Plant Cell-an introduction

CHAPTER: 2

Plant Cell-an introduction

Plant Cell:

•      Cell is the basic Structural and functional unit of living organism. It consists of microscopic mass of protoplasm surrounded by a cell membrane.

•      It is the smallest unit that can carry on all the process of life.

•      In a unicellular organisms, the single cell performs all metabolic functions such as growth , respiration, reproduction , digestion, excretion, etc.

•      In multicellular  organisms all the vital activities are the result of combined action of these cells. Hence, cell is considered as a basic unit of life.

•      From the physiological point of view, a plant cell may be defined as an organized mass of protoplasm constituting the physiological unit of the plant, capable of exhibiting all the characteristics of life.

Structure of plant cell:

•      Plant cells are usually rectangular in shape.

•      In a plant cell the cell membrane forms the outermost covering of the cytoplasm beneath cell wall.

•      The protoplasm consists of the cytoplasm and the nucleus.

•      The cytoplasm consists several living organelles and non-living inclusions.

Ø  The structure of plant cell can be described under following headings:

1. Cell wall:

•      The cell walls of plants are generally thick, strong, porous ,rigid and non- 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 cell wall is primarily made up of cellulose and pectin, but some additional materials are deposited for specific functions.

Structure of cell wall :

Ø  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.

2. Secondary wall -After maturity, more layers of cells are added on the inner side of the primary wall formed secondary cell wall.

    3. Middle lamella -It is the first structure that is formed from cell plate between the newly formed daughter cells at the time of cytokinesis.

Functions of Cell Wall :-

•       Cell wall provides shape and rigidity to cell.

•       It protects the protoplasm from external injury.

2. Cell membrane (Plasma membrane):

•      All living cells are enclosed by a selectively permeable, thin molecular layer termed as plasma membrane or plasmalemma.

•      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.

•      The carbohydrates occur only at the outer surface of the membrane.

Function of plasma membrane :

•      Cell membrane of 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.

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

3.Cytoplasm:

•      It 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..

4.Vacuole:-

Ø  A vacuole is a large fluid-filled sac in the cytoplasm, bounded by a single membrane called the tonoplast.

Ø  Vacuoles are filled with watery fluid termed as cell sap. It is acidic.

Ø  The chemical composition of cell sap differs markedly from that of the surrounding cytoplasm.

Ø  In most cases mature plant cells have single large central vacuole.

Ø  So that the plant cytoplasm lies as a thin layer positioned between the plasma membrane and the tonoplast.

Ø  The vacuole contains nutrients, metabolites, pigments and waste products. Pigment anthocyans are present ,which is responsible for the colors in flowers.

Function of vacuole:-

1. structural support, 2. water storage;  3. hydrolytic enzymes; 4. osmoregulation

5. Storage,

6. Turgor Pressure: 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

5. Lysosomes :

•      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.

•      The enzymes contained in the lysosome are synthesized on rough endoplasmic reticulum and are transported to golgi body.

•      Its functions are defense 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.

6. Endoplasmic reticulum:

Ø  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.

 Ø  Endoplasmic reticulum is of two types:- 

1. 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.
2. Smooth endoplasmic reticulum (SER): It 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: 

Ø  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.

Ø  This is the conveyor belt of the cell.

Ø  The large net work of endoplasmic reticulum provide increased surface for enzyme synthetic activity.

7. 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.

•      Ribosomes always have two subunits which interlock and behave as a single entity.

•      The larger subunits is called corn or dome-shaped and the smaller subunit is cap –like or oblate.

•      In plant cell the larger subunit is of 60s type and smaller subunit is 40s type.

•      It is composed of approximately 60 percent ribosomal RNA (rRNA) and 40 percent protein.

•      Ribosomes are sometimes referred to as simply RNA.

Function of ribosome:-

Ø  Ribosomes are actively involved in the protein synthesis, the process that generates organic tissue. Ribosomes are thus a kind of protein-synthesis "machine." This process of using the information in RNA to make a protein is called translation.

8. Golgi Apparatus (Golgi complex, or Golgi body or Dictyosomes) :

•      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.

Function of golgi body:-

•      Its functions are 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 pectin and other polysaccharides specifically needed by for plant structure and metabolism.

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.

9. Mitochondria:

•      These 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.

Structure:

•      The mitochondrion is bound by double layered (outer & inner) membrane.

•      The membranes are lipo-protein in nature.

•      The outer membrane is smooth but the inner membrane folds inwards at place to form plate like or finger like, may be branched or unbranched structure, the cristae.

•      The cristae increases metabolic surface area of the mitochondria and contains all enzymes for kreb’s cycle.

•      The cavity of mitochondrion contains homogenous granular matrix. The matrix contains lipids, proteins, DNA, RNA, ribosomes, granules and enzymes.

•      The outer surface of outer membrane and inner surface of inner membrane contain numerous minute structures called oxysomes or elementary particles.

•      The oxysomes present on the outer surface are simple or globular, whereas the oxysomes present on the inner surface contain a base, a stalk and a spherical head.

•      Oxysomes are proteinaceous in nature.

