Introduction to botany  © 2007 punam kumar

 

CHAPTER 2:

The Plant Cell

Subsections --"A"

Introduction

All plants are made up of one or more units called cells. The cell also bring structural and functional stability to plants.
Plants are of two types:-

Unicellular plants Many plants consist of single cell e.g. Chlamydomonas(fig-1 singlecelled Chlamydomonas), Anacystis, Chroococcus, Synechococcus.


image207

In these plants all the vital functions are carried out by the single cell. This type is also known as acellular.

Multicellular plants:- Most plants consists of many cells. All the function of life are carried out by one or other group of cells. e.g. Spirogyra(fig2) ,Sphagnum, Lycopodium etc.


image228

In multicellular living things, a collection of cells that work together to perform similar functions is called a tissue; various tissues that perform coordinated functions form organs; and organs that work together to perform general processes form body systems.
A cell is a basic structural and functional unit of plants.

Discovery of Cell


Marcello Malpighi (1661),found that plants consist of separate structural units “utricles“.
Robert Hooke(16650) observed a thin slice of cork under a very primitive microscope. Cork was composed of box-like compartments, forming a honey comb structure. He named it as cells(Latin cellula-a hallow space).
Antony van Leenwenhoek(1674) observed under improved microscope living cells-bacteria, protozoa, spermatozoa etc.
Nehemiah Grew(1682). Cells in the forms of cavities with cellulose walls were discovered in different plants. He wrote The Anatomy of Plants.
H.J.Dutrochet (1824), expressed the idea of individuality of cells i.e., cells were not just spaces between a network of fibres,but these were separate and separable units.

Cell Theory


The cell theory or cell doctrine was proposed by Schleiden and Schwann in 1839 describe that all organisms are composed of similar units of organisation, called cells.
In 1838 Matthias Schleiden found that all plant cells have basically similar structure.
T. Schwann(1839) observed that animal cells differ from plant cells in lacking cell wall but are otherwise similar amongst themselves. He declared that all animals and plants are made up of cells.
Schleiden (1838) summarized his observations into three conclusions about cells:
  1. The cell is the unit of structure, physiology, and organization in living things.
  2. The cell retains a dual existence as a distinct entity and a building block in the construction of organisms.
  3. Cells form by free-cell formation, similar to the formation of crystals (spontaneous generation).

Rudolph Virchow(1858) states that "Omnis cellula e cellula"-that the new cells arise from pre-existing cells only. The main features of cell theory as known nowadays are
  1. All living organism are composed of cells. Therefore cell is structural unit of living organisms.
  2. All cells arise from the pre-existing cells of similar type. Thus cell is a unit of heredity.
  3. All cells are basically alike in chemical composition and metabolism activities.
  4. The function of an organism as a whole is the result of the activities and interactions of its cells.

Exceptions to the Cell Theory

  1. Virus- Protoplasm and nucleus absent. DNA and RNA is the genetic material.
  2. Bacteria and Blue Green Algae:- Nuclear membrane is absent, thus the nuclear content is in direct contact with the cytoplasm. No true cells.
  3. Some mould fungi:- Body is made-up of undivided mass of protoplasm in which several nuclei are scattered.

Cell size
Cell size varies widely in various plants. The size varies from micron to mm (a micron is a one thousandth part of a millimeter). The smallest cells are found in bacteria :-the cell of Mycoplasma laidlawii with a diameter of about 0.1-0.15 micron is considered to be the smallest. In plants the longest cells are fibres producing e.g. Jute, Cotton, ramie, which are about 55cm long. A single celled alga Acetabularia measures about 10 cm in height.


image208


Cell shape
The shape of the cells varies with their position in plants. It can be polygonal, spherical, elliptical, spindle-shaped, cuboidal, plate-like or irregular.
In multicellular plants cell shape also depends on their location and nature of function. The cell shapes are influenced by the surface tension and viscosity of the protoplasm, pressure exerted by the surrounding cells and the rigidity of cell membrane.


