesis, nitrogen fixation. Respiratory enzymes are present in mitochondria, photosynthesis takes place by plastids. These are unicellular organisms, may live in colony form. They are unicellular as well as multi-cellular organisms. Examples are bacteria and cyan bacteria. Examples are plants, animals, fungi, etc. STRUCTURE OF EUKARYOTIC CELL: A typical eukaryotic cell consists of following parts. Plasma membrane. Cell wall (absent in animals) Nucleus Cytoplasm and cytoplasmic organelles. (The image contains a diagram labeled "The structure of the plasma membrane," showing various components such as extrinsic protein, intrinsic protein, lipids, glycoproteid, phospholipid, cholesterol, integral protein, hydrophobic α-helix, etc.) PLASMA MEMBRANE OR CELL MEMBRANE: The nucleus and cytoplasm in all types of cells is enclosed in a membrane called plasma membrane. In plants and bacterial cell membrane is surrounded by cell wall. It is thin, delicate, and elastic and has capability of limited self-repair. Molecular Structure Of Plasma, Membrane: All biological membranes have the same molecular organization. They consist of a double layer (bilayer) of phospholipids interspersed with proteins. Phospholipid molecules are arranged in two parallel layers. Their nonpolar hydrophobic ends face each other, whereas their polar hydrophilic ends are associated with carbohydrates, protein, etc. Fluid Mosaic Model Of Plasma Membrane: In 1972 Singer and Nicolson proposed a working model of the Plasma membrane known as the fluid mosaic model. According to it, the cell membrane consists of: Lipid Bilayer: It is retained as a core of the cell membrane. These lipid molecules are present in a fluid state. Protein Molecules: The proteins are present in the lipid layer like floating icebergs. The proteins occur as a mosaic of discontinuous particles that penetrate deeply into and even completely through the lipid sheet. The proteins associated with the lipid bilayer can be divided into two groups. Integral Proteins: (Intrinsic Proteins) A class of proteins that are directly incorporated within the lipid bilayer. Peripheral Proteins: (Extrinsic Proteins) A class of proteins located entirely outside of the lipid bilayer. Functions Of Plasma Membrane: The main and most important functions of the plasma membrane: Protection of Cell. To regulate the flow of solution and material in and out of the cell with certain limitations called differential or selective permeability. This transport is necessary to maintain suitable pH, ionic concentration for enzyme activity and excrete toxic substances. Types Of Transport: Passive Transport Active Transport The passive transport includes processes of diffusion and osmosis, which do not require energy. The active transport includes endocytosis and exocytosis. The active transport requires energy. Endocytosis: Endocytosis is the process of taking in material in a cell. It may be phagocytosis (cell eating process) in which solid particles are ingested by the cell, e.g., destroying of harmful bacteria by W. B. C. or pinocytosis (cell drinking process) when liquid material is taken in. Exocytosis: The process by which material moves outside of a cell. CELL WALL: Plant cell possesses a cell wall which lies outside the membrane. It is a non-living component of the cell, secreted by the protoplasm. In the plant cell, it is mainly composed of cellulose, a polysaccharide. Pectin and a few other compounds may also be found in the cell wall. Structure: Ultra microscopic structure of the cell wall shows that cellulose makes the fibers which lie at different angles. The cell wall is composed of three layers. Middle Lamella Primary wall Secondary wall Middle lamella: The first formed cell plate works as a cementing layer between two daughter cells and is called the middle lamella. It is composed of calcium and magnesium pectate. Primary Wall: The primary wall is the first product of the cell, synthesized by protoplast. It is 1.3 m thick and elastic. The primary wall contains up to 50% hemicellulose, cellulose up to 25%, and a small amount of pectic substances. Hemicellulose forms a matrix of the wall in which cellulose microfibrils are embedded. Secondary Wall: It is produced inside the primary wall by the deposition of cellulose. It is 5 - 10 m thick and rigid. It mainly consists of cellulose or varying mixtures of cellulose. Plasmodesmata: At places in the cell wall, the deposition of wall material does not take place. These places are known as plasmodesmata, through which cellular contents of neighboring cells remain in communication with each other. Functions Of Cell Wall: The framework around the plasma membrane, affords protection, gives a definite shape, and provides rigidity to the plant body as a whole. Being hydrophilic, cellulose is capable of imbibing water. The cell wall acts as a permeable structure that helps