viruses, history and origin of viruses, types of viruses, semester 3, botany notes,

  In nature, there are many ultramicroscopic particles known as viruses. A virus is a small particle composed of two types of substances, protein and nucleic acid; they are seen only with the help of electron microscope. Viruses have an important position in comparative study of living and nonliving, because they are entities sharing the characters of both they cause wide range of disease in plant and animals. The study of viruses has become so important and detailed that their study is treated in a new branch known as Virology.

  VIRUSES  

Definition: -

Various definition of viruses has been given, some of them are as follow: -

"A virus particle is in effect, a minute package of DNA or RNA surrounded by a protein coat or capsid." 

"A genetic element enclosed in a protein coat". 

"A core of nucleic acid either DNA or RNA surrounded by a protein coat shell".

 Green (1935) define viruses as "the smallest unit showing reproductive properties considered typical of life".

Bawden (1949) defined viruses as "obligate parasite too small to be seen".

According to Luria (1953) viruses are "submicroscopic entities capable of being introduced into a specific living cell and reproducing inside such cells only."

HISTORY: -

In 1886 Mayer described Tobacco Mosaic Disease but he could not find any casual agent. Credit for the discovery of viruses goes to the Russian botanist Iwanowski (1892) who found the causes of tobacco. He reported that the sap of infected plant filtered through bacteria proof filter was equally potent in causing the disease and conclude that the cause of disease was not bacterium but some other a smaller particle known as Virus.

Beijerinck (1898) called them "Contagium vivum fluidum " (i.e, living fluid infectant).

• In the following year the other filterable viruses" were discovered that were responsible for infection in plant animals and bacteria Bacteriophages (Virus infecting bacteria) were discovered by Twort (1915) and d. Herelle (1917). Shafferman and Moris (1963) discover Cyanophage that cause infection in blue green algae. In 1935 W.M. stanley crystallized TMV and showed that crystals retained their infectivity even when stored for indefinite period in a bottle. That the viruses could be crystallized showed that were not cells, but much simpler chemical entities. 
• Bawden and Pirie (1937) studies the chemical nature of TMV particles and reported that they are nucleoproteins.

CHARACTER OF VIRUSES: -

• Viruses are ultramicroscopic infections particles ranging from 20 to 35 nanometer in diameter.
• They're nucleoproteins.
• They contain DNA or RNA but not both which is either single standard or double standard.
• They DNA or RNA is surrounded by a protein coat that may be attached to more complicated structures.
• They are highly pathogenic cause disease in plants, animals, and bacteria.
• They are without protoplasm.
• They grow multiply and undergo mutation only within living host cells.
• They cause infection in host by nucleic acids.
• The genome has four gene, one for the core protein, two for the replicase enzyme and the fourth for a protein that probably enable the virus to spread from cell to cell in the plant.
• They are easily transmitted from one organism to another.
• They are not affected by antibodies.
• They can be crystallized.
• They are perfected obligate intracellular parasites that is they depend upon a specific host for their production and development the cells of animals, plants, and bacteria can serve as host to virus. Outside the host cell viruses exit as individual particles called Virions.

👉On the basis of the characters, given above it is not easy to say that viruses are living or nonliving. They have characters of both.

NOTE: -with few exceptions plant viruses have only RNA and a single coat made of one or a few kinds of proteins.

(a)character of non-livings: -

1. They do not have protoplasm. 
2.  They do not have enzyme system.
3.  They do not have respire.
4. They can be crystallized.
5.  Attempts to culture viruses in different types of culture media (outside the cells) have failed.

(b)Characters of living beings: -

1. They replicate, although inside the living cells.
2.  Nucleic acid present in their body, are capable of synthesizing protein for their coat, although they use ribosomes of the host for the propose.
3. Nucleic acid shows similar gene mutation as chromosome of the living organism.
4. They cause disease lack bacteria and fungi.

 However, on the basis of the above given characters many scientists considered a virus as bridge between living and nonliving or in other words they represent the transitional form of life, lying on the border line between the living and nonliving because viruses lack many features associated with life, they are referred to as particles.

CLASSIFICATION: -

According to Holmes (1948), viruses are classified on the basis of their host into the following kinds: -

(a)Animal viruses ( Zoophagineae): -They infect, fowl, pigeon, parrot, dog, crow and arthropods insect, etc. They've usually DNA or many also have RNA.

(b)Plant viruses (Phytophaginae): - They impact angiosperms, like potato, tobacco, sugarcane, cucurbits, beans and any other higher plants. They have RNA.

(c)Bacterial viruses (Phagineae): - They are commonly called bacteriophages or phases i.e, eaters of bacteria they have DNA.

 

I)TOBACCO MOSAIC VIRUS: - It is the most thoroughly studied rod shaped virus. It was isolated in the crystalline form by W.M.stanley(1935). Nobel prize was awarded to him for this discovery. Electron microscope a study have related that the TMV is a road shaped structure, 18 nanometer is diameter and 300 nanometer long. The protein coat (capsid or shell) consists of 2,130 identical subunits (capsomeres), which are arrange in a helix around a central hole of 4 nanometers in diameter. The protein constitutes 94.5% of virus. TMV contains a single chain of RNA of about 330nm in length and runs the entire length of the TMV rod. It contains about 7300 nucleotides. RNA is not present in the center of the hole but in terms with the protein subunit. It protrudes from one end of the virus. It is 5.5% of the virus. Frankel Conart and Schramm (1955) found that RNA alone is capable of causing infection. 

mitochondria, history and origin of mitochondria, function of mitochondria,

 

  MITOCHONDRIA  

Introduction: - 

Mitochondria (Gr., mito, thread; chondrion, granule) are thread like or granular structures of eukaryotic cells. These may assume rod-like shape called chondriosomes which may enlarge or aggregate to form massive spheroidal bodies called chondriospheres. These are not present in bacterial cells. Mitochondria are the 'power plants' which by oxidation release the energy contained in the fuel molecules or nutrients and make other forms of chemical energy. The main function of mitochondria is oxidative phosphorylation, which is an exergonic reaction, meaning that it releases energy. In prokaryotes, oxidation of organic material is carried out by plasma membrane enzymes. 

