Free Download Cell Biology-A Short Course by Stephen R. Bolsover

Free Download Cell Biology-A Short Course by Stephen R. Bolsover This text tells the story of cells as the unit of life in a colorful and student-friendly manner, taking an "essentials only" approach. By using the successful model of previously published Short Courses, this text succeeds in conveying the key points without overburdening readers with secondary information. The authors (all active researchers and educators) skillfully present concepts by illustrating them with clear diagrams and examples from current research. Special boxed sections focus on the importance of cell biology in medicine and industry today. This text is a completely revised, reorganized, and enhanced revision of From Genes to Cells. To download this book click on download button

Free Download Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

Free Download Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

The multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology is the outcome of sincere and combined efforts of the authors and editors (namely Shishir Bhatnagar, Shubha Pradhan, Malini Kothiyal) and young but talented persons of DTP of S.Chand & Company Ltd. Their main motive remained to provide relevant coloured photographs explaining various intricate biological
topics. Multicoloured figures and photographs of this edition would help our target readers to understand and fully appreciate the very gist of the subject matter. Authors and editors have remained quite choosy and vigilant regarding relevance and authenticity of each and every illustration/picture finding its place in this textbook.

Authors earnestly hope that this multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology will enhance the curiosity of our target readers to know more and more about the subject. It will arm them with latest information for facing any type of exam quite adequately.

This book is meant for students of B.Sc., B.Sc. (Hons.) and M.Sc. of biological group. Students appearing in entrance exams of C.P.M.T., I.F.S., P.C.S. and I.A.S., etc, may be immensely benefited by this book.

                                     To download this book click on download button

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Cell Biology-A Short Course

Cell Biology-A Short Course

Cell Biology-A Short Course********************************************

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Book Description

This text tells the story of cells as the unit of life in a colorful and student-friendly manner, taking an “essentials only” approach. By using the successful model of previously published Short Courses, this text succeeds in conveying the key points without overburdening readers with secondary information.

The authors (all active researchers and educators) skillfully present concepts by illustrating them with clear diagrams and examples from current research. Special boxed sections focus on the importance of cell biology in medicine and industry today. This text is a completely revised, reorganized, and enhanced revision of From Genes to Cells.

Cell Biology-A Short Course

Bacteria in Biology, Biotechnology and Medicine By Paul Singleton

Bacteria in Biology, Biotechnology and Medicine By Paul Singleton

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Bacteria in Biology, Biotechnology and Medicine By Paul Singleton

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Bacteria in Biology, Biotechnology and Medicine is a broadly based textbook of pure and applied bacteriology. Written in clear language, the up-to-date text gives readers access to new ideas and developments in the current literature. The book is intended primarily for undergraduates and postgraduates in biology, biotechnology, medicine, veterinary science, pharmacology, microbiology, food science, environmental science and agriculture; no prior knowledge of bacteria is assumed.

The sixth edition has been extensively updated; much of the text is new, or re-written, and there are many new references. Over 70 genera of bacteria, listed alphabetically, are described in the Appendix. Cross-references and a detailed index, maximise…
Reviews of previous editions:
“….a useful survey of the subject for students contemplating specialization.” —Nature
“Singleton assumes the reader has no prior knowledge of DNA and gene expression, and does an extraordinary job of explaining things from scratch.” —Quarterly Review of Biology
“….recommended to undergraduates and those seeking clear explanations of basic concepts of bacteriology.” —Journal of Medical Microbiology

Bacteria in Biology, Biotechnology and Medicine By Paul Singleton

Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

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Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

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The multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology is the outcome of sincere and combined efforts of the authors and editors (namely Shishir Bhatnagar, Shubha Pradhan, Malini Kothiyal) and young but talented persons of DTP of S.Chand & Company Ltd. Their main motive remained to provide relevant coloured photographs explaining various intricate biological
topics. Multicoloured figures and photographs of this edition would help our target readers to understand and fully appreciate the very gist of the subject matter. Authors and editors have remained quite choosy and vigilant regarding relevance and authenticity of each and every illustration/picture finding its place in this textbook.

