Study materials for Degree Part 3 students

Related to cytology: BY Anand Mohan Mishra;Department of Botany

M.L.S.M.College, Darbhanga.

MITOCHONDRIA

The term mitochondria has been derived from a greek word which means thread like structure. It is considered as the power house of cell because it generates energy for the different cell organales for performing their work. It is found in eukaryotic cell and becomes absent in prokaryotic cell. First of all Mitochondria was discovered by Richard Altman (1890) and was named as mitochondria by Benda (1897). In plant the existance of mitochondria was discovered by F. Meves(1904).

Shape: The shape of mitochondria is variable and is influenced by different physiological and environmental conditions. They may be grannular, club shaped, tannis racket shaped , vesicular or rod shaped.

Size: Like shape the size of mitochondria is also variable. The rod like mitochondria become 0.5u to 2u but may attain a height of 7u .

Number: The number of mitochondria per cell also varies eg.in the cell of rat liver , 500, 1000 and 2500 mitochyondria some rtimes have been reported . In sea urchin the number have been reported varying between 14000 to 1, 50, 000 per cell.

Structure: Based on the electron microscopic study a typical mitochondria is consisted of two unit memembrane consisted of lipid and protein, lying parallel to one another. The outer membrane appears to be smooth and sculptured but the inner membrane forms involution inside the lumen of mitochondria which are called cristae. The cristae may be branched or unbranche3d . In plant the cristae bay become tubular or intermediate. On the outer surface of outer membrane and inner surface of the inner membrane are found numerous minute structures called as microbodies. The microbodies are globular, simple and pin headed of outer membrane where as they are stalked or bacteriophase like of inner membrane. They are placed at intervals of 100 A0

along the membrane. These structures are also called as elementaryu particle

or inner lamellae spheres. Lehninger (1964) called them by the name oxysome. The cavity of mitochondria is filled with mitochondrial matrix which are grannular or homogenous in out line

Chemical composition : Mitochondria is consisted of lipid and protein which shares 90 to 99 % of the body of mitochondria. On dry weight basis its chemical composition is as follows:

Protein-65 to 70%, Phospholipid- 25 to 30 %, RNA-0.5%, DNA- small amount.

Function : Mitochondria is considfered as the power house cell because it makes available the readily formed energy to all vital orgains for their biological activities. The energy transported to the different cell organales for function is in the form of ATP ( Adenosine Triphosphate) which is generated during the oxidation. The food that we eat is broken into simpler molecules like carbohydrates, fats, etc., in our bodies. These are sent to the mitochondrion where they are further processed to produce charged molecules that combine with oxygen and produce ATP molecules. This entire process is known as oxidative phosphorylation.It is important to maintain proper concentration of calcium ions within the

various compartments of the cell. Mitochondria help the cells to achieve this goal by serving as storage tanks of calcium ions.

They also help in the building of certain parts of the blood, and hormones like testosterone and estrogen.

Mitochondria in the liver cells have enzymes that detoxify ammonia.

They play an important role in the process of programmed cell death. Unwanted and excess cells are pruned away during the development of an organism. The process is known as apoptosis. Abnormal cell death due to mitochondrial dysfunction can affect the function of the organ.

Origin of mitochondria: Regarding the origin of mitochondria there are two views and they are:

(1) Autonomous replication theory(2) Promitochondrial origin theory:

Autonomous replication theory: According to this theory the mitochondria is an autonomous body with independent hereditary action and lives inside the cell symbiotically. The concept may appeal but its universal acceptance is doubtful.

Promitochondrial origin theory: According to this theory the mitochondria develops from a smaller particles known as promitochondria which are present in considerable number inside the meristematic cells. With the growth of the cell promitochondrial particle starts increasing in size and tyheir inner membrane forms folds at right angle to the surface. Later these folds converts into ctristae.Some biologists consider that promitochondria may arise from nuclear envelop. There fore it would be more proper to say that the mitochondria is nuclear in origin.

