Structure and Functions of Cell Organelles

Cell as a unit of life

Cytology : (Gk Kyios = cell logas = study) It is the branch of biology. Which comprises the study of cell structure and function. “Cell is the structural and functional unit of all living beings”. Study of metabolic aspects of cell components is called cell biology.

Robert Hooke (1665) discovered hollow cavities (empty boxes) like compartments in a very thin slice of cork (cell wall) under his microscope. He wrote a book “Micrographia” and coined the term cellula, which was later changed into cell. Grew and Malpighi also observed small structures in slice of plants and animals. Leeuwenhoek was the first to see free cells and called them “wild animalcules” and published a book “The secret of nature”. He observed bacteria, protozoa, RBCs, sperms, etc. under his microscope.

Cell theory : H.J. Dutrochet (1824) a French worker gave the idea of cell theory.

The actual credit for cell theory goes to two German scientists, a Botanist M.J. Schleiden (1838) and a Zoologist T. Schwann (1839). They gave the concept 𠇊ll living organisms are composed of cell”. Schleiden and Schwann both supported the theory of “spontaneous generation”. They also mentioned that “the new cell arises from nucleus by budding”.

Exceptions to the cell theory : Viruses, viroids and prions are an exception to the cell theory as they are obligate parasites (sub-cellular in nature).

Modification of cell theory : Modification of cell theory was done by Rudolf Virchow (1855). He proposed the “law of cell lineage” which states that cell originates from pre-existing cells. i.e., (omnis cellula-e-cellula). It is also called �ll principle” or �ll doctrine”. It states :  

(1) Life exists only in cells.

(2) Membrane bound cell organelles of the protoplasm do not survive alone or outside the protoplasm.

(3) Cells never arise de novo. The new cells are like the parent cell in all respect.

(4) All cells have similar fundamental structure and metabolic reactions.

(5) Cells display homeostasis and remain alive.

(6) Genetic information is stored in DNA and expressed within the cells.

(7) DNA controls structure and working of a cell.

The cell as a self contained unit : Autonomy of a cell is believed due to presence of DNA and its expressibility, otherwise, cell components have different shape and function. It has two positions.

(1) Autonomy in unicellular organisms : Unicellular organisms leads to a totally independent life due to different shape, size and role of different organelles shows division of labour. All these display homeostasis. Unicellular organisms are more active due to large surface volume ratio.

(2) Autonomy in multicellular organisms : In multicellular organisms life activities are displayed by each of the cells independently. Multicellular organisms have one thing advantage over unicellular organisms is division of labour.

Cellular totipotency : Totipotency was suggested by Haberlandt (1902). When cells have tendency or ability to divide and redivide the condition of the cell is called totipotent and this phenomenon is called totipotency. Steward et.al. showed the phenomenon of cellular totipotency in carrot culture.

Surface volume ratio : Metabolically active cells are small, as small cells have higher nucleocytoplasmic ratio for better control and higher surface volume ratio for quicker exchange of materials between the cell and its outside environment.  Larger cells have lower surface volume ratio as well as lower nucleocytoplasmic ratio. Surface volume ratio decreases if cell size increases.

Types of cells

Chatton gave the term prokaryote and eukaryote. Depending upon the nature of nucleus cells are classified. Incipient nucleus is present in prokaryotes, where as in eukaryotes well organised nucleus is present.

Mesokaryon : Dodge gave the term ‘Mesokaryon’ for dinoflagellates. These are intermediate type of cell organisation in dinophyceae of algae. In mesokaryotic there is present a true or eukaryotic nucleus with definite nuclear membrane and chromosomes.

Cell wall

Discovery : It was first discovered by Robert Hooke in 1665 in Cork. Cell wall is the outer most, rigid, protective, non living and supportive layer found in all the plant cells, bacteria, cyanobacteria and some protists. It is not found in animal cells.

Chemical composition : Mainly cell wall consists of two parts, matrix and cellulosic fibres (microfibrils). Matrix consists of hemicellulose, pectin, glycoproteins, lipids and water.In most of the plants cell wall is made up of cellulose $(C_6{H}_{10}{O}_5{)}_n,$a polymer made-up of unbranched chain of glucose molecule linked by $\beta ,1-4$glycosidic bond. About 100 molecules of cellulose form a micelle, about 20 micelle form a microfibril and approx 200 microfibril form a fibril.

The cell wall of bacteria and the inner layer of blue green algae is made-up mucopeptide. It is a polymer of two amino sugars namely N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) held alternately in $\beta$ -1,4- linkage. In higher fungi, the cell wall is made up of chitin, polymer of glucosamine.

Structure : Cell wall consists of middle lamella, primary wall, secondary wall, tertiary wall.

(1) Middle lamella : Middle lamella is the outermost region which functions as a common cementing layer between two cells. It is absent on the outer free surface. It ruptures to create intercellular spaces. Middle lamella is formed of calcium and magnecium pectate. Fruit softening is due to gelatinisation of pectic compounds of middle lamella. Pectin is used as commercial jellying agent. Which is present outside the primary wall.

(2) Primary wall : A young plant cell forms a single layer of wall material. This layer is known as the primary cell wall. The primary wall is thin, elastic and capable of expansion in a growing cell. It grows by intussusception. Meristematic and parenchymatous cells have primary cell wall only. The cells of leaves and fruits too have only primary wall. It has more hemicellulose and less cellulose.

(3) Secondary wall : In mature cell, more layers of wall material are added internal to the primary wall. These are called the secondary cell wall. Growth by addition of new wall material on the primary wall is called accretion. It has more cellulose and less hemicellulose. The secondary wall is thick and rigid. It usually consists of three layers, which are often named $S_1,S_2\mathrm{and}{\mathrm{S}}_3.$ It is found in collenchyma and sclerenchyma cells, xylem vesseles.

(4) Tertiary wall : Sometimes tertiary wall is laid down on secondary wall, e.g., tracheids of gymnosperms. It is composed of cellulose and xylan.

Origin : ਊ cell wall is originate at telophase stage of cell division. The plane and place of cell wall is determined by the microtubules. Fragments of ER and vesicles of golgi body alligned at the equator, called as phragmoplast, later which forms the cell plate. The synthesis of cellulose takes place by the help of enzyme cellulose synthase present in the plasma membrane. The cell plate forms the cell wall.

Growth of cell wall

(1) By intussuception : As the cell wall stretches in one or more directions, new cell wall material secreted by protoplasm gets embedded within the original wall.

(2) By apposition : In this method new cell wall material secreted by protoplasm is deposited by definite thin plates one after the other.

Thickenings of cell wall : In many secondary walls specially those of xylem the cell wall becomes hard and thick due to the deposition of lignin. With the increasing amount of lignin, deposition protoplasm is lost. First the lignin is deposited in middle lamella and primary wall and later on in secondary wall.

Pits : Secondary walls may have irregular thickenings at some places and these places are called pits. Pits are of five types :

(1) Simple pit : In which pit chamber is uniform in diameter.

(2) Bordered pit : In which pit chamber is flask shaped in tracheids of gymnosperm and vessels of angiosperms.

(3) Blind pit : A pit without any corresponding pit on the adjacent wall is called blind pit.

(4) Half bordered pit : A pit with half border and the rest half with a simple pit.

(5) Aspirated pit : It is a non-functional pit in which the pit aperture is blocked permanently by torus. 

Plasmodesmata : Tangle (1879) first of all discovered them and were studied elaborately by Strasburger (1901). A number of plasmodesmata or cytoplasmic strands are present in pit through which the cytoplasm of one cell is in contact with another. Endoplasmic reticulum plays a role in origin of plasmodesmata.

Functions of cell wall

(1) It maintains shape of the plant cells and protect the cells from mechanical injury.

(2) It wards off the attacks of pathogens (viruses, bacteria, fungi, protozoans).

(3) It provides mechanical support against gravity. It is due to the rigid cell wall that the aerial parts of the plants are able to keep erect and expose their leaves to sunlight.

(4) The cell wall prevents undue expansion of the cell when water enters by osmosis to compensate for the lack of contractile vacuole. This prevents bursting of cells.

(5) Though permeable, the cell wall plays some regulatory role on the passage of materials into and out of the cell.

(6) Pores in the cell walls permit plasmodesmata to link up all the protoplasts into a system called symplast (symplasm).

(7) Cell wall and intercellular spaces constitute a nonliving component of plant body known as apoplasm.

