tissue

TISSUES - Class 9 Science Notes

Overview of Tissues

  • Definition of Tissue: A group of similar cells in structure and/or function, working together to perform a specific function.

  • Hierarchy of Biological Organization: Cells → Tissues → Organs → Organ Systems → Organism.

Types of Tissues

1. Plant Tissues

  • Types of Plant Tissues:
      - Meristematic Tissues
      - Permanent Tissues

A. Meristematic Tissues

  • Definition: A group of undifferentiated plant cells capable of continuous division, responsible for growth in specific regions of the plant.

  • Locations:
      - Apical Meristem: Tips of roots and shoots; Function: Growth in length (Example: Root tip, shoot tip).
      - Intercalary Meristem: Base of leaves or internodes (mainly in grasses); Function: Regrowth of parts after cutting or grazing (Example: Grass stems).
      - Lateral Meristem (Cambium): Along the sides of stems and roots (in vascular bundles); Function: Growth in thickness/girth (Example: Vascular cambium, cork cambium).

  • Functions of Meristematic Tissue:
      - Enables growth in plants (length, girth, regeneration).
      - Produces new cells that later become specialized (differentiation).
      - Helps in wound healing and regeneration in plants.

  • Differences Between Meristematic and Permanent Tissue:
      - Meristematic tissues can divide, while permanent tissues cannot.
      - Cells of meristematic tissue are typically small and isodiametric while permanent tissue cells are large and shape varies.
      - Vacuoles are usually absent in meristematic tissue; present in living cells of permanent tissue.
      - Intercellular spaces are generally absent in meristematic tissue, while visible intercellular spaces are present in permanent tissue.

B. Permanent Tissue

  • Definition: Plant tissues formed from meristematic tissues by differentiation; cells lose ability to divide and perform specific functions.

(i) Simple Permanent Tissue

  1. Parenchyma:
       - Structure: Thin-walled, living cells; loosely packed with large intercellular spaces.
       - Functions: Stores food; Photosynthesis (when it contains chlorophyll); supports repair & healing.
         - Examples:
            - Chlorenchyma: Contains chloroplasts, involved in photosynthesis.
            - Aerenchyma: Contains large air cavities for buoyancy in aquatic plants.

  2. Collenchyma:
       - Structure: Living, elongated cells with unevenly thickened corners, very little intercellular space.
       - Functions: Provides flexibility (enabling bending without breaking); provides mechanical support.
       - Examples/Location: Leaf stalks, stems of climbers, below epidermis.

  3. Sclerenchyma:
       - Structure: Dead, long, narrow cells; cell walls thickened with lignin; lumen almost absent.
       - Functions: Provides strength, stiffness & hardness to plant parts.
       - Examples/Location: Coconut husk (fibres), seed/nut coverings, veins of leaves, around vascular bundles.

(ii) Complex Permanent Tissue

  1. Xylem:
       - Function: Transports water & minerals from roots to aerial parts (upward).
       - Structure: Composed of tracheids, vessels, xylem parenchyma, xylem fibres.
         - Types of cells:
           - Tracheids: Tubular, thick-walled, dead; help in conduction & support.
           - Vessels: Long, tubular, dead; main conducting elements for water and minerals vertically.
           - Xylem Parenchyma: Living cells that store food.
           - Xylem Fibres: Dead cells that provide mechanical support.

  2. Phloem:
       - Function: Transports food from leaves to other parts (upward & downward).
       - Structure: Composed of sieve tubes, companion cells, phloem parenchyma, phloem fibres.
         - Types of cells:
           - Sieve Tubes: Tubular cells with perforated walls (sieve plates).
           - Companion Cells: Living cells that assist sieve tubes in functioning.
           - Phloem Parenchyma: Living cells that store food.
           - Phloem Fibres: Dead cells providing mechanical support.

2. Protective Tissues

  • Epidermis:
      - Structure: Outermost protective covering, usually a single layer of cells.
      - Functions: Protects against water loss, injury, and fungal infection.
      - Special Features:
        - Stomata: Small pores in leaf epidermis guarded by 2 kidney-shaped guard cells, allowing gaseous exchange & transpiration.
        - Root Hairs: Increase surface area for water absorption.

  • Cork (Phellem):
      - Structure: Formed in older stems/roots by secondary meristem (cork cambium).
      - Cells: Dead, compactly arranged, no intercellular spaces; walls contain suberin making them waterproof and impervious to gases.
      - Function: Provides protection in older plants.

3. Animal Tissues
A. Epithelial Tissue

  • Definition: Covering/protective tissue in animals; covers organs, cavities, skin, blood vessels, etc.

  • Characteristics: Cells tightly packed, form a continuous sheet with very little intercellular space.

  • Functions: Protective covering, separation of systems, exchange of materials (absorption, secretion, transport).

  • Types:
      - Squamous Epithelium: Thin, flat; lining of blood vessels, alveoli; ideal for transport/exchange.
      - Stratified Squamous Epithelium: Many layers; skin, mouth; serves protective function against wear & tear.
      - Cuboidal Epithelium: Cube-shaped; found in kidney tubules and salivary ducts; provides mechanical support.
      - Columnar Epithelium: Tall cells; lining of intestines; involved in absorption & secretion.
      - Ciliated Epithelium: Columnar + cilia; present in the respiratory tract; aids in clearing mucus.

