Introduction
In unicellular organisms, a single cell performs all basic functions such as ingestion, digestion, absorption, reproduction, excretion among others. There are no specialized organs to perform these functions.
In multi-cellular organisms however, there are specialized group of cells that perform these functions and are called tissues. Each specialized function is taken up by a different group of cells. Since these cells carry out only a particular function, they do it very efficiently.
What is a Tissue?
There is division of labour in multi-cellular organisms -
This means that a particular function is carried out by a cluster of cells at a definite place in the body. This cluster of cells, called a tissue, is arranged and designed so as to give the highest possible efficiency of function.
A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue
Examples:
- Muscle cells contract and relax to help in movement
- Nerve cells carry messages
- RBCs carry oxygen and blood to various organs
In Plants also there are specialized tissues to help in movement of food and water.
Difference between Plant and Animal Tissues
Plant Tissues | Animal Tissues |
They are stationery or fixed | They are motile and can move around |
They have a lot of supportive tissues as they have to be upright | The supportive tissues are present in comparatively lesser quantities and the tissues are flexible to help in movement |
The supportive tissues are generally made up of dead cells | The supportive tissues are generally living |
The growth is limited to certain regions | The growth is not limited to specific regions |
Certain tissues grow and divide throughout the life of the plant but these tissues are limited to certain regions | The growth of tissues is more uniform compared to plants |
The organization of tissues is not very complex | The organization of tissues is complex and specialized |
Plant Tissues
Based on the dividing capacity of the tissues, various plant tissues can be classified as:
1. Meristematic tissue or Growing tissues
2. Permanent tissue.
1. Meristematic Tissues:
These are simple living tissues having thin walled compactly arranged immature cells which are capable of division and formation of new cells. They are present in specific locations of the plants and help in growth through division.
As they grow and mature, their characteristics slowly change and they become differentiated as components of other tissues.
Main features of Meristematic tissues are:
• Thin primary cell wall (Made of cellulose)
• Intercellular spaces are absent (Compact tissue).
• Generally vacuoles are absent, dense cytoplasm & prominent nuclei are present. (Why?)
• Active metabolic state, stored food is absent and therefore there is no need for vacuoles.
• Large numbers of cell organelles are present.
• Actively dividing cells are present in growing regions of plants e.g., root & shoot tips.
Depending on their location they can be classified into:
a) Apical Meristem- Present at the growing tips of stems and roots and increases the length of the stem and the root.
b) Lateral Meristem- They are present on the sides of the plants and are responsible in increasing the girth (width) of the stems and the roots.
They are also called as cambium tissues and are present at boundary of tissues that transport water(Xylem) and food (Phloem)
It gives rise to the vascular tissues which are responsible for transport of materials in plants
c) Intercalary Meristem- They can be seen in some plants is located near the node.
These are present at the base of leaf & inter-node region.
These lead to the increase in the length of leaf
2. Permanent Tissues:
Some cells of Meristematic tissue once developed stop dividing and take up a specific role.
These tissues are called as permanent tissues.
This process of taking up a permanent shape, size, and a function is called differentiation.
Differentiation leads to the development of various types of permanent tissues.
Permanent Tissue can also be of 2 types:
i) Simple Permanent Tissues ii) Complex Permanent Tissues
i) Simple Permanent Tissues:
These type of tissues are made up of same type of cells which are similar in function as well as structure.
Simple permanent tissues can be categorized into:-
-Protective Tissues: These tissues are protective in function and generally serve as a boundary between the plant and the environment.
a) Epidermis b) Cork or Phellem
a ) Epidermis: It is generally one celled thick outermost layer of the various parts of the plant.
- It is covered by a waxy layer called cuticle which is made up of cutin secreted by the epidermal cells.
- In xerophytic plants that live in an environment with little water the cuticle is thick to prevent the loss of water.
- The epidermal cells are not continuous and have small pores called stomata which are protected by bean shaped guard cells. The guard cells are the only cells that have chloroplast in them and the rest of them are colorless.
