Bacteria

Bacteria are small, microscopic, unicellular organisms that exhibit a great diversity in habitat, morphology, nutrition, metabolism and reproduction. The term bacterium was first used by Ehrenberg which comes from a Greek word bacterion which means little rod or stick. The study of bacteria is known as bacteriology.

A Dutch man, Anton van Leeuwenhoek, was the first to discover the bacteria in 1675, but he thought them to be animalcules. For about two centuries, thereafter, the bacteria did not receive much scientific attention. Later Louis Pasteur in 1864 and Robert Koch in 1876 reported that these microorganisms could cause disease.

Louis Pasteur declared that all putrefaction (decomposition of organic substances) is due to action of microorganisms. He also established that fermentation is also caused by microorganisms and developed the method of preservation of liquids (milk, wine etc) called the pasteurisation. Heating of liquids upto less than 100^o C temperature to kill the microorganisms is called the pasteurisation.

Robert Koch showed that anthrax disease is transmitted through spores. Koch proposed that all bacterial disease must follow some set conditions: i) that the organisms must be present in the lesions of the disease, ii) that it must be possible to grow these organisms in pure cultures outside the host body, iii) that the culture should be able to produce the typical symptoms of the disease, iv) that the same organism must invariably be present in the disease caused by the inoculum.

General Characters of Bacteria:

1. They are omnipresent, found in all possible habitats, which can support life.

2. They are unicellular and morphologically least complex of all living organisms.

3. Cell is prokaryotic – a well organised nucleus is absent.  Membrane bound cell organelles like endoplasmic reticulum, Golgi bodies, Mitochondria etc are absent.

4. Most of the bacteria have heterotrophic mode of nutrition i.e., they obtain their food directly from external source. They may live as saphrophytes, parasities or symbionts. Some bacteria are autotrophic, as they possess bacteriochlorophyll, the photosynthetic pigment.

5. Binary fission is the most common method of reproduction.

6. True sexual reproduction is absent. However, recombination of genetic material occurs by conjugation, transformation and transduction.

Occurrence and Distribution:

Bacteria are among the most numerous of all living organisms being present in almost very conceivable environment. There are about 2,000 known species.

Bacteria are ubiquitous organisms and occur in all natural habitats. They occur in abundance in both fresh waters of rivers, ponds, reservoirs and lakes and alo in saline waters of Oceans and Seas. They are found in all soil and above the earth.

Some thrive well in hot water springs (75^oC) and can also survive below freezing point in the Atlantic ice. Similarly, their tolerance to hydrogen concentration ranges from pH near to 0 to pH 11.

Many species of bacteria are free-living, and others are symbiotic, parasitic or saphrophytic. They are found within and upon the bodies of plants and animals. They flourish well on our skin, mouth and intestine.

The features which contribute to their universal distribution are:

1. Extremely simple structure

2. Small size, cell division occur rapidly.

3. Resistance of vegetative cells to adverse environmental factors

4. Formation of highly resistant endospores

5. Diversity of their modes of nutrition.

Size:

Bacteria are very small microscopic, unicellular organisms. The size of bacteria varies from 0.2µ (1 micron = 0.0001mm) in diameter and less than 2-10 microns in length.

Thiomargarita namibienxsis and Epulopischum fishelsoni are upto ½ a millimetre long and are visible to the unaided eye.

Forms or Shapes of Bacteria:

The bacteria vary in their shape which is usually constant for each species and helps in their identification.

I. Coccus Bacteria

Spherical or ellipsoidal bacteria are called cocci bacteria. Almost all coccal forms lack flagella (non-motile). The spherical cells may occur singly but often tend to stick together and occur in pairs, chains etc. On the basis of arrangement of cells in a cluster, the cocci are classified into the following six groups:

1. Monococcus: single spherical shaped cells. E.g., Micrococcus luteus

2. Diplococcus: a pair of spherical shaped cells (result when cocci divide and remain together to form pairs) e.g., Neisseria, Diplococcus pneumonia

3. Streptococcus: Cocci cells occur in long chains. Result when cells adhere after repeated divisions in one plane. E.g., Streptococcus lactis (milk bacterium).

4. Staphylococcus: Irregular group of many spherical bacteria arranged in bunches(grapes like). Result when cocci divide in radom planes. E.g., Staphylococcus albus (causes abscesses, wound infections).

5. Sarcinia: cuboidal arrangement of eight cocci cells. Results when cocci cells divide regularly in three planes. E.g., Sarcinae lutea

II. Bacillus Bacteria

This is the probably the most common form of bacteria. These are rod-shaped or hyhens (-) like bacteria are called the bacillus bacteria. Some are motile and some non-motile.