Authors earnestly hope that this multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology will enhance the curiosity of our target readers to know more and more about the subject. It will arm them with latest information for facing any type of exam quite adequately.

This book is meant for students of B.Sc., B.Sc. (Hons.) and M.Sc. of biological group. Students appearing in entrance exams of C.P.M.T., I.F.S., P.C.S. and I.A.S., etc, may be immensely benefited by this book.

Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

********************************************

Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

Click here to Download (Box Link)

OR

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Book Description

The multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology is the outcome of sincere and combined efforts of the authors and editors (namely Shishir Bhatnagar, Shubha Pradhan, Malini Kothiyal) and young but talented persons of DTP of S.Chand & Company Ltd. Their main motive remained to provide relevant coloured photographs explaining various intricate biological
topics. Multicoloured figures and photographs of this edition would help our target readers to understand and fully appreciate the very gist of the subject matter. Authors and editors have remained quite choosy and vigilant regarding relevance and authenticity of each and every illustration/picture finding its place in this textbook.

Authors earnestly hope that this multicoloured edition of the textbook of Cell Biology, Genetics, Molecular Biology, Evolution and Ecology will enhance the curiosity of our target readers to know more and more about the subject. It will arm them with latest information for facing any type of exam quite adequately.

This book is meant for students of B.Sc., B.Sc. (Hons.) and M.Sc. of biological group. Students appearing in entrance exams of C.P.M.T., I.F.S., P.C.S. and I.A.S., etc, may be immensely benefited by this book.

Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal

How Nucleotides are added in DNA Replication

DNA replication is the process of producing two identical replicas from one original DNA molecule. This biological process occurs in all living organisms and is the basis for biological inheritance. DNA is made up of two strands and each strand of the original DNA molecule serves as template for the production of the complementary strand, a process referred to as semiconservative replication. Cellular proofreading and error-checking mechanisms ensure near perfect fidelity for DNA replication.

In a cell, DNA replication begins at specific locations, or origins of replication, in the genome.Unwinding of DNA at the origin and synthesis of new strands results in replication forks growing bidirectional from the origin. A number of proteins are associated with the replication fork which helps in terms of the initiation and continuation of DNA synthesis. Most prominently, DNA polymerase synthesizes the new DNA by adding complementary nucleotides to the template strand.

DNA replication can also be performed in vitro (artificially, outside a cell). DNA polymerases isolated from cells and artificial DNA primers can be used to initiate DNA synthesis at known sequences in a template DNA molecule. The polymerase chain reaction (PCR), a common laboratory technique, cyclically applies such artificial synthesis to amplify a specific target DNA fragment from a pool of DNA.

Photosynthetic Electron Transport and ATP Synthesis

An electron transport chain (ETC) is a series of compounds that transfer electrons from electron donors to electron acceptors via redox reactions, and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. This creates an electrochemical proton gradient that drives ATP synthesis, or the generation of chemical energy in the form of adenosine triphosphate (ATP). Final acceptor of electrons in electron transport chain is molecular oxygen.

Electron transport chains are used for extracting energy via redox reactions from sunlight in photosynthesis or, such as in the case of the oxidation of sugars, cellular respiration. In eukaryotes, an important electron transport chain is found in the inner mitochondrial membrane where it serves as the site of oxidative phosphorylation through the use of ATP synthase. It is also found in the thylakoid membrane of the chloroplast in photosynthetic eukaryotes. In bacteria, the electron transport chain is located in their cell membrane.

In chloroplasts, light drives the conversion of water to oxygen and NADP+ to NADPH with transfer of H+ ions across chloroplast membranes. In mitochondria, it is the conversion of oxygen to water, NADH to NAD+ and succinate to fumarate that are required to generate the proton gradient.