Cell types
Two types of living cells were recognized in plants by Dougherty(1957). This division is based on internal complexity

  • Prokaryotic (pro = primitive, karyon = nucleus) cell : a. The nuclear material is not bounded by nuclear membrane , the nuclear materials remain diffused with cytoplasm. b. a organized nucleus is absent in the protoplasm, i.e., the nucleus is without nuclear membrane, nuclear reticulum, nucleoplasm and nucleolus. Only monera i.e. Blue green algae (Cyanobacteria) and Bacteria are prokaryotic cell known as prokaryotes. c. the genetic material contain DNA, histone absent


    image229

  • Eukaryotic cell (eu =true, karyon =nucleus): This type of cells have:- a. A true nucleus with a definite nuclear membrane by which nuclear material is bounded. b. the genetic material contains DNA complex with histone proteins to form well-organized chromosomes. c. Nucleoli present. The majority of plants are composed of eukaryotic cells. Distinct organelles, the small structures that each perform a specific set of functions, are present within eukaryotes

    Comparison of prokaryotic and eukaryotic plants

    feature Prokaryotic Cells
    Bacteria, and Cynobacteria
    Eukaryotic Cells
    Protists Fungi, Plants
    Cell size and cell organization 1-10 mm: unicellular 5-100 mm: multicellular
    Organelles Few or none nucleus, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, etc
    DNA Circular (or linear) DNA organized in a nucleoid Linear DNA molecules organized in chromosomes bounded in a nucleus
    Synthesis of RNA and proteinsRNA and proteins synthesis occur in same place, RNA synthesis occur in nucleus and proteins synthesis in the cytoplasm
    ChromosomeSingle Multiple
    Ribosomes70s(50s+20s) 80s(60s+40s)
    Metabolic activity Photosynthetic and respiratory enzymes are found on plasma membrane Photosynthetic and respiratory enzymes are found on chloroplast and mitochondria respectively
    Cytoskeleton Not present Present
    Cell division Amitosis, spindle apparatus absent during cell division; replicated DNA pulled by attachment to plasma membrane Mitosis, meiosis, spindle apparatus present and chromosomes pulled by it
    Metabolic activity Anaerobic( respiration in absence of oxygen) or aerobic Aerobic (in presence of oxygen)

    Subsections---"B"

    Cell Structure

    All plant cells are surrounded and confined by a rigid cell wall. Each cell has usually one nucleus which is surrounded by cytoplasm. In higher plants the nucleus is enclosed by nuclear membrane.(parts of a cell)


    image227


    image211

    Cell wall - the cell walls of plants are generally thick, strong, porous ,rigid and none living structures, which enclose the cells including the plasma membranes. Cell wall is secreted by the living matter of cell. Cell wall is highly permeable to water, solutes and gases. This structure making the outermost boundary of the cell, is known as extra cellular matrix(ECM). The function of extra cellular matrix is - Transport of a variety of materials on either side Recognition of a variety of stimuli with the help of receptor proteins and Cell adhesions(including cell to cell adhesions and attachment to the substratum).

    Structure of cell wall - The cell wall of a mature plant cell consists of:-

    1. Primary wall is the first formed wall. It is thin, permeable, elastic(only semi rigid) and capable of growth. Primary walls surround growing and dividing plant cells such as meristematic cells, parenchymatous cells and root hairs.

      Chemical composition of primary wall -It is made up of cellulose micro fibrils running through a matrix of complex polysaccharides(hemicellulose)and pectic material.


      image231

    2. Secondary wall -After maturity, more layers of cells are added on the inner side of the primary wall formed secondary cell wall .At certain areas, secondary wall is not formed, these places are called pits. Secondary wall is rigid and thicker. The cells of sclerenchyma collenchyma xylem fibers, tracheids, and sclereids have secondary deposits of lignified cellulose which provide mechanical strength to the tissue. In tracheids of conifers, secondary wall divided into three layers termed as S1 , S2, S3. S2 is relatively thick.

      Chemical composition of Secondary wall is same as primary wall. secondary cell walls that are comprised mostly of cellulose and cross-linking glycans and can also be lignified.

