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1_Cell as a Basic Unit of Life – class 10

 

 

BIOLOGY (class-IX)

 

Chapter-1:          (THE FUNDAMENTAL UNIT OF LIFE)

 

Introduction

The term cytology is concerned with the study of the structure of the cell and its organelles, whereas cell biology is concerned with the physiological and biochemical aspects of the cell and its components.

Cell is the structural and functional unit of the living organism. It is also called as the building block of life. Each cell is comprised of living substance called protoplasm (the living content of the cell), which is composed of various constituents like water, ions, salts, organic molecules (proteins, carbohydrates, fats) and nucleic acids (DNA and RNA) etc.

The protoplasm contains the living constituents within the plasma membrane, including nucleus and cytoplasm while cytoplasm is the viscous semi-fluid like material enclosed within the plasma membrane or plasmalemma, embedded with in it are the various organelles.

HISTORICAL PERSPECTIVE OF THE CELL

  • The first scientist to discover cells (dead cells) was Robert Hooke, who observed cell wall thickenings in a slice of cork, with his primitive microscope in 1665. He coined these box-like compartments as ‘cells’. Leeuwenhoek observed the first living cell, earlier to Robert Hooke.
  • During the same period Marcello Malpighi studied the animal tissues and observed them to be made up of simple constituents, but he didn’t term it as cells.
  • V. Leeuwenhoek (1632-1723) discovered animalcules, and made microscopic observations of protozoa, spermatozoa, RBC, muscles, nerves, skin, teeth and certain plants. The 19th Century he saw a very extensive study of the cell leading to the foundation of cell biology.
  • Rene Dutrochet (1776-1847) studied plant and animal materials very carefully and in 1824 was the first to propose that all animal and plant tissues were “aggregates of globular cells”.
  • Robert Hooke (1665) coined the term cell in his book entitled Micrographia.
  • Malpighi (1671) and Grew (1682) first observed the animal and plants tissues respectively.
  • Dujardin (1835) observed a living juice in animal cell and named it sarcode.
  • Purkinje (1840) first used the terms protoplasm for the material found in living eggs.
  • Hugo Von Mohl (1846) studied the nature of protoplasm present in the embryonic cell of plants and explained the importance of protoplasm in the cell division.
  • Haeckel (1866) said that nucleus is responsible for storing and transmission of hereditary characters.

Tools and Techniques for Study of Cell

  • Microscope is the instrument that is used to see objects which is not visible to the naked (unaided) eye.
  • The study of fine structure of an object under a microscope is called microscopy, and the person who studies it is termed The microscope magnifies as well as resolves the objects seen through it.
  • The cells of most animals and plants are too small to be seen by the naked eye. Their size ranges from 1.0 micrometer (mm) to 1.0 millimeter (mm).
  • The human eye cannot resolve objects as separate entities if they are closer together than 0.1 mm, besides the living cells are transparent in ordinary light. Hence, it becomes difficult to discriminate among various cellular components.
  • To overcome these practical difficulties, early cytologists investigated various methods such as slide preparation, that included steps like cutting tissues into thin sections, staining, mounting, magnifying these organelles.

Compound Microscope

  • Magnification: Magnification (M) brought about by a microscope may be defined as the ratio of the visible size of an object to its actual size.

 

  • Resolving Power: Human eye is unable to see objects smaller than 100 microns or m This means that two points less than 100 mm apart appear as one point to our eyes. The ability to see two close points as distinct points is called resolving power. Thus, the resolving power of the human eye is 0.1 µm.

Structure of Living Organisms

  • All cells, whether they exists as one celled organisms (unicellular organisms) or as a part of multicellular organisms are capable of carrying out certain basic functions such as nutrition, respiration, growth and reproduction. These functions are essential for the survival of the cells.
Unicellular Organisms Multicellular Organisms
·           A unicellular organism is represented by a single cell. ·           A multicellular organism consists of large number of cells.
·           The life span of an individual short. ·           The life span of an individual is long.
·           Reproduction through a single cell. ·           Only some cells of the body called germ cells take part in reproduction. Other cells (somatic cells) remain intact.
·           There is no division of labour as the single cell performs all life activities. ·           Cells are specialized to perform different functions of the body so that there is a division of labour within cells.
·           All activities of the organisms are performed by a single cell. ·           A single cell performs one or few activities of the organisms.