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RIBOSOME

Introduction: Ribosome is the most important cell organale found inside the living organism. It is considered as the strucure inside which the protein synthesis takes place. It is found both in prokaryotic and eukaryotic cell but shows typical difference in their structure. It was first discovered by Palade (1953) and the name ribosome was given by Roberts( ).

Prokaryotic ribosomes become consisted of two subunits 50s and 30s together with constituting the 70s ( Svedbergs unit) . Boththe subunits are spaced at an interval with the help of magnesium ions. In eukaryote the ribosome appears to be 80s consisted of 60s and 40s subunits. In exceedingly minute quantities of cu and p may also become present. Peterman (1964) with the help of electron microscope studied the structure of ribosome. He observed that in E. coli bacteria 30s subunit

becomes asymmetrical and is represented by a prolate ellipsoid 140x 170 A0 structure and 50s is dome shaped. The ribosomes of eukaryotes are comparatively spherical than those found in bacteria and are more highly hydrated.

Three dimensional structure of ribosome of E. coli bacteria .

Types of ribosome: Ribosomes are of two types-

Free Ribosomes:

Free ribosomes can move about anywhere in the cytosol, but are excluded from the cell nucleus and other organelles. Proteins that are formed from free ribosomes are released into the cytosol and used within the cell. Since the cytosol contains high concentrations of glutathione and is, therefore, a reducing environment, proteins containing disulfide bonds, which are formed from oxidized cysteine residues, cannot be produced within it.

Membrane-bound ribosome:When a ribosome begins to synthesize proteins that are needed in some organelles, the ribosome making this protein can become "membrane-bound". In eukaryotic cells this happens in a region of the endoplasmic reticulum (ER) called the "rough ER". The newly produced polypeptide chains are inserted directly into the ER by the ribosome undertaking vectorial synthesis and are then transported to their destinations, through the secretory pathway. Bound ribosomes usually produce proteins that are used within the plasma membrane or are expelled from the cell via exocytosis.

Biogenesis:

In bacterial cells, ribosomes are synthesized in the cytoplasm through the transcription of multiple ribosome gene operons. In eukaryotes, the process takes place both in the cell cytoplasm and in the nucleolus, which is a region

within the cell nucleus. The assembly process involves the coordinated function of over 200 proteins in the synthesis and processing of the four rRNAs, as well as assembly of those rRNAs with the ribosomal proteins.

Origin:

The ribosome may have first originated in an RNA world, appearing as a self-replicating complex that only later evolved the ability to synthesize proteins when amino acids began to appear.Studies suggest that ancient ribosomes constructed solely of rRNA could have developed the ability to synthesize peptide bonds.In addition, evidence strongly points to ancient ribosomes as self-replicating complexes, where the rRNA in the ribosomes had informational, structural, and catalytic purposes because it could have coded for tRNAs and proteins needed for ribosomal self-replication. As amino acids gradually appeared in the RNA world under prebiotic conditions,their interactions with catalytic RNA would increase both the range and efficiency of function of catalytic RNA molecules.Thus, the driving force for the evolution of the ribosome from an ancient self-replicating machine into its current form as a translational machine may have been the selective pressure to incorporate proteins into the ribosome’s self-replicating mechanisms, so as to increase its capacity for self-replication.

Specialized ribosomes:

Heterogeneity in ribosome composition has been proposed to be involved in translational control of protein synthesis. Vincent Mauro and Gerald Edelman proposed the ribosome filter hypothesis to explain the regulatory functions of ribosomes. Emerging evidence has shown that specialized ribosomes specific to different cell populations can affect how genes are translated. Some ribosomal proteins exchange from the assembled complex with cytosolic copies suggesting that the structure of the in vivo ribosome can be modified without synthesizing an entire new ribosome.Ribosomes are composed of ribonucleic acid (abbreviated as RNA) and proteins, in nearly equal amounts. The ribonucleic acid is derived from the nucleolus, where ribosomes are synthesized in a cell.A simple prokaryotic cell (for example, bacteria) consists of a few thousands of ribosomes, whereas a highly developed eukaryotic cell (for example, human cell) has a few millions of ribosomes. Prokaryotic ribosomes are smaller in size as compared to the eukaryotic ones.