Based on the number of cells and their arrangement bacillus are classified into following :

1. Monobacillus: single rod shaped cells. E.g., Pseudomonas, Clostridium.

2. Diplobacillus:a pair of rod shaped cells. E.g. Corynebacterium diphtheria, Yersinia pestis (plague bacterium)

3. Streptobacillus: chain of bacillus bacteria appearing like straws. E.g., Bacillus tuberculosis, Lactobacillus.

III. Helical Bacteria : the helical forms are of two types – vibrioid and spirillum..

1. Vibrioid: rod shaped bacteria are slightly curved and look like a comma. A vibrio bears a single flagellum at its tip. E.g., Vibrio cholearae

2. Spirillum: the bacteria having the body twisted spirally or into helix. The spirilla forms usually bear two or more flagella at one or both poles. They occur as single cells or in short chains but do not form colonies. E.g., Spirillum volutans.

IV. Filamentous Bacteria:

Some bacteria are filamentous and branched. E.g., Beggiatoa, Thiothrix.

V. Pleomorphic Bacteria:

Some bacteria are able to change shape and size temporarily in response to changes in the environment. For example, Acetobacter, a group of vinegar bacteria occus as small rods, long rods, ellipsoids or a chain of small rods. Such bacteria are called pleomorphic.

Staining of Bacteria:       

Physician Christian Gram, in 1884 devised a differential staining procedure for identification of bacteria called the Gram staining.

A thin smear of bacterial cell is prepared on a glass slide. To the smear, Crystal Violet solution is applied for 30 seconds – step I. The slide is then rinsed in clean water. Then an Iodine solution is applied for 30 seconds – step II. This, inturn, is rinsed off with clean water.

Examination of the smear with the microscope reveals that all the bacteria have taken the purple colour. The next step in the staining procedure is to apply 95% Ethyl alcohol to the smear until the all the except the thickest layers of the smear have ceased to give off the dye.

Examination of the smear at this stage reveals that many of the bacteria still retain the original purple colour, whereas from others it has been washed off.

The bacteria that retain the original purple stain are called gram-positive bacteria. Those which loss the purple colour and become decolourised are called gram-negative bacteria.

The decolourised gram-negative bacteria are nearly invisible. To study their details counter stains like eosine or safranine are added to the smear.

Structure of Bacterial Cell

Bacteria are unicellular organisms. The ultra structure of bacteria is a typical example for the prokaryotic cell that lacks membrane bound nucleus and other cell organelles.

Like other living plant cells, the bacterial cell comprises of a cell wall and protoplast. Structurally, bacterial cell consists of following components:

1. Structures external to cell wall – flagella, fimbriae, pilli, capsule or slime layer

2. The cell Wall

3. Structures internal to cell wall – cell membrane, mesosomes, ribosomes, nucleoid

4. Special cell structures – storage granules, gas vacuoles, carboxysomes, magnetosomes, endospore

Structures External to Cell Wall

Slime layer:

The gelatinous layer present on the outer surface of the cell wall. It is secreted by the cell protoplast. The slime diffuses through the cell wall and deposits in form of a thin, extracellular, viscous layer external to the cell wall.

It is usually composed of polysaccharides or polypeptides or both. When it is made up of only polysaccharides, which forms a viscous layer and loosely attached to the cell wall, it is called Slime layer.

Under certain conditions of growth, the slime accumulates in form of a well-defined, thick , persistent  layer, firmly attached to the cell wall. It is called the sheath or capsule. E.g., Streptococcus pneumonia, Bacillus anthracis.

The sheath protects the bacteria against dessication and antibodies. The encapsulated pathogenic bacteria can with stand phagocytosis.

Some bacteria produce diffuse slime which spreads into the environment and encloses bacterial colonies. This is known as loose slime.

The bacterial cell with a capsule normally does not bear flagella.

Functions:

Provides protection under unfavourable conditions.

Function in the storage of food substances, and as storage site for the disposal of waste material.

Helps bacterium in attaching to various surfaces in order to survive in its natural environment such as streams, roots, teeth and tissues.

Flagella:

Mobility is universal among the spirillum bacteria, common among the bacillus bacteria but lacking or rare in coccal forms.

The motile bacteria have small whip or hair-like appendages called the flagella. They are about 120A⁰ – 150A⁰ thick and 4 0r 5 µ long. They are cytoplasmic in nature. Each flagellum arises within the cytoplasmic membrane and passes out through the cell wall.

Structure

Bacterial flagellum is composed of three parts – basal body, the hook and the filament.