Plasma membrane

Every living cell is externally covered by a thin transparent electron microscopic, elastic regenerative and selective permeable membrane called plasma membrane. It is quasifluid in nature. Membranes also occur inside the cells. They are collectively called biomembranes. The term cell membrane was given by C. Nageli and C. Cramer (1855) for outer membrane covering of the portoplast. It was replaced by the term plasmalemma by Plower (1931).

Chemical composition : Proteins lipoprotein (Lipid +Protein) are the major component forming 60% of the plasma membrane. Proteins provide mechanical strength and responsible for transportation of different substances. Proteins also act as enzyme. Lipids account may 28%-79% depending upon the type of cell and organism involved (in humans, myelin 79%). The lipids of plasma membrane are of three types namely phospholipids, glycolipids and sterols. The sterol found in the membrane may be cholesterol (Animals), phytosterol (Plants) or ergosterol (Microorganisms).

Carbohydrates form 2%-10%. Oligosaccharides are the main carbohydrates present in plasma membrane. The carbohydrates of plasma membrane are covalently linked to both lipid and protein components.

Ultrastructure : Under electron microscope the plasma membrane appears three layered, i.e., trilaminar or tripartite. One optically light layer is of lipid and on both sides two optically dense protein layers are present.

Molecular structure and different models : Several models have been proposed to explain the structure and function of the plasma membrane.

(1) Overton’s model : It suggests that the plasma membrane is composed of a thin lipid single layer.

(2) Sandwitch model : It was proposed by Davson and Danielli (1935). According to this model the light biomolecular lipid layer is sandwitched between two dense protein layers (globular $\alpha$ type protein). This model was also said to be unit membrane hypothesis.

(3) Robertson’s unit membrane model : It states that all cytoplasmic membranes have a similar structure of three layers with and electron transparent phospholipid bilayer being sandwitched between two electron dense layer of proteins (extended or $\beta$ type protein).

Its thickness is about 75 Å with a central lipid layer of 35 Å thick and two peripheral protein layers of 20 Å thick.

(4) Fluid mosaic model : The most important and widely accepted latest model for plasma membrane was given by Singer and Nicolson in 1972. According to them it is “protein iceberg in a sea of lipids.”

According to this model, the cell membrane consists of a highly viscous fluid matrix of two layers of phospholipid molecules. Protein molecules occur as separate particles asymmetrical arranged in a mosaic pattern.

Some of these are loosely bound at the polar surfaces of lipid layers, called peripheral or extrinsic proteins. Others penetrate deeply into the lipid layer called integral or intrinsic proteins. Some of the integral proteins penetrate through the phospholipid layers and project on both the surface. These are called trans membrane or tunnel proteins (glycophorins). Singly or in groups, they function as channels for passage of water ions and other solutes.

The carbohydrates occur only at the outer surface of the membrane. Their molecules are covalently linked to the polar heads of some lipid molecules (forming glycolipids) and most of the proteins exposed at outer surface (forming glycoproteins). Through glycoproteins, bacteria recognise each other. e.g., female bacteria are recognised by male bacteria.

Modification of plasma membrane

(1) Microvilli : They are fingers like evaginations of 0.1 $\mu m$ diameter, engaged in absorption. e.g., intestinal cells, hepatic cells, mesothelial cells. The surface having microvilli is called striated border or brush border.

(2) Lomasomes : They are plasmalemma foldings found in fungal cells. These were reported by Moore and Maclean.

(3) Mesosomes : It serves as site for cellular respiration in prokaryotes.

(4) Tight junctions or (Zonulae occludents) : Plasma membrane of two adjacent cells are fused at a series of points with a network of ridges or sealing strands. e.g., capillaries, brain cells collecting tubules etc.

(5) Desmosomes : Concerned with cell adherence.

(6) Transosomes : It is found in follicular cells of ovary of birds and have triple unit membrane. First reported by Press (1964).

Functions

(1) It is not only provides mechenical strength but also acts as a protective layer.

(2) Plasma membrane is responsible for the transportation of materials, molecules, ions etc.

(3) It helps in osmoregulation.

(4) Diffusion of gases (O₂ and CO₂) take place through plasma membrane by simple and facilitated diffusion.

(5) Water as well as some solute molecules and ion pass through membranes pores pores are always bordered by channel proteins.

Membrane transport

It is passage of metabolites, by-products and biochemicals across biomembrane. Membrane transport occurs through four methods-passive, facilitated, active and bulk. Size of the particles passing through plasmalemma is generally 1 - 15 Å.

Passive transport : No energy spent. Passive transport occurs through diffusion and osmosis.

(1) Diffusion : It is movement of particles from the region of their higher concentration or electrochemical potential to the region of their lower concentration or electrochemical potential. Electrochemical potential operates in case of charged particles like ions. Simple diffusion does not require carrier molecules.

(2) Osmosis : It is diffusion of water across a semipermeable membrane that occurs under the influence of an osmotically active solution.

Mechanism of passive transport : Passive transport can continue to occur if the absorbed solute is immobilised. Cations have a tendency to passively pass from electropositive to electronegative side. While anions can pass from electronegative to electropositive side. There are two modes of passive transport.

(1) Lipid matrix permeability : Lipid soluble substances pass through the cell membrane according to their solubility and concentration gradient, e.g., triethyl citrate, ethyl alcohol, methane.

(2) Hydrophillic membrane channels : They are narrow channels formed in the membrane by tunnel proteins. The channels make the membrane semipermeable. Water passes inwardly or outwardly from a cell through these channels according to osmotic gradients. $C{O}_2$ and $O_2$ also diffuse through these channels as per their concentration gradients.

Facilitated transport or Facilitated diffusion : It is passage of substances along the concentration gradient without expenditure of energy that occurs with the help of special permeating substances called permeases. Permeases form pathways for movement of certain substances without involving any expenditure of energy. Facilitated transport occurs in case of some sugars, amino acids and nucleotides.

Active transport : It occurs with the help of energy, usually against concentration gradient. For this, cell membranes possess carriers and gated channels. At times certain substances are transported alongwith the ones requiring active transport. The latter phenomenon called cotransport.

(1) Carrier particles or Proteins : They are integral protein particles which have affinity for specific solutes. A solute particles combines with a carrier to form carrier solute complex. The latter undergoes conformational change in such a way as to transport the solute to the inner side where it is released into cytoplasm.

(2) Gated channels : The channels are opened by either change in electrical potential or specific substances, e.g., Calcium channels.

Active transport systems are also called pumps. The pumps operate with the help of ATP.$K^{+}-H^{+}$exchange pump occurs in guard cells. $N{a}^{+}-K^{+}$exchange pump operates across many animal membranes.

Active transport of one substance is often accompanied by permeation of other substances. The phenomenon is called secondary active transport. It is of two main types, cotransport (e.g., glucose and some amino acids alongwith inward pushing of excess $N{a}^{+})$ and counter-transport $(C{a}^{2+}$and $H^{+}$movement outwardly as excess $N{a}^{+}$passes inwardly).

Bulk transport : It is transport of large quantities of micromolecules, macromolecules and food particles through the membrane. It is accompanied by formation of transport or carrier vesicles. The latter are endocytotic and perform bulk transport inwardly. The phenomenon is called endocytosis. Endocytosis is of two types, pinocytosis and phagocytosis. Exocytic vesicles perform bulk transport outwardly. It is called exocytosis. Exocytosis performs secretion, excretion and ephagy.

(1) Pinocytosis : (Lewis, 1931). It is bulk intake of fluid, ions and molecules through development of small endocytotic vesicles of 100 - 200 nm in diameter. ATP, $C{a}^{2+},$fibrillar protein clathrin and contractile protein actin are required. Fluid-phase pinocytosis is also called cell drinking. After coming in contact with specific substance, the area of plasma membrane having adsorptive sites, invaginates and forms vesicle. The vesicle separates. It is called pinosome. Pinosome may burst in cytosol, come in contact with tonoplast and pass its contents into vacuole, form digestive vacuole with lysosome or deliver its contents to Golgi apparatus when it is called receptosome.

(2) Phagocytosis : (Metchnikoff, 1883). It is cell eating or ingestion of large particles by living cells, e.g., white blood corpuscles (neutrophils, monocytes), Kupffer’s cells of liver, reticular cells of spleen, histiocytes of connective tissues, macrophages, Amoeba and some other protists, feeding cells of sponges and coelenterates. Plasma membrane has receptors. As soon as the food particle comes in contact with the receptor site, the edges of the latter evaginate, form a vesicle which pinches off as phagosome.