Structure of a Cell

  • All cells have following three major functional regions :

(i) The cell membrane or plasma membrane, and cell wall.

(ii) The nucleus and

(iii) The cytoplasm

The outer boundary of the cell is the plasma membrane. Inside it lies the cytoplasm. Various cellular or cell organelles and inclusions are suspended in the cytoplasm. All activities inside the cell and interaction of the cell with its environment is possible due to these features. Out of these organelles, nucleus is visible under a light microscope. The other organelles can be seen under an electron microscope only.

Eukaryotic Plant Cell Eukaryotic Animal Cell

Structural Organization of a Cell

  • All living organisms present on Earth can be classified into following two types :
  • Non-cellular Organisms : They do not obtain any cell in their body organization, e.g., viruses. Viruses lack any membrane and hence do not show characteristics of life until they enter a living body (i.e., prokaryotic cell or eukaryotic cell) to use its cell machinery to reproduce.
  • Cellular Organisms : They contain either one or many cells in their bodies, e.g. bacteria, plants and animals.

Cellular organisms are again divided into following two main types :

  • Prokaryotic Cell   (b) Eukaryotic Cells
  • Difference between Prokaryotic Cell and Eukaryotic Cell :
Prokaryotic cell Eukaryotic cell
·           Size of the cell is generally small (1-10 mm) ·           Size of cell is generally large (5–100 mm).
·           Nucleus is absent (Nuclear region ‘nucleoid’ is not surrounded by a nuclear membrane). ·           Nucleus is present (Nuclear material is surrounded by a nuclear membrane).
·           It contains single chromosome. ·           It contains more than one chromosome.
·           Nucleolus is absent. ·           Nucleolus is present.
·           Membrane bound cell organelles are absent. ·           Cell organelles such as mitochondria, plastids, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, etc., are present.
·           Cell division takes place by fission or budding. ·         Cell division occurs by mitotic or meiotic cell division.
·           Ribosome is 70 S. ·           Ribosome is 80 S & 70 S type in chloroplast & mitochondria.
·           Cell wall surrounds the plasma membrane in most cases. It is composed of peptidoglycans comprising polysaccharides linked to amino acids. Strengthening material is murein. ·           Cell wall surrounds the plasma membrane in some protists, most fungi and all plants. It is composed of polysaccharides. Main strengthening material is chitin in most fungi and cellulose in others. Animal cells lack cell wall.
·           Cell membrane bears respiratory enzymes. ·         Cell membrane lacks respiratory enzymes.
·           Cell membrane may infold to form mesosomes or photosynthetic lamellae (thylakoids). The latter occur free in the cytoplasm. ·         Cell membrane does not form mesosomes or photosynthetic lamellae. Thylakoids, if present, occur within the chloroplasts.
·           They include one kingdom only such as monera. e.g. Bacteria. ·       They include four kingdoms such as Protista, fungi, Plantae, and Animalia.
·           Cytoplasm lacks organelles (endoplasmic reticulum, mitochondria, Golgi apparatus, centrosome, microfilaments, microtubules, intermediate fibres, microbodies), except ribosomes. ·         Cytoplasm contains organelles, viz. endoplasmic reticulum, mitochondria, Golgi apparatus, lysosomes, centrosome, microfilaments, intermediate fibres, microtubules and microbodies, besides ribosomes.

 

Prokaryotic Cell

 

Eukaryotic Cell

The Cell Theory

Mathias J. Schleiden (1838) and Theodore Schwann (1839), proposed cell theory as per their observation on plant and animal cells respectively:

  • All living things are composed of cells and their products.
  • All cells arise from pre-existing cells.
  • All cells are basically alike in chemical composition and metabolic activities.
  • The function of an organism as a whole is the outcome of the activities and interaction of the constituent of cells.
  • Rudolf Virchow (1821–1902) gave his famous expression Omnis cellula e cellula, i.e., all cells arise from pre-existing cells.