Function: The main function of ribosome is to provide a larger surface and to facilitate translation during protein synthesis.

In a cell, ribosomes are located in two regions of the cytoplasm. Some ribosomes are found scattered in the cytoplasm (referred to as free ribosomes), while others are attached to the endoplasmic reticulum (bound ribosomes). Accordingly, the surface of endoplasmic reticulum when bound with ribosomes is called rough endoplasmic reticulum (RER). Both the free ribosomes and bound ribosomes have similar structure and are responsible for production of proteins.

Speaking about the main functions of ribosomes, they play the role of assembling amino acids to form specific proteins, which in turn are essential for carrying out the cell's activities. As we all have a fair idea regarding production of proteins, the deoxyribonucleic acid (DNA) first produces RNA (messenger RNA or mRNA) by the process of DNA transcription, after which genetic message from the mRNA is translated into proteins during DNA translation.To be more precise about protein synthesis by ribosomes, the sequence for assembling amino acids during protein synthesis are specified in the mRNA. The mRNA synthesized in the nucleus is then transported to the cytoplasm for further continuation of protein synthesis. In the cytoplasm, the two subunits of ribosomes bind around the mRNA polymers and synthesize proteins with the help of transfer RNA (tRNA), as per the genetic code. This whole process of protein synthesis is also referred to as central dogma.Usually, the proteins synthesized by the free ribosomes are utilized in the cytoplasm itself, while the protein molecules produced by the bound ribosomes are transported outside the cell. Considering the primary function of ribosomes in constructing proteins, it is understandable that a cell cannot function without ribosomes.

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ENDOPLSMIC RETICULUM

The name endoplasmic reticulum was given by Porter( 1955) for the organ corresponding to the basophilic component of cytoplasm, and is the most facinating discovery of elctron microscopy. It is present in all living cells of plants and animals. In cross section , the whole meshwork of ER looks like organised channels but when studied with 3D technique it was nfound that it was composed of flat sacular expansions called cisternae. The cisternae may occur singly but more often they are aggregated to form lamellae system roughly parallel tyo one another. These parallel running systernae are interconnected through partitions dotng compartmentalisation in ER. The peripheral region of cisternae may be entire or composed of meshed network of anstomosing tubules of 400-700 A0 in diameter. On the other end of tubules may present vesicles that in many cases do not form a continuous part of ER but never has been considered as separate portion.

Types of ER: On the basis of surface structure , two types of ER has been considered .(i) Smooth walledER(ii) Rough walled ER

Smooth walled ER: It is fine tubular structure which wall becomes smoothRough walled ER: The wall of this type of ER becomes rough due to coated with large number of granular ribosomes

Strucuture of ER: ER is composed of three different types of structures and these are: cisternae, vesicles and tubules.Cisternae: are long , flat and unbranched plates or lamellae arranged in parallel rows.Vesic;les: are usually round or ovoid sacs , ranging in diameter from 25 to 500mu. They often occur isolated in cytoplasm.Tubules: are irregularly branched tube like structures having a diameter of 50- 100mu. These structures are surrounded by thin unit membrane of 50-60 A0 in thickness and their lumen is filled with secretary products of cell.

ROUGH ENDOPLASMIC RETICULUM

The rough endoplasmic reticulum manufactures membranes and secretory proteins. The ribosomes attached to the rough ER synthesize proteins by the process of translation. In certain leukocytes (white blood cells), the rough ER produces antibodies. In pancreatic cells, the rough ER produces insulin. The rough and smooth ER are usually interconnected and the proteins and membranes made by the rough ER move into the smooth ER to be transferred to other locations. Some proteins are sent to the Golgi apparatus by special transport vesicles. After the proteins have been modified in the Golgi, they are transported to their proper destinations within the cell or exported from the cell by exocytosis.