 History: - 

Kölliker (1880) was the first who observed the mitochondria in insects muscle cells. He called them as 'sarcosomes'. Flemming (1882) named the mitochondria as 'fila'. Altmannin 1894 observed them and named them Altmann's granules or bioblasts. The term 'mitochondria' was applied by Benda (1897-98). They were recognized as the sites of respiration by Hogeboom and his coworkers in 1948. Lehninger and Kennedy (1948) reported that the mitochondria catalyze all the reactions of the citric acid cycle, fatty acid oxidation and coupled phosphorylation.

Morphology of Mitochondria: - 

 Morphologically mitochondria may be in the form of filaments or small granules. These may assume rod-like shape called chondriosomes which may enlarge or aggregate to form massive spheroid bodies called chondriospheres.

1. Position- Mitochondria lie freely in cytoplasm, possessing power of independent movement and may take the form of filaments. In some cells they can move freely, carrying ATP where needed, but in others they are located permanently near the region of the cell where more energy is needed. E.g., in the rod and cone cells of retina mitochondria are located in the inner segment, in cells of kidney tubules they occur in the folds of basal regions near plasma membrane, in neurons they are located in the transmitting region of impulse, in certain muscle cells (e.g., diaphragm), mitochondria are grouped like rings or bracers around the I-band of myofibril. During cell division they get concentrated around the spindle. 

2. Number- The number of mitochondria varies a good deal from cell to cell and from species to species. A few algae and some protozoans have only single mitochondria. Their number is related to the activity, age and type of the cell. Growing, dividing and actively synthesizing cells contain more mitochondria than the other cells. In Amoeba (Chaos chaos), there may be as many as 50,000 mitochondria. In rat liver cells, these are few in number, about 1000 to 1600. Some Oocytes contain as many as 3, 00,000 mitochondria. 

3. Size- The average size of mitochondria is 0.5-1.0µ in diameter and about 2-8 µ in length. In exocrine cells of mammalian pancreas, they are about 10 µ long and in oocytes of amphibian Rana pipiens are 20-40µ long. Yeast cells have the smallest mitochondria.

 Ultra structure of Mitochondria: - 

The electron microscope shows the mitochondrion as the vesicles bounded by an envelope of two unit membranes and filled with a fluid matrix. Granule Inner Membrane, Outer Chamber F1 Particle DNA Matrix Cristae Ribosome Outer Membrane 

1. Membranes- Both the inner and the outer mitochondrial membranes resemble the plasma membrane in molecular structure. Each of them is 60-70Å, tri lamellar and composed of two layers of phospholipid molecules sandwiched between two layers of protein molecules. However, the two membranes differ in the kinds of protein and lipids they have and also in their properties. Both the outer and the inner membranes contain specific pumps or channels, for the transport of molecules through them. The membranes may be connected at adhesion sites through which proteins are transferred from the outer to the inner membrane. The outer and the inner membrane are separated from each other by a narrow space called the inter-membrane space or outer chamber or peri-mitochondrial space. It is about 80Å wide. It contains a clear homogeneous fluid. 

(i) Outer Membrane- The outer membrane is smooth permeable to most small molecules, having trans-membrane channels formed by the protein 'porin'. It consists of about 50% lipid, including a large amount of cholesterol. It contains some enzymes but is poor in protein.

(ii) Inner Membrane- The inner membrane is selectively permeable and regulates the movement of materials into and out of the mitochondrion. It is rich in enzymes and carrier proteins permease. It has a very high protein/lipid ratio (about 4:1 by weight). It lacks cholesterol. Cardiolipin is closely associated with certain integral proteins and is apparently required for their activity. 

2. Matrix- The space between the cristae called the inner chamber is filled with a gel like material termed the mitochondrial matrix. It contains proteins, lipids, some ribosomes, RNA, one or two DNA molecules and certain fibrils, crystals and dense granules. 

3. Cristae- The inner mitochondrial membrane bears plate like infoldings called the cristae. They extend inwards to varying degrees, and may fuse with those from the opposite side, dividing the mitochondrion into compartments. They are arranged in a characteristic manner in different cells. Normally they run at right angles to the long axis of the rod- shaped mitochondria. In cells of the proximal parts of the kidney tubules, the cristae are longitudinal folds parallel to the long axis of mitochondrion. In many protozoans, in insect flight muscles cells and in adrenal endocrine cells the cristae are tubular. Cristae are lamellar in hepatocytes. In heart muscle cells cristae are zigzag.  They also vary in number. The active cells may have numerous cristae whereas the inactive cells may have only a few. The cristae have in them a narrow intra-crista space. It is continuous with the inter-membrane space. The cristae greatly increase the inner surface of the mitochondrion to provide enough space for housing enzyme assemblies. The cristae also allow for expansion or swelling of mitochondria under different metabolic and environmental conditions.