B. Muscular Tissue

  • Definition: Made of elongated cells (muscle fibres); responsible for movement.

  • Types:
      - Striated (Skeletal) Muscle:
        - Location: Attached to bones; voluntary movement.
        - Structure: Long, cylindrical, unbranched fibres; multinucleate; show striations (light & dark bands).
        - Functions: Rapid, voluntary actions, but fatigues quickly.

  - Smooth (Unstriated) Muscle:
    - Location: Found in alimentary canal, blood vessels, iris, bronchi; involuntary.
    - Structure: Spindle-shaped fibres with pointed ends; uninucleate; no striations.
    - Functions: Slow, sustained involuntary actions; does not fatigue easily.

  - Cardiac Muscle:
    - Location: Found in heart; involuntary.
    - Structure: Cylindrical, branched fibres; uninucleate; show faint striations.
    - Functions: Rhythmic, continuous contraction & relaxation; does not fatigue throughout life.

C. Connective Tissue

  • Definition: Connective tissues connect, support, and bind other body tissues; provides structural framework;

  • Types and Their Functions:
      1. Tendons: Connective tissue that attaches muscles to bones; tough and non-elastic; helps in movement.
      2. Ligaments: Connective tissue that attaches bones to bones; elastic and strong; helps in stability.
      3. Bone: Hard and non-flexible; porous; contains blood vessels and matrix made of proteins and mineral salts.
      4. Cartilage: Flexible and non-porous; contains matrix made of proteins, without blood vessels.
      5. Areolar Tissue: Most widely distributed connective tissue; fills spaces between organs; helps in tissue repair.
      6. Adipose Tissue: Made of adipocytes (fat cells); stores fat; helps in temperature regulation and is found beneath skin and around organs.
      7. Blood: Fluid connective tissue that transports gases, nutrients, hormones, and waste materials; provides protection against diseases.
         - Components of Blood:
           - Plasma: Fluid part carrying dissolved substances.
           - Red Blood Cells (RBCs/Erythrocytes): Contain hemoglobin; transport oxygen.
           - White Blood Cells (WBCs/Leucocytes): Provide defense against infection.
           - Platelets (Thrombocytes): Help in blood clotting.

4. Nervous Tissue

  • Definition: Specialized to respond to stimuli and transmit messages throughout the body.

  • Basic Unit: Neuron (nerve cell).

  • Structure of Neurons:
      - Dendrites: Branched structures receiving signals from previous neurons and pass them to the cell body.
      - Cell Body (Soma): Contains nucleus and organelles; controls cell activities.
      - Axon: Long fibre carrying nerve impulses away from the cell body to nerve endings.
      - Nerve Endings: Terminal parts of axon transmitting signals to the next neuron/target cell.
      - Nerve Impulse: Signal traveling along a nerve fibre; passes from axon terminals to dendrites of the next neuron.
      - Synapse: Gap between axon of one neuron and dendrite of the next neuron; the signal changes from electrical to chemical and back to electrical in the next neuron.

Revision Questions

  • Question 1: Which connective tissue connects muscles to bones?
      - A) Ligament
      - B) Tendon
      - C) Cartilage
      - D) Areolar

  • Question 2: Which permanent tissue provides flexibility to plants?
      - (a) Parenchyma
      - (b) Collenchyma
      - (c) Sclerenchyma
      - (d) Phloem

Definition of Tissue: A group of similar cells in structure and/or function, working together to perform a specific function.

Types of Tissues

  1. Plant Tissues 1.1 Meristematic Tissues:

    • Undifferentiated and can divide for growth.

    • Apical: Growth in length (e.g., root and shoot tips).

    • Intercalary: Regrowth after cutting (e.g., grass stems).

    • Lateral: Growth in thickness (e.g., vascular cambium).
      1.2 Permanent Tissues:

    • Differentiated and perform specific functions; cannot divide.

    • Simple Permanent Tissues:

      • Parenchyma: Stores food, performs photosynthesis.

      • Collenchyma: Provides flexibility and mechanical support.

      • Sclerenchyma: Provides strength and hardness.

    • Complex Permanent Tissues:

      • Xylem: Transports water and minerals upward.

      • Phloem: Transports food upward and downward.

  2. Protective Tissues:

    • Epidermis: Protects against water loss and injury.

    • Cork: Protective layer for older stems/roots.

  3. Animal Tissues 3.1 Epithelial Tissue: Covers organs and cavities, involved in absorption/secretion. 3.2 Muscular Tissue: Responsible for movement. Types:

    • Striated: Voluntary, rapid movements (e.g., skeletal).

    • Smooth: Involuntary, sustained actions (e.g., digestive tract).

    • Cardiac: Involuntary, rhythmic contractions (heart).
      3.3 Connective Tissue: Supports, binds, and connects other tissues. Types include tendons, ligaments, bone, and blood.
      3.4 Nervous Tissue: Transmits signals and responds to stimuli; neurons are basic units.