 

Plasma Membrane or Cell Membrane

·           Every cell is bound by membranous covering which separates it from the surrounding and provide identity to the cell.

·           Its presence was first recognised by Naigeli & Cramer (1855), but was experimentally confirmed by E. Overton (1899) suggesting that it is made of lipid.

·           Gorter & Grendel (1925) postulated that it is made of bimolecular lipid layer and the lipid molecule in amphipathic having polar (hydrophilic) head and non-polar (hydrophobic) tail part.

·           It is a thin, delicate membrane of about 70 AO to 100 AO thickness.

Þ       Fluid mosaic model by S..Jonathan Singer and Nicholson (1972).

Membrane’s model

 

Structure of Plasma Membrane

As per this model membrane this is two dimensional solutions of oriented globular proteins and lipids. The lipid bilayer is a fluid in which the proteins are dispersed to give a mosaic appearance. The lipid bilayer is, in the membrane. They are amphipathic i.e. they are structurally asymmetric with polar hydrophilic and non-polar hydrophobic groups. On the outer side some of the lipids have sugar chains attached on their polar heads and are hence known as glycolipids.

Proteins are of two kinds:-

  • Peripheral or extrinsic proteins: They are completely external and are loosely attached to the polar heads of the lipids on both the surfaces.
  • Integral or the intrinsic proteins: The small integral proteins partially project from either surface of the membrane while the larger ones span the entire thickness of the membrane and project from one or both the surface.
  • The proteins are present not to give strength to the membrane but to serve as (a) enzymes (catalyse chemical reactions within the membrane), (b) transport proteins or permeases (for movement of water soluble ions); (c) pumps (for active transport) and (d) receptor proteins (for endocytosis). Presence of lipids and proteins provides flexibility to the plasma membrane. This property of flexibility of the plasma membrane helps in endocytosis.

Functions:

  • Biomembranes compartmentalize the cells. This separates (not isolates) the cell interior from the external environment. The membranes also allow the adjacent intra-cellular compartments (e.g., mitochondria, chloroplasts) to maintain their distinct identity and physiology.
  • Plasma membrane binds the semi-fluid protoplasmic contents of the cell.
  • Plasma membrane controls the substances to be passed inwardly and outwardly from the cell (selective permeability).
  • Plasma membrane controls the entry and exit of molecules thus acting as a selectively permeable membrane.
  • Almost all the cells maintain a difference in ionic concentration on the inside and outside of the cell membrane. The usual ions are Na+ and K+, with a higher concentration of Na+ outside the cell, than inside. The ionic concentration of Na+ and K+ is altered when impulse arrives at a cell. The impulse is then conducted along the length of the cell such as nerve or muscle cell.
  • Plasma membrane is capable of ingesting liquids (pinocytosis) and solid materials (phagocytosis) by the formation of an invagination and later absorbing then into the cell.
  • Secretory materials and waste products are thrown out of the cell by plasma membrane by exocytosis.
  • The function of plasma membrane is performs certain physical activities, such as diffusion and osmosis for the intake of some substances.
  • Diffusion : Some substances (molecules, ions) such as carbon dioxide (CO2), oxygen (O2), water etc., can move across the plasma membrane through a process called diffusion. The diffusion is the spontaneous of molecules from a region of high concentration to one of lower concentration, until uniform concentration is finally achieved. Diffusion is faster in the gaseous phase than in liquids and solids.

Diffusion of (O2) and (CO2) across the

plasma membrane in amoeba

  • Osmosis : The spontaneous movement of water molecules through a  selective permeable membrane (e.g. plasma membrane) is called osmosis. The movement of water across the plasma membrane of the cell is affected by amount of substance dissolved in water. Thus, osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentrations. Osmosis is purely a mechanical diffusion process by which cells absorb water without spending any amount of energy.

Let us see what will happen if you put an animal cell {e.g., red blood cells or RBCs) or plant cells {e.g., Rheo leaves) into a solution of sugar or salt prepared in water.

