Category: Biology

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Nuclear Divisions – Definition and its Difference

There are two types of nuclear division, as mitosis and meiosis. In mitosis, the daughter cells formed will have the same number of chromosomes as the parent cell, typically diploid (2n) state. Mitosis is the nuclear division that occurs when cells grow or when cells need to be replaced and when organism reproduces asexually.

In meiosis, the daughter cells contain half the number of chromosomes of the parent cell and is known as haploid state (n). Whichever division takes place, it is normally followed by division of the cytoplasm to form separate cells, called as cytokinesis.

The process by which a nucleus divides, resulting in the segregation of the genome to opposite poles of a dividing cell. Supplement, Nuclear divisions occur in both mitosis and meiosis. In mitosis, the result is the division of duplicated copies of genome into two.

There are two kinds of nuclear division-mitosis and meiosis. Mitosis divides the nucleus so that both daughter cells are genetically identical. In contrast, meiosis is a reduction division, producing daughter cells that contain half the genetic information of the parent cell.

Mitosis is a process of nuclear division in eukaryotic cells that occurs when a parent cell divides to produce two identical daughter cells. Mitosis is conventionally divided into five stages known as prophase, prometaphase, metaphase, anaphase, and telophase.

Mitosis is a single nuclear division that results in two nuclei, usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original. They have the same number of sets of chromosomes: one in the case of haploid cells, and two in the case of diploid cells. Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new daughter cells.

The process by which a nucleus divides, resulting in the segregation of the genome to opposite poles of a dividing cell. Nuclear divisions occur in both mitosis and meiosis. In mitosis, the result is the division of duplicated copies of genome into two.

Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells. It occurs concurrently with two types of nuclear division called mitosis and meiosis, which occur in animal cells.

Meiosis I, the first meiotic division, begins with prophase I. During prophase I, the complex of DNA and protein known as chromatin condenses to form chromosomes. The pairs of replicated chromosomes are known as sister chromatids, and they remain joined at a central point called the centromere.

Under the microscope, you will now see the chromosomes lined up in the middle of the cell. You will probably also see thin-stranded structures that appear to radiate outward from the chromosomes to the outer poles of the cell.

Nuclear division occures twice during meiosis as four haploid gametes are produced; each of which are genetically different from each other. In both processes the nuclear envelope is fragmented and completley broken down into small vesicles during prophase, to allow the chromosomes to segregate. Cell division occurs during phase, which consists of nuclear division (mitosis) followed by cytoplasmic division (cytokinesis).

They are also genetically identical to the parental cell. Mitosis has five different stages: interphase, prophase, metaphase, anaphase and telophase. The process of cell division is only complete after cytokinesis, which takes place during anaphase and telophase.

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Flagella – Definition Structure and its Types

Prokaryotic Flagellum

Bacterial flagella are helical appendages helps in motility. They are much thinner than flagella or cilia of eukaryotes. The filament contains a protein called flagellin. The structure consists of a basal body associated with cytoplasmic membrane and cell wall with short hook and helical filament. Bacteria rotates their helical flagella and propels rings present in the basal body which are involved in the rotary motor that spins the flagellum.

Structure of Flagella in Bacteria

The gram positive bacteria contain only two basal rings. S-ring is attached to the inside of peptidoglycan and M-ring is attached to the cell membrane. In Gram negative bacteria two pairs of rings proximal and distal ring are connected by a central rod.

They are L-Lipopolysaccharide ring, P-Peptidoglycan ring, S-Super membrane ring and M-membrane ring. The outer pair L and P rings is attached to cell wall and the inner pair S and M rings attached to cell membrane (Figure 6.27).

Mechanism of Flagellar Movement – Proton Motive Force

In flagellar rotation only proton movements are involved and not ATP. Protons flowing back into the cell through the basal body rings of each flagellum drives it to rotate. These rings constitute the rotary motor.The proton motive force (The force derived from the electrical potential and the hydrogen ion gradient across the cytoplasmic membrane) drives the flagellar motor.

For the rotation of flagellum the energy is derived from proton gradient across the plasma membrane generated by oxidative phosphorylation. In bacteria flagellar motor is located in the plasma membrane where the oxidative phosphorylation takes place. Therefore, plasma membrane is a site of generation of proton motive force.

Eukaryotic Flagellum – Cell Motility Structure

Eukaryotic Flagella are enclosed by unit membrane and it arises from a basal body. Flagella is composed of outer nine pairs of microtubules with two microtubules in its centre (9+2 arrangement). Flagella are microtubule projection of the plasma membrane. Flagellum is longer than cilium (as long as 200µm). The structure of flagellum has an axoneme made up microtubules and protein tubulin (Figure 6.28)

Movement

Outer microtubule doublet is associated with axonemal dynein which generates force for movement. The movement is ATP driven. The interaction between tubulin and dynein is the mechanism for the contraction of cilia and flagella. Dynein molecules uses energy from ATP to shift the adjacent microtubules. This movement bends the cilium or flagellum.

Cilia

Cilia (plural) are short cellular, numerous microtubule bound projections of plasma membrane. Cilium (singular) is membrane bound structure made up of basal body, rootlets, basal plate and shaft.

The shaft or axoneme consists of nine pairs of microtubule doublets, arranged in a circle along the periphery with a two central tubules, (9+2) arrangement of microtubules is present. Microtubules are made up of tubulin. The motor protein dynein connects the outer microtubule pair and links them to the central pair. Nexin links the peripheral doublets of microtubules (Figure 6.29).

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Nucleus Definition and Various Types of Functions

Nucleus is an important unit of cell which controls all activities of the cell. Nucleus holds the hereditary information. It is the largest among all cell organelles. It may be spherical, cuboidal, ellipsoidal or discoidal. It is surrounded by a double membrane structure called nuclear envelope, which has the inner and outer membrane.

The inner membrane is smooth without ribosomes and the outer membrane is rough by the presence of ribosomes and it continues with irregular and infrequent intervals with the endoplasmic reticulum.

The membrane is perforated by pores known as nuclear pores which allows materials such as mRNA, ribosomal units, proteins and other macromolecules to pass in and out of the nucleus. The pores enclosed by circular structures called annuli. The pore and annuli form the pore complex. The space between two membranes is called perinuclear space.

Nuclear space is filled with nucleoplasm, a gelatinous matrix has uncondensed chromatin network and a conspicuous nucleolius. The Chromatin network is an uncoiled, indistinct and remain thread like during the interphase. It has little amount of RNA and DNA bound to histone proteins in eukaryotic cells (Figure 6.22).

