2_Tissues System

 

 

BIOLOGY (class-ix)

 

Chapter-2:                          TISSUEs

 

Plant Tissues

Based on the capacity to divide, the plant tissues have been classified into two fundamental types, meristematic and permanent.

Figure 2. 1  Plant Tissues

 

Þ   Meristematic Tissues: A meristem or meristematic tissue (Gk. meristos – divided) is a simple tissue composed of a group of similar and immature cells (meristematic cells) which can divide and form new cells.

Characteristics of Meristematic Cells ·           Ability to grow and divide ·           Small immature cells. ·           Isodiametric, rounded, oval or polygonal. ·           Absence of intercellular spaces. ·           Walls are thin, elastic and made of cellulose. ·           Nucleus conspicuous. ·           Cytoplasm dense. l     Vacuoles absent or very small ·           Crystals absent ·           Endoplasmic reticulum small. ·           Proplastids are present instead of plastids. ·           Mitochondria have simple structure. ·           Rate of respiration is very high

Figure 2. 2 Section of root showing Meristematic Tissue

 Þ  Permanent Tissues

       They are those tissues, the cells of which have lost the capacity to divide and have attained a permanent shape, size and function due to morphological, biochemical and physiological differentiation. Depending upon their origin, permanent tissues are of two types, primary  (derived from apical and intercalary meristem) and secondary (derived from a lateral meristem). On the basis of composition, permanent tissues can be simple, complex and special (e.g., seretory)

       Simple Permanent Tissues: A simple permanent tissue is that tissue which is made up of similar permanent cells that carry out the same function or similar set of functions. Simple permanent tissues are of three types – parenchyma, collenchyma and sclerenchyma.

Parenchyma (Gk. para – beside, engchyma – tissue) Parenchyma is a simple permanent living tissue which is made up of thin-walled similar isodiametric cells. It is the most abundant and common tissue of plants. Typically the cells are isodiametric (all sides equal). They may be oval, rounded or polygonal in outline. The cell wall is made up of cellulose. Cells may be closely packed or have small intercellular spaces for exchange of gases. Internally each cell encloses a large central vacuole and a peripheral cytoplasm containing nucleus. The adjacent parenchyma cells are connected with one another by plasmodesmata. They, therefore, form symplasm or living continuum.

Functions

• Storage of food
• Providing turgidity to softer parts.
• Checking water loss in the form of epidermis.
• Formation of water absorbing epiblema in root.
• Photosynthesis in the form of chlorenchyma.
• Secretion

       Collenchyma

(Gk. kola – glue, enchyma – tissue)

Collenchyma is a simple permanent tissue of refractile nonlignified living cells which possess pectocellulose thickenings in specific areas of their walls. The cells appear conspicuous under the microscope due to their higher refractive index. The cells are often enlongated. They are circular, oval or angular in transverse section. Internally each cell possesses a large central vacuole and a peripheral cytoplasm. Wall possesses uneven longitudinal thickenings in specific area.

Collenchyma is found below the epidermis in the petiole, leaves and stems of herbaceous dicots, especially in the region of ridges (e.g., Gourd).

Functions

• It provides mechanical strength to young dicot stems, petioles and leaves.
• While providing mechanical strength, collenchyma also provides flexibility to the organs and allows their bending, g.., Cucurbita stems.
• It prevents tearing of leaves
• Collenchyma allows growth and elongation of organs.
• Being living, its cells store food.

       Sclerenchyma                                                  

The cells of sclerenchyma are long, narrow, thick and lignified. These are closely packed without intercellular spaces. They are usually pointed at both ends. Their walls are usually pointed at both ends. The walls of their cells are often so thick that the cell cavity or lumen is nearly absent. Often oblique thin areas are found in their walls. These are called pits. The middle lamella that is the wall between adjacent cells is conspicuous. Sclerenchyma cells are dead cells devoid of protoplasm. Sclerenchymatous cells are found abundantly in plants. The length of sclerenchyma cells varies from 1 mm to 550 mm in different plants. Their main function is to give mechanical support to plants. Sclerenchyma is of two types, sclerenchyma fibres and sclereids.