The outermost, long, hair-like, cylindrical, hollow structure of the flagellum is called filament or shaft. It has a constant diameter of 120-200A⁰. It is made up of globular proteins, the flagellin.

The flagellum is composed of several chains of flagellin subunits forming a hollow cylindrical thread. Unlike the Eukaryotes, the filaments are not covered by a membrane or sheath.

Hook is short, curved, thicker segment of the flagellum. It is made up of a single type of protein. It connect the basal bodies to the filament of the flagellum. It passes out through the cell wall.

The basal body attaches the flagellum to the cell wall and plasma membrane. It is composed of a small central rod inserted into a series of rings. In Gram negative bacteria two pairs of rings, the proximal ring and the distal ring, are connected by a central rod.

The outer pair of rings i.e., L-ring (Lipopolysaccharide ring) and P- ring (Peptidoglycon ring) are attached to respective polysaccharide and  peptidoglycan layers of the cell wall. The inner pair of rings i.e., M-ring (Membrane ring) and S-ring (Super membrane ring) are attached with the cell membrane.

In Gram positive bacteria only inner pair of rings are present. The S ring is attached to the peptidoglycan layer and M-ring is attached to the cell membrane.

Flagellation:

The number, position and arrangement of flagella varies with the species. The flagella may be restricted to one or both ends of the bacterium cell (polar flagellation) or may be distributed uniformly all over the body surface (non-polar flagellation).

On the basis of number and arrangement of flagella, bacteria are classified into the following types:

Polar Flagellatoin:

This type of flagellation is found in bacilli and spirillum bacteria. They are all gram-negative. Polar flagellation are following types:

I. Monontrichous: bacteria with a single flagellum located at one end or pole. All Vibrios are monotrichous. Eg., V.cholerae

II. Amphitrichous: bacteria with one flagellum at each pole or end of the cell.

III. Cephalotrichous : two or more flagella at one pole of the bacterium cell. Eg., Pseudomonas fluorescenes. The flagellum shows more than two curves.

IV. Lophotrichous: Bacteria with two or more flagella at both the poles of the cell. Eg., Spirillum volutans.

Non-Polar Flagellation:

Flagella are found all over the cell surface. It is of one type only.

Peritrichous: bacterial cell with flagella evenly distributed all over the cell surface. Eg., Bacillus tylphosus, Proteus vulgaris.

Atrichous: all bacteria which lack flagella are called atrichous. Eg., Diptheria bacilli, Lactobacillus.

Pili:

Minute, straight, hair-like, non-flagellar appendages found on the cell wall of many gram negative bacteria. They are rigid and composed of a protein called pilin. They may be several microns long but are smaller in diameter around 30-50 A⁰.

There may be 100-400 pilli distributed over the surface of the bacterium cell.

The structures which are shorter than the pili are called the fimbriea.

Function:

Some pili are used for the attachment.

Pathogenic bacteria use them to identify and to attach their host cells.

F or sex pili are involved in the transfer of the DNA during bacterial conjugation.

Cell Wall:

Enveloping the protoplast of bacterial cell, there is a thin, firm cell wall. It gives rigidity and shape to bacterial cell.

The bacterial cell wall is composed of  4 layers. The outer most layer is L₄, it is wavy and electron dense. Within this is a lighter layer called the L₃. Next is the inner dense layer – L₂ made up of mucopeptide. The inner most layer is the L₁.

The most important component of bacterial cell wall is peptidoglycon or murein (mucopeptide or muramic acid, Gr. Murus=wall). This polymer is made up of chains of alternating molecule of N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM). NAG and NAM are linked by 1, 4, β-glycosidic bonds.

The cell wall of Gram +ve bacteria (Clostridium, Bacillus) is thick, homogenous, mainly consists of peptidoglycon (90%). The remainder being made up of proteins, polysaccharides and teichoic acid. Lipid content is very less in Gram +ve bacteria. The cell wall of Gram +ve bacteria is more sensitive to pencillin.

The cell wall of Gram –ve bacteria is chemically more complex and heterogenous. Peptidoglycan comprises up to 10% of the cell wall. The outer layer that surrounds peptidoglycan is two layered and consists of phospholipids, proteins and lipopolysaccharide mixture. Gram –ve bacteria possess more lipid. Teichoic acid is absent. The cell wall is less sensitive to pencillin.

The protoplast:

Within the cell wall is the living liquid called the protoplast. The protoplast is a clear water or slightly viscous substance.

It is differentiated into the cell or cytoplasmic membrane, cytoplasm and chromatin or nuclear body.