SMOOTH ENDOPLASMIC RETICULUM

The smooth ER has a wide range of functions including carbohydrate and lipidsynthesis. Lipids such as phospholipids and cholesterol are necessary for the construction of cell membranes. Smooth ER also serves as a transitional area for vesicles that transport ER products to various destinations. In liver cells the smooth ER produces enzymes that help to detoxify certain compounds. In muscles the smooth ER assists in the contraction of muscle cells, and in brain cells it synthesizes male and female hormones.

Function of ER: ER is involved in following major functions:

1. Functions common in both smooth and rough walled ER.

(a) Forms skeletal frame work

(b) Active transport

(c) Metyabolic activities due to enzymes

(d) Provide increased surface area

(e) Formation of new nuiclear membrane during cell division.

2, Function of smooth walled ER:

(a) Lipid synthesis

(b) Glycogen synthesis

(c) Steroids like ( cholestrol, progestrone, testostemerone) synthesis.

3.Functions of rough walled ER:

(a) It provides sites for protein synthesis

(b) It helps in transport of proteins.

Golgi apparatus

This structure was first discovered by Camillo Golgi ( 1898) and was named in honour of him, the Golgi Apparatus. In plants and in lower invertebrates they are called as dictyosomes.

Distribution: Golgi Apparatus becomes present in all eukaryotic cells and becomes absent in prokaryotic cells. Amongst eukaryotic cells the Golgi complex is not found in fungi, male gametes of bryophyta ,and pteidophytes ,mature sieve tubes, mature sperms and red blood cells of animals.

Form, size and number: The form and size of Golgi complex depends upon the types of cells in which it occurs. Generally they become polymorphic but changes from one shape to another. In some algae only one Golgi body becomes present however in a generalized plat cell these may number upto 25000.

Structure: Golgi complex is an assemblage of flat or curved saccules cisternae lying one above the other in close parallel array. Each cisternal unit corresponds to an individual lamella which bounds a thin closed cavity of about 150 A0

across and of variable length. Four to eight such cisternal units are commonly spaced 200-300 A0 apart and thus form multi layered cisternae. On their convex outer surface becomes present small myriad vesicles , 440-800 A 0 in diameter which evolve from the edges of cisternae.

Function: Golgi complex performs the following major functions:

(i) Synthesis of polysachharides.(ii) Formation of glycoproteins by combining carbohydrates with proteins.(iii) Synthesis of pectins and other carbohydrates necessary for cell wall

formation. Secretary vesicle of Golgi complex get fused to form cell membrane. Similarly in plant cells , Golgi apparatus deposites pectic substances and cellulose microfibrilis to form cell plates during cell division.

(iv) It also secretes gum and mucilage.(v) It is associated with storage, condensation, packaging and

transportation of various substances.

(vi) It is also active in the transformation of one type of membrane to another.

(vii) Secretary vesicles and lysosomes are originated from Golgi apparatus.

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Plastids

The term plastids has been derived from a Greek word p-lastikos meaning there by sac like or bag like. It was used by Schimper( 1885) for the first time. It is found inside the cells of higher eukaryotic plant cells.

Types of plastids: Three major types of plastids are recognised on the basis of their colour.

(i) Chloroplast – green coloured plastids(ii) Chromoplast- coloured other than green(iii) Leucoplst- colourless plastids.

