CLASS 9 Science CHAPTER 6 Tissues (NCERT Notes)
Unicellular organisms
Unicellular organisms are made up of single cell performs all basic functions. There in no division of labour between cell .For example, in Amoeba, a single cell carries out movement, intake of food, gaseous exchange and excretion.
Multi- cellular organisms
Multi- cellular organisms there are millions of cells. Most of these cells are specialised to carry out specific functions. 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. So, multi-cellular organisms show division of labour.
Cells specialising in one function are often grouped together in the body to carry out particular function by the cells are called tissue. Or 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 are all examples of tissues.
Are Plants and Animals Made of Same Types of Tissues?
There are noticeable differences between the two.
Plants are stationary or fixed – they don’t move. Since they have to be upright, they have a large quantity of supportive tissue. The supportive tissue generally has dead cells. 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. Based on the dividing capacity of the tissues, various
plant tissues can be classified as growing or meristematic tissue and permanent tissue.
Animals have 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. The growth in animals is limited. Cell
growth in animals is more uniform. The structural organisation of organs and organ systems is far more specialised and localised in complex animals than even in very complex plants.
Plant Tissues
MERISTEMATIC TISSUE
The growth of plants occurs only in certain specific regions. This is because the dividing tissue, also known as meristematic tissue, is located only at these points.
Depending on the region where they are present, meristematic tissues are classified as apical,
lateral and intercalary .
New cells produced by meristem as they grow and mature, their characteristics slowly change and they become differentiated as components of other tissues.
Apical meristem is present at the growing tips of stems and roots and increases the length of the stem and the root. The girth of the stem or root increases due to lateral meristem (cambium).
Intercalary meristem seen in some plants is located near the node.
Cells of meristematic tissue are very active, they have dense cytoplasm, thin cellulose walls and prominent nuclei. They lack vacuoles.
PERMANENT TISSUE
They take up a specific role and lose the ability to divide. As a result, they form a permanent tissue. 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.
(i) SIMPLE PERMANENT TISSUE
Cells beneath the epidermis are generally simple permanent tissue. Parenchyma is the most common simple permanent tissue. It consists of unspecialised 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.
In some situations, it contains chlorophyll and performs photosynthesis are called chlorenchyma.
In aquatic plants, large air cavities are present in parenchyma to help them float. Such a parenchyma type is called aerenchyma. The flexibility in plants is due to another permanent tissue, collenchyma. It allows bending of various parts of a plant like tendrils and stems of climbers without breaking. It
also provides mechanical support.
Tissue in leaf stalks below the epidermis are living, elongated and irregularly thickened at the
corners. There is very little intercellular space.
Sclerenchyma is the tissue which makes the plant hard and stiff. The husk of a coconut is made of sclerenchymatous tissue. The cells of this tissue are dead. They are long and narrow as the walls are thickened due to lignin. Often these walls are so thick that 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. It provides strength to the plant parts.
The outermost layer of cells, called epidermis. The epidermis is usually made of a single layer of cells. In some plants living in very dry habitats, the epidermis may be thicker since protection against water
loss is critical. The entire surface of a plant has an outer covering epidermis. It protects all the
parts of the plant.
Epidermal cells on the aerial parts of the plant often 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 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.
The epidermis of the leaf has small pores are called stomata . Stomata are enclosed by two kidney-shaped cells called guard cells. They are necessary for exchanging gases with the atmosphere.
Transpiration (loss of water in the form of water vapour) also takes place through stomata .
Epidermal cells of the roots, whose function is water absorption, commonly bear long hair-like parts that greatly increase the total absorptive surface area.
In some plants like desert plants, epidermis has a thick waxy coating of cutin (chemical substance with waterproof quality) on its outer surface.
As plants grow older, the outer protective tissue undergoes certain changes. A strip of secondary meristem located in the cortex forms layers of cells which constitute the cork. Cells of cork are dead and compactly arranged without intercellular spaces. They also have a substance called suberin in their
walls that makes them impervious to gases and water.
(ii) COMPLEX PERMANENT TISSUE
Complex tissues are made of more than one type of cells. All these cells coordinate to perform a common function. Xylem and phloem are examples of such complex tissues. They are both conducting tissues and constitute a vascular bundle. Vascular tissue is a distinctive feature of the complex plants,
one that has made possible their survival in the terrestrial environment.
Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres.
Tracheids and vessels have thick walls, and many are dead cells when mature. Tracheids and vessels are tubular structures. This allows them to transport water and minerals vertically. The parenchyma stores food. Xylem fibres are mainly supportive in function.