During cell division chromatin is condensed into an organized form called chromosome. The portion an eukaryotic chromosome which is transcribed into mRNA contains active genes that are nottightly condensed during interphase is called Euchromatin.

The portion of an eukaryotic chromosome that is not transcribed into mRNA which remains condensed during interphase and stains intensely is called Heterochromatin. Nucleolus is a small, dense, spherical structure either present singly or in multiples inside the nucleus and it’s not membrane bound. Nucleoli possess genes for rRNA and tRNA.

Functions of the Nucleus

• Controlling all cellular activities
• Storing the genetic or hereditary information.
• Coding the information from DNA for the production of enzymes and proteins.
• DNA duplication and transcription takes place in the nucleus.
• In nucleolus ribosomal biogenesis takes place.

Chromosomes

Strasburger (1875) first reported its present in eukaryotic cell and the term ‘chromosome’ was introduced byWaldeyerin 1888. Bridges (1916) first proved that chromosomes are the physical carriers of genes. It is made up of DNA and associated proteins.

Structure of Chromosome

The chromosomes are composed of thread like strands called chromatin which is made up of DNA, protein and RNA. Each chromosome consists of two symmetrical structures called chromatids. During cell division the chromatids forms a well organized chromosomes with definite size and shape.

They are identical and are called sister chromatids. A typical chromosome has narrow zones called constrictions. There are two types of constrictions, namely primary constriction and secondary constriction. The primary constriction is made up of centromere and kinetochore.

Both the chromatids are united at centromere, whose number varies. The monocentric chromosome has one centromere and the polycentric chromosome has many centromeres. Centromere contains a complex system of protein fibres called kinetochore. Kinetochore is the region of chromosome which is attached to the spindle fibre during mitosis.

Besides primary there are few secondary constrictions, are present. Nucleoli develop from these secondary constrictions are called nucleolar organizers. Secondary constrictions contain the genes for ribosomal RNA which induce the formation of nucleoli and are called nucleolar organizer regions (Figure 6.23).

A satellite or SAT Chromosome is a short chromosomal segment or rounded body separated from main chromosome by a relatively elongated secondary constriction. It is a morphological entity in certain chromosomes.

Telomere is the terminal part of chromosome. It offers stability to the chromosome. DNA of the telomere has specific sequence of nucleotides. Telomere in all eukaryotes are composed of many repeats of short DNA sequences (5’TTAGGG3’ sequence in Neurospora crassa and human beings).

Maintenance of telomeres appears to be an important factor in determining the life span and reproductive capacity of cells, so studies of telomeres and telomerase have the promise of providing new insights into conditions such as ageing and cancer. Telomeres prevent the fusion of chromosomal ends with one another.

Types of Chromosomes

Based on the position of centromere, chromosomes are called telocentric (terminal centromere), acrocentric (terminal centromere capped by telomere), sub metacentric (centromere subterminal) and metacentric (centromere median). The eukaryotic chromosome may be rod shaped (telocentric and acrocentric), L-shaped (sub-metacentric) and V-shaped (metacentric) (Figure 6.24).

Based on the functions of chromosome it can be divided into autosomes and sex chromosomes. Autosomes are present in all cells controlling somatic characteristics of an organism. In human diploid cell, 44 chromosomes are autosomes whereas two are sex chromosomes. Sex chromosomes are involved in the determination of sex.

Special Types of Chromosomes

These chromosomes are larger in size and are called giant chromosomes in certain plants and they are found in the suspensors of the embryo. The polytene chromosome and lamp brush chromosome occur in animals and are also called as giant chromosomes.

Polytene chromosomes observed in the salivary glands of Drosophila (fruit fly) by E.G. Balbiani in 1881. In larvae of many flies, midges (Dipthera) and some insects the interphase chromosomes duplicates and reduplicates without nuclear division.

A single chromosome which is present in multiple copies form a structure called polytene chromosome which can be seen in light microscope. They are genetically active. There is a distinct alternating dark bands and light inter-bands. About 95% of DNA are present in bands and 5% in inter-bands.

The polytene chromosome has extremely large puff called Balbiani rings which is seen in Chironomous larvae. It is also known as chromosomal puff. Puffing of bands are the sites of intense RNA synthesis. As this chromosome occurs in the salivary gland it is known as salivary gland chromosomes. Gene expression, transcription of genes and RNA synthesis occurs in the bands along the polytene chromosomes.

Lampbrush chromosomes occur at the diplotene stage of first meiotic prophase in oocytes of an animal Salamandar and in giant nucleus of the unicellular alga Acetabularia. It was first observed by Flemming in 1882. The highly condensed chromosome forms the chromosomal axis, from which lateral loops of DNA extend as a result of intense RNA synthesis.

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Cell Organelles Definition, Functions and Various Types of Organelles

Endomembrane System

System of membranes in a eukaryotic cell, comprises the plasma membrane, nuclear membrane, endoplasmic reticulum, golgi apparatus, lysosomes and vacuolar membranes (tonoplast). Endomembranes are made up of phospholipids with embedded proteins that are similar to cell membrane which occur within the cytoplasm. The endomembrane system is evolved from the inward growth of cell membrane in the ancestors of the first eukaryotes (Figure 6.12).

Endoplasmic Reticulum

The largest of the internal membranes is called the endoplasmic reticulum (ER). The name endoplasmic reticulum was given by K.R. Porter (1948). It consists of double membrane. Morphologically the structure of endoplasmic reticulum consists of the following:

1. Cisternae are long, broad, flat, sac like structures arranged in parallel bundles or stacks to form lamella. The space between membranes of cisternae is filled with fluid.
2. Vesicles are oval membrane bound vacuolar structure.
3. Tubules are irregular in shape, branched, smooth walled, enclose a space.

Endoplasmic reticulum is associated with nuclear membrane and cell surface membrane. It forms a network in cytoplasm and gives mechanical support to the cell. Its chemical environment enables protein folding and undergo modification necessary for their function. Misfolded proteins are pulled out and are degraded in endoplasmic reticulum.

When ribosomes are present in the outer surface of the membrane it is called as rough endoplasmic reticulum(RER), when the ribosomes are absent in the endoplasmic reticulum it is called as smooth Endoplasmic reticulum(SER).

Rough endoplasmic reticulum is involved in protein synthesis and smooth endoplasmic reticulum are the sites of lipid synthesis. The smooth endoplasmic reticulum contains enzymes that detoxify lipid soluble drugs, certain chemicals and other harmful compounds.

Golgi Body (Dictyosomes)

In 1898, Camillo Golgi visualized a netlike reticulum of fibrils near the nucleus, were named as Golgi bodies. In plant cells they are found as smaller vesicles termed as dictyosomes. Golgi apparatus is a stack of flat membrane enclosed sacs.