Cell or Cytoplasmic or Plasma Membrane:

At the surface of protoplast is a differentiated, delicate, fine membrane known as cell membrane or cytoplasmic membrane. It is about 75A⁰ thick. It lies close to the cell wall. Chemically it consists of phospholilpids with proteins and polysaccharides.

It is a selective permeable membrane which controls the passage of dissolved substances in an out of the cell. The cytoplasmic membrane also contains enzymes involved in enzymes and synthesis of capsule and cell wall material. It is also the site of ATP synthesis and the centre for control of flagellar motility.

Mesosomes

The plasma membrane of bacteria, at certain places, forms infoldings to form vesicle or tubular structures called the mesosomes. The term mesosomes was coined by Fitzjames. They increase the surface area of the membrane. They have respiratory enzymes, but they are not analogous to mitochondria. During division, they play a role in the DNA replication, septa formation, cell wall synthesis and spore formation.

Cytoplasm:

The bacterial cytoplasm is a complex mixtures composed of 70-80% of water and proteins, carbohydrates, lipids, inorganic ions. Organic matter is in the colloidal state.

The cytoplasm shows no streaming movement and contain no vacuoles. Cytoplasm is dense and transparent.

Bacterial cytoplasm contains nucleoid, plasmid, and ribosomes. Other membrane bound cell organelles like mitochondria, Endoplasmic reticulum, Golgi bodies are absent. Although a well organised chloroplast is absent, the photosynthetic bacteria have chromatophores.

The non-living inclusions are the storage granules of volutin, glycogen, lipid globules, poly-β-hydroxybutyrate (PHB).

Ribosomes:

In electron micrographs the bacterial cytoplasm is granular and contains numerous, minute, nearly spherical, hollow bodies called the ribosomes. They are composed of rRNA and proteins. Their number varies from 10,000 to 15,000 in a cell, which account upto 30% of total dry weight of the cell.

Bacterial ribosomes are of 70S type and is made up of two sub-units – 50S and 30S. Ribosomes are functional only when the two subunits are together. The association and dissociation of two subunits of ribosomes depends on the concentration of Mg²⁺ ions.

The 50S subunit has one molecule of 23S rRNA (3200 nucleotides), one molecule of 5S rRNA (120 nucleotides) and 34 different proteins. 30s subunit contains one molecule of 16s rRNA (1600 nucleotides) and 23 different proteins.

The ribosomes lie free in the cytoplasm but sometimes they occur in small groups called polyribosomes held by a strand of messenger RNA.

The ribosomes are sites for protein synthesis. Several antibiotic like streptomycin, neomycin and tetracycline inhibit protein synthesis on the ribosomes.

Nucleoid:

Area in the bacterial cytoplasm where the genetic material or DNA is present is called as nucleoid or genophore. Usually it is centrally located in bacterial cells and is equivalent to the nucleus in eukaryotes. However, it lacks nuclear membrane and nucleoli.

The shape of the nucleoid varies, often it is irregular, sometimes spherical or oval but elongated. The nucleoid is composed of about 60% DNA, 30% RNA and 10% proteins.

The DNA is single, long, circular, double stranded approximately 5X10⁹ base pairs and molecular weight of about 3X10⁹ daltons. The nucleoid can be stained with Feulgen stains.

Plasmids:

In addition to the nucleoid, bacterial cytoplasm normally contains one or more molecules of circular, double stranded DNA. Small, circular, self replicating and double stranded DNA molecule present in bacterial cell, in addition to nucleoid are called as plasmids. Lederberg coined the term plasmid.

They replicate autonomously, and inherited by both the daughter cells during the cell division. Some plasmids get integrated  into the bacterial DNA or chromosome. They are called episomes by Jacob Schaeffer and Wollman.

The number of plasmids ranges from one to hundred or more per bacterial cell. A plasmid contains 5-100 genes that govern several biological functions. Plasmids are not essential for the growth and survival of the bacterial cell, but they confer special characteristics like drug resistance, toxigenecity to the bacteria.

On the basis of functions, plasmids are divided into several types:-

i. F-factor (fertility) plasmids,

ii. R-factor (Resistance) plasmids

iii. col-factor (calicinogeny) plasmids

iv. Virulance plasmids conferring pathogenecity to mammals

v. Ti- plasmids of Agrobacterium tumifacines

vi. Ri-plasmids (root inducing) if Agrobacterium rhizogenes

Plasma Membrane

The cell has a different internal milieu from that of its external environment. This difference is maintained throughout the life of the cell by the thin surface membrane, the plasmalemma or plasma membrane. It controls the entrance and exit of molecules and ions.