 

One of the following three conditions could happen:

 

  1. If the medium surrounding the cell has a higher water concentration than the cell, e., if solution is a very dilute solution, the cell will gain water by osmosis. Such a dilute solution is called hypotonic solution.

While water molecules are free to pass across the plasma membrane in both directions, more water will enter the cell than leave. The net (over all) result is that water enters the cell. In such a situation, cell is likely to swell up, i.e., become inflated or turgid. Such swollen RBCs may ultimately burst, i.e., haemolysed.

  1. If the medium surrounding the cell is of exactly the same water concentration as the cell, there will be no net movement of water across the plasma membrane. Such a solution is called isotonic solution {e.g., Ringer’s solution is an isotonic solution for the animal cells).

In this case, water crosses the plasma membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. In such a situation, the cell will maintain the same size.

  1. If the medium has a lower concentration of water than the cell, i.e., if it is very concentrated solution, the cell will lose water by osmosis. Such a concentrated solution is called hypertonic solution.

In this case too, water crosses the plasma membrane in both directions, but this time more water leaves the cell than enters it. Therefore, the cell will shrink. In this situation, plant cell is said to be plasmolysis.

Conditions in Osmosis

Cell Wall

 

    It is outer rigid protective supportive and semi-transparent covering of plant cells, fungi and some protists.

  • Cell wall was first seen in cork cells by Hooke in 1665. Its thickness varies in different types of cells from 0.1 m
  • In plant cells, there occurs a rigid cell wall which lies outside the plasma membrane. Cell wall is non-living and freely permeable and is secreted by the cell itself for the protection of its plasma membrane and cytoplasm. It determines the shape of a plant cell and prevents desiccation of cells. It is made up of a fibrous polysaccharide (carbohydrate) called cellulose. The plant cell wall, thus, consists of tiny cellulose fibres called microfibrils, glued together by a mixture of polysaccharides. Each microfibril is made up of thousands of cellulose molecules bound together by pectins and hemicellulose.
  • It is laid down during development of the cell and starts as a thin organic material called pectin, beneath which, cellulose secreted by the outer part of cytoplasm is laid down (primary wall). Further layers of cellulose constitute the secondary wall.
  • Cell wall is a non-living extracellular secretion or matrix of the cell which is closely attached to it. Cell wall is metabolically active and is capable of growth.
  • Cell wall is present only in plant cells. It is the non-living component and it is secreted by the cell itself.

Functions

  • Protects the protoplasm against mechanical injury
  • Protects the cell from attack of pathogens.
  • Counteracts osmotic pressure.
  • Walls of sieve tubes, tracheid and vessels are specialized for long distance transport.
  • Cutin and suberin of the cell wall reduce the loss of water through transpiration.
  • Cell wall gives definite shape to the plant cells.
  • It provides mechanical strength and protection to the cell.
  • Cell wall prevents the cell from desiccation.
  • Plasmolysis : When a living plant cell loses water through osmosis, there is a shrinkage or contraction of the protoplasm away from the cell wall. This phenomenon is called

Differences between Cell Wall and Plasma Membrane

Cell Wall Plasma Membrane
·           It occurs in plant cells. ·           It is found in both plant and animal cells.
·           Its major function is to provide protection and strength to the cell. ·           Its major function is to hold cellular contents and control passage of materials in and out of the cell.
·           It lies outside of the cells ·           It lies on the outside of animal cells and inner to cell wall in plant cells.
·           It is non-living and quite thick in plant cells. ·           It is living and quite thin.
·           It is rigid. ·           It is flexible
·           It is generally permeable. ·           It is selectively permeable.
·           It is formed of cellulose, hemicellulose and pectin. ·           It is formed of lipids and proteins and small number of small carbohydrates (i.e., oligosaccharides).

 

NUCLEUS

       The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. Chromosomes contains information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. Chromosomes are composed of DNA and protein. The DNA molecules contain the information necessary for constructing and organizing cells. Functional segments of DNA are called genes. In a cell which is not dividing, this DNA is present as part of chromatin material. Chromatin material is visible entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organized into chromosomes.