Þ   Protective Tissues

       These tissues are usually present in the outermost layer of the plant body such as leaves, flowers, stem and roots. This layer is one cell thick and may be covered with cutin. These tissues protect the inner tissues present in the plant body.

As roots and stem grow older with time, tissues at the periphery become cork cells. Cork cells are dead and do not have any intercellular spaces. The walls of these cells are heavily thickened by the deposition of suberin. They prevent loss of water.

       Examples : Epidermis and Periderm (usually many cell layered, replaces old epidermis as plant grows, produced by cork cambium).

VASCULAR TISSUE SYSTEM

Vascular plants have specialized tissue called vascular tissue. Vascular tissue carries water and nutrients throughout the plant and helps support the plant.

There are two kinds of vascular tissue; both kinds of vascular tissue contain specialized conducting cells:

 

xylem

• Moves water and minerals upward from roots to leaves.
• When water and minerals are absorbed by the roots of a plant, these substances must be transported up to the plant’s stems and leaves. Xylem is the tissue that carries water and dissolved substances upward in the plant.
• Xylem consists of tracheids vessels, xylem parenchyma and xylem fibres.
• Tracheids are long, thick walled sclerenchyma, narrow cells of xylem with thin separations between them. Water moves from one tracheid to another through pits, which are thin, porous areas of the cell wall.
• Vessel elements are short, sclerenchyma, wide cells of xylem with no end walls. Vessel elements do not have separations between them, they are arranged end to end liked barrels staked on top of each other. These vessels are wider than tracheids, and more water moves through them.
• The function of xylem parenchyma is storage of food and side ways conduction of water.
• Fibres are mainly supportive in function.
• Angiosperms, or flowering plants, contain tracheids and vessel elements.
• Gymnosperms, or cone bearing seed plants, contain only tracheids.

PHLOEM

l     Moves sugars or saps in both directions throughout the plant originating in the leaves. ·           Sugars made in the leaves of a plant by photosynthesis must be transported throughout the plant. ·           Phloem tissue conducts sugars upward and downward in a plant. ·           The sugars move as sugary sap. ·           Phleom consitrs of sieve tubes, companion cells, phloem fibres and phloem paranechyma. ·           Sieve tubes members  are cells of phloem that conduct sap. Sieve tube members are stacked to form long sieve tubes. Compounds move from cell to cell through end walls called sieve plates. ·           Companion cells are parenchyma cells of phloem that enable (assist) the sieve tube elements to function. ·           Phleom paranchyma are living parenchyma cells found in between sieve tubes. Their function, is storage food.

• Phleom fibres are only non living components of phloem and their function is providing mechanical supper.
• Each sieve tube element has a companion cell. Companion cells control the movement of substances through the sieve tubes.

l  The partnership between these two cells is vital; neither cell can live without the other.

 

ANIMAL TISSUES

Figure 2. 3 Animal Tissues

Introduction

The term “tissue” was given by a French scientist, Bichat (1771-1802) It is the “group of cells of similar origin, structure and function”. The term histology, (study of tissue) was given by Mayer (1819), a German scientist. Marcello Malpighi (1828-1694) was the first to make this kind of study hence he is known as founder of histology.

Histology is the branch of science which deals with the structures and functions of tissues. All multicellular organisms are made up of different types of tissues.

Animal tissues are divided into four major classes on the basis of their functions.

Epithelial tissue : Covers or lines the free surfaces of other tissues. It serves several functions such as protection, secretion, excretion and also forms receptors.

Connective tissue : Supports and connects various tissues.

       Muscular tissue : Causes movement of the skeleton and other internal organs and contraction and relaxation.

       Nervous tissue : Transmits messages in the form of impulses, thus coordinating the activities of body.

EPITHELIAL TISSUE

One of the simplest animal tissues is epithelium. Epithelium is a lining tissue. In its simplest form it consists of a single layer of cells covering the surface of the animals and the organs, cavities and tubes within it. The epithelium lining the inside of heart, blood vessels and lymph vessels is referred to as endothelium.

       Structure : Typically all individual cells of epithelium are firmly attached with each other. These cells rest on a non-cellular basement membrane.

