Class 9th Tissues Lecture Overview

Introduction to Tissues and Structural Organization

• Definition of Tissue: A group of cells having similar shape and function that act together to perform a specific function is known as a tissue.

• Hierarchy of Structural Organization: The organization of living organisms follows a specific hierarchy:

  • $CELLS \rightarrow TISSUES \rightarrow ORGANS \rightarrow ORGAN SYSTEM \rightarrow ORGANISM$

• Utility in Multicellular Organisms: Tissues provide division of labor. In multicellular organisms, different groups of cells are specialized to carry out specific functions efficiently, ensuring the survival and coordination of the organism.

Plant Tissues: Classification and Meristematic Tissue

Plant tissues are broadly classified into two categories: Meristematic Tissue and Permanent Tissue.

• Meristematic Tissue: These tissues consist of cells that are actively dividing and contributing to the growth of the plant.

  • Characteristics of Meristematic Cells:

    • The cells have the capacity to divide continuously.

    • They are small in size and isodiametric in shape.

    • Vacuoles are usually absent because these cells are metabolically active and do not need to store food or waste.

    • Intercellular spaces are generally absent; cells are compactly arranged.

    • Cell walls are thin and made of cellulose.

    • They possess a prominent nucleus and dense cytoplasm.

  • Classification Based on Location:

    • Apical Meristem: Found at the growing tips of stems and roots (Shoot Apical Meristem - SAM and Root Apical Meristem - RAM). They facilitate the increase in length of the plant, a process known as primary growth.

    • Intercalary Meristem: Located at the base of leaves or between nodes on branches. They help in the elongation of internodes and leaves.

    • Lateral Meristem: Located on the lateral sides of stems and roots. They help the plant parts become thicker, which is known as secondary growth. This increases the girth of the stem or root.

Permanent Plant Tissues

• Definition: Permanent tissues are composed of cells that have lost their capability to divide and have attained a definite shape, size, and function through the process of differentiation.

• Characteristics:

  • Cells may be living or dead.

  • Vacuoles are present in living cells.

  • Intercellular spaces are often visible.

  • Cell walls can be either thin or thick.

• Simple Permanent Tissue: Composed of only one type of cell.

  • Parenchyma:

    • Forms the basic packing tissue.

    • Cells are relatively unspecialized, living, and have thin cell walls.

    • Usually loosely packed with large intercellular spaces.

    • Functions: Provides support and stores food in roots and stems.

    • Specialized Types:

      • Chlorenchyma: Parenchyma containing chlorophyll that performs photosynthesis.

      • Aerenchyma: Found in aquatic plants; contains large air spaces to provide buoyancy, helping them float.

  • Collenchyma:

    • Found in leaf stalks (petioles) just below the epidermis.

    • Cells are living, elongated, and irregularly thickened at the corners.

    • Very little intercellular space exists.

    • Function: Provides mechanical support and flexibility, allowing plant parts to bend without breaking.

  • Sclerenchyma:

    • Makes the plant hard and stiff (e.g., the husk of a coconut).

    • Cells are dead, long, and narrow.

    • Cell walls are heavily thickened with lignin, a chemical substance that acts as cement.

    • There is no internal space inside the cells due to the thick walls.

    • Location: Found in stems, around vascular bundles, in leaf veins, and hard coverings of seeds and nuts.

Complex Permanent Tissue: Xylem and Phloem

Complex tissues are made of more than one type of cell that work together as a unit.

• Xylem (Wood):

  • Function: Conducts water and minerals vertically from roots to other parts. Mostly unidirectional.

  • Elements of Xylem:

    • Tracheids: Elongated, angular, dead cells with pits. Primitive elements mainly responsible for water conduction in gymnosperms.

    • Vessels: Advanced elements (found in angiosperms); cylindrical, tube-like structures joined end-to-end to form a continuous channel.

    • Xylem Parenchyma: Small, thick-walled living cells involved in the storage of starch (food) and lateral conduction.

    • Xylem Sclerenchyma (Xylem Fibres): Non-living fibres with thick walls that provide mechanical support.

  • Note: All xylem elements are dead except for xylem parenchyma.

  • Annual Rings: The rings seen in a tree trunk are xylem rings; the age of a tree can be determined by counting these rings.

• Phloem:

  • Function: Transports organic solutes (food) from leaves to storage organs and growing parts. Direction of conduction is bidirectional.

  • Elements of Phloem:

    • Sieve Tubes: Slender, tube-like structures with thin walls and perforated end walls called sieve plates. Mature sieve cells lack a nucleus but keep their cytoplasm.

    • Companion Cells: Living cells with dense cytoplasm and prominent nuclei. They are "sister cells" to sieve tubes and help regulate their function through plasmodesmata.

    • Phloem Parenchyma: Living cells that store food and assist in radial conduction.

    • Phloem Fibre: The only dead element in phloem; provides mechanical support to the sieve tubes.

Protective Plant Tissues

• Epidermis:

  • The outermost single layer of cells covering the entire plant.

  • Functions: Protects against water loss, mechanical injury, and invasion by parasitic fungi.

  • Adaptations: Desert plants (xerophytes) have a thick waxy coating of cutin (cuticle) to reduce transpiration. In roots, epidermal cells often have long hair-like structures to increase surface area for water absorption.

  • Stomata: Small pores in the epidermis of leaves, enclosed by two kidney-shaped guard cells, which regulate gas exchange and transpiration.