    3. Tertiary wall -It is rarely present and is foun on inner side of secondary wall. It is made up of cellulose and xylan.
    4. Middle lamella -It is the first structure that is formed from cell plate between the newly formed daughter cells at the time of cytokinesis Adjacent cells in plant tissues are held together and separated from one another by cementing material called middle lamella and also known as intercellular substance.It is primarily composed of pectic substance.

      The cell wall is primarily made up of cellulose and pectin, but some additional materials are deposited for specific functions Lignin(lignification) in secondary walls of the cells of woody plant Cutin(cutinisation) on exposed surface of cell walls as in leaf surface Suberin(suberisation) in the secondary wall of cork cells Silica in cell wall of grasses.

      Plasmodesmata(single plasmodesma) are living cytoplasm strands which establish connections between two adjacent cells through very fine pores in adjacent cell walls.


      image232

      Functions of Cell Wall :- Cell wall provides shape and rigidity to cell It protects the protoplasm from external injury.

      Cell Membrane Cell Membrane(C. Nageli and C. Cramer in1855) or Plasma Membrane:- All living cells are enclosed by a selectively permeable, thin molecular layer termed as plasma membrane or plasma lemma(J. Q. Plowe in 1931). The plasma membrane separates the cell from its surroundings, protects it from changes in the chemical and physical environment, and regulates movements of molecules into and out of the cell. The study of plasma membrane is based on the Fluid Mosaic Model .To perform the function of the organelle, plasma membrane is composed primarily of two types of molecules—lipids, which are fatty or oily molecules, and proteins.All membranes have a common general structure in which two layered sheets or bilayer of lipid molecules have proteins embedded in them The most common lipids in the plasma membrane are the phospholipid, which has a polar (hydrophilic it can mix with water) head and two nonpolar (hydrophobic they do not mix well with water) tails. In the plasma membrane’s bilayer construction, phospholipid molecules are arranged so that their hydrophilic heads point outward on either side of the membrane, and their hydrophobic tails point toward each other in the middle of the membrane. By bridging both the hydrophilic and hydrophobic regions of the membrane, these proteins are "bound" to the membrane. Most interestingly, however, is that these bridging proteins can form channels through the membrane for a particular kind of chemical to pass. In most cells, the plasma membrane is about 40 percent lipid and 60 percent protein, but these proportions vary greatly, from as little as 20 percent to as much as 75 percent protein depending on the type of cell.

    The carbohydrates occur only at the outer surface of the membrane. Their molecules are covalently linked to

    1. the polar heads of some lipid molecules (forming glycolipids) and
    2. most of the proteins exposed at outer surface (forming glycoproteins).
    The carbohydrates so bound to membrane components constitute the glycocalyx of cell surface.


    image234

    Function of plasma membrane -Cell membraneof the cell is the important import/export control area . It can "pump" other substance into the cell against the concentration gradient or pump other "wastes" etc. out of the cell is known Membrane transport . The difference between the concentration of a substance in two different areas is known as a concentration gradient. Membrane transport is of two types

    1. Passive transport Passive transport does not require an expenditure of metabolic energy, and materials flow down the concentration gradient. Examples of passive transport are(a) diffusion-movement of molecules down a concentration gradient (b)osmosis is the diffusion of water through a semipermeable membrane and(c) facilitated diffusionis the diffusion of a substance across a membrane.
    2. Active transport Active transport uses energy (in the form of ATP), and materials flow against the concentration gradient.

    Endocytosis, the plasma membrane folds inward, forming a pouch that traps molecules. The pouch continues to press inward until it forms a closed sac that breaks loose from the plasma membrane and sinks into the cell. Exocytosis, is a reversal of endocytosis. A sac inside the cell containing proteins and other molecules moves toward the outer edge of the cell until it touches the plasma membrane. The membrane of the sac then joins with the plasma membrane, and the contents of the sac are released from the cell.

    In multicellular organisms, the plasma membrane also plays a critical role in communication between cells.

    Subsections----"C"

    Cytoplasm(Strasburger, 1882) is a homogeneous, mostly clear jelly-like liquid lies between the nucleus and cell membrane, consisting of cytosol and the cellular organelles.

