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CBSE Class 9 Science Chapter 6 Notes Tissues
Tissues Class 9 Notes Understanding the Lesson
1. A single cell performs all the basic functions like digestion, respiration, excretion, etc. in order to sustain life in the unicellular organisms like Amoeba. In multicellular organisms like human beings, each specialised function to sustain life is taken up by a different group of cells.
2. Division of labour: The multicellular organisms show division of labour as each function is carried out by a cluster of specialised cells at a definite place in the body.
For example:
In human beings, muscle cells contract and relax to cause movement, nerve cells carry messages, blood flows to transport oxygen, etc. In plants, vascular tissues called xylem and phloem conduct water and food respectively from one part of the plant to the other parts.
3. Tissue: A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue. Example: Blood, phloem and muscle. Plant tissues are classified as growing or meristematic tissue and permanent tissue.
4. Meristematic tissue: This tissue consists of cells which continuously divide to produce new cells. The cells of this tissue are very active, lack vacuoles, have dense cytoplasm, thin cellulosic cell walls and prominent nuclei.
5. Location of meristematic tissue: This tissue is present only at specific regions of the plant like the root tip, shoot tip and at the base of intemodes and leaves.
6. Types of meristematic tissue: They are classified as apical, lateral and intercalary meristematic tissue based on the region where they are present.
- Apical meristem: It is present at the growing tips of stems and roots and results in increase in the length of the stem and the root.
- Lateral meristem (cambium): It helps to increase the girth of the stem or root.
- Intercalary meristem: It is present at the base of the leaves or intemodes.
5. Permanent tissue: Consists of cells which have taken up a specific role and lost the ability to divide.
It is of two types:
- Simple tissue: It is made up of only one type of cells. Its three types are: parenchyma, collenchyma and sclerenchyma.
- Complex tissue: It is made up of more than one type of cells. They are the conducting tissues called xylem and phloem.
6. Differentiation: The process of taking up a permanent shape, size, and a function by the cells is called differentiation.
7. Types of simple tissue:
(a) Parenchyma: They are loosely packed living cells, with thin cell walls and large intercellular spaces. They provide support to plants and store food. It is called chlorenchyma if it contains chlorophyll and performs photosynthesis. The parenchyma of aquatic plants have large cavities to provide buoyancy to the plants to help them float. Such type of parenchyma is called aerenchyma.
(b) Collenchyma: It consists of living, elongated cells that are irregularly thickened at the corners and have a very little intercellular space. It allows easy bending in various parts of a plant (leaf, stem) without breaking. It also provides mechanical support to plants like in the leaf stalks below the epidermis.
(c) Sclerenchyma: This tissue consists of dead cells which makes the plant hard and stiff. The cells are long and narrow as the walls are thickened (often so thick that there is no internal space inside the cell) due to lignin (a chemical substance which acts as cement and hardens them). This tissue provides strength to the plants and is present in stems, around vascular bundles, in the veins of leaves and in the hard covering of seeds and nuts.
8. Epidermis: The outermost layer of cells covering an organism is called epidermis. It is usually made up of a single layer of cells and gives protection. The epidermis may be thicker in some plants living in dry habitats or often secrete a waxy, water-resistant layer on their outer surface called cutin (chemical substance with waterproof quality) to prevent water loss.
The epidermis of leaves have small pores called stomata which are enclosed by two kidney shaped cells called guard cells. Stomata help in gaseous exchange and transpiration. The epidermal cells of roots bear root hairs that greatly increase the total absorptive surface area of the roots for absorption of water.
9. Cork: A strip of secondary meristem replaces the epidermis of the older stem and cuts off cells towards outside to form a several-layer thick cork or the bark of the tree. Cells of cork are dead, compactly arranged without intercellular spaces and have a chemical called suberin in their walls that makes them impervious to gases and water.
10. Complex Permanent Tissue: These tissues are made of more than one type of cells which coordinate to perform a common function, e.g., Xylem and phloem. They are mainly conducting tissues and constitute a vascular bundle.
(a) Xylem: Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres. All the cells of xylem except the xylem parenchyma are dead. Xylem helps to transport water and minerals. Tracheids and vessels help in vertical transport whereas the parenchyma stores food and helps in the sideways conduction of water. Fibres are mainly supportive in function.
(b) Phloem: Phloem has four elements called sieve tubes, companion cells, phloem fibres and the phloem parenchyma. All cells of phloem are living except the phloem fibres. Phloem transports food from leaves to other parts of the plant.
11. Animal Tissues: The animal tissues are of four types: epithelial tissue, connective tissue, muscular tissue and nervous tissue.
12. Epithelial Tissue: They are the covering or protective tissues and cover most organs and cavities in the animal body. These cells are tightly packed, form a continuous sheet and are almost without any intercellular spaces between them. e.g., skin, the lining of the mouth, the lining of blood vessels, lung alveoli and kidney tubules are all made of epithelial tissue. All epithelium is usually separated from the underlying tissue by an extracellular fibrous basement membrane. The types of epithelium on the basis of their structure and functions are:
(a) Squamous epithelium: Consists of flattened cells. Present in oesophagus and lining of the mouth. Skin epithelial cells are arranged in many layers to prevent wear and tear and are called as stratified squamous epithelium.