       Functions : Functions are varied and many and involves protection of underlying tissues, production of motion (ciliated epithelium), absorption of digested material, secretory and also sensory function (olfactory region of nose, taste buds of tongue, retina of eye are all example of sensory functions of epithelium).

(a)   Simple squamous epithelium : In such a type of epithelium, the cells are flattened. This gives a sheet like appearance in surface view. It is found in places where the protective covering also needs to be readily permeable to molecules in solution for e.g., the lining of capillaries, alveoli in lungs.

It is also called as pavement epithelium or tessellated epithelium as cells arranged like tiles.        Examples: Lining of blood vessel’s wall (endothelium), mesothelium, Bowman’s capsule, loop of Henle, alveoli of lungs, terminal bronchiole, etc.

Figure 2. 4 Squamous epithelium

(b)   Columnar epithelium

l  Cells tall, pillar-like with subcentric nucleus which lie in the same line in all cells l     No intercellular space at apical surface but may be little at basal surface. l  Meant for absorption, storage, synthesis and secretion.        Ex. Lining (mucosa) of stomach and small              intestine. l  In intestine gives typical brush border look due to microvilli at apical surface.

Figure 2. 5 Cuboidal and columnar epithelium

(c)   Cuboidal epithelium

       Cells are cubical in shape. Normally this epithelium forms the lining of glands or their ducts. A classical example is the epithelial lining of the follicles of thyroid.

Simple Cuboidal Epithelium

(d)   Ciliated epithelium

       A specialized form of lining tissue is ciliated epithelium. Usually columnar in shape, the free surfaces of each cell bears numerous cilia capable of beating rapidly and rhythmically. This helps in causing motions producing currents. In mammals ciliated epithelium lines tubes and cavities in which materials have to be moved. e.g., ciliated epithelium is found in the lining of respiratory tract for expelling dust particles; in oviduct ciliated epithelium causes movement of ova.

(e)   Glandular epithelium

l  Cells are secretory so they are spacious.

l  The cells of this tissue are of any shape like cuboidal, columnar, oval, spherical or irregular.

l  Cells together may take different shape or remain single to act as gland, hence classified as two main types.

Simple glandular epithelium (single cell individually acts as gland e.g., Goblet cells).

Compound glandular epithelium (many cells together form a unit gland e.g., all common glands).

l     Tubulo-alveolar or racemose gland: This type of gland comprises of both alveolar (as secretory) and            tubular (as duct) parts. Examples: All common exocrine glands (salivary gland, mammary gland,          pancreas, Cowper’s gland).

EXOCRINE AND ENDOCRINE GLANDS

l     Both are formed by the invagination of epithelial lining into connective tissue part.

l  Exocrine gland retains the connection with upper epithelial layer that forms duct; product is released outside i.e. at the site of action through its own duct.

l  Endocrine gland later loses connection from parent (upper) epithelium and becomes ductless, product (hormones) is released inside i.e., into the blood, which carries it to the site of action.

(h)   Pseudostratified epithelium

l  This type of epithelium is basically of ciliated type;

l  Cells belong to same layer, but due to excess and early growth of neighbouring
cells some cells remain subdued and can not reach the surface hence appear to be in another layer. Examples: Patches in the lining of pharynx, nasal chamber, trachea, covering of epiglottis, same
part of vasa deferentia and oviducal funnel, etc.

Compound Epithelial Tissue

(i)    Stratified Epithelium

The cells of stratified epithelium are arranged in many layers. The deepest layer, resting on a basement membrane, consists of columnar cells. The intermediate cells are polyhedral or cubical and the superficial layers are made up of flattened cells.

(j)   Stratified Squamous       

This is found on surfaces which may be subjected to friction, mechanical injury or desiccation. The layering may vary from a few layers (e.g., corneal epithelium) to many layers (e.g., epidermis). Stratified squamous epithelium is of 2 types :

       Non-keratinized, and keratinized.

Non-keratinized epithelium is found on covering surfaces, which are not subjected to desiccation while the keratinized variety is present on exposed skin surfaces.

l     Non-keratinized

Lining of mouth, pharynx, oesophagus and anus.