One or more lysosomes fuse with a phagosome, form digestive vacuole or food vacuole. Digestion occurs inside the vacuole. The digested substances diffuse out, while the residual vacuole passes out, comes in contact with plasma membrane for throwing out its contents through exocytosis or ephagy.

Protoplasm (Proto = first, plasm = fluid)

Protoplasm is a complex, granular, elastic, viscous and colourless substance. It is selectively or differentially permeable. It is considered as “Polyphasic colloidal system”.

Discoveries

(1) J. Huxley defined it as “physical basis of life”.

(2) Dujardin (1835) discovered it and called them “sarcode”.

(3) Purkinje (1837) renamed it as “Protoplasm”.

(4) Hugo Von Mohl (1844) gave the significance of it.

(5) Max Schultz (1861) gave the protoplasmic theory for plants.

(6) Fischer (1894) and Hardy (1899) showed its colloidal nature.

(7) Altman (1893) suggested protoplasm as granular.

Maximum water content in protoplasm is found in hydrophytes, i.e., 95% where as minimum in seeds, spores (dormant organs) i.e., 10 - 15%. In animals water is less (about 65%) and proteins are more (about 15%).

Properties of protoplasm

(1) Cyclosis movement : These are shown by protoplasm. These are of two types.

Rotation : In one direction, either clockwise or anticlockwise e.g., Hydrilla, Vallisneria. Found only in eukaryotes.

Circulation : Multidirectional movements around vacuole e.g., Tradescantia.

(2) It shows stimulation or irritability, Sol-gel transformation and Brownian movements.

(3) It is highly viscous and coagulates at 60^o C or above or if treated with concentrated acids or bases.

(4) It’s pH is on acidic side, but different vital activities occur at neutral pH which is considered as 7, injury decreases the pH of the cell (i.e., 5.2 - 5.5) and if it remains for a long time, the cell dies.

Cytoplasm

The substance occur around the nucleus and inside the plasma membrane containing various organelles and inclusions is called cytoplasm.

(1) The cytoplasm is a semisolid, jelly - like material. It consists of an aqueous, structureless ground substance called cytoplasmic matrix or hyaloplasm or cytosol.

(2) It forms about half of the cell’s volume and about 90% of it is water.

(3) It contains ions, biomolecules, such as sugar, amino acid, nucleotide, tRNA, enzyme, vitamins, etc.

(4) The cytosol also contains storage products such as glycogen/starch, fats and proteins in colloidal state.

(5) It also forms crystallo - colloidal system.

(6) Cytomatrix is differentiated into ectoplasm or plasmagel (outer) and endoplasm or plasmasol (inner).

(7) Cytomatrix is three dimensional structure appear like a network of fine threads and these threads are called microfilaments (now called actin filaments or microtrabecular lattice) and it is believed to be a part of cytoskeleton. It also contains microtubules and inter mediate cytoplasmic filaments.

(8) Hyaloplasm contains metabolically inactive products or cell inclusions called deutoplast or metaplasts.

(9) Cytoplasmic organelles are plastid, lysosome, sphaerosome, peroxisome, glyoxysomes, mitochondria, ribosome, centrosome, flagellum or cilia etc.

(10) The movement of cytoplasm is termed as cyclosis (absent in plant cells).

Mitochondria

Mitochondria (Gk. Mito = thread chondrion = granule) are semi autonomous having hollow sac like structures present in all eukaryotes except mature RBCs of mammals and sieve tubes of phloem. Mesosomes of prokaryotes (bacteria) is analogous to mitochondrion in eukaryotes.

Mitochondria are also called chondriosome, chondrioplast, plasmosomes, plastosomes and plastochondriane.

Discoveries

(1) These were first observed in striated muscles (Voluntary) of insects as granules by Kolliker (1880), he called them “sarcosomes”.

(2) Flemming (1882) called them 𠇏ila” for thread like structure.

(3) Altman (1890) called them 𠇋ioplast”.

(4) C. Benda (1897) gave the term mitochondria.

(5) F. Meves (1904) observed mitochondria in plant (Nymphaea).

(6) Michaelis (1898) demonstrated that mitochondria play a significant role in respiration.

(7) Bensley and Hoerr (1934) isolated mitochondria from liver cells.

(8) Seekevitz called them “Power house of the cell”.

(9) Nass and Afzelius (1965) observed first DNA in mitochondria.

Number of mitochondria : Presence of mitochondria depends upon the metabolic activity of the cell. Higher is the metabolic activity, higher is the number e.g., in germinating seeds.

(1) Minimum number of mitochondria is one in Microasterias, Trypanosoma, Chlorella, Chlamydomonas (green alga) and Micromonas. Maximum numbers are found (up to 500000) in flight muscle cell, (up to 50000) in giant Amoeba called Chaos - Chaos. These are 25 in human sperm, 300 - 400 in kidney cells and 1000 - 1600 in liver cells.

(2) Mitochondria of a cell are collectively called chondriome.

Size of mitochondria : Average size is 0.5-1.00 $\mu$m and length up to 1 - 10 $\mu$ m. Smallest sized mitochondria in yeast cells $(1\mu m^3).$ and largest sized are found in oocytes of Rana pipiens and are 20 - 40 $\mu m.$

Ultrastructure : Mitochondrion is bounded by two unit membranes separated by perimitochondrial space (6 - 10nm wide). The outer membrane is specially permeable because of presence of integral proteins called porins. The inner membrane is selective permeable. The inner membrane is folded or convoluted to form mitochondrial crests. In animals these are called cristae and in plants these folding are called tubuli or microvili.

The matrix facing face is called ‘M’ face and face towards perimitochondrial space is called 𠆌’ face. The ‘M’ face have some small stalked particles called oxysomes or F₁ particle or elementory particle or Fernandez - Moran Particles (10⁴-10⁵ per mitochondria). Each particle is made up of base, stalk and head and is about 10nm in length.

Oxysomes have ATPase enzyme molecule (Packer, 1967) and therefore, responsible for ATP synthesis. These elementary particles are also called F₀ - F₁ particles. The F₁ particle is made up of five types of subunits namely $\alpha ,\beta ,\gamma ,\delta$and $\epsilon .$of these $\alpha$is heaviest and$\epsilon$is lightest. F₀ particles synthesize all the enzymes required to operate Kreb’s cycle.

Semi-autonomous nature of mitochondrion : Mitochondria contain all requirements of protein synthesis :

(1) 70 S ribosomes.

(2) DNA molecules (rich in G-C ratio) to form mRNA and also replicate.

(3) ATP molecules to provide energy.

The mitochondria can form some of the required proteins but for most of proteins, these are dependent upon nuclear DNA and cytoplasmic ribosomes, so the mitochondria are called semi-autonomous organelles.

According to endosymbiotic origin of mitochondria by Kirns Altman, mitochondria were intially a free living, aerobic bacteria which during to the process of evolution entered an anaerobic cell and become established as mitochondria. This theory is supported by many similarities which exist between bacteria and mitochondria.

Chemical composition : Cohn gave the chemical composition of mitochondrion :

Proteins = 65 - 70% Lipids = 25 - 30% (90% phospholipids and 10% cholesterol, Vit. E., etc.) RNA = 5 - 7%. Some amount of DNA 2 - 5%.

The mitochondrial matrix has many catabolic enzymes like cytochrome oxidase and reductases, fatty acid oxidase, transaminase, etc.

Enzymes of Mitochondria

(1) Outer membrane : Monoamine oxidase, glycerophosphatase, acyltransferase, phospholipase A.

(2) Inner membrane : Cytochrome b,c₁,c,a, (cyt.b, cyt.c₁, cyt.c, cyt.a, cyt.a₃) NADH, dehydrogenase, succinate dehydrogenase, ubiquinone, flavoprotein, ATPase.

(3) Perimitochondrial space : Adenylate kinase, nucleoside diphosphokinase.

(4) Inner matrix : Pyruvate dehydrogenase, citrate synthase, aconitase, isocitrate dehydrogenase, fumarase, \alpha#xA0-Ketoglutarate dehydrogenase, malate dehydrogenase.

