CBSE NOTES CLASS 9 SCIENCE CHAPTER 6
A group of cells that are similar in structure and/or work together to achieve a particular function form a tissue.
Difference in Body Structure of Unicellular and Multi Cellular Organism
In unicellular organisms, a single cell performs all the basic functions of life, like movement, respiration, digestion, reproduction, etc.
For example, in Amoeba, a single cell carries out movement, intake of food and respiratory gases, respiration and excretion.
But 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. Since these cells carry out only a particular function, they do it very efficiently.
For example in human beings, muscle cells contract and relax to cause movement; nerve cells carry messages; blood flows to transport oxygen, food, hormones and waste material and so on.
In plants, vascular tissues conduct food and water from one part of the plant to other parts.
Thus, multi-cellular organisms show division of labour between different types of cells. The tissues are arranged and designed so as to give the highest possible efficiency of function.
Difference between Plants & Animal Tissues
Plants are stationary or fixed – they don’t move. Most of the tissues in plants are supportive, which provide them with structural strength. Most of these tissues are dead. Since dead cells can provide mechanical strength as easily as live ones, and need less maintenance.
Animals on the other hand move around in search of food, mates and shelter. They consume more energy as compared to plants. Most of the tissues they contain are living.
2. Pattern of Growth.
The growth in plants is limited to certain regions. There are some tissues in plants that divide throughout their life. These tissues are localised in certain regions.
Cell growth in animals is more uniform. So, there is no such demarcation of dividing and non-dividing regions in animals
3. Structural organisation of organs and systems
The structural organisation of organs and organ systems is far more specialised and localised in complex animals than even in very complex plants.
Types of Plant Tissues
1. Meristematic tissues or Meristems
The Meristems are the tissues having the capabilities of cell division. These are found on regions of the plant where growth takes place.
The cells of meristems are undifferentiated. New cells produced by meristem are initially like those of meristem itself, but as they grow and mature, their characteristics slowly change and they become differentiated as components of other tissues.
Types of Meristems
(a) The Apical Meristem is present at the growing tip of the stem and roots and increases the length.
(b) The Lateral Meristem is present at the lateral side of stem and roots (cambium) and increases the girth.
(c) The Intercalary Meristem is present at internodes or base of the leaves and increases the length between the nodes.
The cells of meristematic tissue are very active and hence they have dense cytoplasm, thin cellulose walls and prominent nuclei. They lack vacuoles.
The vacuoles causes hindrance in cell division as it is full of cell sap to provide turgidity and rigidity to the cell. They also do not have any waste material to store so vacuoles are usually absent in the meristematic cells.
2. Permanent tissues
When the cells formed by meristematic tissue take up specific roles and lose the ability to divide, they form a permanent tissue.
The process of taking up a permanent shape, size, and function is called differentiation. Cells of meristematic tissue differentiate to form different types of permanent tissue.
There are two types such permanent tissues.
(A) Simple permanent tissues
Tissues which are made of only one type of cells, which look like each other, are called simple permanent tissues.
- It consists of relatively unspecialised cells with thin cell walls. These cells are live cells. The cells are usually loosely packed, so that large spaces between cells (intercellular spaces) are found in this tissue.
- This tissue provides support to plants and also stores food.
- Chlorenchyma - The parenchyma which contains chlorophyll and performs photosynthesis is called chlorenchyma.
- Aerenchyma - The parenchyma with large air cavities in aquatic plants, are called aerenchyma. The air cavities provide buoyancy to the plants to help them float.
- The parenchyma of stems and roots also stores nutrients and water.
- This tissue is 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
- This tissue provides mechanical support. It also provides flexibility and easy bending of various parts of plants, without breaking.
- The cells of this tissue are dead. They are long and narrow as the walls are thickened due to lignin (a chemical substance which acts as cement and hardens them). There is no internal space inside the cell.