Urethra (near outlet).

Vagina

l     Keratinized

Epidermis covering the whole surface of the body.

connective TISSUE

       The connective tissues of the body connect and anchor parts. They are often referred to as supporting tissues because they give support to the body and its organs. All of the supporting tissues possess two  characteristics in common :

l  They are all developed from the embryonic mesenchyme, which is itself derived from the primitive mesoderm.

(i)   Connective tissue proper,

(ii)  Skeletal connective tissue

(iii) Fluid connective tissue

1. CONNECTIVE TISSUE PROPER

The connective tissue as such refers to connective tissue proper. Features of typical connective tissue are as follows:

(a)  Areolar Tissue :

Aerolar tissue is the most typical kind and can be regarded as the general form from which the other varieties are specialized. It is a loose; irregular connective tissue which has a very widespread distribution. Areolar tissue connects the skin to the underlying structures and fills any unoccupied spaces between organs. It penetrates with the blood vessels and nerves into the various tissues and organs. In the fresh conditions it is soft and transparent and contains numerous potential cavities. The presence of these spaces is responsible for the name (areolar – “little areas” or spaces). Pathologically, these spaces may become filled with fluid resulting in oedema (swelling).

Structure

Areolar tissue consists of a matrix that contains various kinds of cells and fibres namely collagen and elastic fibres.

Intercellular Matrix

In a fresh spread preparation of areolar connective tissue, the matrix is optically homogenous and transparent. The intercellular matrix contains several protein polysaccharide complex.

(b)  Adipose connective tissue

l  Fat (food) storing tissue, mainly in subcutaneous layer and termed as panniculus adiposus.

l  Made of adipocytes, interstitial cells give rise to adipocytes and some elastic fibres which provide   strength.

 

Adipose Tissue

l     Increase in the amount of this tissue is called obesity

l  Fat is deposited in three forms: olein, stearin and palmitin, which is more influenced by female sex hormone (estrogen).

l  As secondary sexual character, it gives curvy look to the female body.

l  Acts as thermal insulator hence specially important for homoeotherms.

l  As shock absorber, it forms the padding around internal organs. Examples: Mammary gland, Camel’s hump, Blubber of Whale, Dolphin, etc. These are the type of  common white fat.

l  Protects the baby from temperature-shock after birth, and provides extra energy and heat.

White fibrous connective tissue

l  White fibrous connective tissue is cord like structure, mainly made of collagenous fibres. Examples: Tendon which connects muscle to bone.

Yellow elastic connective tissue

l  Yellow elastic connective tissue is cord like and made of mainly elastic fibres. Examples: Ligament which connects bone to bone.

1. SKELETAL CONNECTIVE TISSUE

l     The skeletal connective tissue forms framework of the body and provides surface for the attachment of muscle.

l     It is mainly of two types: cartilage and bone

Cartilage

l  Skeleton in vertebrates initially formed as cartilage hence is primitive to bone.

l  The mesenchyme differentiates into chondrioblasts which give rise to chondriocytes; the fibrous covering is perichondrium.

l  Chondriocytes exist as singlet, doublet and cell nests within the lacunae

l     The cartilaginous tissues are classified as follows:

l  Hyaline cartilage is transparent, smooth like glass; with less or no fibres. Typical and major type cartilage in the body. Examples: Cartilages of larynx, tracheal rings, epiphysis and hyoid apparatus of frog, etc. l  Fibrous cartilage is with abundant collagenous fibres hence, appears opaque; strongest cartilage present between two bones. Examples: Intervertebral disc and pubic symphysis of mammals, Ischium, pubis and mentomeckelian cartilage of frog. l  Elastic cartilage consists mainly of elastic fibres, hence forms thin and elastic parts like. Examples: Pinna, nose tips, epiglottis in mammals, eustachian tube, etc.

Figure 2. 6 Hyaline cartilage

l  Calcareous cartilage has extra deposition of CaCO3, hard like bone. Examples:  Supra scapula in pectoral girdle of frog.

