CBSE Notes Class 9 Science Chapter 6 - Tissue

Tissues are groups of similar cells that perform a common function. For example, blood, muscle, and Phloem are all tissues that are specialized for particular tasks. In essence, tissues are clusters of cells designed to work together to achieve maximum efficiency in their functions.

1.0Plant Tissues

• Plants are stationary so they require more supportive tissue for obtaining structural strength. 
• Most of these tissues are dead and hence they provide more mechanical strength and they require less energy for their maintenance. 
• The growth of plants is limited to certain regions. In these certain regions, growing tissues are present, which have the capacity to divide throughout life. 

2.0Classification of Plant Tissues

Meristematic tissue

• These are living tissues which are capable of division throughout life. These tissues are found in growing regions of plants. 
• Cells have thin cell walls, and are compactly packed with no intercellular space. Cells contain dense cytoplasm and do not have vacuoles. 
• Cells contain prominent and large nucleus.

Classification of meristematic tissue

(i) Apical meristem - It is present at the growing tips of shoot and roots. They are responsible for increase in the length of shoot and root.

(ii) Intercalary meristem - These are present at the base of leaf or internode (on either side of the node).They are responsible for the growth in length of plant organs.

(iii) Lateral meristem (cambium) - It lies on the lateral sides of stem and root or occurs along the sides of the longitudinal axis of the plant. It helps in increasing the diameter (girth) of the plant, hence helps in secondary growth. 

Permanent tissue 

• They are formed by division and differentiation of meristematic tissue. 
• They are composed of those cells which have lost the power of division (temporarily or permanently) and attain a permanent shape, size and function. 
• Cells may be living or dead. Permanent tissues are of two types, simple permanent tissue and compound or complex permanent tissue.

1. Simple Permanent Tissue

• Simple permanent tissue is made of one cell type and serves functions like support and storage. 
• The most common type is Parenchyma, which consists of loosely arranged living cells with thin cell walls. 
• It often stores food or contains Chlorophyll for photosynthesis, called Chlorenchyma. 
• Aerenchyma—a type of Parenchyma with air cavities—helps the plant float in aquatic plants.
• Collenchyma is another simple permanent tissue that provides flexibility and mechanical support to plants. 
• It allows the bending of plant parts like tendrils and climbers without breaking. 
• Lastly, sclerenchyma is made of dead cells, giving plants a hard, stiff structure. 
• It is found in stems, seed coverings, and other tough plant parts.

2. Complex Permanent Tissue

• Complex permanent tissue consists of more than one type of cell. 
• Examples include Xylem and Phloem, which transport water, nutrients, and food throughout the plant. 
• The xylem comprises tracheids, vessels, xylem parenchyma, and fibers, while the Phloem comprises sieve cells, companion cells, and phloem parenchyma. 
• These tissues work together to support the plant's survival in a terrestrial environment.
• Xylem or wood : Xylem is made up of four types of cells.
• Tracheids : Tracheids are elongated cells with tapering ends. Since tracheids do not have open ends like the vessels, the water has to pass from cell to cell via the pits. Tracheids are the chief water conducting elements in non-flowering plants. i.e. gymnosperms. 
• Vessels or tracheae : Very long tube-like structures formed by a row of cells placed end to end. Vessels are present in flowering plants i.e. angiosperms only. The transverse walls between the vessels are completely dissolved to form continuous channels or water-pipes. Tracheids and vessels help in long distance conduction of water and minerals vertically upward from the root system to various parts of the plant. Tracheids and vessels also provide mechanical support. 
• Xylem fibre : These are dead and lignified sclerenchymatous cells which are mainly supportive in function. 
• Xylem parenchyma : It is formed of living parenchymatous cells which helps in storage of food and radial conduction of water and minerals.
• Phloem : It is also made up of four types of cells
• 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. Wall of the sieve tube is perforated. 
• 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. 
• Phloem parenchyma : These are living and thin walled cells which helps in sideways conduction of food. It stores various materials like resin, latex and mucilage. 
• Phloem fibre : Phloem fibres are dead sclerenchymatous cells. Phloem or bast fibres of some plants are the source of commercial fibres e.g. Jute, Hemp, Flax.

Protective tissue

1. Epidermis 

• It is the outermost layer of all organs of the plant body which is formed from parenchymal cells. In epidermal cells outer walls are thicker than inner walls. 
• It is mostly single layer but in desert plants it is multilayered for protection against water loss. 
• It protects the internal tissue from mechanical injuries and entry of germs.

2. Cork or phellem 

• Cork is the peripheral tissue of old stems and roots of woody trees and is formed due to activity of cork cambium or phellogen (secondary lateral meristem). 
• Cork cambium (phellogen) produces new cells on its both sides, thus forming cork (phellem) on the outer side and the secondary cortex or phelloderm on the inner side. 
• It is made up of dead cells with thick walls but no intercellular spaces. 

3.0Animal Tissues

Every animal, whether it is unicellular or multicellular, is capable of performing all vital functions such as respiration, ingestion, excretion and reproduction. Types of animal tissue : Based on the location and function, the animal tissues are classified into four type

1. Epithelial Tissue

• Word epithelium is composed of two words Epi-upon, Thelio-grows. (Means - A tissue which grows upon another tissue is called epithelium).
• It is the simplest tissue. It is the protective tissue of an animal's body. 
• It covers most organs and cavities within the body. It also forms a barrier to keep different body systems separate. 
• Epithelial cells are closely packed and have a small amount of cementing material, so there are very little intercellular spaces present between the cells. 
• Due to absence or less of intercellular spaces; blood vessels, lymph vessels and capillaries are unable to pierce this tissue, so blood circulation is absent in epithelium. 
• Hence cells depend for their nutrients on the underlying connective tissue. 

