Chapter-6. Tissues

 

Main-points: 

• In unicellular organisms, a single cell performs all basic functions. For example, in Amoeba, a single cell carries out movement, intake of food and respiratory gases, respiration and excretion.
• In multi-cellular organisms there are millions of cells. Most of these cells are specialised to carry out a few functions. Each specialised function is taken up by a different group of cells.
• Multi-cellular organisms show division of labour.
• Cells specialising in one function are often grouped together in the body.
• A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue.

In Human being:

Muscle Cells: contract and relax to cause movement.

Nerve cells: carry messages.

Blood cells: Form blood and blood flows to transport oxygen food hormones and waste material.

In plants:

Vascular tissues conduct food and water from one part of the plant to other parts.

Tissue: A group of cells having similar functions and similar structure is known as tissue.

Plant Tissues:

(i)  Plants are stationary or fixed – they don’t move.

(ii)  Most of the tissues they have are supportive, which provides them with structural strength.

(iii) Most of these tissues are dead, since dead cells can provide mechanical strength as easily as live ones, and need less maintenance.

(iv) The growth in plants is limited to certain regions.

(v) There are some tissues in plants that divide throughout their life. These tissues are localised in certain regions. Based on the dividing capacity of the tissues, various plant tissues can be classified as growing or meristematic tissue and permanent tissue.

Animal Tissues:

(i) Animals on the other hand move around in search of food, mates and shelter.

(ii) They consume more energy as compared to plants.

(iii) Most of the tissues they contain are living.

(iv) Cell growth in animals is more uniform. So, there is no such demarcation of dividing and non-dividing regions in animals.

 

Type of Tissue:

(1)  MERISTEMATIC TISSUE:

The growth of plants occurs only in certain specific regions. This is because the dividing tissue, such dividing tissues are located at growing parts of the plants. Such tissues are known as meristematic tissue.

 

Classification of Meristematic Tissue:

(A) Apical Meristem: Apical meristem is present at the growing tips of stems and roots and increases the length of the stem and the root.

 

(B) Lateral Meristem: The girth of the stem or root increases due to lateral meristem (cambium).

 

(C) Intercalary meristem: Intercalary meristem is the meristem at the base of the leaves or internodes (on either side of the node) on twigs.

 

Properties Of Meristematic Tissue:

(i)The cells of this tissue are very active,

(ii) They have dense cytoplasm, thin cellulose walls and prominent (Clear) nuclei.

(iii) They have lack in vacuoles.

IMAGE

(2) PERMANENT TISSUE :

The tissues developed or formed by meristematic tissues take up a specific role and lose the ability to divide further. As a result, they form a permanent tissue.

Cells of meristematic tissue differentiate to form different types of permanent tissue.

Definition: The cells which are differentiated to perform specific function and lose the ability to divide are called permanent tissues.

Differentiation: This process of taking up a permanent shape, size, and a function is called differentiation.

Type of permanent tissue:

(A) Simple permanent tissue:

It compose of single type of cell, which look like each other. Such tissues are called simple permanent tissue. 

E.g: Parenchyma, collenchymas, sclerenchyma etc.

 

Type of simple permanent tissues:

(1) Parenchyma: A type of permanent tissue, whose few layer of cells form basic packing of tissue. These are called parenchyma.

Features:

(i)  It consists of relatively unspecialised cells with thin cell walls.

(ii) They are live cells.

(iii) They are usually loosely packed.

(iv) therefore large spaces between cells (intercellular spaces) are found in this tissue.

(v) This tissue provides support to plants and also stores food.

(vi) The parenchyma of stems and roots also stores nutrients and water.

 

(i) Chlorenchyma: In some other parenchyma, it contains chlorophyll and performs photosynthesis, and then it is called chlorenchyma.

(ii) Aerenchyma: In aquatic plants, large air cavities are present in parenchyma to give buoyancy to the plants to help them float. Such a parenchyma type is called aerenchyma.

 

(2) Collenchyma: This is another simple permanent tissue which gives flexibility in plants. It allows easy bending in various parts of a plant (leaf, stem) without breaking.  Such type of permanent tissue is called collenchymas.

 

We can find this tissue in leaf stalks below the epidermis.

Features:

(i)  It gives flexibility to plants.

(ii) It allows easy bending in various parts of a plant (leaf, stem) without breaking.

(iii) It also provides mechanical support to plants.

(iv) The cells of this tissue are living, elongated and irregularly thickened at the corners.

(v)  There is very little intercellular space.

 

(3) sclerenchyma: This also another type of simple permanent tissue, which makes the plants hard and stuff. Such a simple permanent tissue is called sclerenchyma. Example : The husk of a coconut.

This tissue is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts.

Features:

 

(i) The cells of this tissue are dead.

(ii) They are long and narrow as the walls are thickened due to lignin.

(iii) Often these walls are so thick that there is no internal space inside the cell.