    Cytosol is composed of water, salts, organic molecules, and the many enzymes necessary for the cell to catalyze reactions. 80% of the cytoplasm is aqueous and composed of ions and soluble, primarily organic, macromolecules.The 20% of the cytoplasm that is not aqueous is made up of organelles, mitochondria, chloroplasts, vacuoles, the cytoskeleton, and endoplasmic reticulum. The aqueous part is also called hyaloplasm. It behaves like a gel sometimes, depending on the activity phase of the cell; in this state, it is called cytogel. Cytogel lines the plasma lemma and known as ectoplasm. When instead it behaves like a liquid, it's called cytosol.It is known as endoplasm..

    Vacuole:-A vacuole is a large fluid-filled sac in the cytoplasm, bounded by a single membrane called the tonoplast, it plays the major role in all these functional activities of the vacuole Vacuoles are filled with watery fluid termed as cell sap.It is acidic. The chemical composition of cell sap differs markedly from that of the surrounding cytoplasm. In most cases mature plant cells have single large central vacuole. So that the plant cytoplasm lies as a thin layer positioned between the plasma membrane and the tonoplast. The vacuole contains nutrients, metabolites, pigments and waste products. Pigment anthocyans are present ,which is mainly responsible for the colours in flowers.

    Function of vacuole:- In mature plant cells, vacuoles tend to be very large and are extremely important in providing structural support, as well as serving functions such as:-1 ) storage; 2) water storage; 3) hydrolytic enzymes; 4) osmoregulation and 5) turgor Pressure.Osmoregulation:- Most of the water in mature plant cells occursin the vacuoles Water enters the cell sap by osmosis(osmosis is diffusion of water across a membrane) through the differentially permeable tonoplast.water enters the cell (cell vacuole) and creates turgor pressure . Turgor pressure drives cell growth . Opening and closing of stomata is due to changes in turgor pressure of guard cells The pressure applied by the vacuole, called turgor, is necessary to maintain the size of the cell. Plant cells don't increase in size by expanding the cytosplasm, rather they increase the size of their vacuoles.

    Lysosomes (DE Duve, 1955)Lysosomes are a single membrane bound small vesicular organelles. Shape and size of lysosome is variable. A granulated stroma and a vacuole are located inside the membrane. Lysosomes are found in fungi and root tips of cells.Lysosomes are little sacs of hydrolyzing enzymes, acid hydrolases that are used by the cell to break down food as well as debris and the byproducts are used in biosynthetic activity of cells.. These sacs are separate from the cell because the enzymes could destroy the cell if they were mixed with the cytoplasm. Only the waste products or nutrients leave the lysosome. Lysosomes are involved amongst other things in the intracellular digestions of particles scavenged by some types of cell during endocytosis. The enzymes contained in the lysosome are synthesized on rough endoplasmic reticulum and are transported to golgi body.

    Lysosomes are of four types depending upon their function :-

    • Primary lysosome ,
    • Secondary lysosome,
    • Residual bodies,
    • Autophagic vacuoles.

    Its functions are defence against bacteria and viruses and in destroying old and worn out organelles. They have been found occasionally to be digesting the whole cell or part of cell(autolysis or self-destruction); therefore lysosomes are called suicide bags.

    Endoplasmic reticulum-Some of the floating membranes in the cytosol include a network of tiny sacs, tubules and vesicles that interconnect throughout the inside of the cell. This network is called endoplasmic reticulum(Porter et al.in1945 and Thompson in1945).According to the metabolic activities of cell endoplasmic reticulum assumes three forms in different cell :-


    image216

    The endoplasmic reticulum forms a continuous sheet enclosing internal space or a narrow lumen called endoplasmic reticulum lumen or endoplasmic reticulum cisternal space.

    Endoplasmic reticulum is of two types:- Rough endoplasmic reticulum(R.E.R)The membrane of endoplasmic reticulum on its external surface carries granular structures known as ribosomes, these regions are known as rough endoplasmic reticulum These ribosomes are made of protein and RNA and are actively involved in the synthesis of proteins.


    image237

    Smooth endoplasmic reticulum (SER), which has a tubular structure and no ribosomes attached to the membrane surface. It consists of smooth membrane segments and found in regions poor in protein synthesis.