4. Oxysomes- The inner mitochondrial membrane bears minute regularly spaced particles known as the inner membrane subunits or elementary particles (EP) or oxysomes. An oxysome consists of three parts- a rounded head piece or F1 subunit joined by a short stalk to a base piece or F0 subunit located in the inner membrane. There may be 100,000 to 1000,000 oxysomes in a single mitochondrion.

Biogenesis of Mitochondria: - 

The formation of new mitochondria has been explained with the following hypothesis. 

1. De Novo Synthesis- According to this hypothesis mitochondria arises de novo from precursors in the cytoplasm. 

2. Origin from membrane- This hypothesis proposes that the mitochondria arises from the invaginations of plasma membrane, endoplasmic reticulum, Golgi apparatus or nuclear envelop. The membrane invaginates and extends into the cytoplasm as a tubular structure. It gradually becomes curved and folded and forms a double walled structure, the mitochondrion. 

3. Develop from Micro bodies- It is held that they mitochondria are developed by the accumulation of micro bodies in the cytoplasm. A micro body consists of a single outer membrane and a dense matrix with a few cristae which eventually develops into fully formed mitochondria. 

4. Prokaryotic Origin- It is believed that mitochondria are originated from bacteria. It is supported by many evidences. 

(i) First is the localization of enzymes of respiratory chain, which in case of bacteria, are localized in plasma membrane which can be compared with the inner membrane of the mitochondrion. 

(ii) In some bacteria, plasma membrane forms membranous projections (called mesosomes) like cristae of mitochondria. These mesosomes possess respiratory chain enzymes. 

(iii) The mitochondrial DNA is circular as it is in bacteria. Replication process of mitochondria is similar to bacteria. 

(iv) Ribosomes in mitochondria are smaller and similar in size to that of bacterial ribosomes. Chloramphenicol inhibits the synthesis of protein in mitochondria as well as in bacteria. Furthermore, in the process of protein synthesis, mitochondria depend partially on mitochondrial matrix and DNA and partially on nucleus and cytoplasm of the eukaryotic cells. It exhibits the symbiotic nature of mitochondria. These evidences support the prokaryotic origin of mitochondria. 

5. Replication- It is held that mitochondria are self-replicating organelles. New mitochondria arise by some type of splitting process from pre-existing mitochondria. The last hypothesis seems probable. Since the mitochondria have their own DNA and ribosomes, they can replicate new mitochondria. However, there is a nuclear control over the process as the mitochondria synthesize some of their proteins themselves and get others from the cytoplasm of the cell formed under the direction of the nuclear DNA

Functions of Mitochondria: - 

Mitochondria perform the following functions: - 

1. Cell respiration takes place in mitochondria and so they are known as the 'powerhouse' of the cell. They bring about stepwise oxidation of food stuffs or "low-grade" fuel of the cell and transfer the energy so released to the energy carrier ATP, the "high-grade" fuel of the cell. ATP is used to bring about the energy-requiring activities in the cells, namely, biosynthesis, active transport, transmission of nerve impulse, muscle contraction, cell growth and division and bioluminescence. 

2. Mitochondria provide intermediates for the synthesis of important biomolecules such as chlorophyll, cytochromes, steroids etc. 

3. Some amino acids are also formed in the mitochondria. 

4. Mitochondria actively accumulate calcium ions as calcium phosphate precipitate. They regulate the calcium ions concentration in the cytoplasm by storing and releasing Ca+ The calcium ions regulate numerous biochemical activities in the cell

 Electron Transport Mechanism: - 

In the electron transport chain electrons are transferred from a donor molecule to an acceptor molecule, thus, it consists of a several electron receptors. Molecular oxygen is the final hydrogen acceptor. The respiratory chain is located in the inner mitochondrial membrane. In the respiratory chain, the electron transfer is done in stepwise fashion in which the electron pairs are passed from one acceptor to another, thus, delivering energy more gradually. Flow of electrons in mitochondria occurs as follows: 

Self-Assessment Questions and Possible Answers: - 

Multiple Choice Questions: - 

1. Cell's power houses are its: 

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

2. Mitochondrion is bounded by: 

 (a) A single unit membrane (b) Two-unit membranes (c) No membranes (d) Plasma membranes 

3. New mitochondria arise: 

 (a) De novo (b) By replication (c) From plasma membrane (d) from nuclear envelop 

4. The ATPase enzyme is located in the mitochondria in: 

 (a) Oxysomes (b) Outer membrane (c) Inner membranes (d) Matrix 

5. The name mitochondria were given by: 

 (a) Altman (b) Flemming (c) Benda (d) Kollikar 

6. ETS is located in: 

 (a) Outer mitochondrial membrane (b) Inter membrane space(c) Inner mitochondrial membrane   (d)mitochondrial matrix 

ANSWERS 

1. (b) 2. (b) 3. (b) 4. (a) 5. (c) 6. (c)

Very short questions: 