in the transportation of material. NUCLEUS: The nucleus is the most significant, important, and prominent part of the cell, which controls all its activities. Discovery: The nucleus was discovered by Robert Brown in 1831. Shape: It is commonly spherical or oval in shape but may be lobed or elongated. Size: Its size varies between 5 to 25 μm. Numbers: Most cells have one, some have two or more nuclei. Some small organisms have several small nuclei per cell (coenocytic). Position: In animal cells, the nucleus is present in the center of the cell. In plant cells, it is located in the center in a young cell, while in a mature cell, it comes to lie on one side due to a large vacuole. Structure Of Nucleus: The nucleus consists of the following parts: Nuclear membrane Nucleoplasm Nucleolus Image: Structure of nucleus and its components. Nuclear membrane: The nucleus is enveloped by a double membrane called the nuclear membrane. It is perforated by nuclear pores through which certain substances pass freely. Nucleoplasm Or Karyolymph: The nucleus is filled with a gel-like protein-rich substance called nucleoplasm or karyolymph. It is granular in nature and denser than the cytoplasm. In the nucleoplasm, a network of fine loosely connected threads is present, called the chromatin network or nuclear reticulum, which is composed of DNA and protein. From this network, chromosomes are produced during cell division.  Chromosomes: Chromosomes are thread-like structures present inside the nucleus, bearer of hereditary characters in the form of genes, present in pairs in an individual, and their number remains constant from generation to generation in a given species. Number Of Chromosomes: Fruit fly: 8 Human: 46 Corn: 20 Structure Of Chromosomes: A typical chromosome is composed of two parts. Chromatid In mature cells, each chromosome consists of two very thin threads called chromatids. Chromatid consists of one or more threads called chromonema, which have bead-like areas called chromomeres. Centromere: Both chromatids are joined at a point called the centromere. The centromere has a disc-shaped protein called kinetochore. Spindle fibers attach to the kinetochore. Types Of Chromosomes: Types of chromosomes depend on the position of the centromere. Metacentric: In these chromosomes, the centromere is present almost in the center. These chromosomes have two equal arms resembling the letter V. Submetacentric: The position of the centromere is just away from the center. Chromosomes have two unequal arms. These are J or L-shaped chromosomes. Acrocentric Or Subtelocentric: The centromere is subterminal. These rod-shaped chromosomes have one arm very small and the other very long. Telocentric: The location of the centromere is at the end of the chromosome. Nucleolus: Inside the nucleus, a spherical body called the nucleolus is present. The nucleolus synthesizes RNA and ribosomes in eukaryotic cells. Functions Of Nucleus: Controlling Centre: The nucleus controls all the activities of the cell. Inheritance Of Characters: Inside the nucleus, chromosomes are formed during cell division, containing DNA as hereditary material. Therefore, it plays a significant role in the inheritance of characters from parents to offspring. CYTOPLASM & CYTOPLASMIC ORGANELLES: Cytoplasm: The protoplasm outside the nucleus is called cytoplasm. It is a semifluid colloid. The cytoplasm exhibits active streaming movement called cyclosis. Cytoplasm divides into two parts: Cytosol Cytoplasmic organelles Cytosol: It is a fluid matrix part of the cytoplasm. It is a cell solution in which organelles reside. Cytosol is a watery solution of salts, sugar, amino acids, proteins, fatty acids, nucleotides, and other materials. The cytosol is organized into a three-dimensional network of fibrous proteins called the cytoskeleton. Cytoplasmic Organelles: There are many minute bodies present in the cytoplasm called organelles. Organelles reside in the cytoplasm, occupying as much as half of the volume of the cell. A variety of cytoplasmic organelles are present in cells. ENDOPLASMIC RETICULUM: It is a complex network of channels, which extends from the plasma membrane to the nuclear membrane. Composition: They are composed of lipoprotein. Types Of Endoplasmic Reticulum: There are two types of endoplasmic reticulum. A Granulated Or Smooth Endoplasmic Reticulum (SER): It is not associated with ribosomes. It is present in steroid-producing cells like adipose cells (fat cells), interstitial cells, glycogen-storing cells (liver), and muscle cells. Granulated Or Rough Endoplasmic Reticulum (RER): It is heavily coated with ribosomes on its outer surface. It is mostly present in protein-synthesizing cells, e.g., mammalian salivary glands and pancreas. Functions: It serves as a supporting platform for the ribosomes. It forms a structural framework of the cell. It contains enzymes and takes