Electron transport chains are major sites of premature electron leakage to oxygen, generating superoxide and potentially resulting in increased oxidative stress

Slipped strand mispairing

Slipped strand mispairing (SSM) is a mutation process which occurs during DNA replication. It involves denaturation and displacement of the DNA strands, resulting in mispairing of the complementary bases. Slipped strand mispairing is one explanation for the origin and evolution of repetitive DNA sequences. Slipped strand mispairing has also been shown to function as a phase variation mechanism in certain bacteria

The Tryptophan Repressor

Tryptophan repressor (or trp repressor) is a transcription factor involved in controlling amino acid metabolism. It has been best studied in Escherichia coli, where it is a dimeric protein that regulates transcription of the 5 genes in the tryptophan operon.When the amino acid tryptophan is plentiful in the cell, it binds to the protein, which causes a conformational change in the protein.The repressor complex then binds to its operator sequence in the genes it regulates, shutting off the genes.

One of the genes regulated by trp repressor, trpR, codes for the tryptophan repressor protein itself. This is a form of feedback regulation.

The (tryptophan) repressor is a 25 kD protein homodimer which regulates transcription of the tryptophan biosynthetic pathway in bacteria. There are 5 operons which are regulated by trpR: the trpEDCBA, trpR, AroH, AroL, and mtr operons.

Mechanism

When the amino acid tryptophan is in plentiful supply in the cell, trpR binds 2 molecules of tryptophan, which alters its structure and dynamics so that it becomes able to bind to operator DNA. When this occurs, transcription of the DNA is prevented, suppressing the products of the gene - proteins which make more tryptophan. When the cellular levels of tryptophan decline, the tryptophan molecules on the repressor fall off, allowing the repressor to return to its inactive form.

trpR also controls the regulation of its own production, through regulation of the trpR gene.

The structure of the ligand-bound holorepressor, and the ligand-free forms have been determined by both X-ray crystallography and NMR.

The trp operon consists of a regulatory gene, a promoter, an operator, and a terminator. The trp operon is active only when cellular tryptophan is scarce. If there isn't enough tryptophan, the repressor protein breaks off from the operator (where the repressor is normally bound) and RNA polymerase can complete its reading of the strand of DNA. If the RNA polymerase reaches the terminator (at the end of the DNA strand), the enzyme for tryptophan is made.

How Facilitated Diffusion Works

Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is the process of spontaneous passive transport (as opposed to active transport) of molecules or ions across a biological membrane via specific transmembrane integral proteins. Being passive, facilitated transport does not directly require chemical energy from ATP hydrolysis in the transport step itself; rather, molecules and ions move down their concentration gradient reflecting its diffusive nature.

Facilitated diffusion is different from free diffusion in several ways. First, the transport relies on molecular binding between the cargo and the membrane-embedded channel or carrier protein. Second, the rate of facilitated diffusion is saturable with respect to the concentration difference between the two phases; unlike free diffusion which is linear in the concentration difference. Third, the temperature dependence of facilitated transport is substantially different due to the presence of an activated binding event, as compared to free diffusion where the dependence on temperature is mild.

3D rendering of facilitated diffusion

Polar molecules and large ions dissolved in water cannot diffuse freely across the plasma membrane due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the lipid bilayer. Only small, non-polar molecules, such as oxygen and carbon dioxide, can diffuse easily across the membrane. Hence, all polar molecules are transported by proteins in the form of transmembrane channels. These channels are gated, meaning that they open and close, and thus regulate the flow of ions or small polar molecules across membranes, sometimes against the osmotic gradient. Larger molecules are transported by transmembrane carrier proteins, such as permeases, that change their conformation as the molecules are carried across (e.g. glucose or amino acids). Non-polar molecules, such as retinol or lipids, are poorly soluble in water. They are transported through aqueous compartments of cells or through extracellular space by water-soluble carriers (e.g. retinol binding protein). The metabolites are not altered because no energy is required for facilitated diffusion. Only permease changes its shape in order to transport metabolites. The form of transport through a cell membrane in which a metabolite is modified is called group translocation transportation.