1. Where are ETS enzymes located in mitochondria? 

2. Give the function of mitochondria. 

3. What are cristae? 

4. What type of DNA do mitochondria have? 

5. Mention three parts of oxysome. 

6. Who named mitochondria? 

7. What kind of enzymes is present in the mitochondria? 

8. Name the enzymes oxysomes represent. 

9. Which is the most common energy carrier in cells? 

10. Give alternative names of oxysomes. 

ANSWERS: -

1. Inner membrane 

 2. ATP formation 

 3. Infolds of inner mitochondrial membrane 

 4. Circular, single molecule and double stranded 

 5. Head piece, stalk and base piece (FO & F1) 

 6. Benda 

 7. Respiratory enzymes 

 8. ATPase (ATP Synthetase) 

 9. ATP 

 10. Elementary particles, inner membrane subunits, F0-F1 Complex.

plasma membrane, history of plasma membrane, structure of plasma membrane, chemical composition of plasma membrane,

Every cell, prokaryotic or eukaryotic, is surrounded by a thin layer of outermost boundary called the plasma membrane or cell membrane or plasma - lemma. The plasma membrane is a discrete structure and is remarkably complex in its molecular organization. It maintains the difference of the internal environment of the cell from its external environment by controlling the entrance and exit of the molecules and ions. It checks the loss of metabolically useful substances and encourages the release of toxic metabolic byproducts of the cell. Thus, it functions as semi-permeable or selectively permeable membrane. It is about 70-100Å in thickness. In plant cells plasma lemma is further covered by cellulosic cell wall. It is an important cell organelle composed of lipids and proteins. It possesses devices for attachment to other cells for cell-to-cell communications, ion pumps for controlling internal milieu of the cell, receptors for hormones and mechanisms for the production of secondary messengers that activates the cell's physiological response.  

  Plasma Membrane  

History and Origin 

It had been shown by Karl W. Nageli (1817-1891) that the cell membrane is semipermeable and is responsible for the osmotic and other related phenomena exhibited by living cells. Before 1855, he used the term Zellen membrane in his early papers. The term plasma membrane was used in 1855 by him to describe the membrane as a firm protective film that is formed by out flowing cytoplasm of an injured cell when protein rich cell sap came in contact with water.

Ultra-Structure of Plasma Membrane: -

Symmetrical Molecular Structure of Plasma membrane: -

Plasma membrane is a tripartite structure and is made up of three layers, having total thickness of 75Å. Two di-electronic layers are there, each of 25Å thickness, enclosing a middle dielectronic layer which is also 25Å thick. The middle layer is a trimolecular layer of lipids having its non-polar hydrophobic groups facing inwards, whereas polar hydrophilic groups facing outwards. The hydrophilic polar groups are covered by a protein layer which is 20 to 25Å thick. The protein chains lie at right angles to the lipids.

Asymmetrical Molecular Structure of Plasma Membrane: -

It is also a tripartite structure having a thick inner dielectronic component of 35-40 Å, a narrow outer dielectronic component of 25Å thickness, and a central dielectronic layer (bimolecular layer of lipids) which is 30Å wide; thus, total thickness comes to 90-95Å. In different types of cells, the thickness of plasma membrane varies. For example, in red blood corpuscles of rabbit, the plasma membrane is about 215 Å thick whereas, in intestinal epithelial cells it is 105 Å in thickness. Very small pores measuring about 10Å in diameter (smaller than pores of nuclear membrane) have been discovered in the membranes.

Chemical Composition of the Plasma Membrane: -

Plasma membrane is primarily composed of protein and lipid, although carbohydrate is often present in association with protein (as glycoprotein) or lipid (as glycolipid). However, the relative proportions of protein and lipid vary considerably in membranes from different sources.

Lipids: -

The plasma membrane contains about 20 to 79% lipids mainly of three types like 

phospholipids, cholesterol and glycolipids. The phospholipids which make up between 55% and 75% of the total lipid content, consists chiefly of lecithin and cephalin. The remainder consists of sphingolipids (with an amino group) and glycolipid conjugates with 

carbohydrates. Phospholipids derived from glycerol are called phosphoglycerates. A phosphoglycerate is made up of two fatty acid chains, a glycerol backbone and a phosphorylated alcohol. The outer layer of phospholipids consists mainly of lecithin and sphingomyelin, while the inner layer is composed mainly of phosphatidyl ethanolamine and phosphatidyl serine (both are phosphoglycerides). The glycolipids (sugar containing lipids) are mainly in the outer half of the bilayer.

Cholesterol is present in eukaryotes but not in prokaryotes. Plasma membrane of cells such as erythrocyte, liver cells and myelinated nerve cells are rich in cholesterol Membrane lipids are amphipathic molecules, they contain both a hydrophobic and hydrophilic moiety. Hydrophilic unit is also called the polar head groups, is represented by a circle and their hydrocarbon tails are depicted by straight or wavy lines. Polar head groups have affinity for water, whereas there. hydrocarbons tails avoid water. This can be accomplished by forming a micelle, in which polar head groups are on the surface and hydrocarbon tails are directed inside. 

 

Another arrangement of lipid molecule in a membrane is a bimolecular sheet, which is also called a lipid bilayer. Phospholipids and glycolipids are key membrane constituents of bimolecular sheets. Hydrophobic interactions are the major driving force for the formation of lipid bilayer. The lipid bilayer of the membrane is interrupted only by the proteins that 

traverse it. This bilayer consists primarily of:

(a) Neutral Phospholipids and Cholesterol: These includes phosphatidylenoline , lectin 

cerebroside, and sphingomyelin and phosphatidyl ethanolamine. They are without any electric charge at neutral pH and are closely packed in the bilayer along with cholesterol.