The limiting membrane of the cell organelles like Golgi complex, endoplasmic reticulum, nuclear membrane etc., is known as cell membrane. The cell membrane and plasma membrane are collectively known as biological membranes.

Cell membranes may be defined as the membranes that serve as boundary of cytoplasm or organelles, exerts selective influence on the molecules and ions entering or leaving the cell or organelles, help in controlling intracellular melieu and influence the cell or organelle behaviour.

In some eukaryotic cells the different membrane systems make up as much as 80 percent of the total dry cell mass. Membranes serve not only as barriers separating aqueous compartments with different solute composition but also as the structural base to which certain enzymes and transport systems are firmly bound. They are very thin (about 8 nm) and flexible.

Composition of membrane:

The cell membrane mainly consists of proteins and lipid or lipoproteins. In certain cases polysaccharides, sialic acid, RNA and DNA have also been found. The ratio of protein to lipid varies from 1:0.8 to 1:4 and it depends, on the type of membrane, type of cell and type of organism.

All these chemical components are held together in a thin film – like pliable sheet of about 75A⁰ thickness, in three concentric layers. The middle layer is about 35A⁰ and outer layer on either side are 20 A⁰ thick.

Lipids:

The amount of lipids found in the membrane varies from 30 percent (mitochondria) to 70 percent of the total dry matter of cell membrane.

The most common lipids found in the membrane are phospholipids, glycolipids and sterols. All of them are amphipathic i.e., they have both hydrophilic and hydrophobic portions within a molecule.

The major proportion of membrane phospholipids is represented by phosphatidylcholine, phosphatidylethanolamine and sphingomylein. Glycolipids contain a sugar (glucose or galactose) in addition to fatty acids and sphingosine. The examples of sterols are cholesterols present in animal tissues, phytosterols present in plant cells and ergosterol present in ergot and  yeast.

Membrane Proteins:

Proteins represent the main component of most biological membranes. They play an important role, not only in the structure of the membrane, but also as carriers or channels, serving for transport, they may also be involved in regulatory or ligand-recognition properties. Numerous enzymes, antigens, and various kinds of receptor molecules are present in plasma membranes.

The different types of proteins present in membranes are:

i. Structural proteins: that form the backbone of the membrane and are strongly lipophilic in nature.

ii. Functional proteins: like enzymes that catalyse several physiological reactions (e.g., glucose 6 phosphatase found in membranes of endoplasmic reticulum and cytochrome oxidase present in mitochondria)

iii. Carrier proteins: that bring about the diffusion of substances across the membrane.

The proteins present in the plasma membrane are further differentiated into integral or intrinsic or integral proteins and peripheral or extrinsic proteins.

The intrinsic proteins make up 70 percent or more of the total membrane proteins. They are very tightly bound to the lipid portion and can be removed only by drastic procedures for isolation. The intrinsic proteins are highly insoluble in water solutions and are associated with lipids. These proteins may be attached to oligosaccharides , thus forming glycoproteins.

The peripheral or extrinsic proteins are loosely attached to the membrane surface, free of lipids and can easily be removed in soluble form by mild extraction procedures.

Structure of Plasma Membrane or Molecular models of Cell Membrane

Since substances soluble in lipid solvents penetrate the plasma membrane easily, in 1902 Overton postulated that the plasma membrane is composed of a thin layer of lipid.

Lipid Bilayer Model:

In 1925, E.Gorter and F.Grendel made the first proposal that the plasma membrane might contain a lipid bilayer. They extracted the lipids from human erythrocytes and concluded that the plasma membrane contained a bimolecular layer of lipids, or simply a lipid bilayer.

They also suggested that the polar groups of each molecular layer were directed towards the outside of the bilayer.

Later, it was found that some water soluble molecules could move across the cell surface and this resulted in the suggestion that the  plasma membrane was a mosaic of lipid and non-lipid substances.

Danielli – Davson Model:

In 1935, James Danielli and Hugh Davson studied the starfish egg membrane and proposed that the plasma membrane is made up of two layers of lipids sandwiched between two continuous layers of proteins.

According to this model, the protein and lipid molecule layers are electrostatically held together the former being the charged and the latter negatively charged. The lipophilic(hydrophobic) tails of fatty acids face one another in the centre and their hydrophilic heads face outwards.

They further envisioned the polar ends of the lipid molecules as being associated with a monomolecular layer of polar globular proteins. Each protein layer is found to be 20 A⁰ thick and lipd layers 35A⁰ thick, making the membrane 75 – 100 A⁰ thick.

This model cannot be applied to all membranes, as it reveals the definite proportion of lipids and proteins, which has not been found true. Moreover, recent studies reveal that proteins are found in form of particles and not as continuous layer.

