Endomembrane System II

Ribosomes

(1) Definition : 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.

(2) 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). It was 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).

(3) Occurrence : These are found in both prokaryotes as well as eukaryotes these are present 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.

(4) Number : The number of ribosomes depends upon the RNA contents of the cell. These are more in plasma cells, liver cells, Nissl’s granules of nerve cells, meristematic cells and cancerous cells.

(5) Types of ribosomes : It is determined on the basis of sedimentation coefficient measured in Svedberg unit or ‘S’ unit and their size. Velocity of sedimentation is 1 X 10^-3 cm/sec/dyne/gm.

(i) 70S ribosomes : Found in prokaryotes, mitochondria and plastid of eukaryotes. Each is about 200 – 290Å × 170 – 210Å in size and 2.7 ×10⁶ dalton in molecular weight.

(ii) 80S ribosomes : Found in cytoplasm of eukaryotes. Each is about 300 – 340 Å × 200 – 240 Å in size and 4.5 – 5.0 ×10⁶ daltons in molecular weight.

(iii) 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.

(6) 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 subunit is dome shaped and attached to ER by glycoproteins called “ribophorins”.

(7) Function :

(i) Ribosomes are also called protein factory of the cell or work branch of proteins.

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

(iii) Ribosomes are essential for protein synthesis.

(iv) Help in the process of photosynthesis.

(v) They are found numerously in actively synthesizing cells like liver cells, pancreas, endocrine, yeast cells and meristematic cells.

(vi) Ribosomes also store the proteins temporarily.

(vii) These also store rRNAs, which helps in protein synthesis.

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

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

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 :

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

(ii) Position : The microtubules are electron-microscopic structures found only in the eukaryotic cellular structures like cilia, flagella, centriole, basal-body, astral fibres, spindle fibres, sperms tail, neuraxis of nerve fibres etc. These are absent from amoebae, slime-moulds and prokaryotes.

(iii) 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, each being formed of a liner series of globular dimeric protein molecules.

(iv) Chemical composition : These are mainly formed of tubulin protein. A tubulin protein is formed of 2 sub-units: ∝-tubulin molecule and ß-tubulin molecule which are alternatively in a helical manner.

(v) Function

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

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

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

(d) These help in distribution of pigment in the chromatophores, so help in skin colouration.

(e) These also form micro-circulatory system of the cell which helps in intracellular transport.

(f) These control the orientation of cellulose microfibrils of the cell wall of plants.

(2) Microfilament

(i) 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 from the prokaryotes. These are also associated with the pseudopodia, plasma membrane of fibroblats, etc. These are either scattered or organized into network or parallel arrays in the cytoplasmic matrix.

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

(iii) 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.

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

(v) Functions

(a) The microfilaments form a part of cytoskeleton to support the relatively fluid matrix.

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

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

(d) The microfilaments also cause cleavage of animal cells which is brought about by contraction of a ring of microfilaments.

(e) The microfilaments also participate in gliding amoeboid motion shown by amoebae, leucocytes and macrophages.

(f) The microfilaments are also responsible for the change in cell shape curing development, motility and division.

(g) Myofilaments bring about muscle contraction.

(h) The microfilaments cause movements of villi to quicken absorption of food.

(i) The microfilaments are responsible for the movement of cell membrane during endocytosis and exocytosis.

(j) The microfilaments cause plasma membrane undulations that enable the fibroblasts to move.

(3) Intermediate filaments

(i) Location : They are supportive elements in the cytoplasm of the eukaryotic cells, except the plant cells. They are missing in mammalian RBCs and in the prokaryotes.

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

(iii) Functions

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

(b) They stabilize the epithelia by binding to the spot desmosomes.

(c) They form major structural proteins of skin and hair.

(d) They integrate the muscle cell components into a functional unit.

(e) They provided strength to the axons.

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

Nucleus

(1) Definition : (Karyon = 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.

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

(3) 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 or bacterial chromosome.

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

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

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

(iii) 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.

(5) 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.

(6) Size : The size of nucleus is variable i.e. 5 – 30m. In metabolically active cells size of the nucleus is larger than metabolically inactive cells. The size depends upon metabolic activity of the cells. It is directly proportional to number of chromosomes.

(7) Chemical composition of nucleus

Proteins = 80% (65% acidic, neutral and enzymatic proteins; 15% basic proteins-histones)

DNA     = 12%

RNA     = 5%

Lipids    = 3%

Enzymes like polymerases are abundantly present and help in synthesis of DNA and RNA. Minerals like Ca²⁺, Mg²⁺, Na⁺ and K⁺ are present in traces.