    Function of endoplasmic reticulum The endoplasmic reticulum near the ribosomes is responsible for moving much of the protein through its tubules to other parts of the cell. and maintains the integrity of the membranes surrounding the nucleus. This is the conveyor belt of the cell. The large net work of endoplasmic reticulum provide increased surface for enzyme synthetic activity.

    Ribosomes - Ribosomes are small ,non membranous, spherical bodies mainly found bound to the endoplasmic reticulum as well as freely scattered throughout the cytoplasm, in all types of cells. Certain organelles within the cell, chloroplast and mitochondria, have their own distinct ribosomes as well.Ribosomes always have two subunits which interlock and behave as a single entity.It is composed of approximately 60 percent ribosomal RNA (rRNA) and 40 percent protein. Ribosomes are sometimes referred to as simply RNA. Like DNA, they are long chains of amino acids, but their base pairs are different and they are usually not as long as DNA.

    Function of ribosome:-Their main function is to produce a variety of proteins from simple genetic instructions which propagate outwards from the cellular nucleolus in the form of messenger RNA (mRNA). Ribosomes are actively involved in the protein synthesis, the process that generates organic tissue. Genetic instructions for the creation of new proteins come from mRNA. Ribosomes are thus a kind of protein-synthesis "machine." This process of using the information in RNA to make a protein is called translation; it is the complement of transcription. The protein products of the translation include the enzymes ("workers") of the cell. These proteins are shipped throughout the cell for functional uses.

    Golgi Apparatus (Golgi complex, orGolgi body or Dictyosomes) in 1898 by Camillo Golgi:- the Golgi body consists of a series of five to eight cup-shaped, membrane-bounded, flattened sacs. called cisternae. The Golgi body are surrounded by numerous, small, membrane-bounded vesicles. 60 cisternae may combine to make up the Golgi apparatus in some unicellular flagellates and the number of Golgi bodies in a cell varies according to its function. This complex is usually located close to the cell nucleus.


    image238

    Function of golgi body:-The Golgi body and its vesicles function in the sorting, modifying, and packaging of macro-molecules that are secreted by the cell or used within the cell for various functions. The Golgi complex in plant cells produces pectins and other polysaccharides specifically needed by for plant structure and metabolism. The products exported by the Golgi apparatus through the trans face eventually fuse with the plasma membrane of the cell. Among the most important duties of the Golgi apparatus is to sort the wide variety of macromolecules produced by the cell and target them for distribution to their proper location. Specialized molecular identification labels or tags, such as phosphate groups, are added by the Golgi enzymes to aid in this sorting effort. . Each Golgi body has two faces , a cis face- entry face, Golgi body receives macromolecules synthesized in the endoplasmic reticulum encased within vesicles. The trans face- exit face, the modified and packaged macromolecules are transported to their destinations in the form of smaller detached vesicles. The cis face is found near the endoplasmic reticulum, from where most of the material it receives comes, and the trans face is positioned near the plasma membrane of the cell, to where many of the substances it modifies are shipped.

    Mitochondria(singlular- mitochondrion; Benda, -1897 ) are rod-shaped, filamentous or granular structures distributed through the cytosol of most eukaryotic cells. Their number within the cell depends upon the metabolic activity of that cell, and may range from a single large mitochondrion to thousands of the organelles. Each mitochondrion is surrounded by two layered membranes forming 5 distinct compartments: outermembrane, intermembrane space, inner membrane, cristae space (formed by invaginations of the inner membrane), and the matrix (space within the inner membrane). Much of the activity of the mitochondria occurs within the inner mitochondrial membrane.


    image239

    The number of mitochondria in a cell can:-
    • Increase by their fission (e.g. following mitosis);
    • Decrease by their fusing together.

    The biochemical activities of the mitochondria are regulated by a set of enzymes.