Bone

Figure 2. 7

l     The outer most fibrous layer of bone is periosteum, osteoblasts give rise to osteocytes or bone cells. These cells are arranged in concentric layers around the central cavity, which contains cavity, through this cavity also pass nerves and blood vessels.

l  Fibres of Sharpey are the radial fibres traversing inside from outer layer the strength of the bone. Its number increases with age.

Bone

l  Other characteristics are comparable to cartilage are as follows:

Table: Comparision between cartilage and bones

Characters	Cartilage	Bone
1.    Content of matrix	Mainly (90%) organic substance	Mainly inorganic substance (65-70%) and rest organic substance (30-35%)
2.    Main organic substance	Chondroitan sulphate	Collagen substance
3.    Special protein	Chondrin	Ossein
4.    Inorganic substance	Mainly CaCO3	Mainly Ca(PO4)2
5.    Cavity	Absent	Central cavity called as bone marrow
6.    Position of basic components	Chondriocytes are scattered in matrix	Osteocytes are arranged in concentric layers
7.    Layers of matrix	Absent	Layers form lamellae
Characters	Cartilage	Bone
8.    Lacunae	Without canaliculi	With canaliculi
9.    Coverings	Only perichondrium	Both periosteum and endosteum
10.  Blood supply & Nerve supply	Absent	Both types of supplies takes place through a special cavity and canals
11.  Mode of nutrition	Received by simple diffusion	Received directly through blood supply

l  Birds bone is pneumatic, sustain air cavities to reduce the weight of bone as flight adaptation.

Mammalian bone

l  Consists of many longitudinal Haversian canals parallel to central cavity and transverse Volkmann’s canals connecting them. Through these pass blood vessels and nerve.

l  Osteocytes get arranged around Haversian canals instead of central cavity.

l  All these together form Haversian system or osteon, a structural unit of mammalian bone, each consists of osteocyte lamella.

III.  FLUID CONNECTIVE TISSUE OR BLOOD VASCULAR TISSUE

l  In most animals blood collects excretory and other wastes from the body and flows within closed vessels except few invertebrates.

l  Transport entire materials like nutritive substance, respiratory gases, hormones, etc. throughout the body

l  Constitutes internal environment of the body and defense system.

l  In human body the amount is 5 to 6 litre, i.e. about 8% of total body weight (90 ml/kg).

l  pH is slightly alkaline, strictly maintained; 7.2 for venous and 7.4 in arterial blood.

l  Color-Venous blood bluish; Arterial blood bright red.

l  Increasing bluishness due to accumulation of venous blood in any part is called as cyanosis.

l  Blood is described by separating its two components: (i) Intercellular substance or matrix, called plasma (54-60%), (ii) Cellular component or formed element includes RBC, WBC and Platelets
(40-46%).

Plasma 

l     A viscous fluid medium in which blood cells remain suspended hence also called as ground substance
of blood.

l  Pale yellow but transparent fluid, constituting 4.5 to 5% of total body weight.

l     Plasma consists of mainly water (90 – 92%), Proteins (6 – 7%), minerals (0.9 – 1%), and glucose (0.1%)
in almost fixed ratio.

l  Other substances whose ratio vary from place to place and time to time include hormones, enzymes,
lipids, amino acids, antitoxins, vitamins, urea, uric acid, creatinine, and gases like NH3, CO2, O2, and N2.

l  Inorganic salts (minerals) are mainly as bicarbonates and chlorides of sodium, other salts like –
phosphates, carbonates, sulphates and iodides of Ca, Mg, K and Fe are present in traces.

 

Cellular components

Erythrocytes (RBC)

l  Erythropoiesce is the process by which RBC’s are produced.

l  As completely differentiated cell it doesn’t divide and has no centrosome

l  These are also called as terminated cell for the loss of its structural functional organisation and short life span.

l  Simply as sac of haemoglobin this cell is present in only vertebrate blood and have evolved for
the transport of gases.

l  Mammalian RBC is without the nucleus, mitochondria, ribosomes, ER and Golgi bodies.

l  Viability and source of energy depend upon its membrane and glycolysis.

l  Very elastic and flexible, undergoes deformation to pass through narrow capillaries.

l  Contains antigen of blood group and an enzyme, carbonic anhydrase, which helps in Co₂ transport.

l  Number of RBC primarily depends upon physical activity of the organism.