- This tissue is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts. This tissue makes the plant hard and stiff and provides strength to the plant parts.
- This tissue is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts. Example is husk of a coconut.
(iv) Epidermal Tissue
- The entire surface of a plant has outer covering of epidermis.
- The epidermis is made of a single layer of cells. In some plants living in very dry habitats, the epidermis may be thicker to protect against water loss.
- Epidermis protects all the parts of the plant.
- Epidermal cells on the aerial parts of the plant secrete a waxy, water-resistant layer on their outer surface. This aids in protection against loss of water, mechanical injury and invasion by parasitic fungi.
- Since it has to protect the plant, cells of epidermal tissue form a continuous layer without intercellular spaces. Epidermal cells are relatively flat. Their outer and side walls are thicker than the inner wall.
- Stomata and guard cells: Small pores in the epidermis of the leaf are called stomata. Stomata are enclosed by two kidney-shaped cells called guard cells.
Functions of Stomata
- Exchange of gases, particularly CO2 and O2, with atmosphere.
- Loss of water in the form of water vapour during transpiration. This is important for rise of water in the xylem vascular tissue.
CO2 and water are required for photosynthesis and O2 is given out during day. O2 is taken in and CO2 is given out during respiration.
- Epidermal cells of the roots, whose function is water absorption, commonly bear long hair-like parts that greatly increase the total absorptive surface area.
- Cutin - In desert plants, epidermis has a thick waxy coating of cutin (chemical substance with waterproof quality) on its outer surface. This protects loss of water.
- Cork - 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. They also have a chemical called suberin in their walls that makes them impervious (not allowing passage of) to gases and water.
(B) Complex Permanent Tissues
- Complex permanent tissues are made of more than one type of cells. All these cells coordinate to perform a common function.
- Examples are Xylem and Phloem. They are both conducting tissues and constitute a vascular bundle.
- Xylem conducts of water and minerals from roots to leaves.
- It consists of tracheids, vessels, xylem parenchyma and xylem fibers.
- The cells have thick walls, and many of them are dead cells.
- Tracheids and vessels are tubular structures. This allows them to transport water and minerals vertically.
- The parenchyma stores food and helps in the sideways conduction of water.
- Fibres are mainly supportive in function.
- Phloem transports food from leaves to other parts of the plant.
- Phloem is unlike xylem in that materials can move in both directions in it.
- Phloem is made up of four types of elements: sieve tubes, companion cells, phloem fibres and the phloem parenchyma
- Sieve tubes are tubular cells with perforated walls. A companion cell is closely associated with a sieve element. It provides supportive functions and regulates the sieve.
- Except for phloem fibres, phloem cells are living cells.
(i) Epithelial tissue
Epithelium covers the outer body and most organs and cavities within the body. It forms a barrier to keep different body systems separate.
Examples: The skin, the lining of the mouth, the lining of blood vessels, lung alveoli and kidney tubules are all made of epithelial tissue.
- Epithelial tissue cells are tightly packed and form a continuous sheet. They have only a small amount of cementing material between them and almost no intercellular spaces.
- The cells of epithelia are permeable for regulation of exchange of materials between the body and the external environment and also between different parts of the body.
- Epithelium is separated from the underlying tissue by an extracellular fibrous basement membrane.
Simple Squamous Epithelium is extremely thin and flat and forms a delicate lining.
Examples - oesophagus and the lining of the mouth.
Stratified Squamous Epithelium is arranged in pattern of layers to prevent wear and tear.
Example - skin, which protects the body, is made of stratified squamous epithelium.
Columnar Epithelium consists of tall column like cells. The Columnar cells help in absorption and secretion.
Example – inner lining of the intestine.
Ciliated Epithelium: If the columnar epithelium also has cilia, hair-like projections on the outer surfaces of epithelial cells, it is called Ciliated Epithelium. The cilia can move, and their movement pushes the mucus forward to clear it.