Centrosome

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

(2) 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.

(3) 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 or Kinetoplasm.

(4) Functions

(i) The centrioles help organising the spindle fibres and astral rays during cell division. Therefore, they are called microtubules organising centres. The cells of higher plants lack centrioles and still form a spindle.

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

(iii) The distal centriole of a spermatozoan give rise to the axial filament of the tail.

Cilia and Flagella

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

(2) Definition : Cilia and flagella are microscopic, hair or thread-like motile structures present extra-cellularly but originate intra-cellularly from the basal body and help in movements, locomotion, feeding, circulation etc.

(3) Occurrence : Cilia are found in all the ciliate protozoans e.g., Paramecium, Vorticella etc. flame cells of flat worms; in some larval forms e.g., Trochophore larva of Nereis, Bipinnaria larva of starfish etc.; in some body structures e.g. wind-pipe, fallopian tubes, kidney-nephrons etc.

(4) Function

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

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

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

(iv) Ciliated larvae take part in dispersal of the species.

(v) The cilia of respiratory tract remove solid particles from it. Long term smoking damages the ciliated epithelium, allowing dust and smoke particles to enter the long alveoli.

(vi) The cilia of urinary and genital tracts drive out urine and gametes.

Microbodies

Microbodies are single membrane bounded small spherical or oval organelles, which take part in oxidation reactions other than those of respiration. They can only be seen by electron microscope. Microbodies possess a crystalline core and granules matrix. They are following types :–

(1) Sphaerosomes

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

(ii) Occurrence : These are found in all the plant cells which involves in the synthesis and storage of lipids i.e. endosperm and cotyledon.

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

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

(2) Peroxisomes (Uricosomes)

(i) Discovery : These were first discovered by J. Rhodin (1954) in the cells of mouse kidney with the help of electron microscope, 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).

(ii) Occurrence : These are found in photosynthetic cells of plants. In animals peroxisomes are abundant in the liver and kidney cells of vertebrates. They are also found in other organs like brain, small intestine, testis and adrenal cortex. They also occur in invertebrates and protozoans e.g., Paramecium.

(iii) Shape, size and structure : These are spherical in shape, about 1.5 mm in size. They are bounded by a single unit membrane. They contain granular consents condensing in the centre.

(iv) Function : These are involved in the formation  and degrading of H₂O₂. Plant peroxisomes are also involved in photorespiration. In which glycolic acid oxidase enzyme oxidises the glycolic acid to glyoxylic acid. In case of plants peroxisomes is also known as glyoxisomes.

(3) Glyoxysomes

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

(ii) Occurrence : These are found in fungi, some protists and germinating seeds especially in germinating fatty seeds where insoluble lipid food reserves must be turned into soluble sugars.

Animals cannot execute this conversion because they do not possess glyoxylate enzymes.

(iii) Shape, size and structure : These are spherical in shape, about 0.5-1 mm in size, they contain enzymes of metabolism of glycolic acid via glyoxylate cycle and bounded by a unit membrane. These also contain enzymes for b-oxidation of fatty acids.

(iv) 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.

Q.1. Respiratory enzymes are present in      [AFMC 1985; CPMT 2001]       

(a)     Mitochondria          (b)     Chloroplasts (c)     Golgi bodies (d)     Lysosomes

Q.2. Oxidative enzymes occurs mostly in     [AFMC 1995]        

(a)     Lysosomes  (b)     Golgi bodies (c)     Mitochondria          (d)     Ribosomes

Q.3. Glycogen occurs in [CPMT 1994]        

(a)     Mitochondria          (b)     Krab’s cycle (c)     Cytoplasm   (d)     None of these

Q.4. What is the energy coin of a cell [Pb.  PMT 1999; MP PMT 2002]        

(a)     DNA   (b)     RNA   (c)     ATP   (d)     Minerals

Q.5. Which of the following contains DNA   [MP PAT 1995]    

(a)        Mitochondria          (b)     Lysosomes  (c)     Golgi bodies (d)     Ribosomes

Q.6. Chondrisome was discovered by [BHU 1997]

(a)     Tatum          (b)     Palade         (c)     Sutton          (d)     Benda

Q.7. Foldings of inner mitochondrial membrane are called [JIPMER 1992, BHU 1984 Bihar PMT 1996]

(a)     Grana (b)     Thylakoids   (c)     Cristae        (d)     Structures

Q.8. Cristae control         [CPMT 1996]        

(a)     Photo-oxidation      (b)     Photosynthesis      (c)     Absorption   (d)     Dark respiration

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