(b) Acidic Phospholipids: These constitute about 5% to 20% fractions of the total phospholipids of plasma membrane. They are negatively charge and are associated with proteins by way of lipid - protein intentions. Common examples are phosphatidyl inositol, phosphatidylserine, sulpholipids, phosphatidyl glycerol and Cardiolipin. In plasma membrane, lipid fractions form permeability Barrie and structural framework

Proteins: - 

Proteins are the main component of plasma membrane. Myelin sheath (membrane surrounding some nerve axons) is composed of about 80% lipids and 20% protein and presence of lipid makes myelin an excellent insulator. Eukaryotes membrane which serves 

primarily as permeability barriers possesses about 50% proteins and 50% lipid. Plasma membrane that are actively involved in energy transfer, such as inner membrane of 

mitochondria, chloroplasts and membranes of aerobic prokaryotes have large amounts of 

proteins i.e., about 75%. They not only provide mechanical support but also act as carriers or channels, serving for transport. In addition, numerous enzymes, antigens and various kinds of receptor molecules are present in plasma membranes. Membrane proteins are classified as integral (intrinsic) or peripheral (extrinsic) according to the degree of their association with the membrane (Singer, 1971). 

(a) Peripheral Proteins: They are also called extrinsic proteins associated with membrane 

surface. These can be separated by addition of salts, soluble in aqueous solutions and usually free of lipids. They are bound to the surface by electrostatic and hydrogen bond interactions. They form outer and inner layers of the lipid bilayer of plasma membrane. Common examples are cytochrome-C found in mitochondria, acetyl cholinesterase in electroplax membrane and spectrum found in erythrocytes. 

(b) Integral or Intrinsic Proteins: These proteins penetrate the lipid layer wholly or partially and represent more than 70% of the two protein types. Their polar ends protrude from the membrane surface while non-polar regions are embedded in the interior of the membrane. Usually, they are insoluble in water solutions and can be separate them from the membrane by detergents or organic solvents. The major integral proteins span the thickness of the membrane and have a small amount of carbohydrates on the pole at the outer surface. This protein appears to be involved in the diffusion of anions across the membrane. Integral proteins may be attached to the oligosaccharides to form glycoprotein or to phospholipid to form lipoproteins or proteolipids. Common intrinsic proteins are rhodopsin found in retinal rod cells and cytochrome oxidase found in mitochondrial membranes. Every protein in the cell membrane is distributed asymmetrically with respect to the lipid bilayer.

Enzymes: -

About 30 enzymes have been found in various membranes. Those most constantly found are 5'-nucleotidase, Na+, K+ activated ATPase, alkaline phosphatase, adenyl cyclase, RNAs and acid phosphomonoesters. Na+-K+ activated Mg+ ATPase plays an important role in the ionic exchange and may also act as carrier protein or permease across the plasma membrane. Some enzymes have a preferential localization. For example, alkaline phosphatase and ATPase are more abundant in bile capillaries, while disaccharides are present in microvilli of the intestine. Enzymes are asymmetrically distributed, for example in the outer surface of erythrocytes there are acetylcholinesterase, nicotinamide adenine dinucleotides and Na+-K+ ATPase. In the inner surface there is NADH-diaphorase, G3PD, adenylate cyclase, protein kinase and ATPase.

 Carbohydrates: -

The membranes of eukaryotic cells usually contain 2% to 10% carbohydrates in the 

form of glycolipids and glycoproteins. Hexose, hexosamine, fucose and sialic acid are the 

commonest carbohydrates found in the membrane. Plasma membranes of neuronal surface 

contain gangliosides (Lapertina, 1967) and are probably involve d in the ion transfer .the  

distribution of oligosaccharides is also highly asymmetrical.

 Salts and water: -

They are also present in cell membranes. Water in cell membranes forms parts of membrane structure as it does in all cell constituents.

Functions of Plasma Membrane: - 

The plasma membrane serves many functions such as: 

•  It maintains the individuality and form of the cell. 
• It keeps the cell contents in place and distinct from the environmental materials. 
•  It protects the cell from injury. 
•  It regulates the flow of materials into and out of the cell to maintain the concentration and kinds of molecules and ions in the cell. A cell remains alive as long as the cell membrane is able to determine which materials should enter or leave the cell.  It forms organelles within the cytoplasm. 
•  Its junctions keep the cells together. 
•  It’s infolds help in the intake of materials by endocytosis (pinocytosis and phagocytosis).  It’s out folds (microvilli) increase the surface area for absorption of nutrients. The out folds also form protective sheaths around cilia and flagella. 
•  Its receptor molecules permit flow of information into the cell. 
•  Its oligosaccharide molecule helps in recognizing self from non-self. 
•  By controlling flow of material and information into the cell, the plasma membrane makes metabolism possible. 
•  It permits exit of secretions and wastes by exocytosis. 
•  It controls cellular interactions necessary for tissue formation and defense against microbes. 
•  It helps certain cells in movement by forming pseudopodia as in Amoeba and eucocytes. 

The bio-membranes around the organelles help the latter to: 

(1) Maintain their identity and functional individuality. 

(2) Receive and turn out required material.