Functions of Epidermis
• The main function of epidermis is to protect the plant from infection.
• Cuticle of epidermis cuts the rate of transpiration and evaporation of water and prevents wilting or bending of leaves
• Stomata in epidermis allow gaseous exchange to occur during photosynthesis respiration.
• Stomata also helps in transpiration.
b) Cork Cells: As plant becomes older the lateral meristem at the periphery become cork cells or phellem cells.
It is made up to dead cells that has thick walls and no intercellular spaces.
The cell walls in cork have waxy substance called as suberin that do not allow gases and water to pass through
The cork cells are without any protoplasm but are filled with resins or tannins.
Functions of Cork:
- Cork is protective in function. Cork cells prevent infection and mechanical injury.
- Since it is imperviousness, light, tough, compressible and elastic, it has a lot of commercial value.
- Cork is used for insulation, as shock absorber.
- Cork is used in the making of a variety of sport goods such as cricket balls, table tennis, shuttle cocks, wooden paddles etc.
- Supportive Tissues:
a) Parenchyma b) Collenchyma c) Sclerenchyma
a) Parenchyma: It is the most common simple permanent tissue.
It consists of relatively unspecialized cells with thin cell walls.
They are living cells.
They are usually loosely arranged, thus large spaces between cells (intercellular spaces) are found in this tissue
This tissue generally stores food and thus has vacuoles.
Chlorenchyma : Parenchyma tissues that contain chlorophyll are called as Chlorenchyma
Aerenchyma : Parenchyma tissues that have air spaces in them to help them float are called as Aerenchyma
b) Collenchyma: This tissue provides flexibility in plants and allows bending of various parts of a plant like tendrils and stems of climbers without breaking.
It also provides mechanical support.
They are found in leaf stalks below the epidermis.
The cells of this tissue are living, elongated and irregularly thickened at the corners.
There is very little intercellular space
c) Sclerenchyma: It is responsible for hardness and stiffness in plants and provides strength.
They have thick walls due to lignin with no inter-cellular space
The cells of this tissue are dead.
This tissue is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts.
The husk of coconut is made up this type of tissue.
(ii) Complex Tissues
The complex tissues consist of more than one type of cells. All these cell co-ordinate to perform a common function. Complex tissues transport water, mineral salts (nutrients) and food material to various parts of plant body.
Complex tissues are of the following two types :
(i) Xylem (ii) Phloem
(i) Xylem
Xylem is made up of four types of cells -
1.Tracheids
2.Vessels
3. Xylem fibers
4. Xylem parenchyma Living element
1. Tracheids: Tracheids are elongated cells with tapering ends. They also conduct water. Since tracheids do not have open ends like the vessels, so the water has to pass from cell to cell via the pits.
2. Vessels or tracheae: Very long tube like structures formed by a row of cells placed end to end. The transverse walls between the vessels are completely dissolved to form continuous channels or water-pipes.
3. Xylem fibre: These are dead and lignified sclerenchymatous cells which are mainly supportive in function.
4. Xylem parenchyma: It is formed of living parenchymatous cells which helps in storage of food and lateral conduction of water and minerals.
Functions:
1. Tracheids and vessels help in long distance conduction of water and minerals upward from the root system to various parts of plant.
2. Tracheids and vessels provides mechanical support.
(ii) Phloem:
It is also made up of four types of cells-
1. Sieve tubes
2. Companion cells living elements
3. Phloem parenchyma
4. Phloem fibers
1. Sieve tubes: Sieve tubes are slender, tube-like structures composed of elongated thin-walled cells, placed end to end. Their end walls are perforated by numerous pores and are called sieve plates. Walls of sieve tubes are perforated. The nucleus of each sieve cell degenerates at maturity, however, cytoplasm persists in the mature cell. Thus, nuclei are absent in mature sieve tube elements. The cytoplasm of one sieve tube element is continuous with those of the sieve elements above and below by cytoplasmic connections passing through the pores of the sieve plate.