Nucleus

Functions

  • It controls all the cellular activities of the cell. If the nucleus is removed from a cell, the protoplasm ultimately dries up and dies.
  • It regulates the cell cycle.
  • The nucleus contains chromosomes having a characters which are passes from parents to their offsprings.

 

Chromosome Structure

 

  • A chromosome is an organized structure of DNA and protein found in cells.
  • It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.
  • Chromosomes vary widely between different organisms. The DNA molecule may be circular or linear, and can be composed of 100,000 to 10,000,000,000 nucleotides in a long chain.
  • Eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have small circular chromosomes.
  • Cells may contain more than one type of chromosome; for example, mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes. In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin.
  • Chromosomes also have banding patterns, unique to each one. These bands are visible or stained by certain dyes. Chromosome banding can help to determine homologs of a karyotype.

Chromosome Numbers

    Somatic Cells – body cells, such as muscle, skin, blood etc. These cells contain a complete set of chromosomes (46 in humans) and are called diploid or 2n.

  • Sex Cells – also known as gametes (sperm and egg). These cells contain half the number of chromosomes as body cells and are called haploid or n.
  • Chromosomes arranged in pairs, called homologous pairs (or homologs). Imagine homologs as a matching set, but they are not exactly alike, like a pair of shoes.
  • In humans, Diploid cells have 23 homologous pairs = total of 46.
  • In humans, Haploid cells have 23 chromosomes (that are not paired) = total of 23

 

CYTOPLASM

 

  • The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialized cell organelles. Each of these organelles performs a specific function for the cell.
  • In a broad sense, a cell is differentiated into cytoplasm and nucleus. Cytoplasm is surrounded by cell or plasma membrane and the nucleus by nuclear membrane. A typical animal cell or plant cell consists of plasma membrane, cell wall (only in plant cells) cytoplasm and nucleus. Cytoplasm is filled with both minute and large dispersed particles and organelles. The portion of cytoplasm immediately below the cell membrane is gel like and is called The cytoplasm between ectoplasm and nuclear membrane is liquefied and is called endoplasm.
  • Embedded in the cytoplasm are the cell organelles, the endoplasmic reticulum, ribosomes, Golgi bodies, lysosomes, vacuoles, centrioles and nucleus.
  • A part from the cell organelles, several inorganic and organic substances are present in the cytoplasm. The different substances that make up the cell are collectively called protoplasm.
  • The varied substances that constitute the cytoplasm are water (70 – 85%), electrolytes (K+, Mg++, , , , NaCl and Ca), proteins, lipids and carbohydrates.

Functions

  • Cytoplasm acts as a store of vital chemicals such as amino acids, glucose, vitamins, ions, etc.
  • It is the site of certain metabolic pathways, such as glycolysis. Synthesis of fatty acids, nucleotides, and some amino acids also take place in the cytosol.
  • Living cytoplasm is always in a state of movement.

Cell Organelles

  • Every cell has a membrane around it to keep its content separate from the external environment. The different components of cell perform different function and these components are called cell organelles.
  • A cell has to perform different functions with the help of its various membrane-bound organelles

(a)  It has to synthesize substances, e.g., protein synthesis by ribosomes, lipid synthesis on the surface of smooth endoplasmic reticulum (SER), and photosynthesis of food (e.g., glucose, starch) by chloroplasts.

(b)  It has to secrete cell products, e.g., enzymes, hormones, mucus, etc.

(c)  It has to digest those substances which are taken up by the cell during endocytosis. Such intracellular digestion is done by enzymes of lysosomes.

(d)  It has to generate energy, e.g., synthesis of energy-rich ATP (adenosine triphosphate) by mitochondria.

  • Every cell is bounded by a membrane and thus, keeps its own contents separate from the external environment. Larger or more evolved cells, or cells from multicellular organisms, have a great deal of metabolic activities to support their complicated structure or function. To keep metabolic activities of different types separate from each other, cells have membrane bound organelles within themselves. Cell organelles are “small organs” of the cell and are found embedded in the cytosol. They form living part of the cell and each of them has a definite shape, structure, and function. Examples of such organelles are nucleus, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, ribosomes, etc. We have already discussed about the nucleus in a previous section. In this section, we will discuss the cellular organelles one by one.