Unit Membrane Concept:

It was proposed by Robertson . This concept was based on the study of the myelin sheath of a nerve fibre.

Robertson studied the structure of membrane with the help of electron microscope and found that all biological membranes show three layers (trilaminar) i.e., two outer layers  (electron dense) separated by a lighter middle layer. These correspond to two layers of proteins and a middle layer of phospholipids.

The molecular chains of lipids remain parallel to each other thus forming a bimolecular structure. The two chains remain linked by non-polar and hydrophobic inner ends of lipid molecules. Van der Waals forces bind the two lipid layers together.

The protein layer provides elasticity and mechanical resistance to plasma membrane. The unit membrane model is similar to Danielli-Davson model except that the protein layers at the exterior and interior are different. The outer surface has mucoproteins, while the inner surface has un-conjugated protein molecules are present. This shows the asymmetry of the protein surfaces on either side of the lipid bilayer.

Robertson provided evidence that the membranes of other cell organelles like endoplasmic reticulum, mitochondria, golgi complex have the unit membrane structure.

The Robertson’s unit membrane model was widely approved but remained unsatisfactory because it could not explain the dynamic nature and functional specificity of the membrane.

Fluid Mosaic Model

The most satisfactory model of membrane structure was postulated by S.J. Singer and G.L. Nicolson called the fluid-mosaic model.

This model postulates that the phospholipids of membranes are arranged in a bilayer to form a fluid, liquid-crystalline matrix or core. In this bilayer individual lipid molecules can move laterally, endowing the bilayer with fluidity, flexibility and a characteristically high electrical resistance and relative impermeability to highly polar molecules.

The fluid mosaic model postulates that that the membrane proteins are globular and are dispersed in the lipid bilayer. Thus, the proteins would form a mosaic like structure in the fluid phospholipid bilayer. In a way, these protein molecules resemble floating icebergs in the sea of lipids.

This mosaic is not fixed or static, since the proteins are free to diffuse laterally. Some of the proteins are partially embedded in the membrane, penetrating in the lipid layer (integral or intrinsic proteins) and some are superficially attached (peripheral or extrinsic proteins.

The important concepts of fluid mosaic model can be summarized as i) that biological membranes are quasi-fluid structures in which both the lipids and integral proteins are able to perform translational movements within the bilayer, ii) that the lipid and integral proteins are disposed in a kind of mosaic arrangement.

The fluid mosaic model accounts satisfactorily for many features and a properties of biological membranes. It provides for membranes with widely different protein content, varying thickness of different types of membranes, it account for the asymmetry of natural membranes, it accounts for the electrical properties and permeability of membranes.

The Micellar Model:

Many cell biologists suggested that membrane might consists of closely packed repeating units which are similar in structure.

In 1953, Hillier and Hoffman suggested that the membranes have a non-lamellar pattern, consisting of globular subunits known as micelles.

Each micelle is about 40-70A⁰ in diameter with a lipid core and a hydrophilic shell of polar groups. The protein components of the membrane form a monolayer on either side of the plane of lipid micelles.

The space between micelles represent water filled pore (4A⁰). These pores are lined partly by the polar groups of micelles and partly by the polar groups of associated proteins.

Functions of Biological Membranes

1. Membranes are living boundaries of the cytoplasm and cellular organelles. They form and continuous and unbroken sheets that divide the living matter into self-sustaining units to perform specialised functions.

2. Acts as a selectively permeable membrane- allowing appropriate substances into the cell and removing the inappropriate substances out of cell.

3. Allow the transport of metabolic essential substances

4. membranes have on their outer surface specific protein molecules acting as receptors, which unit with complementary substances or ligands providing external stimuli to the cell.

5. Perform different biochemical activities in the presence of enzymes.

Stelar Evolution in Pteridophytes

Write as second assignment

Stelar Evolution in Pteridophytes

Central cylinder or core of vascular tissue consisting of xylem, phloem, pericycle and sometimes medullary rays and pith is called Stele.
The word stele is derived from Greek which means  central pillar. The Stele is the central cylinder and is separated from the cortex by the endodermis.  The endodermis is the innermost layer of the cortex and pericycle is the outermost portion of the Stele.

Van Tiegam and Douliot proposed Stellar Theory in 1886.
According to to the the stellar theory there is no fundamental difference in the gross anatomy of the stem and root because in both of them is Stele surrounded by cortex is present
According to this theory the stele is not only made up of xylem and phloem but the tissue like pericycle, vascular rays and pith are also associated with it.