2. Companion cells: These are associated with sieve tubes. These are smaller cells having dense cytoplasm and prominent nucleus. The companion cells help the sieve tubes in the conduction of food material. Sieve cells & companion cells are so called sister cell because they originate from single mother cell.
3. Phloem parenchyma: These are living and thin walled cells. It is also known as bast parenchyma. It helps in conduction of food in radial direction. It store various materials e.g. Resin, Latex etc.
4. Phloem fibers: These are dead and sclerenchymatous cells. Phloem or bast fibres of some plants are source of commercial fibres e.g. Jute, Hemp, Flex. They provide mechanical support to the conducting elements.
Function of phloem:
Phloem transport photosynthetically prepared food materials from the leaves to the storage organs and latter from storage organs to the growing regions of the plant body.
Xylem and Phloem: Transport in Plants:-
Animal Tissues
1. Epithelial Tissue
-These are protective tissues and cover most organs and cavities in organisms. They are usually single layered thick.
- The act as barrier keeping different body systems separate from each other
- They are tightly packed and are continuous and hence no intercellular spaces.
- They are selectively permeable and help in regulation of exchange of material between the body and the environment.
- Different types of epithelial tissues have different permeability.
- They are always found above another tissue and are separated by extracellular fibrous membrane
- The skin, the lining of the mouth, the lining of blood vessels, lung alveoli and kidney tubules are all made of epithelial tissue.
Depending upon their structure, they perform different functions and are accordingly present in various parts of the body:
(a) Simple Squamous Epithelial Tissue: These are simple flat kind of epithelial tissues and are usually found in places where movement of substances through a selectively permeable surface occurs.
(b) Stratified Squamous Epithelial Tissue: When epithelial cells are arranged in many layers to prevent wear and tear they are called stratified squamous epithelium.
- The word ‘Squama’ means scale of skin
- The Oesophagus and the lining of the mouth are also covered with squamous epithelium. The skin, which protects the body, is also made of squamous epithelium.
Stratified Squamous Tissues are usually found in harder parts of the skin.
(c) Cuboidal Epithelium: They are cube like cells that fit closely, cells look like squares in section, but free surface appears hexagonal. They are found in places that involve absorption, excretion & secretion.
It also provides mechanical support.
It is found in kidney tubules, thyroid vesicles & in glands (salivary glands, sweat glands).
It forms germinal epithelium of gonads (testes & ovaries).
(d) Glandular Epithelium: Cuboidal Epithelial cells often acquire additional specialization as gland cells, which can secrete substances at the epithelial surface.
Sometimes a portion of the epithelial tissue folds inward, and a multicellular gland is formed. This is glandular epithelium.
(e) Columnar Epithelial: Columnar means ‘pillar-like’ epithelium. It forms lining of stomach. They are also found in places where absorption and secretion occur.
Usually found in small intestine & colon, forming mucous membranes and they are also found on the border of micro villi which is present at the free surface end of each cell which increases absorption efficiency in small intestine.
(f) Ciliated Epithelial: Cuboidal or Columnar epithelial tissues having hair-like projections on the outer surfaces of epithelial cells are called as ciliated epithelial tissues. These cilia (hair-like projections) can move, and their movement pushes the mucus in the respiratory tract forward to clear it.
It helps in the movement of ova in the fallopian tube.
2. Connective Tissue:
The cells of the connective tissue are widely spaced and embedded in an intercellular matrix.
Matrix can be any material which acts as a fluid material in which other things may be dissolved or floating.
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.
Their basic function is to provide support to different organs & keeping them in place.
i) Fluid or Vascular Connective Tissue: Blood and Lymph are fluid connective tissues.
In blood, plasma is the liquid matrix which has several wandering and floating cells called corpuscles.
These corpuscles help in transportation of various materials such as nutritive substances, gases, waste products thorough the body
Plasma: The plasma contains proteins, salts and hormones. It consists of 90% water and the rest is protein and salts.