Origin : Mitochondria are self-duplicating organelles due to presence of DNA molecules so new mitochondria are always formed by growth and division of pre-existing mitochondria by binary fission.

Functions

(1) Mitochondria are called power house or storage batteries or ATP mills formation of ATP is called oxidative phosphorylation.

(2) Intermediate products of cell respiration are used in the formation of steroids, cytochromes, chlorophyll, etc.

(3) These are also seat of some amino acid biosynthesis.

(4) Mitochondria also regulate the calcium ion concentration inside the cell.

(5) Site of thermiogenesis.

(6) Yolk nucleus (a mitochondrial cloud and golgi bodies) controls vitellogenesis.

(7) Mitochondria of spermatid form nebenkern (middle piece) of sperm during spermiogenesis.

(8) Mitochondria release energy during respiration.

(9) Mitochondria contain electron transport system.

Plastids

Plastids are semiautonomous organelles having DNA, RNA, Ribosomes and double membrane envelope. These are largest cell organelles in plant cell.

History

(1) Haeckel (1865) discovered plastid, but the term was first time used by Schimper (1883).

(2) A well organised system of grana and stroma in plastid of normal barley plant was reported by de Von Wettstein.

(3) Park and Biggins (1964) gave the concept of quantasomes.

(4) The term chlorophyll was given by Pelletier and Caventou, and structural details were given by Willstatter and Stall.

(5) The term thylakoid was given by Menke (1962).

(6) Fine structure was given by Mayer.

(7) Ris and Plaut (1962) reported DNA in chloroplast and was called plastidome.

Types of plastids : According to Schimper, Plastids are of 3 types: Leucoplasts, Chromoplasts and Chloroplasts.

Leucoplasts : They are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae. Grana and photosynthetic pigments are absent. They mainly store food materials and occur in the cells not exposed to sunlight e.g., seeds, underground stems, roots, tubers, rhizomes etc. These are of three types.

(1) Amyloplast : Synthesize and store starch grains. e.g., potato tubers, wheat and rice grains.

(2) Elaioplast (Lipidoplast, Oleoplast) : They store lipids and oils e.g., castor endosperm, tube rose, etc.

(3) Aleuroplast (Proteinoplast) : Store proteins e.g., aleurone cells of maize grains.

Chromoplasts : Coloured plastids other than green are kown as chromoplasts. These are present in petals and fruits. These also carry on photosynthesis. These may arise from the chloroplasts due to replacement of chlorophyll by other pigments.

Green tomatoes and chillies turn red on ripening because of replacement of chlorophyll molecule in chloroplasts by the red pigment lycopene in tomato and capsanthin in chillies. Thus, chloroplasts are changed into chromatoplast.

All colours (except green) are produced by flavins, flavenoids and cyanin. Cyanin pigment is of two types one is anthocyanin (blue) and another is erythrocyanin (red). Anthocyanin are water soluble pigments and found in cell sap of vacoule.

Chloroplast : Discovered by Sachs and named by Schimper. They are greenish plastids which possess photosynthetic pigments.

Number : It is variable. Number of chloroplast is 1 in Spirogyra indica, 2 in Zygnema, 16 in S.rectospora, up to 100 in mesophyll cells. The minimum number of one chloroplast per cell is found in Ulothrix and species of Chlamydomonas.

Size : It ranges from 3 - 10$\mu m$ (average 5$\mu m)$ in diameter. The discoid chloroplast of higher plants are 4 - 10$\mu m$ in length and 2- 4$\mu$m in breadth. Chloroplast of Spirogyra may reach a length of 1 mm. Sciophytes (Shade plant) have larger chloroplast.

Chemical composition : Proteins 50 - 60% Lipids 25 - 30% Chlorophyll - 5- 10 % Carotenoids (carotenes and xanthophylls) 1 -2% DNA - 0.5%, RNA 2 - 3% Vitamins K and E Quinines, Mg, Fe, Co, Mn, P, etc. in traces.

Ultrastructure : It is double membrane structure. Both membranes are smooth. The inner membrane is less permeable than outer but rich in proteins especially carrier proteins. Each membrane is 90 - 100 Å thick. The inter-membrane space is called the periplastidial space. Inner to membranes, matrix is present, which is divided into two parts.

(1) Grana : Inner plastidial membrane of the chloroplast is invaginated to form a series of parallel membranous sheets, called lamellae, which form a number of oval - shaped closed sacs, called thylakoids. Thylakoids are structural and functional elements of chloroplasts.

Along the inner side of thylakoid membrane, there are number of small rounded para-crystalline bodies, called quantasomes (a quantasome is the photosynthetic unit).

Each quantasome contains about 230 chlorophyll molecules (160 chl. ‘a’ and 70 chl. ‘b’) and 50 carotenoid molecules.

In eukaryotic plant cells, a number of thylakoids are superimposed like a pile of coins to form a granum. The number of thylakoids in a granum ranges from 10-100 (average number is 20-50). Adjacent grana are interconnected by branched tubules, called stromal lamellae or Fret-channel or Fret membrane’s.

(2) Stroma : It is transparent, proteinaceous and watery substance. Dark reaction of photosynthesis occurs in this portion. Stroma is almost filled with “Rubisco” (about 15% of total enzyme, protein) enzyme CO₂ is accepted by this enzyme. CO₂ assimilation results in carbohydrate formation. It has 20 - 60 copies of naked circular double stranded DNA.

$C_{55}{H}_{72}{O}_5{N}_{4}Mg$ (with methyl group)

Chlorophyll b : $(C_{55}{H}_{74}{O}_6{N}_{4}Mg)$(with aldehyde group)

Chlorophyll c : $C_{35}{H}_{32}{O}_5{N}_{4}Mg$

Chlorophyll d : $\alpha ,\beta ,\gamma$

Bacteriochlorophyll $(C_{55}{H}_{74}{O}_6{N}_{4}Mg)$ or chlorobium chlorophyll present in photosynthetic bacteria. These pigment are red in acidic and blue in alkaline medium.

Carotenoids : These are hydrocarbons, soluble in organic solvents. These are of two types :

(1) Carotenes : $C_{40}{H}_{56}{O}_2,$derivatives of vitamin A. Carrot coloured $\alpha ,\beta ,\gamma$ carotene, lycopene, etc. $\beta$ - carotene most common.

(2) Xanthophyll : $C_{40}{H}_{56}{O}_2,$yellowish in colour, fucoxanthin, violaxanthin. Molar ratio of carotene and xanthophyll in young leaves is 2 : 1.

Origin of chloroplast : Plastids, like the mitochondria, are self duplicating organelles. These develop from colourless precursors, called proplastids. They are believed to be evolved from endosymbiont origination.

Functions

(1) It is the site of photosynthesis, (light and dark reaction).

(2) Photolysis of water, reduction of NADP to NADPH₂ take place in granum.

(3) Photophosphorylation through cytochrome b₆ f, plastocyanine and plastoquinone etc.

(4) They store starch or factory of synthesis of sugars.

(5) Chloroplast store fat in the form of plastoglobuli.

(6) They maintain the percentage of CO₂ and O₂ in atmosphere.

Endoplasmic reticulum (ER)

It is well developed electron microscopic network of interconnected cisternae, tubules and vesicles present throughout the cytoplasm, especially in the endoplasm.

Discovery : Garnier (1897) was first to observe the ergastoplasm in a cell. The ER was first noted by Porter, Claude, and Fullman in 1945 as a network. It was named by Porter in 1953.

Occurrence : The ER is present in almost all eukaryotic cells. A few cells such as ova, embryonic cells, and mature RBCs, however, lack ER. It is also absent in prokaryotic cell. In rapidly dividing cells endoplasmic reticulum is poorly developed.

Ultrastructure : The ER is made up of three components. All the three structures are bound by a single unit membrane.

(1) Cisternae : These are flattened, unbranched, sac like structures. They lie in stacks (piles) parallel to one another. They bear ribosomes. They contain glycoproteins named ribophorin-I and ribophorin-II that bind the ribosomes. Found in protein forming cells.

(2) Vesicles : These are oval or rounded, vacuole like elements, scattered in cytoplasm. These are also studded with ribosomes.

(3) Tubules : Wider, tubular, branched elements mainly present near the cell membrane. They are free from ribosomes. These are more in lipid forming cells.