an active part in various metabolic reactions in the cell. It provides a conducting pathway for the circulation of various substances within the cell. It provides passage for RNA to pass from the nucleus to the cytoplasm. It involves protein and lipid synthesis. It helps in the detoxification of harmful drugs. It involves the storage and release of Ca²⁺ ions. MITOCHONDRIA: Mitochondria are one of the cell’s most important organelles. They are the center of aerobic respiration. Position: They are universally present in the cytoplasm of plant and animal cells. Shape: They appear as minute granules, vesicles, rodlets, threads, or strings. Size: Each mitochondrion is 0.2 to 1.0 nm in diameter and about 10 µm long. Structure: Each mitochondrion is bounded by two thin membranes, made of lipids and proteins. The outer membrane is smooth, while the inner membrane forms irregular, incomplete partitions called cristae. On these cristae, enzymes and co-enzymes are present, which metabolize carbohydrates (starch), fatty acids (lipids), and amino acids (protein). Energy is released in the form of ATP and stored within mitochondria. Inside the mitochondria, granular fluid like organic matrix is present, consisting of numerous chemical compounds. Functions: Cellular respiration takes place in the mitochondria. They form and release ATP (an energy-rich compound). Mitochondria are known as the “Powerhouse” of the cell, where energy is stored and released whenever and wherever required by a living body. GOLGI APPARATUS: (DICTYOSOME) It was discovered by Camello Golgi in 1898. It is a canalicular system with sacs. It has parallel arranged, flattened, membrane-bound vesicles which lack ribosomes. Structure: Each of them is disc-shaped and consists of central, flattened, plate-like compartments called cisternae. A peripheral network of interconnecting tubules and vesicles and Golgian vacuoles. Functions: They perform the function of collection, packaging, and distribution. Manufacture certain macromolecules. Cell wall and cell plate material is Golgi product. LYSOSOMES: These are spherical bodies, a few micrometers in diameter. These were isolated by De Duve in 1949. They are most abundant in those animal cells which exhibit phagocytic activity. Each lysosome is bounded by a single membrane. Lysosomes originate from the Golgi apparatus. Lysosomes contain acid phosphatase enzymes and hydrolytic enzymes (digestive enzymes). These enzymes can digest the phagocytosed food particles. They are also involved in autophagy (self-eating). During this process, some parts of the cell are digested, resulting in the degeneration of the cell, so lysosomes have been referred to as “suicide sacs”. Lysosomal Storage Diseases: Disturbance in lysosome function influences human health. Diseases characterized by the deficiency of a lysosomal enzyme and the corresponding accumulation of undegraded substrate are called lysosomal storage disorders. In 1965, W. G. Hers explained that the absence of a lysosomal enzyme, α-glucosidase, leads to storage of undigested glycogen, causing swelling of organelles and damage of the cell and tissues. More than 30 disorders have been diagnosed. Some are described as under: Disease Consequences Tay-Sachs disease Mental retardation, blindness, death by age 3. Gaucher’s disease Liver and spleen enlargement, erosion of bones. Krabbe’s disease Long bones, mental retardation in infantile form only. PLASTIDS: Plastids are special protoplasmic membrane-bound organelles. These are chemical synthesizers and food storage bodies. Position: Plastids are present in plant cells and in some protists. Types Of Plastids: Plastids are of three types. Chloroplast: The chloroplasts are green plastids containing chlorophyll. They are found in all green parts of the plant, particularly in leaves. Functions: They have the ability to convert solar energy into chemical or food energy, therefore manufacturing carbohydrates by the process of photosynthesis. Chromoplast: These plastids are present in all colored parts of plants except green. These are mostly present in petals of flowers and fruits; they have pigments like xanthophylls and carotene and anthocyanin. Functions: Due to the chromoplast, insects are attracted towards flowers to take part in pollination. Leucoplast: The leucoplasts are usually colorless and contain starch. These plastids develop in the absence of sunlight and are commonly found in all underground parts of plants. Functions: They convert sugar into starch and store food material as carbohydrates, lipids, and proteins. Chloroplast As Energy Converting Organelles: Chloroplasts are the most common type of plastids in plants. They have the ability to convert solar energy (light energy) into chemical or food energy by the process of photosynthesis. Structure Of Chloroplast: Each chloroplast consists of three parts. An outer double