Glucose, sodium ions and chloride ions are just a few examples of molecules and ions that must efficiently cross the plasma membrane but to which the lipid bilayer of the membrane is virtually impermeable. Their transport must therefore be "facilitated" by proteins that span the membrane and provide an alternative route or bypass mechanism.

Various attempts have been made by engineers to mimic the process of facilitated transport in synthetic (i.e., non-biological) membranes for use in industrial-scale gas and liquid separations, but these have met with limited success to date, most often for reasons related to poor carrier stability and/or dissociation of the carrier from the membrane.

Facilitated Diffusion has to do with passive and active transport. Passive Transport is the movement of molecules across the cell that does not require expenditure of energy, however active transport is the movement of substances across a membrane involving a carrier protein and energy from respiration(ATP).

Antibiotics Protein

A protein synthesis inhibitor is a substance that stops or slows the growth or proliferation of cells by disrupting the processes that lead directly to the generation of new proteins.

interpretation of this definition could be used to describe nearly any antibiotic, in practice, it usually refers to substances that act at the ribosome level (either the ribosome itself or the translation factor),[2] taking advantages of the major differences between prokaryotic and eukaryotic ribosome structures.

Toxins such as ricin also function via protein synthesis inhibition. Ricin acts at the eukaryotic 60S.

Examples:

Neomycin

Geneticin, also called G418

Diffusion

Diffusion is the net movement of a substance (e.g., an atom, ion or molecule) from a region of high concentration to a region of low concentration. This is also referred to as the movement of a substance down a concentration gradient. A gradient is the change in the value of a quantity (e.g., concentration, pressure, temperature) with the change in another variable (e.g., distance). For example, a change in concentration over a distance is called a concentration gradient, a change in pressure over a distance is called a pressure gradient, and a change in temperature over a distance is a called a temperature gradient.

The word diffusion is derived from the Latin word, "diffundere", which means "to spread out" (if a substance is “spreading out”, it is moving from an area of high concentration to an area of low concentration). A distinguishing feature of diffusion is that it results in mixing or mass transport, without requiring bulk motion (bulk flow). Thus, diffusion should not be confused with convection, or advection, which are other transport phenomena that utilize bulk motion to move particles from one place to another.

Antibiotics Cell Wall Inhibition

β-Lactam (beta-lactam) and glycopeptide antibiotics work by inhibiting or interfering with cell wall synthesis of the target bacteria.

Two types of antimicrobial drugs work by inhibiting or interfering with cell wall synthesis of the target bacteria. Antibiotics commonly target bacterial cell wall formation (of which peptidoglycan is an important component) because animal cells do not have cell walls. The peptidoglycan layer is important for cell wall structural integrity, being the outermost and primary component of the wall.

The first class of antimicrobial drugs that interfere with cell wall synthesis are the β-Lactam antibiotics (beta-lactam antibiotics), consisting of all antibiotic agents that contains a β-lactam nucleus in their molecular structures. This includes penicillin derivatives (penams), cephalosporins (cephems), monobactams, and carbapenems. β-Lactam antibiotics are bacteriocidal and act by inhibiting the synthesis of the peptidoglycan layer of bacterial cell walls . The final step in the synthesis of the peptidoglycan is facilitated by penicillin-binding proteins (PBPs). PBPs vary in their affinity for binding penicillin or other β-lactam antibiotics.