Types of ER : Depending upon the presence of ribosomes, the ER has been categorised into two types :

(1) A smooth or Agranular endoplasmic reticulum (SER) : It consists mainly of tubules and vesicles. It has no ribosomes associated to it. It is well developed in the muscle cells, adipose tissue cells, interstitial cells, glycogen storing liver cells, etc. and the cells that synthesize and secrete steroids. SER also takes part in synthesis of vitamins, carbohydrates and detoxification. It gives rise to sphaerosomes.

(2) Rough or Granular endoplasmic reticulum (RER) : It mainly consists of cisternae. It has ribosomes attached on its cytoplasmic surface. It is abundant in cells engaged in production and excertion of proteins, e.g., plasma cells, goblets cells, pancreatic acinus cells and certain liver cells. The RER is more stable than SER. The RER is basophilic due to the presence of ribosomes. Ribosomes are attached to ER through hydrophobic interaction.

A third type annulate endoplasmic reticulum was discovered by Mecullo in 1972. It may smooth or rough having pores like nuclear envelop.

Origin : RER is formed from nuclear membrane while SER is formed from RER by loss of ribosomes. Rough vesicles originate only from RER after homogenisation of cell. RER breaks in small fragments (Vesicles) and it is called microsome (This is not a cell organelle).

Functions

(1) Synthesis and secretion of specific proteins via - golgi bodies.

(2) Provides surface for synthesis of cholesterol, steroid, ascorbic acid, visual pigments and hormones e.g., testosterone and estrogen.

(3) It helps in glycogenolysis in the liver cells and brings about detoxification (SER).

(4) ER is a component of cytoskeleton (Spread as a net) of cell and provides mechanical support and shape to the cell.

(5) ER acts as segregation apparatus and divides the cytoplasm into chambers. Compartmentalisation is most necessary for cellular life.

(6) It participates in the formation of cell-plate during cytokinesis in the plant cells by the formation of phragmoplasts.

(7) ER forms 30-60% part of total membranous system.

(8) It gives rise to vacoules.

(9) Sacroplasmic reticulum : It is a modified SER striated muscle fibres (Veratti, 1902) which forms a network of interconnected tubules in the sarcoplasm. It helps in conduction of motor nerve impulses throughout the muscle fibre and in the removal of lactic acid so prevents muscle fatigue. It is called “Sarcoplasmic reticulum” in muscle and “Nissl’sgranules” in nerve cells, mylloid body in retinal cells.

Golgi complex

Golgi complex is made up of various membranous system e.g., cisternae, vesicles and vacuoles. These are also called golgi bodies, golgisomes, lipochondrion, dictyosomes, Dalton complex, idiosomes or Baker’s body and “traffic police” of the cell.

Discovery : First observed by George (1867) but it’s morphological details were given by Camillo Golgi (1898), in nerve cells of barn owl and cat.

Occurence : It is present in all eukaryotic cells. In plants, these are scattered irregularly in the cytoplasm and called as “dictyosomes”. These are absent in bacteria and blue green algae, RBCs, spermatozoa of bryophytes and pteridophytes, and sieve tube cells of phloem of angiosperm. The number of golgi body increased during cell division. Average number 10 - 20 per cell. Golgi body surrounded by a zone of protoplasm which is devoid of cell organelles called zone of exclusion (Morre, 1977).

Structure : Under transmission electron microscope the st. of golgibodies was study by Dalton and Felix (1954), golgi body is made of 4 parts.

(1) Cisternae : Golgi apparatus is made up of stack of flat. Sac like structure called cisternae. The margins of each cisterna are gently curved so that the entire golgi body takes on a cup like appearance. The golgi body has a definite polarity. The cisternae at the convex end of the dictyosome comprises forming face (F. face) or cis face. While the cisternae at the concave end comprises the maturing face (M. face) or trans face. The forming face is located next to either the nucleus or endoplasmic reticulum. The maturing face is usually directed towards the plasma membranes. It is the functional unit of golgi body.

(2) Tubules : These arise due to fenestration of cisternae and it forms a complex of network.

(3) Secretory vesicles : These are small sized components each about 40 Å in diameter presents along convex surface of edges of cisternae. These are smooth and coated type of vesicles.

(4) Golgian vacuoles : They are expanded part of the cisternae which have become modified to form vacuoles. The vacuoles develop from the concave or maturing face. Golgian vacuoles contain amorphous or granular substance. Some of the golgian vacuoles function as lysosomes.

Origin : Most accepted view is that golgi body originates from RER-that has lost its ribosomes from this RER arise transport vesicles that contain Golgi membrane and fuse with the saccule on the forming face of Golgi apparatus. This is why this face is called the forming face.

Functions 

(1) The main function of golgi body is secretion, so it is large sized among the secretory cells.

(2) Glycosidation of lipids i.e., addition of oligosaccharides to produce glycolipids.

(3) Glycosylation of proteins i.e., addition of carbohydrate to produce glycoproteins.

(4) Formation of primary lysosomes.

(5) Golgi body forms the cell plate. During cell division by secreting hemicellulose formation of enzyme and hormones (Thyroxine) etc.

(6) In oocytes of animal, golgi apparatus functions as the centre around which yolk is deposited i.e., vitellogenesis.

(7) Membrane of the vesicles produced by golgi apparatus join in the region of cytokinesis to produce new plasmalemma.

(8) It is also called export house of cell.

(9) Golgi body contains phospholipids, proteins, enzymes and vitamin-c.

(10) The golgi complex gives rise to the acrosome in an animal sperm.

Lysosomes

Lysosomes are electron microscopic, vesicular structures of the cytoplasm, bounded by a single membrane (lipoproteinous) which are involved in intracellular digestive activities, contains hydrolytic enzymes, so called lysosomes.

Discovery

(i) These were first discovered by a Belgian biochemist, Christian de Duve (1955) in the liver cells and were earlier named pericanalicular dense bodies.

(ii) Terms Lysosome was given by Novikoff under the study of electron microscope.

(iii) Matile (1964) was first to demonstrate their presence in plants, particularly in the fungus Neurospora. Polymorphism in lysosomes were described by De Robertis et. al (1971).

Occurrence : These are absent from the prokaryotes but are present in all eukaryotic animal cells except mammalian RBCs. They have been recorded in fungi, Euglena, cotton and pea seeds.

Shape : These are generally spherical in shape but are irregular in plant root tip cells.

Size : Size range is 0.2-0.8 $\mu$m while size is 0.5 $\mu$m (500 nm).

Types of lysosomes : On the basis of their contents, four types of lysosomes are recognised.

(1) Primary Lysosomes : A newly formed lysosome contains enzymes only. It is called the primary lysosomes. Its enzymes are probably in an inactive state.

(2) Secondary Lysosomes : When some material to be digested enters a primary lysosome, the latter is named the secondary lysosome, or phagolysosome or digestive vacuole, or heterophagosome.

(3) Tertiary lysosomes/Residual bodies : A secondary lysosome containing indigestible matter is known as the residual bodies or tertiary lysosome. The latter meets the cell by exocytosis (ephagy).

(4) Autophagosomes/Autolysosomes : A cell may digest its own organelles, such as mitochondria, ER. This process is called autophagy. These are formed of primary lysosomes. The acid hydrolases of lysosomes digest the organelles thus, it is called autophagosome. The lysosome are sometimes called disposal units/suicidal bags. Sometime they get burst and causes the distruction of cell or tissue.

Chemical composition : Matrix of primary lysosome is formed of hydrolases, which is involved in hydrolysis or polymeric compounds, that operate in acidic medium at pH 5, so called acid hydrolases. Upto now 50 types of enzyme have been reported. These are as :

Proteases (cathepsin and collagenase), Nucleases (DNAse and RNAse), Glycosidases ($\beta$-galactosidase, $\beta$-glucoronidase), Phosphatases (ATPase, acid phosphatase /marker enzyme).

Functions

(1) Lysosomes of sperms provide enzyme for breaking limiting membrane of egg e.g., hyaluronidase enzyme.

(2) Lysosomes functions as trigger of cell division or initiate cell division by digesting repressor molecules.

(3) Nucleases (DNAse) of lysosomes may cause gene mutations which may cause disease like leukemia or blood cancer (partial deletion of 21^st chromosome).

(4) Sometimes residual bodies accumulate inside the cells leading to storage diseases e.g., a glycogen storage disease called Pompe’s disease, polynephritis Hurler’s disease (deformed bones due to accumulation of mucopolysaccharides).