 

ENDOPLASMIC RETICULUM

It is an elaborate network of membrane bound          tubules highly concentrated in the endoplasm hence       called endoplasmic reticulum.

It was first observed by        Porter, Claude and Fullmax (1945) under EM       Porter and Kallman (1952) coined the term   ‘endoplasmic reticulum’.

Structure :

It is a fine branched vacuolar system extending from the nucleus through the cytoplasm to the margins of the cell.

In young meristematic cells it forms a continuous system extending from the nuclear envelope to the cell membrane and even to the cell wall. It may even extend to the neighbouring cells. In old cells it may be less prominent and is represented by discontinuous vesicles.

It occurs in one of the three shapes cisternae, tubules and vesicles. The morphology of the ER depends upon the physiological and developmental stages of a cell. It is not a stable structure rather capable of being broken down and reconstructed. The cisternae are large flattened parallel sac like structures interconnected to each other. A cisternae is usually 40 to 50 nm in thickness.

Endoplasmic Reticulum

The tubules are 50 to 100 nm and vesicles 25-500 nm have a diameter. The membrane of the ER                                 is thinner 50-60 Å than the plasma membrane. The cavity of the ER is                                   sometimes so small that the          two membranes come in intimate contact but usually the space is quite       distinct. The plasma cells          and the goblet cells with active protein synthesis show a wider space.    

Function :

  • Storage of glycogen, lipid, calcium.
  • Glyosylation or synthesis of glycoprotein, glycolipids etc.
  • Detoxification of various toxic, waste materials like drugs, pollutants, bilesalts, degraded amino acids and fatty acids.
  • Provides skeletal framework to the cell and give rise to membrane organelles like Golgi bodies, spherosomes (The body wall of any radiate animal) etc.
  • Plays important role in cycling of membrane (membrane biogenesis).
  • Depending upon the nature of its membranes, endoplasmic reticulum is of two main types, smooth and The two types of ER may be continuous with one another, plasma membrane and nuclear envelope. Endoplasmic reticulum may develop from pre-existing E.R., plasmalemma or nuclear envelope.

Differences between SER and RER

SER RER
·           SER or smooth endoplasmic reticulum does not bear ribosomes over the surface of its membranes. ·           RER or rough endoplasmic reticulum possesses ribosomes attached to its membranes.
·           It is mainly formed of vesicles and tubules. ·           It is mainly formed of cisternae and a few tubules.
·           It is engaged in the synthesis of glycogen lipids and steroids. ·           The reticulum takes part in the synthesis of proteins and enzymes.
·           SER gives rise to sphaerosomes. ·           It helps in the formation of lysosomes through the agency of Golgi apparatus.
·           Pores are absent so that materials synthesized by SER do not pass into its channels. ·           RER possesses narrow pores below its ribosomes for the passage of synthesized polypetide into ER channels.
·           SER is often peripheral. It may be connected with plasmalemma. ·           It is often internal and connected with nuclear envelope.
·           Ribophorins are absent. ·           RER contains ribophorins for providing attachment to ribosomes.
·           It may develop from RER through loss of ribosomes.     It may develop from outer membrane of nuclear envelope.
·           It has enzymes for detoxification. ·           The same are absent.

GOLGI APPARATUS

Golgi bodies were first detected in nerve cells of cat and owl by Camillo Golgi (1891). It is found in all cells except RBC of mammals and muscle cells. The Golgi bodies of plants are called ‘dictyosomes’ (dictyos=net) because of their apparent net like structure.

       The number – varies from cell to cell, secretory cells have most numerous. In plant cells number increases during cell division in plant cells.

The shape is also variable, from a single vesicle (=dictyosomes in plants) to large network like (neurons) located in between the nucleus and the secretory surface. Dictyosomes are present in large number on two side of the equator in dividing plant cells.

Structure

As observed under EM a Golgi body consists of three parts.

  • Cisternae, the stacks of flattened membranous sacs.
  • Vesicles of 600 Å diameter.
  • Vacuoles filled with amorphous substance.
 (a) A Golgi Body                          

The cisternae are the most conspicuous part arranged in a stack of parallel sacs. The number of cisternae or saccules in a stack is usually 3 to 8 in animal and 10 to 20 in plant cell.