Penicillin spheroplast generation

Diagram depicting the failure of bacterial cell division in the presence of a cell wall synthesis inhibitor (e.g. penicillin, vancomycin).1- Penicillin (or other cell wall synthesis inhibitor) is added to the growth medium with a dividing bacterium.2- The cell begins to grow, but is unable to synthesize new cell wall to accommodate the expanding cell.3- As cellular growth continues, cytoplasm covered by plasma membrane begins to squeeze out through the gap(s) in the cell wall.4- Cell wall integrity is further violated. The cell continues to increase in size, but is unable to "pinch off" the extra cytoplasmic material into two daughter cells because the formation of a division furrow depends on the ability to synthesize new cell wall.5- The cell wall is shed entirely, forming a spheroplast, which is extremely vulnerable relative to the original cell. The loss of the cell wall also causes the cell to lose control over its shape, so even if the original bacterium were rod-shaped, the sphereoplast is generally spherical. Finally, the fact that the cell has now doubled much of its genetic and metabolic material further disrupts homeostasis, which usually leads to the cell's death.

Bacteria often develop resistance to β-lactam antibiotics by synthesizing a β-lactamase, an enzyme that attacks the β-lactam ring. To overcome this resistance, β-lactam antibiotics are often given with β-lactamase inhibitors such as clavulanic acid.

The second class of antimicrobial drugs that interfere with cell wall synthesis are the glycopeptide antibiotics, which are composed of glycosylated cyclic or polycyclic nonribosomal peptides. Significant glycopeptide antibiotics include vancomycin, teicoplanin, telavancin, bleomycin, ramoplanin, and decaplanin. This class of drugs inhibit the synthesis of cell walls in susceptible microbes by inhibiting peptidoglycan synthesis. They bind to the amino acids within the cell wall preventing the addition of new units to the peptidoglycan .

Penicillin spheroplast generation

Diagram depicting the failure of bacterial cell division in the presence of a cell wall synthesis inhibitor (e.g. penicillin, vancomycin).1- Penicillin (or other cell wall synthesis inhibitor) is added to the growth medium with a dividing bacterium.2- The cell begins to grow, but is unable to synthesize new cell wall to accommodate the expanding cell.3- As cellular growth continues, cytoplasm covered by plasma membrane begins to squeeze out through the gap(s) in the cell wall.4- Cell wall integrity is further violated. The cell continues to increase in size, but is unable to "pinch off" the extra cytoplasmic material into two daughter cells because the formation of a division furrow depends on the ability to synthesize new cell wall.5- The cell wall is shed entirely, forming a spheroplast, which is extremely vulnerable relative to the original cell. The loss of the cell wall also causes the cell to lose control over its shape, so even if the original bacterium were rod-shaped, the sphereoplast is generally spherical. Finally, the fact that the cell has now doubled much of its genetic and metabolic material further disrupts homeostasis, which usually leads to the cell's death.

Catalysis

Catalysis is the increase in the rate of a chemical reaction due to the participation of an additional substance called a catalyst. With a catalyst, reactions occur faster and with less energy. Because catalysts are not consumed, they are recycled. Often only tiny amounts are required.

Atomic Structure

The atom is the smallest unit that defines the chemical elements and their isotopes. Every substance, be it solid, liquid or gas is made up of atoms. The size of atoms is measured in picometers (trillionths of a meter). A single strand of human hair is about one million carbon atoms wide.

Every atom is composed of a nucleus made of protons and neutrons (hydrogen-1 has no neutrons). The nucleus is surrounded by a cloud of electrons. The electrons in an atom are bound to the atom by the electromagnetic force, and the protons and neutrons in the nucleus are bound to each other by the nuclear force. Over 99% of the atom's mass is in the nucleus. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. Normally, an atom's electrons balance out the positive charge of its protons to make it electrically neutral. If an atom has a surplus or deficit of electrons, then it will have an overall charge, and is called an ion.

The number of protons in the nucleus determines what chemical element the atom belongs to (e.g. all copper atoms contain 29 protons). The number of neutrons determines what isotope of the element it is.[2] The electron cloud of the atom determines the atom's chemical properties and strongly influences its magnetic properties.