SELF ASSESSMENT QUESTIONS AND POSSIBLE ANSWERS 

Multiple Choice Questions: 

1. According to Fluid mosaic model, the correct sequences of substances in plasmalemma is: 

 (a) L-P-P-L                                                      (b) P-L-L-P 

 (c) P-P-L-L                                                      (d) L-P-L-P 

2. Membrane occurs in: 

 (a) Chromosomes, nuclei and mitochondria (b) Cytoplasm, chloroplasts and mitochondria 

 (c) Cytoplasm, nuclei and starch grains    (d) Chromosomes, chloroplasts and starch grains 

3. Plasma membrane is: 

 (a) Non-selective barrier (b) Selective barrier 

 (c) Impermeable              (d) made of cellulose

4. What limits Animal cells from outside? 

 (a) Cell wall (b) Basement membrane 

 (c) Shell membrane (d) Plasma membrane 

5. Cell membrane consists of: 

 (a) Protein double layer (b) Phospholipid proteins 

 (c) Phosphoproteins       (d) Glycoproteins 

6. Non-membranous cell organelles are: 

 (a) Ribosomes (b) centrioles and ribosomes 

 (c) E.R.            (d) Mitochondria 

7. Which of the following theories explain that plasma membrane is selectively permeable: 

 (a) Unit membrane theory (b) Cascade theory 

 (c) Sandwich theory (d) Fluid Mosaic theory 

8. The hydrophobic ends of phospholipid molecules are: 

 (a) Polar                   (b) non-polar  

 (c) Neutral               (d) Bipolar 

9. The membrane protein that extend through both sides of lipid bilayer. 

 (a) Acidic protein (b) Glycoprotein 

 (c) Intrinsic protein (d) Glycolic acid 

10. Two plant cells are connected with the help of: 

 (a) Cell wall (b) Plasma membrane 

 (c) Plasmodesmata (d) None of these 

M.C.Q:- ANSWERS 

1. (b) 2. (b) 3. (b) 4. (d) 5. (b) 6. (b) 7. (d) 8. (b) 9. (c) 10. (c) 

💦 Very short questions:

1. What is the thickness of plasma membrane? 

2. Who proposed the fluid mosaic hypothesis for the molecular structure of cell membrane? 

3. What is the structure of plasma membrane?

4. What are the main lipid components of the plasma membrane? 

5. What are the two types of proteins of the plasma membrane on the basis of their association with the membrane and their solubility? 

6. What are tunnel proteins? 

7. Why Na+, -K+ ATPase enzyme is most important? 

8. Who proposed that plasma membrane contained a lipid bilayer and protein adhering to both lipid aqueous interfaces? 

9. Who gave the unit membrane model of plasma membrane? 

10. Give the two alternative name of cell membrane. 

Answers:-

 1. 70 - 100Å. 

 2. Singer and Nicolson. 

 3. It is formed of bilayer of lipids into which protein complexes are embedded in a kind of mosaic arrangement. 

 4. Phospholipids, cholesterol and galactolipids. 

 5. Integral or intrinsic proteins and peripheral or extrinsic proteins. 

 6. Large integral protein molecules that lie throughout the phospholipid matrix and projects on both the surfaces. 

 7. It helps in ion transfer across the plasma membrane. This enzyme is dependent on the presence of lipids and is inactivated when all lipids are extracted. 

8. Danielli and Davson in 1935. 

 9. Robertson, 1959. 

 10. Plasma membrane and plasmalemma. 

cell and molecular biology, cell biology, cell type, types of cells, history of cell, Introduction of cell, function,

A structure containing a mass of cytoplasm surrounded by semi-permeable membrane called plasma membrane is called a cell. It encloses cytoplasm, many cell organelles along with nucleus or nuclear material. On the basis of organization of membranes, variety and structure of cytoplasmic organelles and complexity of nuclear region, the cells are classified into two types: Prokaryotic cell and Eukaryotic cell. These terms were suggested by Hans Ris in 1960s.

  Cell type and History of Origin 

History and Origin: -

A cell was defined as “unit of biological activity delimited by a semi permeable membrane and capable of self-reproduction in a medium free of other living systems” by Loewy and Sinkovitz (1963). The study of cell has been made possible with the help of light microscope. Robert Hooke (1665) with the help of light microscope discovered that a section of cork is made up of small cavities surrounded by firm walls. He used the term “cell” for the first time to describe his investigations on the “texture of a piece of cork”. Later on, A. Van Leeuwenhoek (1632-1723) observed various unicellular organisms and cells like bacteria, protozoan’s, red blood cells and sperm etc. He observed nucleus in some erythrocytes, and all this were made possible with the improved microscopes. In 1809, Marble M. stated that all plant tissues are composed of cells. In the same year, importance of cells in living organisms was described by J.B. Lamarck. Robert Brown in 1831 observed nucleus in certain plant cells. Mimosa cells were boiled in nitric acid by Dutro Chet (1837) to separate the cells to conclude that all organic tissues are composed of globular cells, united by simple adhesive forces. “All living organism is composed of cells” was stated by Schwann, T. (1839) after examining a variety of animals and plant tissues.

BASIC COMPONENTS OF PROKARYOTIC AND EUKARYOTIC CELL: -

Prokaryotic Cells: -

Prokaryotic cells are the most primitive cells and have simple structural organization. It has a single membrane system. They include bacteria, viruses, blue-green algae, mycoplasmas, rickettsia, spirochetes etc. Cyanobacteria or blue green algae are the largest and most complex prokaryote, in which photosynthesis of higher plants type have evolved. Prokaryotes are included in the kingdom Monera and the super kingdom Prokaryotes. The Prokaryotes have the following characters:

1. The size of prokaryotic cells ranges between 1 to 10 µm. They occur in a variety of forms. 

2. Prokaryotic cell consists of three main components: 

(I) Outer covering: It is composed of inner cell or plasma membrane, middle cell wall and outer slimy capsule. 