(iv) It provides strength to the plant parts.

Lignin: a chemical substance which acts as cement and hardens them.

Differentiation among Parenchyma, collenchymas and Sclerenchyma:

 

Parenchyma	collenchymas	sclerenchyma
They are live cells. Cell walls are thin. They have large space between cells. It provides support to plants and stores food.	They are live cells. Cell walls are thick. Intercellular spaces are present. It provides mechanical support to plants.	They are dead cells. Cell walls are thick. Intercellular spaces are absent. It provides strength to the plant parts.

 

 

(B) Complex permanent Tissue:

Complex tissues are made of more than one type of cells, e.i consist of different type of cells. All these cells coordinate to perform a common function.

E.g : xylem and phloem etc.

• They are both xylem and phloem conducting tissues and constitute a vascular bundle.
• Vascular or conductive tissue is a distinctive feature of the complex plants, one that has made possible their survival in the terrestrial environment.

1. Xylem

Xylem is a conducting tissue and constitute a vascular bundle. Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres. 

Forms vascular bundles with phloem and gives mechanical strength to plant due to presence of lignin cells. 

Function of xylem: 

(i) The xylem carries water and minerals obtained from the soil by roots to the other parts plant.

(ii) It is also responsible for supporting the plant as well as storage and long-distance transportation of water and nutrients from root to other parts. 

(iii) Xylem transports materials unilaterally. 

(iv) It also provides mechanical strength to the plants. 

Elements of xylem: 

(i) Tracheids: These are conducting elements. 

(ii) Vessels : These are also conducting elements. 

(iii) Xylem parenchyma 

(iv) Xylem fibres

Dead Cells: Vessels, tracheids and xylem fibres. 

Live cells: Xylem parenchyma. 

• Conducting and tracheary elements are usually dead. 

2. Phloem: 

Phloem is also a conducting tissue and constitutes a vascular bundle. Phloem is made up of four types of elements: sieve tubes, companion cells, phloem fibres and the phloem parenchyma. 

 

Function of phloem: 

(i) Phloem transports products of photosynthesis from the leaves where they are synthesized to other parts of the plant. 

(ii) Phloem can move unlike materials of xylem in both directions. 

(iii) Phloem has no mechanical function. 

Elements of phloem:

(i) Sieve tubes : Sieve tubes are tubular cells with perforated walls.

(ii) Companion cells : These are living cells. 

(iii) Phloem fibres

(iv) Phloem parenchyma 

Dead Cells: Phloem fibres 

Live cells: Sieve tubes, Companion cells and Phloem parenchyma. 

• Conducting cells are living. 

Translocation: Transportation of food and nutrients such as sugar and amino acids from leaves to storage organs and growing parts of plant. This movement of substances is called translocation.

Differences between xylem and phloem: 

Xylem	Phloem
1. It transports water and minerals.  2. It provides mechanical supports to the plants.   3. xylem occupies the centre of the vascular bundle.  4. Materials can move in single direction up only e.i unidirection.   5. Conducting and tracheary elements are usually dead.	1. It transports sugar and amino acids.   2. It does not provide mechanical support   3. Phloem occupies outer side of vascular bundle.   4. Materials can move in both directions up and down, e.i bidirection.   5. Conducting cells are living.

 

Epidermis:

The outer most layer of cells is called epidermis. The epidermis is usually made of a single layer of cells. The epidermis may thicker in plants of dry habitats.

Since, it has a protective role to play, cells of epidermal tissue form a continuous layer without intercellular spaces. Most epidermal cells are relatively flat. Often their outer and side walls are thicker than the inner wall.

Features:

(i) Epidermis may be thicker since protection against water loss is critical.

(ii) The entire surface of a plant has this outer covering of epidermis.

(iii) It protects all the parts of the plant.

(iv) Epidermal cells on the aerial parts of the plant often secrete a waxy, water-resistant layer on their outer surface.

(v) This aids in protection against loss of water, mechanical injury and invasion by parasitic fungi.

Functions Of Epidermis in plants:

(i) This aids in protection against loss of water,

(ii) Mechanical injury and invasion by parasitic fungi.

(iii) Its jelly like substances  forms a water resistant layer. 

Functions of Epidermis in roots:

Epidermal cells of the roots, whose function is water absorption, commonly bear
long hair-like parts that greatly increase the total absorptive surface area.

Role of Epidermis in Desert Plants:

Epidermis has a thick waxy coating of cutin (chemical substance with waterproof quality) on its outer surface.

Cutin: This is a chemical substance with waterproof quality, which is mainly found in epidermis of desert plants. 

Suberin: There is a chemical called suberin in the walls of outer protective
tissue (thick cork) or the bark of the tree that makes them impervious (harmless) to gases and water.

Stomata: There are many small pores in the epidermis (surface) of the leaf. These small pores are called stomata.