Types of steles
Easu and Smith recognise two principal types of stelar organisation among the vascular plants.  These are the Protostele and Siphonostele.
Protostele
The name protostele was suggested by Jeffrey and he regarded it as the most primitive and simplest type of stele.
Steel in which the gas cylinder consists of a solid core of xylem surrounded by phloem pericycle and endodermis is called protostele. There is no pith in a protostele. it is a fundamental type of Steel of vascular plants from which the other type originated in the course of evolution.
Brenner classified the protostele into following types
Haplostele: Protostele with smooth core of  xylem surrounded by uniform layer of phloem is called haplostele.  It is found in several fossil genera like Rynia and Horneophyton and living genera like Selaginella kraussiana.

Actinostele: a protostele in which xylem core is stellate or star shaped with radiating arms is called at actinostele. actor no still the phloem is not present in a continuous manner but in the form of separate groups which alternate with the arms of the star shaped xylem. Example Psilotum,  Lycopodium serratum.

Sometimes the actinostele may show variations due to breaking of xylem tissue in two different forms. The common modifications of actinostele are
Plectostele: The xylem gets broken into a number of more or less parallel plates and such xylem plates alternate with phloem plates.  Example Lycopodium clavatum and Lycopodium volubile.

Mixed protostele in this type of stele xylem appears in the form of irregular crops that are scared scattered in the ground mass of the phloem. E.g., L. cernuum.

Siphonostele : It is a modified form of protostele in which the pith is present in the centre of the stele. A medulated protostele  is called as siphonostele. During the development of siphonostele core of xylem is replaced by parenchymatous pith.
In siphonostele the central pith is surrounded by a cylinder of xylem and phloem. This type of stele is found in members of Pteropsida.
Origin of Siphonostele:
There is a general acceptance that the Siphonostele has evolved from protostele. Two theories have been proposed for evolution of Siphonostele from protostele,
I Extrastelar origin of pith: This theory was put forwarded by Jeffrey. According to him the pith originated as a result of the invasion of cortical parenchymatous cells into the stele. This invasion occurs through the leaf gaps and branch gaps.
II Intrastelar origin of pith:
Boodle, Gwynne-Vaughan, Bower supported this theory of intrastelar origin of pith.
According to this theory the innermost or centrally placed vascular tissue changes into parenchymatous cells which behave as pith. Presence of isolated trachieds in the pith in Botrychium viriginianum, Osmunda regalis support the intrastelar origin of pith.

Types of Siphonostele
Jeffrey classified the siphonostele into following two types on the basis of the position of phloem :
Ectophloic Siphonostele:
in this the earth is surrounded by xylem which in turn is surrounded by phloem, pericycle and endodermis.  So in this case phloem is present only external to the xylem. E.g., Osmunda

Amphiphloic Siphonostele:
In this case the phloem is present on both the sides of xylem i.e., on external as well as internal sides.  The pith is  present in the centre.  Xylem on inner side remains surrounded by inner phloem, inner pericycle, inner endodermis, and centrally placed pith. On the outer side of the xylem are present  the outer phloem,  outer pericycle and outer endodermis. It is found in Marsilea rhizome and   Adiantum.

Modifications of siphonostele
Cladosiphonic siphonostele: A siphonostele with no leaf gap has been termed as clado siphonic siphonostele by Jeffrey. E.g., Selaginella.
Phyllosiphonic siphonostele: It is a siphonostele that remains perforated by smaller or larger leaf gaps.  E.g., members of Fillicophyta.
The point at which the stem bundle diverges from the vascular cylinder toward the leaf is called as leaf gap..
If the stem or the rhizome bears leaves at distance the leaf gaps also appear at a considerable distance from each other. At such places the stele is interrupted and appears horseshoe shaped. The leaf gap closes at a higher level and the Stele again becomes complete and circular.
The siphonostele main assume different forms depending upon the occurrence, number and distribution of leaf gaps. These are as follows:
Solenostele: If the siphonostele is perforated by a single leaf gap only, corresponding to the origin of the leaf trace such a condition is known as solenostele. E
g., Adiantum pedatum.
Dictyostele: In advanced ferns like Dryopteris, Pteris the rhizome or stem is short and leaves overlap each other. This leads to the overlapping of leaf gaps in the stele so that the lower part of one leaf gap is parallel with the upper part of another leaf gap.
In Dictyostele the stele is broken into a network of seperate vascular strands, mainly because of crowded leaf gaps.
Each such separate vascular strand is known as meristele. Each meristele  is of concentric type and consists of a central core of xylem surrounded by phloem, pericycle and endodermis.