Leucocyte (WBC)

l  Leucocytes are regarded as policeman of the body, constitutes defence system.

l  Active and nucleated cells of dividing nature; can change shape; mobile and phagocytotic.

l     Cross through blood capillaries wall or diapedesis since their most functions are outside blood.

l  Number (Total Count) is 5000 to 9000/ mm³, depicts body’s state of infection, it is in higher range in sick person.

l     Leucocytes are classified into following two broad types:

(a) Agranulocytes : No granules in cytoplasm and nucleus large and of definite shape. These are:
(1) Monocyte, (2) Lymphocyte

l     Monocytes : These are largest WBC (diameter 10mto 18m); macrophages (phagocytotic) in tissue fluid, to mop up unwanted material. Number (differential count) is approximately 4 to 11% i.e., about 200 to 700/mm3. Nucleus large, kidney or bean-shaped, may also be horse-shoe shaped.

l  Lymphocyte : These are both small and large sized. Nucleus enormously large, spherical and occupies most area of cell. Cytoplasm is peripheral, rim-like. Its main role is in maintaining the immune system; forming antibodies.

l     Neutrophils or heterophils: They take color with neutral stains (dyes). These are the main WBC type; with maximum number and variety of functions. The number (D.C) is 60–65% of the total WBC i.e., 4000 to 5000/mm³, large sized and phagocytotic in nature. Neutrophils secrete chemicals like pyrogens, toxins (killer substance), antitoxins, inflammatory and anti-inflammatory substance, lysozymes etc. Nucleus is bi-lobed and the number of lobe increase with age.

l  Eosinophil or acidophil: They take color with acidic dye, hence cytoplasm is alkaline. The number increases i.e., eosinophilia (in helminthic and the respiratory tract infections). The nucleus is bi-lobed, also phagocytic, but less motile than neutrophils, granules lysosomal with high peroxidase content.

l  Basophils: They take color with basic dye, thus cytoplasm is acidic. Nucleus is twisted, S-shaped. Cytoplasm contains heparin, histamine and serotonin like mast cell.

Blood platelets

l     These are only found in mammalian blood and protoplasmic fragments are without nucleus, these are formed from megakaryocytes in bone marrow.

l  They contains blood clotting factors, decrease in number, thrombocytopenia, hampers blood clotting.

l     In other vertebrates (frog) and invertebrates, instead of platelets, these are nucleated cells called as
thrombocytes or spindle cells.

Figure 2. 8 Formed elements of blood

Muscular tissue

l  Most specialized tissue, only function is to generate movement or force.

l  Converts chemical energy into mechanical energy like automobile machine.

l  Machinery for movement is the set of actin and myosin proteins (both filamentous) as main component of the cytoplasm.

l  The cell (completely differentiated) doesn’t divide, has no golgi bodies and centrosome and other parts called as: cytoplasm is called as sacroplasm, plasmalemma as sarcolemma, endoplasmic reticulum is sarcoplasmic reticulum (reduced as vesicles)

l  Mitochondria (Sarcosomes) are the main organelle, largest in size and number. In vertebrate there are three types of muscles; (1). Skeletal muscle, (2). Cardiac muscle and (3).  Smooth muscles

Skeletal muscle

l  Associated with skeleton, generates external movement in the body.

l  Cells are coenocytic long and cylindrical called as muscle fibre which is formed by the fusion of many myoblasts.

l  Only parallel fibres are present in a muscle.

Figure 2. 9 Structure of skeletal muscles

l  Striations (alternate dark and light bands) are formed due to particular arrangement pattern of actin
and myosin filaments – as revealed in electron microscopic structure of a myofibril.

l  Lengthwise each myofibril consists of many sarcomeres, which is plate like zigzag structure through which thin filaments pass is the functional unit of muscle and constitute the area between two Z-lines.

l  Dark band (Anisotropic band or A-band) consists of both actin (thin) and myosin (thick) filaments.

l  Light band (Isotropic band or I-band) consists of only actin filaments.