Atoms can attach themselves to each other by chemical bonds to form molecules, network solids, metal alloys, crystals, and other solid solutions. The tendency for atoms to bond and break apart is responsible for most of the physical changes we observe in nature, and this is studied by the science of chemistry.

Atoms and sub-atomic particles behave in peculiar ways that cannot be explained through the classical laws of physics. The field of quantum mechanics was developed to explain the structure and behavior of atoms.

Not all matter is made up of atoms, but atoms do comprise all the types of matter than can be seen and touched. Astronomical observations indicate that most of the Universe's matter is "dark matter", composed of particles of a currently unknown type.

Bacterial Locomotion

Bacterial locomotion 

Locomotion or motility is important characteristic of bacteria. Bacterial locomotion is of three types: Flagellar, Spirochaetal and Gliding movement. The word motility, movement and locomotion are used synonymously. 

Flagellar motility: 

This type of motility is caused by flagella, cell surface appendages. Flagellum has typical structure; it is embedded in cell wall by S ring or stator (hook) and basal body or motor. M ring is attached to the flagellum and acts like a rotor (shaft). P and L are also present and work like bearings or bushers. Basal body is powered by proton energy, which is movement of ions between M and S rings. Transformation of proton energy into work operates flagella in clockwise and counterclockwise directions. Depending upon location of flagella, bacteria can swim smoothly, reverse the movement backward or forward or tumble. Peritrichously flagellated bacteria bear flagella all over the surface move by tumbling or anticlockwise swimming. Polar flagella (mono, bi or multipolar) are present at the ends of cell and bacteria move in one direction and as well as in reversal. Flagellar motility is present in Pseudomonas, Vibrio, Spirillum, Azospirillum, Klebsiella, Salmonella, Proteus and etc. 

Spirochaetal movement: 

Spirochaetal movement is seen in all genera of bacterial group (V), 'The Spirochetes' of Bergey's Manual of Determinative Bacteriology. Important genera include, Spirochaeta, Cristispira, Treponema, Borrelia and Leptospira. Spirochetes are helical bacteria. They have flagella like axial filament buried in space between inner and outer membranes of cell wall. Axial filament is composed of 2 or more fibrils which are embedded in inner membrane and acts like basal body or motor. Spirochetes can perform flexing, swimming, creeping or spinning types of movements. Imagination of motion of flexible helical rod in air will give you an exact idea about spirochetal movement. 

Gliding movement: 

Like spirochetes, gliding motility is represented by special bacteria, 'The Gliding Bacteria' group (II) of Bergey's Manual of Determinative Bacteriology. Bacteria move by gliding on the surface! They do not have flagellar structures either internally or externally but they secrete slimy substance like snails during locomotion. The exact mechanism of gliding locomotion is still unknown but some scientists have suggested presence of fimbriae like appendages at the poles of glider cell. The generation of contractile waves or surface tension or pushing by secreted slime was also proposed as possible mechanisms of gliding. Principle glider genera are Myxococcus, Archangium, Cystobacter, Melittangium, Stigmatella, Polyangium, Nannocystis, Chondromyces, Cytophaga, Flexithrix, Herpetosiphon, Beggiatoa, Saprospira, Thioplaca, Leucothrix, Alysiella, Achroonema and cyanobacterium Oscillatoria. 

Laboratory detection and assay: 

Motility can be directly observed under light microscope by hanging drop in cavity slide or wet mount preparation. It is important to determine true and false motility microscopically. Truly motile bacteria will show propelling action towards definite direction, as if they are pushing themselves with efforts! Nonmotile bacteria also appear to be motile because of bombardment of liquid medium particles or air currents. Motility of nonmotile bacteria is zigzag and directionless. This movement of nonmotile bacteria is actually a Brownian movement; even dead bacteria seem to be moving because of this movement. Craigie's tube or capillary tube can be used by placing them in broth culture for observation of directed movement of bacteria towards chemical or physical gradients with time. All motile bacteria show movement towards chemical or physical gradients. This phenomenon is known as tactic response. Chemical or physical gradient can be attractant or repellent and accordingly, tactic response would be positive or negative. Presence of gradients is sensed by special receptors of bacteria. Thus swimming towards certain glucose concentration present in the medium would be positive chemotactic or chemotaxis. Similarly, motile bacteria exhibit phytotaxis (light intensity) and magnetotaxis (magnetic particles) responses. Motile bacteria are assayed on semisolid agar or broth medium for chemotaxis and are very important in species identification and classification. 