All these kinds of plastids are interconversible means they can be changed from one type to another. Structure of chloroplastThe detailed structure of chloroplast is given below as it is of wide occurance and is of great importance. This type of plastid contains the photosynthetic pigment the chlorophyll. Chloroplast becomes present in all eukaryotic green plant cells. Fungi lack chlorophyll and that is why they are heterotrophic in nature. In prokaryotic cells like blue green alga and bacteria the organised chloroplast becomes absent.1. Form, size and number of chloroplast: The shape of

chloroplast becomes fixed but is variable in case of algae where it becomes of different shapes. In that case it becomes ribbon like, reticulate, cup like, collar like,spiral, stellate, discoid, spherical or ovoid. In higher plant it is discoid in shape. The diameter of chloroplast varies from 4 to 8 u and about 2u thick. Most of the algal cell possess single chloroplast but in higher plant cell the number starts from 20 to 40 per cell.

2. Structure: Chyloroplast is surrounded by two unit membrane, each about 40-60 A0 thick. There is a space in between two membrane called as periplastidaql space. This space measures about 100-300 A0. The internal structure of plastids show two distinct parts-(i) Colourless ground substances callede as stroma and (ii) Closed, flat, sack like membrane system called grana.

Stroma is watery and protinaceous ground substance. It contains starch grains , lipid droplets, RNA,DNA and free ribosomes (70S). The dark reaction of photosynthesis takes place in grana.Grana are densely packed stacks of membrane layers called thyllakoids. Each thyllakoid is bound by a single

membrane but because of flatness of these structures they appear as double membraned structures or lamellae. Large number of grana are formed at frequent intervals by packed stackes of thyllakoids. This is the place where light reaction of photosynthesis takes place. Two adjacent grana are joined together with one another by lamellae called intergranal lamellae, or stroma lamellae or fret

channel. Each chloroplast contains about 40-60 grana.

Structure of chloroplast

A lamellae is made up of bimolecular layer of lipo-pigments sandwiched in between two layers of protein. The lipopigment layer contains chlorophyll and carotenoid pigments along with phospholipids.The chlorophyll molecules are so arranged in this layer that their hydrophilic porphyrin head is close to the inner surface of protein layers.Amongst these lame;llae , distinct groups , each consisting of about 230 to 300 chlorophyll molecules , called quanttasomes are present. Quantasomes are smallest morphologically distinctand photosynthetically functional units. Chloroplasts also have 70s ribosomes which are either free or attached to intergranal lamellae. Pyrenoid is present in the chloroplast of some algaeand other plants.It becomes consisted of a protein core surrounded by peripheral starch plates.Chemical composition: The chemical;lcomposition of chloroplast is as follows:1. Protein 40-50%2. Phospholipids 23-25% 3. Chlorophyll 3-10%4. Carotenoids 1-2%5. RNA 5%6. DNA in trace7. Enzymes in trace

8. Various mineral elements in trace

(Cu, Fe, Mn and Zn etc)

Function of chloroplast:Chloroplast performs the following functions:

1. Absorption of light energy.2. Production of NADPH2 and evolution of oxygen through the process of

Hill reaction.3. Transfer of carbon dioxide obtained from air to 5 carbon sugar ( Ribulose

1,5,di phosphate) .4. Breaking of 6 carbon compound into two molecules of phosphoglyceric

acid.5. Hydrogenation of phosphoglyceric acid to form phosphoglyceraldehyde.

CHROMOPLASTThis is special types of plastid characterised by its specific colour pigments. They can arise either from chloroplast as in petals where it becomes initially green but becomes coloured subsequently or from leucoplast as in carrot root. No perfect structural organisation of these plastids are fairly known. Some times these colouring pigments are present in vacuolar sap as in petals. In this form they provide plants to resist against thermal fluctuations. They are also cold resistant and that is why variously coloured flowers are produced in temperate regions and during winter in tropical regions. At times they are believed to be attracting insects for the process of pollination. LEUCOPLASTThey are colourless plastids because they donot contain any visible pigments. Some times athey are white in colour. They are involved in the storage of various kinds of reserved food materials and accordingly they are named. On that basis they are categorised into different categories:(a) Amyloplast ( Starch)(b)Elaioplast and (Oil or fat)(c) Leucoplast or Aleuroplast. ( Protein)