Cardiac muscle

l  The most versatile muscle is exclusively found in heart.

l     Being striated it is structurally similar to skeletal
muscle except the presence of  –

l  Cross fibres, besides parallel fibres

l  Intercalated discs which represent the joints
of two myoblasts.

l     Contraction pattern is rhythmic and also fast controlled by autonomic nervous system (ANS).

l  Untiring muscle (no fatigue), works ceaselessly
throughout the life.

l  Generates impulse like nerve (i.e. myogenic heart
of vertebrates).

Smooth Muscle

l  Unstriped; hence structurally different from striated and cardiac muscles.

l  Muscle cell spindle-shaped (not cylindrical), uninucleated each formed of single myoblast.

l  Actin filaments remain attached, to dense bodies in the cytoplasm.

l  There is no myofibril, no sarcomere and no Z-line.

l  Actin and Myosin filaments not arranged in any pattern hence no striations.

l  Involuntary muscle, under ANS control is found in visceral organs (visceral muscle). Contraction in such muscles is termed as peristalsis, is rhythmic, slow and prolonged.

Nervous tissue

l  This is coordinating or controlling tissue for entire body’s structural or functional organization.

l  Exhibits highest degree of irritability and conductivity.

l  It is ectodermal in origin and completely differentiated tissue cells do not divide at all (except some glial cells).

l  Receiving, integrating, transforming and transmitting the coded information of stimuli to evoke
response in the body are its specialized functions. This tissue consists of following two types of cells:

l  Neurons – the actual nerve cell,

l  Neuroglia – the supporting cells,

Neurons

l     These are structural and functional units of nervous tissue and highly excitable as well as specialized for generation and conduction of impulse.

l  The longest (upto 1.5 m. or more), the most sophisticated and the most complicated in structure and function.

l  Number is higher in large sized animals 100 billion in human body.

l  Neurons consist of three parts; dendron, cyton and axon. Cyton is the cell body, while dendron and

axon are the branches of the cell.

l  Nissl’s granules remain only in the cyton part and they are the sites for protein synthesis.

l  Neurons possess network of microtubules throughout the cell and constitute a transporting system.

l  Centrosome is absent so neurons are unable to regenerate.

Dendron

l  These are also called as afferent branches and are the receiving branch impulse enters the nerve cells through it. At terminal end each dendron gives rise to fine branches called dendrites.

Figure 2. 10 A. Structure of a neuron, B. L.S. of axon

l  Certain axon of neurons also gives out branches at right angle and they are called as collateral fibres.

l  The cytoplasm of axon is axoplasm with abundant neuro-fibril and mitochondria. The plasma membrane of axon is called as axolemma. Axons are main components of peripheral nervous system (PNS).

l  In myelinated nerve it is covered with myelinated sheath made of Schwann cells. Cut gap at certain intervals in this sheath is called as node of Ranvier.

Cyton (perikaryon or soma or cell body)

l  This part of neuron constitutes the grey matter of CNS, outside CNS in clusters form ganglion.

l  Abundant cytoplasm with almost all organelles and Nissl’s granules as colored body made of RNA, Nissl’s granules are also called as trigoid bodies which are secretory in nature.  Ribososomes and Golgi bodies are also present in the cytonic region.

Synapse

l     It is the joint of two neurons – between the axon of one neuron and the dendron of another neuron.

l  Axon is termed as pre synaptic terminal while, dendron as post synaptic terminal.

l  Synapse are classified into following two types and their classification is based on following distinguishing features:

Neurotransmitters

l  There are more than 30 types of substances discovered so far which are directly or indirectly involve in the conduction of nerve transmission and they are collectively called as neurotransmitters.

l  Acetylcholine, this is excitatory for nerve and general muscle, while, inhibitory for cardiac muscle.

l  Sympathin, Histamine, Noradrenaline (adrenalin) are excitatory for all muscles and nerves.

l  Serotonin, dopamine and GABA (g-amino-butyric acid) are inhibitory for all.

Types of neurons

l  The neurons are classified on the basis of number of dendrons and functionality.

l  On the basis of number of dendrons, neurons are classified into following types:

l   Unipolar neurons: Without dendron, only one branch present is axon.