It consist of cisternae, tubules, vesicles and golgi vacuoles. In plants, the cisternae are 10-20 in number placed in piles separated from each other by a thin layer of inter cisternal cytoplasm often flat or curved.

Peripheral edge of cisternae forms a network of tubules and vesicles. Tubules interconnect cisternae and are 30 – 50nm in dimension. Vesicles are large round or concave sac. They are pinched off from the tubules. They are smooth/secretary or coated type.

Golgi vacuoles are large spherical structures filled with granular or amorphous substance, some function like lysosomes. Golgi apparatus compartmentalises a series of steps leading to the production of functional protein.

Small pieces of rough endoplasmic reticulum are pinched off at the ends to form small vesicles. A number of these vesicles then join up and fuse together to make a Golgi body. Golgi complex plays a major role in post translational modification of proteins and glycosylation of lipids (Figure 6.13 and 6.14).

Functions:

• Production of glycoproteins and glycolipids
• Transporting and storing of lipids.
• Formation of lysosomes.
• Production of digestive enzymes.
• Cell plate and cell wall formation
• Secretion of carbohydrates for the formation of plant cell walls and insect cuticles.
• Zymogen granules (proenzyme/precursor of all enzyme) are synthesised.

Mitochondria

It was first observed by A. Kolliker (1880). Altmann (1894) named it as Bioplasts. Later Benda (1897, 1898), named as mitochondria. They are ovoid, rounded, rod shape and pleomorphic structures. Mitochondrion consists of double membrane, the outer and inner membrane.

The outer membrane is smooth, highly permeable to small molecules and it contains proteins called Porins, which form channels that allows free diffusion of molecules smaller than about 1000 daltons and the inner membrane divides mitochondrion into two compartments, outer chamber between two membranes and the inner chamber is filled with matrix.

The inner membrane is convoluted (infoldings), called crista (plural: cristae). Cristae contain most of the enzymes for electron transport system. Inner chamber of the mitochondrion is filled with proteinaceous material called mitochondrial matrix. The Inner membrane consists of stalked particles called elementary particles or Fernandez Moran particles, F1 particles or Oxysomes.

Each particle consists of a base, stem and a round head. In the head, ATP synthase is present for oxidative phosphorylation. Inner membrane is impermeable to most ions, small molecules and maintains the proton gradient that drives oxidative phosphorylation (Figure 6.15).

Mitochondria contain 73% of proteins, 25-30% of lipids, 5-7% of RNA, DNA (in traces) and enzymes (about 60 types). Mitochondria are called Power house of a cell, as they produce energy rich ATP.

All the enzymes of Kreb’s cycle are found in the matrix except succinate dehydrogenase. Mitochondria consist of circular DNA and 70S ribosome. They multiply by fission and replicates by strand displacement model.

Because of the presence of DNAs it is semiautonomous organelle. Unique characteristic of mitochondria is that they are inherited from female parent only. Mitochondrial DNA comparisons are used to trace human origins. It is also used to track and date recent evolutionary time because it mutates 5 to 10 time faster than DNA in the nucleus.

Plastids

The term plastid is derived from the Greek word Platikas (formed/moulded) and used by A.F.U. Schimper in 1885. He classified plastids into following types according to their structure, pigments and function. Plastids multiply by fission.

According to Schimper, different kind of plastids can transform into one another.

Chloroplast

Chloroplasts are vital organelle found in green plants. Chloroplast has a double membrane the outer membrane and the inner membrane separated by a space called periplastidial space. The space enclosed by the inner membrane of chloroplast is filled with gelatinous matrix, lipo-proteinaceous fluid called stroma. Inside the stroma there are flat interconnected sacs called thylakoid. The membrane of thylakoid enclose a space called thylakoid lumen.

Grana (singular: Granum) are formed when many of these thylakoids are stacked together like pile of coins. Light is absorbed and converted into chemical energy in the granum, which is used in stroma to prepare carbohydrates. Thylakoid contain chlorophyll pigments. The chloroplast contains osmophilic granules, 70s ribosomes, DNA (circular and non histone) and RNA.

These chloroplast genome encodes approximately 30 proteins involved in photosynthesis including the components of photosystem I & II, cytochrome bf complex and ATP synthase. One of the subunits of RuBisco is encoded by chloroplast DNA.

It is the major protein component of chloroplast stroma, single most abundant protein on earth. The thylakoid contain small, rounded photosynthetic units called quantosomes. Chloroplast is a semi-autonomous organelle and divides by fission (Figure 6.16).

Functions:

• Photosynthesis
• Light reactions takes place in granum
• Dark reactions take place in stroma
• Chloroplast is involved in photorespiration.

Ribosome

Ribosomes were first observed by George Palade (1953) as dense particles or granules in the electron microscope. Electron microscopic observation reveals that ribosomes are composed of two rounded sub units, united together to form a complete unit.

Mg²⁺ is required for structural cohesion of ribosomes. Biogenesis of ribosome is a de nova formation, auto replication and nucleolar origin. Each ribosome is made up of one small and one large sub-unit Ribosomes are the sites of protein synthesis in the cell. Ribosome is not a membrane bound organelle (Figure 6.17).

Ribosome Consists of RNA and Protein:

RNA 60% and protein 40%. During protein synthesis, many ribosomes are attached to the single mRNA and is called polysomes or polyribosomes. The function of polysomes is the formation of several copies of a particular polypeptide during protein synthesis. They are free in non-protein synthesising cells. In protein synthesising cells they are linked together with the help of Mg²⁺ ions.

Lysosomes (Suicidal Bags of Cell)

Lysosomes were discovered by Christian de Duve (1953), these are known as suicidal bags. They are spherical bodies enclosed by a single unit membrane. They are found in eukaryotic cell. Lysosomes are small vacuoles formed when small pieces of golgi body are pinched off from its tubules.

They contain a variety of hydrolytic enzymes, that can digest material within the cell. The membrane around lysosome prevent these enzymes from digesting the cell itself (Figure 6.18).

Functions:

Intracellular Digestion:
They digest carbohydrates, proteins and lipids present in cytoplasm.

Autophagy:
During adverse condition they digest their own cell organelles like mitochondria and endoplasmic reticulum.

Autolysis:
Lysosome causes self destruction of cell.

Ageing:
Lysosomes have autolytic enzymes that disrupts intracellular molecules.

Phagocytosis:
Large cells or contents are engulfed and digested by macrophages, thus forming a phagosome in cytoplasm. These phagosome fuse with lysosome for further digestion.