    1. Outer membrane contains:- Monoamine oxidase, NADH-cytochrome C reductase, Kynurenine hydroxylase, Fatty acid Co. A ligase
    2. Intermembrane space contain Adenylate kinase and Nucleoside diphosphokinase
    3. The inner membrane contains 5 complexes of integral membrane proteins: NADH dehydrogenase (Complex I) succinate dehydrogenase (Complex II) cytochrome c reductase (Complex III; also known as the cytochrome b-c1 complex) cytochrome c oxidase (Complex IV) ATP synthase (Complex V)
    4. The matrix contains:- Malate and isocitrate dehydrogenases, Fumarase and aconitase, Citrate synthetase,a-keto acid dehydrogenase, b-oxidation enzymes.

    Function of mitochondria:- The most important role of mitochondria is to synthesize ATP with energy supplied by the electron transport chain and a process called oxidative phosphorylation, therefore mitochondria has been called power house of the cell.

    CH2O + O2 --------> CO2 + H2O + energy.

    This energy is produced through a series of steps. Each one of which is governed by an enzyme (present in cristae and matrix) . Carbohydrates, fat and proteins are broken into smaller molecules which involves(i)glycolysis (ii) oxidative decarboxylation and oxidative phosphorylation including (a)Kreb’s cycle and(b) respiratory chain.

    Subsections---"D"

    Plastids (E. Haeckel, 1866):-these are flat, circular, cytoplasmic cell organelles, only present in plant cells and photosynthetic protests.Which are primarily involved in formation and storage of soluble and insoluble carbohydrates.Plastids are two types on the basis presence or absence of pigment:-

    1. leucoplasts(plastids without pigments):- store reserve food material colourless and occur in the cells which are not exposed to sunlight and also found in embryonic cells, meristematic cells and parenchymatous cells .Leucoplasts are the centers of starch ,oils and proteins synthesis and storage, on that basis leucoplasts are of three types (a) Amyloplasts :-filled with starch found in endo (b)elaioplasts:- store lipids and (c) aleuroplast:- store protein crystals and granules and
    2. chromoplasts and chloroplasts(coloured) Chromoplasts are plastids with pigment colours other than green, their colour is due to two pigments, carotene and xanthophyll.Yellow-to-red colored chromoplasts manufacture carotenoids, These are present in petals and fruits, imparting them different colours(red, yellow etc.). Chromoplasts may develop from chloroplasts due to replacment of chlorophyll by other pigment e.g.green tomatoes and chillies turn red on ripening due to change of chlorophyll in chloroplasts by red pigment lycopin or lycopene in tomato and capsanthin in chillies.Or from leucoplasts by the development of pigments e.g., carrots.Chromoplasta are also involved in photosynthesis.The primary function in the cells of flowers is to attract agents of pollination, and in fruit to attract agents of dispersal

    Chloroplasts are the most important type of plastid having green pigment(chlorophyll), and are typically about 10 micrometers in diameter. The plastids are made up of an outer limiting membrane and inner matrix. The outer membrane is made up of two layers of lipoprotein and separated from one another by a space known as perplastidial space.The outer layer of the double membrane is much more permeable than the inner layer, which features a number of embedded membrane transport proteins. Inner matrix of a chloroplast is differentiated into grana, where light reaction of photosynthesis takes place and the stroma where dark reaction(Calvin cycle) is completed. Grana consist of the lamellar system and stroma is non-membranous both are consist of interconnected sac like structure known as thylakoids.Thylakoids are of two types (1)grana thylakoid and(2) intergranal thyalkoids. The thylakoids in each granum are continuous with those in other grana through intergranal thylakoids. In the chloroplast the thylakoids are embedded, or suspended, in a matrix, the stroma, which has a somewhat granular appearance .


    image230

    The stroma a semi fluid colorless colloidal complex contains DNA fibrils and ribosomes, starch grains, osmiophilic globules and occasional extensive crystal-like structures. The thylakoid membranes contains green pigment chlorophylla and b carotenoids, cytochrome, ATP-synthetase, etc.

    Function of chloroplasts:- Chloroplasts are specialized for photosynthesis, the biological conversion of light energy absorbed by chlorophylls, the green leaf pigments, into potential chemical energy such as carbohydrates. Photosynthesis is the process by which plants make food. In green plants, sunlight captured by chlorophyll enables carbon dioxide from the air to unite with water and minerals from the soil and create food. This process also releases oxygen into the air, that is utilised by people and animals . Energy from the sun splits water molecules into hydrogen and oxygen. The hydrogen joins with carbon from the carbon dioxide to produce sugar.