NUCLEUSThe most important cell organelle is called nucleus because it is the centre of all functions of the cell. First of all it was discovered by Robert Brown ( 1831) as central body or polar body. Distribution:Nucleus is present in all eukaryotic cells. Generally single nucleus becomes present in each cell but this number may vary in different cells. Depe3nding on the number of nucleus the cell is called uni, bi or multi nucleate. Multi nucleate condition is found in case of fungi due to failure of cell plate formation. This condition is known as coenocytic condition.Form of nucleus:The shape and size of nucleus varies with the type of cell . It may become rounded or biscuit shaped but in case of some insects it may become irregular in shape.Structure of nucleus:The nucleus can be easily distinguished into four parts.(1.) Nuclear membrane(2.) Nucleolus(3.) Nucleoplasm and (4.) Chromatin fibres

Structure of Nucleolus

Nuclear membrane: The nucleus remains externally bounded by lipoproteinaceous two unit membranes that run parallel to one another with a gap of about 200 to 400 A0 . This gap is called perinuclear space which is in fact an electron transparent area . Electron microscopic studies have revealed that the boundary between the nucleus and the cytoplasm has a complex double membrane structure . Considering the fact nuclear envelop is now preffered a new term to the nuclear membrane. The outer membrane at places gives rise to tubular structure called as endoplasmic reticulum. In certain cases microtubules on the surface of nuclear membrane is also found. Besides , the nuclear envelop possess here and there openings of 300-600A0 size called as annuli through which nucleoplasm communicates with cytoplasm. Yoo and Bayley(1967) found eight spheroid particles arranged around the cercumference of each annulus in the nuclear envelop of pea seedlings.Nucleoplasm : Encircled by the nuclear envelop is found the nuclear sap ( nucleoplasm or karyolimph) . The nucleoplasm is semifluid substance which contains nuceic acid, nucleoprotein ( both histone and protamines) etc and forms the matrix.Nucleolus : It is found lying in the nucleoplasm. There may be one or more nucleolus per nucleus. These nucleoloi are spherical bodies consisted of protein and RNA. No separate membrane has been detected for nucleolus. The recent studies have revealed that a typical nucleolus has two distinct structural

regions. The core and The cortex. The core is the central region composed of fibrous elements . It remains surrounded by grannular area called as cortex. It is believed that the nucleolus helps in the synthesis of nucleoprotein. Chromatin network :It forms the skeleton of nucleus. Chromatin threads are the sites of main genetic material which controlls all the activities of cell metabolism as well as the heredity. During cell division these chromatids become tightly packed by definite protinaceous matrix and called as chromosome.Chemical composition : Chemically the nucleus becomes consisted of following components:1. Protein 70%2. Phospholipids 10%3. DNA 3-5%4. RNA 1-2% 5. Some RNA Polumerase

Function of nucleus:

Nucleus performs the following functions:

Lysosome

Commonly called as suicidal bag of cell due to having lytic ability . Every eukaryotic cell has a group of cytoplasmic organells , the lysosome, of which the mainm function is intracellular or extra cellular digestion. They may be distinguished from other organelle by their morphology and function.

The concept of lysosome was originated from the development of a cell fractionation technique, by which different subcellular components were isolated. De Duve , 1949 isolated a class of particle which has centrifugal properties intermediate between those of mitochondria and microsomes and it was also reported that they have high content of acid phosphatase and other hydrolytic enzymes. Because of their enzymatic properties in 1955 these particles were named as lysosome.

Types of lysosome:

Lysosome can be grouped into two types

1. Primary lysosome2. Secondary lysosome

Lysosome show considerable polymorphism. The primary lysosome(i.e. storage granules) are dense partcles of about 0.4um surrounded by a single membrane. Their enzymatic content is synthesized by ribosome in ER. And appears in the Golgi region. Secondary lysosome (i.e. digestive vacuoles) results from the asssociation of primary lysosome with vacuoles containing phagocytized materials.