membrane. Inside the chloroplast, a set of flattened sacs called thylakoids form a membranous system. In some regions, thylakoids are stacked, forming structures called grana. The fluid outside the thylakoids is called stroma. During photosynthesis, chlorophyll captures solar energy and transfers it to other molecules in the thylakoid membranes. These molecules transfer the energy to ATP, and other energy carriers diffuse into the stroma, where energy is used to derive the synthesis of sugar. Chloroplast converts solar energy into food or chemical energy through the process of photosynthesis. So, chloroplast is an energy-converting organelle. Proplastids: Proplastids are immature, colorless plastids occurring in cells of meristematic tissues. In mature cells, proplastids develop into chloroplasts, chromoplasts, or leucoplasts. Peroxisome: These are microbodies surrounded by a single membrane. These are present in animal and plant cells. They contain enzymes that transfer hydrogen atoms to oxygen to form hydrogen peroxide (H₂O₂). It is a toxic molecule that is immediately broken down to water by the enzyme catalase. Peroxisomes are found in cells that metabolize alcohol, for example, in liver and kidney cells, where they break down and detoxify fully, half of the alcohol a person drinks. Glyoxysome: They are specialized glyoxysomes, present only in plant cells. Structurally, each glyoxysome is a single-layered membrane-bounded microbody, enclosing a fine granular stroma. Functions: Glyoxysome contains enzymes that initiate the conversion of fatty acids into sugar. Cytoskeleton: The cytosol is organized into a three-dimensional network of fibrous proteins called the cytoskeleton. The cytoskeleton plays vital roles in cell division, mitosis, meiosis, cytokinesis, and cell wall formation. It also maintains the cell shape and differentiation of cells to form different parts. There are three types of cytoskeleton elements found in cells. Microfilaments: Microfilaments are solid, made of protein (two actin chains and some actin with myosin). They can be readily assembled and disassembled. Microfilaments are about 8 nm in diameter and several cm in length. They occur in bundles in the cytoplasm, where they are associated with cell motility, such as cytoplasmic streaming and muscle contraction. Intermediate Filaments: They are solid strands of 8 to 11 nm in diameter and 10 to 100 µm in length. Their shape is rope-like in different tissues. They are made up of at least five different types of proteins. In hair and skin, they consist of keratin protein. They are important in maintaining the cell shape, attachment of muscle cells, and support of nerve cell processes (axon). Microtubules: Hollow tube-like structures with a diameter of 25 nm and more than 50 nm in length. They are composed of protein tubulin. The single microtubule consists of numerous subunits of tubulin arranged in 13 columns, called proto-filaments. They are responsible for the movement of chromosomes during cell division, movement of organelles within the cytoplasm, and movement of cilia and flagella. Ribosome: The ribosomes are small, dense, rounded, and granular particles of ribonucleoprotein. It has 65% ribosomal RNA and 35% proteins. Due to the high concentration of RNA, they are called ribosomes. They were first discovered by G. E. Palade in 1955. They are found freely in the cytoplasm as well as attached to the endoplasmic reticulum. They are formed in the nucleolus inside the nucleus. Structure: Each ribosome is composed of two unequal subunits. The larger subunit is dome-shaped, and the smaller one forms a cap on the flat surface of the larger subunit. The attachment of the two subunits is controlled by Mg. Functions: They serve as sites where proteins are synthesized; hence, they are called protein factories of the cell. Centriole: An animal cell has a centrosome near the nucleus. Each centrosome consists of two rod-like centrioles. Centrioles are short, barrel-shaped structures of microtubules situated at right angles to each other. Each centriole is composed of nine sets of triplet microtubules arranged in a ring. Function: During cell division, the centrioles replicate and move opposite to each other and form a nuclear spindle. Vacuoles: Generally, vacuoles (except food vacuoles) are non-protoplasmic fluid-filled cavities in the cytoplasm. They are surrounded by a membrane called the tonoplast. In animal cells, they are many and small in size. They are temporarily formed at the time of their need, while in plant cells, permanent and large-sized vacuoles are present. Function: In plant cells, vacuoles are filled with cell sap and act as a storage house; they also play a role in plant defense and are necessary for plant cell enlargement. In animal cells, vacuoles are rich in hydrolytic enzymes, including proteases, ribonucleases, and glycosidases.