Exocytosis:
Lysosomes release their enzymes outside the cell to digest other cells (Figure 6.19).

Microbodies

Eukaryotic cells contain many enzyme bearing membrane enclosed vesicles called microbodies. They are single unit membrane bound cell organelles. Example: Peroxisomes and glyoxysomes.

Peroxisomes

Peroxisomes were identified as organelles by Christian de Duve (1967). Peroxisomes are small spherical bodies and single membrane bound organelle. It takes part in photorespiration and associated with glycolate metabolism. In plants, leaf cells have many peroxisomes. It is also commonly found in liver and kidney of mammals. These are also found in cells of protozoa and yeast (Figure 6.20).

Glyoxysomes

Glyoxysome was discovered by Harry Beevers (1961). It is a single membrane bound organelle. It is a sub cellular organelle and contains enzymes of glyoxylate pathway. β-oxidation of fatty acid occurs in glyoxysomes of germinating seeds Example: Castor seeds.

Sphaerosomes

It is spherical in shape and enclosed by single unit membrane. Example: Storage of fat in the endosperm cells of oil seeds.

Centrioles

Centrioles consists of nine triplet peripheral fibrils made up of tubulin. The central part of the centriole is called hub, is connected to the tubules of the peripheral triplets by radial spokes (9+0 pattern). The centriole form the basal body of cilia or flagella and spindle fibers which forms the spindle apparatus in animal cells. The membrane is absent in centriole (non-membranous organelle) (Figure 6.21).

Vacuoles

In plant cells vacuoles are large, bounded by a single unit membrane called Tonoplast. The Vacuoles contain cell sap, which is a solution of sugars, amino acids, mineral salts, waste chemical and anthocyanin pigments. Beetroot cells contain anthocyanin pigments in their vacuoles.

Vacuoles accumulate products like tannins. The osmotic expansion of a cell kept in water is chiefly regulated by vacuole and the water enters the vacuole by osmosis. The major function of plant vacuole is to maintain water pressure known as turgor pressure, which maintains the plant structure. Vacuoles organises itself into a storage/sequestration compartment. Example: Vacuoles store, most of the sucrose of the cell.

• Sugar in Sugar beet and Sugar cane.
• Malic acid in Apple.
• Acids in Citrus fruits.
• Flavonoid pigment cyanidin 3 rutinoside in the petals of Antirrhinum.

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Plant and Animal Cell Structure and its Types

An eukaryotic cell is highly distinct in its organisation. It shows several variations in different organisms. For instance, eukaryotic cells in plants and animals vary greatly (Figure 6.7)

Animal Cell

Animal cells are surrounded by cell membrane or plasma membrane. Inside this membrane a gelatinous matrix called protoplasm is seen to contain nucleus and other organelles which include the endoplasmic reticulum, mitochondria, golgi bodies, centrioles, lysosomes, ribosomes and cytoskeleton.

Plant Cell

A typical plant cell has prominent cell wall, a large central vacuole and plastids in addition to other organelles present in animal cell (Figure 6.8).

Protoplasm

Protoplasm is the living content of cell that is surrounded by plasma membrane. It is a colourless material that exists throughout the cell together with cytoplasm, nucleus and other organelles. Protoplasm is composed of a mixture of small particles, such as ions, amino acids, monosaccharides, water, macromolecules like nucleic acids, proteins, lipids and polysaccharides.

It appears colourless, jelly like gelatinous, viscous elastic and granular. It appears foamy due to the presence of large number of vacuoles. It responds to the stimuli like heat, electric shock, chemicals and so on.

Difference Between Plant and Animal Cells

Plant Cell	Animal Cell
1. Usually they are larger than animal cells	1. Usually smaller than plant cells
2. Cell way present in addition to plasma membrane and consists of middle lamellae, primary and secondary walls	2. Cell wall absent
3. Plasmodesmata present	3. Plasmodesmata absent
4. Chloroplast present	4. Chloroplast absent
5. Vacuole large and permanent	5. Vacuole small and temporary
6. Tonoplast present around vacuole	6. Tonoplast absent
7. Centrioles absent except motile cells of lower plants	7. Centrioles present
8. Nucleus present along the periphery of the cell	8. Nucleus at the centre of the cell
9. Lysosomes are rare	9. Lysosomes present
10. Storage material is starch grains	10. Storage material is a glycogen granules

Cell Wall

Cell wall is the outermost protective cover of the cell. It is present in bacteria, fungi and plants whereas it is absent in animal cell. It was first observed by Robert Hooke. It is an actively growing portion. It is made up of different complex material in various organism.

In bacteria it is composed of peptidoglycan, in fungi chitin and fungal cellulose, in algae cellulose, galactans and mannans. In plants it is made up of cellulose, hemicellulose, pectin, lignin, cutin, suberin and silica.

In plant, cell wall shows three distinct regions

1. Primary Wall
2. Secondary Wall
3. Middle Lamellae (Figure 6.10).

1. Primary Wall

It is the first layer inner to middle lamella, primarily consisting of loose network of cellulose microfibrils in a gel matrix. It is thin, elastic and extensible.In most plants the microfibrils are made up of cellulose oriented differently based on shape and thickness of the wall. The matrix of the primary wall is composed of hemicellulose, pectin, glycoprotein and water. Hemicellulose binds the microfibrils with matrix and glycoproteins control the orientation of microfibrils while pectin serves as filling material of the matrix. Cells such as parenchyma and meristems have only primary wall.

b. Secondary Wall

Secondary wall is laid during maturation of the cell. It plays a key role in determining the shape of a cell. It is thick,inelastic and is made up of cellulose and lignin. The secondary wall is divided into three sublayers termed as S₁, S₂ and S₃ where the cellulose microfibrils are compactly arranged with different orientation forming a laminated structure and the cell wall strength is increased.

c. Middle Lamellae

It is the outermost layer made up of calcium and magnesium pectate, deposited at the time of cytokinesis. It is a thin amorphous layer which cements two adjacent cells. It is optically inactive (isotropic).

Plasmodesmata and Pits

Plasmodesmata act as a channel between the protoplasm of adjacent cells through which many substances pass through. Moreover, at few regions, the secondary wall layer is laid unevenly whereas the primary wall and middle lamellae are laid continuously such regions are called pits. The Pits of adjacent cells are opposite to each other. Each pit has a pit chamber and a pit membrane. The pit membrane has many minute pores and thus they are permeable. The pits are of two types namely simple and bordered pit.

Functions of Cell Wall

The cell wall plays a vital role in holding several important functions given below

1. Offers definite shape and rigidity to the cell.
2. Serves as barrier for several molecules to enter the cells.
3. Provides protection to the internal protoplasm against mechanical injury.
4. Prevents the bursting of cells by maintaining the osmotic pressure.
5. Plays a major role by acting as a mechanism of defense for the cells.