(5) Lysosomes also engulf the carcinogens.

Ribosome

The ribosomes are smallest known electron microscopic without membrane, ribonucleo-protein particles attached either on RER or floating freely in the cytoplasm and are the sites of protein synthesis. 

Discovery : In 1943 Claude observed some basophilic bodies and named them as microsome. Palade (1955) coined the term ribosome (form animal cell). Ribosomes in nucleoplasm were observed by Tsao and Sato (1959). First isolated by Tissieres and Watson (1958) from E. coli. Ribosomes found in groups are termed as polyribosomes or ergosomes (Rich and Warner 1963 observed first time polyribosomes).

Occurrence : In prokaryotes ribosomes are found only in free form in the cytoplasm. While in the eukaryotes the ribosomes are found in two forms in the cytoplasm, free form and bind form (bound on RER and outer nuclear membrane). These are also reported inside some cell organelles like mitochondria and plastids respectively called mitoribosomes and plastidoribosomes.

Types of ribosomes

(1) 70S ribosomes : Found in prokaryotes, mitochondria and plastid of eukaryotes.

(2) 80S ribosomes : Found in cytoplasm of eukaryotes.

(3) 77S, 60S and 55S ribosomes : Levine and Goodenough (1874) observed 77S ribosomes in fungal mitochondria 60S ribosomes in animal mitochondria and 55S in mammalian mitochondria.

Structure : Each ribosome is formed of two unequal subunits, which join only at the time of protein synthesis. In 70S and 80S ribosomes, 50S and 30S, 60S and 40S are larger and smaller subunits respectively. Larger subunits is dome shaped and attached to ER by glycoproteins called “ribophorins”

Smaller subunit is oval shaped and fits as a cap on flat side of larger subunit. Ribosomes are attached to ER through hydrophobic interactions.

Chemical composition : Ribosomes are chemically composed of rRNA and proteins Ribonucleo-Protein (RNP). 70S ribosomes has 60-65% rRNA and 35-40% proteins (ratio is 1.5:1). rRNAs are of three types : 23S type and 5S type rRNAs in 50S and 16S type rRNA in 30S sub-units.

80S ribosome has 45% rRNA and 55% proteins (ratio is about 1 : 1). r-RNA are of four types : 28S, 5S and 5.8S types of rRNAs in 60S and 18S type rRNA in 40S sub-units.

A $1\times 10^{-3}$ (0.001 M) molar concentration of M{g^{ +#xA0+ }} is needed for the structural cohesion of ribosomes i.e., for holding the two subunits together. If this concentration is increased by ten folds, two ribosomes unite to form a dimer. By decreasing the M{g^{ +#xA0+ }} conc. to normal, the dimer breaks into monomers (single ribosomes).

Biogenesis of ribosome

(1) In eukaryotes the ribosomal RNAs like 18S, 5.8S and 28S are synthesized by nucleolus and 5S RNA out of the nucleus.  

(2) In prokaryotes both rRNA and its protein are synthesized as well as assembled by cytoplasm.

Polyribosomes or Polysomes : When many ribosomes (generally 6 - 8) are attached at some mRNA strand. It is called polysome. The distance between adjacent ribosomes is of 90 nucleotides. These are functional unit of protein synthesis.

Functions

(1) Ribosomes are also called protein factories of the cell or work branch of proteins.

(2) Free ribosomes synthesize structural proteins and bounded ribosomes synthesize proteins for transport.

(3) Ribosomes are essential for protein synthesis.

(4) Help in the process of photosynthesis.

(5) Enzyme peptidyl transferase occurs in large subunit of ribosome which helps in protein synthesis.

(6) Newly formed polypeptide is protected from degradation by cytoplasmic enzymes in large sub-unit of ribosomes before releasing it into RER lumen.

Microbodies

(1) Sphaerosomes

Discovery : These were first observed by Hanstein (1880) but discovered by Perner (1953). Term sphaerosomes was given by Dangeard.

Occurrence : These are found in all the plant cells which involves in the synthesis and storage of lipids i.e., endosperm and cotyledon of oil seeds.

Shape, size and structure : These are spherical or oval in shape about 0.5-2.5 $\mu$m in diameter. They contain hydrolytic enzymes like protease, ribonuclease, phosphatase, esterase etc. They are bounded by a single unit membrane.

Function : The main function of sphaerosomes is to help in lipid metabolism. These are also known as plant lysosomes.

(2) Peroxisomes (Uricosomes)

Discovery : These were first discovered by J. Rhodin (1954) in the cells of mouse kidney and were called microbodies. De Duve (1965) isolated certain sac like organelles from various types of animals and plants. These were called peroxisomes because these contain peroxide producing enzymes (oxidases) and peroxide destroying enzymes (catalases).

Occurrence : These are found in photosynthetic cells of plants. In animals peroxisomes are found in vertebrates (cells of liver, kidney), brain, small intestine, testis and adrenal cortex), invertebrates and protozoans e.g., Paramecium.

Shape, size and structure : These are spherical in shape, about 1.5 $\mu$m in size. They are bounded by a single unit membrane.

Their membrane is permeable to amino acids, uric acids, etc. They contain four enzymes of H₂O₂ metabolism. The enzymes urate oxidase, d-amino oxidase, $\alpha$-hydroxy acid oxidase produce $H_2{O}_2$ whereas the catalases plays a significant protective role by degrading H₂O₂ because $H_2{O}_2$ is toxic for cells.

Function : These are involved in the formationਊnd degrading of $H_2{O}_2$. Plant peroxisomes are also involved in photorespiration.

(3) Glyoxysomes

 Discovery : These were discovered by Beevers in 1961 and Briedenbach in 1967.

 Occurrence : These are found in fungi, some protists and germinating fatty seeds where insoluble lipid food reserves must be turned into soluble sugars. Absent in animal cell.

Shape, size and structure : These are spherical in shape, about 0.5-1$\mu$m in size, they contain enzymes of metabolism of glycolic acid via glyoxylate cycle and bounded by a unit membrane. These are also contain enzymes for $\beta$-oxidation of fatty acids. Produced acetyl CoA. The better is metabolised in glyoxlate cycle to produced carbohydrates.

Functions : The main function of glyoxysomes is conversion of fats into carbohydrates.

(4) Lomasomes : These are sac like structures found between cell wall and plasmalemma in the haustoria of fungal hyphae. These were first discovered by Bowen and Berlin. Webster called them border bodies.

Centrosome

Discovery : Centrosome was first discovered by Van Benden (1887) and structure was given by T. Boweri.

Occurrence : It is found in all the animal cell except mature mammalian RBC’s. It is also found in most of protists and motile plant cells like antherozoids of ferns, zoospores of algae and motile algal forms e.g., Chlamydomonas but is absent in prokaryotes, fungi, gymnosperms and angiosperms.

Structure : Centrosome is without unit membrane structure. It is formed of two darkly stained granules called centrioles, which are collectively called diplosome. These centrioles are surrounded by a transparent cytoplasmic area called centrosphere of Kinetoplasm. Centriole and centrosphere are collectively called centrosome. Each centriole is a microtubular structure and is formed of microtubules arranged in 9+0 manner (all the 9 microtubules are peripheral in position). Inside the microtubules, there is an intra-centriolar or cart-wheel structure which is formed of a central hub (about 25Å in diameter) and 9 radial spokes or radial fibres.

Chemical composition : Centrosome is lipoproteinaceous structure. The microtubules of centriole are composed of protein tubulin and some lipids. They are rich in ATPase enzyme.

 Origin : The daughter centriole is formed from the pre-existing centriole in $G_2$ of interphase so called self-replicating organelle.

Functions

(1) The centrioles help organising the spindle fibres and astral rays during cell division.

(2) They provide basal bodies which give rise to cilia and flagella.

Cilia and Flagella

Discovery : Flagellum presence was first reported by Englemann (1868). Jansen (1887) was first scientist to report the structure of sperm flagellum.

Definition : Cilia and flagella are microscopic, hair or thread-like motile structures present extra-cellularly but originate intra-cellularly from the basal body.

Occurrence : Cilia are found in all the ciliate protozoans e.g., Paramecium, Vorticella etc.

Flagella are found in all the flagellate protozoans e.g., Euglena, Trichonympha etc.

Structure : Both cilia flagella are structurally similar and possess similar parts-basal body, rootlets, basal plate and shaft..