Phloem is made up of five types of cells: sieve cells, sieve tubes, companion cells, phloem fibres and the phloem parenchyma. Sieve tubes are tubular cells with perforated walls. Phloem transports food from leaves to other parts of the plant. Except phloem fibres, other phloem cells are living cells.
Animal Tissues
EPITHELIAL TISSUE
The covering or protective tissues in the animal body are epithelial tissues. Epithelium covers most organs and cavities within the body. It also forms a barrier to keep different body systems separate. 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. Anything entering or leaving the body must cross at least one layer of epithelium. Epithelia play an important role in regulating the exchange of materials between the body and the external environment and also between different parts of the body.
Different epithelia show differing structures that correlate with their unique functions.
Simple squamous epithelium: Cell lining blood vessels or lung alveoli, where transportation of substances occurs through a selectively permeable surface, there is a simple flat kind of epithelium. This is called the simple squamous epithelium (squama means scale of skin). Simple squamous epithelial cells are extremely thin and flat and form a delicate lining.
Stratified squamous epithelium. 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. Skin epithelial cells are arranged in many layers to prevent wear and tear. Since they are arranged in a pattern of layers, the epithelium is called stratified squamous epithelium.
Columnar epithelium: Where absorption and secretion occur, as in the inner lining of the intestine, tall epithelial cells are present. This columnar (meaning ‘pillar-like’) epithelium facilitates movement across the epithelial barrier.
Ciliated columnar epithelium: In the respiratory tract, the columnar epithelial tissue also has cilia, which are hair-like projections on the outer surfaces of epithelial cells. These cilia can move, and their movement pushes the mucus forward to clear it. This type of epithelium is thus ciliated columnar epithelium.
Cuboidal epithelium (with cube-shaped cells): It forms the lining of kidney tubules and ducts of salivary glands, where it provides mechanical support.
Epithelial cells often acquire additional specialisation 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.
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.
Blood has a fluid (liquid) matrix called plasma, in which red blood corpuscles (RBCs), white blood corpuscles (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.
Bone is another example of a connective tissue. It forms the framework that supports the body. It also anchors the muscles and supports the main organs of the body. It is a strong and nonflexible tissue . Bone cells are embedded in a hard matrix that is composed of calcium and phosphorus compounds.
Two bones can be connected to each other by another type of connective tissue called the ligament. This tissue is very elastic. It has considerable strength. Ligaments contain very little matrix and connect bones with bones.
Tendons connect muscles to bones and are another type of connective tissue. Tendons are fibrous tissue with great strength but limited flexibility.
Cartilage another type of connective tissue has widely spaced cells. The solid matrix is composed of proteins and sugars. Cartilage smoothens bone surfaces at joints and is also present in the nose, ear, trachea and larynx. We can fold the cartilage of the ears, but we cannot bend the bones in our arms.
Two different types of tissues are :
Areolar connective tissue: It 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.
Adipose tissue: Fats storeing tissue in our body are 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.
MUSCULAR TISSUE
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.
Based on their location, three types of muscles are identified :
Skeletal muscles are closely associated with the skeletal components of the body. They have a striped appearance under the microscope and hence are called striated muscles. As their activities are under the voluntary control of the nervous system, they are known as voluntary muscles too. The cells of this
tissue are long, cylindrical, unbranched and multinucleate (having many nuclei).They are primarily involved in locomotory actions and changes of body postures.
Visceral muscles are located in the inner walls of hollow visceral organs of the body like the alimentary canal, reproductive tract, etc. 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 do not exhibit any striation and are smooth in appearance. Hence, they are called smooth muscles (nonstriated muscle). Their activities are not under the voluntary control of the nervous system and are therefore known as involuntary muscles. They assist, for example, in the transportation of food through the digestive tract and gametes through the genital tract.
Cardiac muscles are the muscles of heart. Many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle. Based on appearance, cardiac muscles are striated. Heart muscle cells are
cylindrical, branched and uninucleate. They are involuntary in nature as the nervous system does not control their activities directly.
NERVOUS TISSUE
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 all composed of the nervous tissue. The cells of this 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. Usually each neuron has a single long part (process), called the axon, and many short, branched parts (processes) called dendrites. An individual nerve cell may be up to a metre long. Many nerve fibres bound together by connective tissue make up a nerve. The signal that passes along the nerve fibre
is called a nerve impulse. Nerve impulses allow us to move our muscles when we want to. The functional combination of nerve and muscle tissue is fundamental to most animals. This combination enables animals to move rapidly in response to stimuli.
( From NCERT Book )