Example – lining of the respiratory tract
Cuboidal Epithelium (with cube-shaped cells) forms the lining of kidney tubules and ducts of salivary glands, where it provides mechanical support.
Glandular epithelium: Epithelial cells which can secrete substances at the epithelial surface are called gland cells. When a portion of the epithelial tissue folds inward, a multicellular gland is formed which is called glandular epithelium.
(ii) Connective Tissue
Tissues that connect, support, bind, or separate other tissues or organs. It has two parts – matrix and the cells embedded in the matrix.
Bone cells are embedded in a hard matrix that is composed of calcium and phosphorus compounds. It is a strong and nonflexible tissue.
It forms the framework that supports the body. It also anchors the muscles and supports the main organs of the body.
The cartilage is a connective tissue with solid matrix composed of proteins and sugars. The cells are widely spaced. It is highly flexible.
Cartilage smoothens bone surfaces at joints and is also present in the nose, ear, trachea and larynx.
Tendons connect bones to muscles tissue. Tendons are fibrous tissue with great strength but limited flexibility.
Two bones can be connected to each other by a type of connective tissue called the ligament. This tissue is very elastic. It has considerable strength. Ligaments contain very little matrix.
Blood has a fluid (liquid) matrix called plasma, in which red blood cells (RBCs), white blood cells (WBCs) and platelets are suspended. The plasma contains proteins, salts and hormones. Blood flows and transports gases, digested food, hormones and waste materials to different parts of the body.
The lymph is formed when the interstitial fluid (the fluid which lies in the intercellular spaces of all body tissues) is collected through lymph capillaries. It is then transported through larger lymphatic vessels to lymph nodes, where it is cleaned by lymphocytes. This blood then is poured into the viens and mixes back with the blood.
(g) Areolar Connective Tissue
Areolar connective tissue is found between the skin and muscles, around blood vessels and nerves and in the bone marrow. It fills the space inside the organs, supports internal organs and helps in repair of tissues.
(h) Adipose Connective Tissue
Adipose tissue is found below the skin and between internal organs. The cells of this tissue are filled with fat globules. Storage of fats also lets it act as an insulator.
(iii) Muscular Tissues
Muscular tissue consists of elongated cells, also called muscle fibres. This tissue is responsible for movement in our body. Muscles contain special proteins called contractile proteins, which contract and relax to cause movement.
Striated muscles/skeletal muscles /voluntary muscles
The muscles, which can be moved by conscious will, are called voluntary muscles. For example muscles present in our limbs move when we want them to, and stop when we so decide.
These muscles are also called skeletal muscles as they are mostly attached to bones and help in body movement.
They have alternate dark and light bands. The cells of this tissue are long, cylindrical, unbranched and multinucleate (having many nuclei).
Unstriated muscles/smooth muscles /involuntary muscles
The muscles that contract and relax without conscious control are called involuntary muscles.
They are called smooth muscles or unstriatiated (dark bands/ light bands are absent).
Examples – Muscles in the alimentary canal, uterus, Iris of an eye.
The cells are long with pointed ends (spindle-shaped) and uninucleate (having a single nucleus).
The involuntary muscles of heart are called cardiac muscles. Heart muscle cells are cylindrical, branched and uninucleate.
Cells of the nervous tissue are highly specialised for being stimulated and then transmitting the stimulus very rapidly from one place to another within the body.
The brain, spinal cord and nerves are composed of the nervous tissue.
Neuron -The cells of nervous tissue are called nerve cells or neurons.
A neuron consists of a cell body with a nucleus and cytoplasm, from which long thin hair-like parts arise.
The long part is called the axon and short, branched parts are called dendrites. The dendrites receive nerve impulses from the receptors and the axon transmits impulses away from the cell body. The neuron is terminated in nerve endings, which transmit the message using chemicals.
An individual nerve cell may be up to a metre long.
Many nerve fibres bound together by connective tissue make up a nerve.