• Cork (Bark):

  • As plants grow older, the outer protective tissue (epidermis) is replaced by a secondary meristem, forming several layers of thick cork.

  • Cork cells are dead and compactly arranged without intercellular spaces.

  • Walls contain suberin, a substance that makes them impervious to gases and water.

  • Function: Protects against infection and water loss.

Animal Tissues: Epithelial and Muscular

• Epithelial Tissue:

  • Protective tissue that covers external surfaces and lines internal organs and cavities.

  • Cells are tightly packed with a non-cellular basement membrane underneath. It is avascular (lacks blood vessels).

  • Types:

    • Squamous Epithelium: Thin, flat cells. Found in the lining of the mouth, esophagus, blood vessels, and lung alveoli.

    • Stratified Squamous Epithelium: Multiple layers to prevent wear and tear; found in the skin.

    • Cuboidal Epithelium: Cube-shaped cells. Found in kidney tubules (nephrons) and salivary glands. Provides mechanical support.

    • Columnar Epithelium: Tall, pillar-like cells. Found where absorption and secretion occur, such as the inner lining of the intestine.

    • Ciliated Epithelium: Columnar cells with hair-like cilia. Found in the respiratory tract (trachea, bronchi) and fallopian tubes to move particles/mucus forward.

• Muscular Tissue:

  • Consists of long cells called muscle fibres. Responsible for movement via contraction and relaxation.

  • Types:

    • Striated Muscle (Skeletal/Voluntary): Attached to bones. Cells are long, cylindrical, unbranched, and multinucleated. Shows light and dark bands (striations). They move according to our will but get tired easily.

    • Non-Striated Muscle (Smooth/Involuntary): Found in the walls of the stomach, intestine, iris, and bronchi. Cells are spindle-shaped (tapering ends) and uninucleated. They control involuntary movements like peristalsis.

    • Cardiac Muscle: Found only in the heart. Cells are cylindrical, branched, and uninucleated with intercalated discs. They perform rhythmic contraction and relaxation throughout life without fatigue.

Connective Tissue

• Blood: A fluid connective tissue consisting of:

  • Plasma ($55\%$): Contains $90-91\%$ water, $7\%$ proteins (Albumin, Fibrinogen, Globulin), and $0.9\%$ inorganic salts.

  • Corpuscles ($45\%$):

    • RBCs (Erythrocytes): Contain haemoglobin for oxygen transport.

    • WBCs (Leukocytes): "Soldiers of the body," phagocytic cells (Monocytes, Lymphocytes, Basophils, Neutrophils, Eosinophils) that protect against infection.

    • Platelets (Thrombocytes): Spindle-shaped cells involved in blood clotting.

• Tendons and Ligaments:

  • Tendons: Connect muscle to bone. They are tough, non-elastic, and white in color.

  • Ligaments: Connect bone to bone. They are strong, elastic, and yellow (due to elastin).

• Bone and Cartilage:

  • Bone: Hard, non-flexible, porous, and has blood vessels. Matrix is made of proteins and mineral salts (Calcium/Phosphorous).

  • Cartilage: Flexible, not very hard, non-porous, and lacks blood vessels. Matrix is made of proteins. Found in the nose, ears, and joints.

• Areolar Tissue: Fills space inside organs; found between skin and muscles, around blood vessels and nerves.

• Adipose Tissue: Stores fat in cells called adipocytes. Acts as an insulator and is found below the skin and around internal organs (e.g., kidneys).

Nervous Tissue

• Function: Specialized to receive and transmit stimuli rapidly.

• Neuron: The structural and functional unit of the nervous system. It is the longest cell in the human body.

• Parts of a Neuron:

  • Dendrites: Branched parts that collect information from other neurons.

  • Cell Body (Cyton/Soma): Contains the nucleus, mitochondria, and other organelles.

  • Axon: A long, tube-like structure that carries information away from the cell body.

  • Nerve Endings: Transmit the signal to the next neuron or target cell.

Comparison: Plant vs. Animal Tissues

Questions & Discussion

• Q: How do meristematic tissues contribute to healing in plants?

  • A: Through rapid cell division, they generate new cells to seal the damaged area and restore structural integrity.

• Q: What is the utility of tissues in multicellular organisms?

  • A: They provide division of labor and efficient functioning by specializing in specific tasks.

• Q: Describe the structural differences between tendons and ligaments.

  • A: Tendons connect muscle to bone and are tough/non-elastic; ligaments connect bone to bone and are strong/elastic.

• Q: Why do meristematic cells lack vacuoles?

  • A: They are in a constant state of division and high metabolic activity, thus they do not need to store food or waste products.

• Q: What determines the age of a tree?

  • A: By counting the number of annual xylem rings in the trunk.

• Fill in the blanks (Solutions):

  1. The lateral meristem helps to increase the girth.

  2. The process of taking permanent shape/size/function is differentiation.

  3. Collenchyma provides flexibility.

  4. Coconut husk is sclerenchyma.

  5. Stomata are enclosed by guard cells.

  6. Phloem parenchyma helps in sideways conduction.

  7. Tendon connects muscles to bones.

  8. Heart muscle cells are cylindrical and branched/uninucleate.

• Matching/Naming:

  • Lining of blood vessels: Squamous epithelium.

  • Lining of small intestine: Columnar epithelium.

  • Lining of kidney tubules: Cuboidal epithelium.

  • Epithelial cells with cilia: Respiratory tract (Trachea/Bronchi).