Cell Membrane

The cell membrane is also called cell surface (or) plasma membrane. It is a thin structure which holds the cytoplasmic content called ‘cytosol’. It is extremely thin (less than 10nm).

Fluid Mosaic Model

Jonathan Singer and Garth Nicolson (1972) proposed fluid mosaic model. It is made up of lipids and proteins together with a little amount of carbohydrate. The lipid membrane is made up of phospholipid. The phospholipid molecule has a hydrophobic tail and hydrophilic head.

The hydrophobic tail repels water and hydrophilic head attracts water. The proteins of the membrane are globular proteins which are found intermingled between the lipid bilayer most of which are projecting beyond the lipid bilayer.

These proteins are called as integral proteins. Few are superficially attached on either surface of the lipid bilayer which are called as peripheral proteins. The proteins are involved in transport of molecules across the membranes and also act as enzymes, receptors (or) antigens.

Carbohydrate molecules of cell membrane are short chain polysaccharides. These are either bound with ‘glycoproteins’ or ‘glycolipids’ and form a ‘glyocalyx’ (Figure 6.11). The movement of membrane lipids from one side of the membrane to the other side by vertical movement is called flip flopping or flip flop movement.

This movement takes place more slowly than lateral diffusion of lipid molecule. The Phospholipids can have flip flop movement because they have smaller polar regions, whereas the proteins cannot flip flop because the polar region is extensive.

Function of Cell Membrane

The functions of the cell membrane is enormous which includes cell signalling, transporting nutrients and water, preventing unwanted substances entering into the cell, and so on.

Cytoplasm

Cytoplasm is the main arena of various activities of a cell. It is the semifluid gelatinous substance that fills the cell. It is made up of eighty percent water and is usually clear and colourless. The cytoplasm is sometimes described as non nuclear content of protoplasm.

The cytoplasm serves as a molecular soup where all the cellular organelles are suspended and bound together by a lipid bilayer plasma membrane. It constitutes dissolved nutrients, numerous salts and acids to dissolve waste products.

It is a very good conductor of electricity. It gives support and protection to the cell organelles. It helps movement of the cellular materials around the cell through a process called cytoplasmic streaming. Further, most cellular activities such as many metabolic pathways including glycolysis and cell division occur in cytoplasm.

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Types of Cells and its Importance

On the basis of the cellular organization and the nuclear characteristics, the cell can be classified into:-

• Prokaryotes
• Mesokaryotes and
• Eukaryotes

Prokaryotes

Those organisms with primitive nucleus are called as prokaryotes (pro – primitive; karyon – nucleus). The DNA lies in the ‘nucleoid’ which is not bound by the nuclear membrane and therefore it is not a true nucleus and is also a primitive type of nuclear material. The DNA is without histone proteins. Example: Bacteria, blue green algae, Mycoplasma, Rickettsiae and Spirochaetae.

Mesokaryotes

In the year 1966, scientist Dodge and his coworkers proposed another kind of organisms called mesokaryotes. These organisms which shares some of the characters of both prokaryotes and eukaryotes. In other words these are organisms intermediate between pro and eukaryotes.

These contains well organized nucleus with nuclear membrane and the DNA is organized into chromosomes but without histone protein components divides through amitosis similar with prokaryotes. Certain Protozoa like Noctiluca, some phytoplanktons like Gymnodinium, Peridinium and Dinoflagellates are representatives of mesokaryotes.

Eukaryotes

Those organisms which have true nucleus are called Eukaryotes (Eu – True; karyon – nucleus). The DNA is associated with histones forming the chromosomes. Membrane bound organelles are present. Few organelles may have risen by endosymbiosis which is a cell living inside another cell. The Organelles like mitochondria and chloroplast well support this theory.

Origin of Eukaryotic cell:

Endosymbiont Theory:
Two eukaryotic organelles believed to be the descendants of the endosymbiotic prokaryotes. The ancestors of the eukaryotic cell engulfed a bacterium and the bacteria continued to function inside the host cell.

Comparison Between Types of Cellular Organisation

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Cell Theory Various Types and its Shapes

In 1833, German botanist Matthias Schleiden and German zoologist Theodor Schwann proposed that all plants and animals are composed of cells and that cells were the basic building blocks of life.

These observations led to the formulation of modern cell theory.

• All organisms are made up of cells.
• New cells are formed by the division of pre-existing cells.
• Cells contains genetic material, which is passed on from parents to daughter cells.
• All metabolic reactions take place inside the cells.

Exception to Cell Theory

Viruses are puzzle in biology. Viruses, viroids and prions are the exception to cell theory. They lack protoplasm, the essential part of the cell and exists as obligate parasites which are sub-cellular in nature.

Protoplasm Theory

Corti first observed protoplasm. Felix Dujardin (1835) observed a living juice in animal cell and called it “Sarcode”. Purkinje (1839) coined the term protoplasm for sap inside a plant cell. Hugo Van Mohl (1846) indicated importance of protoplasm.

Max Schultze (1861) established similarity between Protoplasm and Sarcode and proposed a theory which later on called “Protoplasm Theory” by O. Hertwig (1892). Huxley (1868) proposed Protoplasm as a “physical basis of life”.

Protoplasm as a Colloidal System

Protoplasm is a complex colloidal system which was suggested by Fisher in 1894 and Hardy in 1899. It is primarily made of water and various other solutes of biological importance such as glucose, fatty acids, amino acids, minerals, vitamins, hormones and enzymes. These solutes may be homogeneous (soluble in water) or heterogeneous mass (insoluble in water) which forms the basis for its colloidal nature.

Physical Properties of Protoplasm

The protoplasm exists either in semisolid (jelly-like) state called ‘gel᾿ due to suspended particles and various chemical bonds or may be liquid state called ‘sol᾿. The colloidal protoplasm which is in gel form can change into sol form by solation and the sol can change into gel by gelation. These gel-sol conditions of colloidal system are prime basis for mechanical behaviour of cytoplasm.

1. Protoplasm is translucent, odourless and polyphasic fluid.

2. It is a crystal colloid solution which is a mixture of chemical substances forming crystalloid i.e. true solution (sugars, salts, acids, bases) and others forming colloidal solution (Proteins and lipids).

3. It is the most important property of the protoplasm by which it exhibits three main phenomena namely Brownian movement, amoeboid movement and cytoplasmic streaming or cyclosis. Viscosity of protoplasm is 2-20 centipoises. The Refractive index of the protoplasm is 1.4.