(b) Columnar epithelium: Has tall or ‘pillar-like’ cells. It forms inner lining of the intestine.
(c) Cuboidal epithelium: Has cube-shaped cells. It forms the lining of kidney tubules and ducts of salivary glands, where it provides mechanical support.
(d) Ciliated epithelium: Have cilia on the outer surfaces of epithelial cells. The cilia can move and their movement pushes the mucus in the respiratory tract forward to clear it.
(e) Glandular epithelium: Has gland cells which secrete substances at the epithelial surface.
13. Connective Tissue: The cells of connective tissue are loosely spaced and embedded in an intercellular matrix which may be jelly like, fluid, dense or rigid, e.g., blood, bone, cartilage, etc.
(a) Blood: It has a fluid (liquid) matrix called plasma having red blood cells (RBCs), white blood cells (WBCs) and platelets. Blood helps in the transport of gases, digested food, hormones and waste materials to different parts of the body.
(b) Bone: It has bone cells embedded in a hard matrix composed of calcium and phosphorus compounds. It is a strong and non-flexible tissue which forms a framework that supports the body, anchors the muscles and supports the main organs of the body.
(c) Cartilage: It has widely spaced cells and a solid matrix composed of proteins and sugars. It helps to smoothen bone surfaces at joints and is also present in the nose, ear, trachea and larynx.
(d) Areolar connective tissue: It fills the space inside the organs, supports internal organs and helps in repair of tissues. It is found between the skin and muscles, around blood vessels and nerves and in the bone marrow.
(e) Adipose tissue: It is a fat storing tissue having cells filled with fat globules. It is found below the skin and between the internal organs.
(f) Ligament: It is the connective tissue which connects two bones. This tissue has very little matrix, is very elastic and has considerable strength.
(g) Tendon: It is the connective tissue which connects muscles to bones. It is a fibrous tissue with great strength but limited flexibility.
14. Muscular Tissue: This tissue is responsible for movement in our body and consists of elongated cells, also called muscle fibres. Muscles contain special proteins called contractile proteins, which contract and relax to cause movement.
(а) Striated Muscles: These muscles are also called skeletal muscles as they are mostly attached to bones and help in body movement. These muscles show alternate light and dark bands. These are long, cylindrical, unbranched and multinucleate. These are voluntary muscles as we can move them by conscious will, e.g., muscles of our limbs.
(b) Unstriated Muscles: They are also called smooth muscles or unstriated muscles as they do not have light and dark bands. The cells are long, uninucleate, involuntary in nature and spindle shaped. They are present in iris of the eye, ureters, blood vessels, alimentary canal and bronchi of lungs.
(c) Cardiac Muscles: These are the muscles of the heart which show rhythmic contraction and relaxation throughout life. They are involuntary, cylindrical, branched and uninucleate.
15. Nervous Tissue: The cells of this tissue are called nerve cells or neurons. Each neuron consists of a cell body with a nucleus and cytoplasm, a single long part called the axon, and many short branched parts called dendrites. The cells of the nervous tissue are highly specialised for transmitting the stimulus from one place to another within the body on being stimulated. The brain, spinal cord and nerves are composed of the nervous tissue. A nerve consists of many nerve fibres bound together by connective tissue.
Class 9 Science Chapter 6 Notes Important Terms
Tissue: A group of cells that are similar in structure and/or work together to achieve a particular function.
Meristematic tissue: This tissue that consists of cells which continuously divide to produce new cells.
Permanent tissue: The tissue which consists of cells which have taken up a specific role and lost the ability to divide.
Differentiation: The process of taking up a permanent shape, size, and a function by the cells is called differentiation.
Chlorenchyma: Parenchyma which contains chlorophyll and performs photosynthesis.
Aerenchyma: Parenchyma which contains large cavities to provide buoyancy to the aquatic plants to help them float.
Cambium: It is a lateral meristem which helps in increasing the girth of the stem or root.
Lignin: A chemical substance which acts as cement and hardens the cells of sclerenchyma.
Cutin: A chemical substance with waterproof quality present in epidermis of leaves to prevent water loss by transpiration.
Suberin: A chemical present in the walls of cork cells that makes them impervious to gases and water.
Adipose tissue: It is a fat storing tissue having cells filled with fat globules.
Ligament: It is the connective tissue which connects two bones. This tissue has very little matrix, is very elastic and has considerable strength.
Tendon: It is the connective tissue which connects muscles to bones. It is a fibrous tissue with great strength but limited flexibility.
Contractile proteins: Special proteins present in muscles which contract and relax to cause movement of body parts.
Nerve: A nerve consists of many nerve fibres bound together by connective tissue.