Function of Lysosome:

Lysosome performs the following functions:

1. Digestion of foods or various materials taken by phagocytosis or pinocytosis.

2. Digestion of the parts of acell by a process called as autophagy3. Breakdown of extracellular materials by the release of enzymes

into the surrounding medium.

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CELL MEMBRANE OR PLASMA MEMBRANE

It is also called as bio – membrane which is found just below of the cell wall in case of plant cell and encircles the whole cell or acts like boundary of cell in case of animal cell. It is thin elastic and semipermeable membrane composed of lipid bilyer and bilayer of protein.i) Membrane protein: Three different types of

membrane proteins are found such as structural protein, enzymes and carrier proteins.a) Structural proteins: constituting the backbone of

cell membrane , the structural proteins are extremely lipophilic that display little catalytic activity. In such proteins from one fourth to one third of aminoacid residues show a- helix secondary confirmation while the remainder forming the random coil (Lenard and Singer, 1966) . Thus it is clear that most pert of the cell mebrane becomes vonsisted of structural proteins.

b) Enzymes: In many cases , themajor components of membranes are catalytic protein or enzymes that vary considerably in structure and amount. Such proteins specify the physiologic function of membrane.

c) Carrier proteins: Carrier proteins of mebrane has not yet been defined. Their molecular weight is close to 3.2x 104 daltons in Salmonella typhimurium where it acts as sulphate carrier.

Both carrier and structural protein are of low molecular weights.

ii) Membrane lipids; The amount of lipids present in membrane vary from 30% percent( in mitochondria) to 75 % percent ( in myelin) of the total dry masses of biomembrane. However major lipids are polar lipids and sterols. In animal membranes , the polar lipids present are about 70 to 85 percent consisting of

primarily phosphatidyl choline , phosphatidyl ethanolamine, sphingomylein, phosphatidyl serineand cerebrosides in varying proportions. The mitochondrial membrane is enriched with diphosphatidyl glycerol.

iii) Membrane polysaccharides and glycolipids: The amount of polysaccharides present in biomembrtanes is extremely low. Cerebrosidfe glycolipids are present in mylein and gangliosides are present in many other membrane.

iv) Membrane RNA: Considerable evidences for the occurance of RNA as integral components olf biomembrane have been observed . Bal (1966) while working on Alium cepa noted that RNA is an integral component of nuclear membrane, endoplasmic reticulum, plasma membrane and cell wa,ll. The exact location of RNA is however not clear . Shapot and Pitot (1966) have detected small amount of RNA in the smooth as well as rough membrane of endoplasmic reticulum. RNSA has also been reported from the membrane of chloroplast and mitochondria.

v) Membrane DNA: DNA has been detected from the membranes of mitochondria, chloroplast, yolk granules and plasma membranes. In first three cases the DNA is present in circular form and can be easily separable .

vi) Some other components: Some othjer components playing a vital role in membrane physiology have also been detected such as certain metals( Ca, Zn , Mg) , coenzymes, porphyrines( hematin, chlorophyll) etc. are found present in biomembrane.

Models of Biomembrane structure:

Of the many models proposed for biomembrane few will be discussed here.

1. Danielli-Davson Model: Up to 1936 this model dominated in the mind of biologists, biochemists, and biophysicists as it answered to some extent the problem of permeability of cell. The chief features of the model is one or more bimolecular leaflets of phospholipids sandwitched between two layers of globular proteins in essentialy separated and continuous phase. The phospholipids molecules are closely packed and standing perpendicularto the surface of membranes with their polar heads adjacent to the surface proteins.

2. Unit membrane hypothesis: Robertson(1964) proposed this hypothesis. The model however did not differ from Danielli-Davson model. It restricted the number of bimolecular leaflets of lipid to one and muco protein on outside face and non mucoid protein on the inside fgace of membrane. In both these models the lipids and proteins are held together electrostatically, the former being negatively charged and the later being positively charged.