Each saccule is slightly concave disc with perforation edges separated from each other by a distance of about 115-200 Å. There are several parallel fibrous intercisternal elements of a diameter of 70-80 Å. The saccules produce vesicles around its periphery. Vesicles are of two kinds, one with smooth membranes being more numerous and the other with coated surfaces being generally less numerous.

Functions of the Golgi Apparatus

  • Packaging of proteins trimming, sorting and condensation is made into secretory vesicles for exporting or storing.
  • The synthesis and secretion of polysaccharides.
  • Sulphatation of carbohydrates and proteins if required sulphur is attached.
  • Lipid packaging and secretion.
  • Plasma membrane formation: After releasing its content outside membranes of the vesicles are integrated into the plasma membrane.
  • Cellular synthesis and cell plate formation in plant cell: It synthesizes hemicellulose and pectic groups of polysaccharides especially during cell division. These substances are laid down on the cell plate. Dictyosomes contain a pool of precursors for the synthesis of certain cell wall materials but they are not involved into the synthesis of cellulose itself.
  • Formation of Lysosomes: It gives rise to the primary lysosome. The hydrolytic enzymes produced by RER are modified and concentrated within Golgi bodies before they are packed in a membrane-wrapped structures as lysosome.

LYSOSOMES

  • They are single-membrane spherical bodies having a size of 0.08 to 0.8 m
  • Discovered by Christian de Duve (1955) from liver cells of rats contains about 50 different hydrolysing (digestive) enzymes called acid hydrolases, optimally active in acidic pH.
  • Popularly called as “suicide bags”, they occur in all the animal cells (except R.B.C.), Euglena and plant cells like yeast and fungi.

Functions

  • Primarily meant for heterotrophic nutrition.
  • Being abundant in leucocytes these help in defense against microbes etc.
  • They help in fertilization, differentiation and metamorphosis by performing histolysis.
  • They dispose of the worn out, damaged and dead cells.
  • In osteoclasts they help in replacing cartilage and unwanted part of bone.

MITOCHONDRIA

  • Mitochondria, the power house of the cells as they are involved in ATP generation, storage and supply of ATP according to the need of the cell.
  • Kolliker (1850) first found it in muscle cell and named sarcosomes.
  • Sites of cellular respiration in the cytoplasm (‘Powerhouses’ of the cell)
  • Found at sites of highest metabolism (e.g. muscle cells) to produce energy-rich molecules of ATP (Adenosine Triphosphate)
  • Mitochondria is a double membrane – the outer membrane is smooth and present around the entire mitochondrion, the inner membrane infolds to form many finger like projections called cristae.
  • It is thought that mitochondria in eukaryotic cells may have evolved from ancient symbiotic prokaryotic bacteria that lived inside other larger prokaryotic cells. They have their own DNA and ribosomes, and can reproduce on their own (Semi-autonomous).

Mitochondria

 

PLASTIDS

  • Plastids are present only in Plant They are three types of plastids :
  • Chloroplast : The chloroplast contains chlorophyll molecule by which plants make their food in the form of glucose in the presence of sunlight.
  • Chromoplasts : It imparts colors to the leaves, flowers and fruits. Due to color it helps in pollination by attracting insects.
  • Leucoplasts are found primarily in organelles in which materials such as starch, oils and protein granules are stored.
  • The internal organization of the plastids consists of numerous membrane layers embedded in a material called stroma. Plastids are similar to mitochondria in external structure. Plastids have own DNA and ribosomes.

Structure of chloroplast

  • In EM study of chloroplast shows envelope of two membranes, each 50Å in thickness separated by a clear space. The matrix is termed as stroma.
  • The inner membrane produces extensive enfolding’s which are arranged parallel to each other and are called lamellae organized in the form of discoid sacs called thylakoids 20 to 50 of these are stacked one upon another to form grana. There are 40 to 100 grana in one chloroplast.
  • The thylakoids produce 4-8 projections called fret from their margins. They interlink with the frets of the other. The name stroma lamellae is given to this interconnected network of the intergranum membrane system. The concept of grana with interconnecting fret system was proposed by Weier (1962).