    6 CO2 + 6 H2O + energy (from sunlight) -------->C6H12O6 + 6 O2

    The sugar--together with nitrogen, sulfur, and phosphorus from the soil--helps a plant make the fat, protein, starch, vitamins, and other materials that it needs to survive.

    Nucleus(Robert Brown -1831):-The nucleus is the most prominent structureof cell.Nucleus is present only in cells of higher plants i.e. eukaryotes. Generally there is a single nucleus present in each cell, but there are exceptions also in some fungi e.g., Rhizopus etc. and algae e.g., Vaucheria more than one nucleus are present. Nucleus is absent in simpler one-celled plants -prokaryotes e.g., Viruses, bacteria and cynobacteria. The spherical-shaped nucleus, consists of a semi fluid matrix known as nucleoplasm in which one or more nucleoli, and chromatin threads are suspended. Nucleus controls cellular metabolism of cells and contains all genetic informations and is able to transmit it from one generation to the other. The nucleus is the most prominent of cell. The nuclei of cell consist of four components:-

      Nuclear membrane (or envelope) :-
    1. Nuclear membrane(Erclab-1845: Hertwig, 1893):-The nuclear membrane is a double-layered membrane that encloses the nucleus, and separates the contents of the nucleus from the cellular cytoplasm. The space between the layers is called the perinuclear space and connected with a network of tubules and sacs, called the endoplasmic reticulum, where protein synthesis occurs, and is usually studded with ribosomes. The membrane is perforated by numerous pores called nuclear pores. These pores regulate the flow of molecules between the nucleus and cytoplasm,The nuclear membrane is semi-permeable; permitting selective molecules to pass through the membrane,into and out of the nucleus . The inner nuclear membrane has a protein lining called the nuclear lamina,(Harris and James, 1952) which binds to chromatin and other nuclear components. The nuclear membrane disintegrates during cell division or mitosis, and is reformed as the two cells complete their formation and the chromatin begins to unravel and disperse. The function of the nuclear envelope is to confine the materials necessary for DNA and RNA synthesis inside the nucleus, and controlling movement into and out of the nucleus.
    2. Nucleoplasm(Strasburger, 1882):-The nucleoplasm is semi fluid, granular substance or matrix that fills the interior of the nucleus. The nucleolus and the chromatin network lie suspended in the nucleoplasm. This dense, spherical granule found in the nucleus contains RNA (ribonucleic acid) which is responsible for protein synthesis in the cytoplasm It is mostly composed of water, containing a complex assortment of materials. Nucleoplasm is distinct from cytoplasm due to the high concentration of materials like nucleotides, which are used to make DNA and RNA, and the suite of enzymes which control the DNA and RNA construction reactions .Nucleoplasm contains one or more nucleoli (singlur "nucleolus") , organelles that synthesize protein-producing macromolecular assemblies called ribosomes, and a variety of other smaller components, such as Cajal bodies, GEMS (Gemini of coiled bodies), and interchromatin granule clusters.


      image226

    3. Nucleolus(Bowman, 1848):-The nucleus contains one to four densely granular region called nucleoli (singlur "nucleolus"), but within each species the number of nucleoli is fixed.There is no membrane separating the nucleolus from the rest of the nucleus. The nucleolus, which is important in the formation of ribosomes, appears as a dense mass of RNA (ribonucleic acid), ribosomal RNA, chromatin, and proteins.When a cell reproduces the nucleolus disappearsand reappear after cell formation. The nucleolus made of three morphologically distinct components :-
      • Fibrillar centres (FC), where rRNA genes of several chromosomal loci (termed nucleolar organising regionsNORs) are located;the transcription of rRNA genes(RNA synthesis on DNA template) also takes place in this region.
      • Dense fibrillar component (DFC), which surrounds the fibrillar centrewhich contains actively transcribing rRNA genes and nascent rRNA transcripts; and
      • Granular component (GC),Which is the outermost region, which is the site of late processing events in the biogenesis of rRNAs.The rRNA which is a main element of ribosomes is created in the nucleolus, and the protein portion of the ribosome is synthesized in the cell's cytoplasm. These proteins enter the nucleus through the pores in the nuclear membrane and combine with the rRNA to form the small and large subunits. The completed subunits leave the nucleus and can be found in the cytoplasm or in an organelle known as the endoplasmic reticulum.