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Structure of cell membrane 3. Kavanaus lipid pillars: This model was proposed by J.L.Kavanau (1966) .

This model visualises the the lipid components being organised in descrete pillars of lipid micelles under cwertain conditions and as nearly continuous flattened discs under other conditions.The gaps between micelles serve as water pore for the ion penetration. The protein is however becomes present on both the sides of membrane and consists of two types (i) matching the areas delimited by the bases of pillars and (ii) filling in the spaces among these. The surface of each pillar or disc is comprised of the polar head of phospholipids and the anterior of these is filled with non polar tail of phospholipids and non esterified sterols.

4. Hydrophobic binding models: Benson (1966) while working on chloroplast membrane and Fleischer and Brierly (1961,66) working on mitochondrial membrane observed that in a membrane binding between lipid and prpotein is of hydrophobic rather than electrostatic in

nature, the unit membrane concept has been challenged. Green and Perdue (1966) have also supported his view and visualised that the union between two (protein and lipid ) is too tight to be explained by electrostatic concept. However these authors visualise a strong hydrophobic association among the hydrocarbon side chain of aminoacid residues of membrane protein and the hydrocarbon tails of lipids.

5. Bensons model: Based on the chloroplast membrane, Benson (1966) proposedthat the lipid of membrane sub-units are bound by hydrophobic association of their hydrocarbon tails with complementary hydrophobic regions within the interior of protein leaving ionic chrged groups of the phospholipids on the membrane surface.

6. Lenard and Singers model: The first few proposed models assumed that protein is extended in B- conformation in the membrane. But the study of Lenard and Singer revealed that one fourth to one third proteins of membrane show a- helix of secondary conformation while remainder exists as random coils.

7. Mosaic membrane concepts: Green and Perdue (1966) proposed that membrane comprises of repeating units which are assymmetrical in a direction perpendicular to the surface and these repeating units of membrane are not identical. They have compared these repeating units with the elementary partcles of mitochondria that extended outwards from membrane comprising cristae.

8. Composite model: Schjeide and Lin (1970) visualise that any model proposed to explain the structural organisation of biomembrane should exhibit the following characteristics and then it should be called as composite model.i) The different components can be fixed in physical orientations

ranging from lamellar sheet to aggregation of microtubules.ii) The integral components may include proteins, lipids, glcolipids,

carbohydrates, RNA and DNA.iii) The association between lipids and proteins is mainly of

hydrophobic in nature. However, corpuscular units may be linked by electrostatic bonds.

iv) The polar groups are oriented towards the surface while the hydrophobic groups are mainly located internally.

v) They are usually mosaic and often assymetric.9. Fluid mosaic model: Singer and Nicolson (1972) summerised the modern

concepts concerning biomembrane. The model accomodates the presence of bilayer of lipid molecules containing a variety of protein molecules. Some of the protein molecules penetrate right through the lipid layer , whilst other donot. The function of lipid layer is to provide the basic permeability barrior . The proteins are concerned with two functions, i.e. with the transport of molecules from one side of membrane to other side and with the various enzymatic activities of the membrane.Besides , the recent studies (Lee 1975) on biological membrane have suggested that membrane now should be pictured as highly fluid rather than as a highly ordered and static structure. Besides these some special structures are also found on biomembrane and they are microvilli and desmosomes.

Functions of biomembrane: Some of the known functions of biomembrane may be summerised as follows:(i) It acts as permeability barriors which control and coordinate rates

of substrate transfer and diffusion.

(ii) It acts as cytoskeleton providing mechenical frames on which enzymes can be specifically oriented.

(iii) It acts as vehicles for transport of substances from one organale to other, from inside to the outside of cell and from outside to inside of cell.

(iv) It acts as source of informations of various kinds.(v) It acts as elements capable of synthesizing various

macromolecules.(vi) The function like phagocytosis and pinocytosis are also associated

to it.

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