Protective Layer: As plants grow older, the outer protective tissue undergoes certain changes. A strip of secondary meristem replaces the epidermis of the stem. Cells on the outside are cut off from this layer. This forms the several-layer thick cork or the bark of the tree. Cells of cork are dead and compactly arranged without intercellular spaces

Functions of stomata:

(i) The process of transpiration takes place through stomata. 

(ii) The gaseous exchanges also take place through stomata.

Transpiration: This is a process of loss of water in the form of water vapour.

• Stomata are enclosed by two kidney-shaped cells called guard cells.

Function of guard Cells:

They are necessary for exchanging gases with the atmosphere.

 

Connective Tissue: 

The cells of connective tissue are loosely spaced and embedded in an intercellular matrix. The matrix may be jelly like, fluid, dense or rigid. The nature of matrix differs in concordance with the function of the particular connective tissue.

Examples of Connective Tissue: 

Blood, bones, Cartilage, ligaments and tendons etc. 

Plasma: The fluid (liquid) matrix of blood is called plasma. Plasma contains three types of blood cells.  

(i) RBC - Red blood cells 

(ii) WBC - White blood Cells

(iii) Platelets 

These three types of blood cells are suspended in plasma. Plasma is a yellowish liquid like material. The plasma also contains proteins, salts and hormones. Blood flows and transports gases, digested food, hormones and waste materials to different parts of the body.

Bones: It is also a connective tissue. 

(i) It forms the framework that supports the body.

(ii) It also anchors the muscles and supports the main organs of the body.

(iii) It is a strong and nonflexible tissue.

(iv) Bone cells are embedded in a hard matrix that is composed of calcium and
phosphorus compounds.

Ligaments: Two bones can be connected to each other by another type of connective tissue called the ligament.

(i) This tissue is very elastic.

(ii) It has considerable strength.

(iii) Ligaments contain very little matrix.

Tendons: Tendons connect bones to muscles and are another type of connective
tissue.

(i) Tendons are fibrous tissue with great strength but limited flexibility.

Cartilage: Cartilage is another type of connective tissue, which has widely spaced cells. The solid matrix of this tissue is composed of proteins and sugars.

(i) Cartilage smoothens bone surfaces at joints and is also present in the nose, ear, trachea and larynx.

(ii) Cartilage of ear can be folded. 

Areolar connective tissue: Areolar connective tissue is found between the skin and muscles, around blood vessels and nerves and in the bone marrow.

Functions:

(i) It fills the space inside the organs, supports internal organs and helps in repair of tissues.

Adipose Tissue: Adipose tissue is found below the skin and between internal organs. The cells of this tissue are filled with fat globules.

Function of Adipose Tissue:

(i) It stores fats. 

(ii) it act as an insulator.

Muscular Tissues: 

Muscular tissue consists of elongated cells, also called muscle fibres. This tissue is responsible for movement in our body.

Contractile proteins: Muscles contain special proteins called contractile proteins, which contract and relax to cause movement.

Muscular Tissues are three types: 

(i) Striated Tissue: Voluntary muscles are also called striated tissues. These muscles are also called skeletal muscles as they are mostly attached to bones and help in body movement. Under the microscope, these muscles show alternate light and dark bands or striations when stained appropriately. As a result, they are also called striated muscles.

                

(ii) Unstriated Tissue: Involuntary muscles are made up of unstriated tissues. They are also found in the iris of the eye, in ureters and in the bronchi of the
lungs. The cells are long with pointed ends (spindle-shaped) and uninucleate (having a single nucleus). They are also called unstriated muscles 

                       

(iii) Cardiac Tissues: The muscles of the heart show rhythmic contraction and relaxation throughout life. These involuntary muscles are called cardiac muscles which are made up of cardiac tissue.  Heart muscle cells are cylindrical, branched and uninucleate.

                                  

Type of Muscles: 

(i) Voluntary Muscles: We can move some muscles by conscious will. Muscles present in our limbs move when we want them to, and stop when we so decide.
Such muscles are called voluntary muscles

(ii) Involuntary Muscles: Some muscles cannot be move by concious will. These muscles are called Involuntary muscles. 

(iii) Cardiac Muscles: The muscles of the heart show rhythmic contraction and relaxation throughout life. These involuntary muscles are called cardiac
muscles.

Differences among striated, unstriated and cardiac Muscles: 

Striated Muscles	Unstriated Muscles	Cardiac Muscles
The cells of this tissue are long and cylindrical.	The cells are long with pointed ends and non-cylindrical.	Cells are cylinderical with non-pointed ends.
Cells are multinucleated.	Cells are uninucleated.	Cells are uninucleated.
Cells are not branched.	Cells are not branched.	Cells are branched.
striation are present.	striation or strips are absent.	Cells have faint striation.
Location:
They are found in hands, legs and other skeletal muscles.	They are found in the iris of the eye, in ureters and in the bronchi of the lungs.	They are found in heart.

 

 

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