a. Cell membrane: Cell membrane made up of lipids and proteins, is thin and flexible and controls the movement of molecules across the cell. Respiratory enzymes are carried by it for energy releasing reactions. Mesosomes, the in-folds of plasma membrane bears respiratory enzymes, and these are considered analogous to mitochondria of eukaryotic cells. Similarly, the pigments and enzymes molecules that absorb and convert the light into chemical energy in photosynthetic cells are also associated with the plasma membrane’s in-folds called photosynthetic lamella. These lamellae are analogous to the chloroplast of eukaryotic cells. Plasma membrane plays role in replication and division of nuclear material. Since the in-folds remain continuous with the cell membrane, they are not considered as separate compartments. Thus, prokaryotic cell is non-compartmentalized. 

b. Cell wall: It is a rigid or semi-rigid non-living structure that surrounds the cell membrane and its thickness ranges between 1.5 to 100 µm. Chemically it is composed of peptidoglycans... Some bacteria such as mycoplasmas lack cell wall. 

c. Slimy capsule: A gelatinous coat outside the cell wall is the slimy capsule. It is composed of largely of polysaccharides and sometimes it may have polypeptides and other compounds also. It protects the cell against desiccation, virus attacks, phagocytosis and antibiotics 

(II) Cytoplasm: Prokaryotic cytoplasm contains proteins, lipids, glycogen and inorganic ions along with enzymes for biosynthetic reactions and ribosomes, tRNA and mRNA for protein synthesis. Prokaryotic cytoplasm has some special features as follows:

a. It lacks cell organelles like endoplasmic reticulum, mitochondria, Golgi apparatus, Centrosomes, vacuoles, Lysosomes, microfilaments, intermediate filaments and microtubules. 

b. The only cytoplasmic organelle found in prokaryotic cells is the ribosomes. They are smaller than eukaryotic ribosomes i.e., 70S and lie free in the cytoplasm. They form polyribosomes at the time of protein synthesis. They are the sites of protein synthesis. 

c. Like eukaryotic cells, the cytoplasm of prokaryotic cell does not show streaming movement or cyclosis. 

d. Gas vacuoles are also formed in some prokaryotic cells. 

e. The cell does not show phagocytosis, pinocytosis and exocytose, substances enter and leave the cell through the cell membrane. 

f. They may contain deposits of polysaccharides or inorganic phosphates. 

(III) Nucleoid: Nuclear envelope is absent in prokaryotic cell and the genetic material lies directly into the cytoplasm. Such nuclear material is known as nucleoid. Nucleoid consists of greatly coiled single pro-chromosome. It shows the following special features: 

a. A short and simple pro-chromosome is present which is attached at least at one point on cell membrane. 

b. Mostly there is single copy of chromosome, the prokaryotic cell is haploid. 

c. The DNA is naked as it is not associated with basic histone proteins. It is double stranded, helical and circular. 

d. The amount of DNA is lesser than eukaryotic cell and it codes fewer proteins. Replication of DNA is continuous throughout the cell cycle. Transcription and translation occur in cytoplasm and processing of mRNA is not required. 

e. The processes like meiosis, gamete formation or fertilization are absent. Conjugation is seen in some bacteria. 

f. Mitotic apparatus absent. 

g. There is no nucleolus.

h. Cell membrane folds or mesosomes help to segregate the replicated products of 

chromosomes into daughter cells. 

3. Plasmids: In some prokaryotic cells, in addition to nucleoid, a small circular double stranded DNA molecule is present. It is called plasmid. Plasmids have 1000 to 30,000 base pairs and they generally encode proteins required by the organism to resist antibiotic and other toxic material. 

4. Flagellum: It is a whip like locomotory structure found in many bacteria. It is 150Å thick and 10 to 15µm long. As the flagellum does not have any surrounding membrane, it grows at the tip.  It has two main parts: Filament and basal body. 

(i) Filament- Filament extends out of cell into the medium and it is composed of many intertwined spiral chains of the subunits of a protein called flagellin. Flagellin differs from actins or tubulin.

 (ii) Basal Body- The basal body attaches the flagellum to the cell and generates the force to rotate it. It is composed of many components and numerous proteins. It has two parts: shaft and hook.

Eukaryotic Cells: -

The internal organization of eukaryotic cell is more developed than prokaryotic cells from which they are believed to have been evolved. They are evolved to have double membrane system. Primary membranes are the one that surrounds the cell, celled cell or plasma membrane and the secondary membrane surround the nucleus and other cellular organelles. Eukaryotic cells occur in protists, fungi, plants and animals. Eukaryotic cells have the following characteristics: 

1. Number- In multicellular organisms the numbers of cells are correlated with the body size. The human blood contains about 30 quadrillion (3 × 1015) corpuscles and a 60 kg human being has about 60 × 1015 cells

 

2. Shape- A cell may be spherical, cuboidal, oval, disc-like, polygonal, columnar, spindle like or irregular. Thus, cells acquire a variety of shapes not only in various organisms but also in different tissues of the same organism.

3. Size- Most of the eukaryotic cells is microscopic and their size ranges between 10 to 100µm. Sporozoites of malaria parasite (Plasmodium vivax) is among the smallest cells having the size equal to 2µm long.

 

4. Components of a cell- Three main components of the eukaryotic cells are cell membrane, cytoplasm and nucleus. 

(i) Cell membrane- Cell membrane, plasma membrane or plasmalemma is a thin elastic living covering that surrounds the cell keeping the cell contents in place, provides shape to the cell and controls the transfer of materials across it. It is composed of lipid-protein complex. It lacks respiratory enzymes. In many protists and animal cells it allows endocytosis and exocytosis.