      Chromatin(Flemming, 1879) and Chromosomes(Waldeyer, 1888) The nucleus contains the complex of deoxyribonucleic acid (DNA) and associated proteins, known as chromatin in the uncondensed state and as chromosomes in the condensed stateThe chromatin is embedded in a clear matrix called the nucleoplasm. Chromatin are coiled strands of DNA that are found spread throughout the nucleus, that come together and coil tightly during cell replication.Each DNA strand wraps around groups of small protein molecules called histones, forming a series of bead-like structures, called nucleosomes, connected by the DNA strand.Histones are found only in nucleus and being basic protein interact strongly with deoxyribonucleic acid(DNA) The histone proteins are rich in lysine and arginine.


      image235

      Heterochromatin:- During interphase chromatin becomes dispersed, even then some of its portions remain tightly coiled and are called heterochromatin. Heterochromatin is found in parts of the chromosome where there are few or no genes and are generally inactive.

      Euchromatin:- Loosely coiled are called euchromatin Euchromatin is found in parts of the chromosome that contain many genes. The genes in euchromatin are active.This exposes the euchromatin and makes it available for the transcription process. Chromatin network:- These are very fine thread-like, coiled filaments uniformly distributed in the nucleoplasm.

      Chromosomes At the time of cell division, the chromatin becomes thick and ribbon like and are known as chromosomes.


      image233

      The nucleus of a eukaryotic cell contains a number of chromosomes, which are composed of DNA and histone proteins.A typical chromosomes has two similar parts called chromatids. They are joined to each other by primary constriction or Centromere. Depending upon position of Centromere chromosomes are:-


      image236

      The number, shape and arrangement of genes on chromosomes is characteristic of the species from which the nucleus came. Genes are responsible for storing and transmitting hereditary characteristics from one generation to another. A gene is the functional unit of a chromosome. Genes are arranged in single linear order along the chromosome. One gene may be responsible for a single characteristic, or a single characteristic may be transmitted by a set of genes.Each gene is a set of instructions for the construction of a specific protein.

     

      Quiz

    Which is not correct ?

    1. Robert Brown discovered the cell
    2. Schleiden and Schwann formulated the cell theory
    3. Virchow explained that cells are formed from pre-existing cells
    4. A unicellular organism carries out its life activities within a single cell

    Ans A

    New cells develop from

    1. Bacterial fermentation
    2. Regeneration of old cells
    3. Pre-existing cells
    4. Abiotic material

    Ans C

    Prokaryotic cells are characterized by

    1. Distinct nuclear membrane
    2. Distinct chromosomes
    3. Absence of chromatin material
    4. Absence of nuclear membranes

    Ans D

    Which of the following cell organelle play the most significant role in protein synthesis ?

    1. Lysosome and vacuole
    2. Endoplasmic reticulum and Ribosome
    3. Golgi apparatus and Mitochondria
    4. Lysosome and mitochondria

    Ans B

    Which organelle in the cell, other than nucleus, contains DNA ?

    1. Endoplasmic reticulum
    2. Lysosome
    3. Golgi apparatus
    4. Mitochondria

    Ans D

    The function of a cell wall is

    1. To give definite shape to the cell
    2. To provide mechanical strength and protection to the cell
    3. To prevent the cell from desiccation
    4. All of the above

    Ans D

    Cell membrane is

    1. Permeable
    2. Selectively permeable
    3. Semipermeable
    4. Impermeable

    Ans C

    The powerhouse of a cell is the

    1. Plastid
    2. Chloroplast
    3. Golgi apparatus
    4. Mitochondria

        Practice questions

        Assignments

        Readings

      [Your opinion is important to us. If you have a comment, correction or question pertaining to this chapter please send it to appropriate person listed in contact information or visit forums for this course.]

      Previous: Introduction to Botany Next: The Plant Cell

      21