(1) Basal body : These are also termed as blepharoplast (kinetosome) or basal granule. It is present below the plasma membrane in cytoplasm. The structure is similar to centriole made of 9 triplets of microtubules.

(2) Rootlets : Made of microfilament and providing support to the basal body.

(3) Basal plate : Central fibril develop in this area. It is highly dense and lie above plasma-membrane. The basal body and the shaft at the level of plasma membrane.

(4) Shaft : It is the hair like projecting part of cilia and flagella which remains outside the cytoplasm. It has 9 doublet of microtubules in radial symmetry. These are called axonema. Each axonema has 11 fibrils, 9 in the periphery and 2 in the centre. The arrangement is called 9 + 2 pattern.

Chemical composition : Chemically, the central tubules are formed of dynein protein while the peripheral microtubules are formed of tubulin protein.

Type of flagella : There are two types of flagella.

(1) Tinsel type : In this, flagellum has lateral hair-like processes, called flimmers or mastigonemes.

(2) Whiplash type : In this, flagellum has no flimmers.

Functions

(1) They help in locomotion, respiration, cleaning, circulation, feeding, etc.

(2) Being protoplasmic structure they can function as sensory organs.

(3) They show sensitivity to changes in light, temperature and contact.

Cytoskeleton

In eukaryotic cell, a framework of fibrous protein elements became necessary to support the extensive system of membranes. These elements collectively form cytoskeleton of the cell. There are of three types.

(1) Microtubules : These were first discovered by De Robertis and Franchi (1953) in the axons of medullated nerve fibres and were named neurotubules.

Position : The microtubules are electron-microscopic structures found only in the eukaryotic cellular structures like cilia, flagella, centriole, basal-body, astral fibres, spindle fibres.

Structure : A microtubule is a hollow cylindrical structure of about 250 Å in diameter with about 150 Å luman. Its wall is about 50Å thick. Its walls is formed of 13 parallel, proto-tubules.

Chemical composition : These are mainly formed of tubulin protein. A tubulin protein is formed of 2 sub-units : \alpha#xA0-tubulin molecule and \beta#xA0-tubulin molecule which are alternatively in a helical manner.

Functions

(1) These form a part of cytoskeleton and help in cell-shape and mechanical support.

(2) The microtubules of cilia and flagella help in locomotion and feeding.

(3) The microtubules of asters and spindle fibres of the mitotic apparatus help in the movement of chromosomes towards the opposite poles in cell-division.

(2) Microfilament

Discovery : These were discovered by Paleviz et. al. (1974).

Position : These are electron-microscopic, long, narrow, cylindrical, non-contractile and proteins structures found only in the eukaryotic cytoplasm. These are present in the microvilli, muscle fibres (called myofilaments) etc. But these are absent in prokaryotes.

Structure : Each microfilament is a solid filament of 50-60 Å diameter and is formed of a helical series of globular protein molecules. These are generally grouped to form bundles.

Chemical composition : These are mainly formed of actin-protein (contractile).

Functions

(1) The microfilaments forms a part of cytoskeleton and change the cell shape during development, motility and division.

(2) The microfilaments bring about directed movements of particles and organelles along them in the cell.

(3) The microfilaments also produce streaming movements of cytoplasm.

(4) The microfilaments are responsible for the movement of cell membrane during endocytosisਊnd exocytosis.

(3) Intermediate filaments

Location : They are supportive elements in the cytoplasm of the eukaryotic cells. They are missing in mammalian RBCs.

Structure : The IFs are somewhat larger than the microfilaments and are about 10 nm thick. They are solid, unbranched and composed of nonmotile structural proteins, such as keratin, desmine, vimentin.

Functions

(1) They form a part of cytoskeleton that supports the fluid cytosol and maintains the shape of the cell.

(2) They provided strength to the axons.

(3) They keep nucleus and other organelles in place.

Nucleus

The nucleus also called director of the cell. It is the most important part of the cell which directs and controls all the cellular function.

Discovery : The nucleus was first observed by Robert Brown (1831), in orchid root cells. Nucleus plays determinative (in heredity) role in cell and organism, that was experimentally demonstrated by Hammerling (1934) in surgical experiments with green marine unicelled algae Acetabularia.

Occurence : A true nucleus with definite nuclear membrane and linear chromosome, is present in all the eukaryotes except mature mammalian RBCs, sieve tube cell of phloem, tracheids and vessels of xylem. The prokaryotes have an incipient nucleus, called nucleoid or prokaryon or genophore or false nucleus.

Number : Usually there is a single nucleus per cell i.e., mononucleate condition, e.g., Acetabularia.

(1) Anucleate (without nucleus) : RBCs of mammals, phloem sieve tube, trachids and vessels of xylam.

(2) Binucleate : e.g., Ciliate, Protozoans like Paramecium.

(3) Polynucleate : e.g., fungal hyphae of Rhizopus, Vaucheria. Polynucleate condition may be because of fusion of a number of cells. i.e., syncytium, coconut endosperm or by free nuclear divisions without cytokinesis i.e., coenocyte.

Shape : It varies widely, generally spherical e.g., cuboidal germ cells, oval e.g., columnar cells of intestine, bean shaped in paramecium, horse-shoe shaped in Vorticella, bilobed, e.g., WBCs (acidophils), 3 lobed e.g., basophil, multilobed e.g., neutrophils, long and beaded form (moniliform) e.g., stentor and branched in silk spinning cells of platy phalyx insect larva.

Size : The size of nucleus is variable i.e., 5 - 30$\mu$. In metabolically active cells size of the nucleus is larger than metabolically inactive cells.

Chemical composition

Proteins = 80%, DNA = 12%, RNA= 5%, Lipids = 3%

Enzymes like polymerases are abundantly present and help in synthesis of DNA and RNA.

Ultrastructure : The nucleus is composed of following structure.

(1) Nuclear membrane : It is also called nuclear envelope or nucleolemma or karyotheca, was first discovered by Erclab (1845).

Structure : It is a bilayered envelope. Each membrane is about 60-90Å thick lipoproteinous and trilaminar. Outer membrane, called ectokaryotheca (with ribosome) and inner membrane is called endokaryotheca (without ribosome). Two membranes are separated by a fluid-filled intermembranous perinuclear space (about 100-300Å).

Nuclear membrane is porous and has 1,000-10,000 octagonal nuclear pores. Each nuclear pore is about 400-1,000 Å in diameter (average size is 800 Å). Callan and Tamlin (1950) first to observe nuclear pore in nuclear membrane. The nuclear pares are enclosed by circular structure are called annuli. The pore and annuli together are called pore complex or pore basket.

Origin : It is formed by the fusion of ER elements during the telophase of cell division.

Functions

(i) It regulates the nucleo-cytoplasmic interactions.

(ii) It allows the passage of inorganic ions, small organic molecules, ribosomal subunits, RNAs and proteins through nuclear pores.

(iii) It maintains the shape of the nucleus.

(2) The nucleolus (Little nucleus plasmosome) : It was first observed by Fontana (1781) in the skin cells of an eel. Bowman (1840) coined term ‘nucleolus’. Wagner (1840) gave its light microscopic structure.

Position : It is generally associated with nucleolar organizer region (NOR) of the nucleolar chromosomes. It is absent in muscle fibres, RBC, yeast, sperm and prokaryotes.

Number : Generally, a diploid cell is with two nucleoli but there are five nucleoli in somatic cell of man and about 1000 nucleoli in the oocytes of Xenopus.

Structure : (De Robertis et.al 1971). A nucleolus is distinguishable into following regions :-

(1) Chromatin : The nucleolus is surrounded by perinucleolar chromatin.

(2) Pars fibrosa : Fibrils of 80 - 100 Å size form a part of the nucleolus.

(3) Pars granulosa : Granules of 150 - 200 Å diameter constitute the granular part of the nucleolus. Ribosome formation takes place in this part so it is called assembly line of ribosome.

(4) Pars amorpha : The granules and the fibrils lie dispered in an amorphous proteinaceous matrix. Nucleolus is stained by “pyronine”. It is not bounded by any limiting membrane.

Chemical composition : Nucleolus is mainly formed of RNA and non histone acidic proteins. It is a store house of rRNA.

Origin : A nucleolus is formed at specific sites, called the nucleolar organizers, present on certain chromosomes region (NOR).

Functions

(i) It is seat of biogenesis of rRNA and also stores rRNA.