Functions

  • Chloroplasts trap solar energy and utilizes it to manufacture food for the plant.
  • Chloroplasts impart various colours to flowers to attract insects for pollination.
  • Leucoplasts store food in the form of carbohydrates (starch), fats and proteins.

VACUOLES

  • Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small size in animal cells while in plant cells they are very large in size. The central vacuole of some plant cells may occupy 50-90% of the cell volume.
  • In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell. Many important substances of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids and some proteins. The outer layer of the vacuole is tonoplast.
  • In single-celled organisms like Amoeba. The food vacuole contains the food items that the Amoeba has consumed.

Vacuole

Functions

  • Vacuoles help to maintain the osmotic pressure in a cell (osmoregulation).
  • They store toxic metabolic by-products or end products of plant cells.
  • They provide turgidity and rigidity to the plant cells.

 

Additional Information

Differences between Plant Cell and Animal Cells

Animal Cells Plant Cell
·           Cell wall is absent. ·           The plasma membrane of plant cells is surrounded by a rigid cell wall of cellulose.
·           Except the protozoan Euglena, no animal cell possesses plastids. ·           Plastids are present.
·           Animal cells are generally small in size. ·           Plants cells are larger than animal cells.
·           Vacuoles in animal cells are many, small and temporary. ·           Most mature plant cells have a permanent and large central sap vacuole.
·           Animal cells have a single highly complex and prominent Golgi apparatus. ·           Plant cells have many simpler units of Golgi apparatus, called dictyosomes.
·           Animals cells have centrosome and centrioles. ·           Plant cells lack centrosome and centrioles.

 

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EXERCISE

 

Very Short Answer Questions

  1. What is plasmalemma?
  2. What are the two subunits of 80 S ribosomes?
  3. Which is the principal site for the development of ribosomal RNAs?
  4. What are plasmodesmata?
  5. Name the different types of endoplasmic reticulum?

 

Short Answer Questions

  1. What are the main differences between animal and plant cells?
  2. What organic macromolecules are found in cytoplasm?
  3. Arrange the following from smallest to largest – cell, organ, organism, tissue, system, organelle.
  4. If prokaryotic cells such as bacteria do not have organelles, how do they function?
  5. How does technology advances the cell knowledge?

 

Long Answer Questions

 

  1. Describe the electron microscope structure of biomembrane and the two models proposed to explain it?
  2. What is meant by active transport across a cell membrane?
  3. List the functions of rough and smooth endoplasmic reticulum and Golgi bodies.
  4. Write an account of lysosomes and their role in cellular metabolism.
  5. Distinguish between prokaryotic and eukaryotic cells.

 

MISCELLANEOUS Questions

  1. Why were the scientists not able to observe most of the cell organelles before 1940?
  2. What is the function of the cell wall?
  3. Why are lysosomes called digestive bags or suicide bags?
  4. Why are peroxisomes mostly found in kidney and liver cells?
  5. Why are mitochondria also called ‘The Power House of cell’?
  6. There would be no plant life if chloroplasts did not exist. Justify.
  7. Why is the Golgi apparatus called the secretory organelle of the cell?
  8. Differentiate between smooth and rough endoplasmic reticulum.
  9. Give the importance of cristae in mitochondria.
  10. Give one similarity and one dissimilarity between plastids and mitochondria.
  11. In many plant cells the nucleus and other cell organelles are pushed near the boundary walls.
    Explain.
  12. Differentiate between the cell of an elephant and a plant cell (2 differences).
  13. Why does not a child exactly resemble his father or mother?
  14. Name two components that the cell membrane encloses.
  15. What are the two types of endoplasmic reticulum (ER)?
  16. Define the endoplasmic reticulum.
  17. What is the cell theory? Who proposed it? When?
  18. What are lysosomes?
  19. What are peroxisomes?
  20. Name the important function of peroxisomes.
  21. What is the difference between vacuoles of plant and animal cells?

 

 

 

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