•      The chemical composition of mitochondria on the dry weight basis shows protein(65-70%), lipid (25-30%), RNA (0.5%) and small amount of DNA.

Function of mitochondria:

1. The mitochondria are the centers of respiratory activity and contain all necessary enzymes.
2. Kreb’s cycle takes place in the mitochondria during which food material is completely oxidized into CO₂ and H₂O.
3. The oxysomes lining the cristae and inner membrane function as electron carriers or transporters of hydrogen during kreb’s cycle.
4. Mitochondria can also synthesize structural proteins due to the presence of DNA and RNA.
5. Mitochondria are self replicating and they are passed on from cell to cell.

10. Plastids:-

•      These are flat, circular, cytoplasmic cell organelles, only present in plant cells and photosynthetic organisms.

•      These 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):-

Ø  Store reserve food material, colorless and occur in the cells which are not exposed to sunlight and also found in embryonic cells, meristematic cells and parenchymatous cells .

Ø  Leucoplasts are the centers of starch ,oils and proteins synthesis.

Ø   On that basis of storage, leucoplasts are of three types:

 (a) Amyloplasts :-filled with starch

 (b) elaioplasts:- store lipids

 (c) aleuroplast (proteinoplast):- store protein crystals and granules

2. chromoplasts and chloroplasts(coloured) :-

1. Chromoplasts: 

Ø  Chromoplasts are plastids with pigment colours other than green, their colour is due to two pigments, carotene and xanthophyll.

Ø  Yellow-to-red colored chromoplasts manufacture carotenoids. These are present in petals and fruits, imparting them different colours(red, yellow etc.).

Ø  Chromoplasts may develop from chloroplasts due to replacment of chlorophyll by other pigment.

Ø  For e.g. green tomatoes and chilies turn red on ripening due to change of chlorophyll in chloroplasts by red pigment lycopin or lycopene in tomato and capsanthin in chilies.

2. Chloroplasts: 

Ø  Chloroplasts are the most important type of plastid having green pigment(chlorophyll), and are typically about 10 micrometers in diameter.

Ø  The plastids are made up of an outer limiting membrane and inner matrix.

Ø  The outer membrane is made up of two layers of lipoprotein and separated from one another by a space known as perplastidial space.

Ø  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.

Ø  Grana consist of the lamellar system and stroma is non-membranous both  consist of interconnected sac like structure known as thylakoids.

Ø  Thylakoids are of two types (1)grana thylakoid and(2) intergranal thyalkoids.

Ø  The thylakoids in each granum are continuous with those in other grana through intergranal thylakoids.

Ø  In the chloroplast the thylakoids are embedded, or suspended, in a matrix, the stroma, which has a somewhat granular appearance .

Functions of plastids:

Ø  Leucoplasts are mainly concerned with storage of various kinds of reserve food materials.

Ø  Chromoplasts make flowers and fruits showy and attractive. The flowers attract insects and other animals, which help in pollination and dispersal of fruits and seeds.

Ø  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.

11. Nucleus:

•      The nucleus is the most prominent structure of 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, bacteriaand 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 information 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:-

1. Nuclear Membrane, 2. Nucleoplasm,

      3. Nucleolus,              4. Chromatin.

1. Nuclear membrane:-

•      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.

•      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, which binds to chromatin and other nuclear components.

•      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:-

•      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.

•      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 .

3.  Nucleolus:-

•      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 disappears and reappear after cell formation.

Ø  The nucleolus is composed of four regions:-

1. Granular regions: contains ribonucleoproteins granules
2. Fibrillar regions: contains ribonucleoproteins fibrils
3. Amorphous regions: Contains amorphous protein that hydrolyze the pepsin.
4. Chromatin fibrils: Perinuclear chromatin and intranuclear chromatin are also associated with nucleolus.

4.         Chromatin and Chromosomes:-

•      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 state.

•      The 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.

12. Cytoskeleton:-

•      A complex network of fibrous structures in the cytoplasm is called cytoskeleton. They extend throughout the cytoplasm and maintain the shape of cells. They are concerned with structural frame-work and directly involved in movement of cells. They also provide the machinery for the cyclosis in cytoplasm. Two primary kinds of cytoskeleton are as follows:

1. Microfilaments: are long, thin, very fine filament of protein actin. It plays major role in cyclosis and amoeboid motion. They also help in muscle contarctuin as well as help in cell migration during embryonic development.
2. Microtubules:  are long, hollow, cylindrical, unbranched, tubules. Its major role is to make structural frame-work of cell and maintain the shape of the cell. They also help in intracellular transport of materials.

13. Cytoplasmic Inclusions (Ergastic Substances):-

•      As a result of metabolic activities several kinds of non-living substances are produced within the cell, which are called cytoplasmic inclusions or ergastic substances. They may be in solution, colloidal or solid crystal form.

Ø  Basically, these are classified into three types:

1. Reserve Materials: Sugars, starch, Inulin, Pectin, Cellulose, Glycogen, Nitrogenous matters, Fats and oils.
2. Excretory Materials:  Tannins, Resins, Latex, Gums, Alkaloids, etc
3. Secretory Materials:  Nectar, Anthocyanins and anthoxanthins, Hormones