4. The pH of the protoplasm is around 6.8, contain 90% water (10% in dormant seeds)

5. Approximately 34 elements are present in protoplasm but only 13 elements are main or universal elements i.e. C, H, O, N, Cl, Ca, P, Na, K, S, Mg, I and Fe. Carbon, Hydrogen, Oxygen and Nitrogen form the 96% of protoplasm.

6. Protoplasm is neither a good nor a bad conductor of electricity. It forms a delimiting membrane in contact with water and solidifies when heated.

7. Cohesiveness:
Particles or molecules of protoplasm are adhered with each other by forces, such as Vander Waal’s bonds, that hold long chains of molecules together. This property varies with the strength of these forces.

8. Contractility:
The contractility of protoplasm is important for the absorption and removal of water especially for stomatal operations.

9. Surface tension:
The proteins and lipids of the protoplasm have less surface tension, hence they are found at the surface forming the membrane. On the other hand the chemical substances (NaCl) have high surface tension, so they occur in deeper parts of the protoplasm.

Cell Sizes and Shapes

Cell greatly vary in size, shape and also in function. Group of cells with similar structures are called tissue they integrate together to perform similar function, group of tissue join together to perform similar function called organ, group of organs with related function called organ system, organ system coordinating together to form an organism.

Shape

The shape of cell vary greatly from organism to organism and within the organism itself. In bacteria, cell shape vary from round (cocci) to rectangular (rod). In virus, shape of the envelope varies from round to hexagonal or ‘T’ shaped. In fungi, globular to elongated cylindrical cells and the spores of fungi vary greatly in shape. In plants and animals cells vary in shape according to cell types such as parenchyma, mesophyll, palisade, tracheid, fiber, epithelium and others (Figure 6.6).

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Microscopy – Bright Field Microscope and Electron Microscope

Microscope is an inevitable instrument in studying the cell and subcellular structures. It offers scope in studying microscopic organisms therefore it is named as microscope (mikros – small; skipein – to see) in Greek terminology. Compound microscope was invented by Z. Jansen.

Microscope basically works on the lens system and its properties of light and lens such as reflection, magnification and numerical aperture. The common light microscope which has many lenses are called as compound microscope. The microscope transmits visible light from sources to eye or camera through sample.

Bright Field Microscope

Bright field microscope is the routinely used microscope in studying various aspects of cells. It allows light to pass directly through specimen and shows a well distinguished image from different portions of the specimen. The contrast can be increased by staining the specimen with reagent that reacts with cells and tissue components of the object.

The light rays are focused by condenser on to the specimen on a microslide placed upon the adjustable platform called stage. Light comes from the Compact Flourescent Lamp (CFL) or Light Emitting Diode (LED). Then it passes through two lens systems namely objective lens (closer to the object) and the eye piece (closer to eye).

There are four objective lenses (5X, 10X, 45X and 100X) which can be rotated and fixed at certain point to get required magnification. It works on the principle of numerical aperture value and its own resolving power.

The first magnification of the microscope is done by the objective lens which is called primary magnification and it is real, inverted image. The second magnification of the microscope is obtained through eye piece lens called as secondary magnification and it is virtual and inverted image (Figure 6.2 a, b and c).

Electron Microscope

Electron Microscope was first introduced by Ernest Ruska (1931) and developed by G Binning and H Roher (1981). It is used to analyse the fine details of cell and organelles called ultrastructure. It uses beam of accelerated electrons as source of illumination and therefore the resolving power is 1,00,000 times greater than that of light microscope.

The specimen to be viewed under electron microscope is dehydrated and impregnated with electron opaque chemicals like gold or palladium. This is essential for withstanding electrons and also for contrast of the image.

There are two kinds of electron microscopes namely:

1. Transmission Electron Microscope (TEM)
2. Scanning Electron Microscope (SEM)

1. Transmission Electron Microscope:

This is the most commonly used electron microscope which provides two dimensional image. The components of the microscope are as follows:

• Electron generating system
• Electron condensor
• Specimen objective
• Tube lens
• Projector

A beam of electron passes through the specimen to form an image on fluorescent screen. The magnification is 1-3 lakhs times and resolving power is 2-10 Å. It is used for studying detailed structrue of viruses, mycoplasma, cellular organelles, etc (Figure 6.3 a and b).

2. Scanning Electron Microscope:

This is used to obtain three dimensional image and has a lower resolving power than TEM. In this, electrons are focused by means of lenses into a very fine point.

The interaction of electrons with the specimen results in the release of different forms of radiation (such as auger electrons, secondary electrons, back scattered electrons) from the surface of the specimen. These radiations are then captured by an appropriate detector, amplified and then imaged on fluorescent screen. The magnification is 2,00,000 times and resolution is 5-20 nm (Figure 6.4 a and b).

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Discovery of a Cell Definition and its Structure

Aristotle (384 – 322BC), was the one who first recognised that animals and plants consists of organised structural units but unable to explain what it was. In 1660’s Robert Hooke observed something which looks like ‘honeycomb with a great numbers of little boxes’ which was later called as ‘cell’ from the cork tissue. In 1665, He compiled his work as Micrographia.

Later, Anton Van Leeuwenhoek observed unicellular particles which he named as ‘animalcules’. Robert Brown (1831 – 39) described the spherical body in plant cell as nucleus. H. J. Dutrochet (1824), a French scientist, was the first to give an idea on cell theory. Later, Matthias Schleiden (German Botanist) and Theodor Schwann (German Zoologist) (1833) outlined the basic features of the cell theory.

Rudolf Virchow (1858) explained the cell theory by adding a feature stating that all living cells arise from pre-existing living cells by ‘cell division’. Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope. The cell theory, that all the plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839).

A cell is the smallest and most basic form of life. Robert Hooke, one of the first scientists to use a light microscope, discovered the cell in 1665. In all life forms, including bacteria, plants, animals, and humans, the cell was defined as the most basic structural and functional unit.

The cell (from Latin cella, meaning “small room”) is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to as the “building blocks of life”. The study of cells is called cell biology, cellular biology, or cytology.

The levels, from smallest to largest, are: molecule, cell, tissue, organ, organ system, organism, population, community, ecosystem, biosphere.

A cell consists of a nucleus and cytoplasm and is contained within the cell membrane, which regulates what passes in and out. The nucleus contains chromosomes, which are the cell’s genetic material, and a nucleolus, which produces ribosomes.

The cell is the smallest structural and functional unit of living organisms, which can exist on its own. Therefore, it is sometimes called the building block of life.