(ii) It plays important role in spindle formation during cell division.

(iii) It receives the ribosomal proteins from the cytoplasm, combines the rRNAs and ribosomal proteins to form ribosomal subunits. So it is also called ribosome producing machine or factory.

(3) Nucleoplasm : It is also called karyolymph. It is transparent, homogenous, semifluid, colloidal, ground substance present inside the nuclear membrane. It contains Nucleic acid (DNA and RNA), Proteins (Basic proteins and acidic protein), Enzyme (DNA and RNA polymerase, NAD synthetase etc.), Minerals (K, Na, Ca, Mg etc.) and Ribonucleoproteins.

The nucleoplasm helps in maintaining the shape of nucleus formation of spindle protein of NAD, ATP, DNA, RNAs and ribosomal subunits. Plasmosome and karyosome combindly called “amphinucleoli”.

(4) Chromatin fibres /Nuclear chromatin : The nucleoplasm contains many thread like, coiled and much elongated structures which take readily the basic stains such as �sic fuchsine”. These thread like structures are known as chromatin fibre. They are uniformly distributed in the nucleoplasm. They are observed only in the “interphase stage”.

(5) Nuclear matrix : It is network of proteinaceous fibrils. It is outer thicker part is called fibrous lamina (Haris and James, 1952).

Chromosome (Gr. chroma = colour soma = body)

Chromosome were discover by Hofmeister (1848) in filament of pollen mother cells of tradescantia (Rhoeodiscolour) studied by strasburger (1875) and given the persent name by Waldeyer (1888).

During interphase, chromatin threads are present in the form of a network called chromatin reticulum. At the time of cell division, these thread like structures of chromatin become visible as independent structures, called chromosomes. The haploid set of chromosomes is define as genome.

Structure : Each chromosome consists of two coiled filaments throughout its length called chromonemata by Vejdovsky. These have bead like structures called chromomeres which bear genes. Chromatid is a half chromosome or daughter chromosome. The two chromatids are connected at the centromere or primary constriction. Primary constriction (centromere) and secondary constriction gives rise to satellite. The secondary constriction consists of genes which code for ribosomal RNA and nucleolus hence it is called as “nucleolar organizer region”. Chromosomes having satellite are called SAT chromosomes. The ends of chromosomes are called “telomeres” (which do not unite with any other structure).

In 1928 Emile Heitz developed a technique for stainning of chromosomes. Staining property of chromosomes is called as heteropycnosis. Chromosomes can be stained with basic dye like janus green there are two types of regions are seen :-

(1) Heterochromatin : It is formed of thick regions which are more darkly stained than others areas. It is with condensed RNA which is transcriptionally inactive and late replicating. It generally lies near the nuclear lamina. It is of two type :

(i) Constitutive hetrochromative : Occurs in all cells in all stages. e.g., Centromere.

(ii) Facultative hetrochromative : Formed by inactivation of some gene in some cell in some stages. e.g., Barr body. 

(2) Euchromatin : It is true chromatin and is formed of thin, less darkly stainedਊreas. It is with loose DNA which is transcriptionally active and early replicating.

Chemical chomposition : DNA - 40%. Histone - 50%. Other (acid) Proteins - 8.5%. RNA - 1.5%. Traces of lipids, Ca, Mg and Fe. Histone are low molecular weight basic proteins which occur alongwith DNA in $1:1$ratio. Nonhistone chromosomal or NHC proteins are of three types- structural, enzymatic and regulatory. Structural NHC proteins form the core or axis of the chromosome. They are also called scaffold proteins.

Metabolically inactive cell inclusions

Within the cytoplasm of a cell there occur many different kinds of non-living structures which are called inclusions or ergastic / Deutoplasmic substances.

(1) Vacuoles : The vacuole in plants was discovered by Spallanzani. It is a non-living reservoir, bounded by a differentially or selectively permeable membrane, the tonoplast. The vacuole is filled with cell sap or tonoplasm. They contain water, minerals and anthocyanin pigments.

Some protozoans have contractile vacuoles which enlarge by accumulation of fluid or collapse by expelling them from the cell. The vacuoles may be sap vacuoles, contractile vacuoles or gas vacuoles (pseudo vacuoles).

Function of vacuoles : Vacuole maintains osmotic relation of cell which is helpful in absorption of water. Turgidity and flaccid stages of a cell are due to the concentrations of sap in the vacuole.

(2) Reserve food material

The reserve food material may be classified as follows :

(i) Carbohydrates : Non-nitrogenous, soluble or non- soluble important reserve food material. Starch cellulose and glycogen are all insoluble.

(a) Starch : Found in plants in the form of minute solid grains. Starch grains are of two types :

Assimilation starch : It is formed as a result of photosynthesis of chloroplasts.

Reserve starch : Thick layers are deposited around an organic centre called hilum.

(b) Glycogen : Glycogen or animal starch occurs only in colourless plants like fungi.

(c) Inulin : It is a complex type of polysaccharide, soluble and found dissolved in cell sap of roots of Dahlia, Jaruslem, Artichoke, Dandelion and members of compositae.

(d) Sugars : A number of sugars are found in solution of cell sap. These include glucose, fructose, sucrose, etc.

(e) Cellulose : Chemical formula is $(C_6{H}_{10}{O}_5{)}_n$. The cell wall is made up of cellulose. It is insoluble in water.

(ii) Fats and Oils : These are important reserve food material. These are always decomposed into glycerol and fatty acids by enzymatic action. Fat is usually abundant in cotyledons than in the endosperm. e.g., flax seed produce linseed oil, castor produce castor oil, cotton seeds produce cottonseed oil, etc.

(iii) Proteins and Amides (Aleurone grains) : Storage organ usually contain protein in the form of crystalline bodies known as crystalloids (potato). Proteins may be in the form of aleurone grains as in pea, maize, castor, wheat, etc.

(3) Excretory Products : The organic waste products of plants are by-product of metabolism. They are classified as :

(i) Resins : They are believed to be aromatic compounds consisting of carbon, hydrogen and oxygen and are acidic in nature. Sometimes they are found in combination with gums and are called gum resin. e.g., Asafoetida (heeng).

(ii) Tannins : They are complex nitrogenous compounds of acid nature having an astringent taste. Presence of tannin in plants makes its wood hard durable and germ proof.

(iii) Alkaloids : These are organic, basic, nitrogenous substance. They occur in combination with organic acids and most of them are poisonous. From plants, cocaine, hyoscine, morphine, nicotine, quinine, atropine, strychnine and daturine etc. are extracted.

(iv) Glucosides : Some glucosides or glycosides function as storage substance e.g., amygdaline of the bitter almond.

(v) Etherial and Essential oils : These consist mixture of various hydrocarbons known as tarpenes and their oxygen derivatives. They are responsible for flavor of many fruits and scent of many flowers etc. They are volatile and are soluble in water, ether, petroleum etc. e.g., lavender, mint, clove oil, eucalyptus oil, theme oil etc.

(vi) Mineral matter : Many minerals are waste products in plants.

(a) Calcium oxalate : It occurs in the form of crystals of various shapes.

Raphides : Needle shaped crystals are known as raphides. e.g., in plants like jamikand, Colocasia, water hyacinth (Jal kumbhi) etc.

Rosette or Sphaeraphides : Star shaped crystals. They occur in special mucilaginous parenchyma cells of the petiole of arum, water hyacinth, etc. Crystals in the form of cubes are found in tunic of onion bulb. In the leaf of belladona, these crystals are in the form of sand and also called as sand crystals.

Calcium oxalate crystals : In members of family solanaceae. They are found as cubics, rods and prisms.

(b) Calcium carbonate : It is deposited in the form of crystalline masses hanging from a cellulose stalk in enlarged epidermal cells of leaves of Ficus elastica (Indian rubber plant) and is called as cystolith.

(vii) Latex : It is an emulsion in water having many substances either in suspension or in true solution. It may contain sugars, alkaloids and oils. It is watery in banana, milky white in Euphorbia, yellow or orange red in opium (poppy) is dried latex.

(viii) Organic acids : Tartaric acid in tamarind, and grapes, citric acid in lemon, orange etc. malic acid in apple and Bryophyllum. Oxalic acid in the form of crystals.

(ix) Gums : It is formed by decomposition of cellulose cell wall. Gum arabic of commerce is obtained from Acacia senegal.