Examples: Embryonic brain cells.

l   Bipolar neurons: Only one dendron and one axon hence with two branches.

Examples: Neurons present in the retina of eyes.

• Multipolar neurons: More than one dendron, hence, has many branches.

Examples: All common           neurons.

l     On the basis of functional characteristics, neurons are of following three types:

l   Sensory neuron: Carries impulse from sensory organ to brain, also called as afferent neuron.

• Motor neuron: Carries impulse from brain to effector organ (i.e. muscles) hence, also
  called efferent neuron.

l   Interneuron: Acts as adaptor between sensory and motor neurons in the CNS.

 

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Very Short Answer Questions

1. Name the most durable wood?
2. What is a conjuctive tissue?
3. What makes the apical meristem of the root sub-terminal?
4. Which protein constitutes bone matrix?
5. Neurons are packed around with some cells. Name?

Short Answer Questions

1. Name the difference in the function of collenchyma and sclerenchyma.
2. Define meristems
3. Name two types of sieve elements found in phloem.
4. What are the two types of fibres of the connective tissue? Differentiate between the two.
5. Name the tissue that lines the urinary bladder. State any one advantage of this tissue being present there.

Long Answer Questions

1. Name the main components of xylem. Which of these is most suitable for carrying water?
2. Sketch the elements of (a) Xylem and (b) Phloem.
3. Write the function of blood plasma?
4. Describe various types of epithelial tissues with the help of labeled diagrams.
5. Name the tissues which perform the following functions.

(i)   Haemopoeisis                                                  (ii)   Coagulation

(iii) Transmission of message                                (iv)  Locomotion

• Formation of antibodies.

 

Hot Questions :

1. State the location and function of different types of meristems.
2. Cork cambium forms tissues that form the cork. Do you agree with this statement?
   Explain.
3. Explain the process of secondary growth in the stems of woody angiosperms with the
   help of schematic diagrams. What is its significance?
4. Draw illustrations to bring out the anatomical difference between

(a) Monocot root and Dicot root

(b) Monocot stem and Dicot stem

5. Cut a transverse section of young stem of a plant from your school garden and observe
   it under the microscope. How would you ascertain whether it is a monocot stem or a
   dicot stem? Give reasons.
6. The transverse section of a plant material shows the following anatomical features –

(a)        the vascular bundles are conjoint, scattered and surrounded by a
sclerenchymatous bundle sheaths.

(b)        phloem parenchyma is absent. What will you identify it as?

7. Why are xylem and phloem called complex tissues?
8. What is stomatal apparatus? Explain the structure of stomata with a labeled diagram.
9. Name the three basic tissue systems in the flowering plants. Give the tissue names
   under each system.
10. What is periderm? How does periderm formation take place in the dicot stems?
11. Describe the internal structure of a dorsiventral leaf with the help of labeled diagrams.
12. Define Tissue. Write the names of four types of animal tissues and describe them
    briefly.
13. Give the function of Leucoplast in plants.
14. Give one function of SER and RER each.
15. Which epithelial tissue is found in surfaces where absorption and secretion occurs.
16. Draw a neat labeled diagram of any type of complex tissue in Plants.
17. Give hierarchy of classification. Who gave five kingdom classification.
18. Differentiate Thallophyta and Bryophyta. Give one example each.
19. What is the difference between tendon and ligament.
20. What is a nucleoid. Which type of cell have it. List their one structural feature too which
    differentiate them from animal cells.
21. Compare three types of muscle fibres structurally and functionally.
22. What is the difference between Meristematic and Permanent tissue. Show the location of
    various types of Meristematic tissues via schematic diagram.
23. Name a eukaryotic unicellular organism. Also name the kingdom it belongs to and what is
    its mode of nutrition.
24. Name the following.
    (a) Tissue that forms the inner lining of our mouth.
    (b) Tissue that connects muscle to bone in humans.
    (c) Tissue that transports food in plants.
    (d) Tissue that stores fat in our body.
    (e) Connective tissue with a fluid matrix.
    (f) Tissue present in the brain.