Red Blood Corpuscles (RBCs): They contain red colored pigment called Hemoglobin which helps in transportation of oxygen. They are also called as Erythrocytes.
White Blood Corpuscles (WBCs): They are referred to as soldiers of the body and since they do not have any colored pigment, they are also called as Leucocytes.
They are irregular, amoeboid, phagocyte cells that protect our body by engulfing bacterial & other foreign particles.
They are of five types : Monocytes, Lymphocytes, Basophiles, Neutrophils, Eosinophils.
Platelets: They are spindle shaped cells which are involved in clotting of blood.
ii) Skeletal Connective Tissue:
Skeletal connective tissue is hard and supports the framework of the body.
It is of two types: Bones and Cartilage
Bone:
It is a strong and nonflexible tissue having a dense matrix composed of calcium and phosphorus compounds such as Calcium Phosphate.
Bone cells are called as Osteoblasts and are present in the hard dense matrix of bones.
Matrix is deposited in the form of concentric layers around a central canal, the bone cells occupy small spaces between the concentric layers of matrix.
The long bones are usually hollow containing cavity called as marrow cavity. It is full of bone marrow.
Cartilage:
This tissue is elastic, less harder as compared to bones. The elasticity is due to presence of chondrin – a type of protein.
It provides flexibility, smoothness and great tensile strength. Its solid matrix is composed of proteins and sugars.
Cells are called as chondroblast, which are widely spaced and matrix is reinforced by fibres.
It occurs at joint of bones, in the nose, ear, trachea & larynx.
iii) Supporting Tissue: It is the most abundant type of connective tissue. It is further divided into two types :
Ligament: It is a yellow fibrous connective tissue that connects bone to bone.
They are very elastic due to the presence of a network of yellow fibres in its matrix.
Tendon: It is a white fibrous connective tissue that connects bones to muscles.
They have very little matrix containing a lot of white fibres.
iv) Areolar Tissue: Areolar tissue are a type of loose connective tissues and fills spaces inside organs and is found between the skin & muscles, around blood vessels, nerves and in the bone marrow.
They help in holding the organs in place and attaches the epithelial tissues to other underlying tissues.
It act as a reservoir of water and salts for surrounding tissues helps in repair of tissues.
v) Adipose Tissue: These are oval and round cells, filled with fat globules.
It is found below the skin, between organs, around the heart, brain and below the eyeballs.
It acts as an insulator and prevents loss of heat from the body.
3. Muscular Tissue:
It consists of elongated cells called muscle fibres which are responsible for movement. Due to presence of special proteins they can contract and relax causing movement
Striated muscles – Voluntary Muscles
They are also called as voluntary muscles because these are under the control of one’s will. Since they attach to the bones, they are also called as Skeletal Muscles. They have alternate light and dark striations (bands) and are thus called striated muscles. Muscle fibres or cells are long, cylindrical, multinucleated and unbranched.
Non-striated muscles
They are involuntary muscles also called as smooth muscles because they do not have striations. The muscle fibres are uni-nucleated and spindle shaped. Such muscles are found in the walls of stomach, intestine, urinary bladder, bronchi, iris of eye etc. Peristaltic movements in alimentary canal are brought about by smooth muscles.
Cardiac muscle fibres
They are involuntary muscles found only in the walls of heart. Their structure is in between the striated and non-striated muscles. They are uni-nucleated and branched. Branches are united by intercalated disc. In these muscles rhythmic contraction and relaxation occurs throughout the life.
4. Nervous Tissue:
They are highly specialized tissue due to which the animals are able to perceive and respond to the stimuli.
Their functional unit is called as nerve cell or neuron.
Cell body is cyton covered by plasma membrane.
Short hair like extensions rising from cyton are Dendron which are further subdivided into dendrites.
Axon is long, tail like cylindrical process with fine branches at the end. Axon is covered by a sheath.
Axon of one neuron is very closely placed to the dendrons of another neuron to carry impulses from one to another neuron in the form of electrochemical waves. This close proximity is called as synapse.