Importance of bacterial locomotion: 

Chemotactic behavior and survival: 

Motility confers bacteria an ability to change direction. This is important when bacteria require moving away or towards repellents or attractants respectively. It avoids unfavorable conditions of habitat and offers protection. It is thus important in the survival and offers to choose favorable environment containing positive stimuli, light, gravity or chemicals for bacteria. 

Root colonization: 

Root colonization is perquisite for establishment of bacteria in the rhizosphere region. Motile bacteria are effective root colonizers and can swim towards root exudates or other nutrient gradients earlier than nonmotile bacteria. Pseudomonads and Azospirilla are very efficient in attachment and subsequent root colonization of their host plants. 

Pathogenesis: Most human pathogenic bacteria (Campylobacter, Salmonella and Vibrio) and saprophytes or opportunists (Escherichia) are motile and motility is important for attachment and colonization of cell wall of intestine and other vital organs. 

Motile versus nonmotile: 

Some bacteria like Acinetobacter spp. show twitching or jumping type of motility even though flagella are absent in them. These bacteria show the twitching particularly on semisolid media and also present chemotactic response. Twitching motility is thought to be because of piliated cell surface. It is the favorite topic of interest and research that why some bacteria are nonmotile? It has been found that in some bacterial genera that nonmotile species are equally efficient like their motile species. These nonmotile bacteria also possessed flagellar appendages; but basal body or motor function was found to be impaired or paralyzed. Reason for their efficiency even in absence of motility however could not be explained. 

Covalent Bond

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding. For many molecules, the sharing of electrons allows each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration.

Covalent bonding includes many kinds of interactions, including σ-bonding, π-bonding, metal-to-metal bonding, agostic interactions, and three-center two-electron bonds. The term covalent bond dates from 1939. The prefix co- means jointly, associated in action, partnered to a lesser degree, etc.; thus a "co-valent bond", in essence, means that the atoms share "valence", such as is discussed in valence bond theory.

In the molecule H

2, the hydrogen atoms share the two electrons via covalent bonding.Covalency is greatest between atoms of similar electronegativities. Thus, covalent bonding does not necessarily require that the two atoms be of the same elements, only that they be of comparable electronegativity. Covalent bonding that entails sharing of electrons over more than two atoms is said to be delocalized.

Chemotaxis in E coli

Chemotaxis (from chemo- + taxis) is movement of an organism in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food (e.g., glucose) by swimming toward the highest concentration of food molecules, or to flee from poisons (e.g., phenol). In multicellular organisms, chemotaxis is critical to early development (e.g., movement of sperm towards the egg during fertilization) and subsequent phases of development (e.g., migration of neurons or lymphocytes) as well as in normal function. In addition, it has been recognized that mechanisms that allow chemotaxis in animals can be subverted during cancer metastasis.

Positive chemotaxis occurs if the movement is toward a higher concentration of the chemical in question. However, negative chemotaxis occurs if the movement is in the opposite direction. Chemically prompted kinesis (randomly directed or nondirectional) can be called chemokinesis.