The cell is the structural and functional unit of all known living organisms. So, the entire functioning of the living organisms begins from the basic unit called cell. Hence, cell is called the fundamental unit of life.

They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Cells also contain the body’s hereditary material and can make copies of themselves.

The largest cells is an egg cell of ostrich. The longest cell is the nerve cell. The largest cell in the human body is female ovum. Smallest cell in the human body is male gametes, that is, sperm.

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Selected Families of Angiosperms

Dicot Families

Family: Fabaceae (Pea family)

Systematic Position

General Characters

Distribution:
Fabaceae includes about 741 genera and more than 20, 200 species. The members are cosmopolitan in distribution but abundant in tropical and subtropical regions.

Habit:
All types of habits are represented in this family. Mostly herbs (Crotalaria), prostrate (Indigofera enneaphylla) erect (Crotalaria verrucosa), shrubs (Cajanus cajan), small trees (Sesbania), climbers (Clitoria), large tree (Pongamia, Dalbergia), woody climber (Mucuna), hydrophyte (Aeschynomene aspera) commonly called pith plant.

Root:
Tap root system, roots are nodulated, have tubercles containing nitrogen – fixing bacteria (Rhizobium leguminosarum)

Stem:
Aerial, herbaceous, woody (Dalbergia) twining or climbing (Clitoria).

Leaf:
Leaf simple or unifoliate (Desmodium gangeticum) bifoliate (Zornia diphylla,), Trifoliate (Lablab purpureus), alternate, stipulate, leaf base, pulvinate, reticulate venation terminal leaflet modifies into a tendril in Pisum sativum.

Inflorescence:
Raceme (Crotalaria verrucosa), panicle (Dalbergia latifolia) axillary solitary (Clitoria ternatea)

Flowers:
Bracteate, bracteolate, pedicellete, complete, bisexual, pentamerous, heterochlamydeous, zygomorphic hypogynous or sometimes perigynous.

Calyx:
Sepals 5, green, synsepalous, more or less united in a tube and persistant, valvate or imbricate, odd sepal is anterior in position.

Corolla:
Petals 5, apopetalous, unequal and papilionaceous, vexillary or descendingly imbricate aestivation, all petals have claw at the base. The outer most petal is large called standard petal or vexillum, Lateral 2 petals are lanceolate and curved. They are called wing petals or alae. Anterior two petals are partly fused and are called keel petals or carina which encloses the stamens and pistil.

Androecium:
Stamens 10, diadelphous, usually 9+1 (Clitoria ternatea). The odd stamen is posterior in position. In Aeschynomene aspera, the stamens are fused to form two bundles each containing five stamens (5)+(5). Stamens are monadelphous and dimorphic ie. 5 stamens have longer filaments and other 5 stamens have shorter filaments thus the stamens are found at two levels and the shape of anthers also varies in (Crotalaria verrucosa). (5 anthers are long and lanceolate, and the other 5 anthers are short and blunt). Anthers are dithecous, basifixed and dehiscing longitudinally

Gynoecium:
Monocarpellary, unilocular, ovary superior, with two alternating rows of ovules on marginal placentation. Style simple and bent, stigma flattened or feathery.

Fruit:
The characteristic fruit of Fabaceae is a legume (Pisum sativum), sometimes indehiscent and rarely a lomentum (Desmodium). In Arachis hypogea the fruit is geocarpic (fruits develops and matures under the soil). After fertilization the stipe of the ovary becomes meristematic and grows down into the soil. This ovary gets buried into the soil and develops into fruit.

Seed:
Endospermic or non-endospermic (Pisum sativum), mostly reniform.

Botanical Description of Clitoria Ternatea (Sangu Pushpam)

Habit:
Twining climber

Root:
Branched tap root system having nodules.

Stem:
Aerial, weak stem and a twiner

Leaf:
Imparipinnately compound, alternate, stipulate showing reticulate venation. Leaflets are stipellate. Petiolate and stipels are pulvinated.

Inflorescence:
Solitary and axillary

Flower:
Bracteate, bracteolate, bracteoles usually large, pedicellate, heterochlamydeous, complete, bisexual, pentamerous, zygomorphic and hypogynous.

Calyx:
Sepals 5, synsepalous, green showing valvate aestivation. Odd sepal is anterior in position.

Corolla:
Petals 5, white or blue apopetalous, irregular papilionaceous corolla showing descendingly imbricate aestivation.

Androecium:
Stamens 10, diadelphous (9)+1, nine stamens fused to form a bundle and the tenth stamen is free. Anthers are dithecous, basifixed, introse and dechiscing by longitudinal slits.

Gynoecium:
Monocarpellary, unilocular, with many ovules on mariginal placentation, ovary superior, style simple and incurved with feathery stigma.

Fruit:
Legume

Seed:
Non-endospermous, reniform.

Floral Formula:

Economic Importance

Family:
Solanaceae (Potato Family / Night shade family)

Systematic Position

General Characters

Distribution:
Family Solanaceae includes about 88 genera and about 2650 species, of these Solanum is the largest genus of the family with about 1500 species. Plants are worldwide in distribution but more abundant in South America.

Habit:
Mostly annual herbs, shrubs, small trees (Solanum violaceum) lianas with prickles (Solanum trilobatum)

Root:
Branched tap root system.

Stem:
Herbaceous or woody; erect or twining, or creeping; sometimes modified into tubers (Solanum tuberosum) it is covered with Spines (Solanum tuberosum)

Leaves:
Alternate, simple, rarely pinnately compound (Solanum tuberosum and Lycopersicon esculentum, exstipulate, opposite or sub-opposite in upper part, unicostate reticulate venation. Yellowish verbs present in Solanum tuberosum.

Inflorescence:
Generally axillary or terminal cymose (Solanum) or solitary flowers (Datura stramonium). Extra axillary scorpiod cyme called rhiphidium (Solanum americanum) solitary and axillary (Datura and Nicotiana) umbellate cyme (Withania somnifera).

Flowers:
Bracteate or ebracteate, pedicellate, bisexual, heterochlamydeous, pentamerous actinomorphic or weakly zygomorphic due to oblique position of ovary, hypogynous.

Calyx:
Sepals 5, Synsepalous, valvate persistent (Solanum americanum), often accrescent. (Physalis)

Corolla:
Petals 5, sympetalous, rotate, tubular (Solanum) or bell – shaped (Atropa) or infundibuliform (Petunia) usually alternate with sepals; rarely bilipped and zygomorphic (Schizanthus) usually valvate, sometimes convolute (Datura).

Androecium:
Stamens 5, epipetalous, filaments usually unequal in length, stamens only 2 in Schizanthus (others 3 are reduced to staminode), Anthers dithecous, dehisce longitudinally or poricidal.