(ii) Cytoplasm- The cytoplasm or the cytostomes is a semi-fluid, homogeneous, translucent ground substance known as cytoplasmic matrix or cytosol which is present between the cell membrane and the nucleus. 

a. Organelles: The organized structures having the specific functions and capacity of growth and multiplication in some cases are known as organelles.

I. Mitochondria: The rod like or globule shaped structures scattered in the cytoplasm are found singly or in groups. They are bounded by double membrane of lipoproteins. The inner membrane gives out finger like structure known as cristae which partially subdivide the inner chamber of mitochondrion. On the inner surface of cristae are present mushroom like structures, exosomes that are related to phosphorylation. The space between the membranes and its lumen is filled with mitochondrial matrix. Both the membranes and the matrix contain many oxidative enzymes and coenzymes. Since mitochondria contain DNA molecules and ribosomes, they synthesize certain proteins. They produce the energy and reserve it in the form of adenosine triphosphate (ATP). Due to the presence of its own DNA and ability of protein synthesis along with its duplication, the mitochondria are called semi-autonomous organelle. The DNA of mitochondria resembles that of bacterial cell; hence it is also called as endo-symbiotic organelle II. Centrosomes: (9+0) there is a clear zone around centrioles, near the nucleus, that includes a specialized portion of cytoplasm, called centrospheres. Its matrix is called Kino plasm that bears two rounded bodies the “centrioles”. Each centriole consists of nine fibrillar units and each of them is found to contain three microtubules arranged in a circle. Both the centrioles are arranged at right angle to each other.

III. Golgi bodies: These are the stack of flattened parallel-arranged sacs and vesicles found in association of endoplasmic reticulum. They are composed of many lamellae, tubules, vesicles and vacuoles. Their membranes are supposed to be originated from ER and are composed of lipoproteins. In plant cells the Golgi complex is called dictyosome that secretes required materials for the formation of cell wall at the time of cell division. It helps in the formation of acrosome of sperms, 

release of hormones, enzymes and other synthetic materials

IV. Plastids: These organelles are found in plant cells and are absent in animal cells. They may be colored like chloroplast or chromoplasts or colorless like leucoplast. 

V. Metaplasm: The particles like vacuoles, granules and other cytoplasmic bodies such as ribonucleoprotein molecules are represented by it. 

VI. Cilia, basal bodies and flagella: Cilia are the minute structures covering the surface in some cells. Both cilia and flagella originate from the basal bodies or blepharoplast lying-in cytoplasm. They consist of nine outer fibrils with the two larger fibrils in the center. 

VII. Microtubules: The ultra-fine tubules of protein (tubulin) traversing the cytoplasm of plant and animal cells providing the structural framework to the cell, determine the cell shape and general organization of the cytoplasm are known as microtubules.

IX. Ribosome’s: Ribosome is the minute spherical structures that originate in nucleolus and are found attached with the membrane of endoplasmic reticulum and in the cytoplasm. They are mainly composed of ribonucleic acids (RNA) and protein. They are mainly responsible for protein synthesis.

Self-Assessment Questions and Possible Answers 

Multiple Choice Questions: -

1. There is no organized nucleus in: 

 (a) Bacterial cell (b) Green algae cell (c) Animal cell (d) Plant cell 

2. The prokaryotic cells are characterized by: 

(a) A distinct nuclear membrane (b)Absence of chromatin material (c)Distinct chromosome (d) Absence of nuclear membrane 

3. In a prokaryotic cell, DNA is: 

(a) Enclosed by nuclear envelop(b) Lacking(c)Not a genetic material(d)without a membrane

4. Cell wall is found around the: -

  (a) Prokaryotic cell (b) Algal cells (c) Plant cell (d) All of these 

5. Chemical energy of food stuffs is converted into biologically useful forms by: 

 (a) Ribosomes (b) Golgi complex (c) Mitochondria (d) Plastids 

6. Sun radiant energy is converted into chemical energy of organic compound by: 

 (a) Mitochondria (b) Chloroplast (c) Ribosomes (d) Centrosomes 

7. Which structure is present only in animal cell? 

 (a) Cell membrane (b) Lysosomes (c) Centrioles (d) Ribosomes 

8. Single envelope system is characteristic of: 

 (a) Prokaryotic cell (b) Eukaryotic cell (c) None (d) Both 

9. Prokaryote and eukaryotes have the common: 

 (a) Mitotic apparatus (b) Histone (c) Genetic code (d) Mitochondria 

10. Unicellular microscopic organisms were first studied by: 

 (a) Robert Hooke (b) Priestley (c) Pasteur (d) Leeuwenhoek 

ANSWERS: - 

 1. (a) 2. (d)3. (d)  4. (d) 5. (c)   6. (b) 7. (c)   8. (a)    9. (c)  10. (d) 

Very Short Questions: -

1. What are prokaryotes? Give an example. 

2. What are eukaryotes? Give few examples. 

3. Cell is an open dynamic system. Is it correct? 

4. Prokaryotic cells are haploid. Is it so? 

5. What are cyanobacteria? 

6. Give three essential characteristics of cell? 

7. Where is nucleolus found? 

8. What are the power houses of the cell? 

9. Name the protein factories of prokaryotic and eukaryotic cells? 

10. What is the control center of a cell?