Eustele : if the stele is split into distinct collateral vascular bundle it is called eustele. It is a modification of the ectophloic siphonostele. E.g., Dicotyledons.

Atactostele: Differing from eustelic condition, the vascular strands are scattered. Such a stele has been named as atactostele by Esau. It occurs in monocotyledons.

Polycyclic stele: when the vascular tissue is present in the form of two or more concentric rings, such a stele is called a polycyclic stele. E.g., Pteridium aquilinium, Marattia. 

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Equisetum

Equisetum
                                                                                                        Class Sphenopsida
                                                                                                        Order Equisetales
                                                                                                         Family Equisetaceae
                                                                                                          Genus Equisetum
Distribution
Equisetum is the only living representative of the family Equisetaceae. It is commonly known as Horse tail (aerial shoot with closely arranged whorls of branches look like a tail of a horse) or scoring rushes (due to sand papery  texture due to deposition  of silica).
The genus is represented by 32 species, found all over the world except in Australia and New Zealand.  Most species occur in the north Northern temperate zone but they are also known in Arctic, the southern and temperate and the tropics of both the hemispheres.
In India it is represented by about 7 species. E. arvense is a cosmopolitan species. E. debile  grows along the banks of sandy or swampy  soil and is found even in the Gangetic plains. Other Indian species are E. diffusum, E.ramossissimum, E. maximum, E. dubium.

Habitat
Equisetum grows under various habitats but thrives well under damp and amphibious conditions. Commonly they are found along the banks of rivers, streams ,( E. debile, E. protease) and swampy places (E.palustris). E. debile grows under hydrophytic as well as xerophytic conditions. E.avernse grows in open grasslands, railway embankments, exposed sandy and dry places. E. avernse is commonly called field horse tail.
According to Vogt,  few Equisetum species are used as ecological indicators, specially to indicate the mineral content of the soil in which they grow.  According to Benedict, certain Equisetum species accumulate minerals including gold and can therefore well be used to determine the presence of minerals in the soil.
Sporophyte
External morphology
The plant body of Equisetum represents the sporophyte generation. All species are well branched,  perennial herbs, containing a horizontal underground rhizome from which arise many roots towards lower side and many erect aerial shoots towards upper side.
Rhizome
Main stem is an extensively branched, sympodial, dark brown, rhizome. The rhizome penetrates deep into the soil. The rhizome presents a jointed appearance due to presence of distinct nodes and internodes.
On its nodes are present many scaly leaves. The scaly leaves are small, slender and united laterally with each other to form a sheet on the node.
The rhizome gives off  aerial as well as subterranean branches that alternate with the scale leaves.  A branch primordia, alternating with each leaf, is present on the node.
Aerial stem
From the rhizome, the aerial shoots arise towards upper side.  They are conspicuously jointed and are green, rough and stiff. They are abrasive due to deposits of silica.
The  jointed appearance is due to the presence of distinct nodes and internodes.
The nodes bear whorled leaves that are laterally fused at the base.  The internodes are long and hollow and are longitudinally rigid and grooved.
Each ridge corresponds to a leaf at the node above and the ridges in the successive internodes alternate with one another.
Another characteristic feature of the stem is the presence of an intercalary meristem that is present at the base of the internodes and just about the node. The meristematic region is protected by the leaf sheath. The activity of this meristem is responsible for the increase in length of the internode.
The aerial stem maybe branched or unbranched. In E. arvense the Ariel princess or profusely branched the branches arise in whorls at the node. They alternate with leaves.

In most spaces the aerial branches are distinguish as sterile and fertile branches.
Sterile branches are green with whorl of scaly leaves at each node. They are photosynthetic. At each node whorl of lateral branches arise, which are assimilatory in function. Each whorl bears an equal number of branches to that of leaves.
Fertile branches are unbranched, non chlorophyllous and terminate into strobilus at the apex.  In many species, the fertile branches  die as soon as the spores are released. In other species the fertile branch is green and after the cone sheds the spores, it behaves like the sterile branch

Leaves
Equisetum is micro fillers the leaves are simple irregular surface and extremely reduced in size they arise in viral at each node and their number varies with species.all the leaves are late relief use a sheet around the base of the internals the sheet and pointed teeth like tips. the leaves are mainly protective in function and in some species the leaves become dead and scale like at maturity the leaves do not perform any photosynthetic function at each node alternating with the leave there is a branch primordium the leave as a single median when they are usually without chlorophyll
Roots
The roots develop from the nodes of rhizome or from the base of the stem. They are long, slender, well branched and adventitious.
The roots arise indigenously from the pericycle. The roots are extensively branch and have a root cap at their tips