Gynoecium:
Bicarpellary, syncarpous obliquely placed, ovary superior, bilocular but looks tetralocular due to the formation of false septa, numerous ovules in each locule on axile placentation.

Fruit:
A capsule or berry. (Datura & Petunia, Lycopersicon esculentum, Capsicum)

Seed:
Endospermous.

Botanical Description of Datura Metel

Habit:
Large, erect and stout herb.

Root:
Branched tap root system.

Stem:
Stem is hollow, green and herbaceous with strong odour.

Leaf:
Simple, alternate, petiolate, entire or deeply lobed, glabrous exstipulate showing unicostate reticulate venation.

Inflorescence:
Solitary and axillary cyme.

Flower:
Flowers are large, greenish white, bracteate, ebracteolate, pedicellate, complete, heterochlamydeous, pentamerous, regular, actinomorphic, bisexual and hypogynous.

Calyx:
Sepals 5, green synsepalous showing valvate aestivation. Calyx is mostly persistent, odd sepal is posterior in position.

Corolla:
petals 5, greenish white, sympetalous, plicate (folded like a fan) showing twisted aestivation, funnel shaped with wide mouth and 10 lobed.

Androecium:
Stamens 5, free from one another, epipetalous, alternipetalous and are inserted in the middle of the corolla tube. Anthers are basifixed, dithecous, with long filament, introse and longitudinally dehiscent.

Gynoecium:
Ovary bicarpellary, syncarpous superior ovary, basically bilocular but tetralocular due to the formation of false septum. Carpels are obliquely placed and ovules on swollen axile placentation. Style simple long and filiform, stigma two lobed.

Fruit:
Spinescent capsule opening by four apical valves with persistent calyx.

Seed:
Endospermous.

Floral Formula:

Economic Importance of the Family Liliaceae

Family: Liliaceae (Lily Family)

Systematic Position

General Characters

Distribution:
Liliaceae are fairly large family comprising about 15 genera and 550 species. Members of this family are widely distributed over most part of the world.

Habit:
Mostly perennial herbs persisting by means of a sympodial rhizome (Polygonatum), by a bulb (Lilium) corm (Colchicum), shrubby or tree like (Yucca and Dracaena) oody climbers, climbing with the help of stipular tendrils in Smilax. Trees in (Xanthorrhoea), succulents (Aloe).

Root:
Adventitious and firous, and typically contractile.

Stem:
Stems usually bulbous, rhizomatous in some, aerial, erect (Dracaena) or climbing (Smilax) in Ruscus the ultimate branches are modified into phylloclades, In Asparagus stem is modified into cladodes and the leaves are reduced to scales.

Leaf:
Leaves are radical (Lilium) or cauline (Dracaena), usually alternate, opposite (Gloriosa), sometimes fleshy and hollow, reduced to scales (Ruscus and Asparagus). The venation is parallel but in species of Smilax it is reticulate. Leaves are usually exstipulate, but in Smilax, two tendrils arise from the base of the leaf, which are considered modified stipules.

Inflorescence:
Flowers are usually borne in simple or branched racemes (Asphodelus) spikes in Aloe, huge terminal panicle in Yucca, solitary and axillary in Gloriosa, solitary and terminal in Tulipa.

Flowers:
Flowers are often showy, pedicellate, bracteate, ebracteolate, except Dianella and Lilium, bisexual, actinomorphic, trimerous, hypogynous, rarely unisexual (Smilax) and are dioecious, rarely tetramerous (Maianthemum), slightly zygomorphic (Lilium) and hypogynous.

Perianth:
Tepals 6 biseriate arranged in two whorls of 3 each, apotepalous or rarely syntepalous as in Aloe. Usually petaloid or sometimes sepaloid, odd tepal of the outer whorl is anterior in position, valvate or imbricate, tepals more than six in Paris quadrifolia.

Androecium:
Stamens 6, arranged in 2 whorls of 3 each, rarely stamens are 3 (Ruscus), 4 in Maianthemum, or up to 12, apostamenous, opposite to the tepals, sometimes epitepalous; fiaments distinct or connate, anthers dithecous, basified or versatile, extrose, or introse, dehiscing usually by vertical slit and sometimes by terminal pores; rarely synstamenous (Ruscus).

Gynoecium:
Tricarpallary, syncarpous, the odd carpel usually anterior, ovary superior, trilocular, with 2 rows of numerous ovules on axile placextation. Style simple, slender with simple stigma.

Fruit:
A loculicidal capsule

Seed:
Endospermous

Floral Formula:

Economic Importance of the Family Liliaceae

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Cladistics and its Various Types of Classifications

Analysis of the taxonomic data, and the types of characters that are used in classification have changed from time to time. Plants have been classified based on the morphology before the advancement of microscopes, which help in the inclusions of sub microscopic and microscopic features.

A closer study is necessary while classifying closely related plants. Discovery of new fier molecular analytical techniques coupled with advanced software and computers has ushered in a new era of modern or phylogenetic classification.

The method of classifying organisms into monophyletic group of a common ancestor based on shared apomorphic characters is called cladistics (from Greek, kladosbranch).

The outcome of a cladistic analysis is a cladogram, a tree-shaped diagram that represent the best hypothesis of phylogenetic relationships. Earlier generated cladograms were largely on the basis of morphological characters, but now genetic sequencing data and computational softwares are commonly used in phylogenetic analysis.

Cladistic Analysis

Cladistics is one of the primary methods of constructing phylogenies, or evolutionary histories. Cladistics uses shared, derived characters to group organisms into clades.

These clades have atleast one shared, derived character found in their most recent common ancestor that is not found in other groups hence they are considered more closely related to each other. These shared characters can be morphological such as, leaf, flower, fruit, seed and so on; behavioural, like opening of flowers nocturnal/diurnal; molecular like, DNA or protein sequence and more.

Cladistics accept only monophyletic groups. Paraphyletic and polyphyletic taxa are occasionally considered when such taxa conveniently treated as one group for practical purposes. Example: dicots, sterculiaceae. Polyphyletic groups are rejected by cladistics.

(i) Monophyletic Group:
Taxa comprising all the descendants of a common ancestor.

(ii) Paraphyletic Group:
Taxon that includes an ancestor but not all of the descendants of that ancestor.

(iii) Polyphyletic Group:
Taxa that includes members from two different lineages.

Need for Cladistics

1. Cladistics is now the most commonly used and accepted method for creating phylogenetic system of classifications.
2. Cladistics produces a hypothesis about the relationship of organisms to predict the phylogeny
3. Cladistics helps to elucidate mechanism of evolution.