Types of epithelial tissue on the basis of shapes and functions 

Type : Squamous epithelium (scale like) 

• Description : Flattened cells, extremely thin. 
• Common locations : Walls of blood vessels, air sacs of lungs, esophagus, lining of mouth. 
• Function : Diffusion 

Type : Cuboidal epithelium 

• Description : Cube-like cells may have microvilli at its free surface. 
• Common locations : Lining of kidney tubules, ducts of salivary glands. It also forms the germinal epithelium of gonads. 
• Function : Secretion, absorption, mechanical support, excretion. 

Type : Columnar (Pillar like) 

• Description : Tall slender cells ; may have microvilli at its free surface. 
• Common locations : Inner lining of intestine, part of respiratory tract lining. 
• Function : Secretion, absorption. 

Modification of columnar epithelium 

Glandular epithelium 

• Description : Tall, slender cells, some cells from the free surface invaginate inside to form secretory cells – goblet cells. 
• Common location : Lining of intestine & glands, trachea, bronchi. 
• Function : Secretion of mucus and other secretions. 

Ciliated columnar epithelium 

• Description : Tall , slender cells which possess cilia. 
• Common location : Lines the nasal passages, oviducts, terminal bronchioles. 
• Function : Protection, movement of substances in a particular direction for e.g. of mucus in nasal passages, egg in oviduct.

2. Connective tissue -  The cells of connective tissue are loosely spaced and embedded into a non cellular matrix. The matrix may be solid (as in bone), soft (as in loose connective tissue), or liquid (as in blood).

Loose connective tissue

• Loose connective tissue is a mass of widely scattered cells whose matrix is a loose weave of fibres. Many of the fibres are strong protein fibres called collagen. 
• Loose connective tissue is found beneath the skin and between organs. 
• It is a binding and packing material whose main purpose is to provide support to hold other tissues and organs in place.

Adipose tissue

• It consists of adipose cells (Adipocytes) filled with fat globules in loose connective tissue. 
• Each adipose cell stores a large droplet of fat that swells when fat is stored and shrinks when fat is used to provide energy. 
• Adipose tissue pads and insulates the animal body. 

Fluid / vascular connective tissue 

• It is a special type of connective tissue which maintains link among different parts of the body. 
• It receives materials from certain parts of the body and transports them to the other parts. It constitutes the transport system of animals. 
• It consists of two basic components – blood and lymph. Blood Blood is a connective tissue of cells separated by a liquid matrix called plasma. 
• It lacks fibers in its matrix. 
• Blood constitutes 55 percent of plasma and 45 percent blood corpuscles. 
• Plasma contains water, salts, sugars, lipids, amino acids etc. 
• Blood corpuscles are of 3 types : 

(i) Red blood cells or erythrocytes. 

(ii) White blood cells or leukocytes                                                                                             (iii) Platelets or thrombocytes. 

Lymph 

• Lymph is actually filtered blood which is similar to blood in composition except that it is devoid of RBC, platelets and some blood protein. 
• WBC are present in abundance in lymph. Due to the absence of hemoglobin, lymph is colorless.

Fibrous / dense regular connective tissue

• This tissue consists mainly of fibers. The fibers are of 2 types : 
• (1) Collagen white fibers The dominance of the (white inelastic) collagen fibers contributes to the considerable mechanical strength of white fibrous tissue. 
• (2) Elastin yellow fibers 
• It can stretch upto one and a half times their length then snap back to its original length when relaxed. 

Skeletal connective tissue

• It mainly consists of bones and cartilages that provide a strong supportive framework for the body. 
• In these connective tissues, the matrix contains numerous fibers and in some cases, deposits of insoluble calcium salts. 
• Bone It is a rigid connective tissue that has a matrix of collagen fibers and salt of calcium and phosphorous compounds, giving it greater rigidity and strength. 
• Most of the skeletal system is bone. Haversian systems are the structural units of mammalian bone. It consists of Haversian canals, Haversian lamellae and osteocytes.

Muscular Tissue

• Muscular tissue is distinguished from other tissues by its unique ability to contract & relax and thereby perform mechanical work. 
• It is responsible for movement of body organs and locomotion of the body. 
• The structural unit of muscular tissue is the muscle cells which because of its elongated shape is also called muscle fibre. 
• The contractility is due to the presence of contractile proteins (actin & myosin) in the muscle fibre.

Nervous Tissue

• The nervous tissue, containing densely packed cells called nerve cells or neurons, is present in the brain, spinal cord and nerves. 
• The neurons are specialized for conduction of nerve impulses. 
• They receive stimuli from within or outside the body and conduct impulses (signals) which travel from one neuron to another neuron. 
• 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 stimulus. Each neuron has the following 2 parts 
• (i) Cyton or cell body : Contains a central nucleus and cytoplasm with characteristic deeply stained particles called Nissl's granules (i.e. clumps of ribosomes and rough endoplasmic reticulum). 
• (ii) Cell Processes 
• (a) Dendrites : These may be one to many, generally short and branched cytoplasmic processes. Dendrites are afferent processes because they receive impulse from a receptor or other neuron and bring it to the cyton. 
• (b) Axon : It is a single generally long efferent process which conducts impulse away from cyton to other neurons. Longest cell in the body is the neuron because the axon can be more than one meter long. Axon has uniform thickness but it has terminal thin branches called telodendria. Terminal end buttons or synaptic knobs occur at the end of telodendria.