Ionic Bond

Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions. These ions represent atoms that have lost one or more electrons (known as cations) and atoms that have gained one or more electrons (known as an anions). In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complex nature, e.g. molecular ions like NH4+ or SO42-

It is important to recognize that clean ionic bonding – in which one atom "steals" an electron from another – cannot exist: All ionic compounds have some degree of covalent bonding, or electron sharing. Thus, the term "ionic bonding" is given when the ionic character is greater than the covalent character—that is, a bond in which a large electronegativity difference exists between the two atoms, causing the bonding to be more polar (ionic) than in covalent bonding where electrons are shared more equally. Bonds with partially ionic and partially covalent character are called polar covalent bonds.

Ionic compounds conduct electricity when molten or in solution, but typically not as a solid. There are exceptions to this rule, such as rubidium silver iodide, where the silver ion can be quite mobile. Ionic compounds generally have a high melting point, depending on the charge of the ions they consist of. The higher the charges the stronger the cohesive forces and the higher the melting point. They also tend to be soluble in water. Here, the opposite trend roughly holds: The weaker the cohesive forces, the greater the solubility.

Formation

Ionic bonding can result from a redox reaction when atoms of an element (usually metal), whose ionization energy is low, release some of their electrons to achieve a stable electron configuration. In doing so, cations are formed. The atom of another element (usually nonmetal), whose electron affinity is positive, then accepts the electron(s), again to attain a stable electron configuration, and after accepting electron(s) the atom becomes an anion. Typically, the stable electron configuration is one of the noble gases for elements in the s-block and the p-block, and particular stable electron configurations for d-block and f-block elements. The electrostatic attraction between the anions and cations leads to the formation of a solid with a crystallographic lattice in which the ions are stacked in an alternating fashion. In such a lattice, it is usually not possible to distinguish discrete molecular units, so that the compounds formed are not molecular in nature. However, the ions themselves can be complex and form molecular ions like the acetate anion or the ammonium cation.

For example, common table salt is sodium chloride. When sodium (Na) and chlorine (Cl) are combined, the sodium atoms each lose an electron, forming cations (Na+), and the chlorine atoms each gain an electron to form anions (Cl−). These ions are then attracted to each other in a 1:1 ratio to form sodium chloride (NaCl).

Na + Cl → Na+ + Cl− → NaCl

However, to maintain charge neutrality, strict ratios between anions and cations are observed so that ionic compounds, in general, obey the rules of stoichiometry despite not being molecular compounds. For compounds that are transitional to the alloys and possess mixed ionic and metallic bonding, this may not be the case anymore. Many sulfides, e.g., do form non-stoichiometric compounds.

Many ionic compounds are referred to as salts as they can also be formed by the neutralization reaction of an Arrhenius base like NaOH with an Arrhenius acid like HCl

NaOH + HCl → NaCl + H2O

The salt NaCl is then said to consist of the acid rest Cl- and the base rest Na+.

Representation of ionic bonding between lithium and fluorine to form lithium fluoride. Lithium has a low ionization energy and readily gives up its lone valence electron to a fluorine atom, which has a positive electron affinity and accepts the electron that was donated by the lithium atom. The end-result is that lithium is isoelectronic with helium and fluorine is isoelectronic with neon. Electrostatic interaction occurs between the two resulting ions, but typically aggregation is not limited to two of them. Instead, aggregation into a whole lattice held together by ionic bonding is the result.

The removal of electrons from the cation is endothermic, raising the system's overall energy. There may also be energy changes associated with breaking of existing bonds or the addition of more than one electron to form anions. However, the action of the anion's accepting the cation's valence electrons and the subsequent attraction of the ions to each other releases (lattice) energy and, thus, lowers the overall energy of the system.

Ionic bonding will occur only if the overall energy change for the reaction is favorable. In general, the reaction is exothermic, but, e.g., the formation of mercuric oxide (HgO) is endothermic. The charge of the resulting ions is a major factor in the strength of ionic bonding, e.g. a salt C+A- is held together by electrostatic forces roughly four times weaker than C2+A2- according to Coulombs law, where C and A represent a generic cation and anion respectively. Of course the sizes of the ions and the particular packing of the lattice are ignored in this simple argument.