Class 9 Science Notes Chapter 6 (Chapter 6) – Examplar Problem (English) Book

Alright class, let's get started with Chapter 6, 'Tissues'. This is a crucial chapter, forming the basis of how complex organisms like plants and animals are structured and function. Understanding tissues is vital not just for your Class 9 exams, but also for various competitive government exams where biology is a component. We'll cover the key concepts from your NCERT Exemplar book.

Chapter 6: Tissues - Detailed Notes for Government Exam Preparation

1. Introduction to Tissues

• Definition: A tissue is a group of cells similar in structure and/or working together to perform a particular function.
• Importance: In multicellular organisms, cells specialize to perform specific functions efficiently. This specialization leads to the formation of tissues, organs, and organ systems, enabling a division of labour and higher organization.
• Difference between Plant and Animal Tissues: Key differences arise due to their different modes of life (sedentary vs. mobile) and nutrition (autotrophic vs. heterotrophic).
  • Plants: Have tissues for support (often dead cells providing mechanical strength) and growth restricted to specific regions (meristems). Energy needs are lower.
  • Animals: Have tissues for movement, requiring more energy. Growth is generally more uniform. Tissues are adapted for locomotion, feeding, and response to stimuli.

2. Plant Tissues

Plant tissues are broadly classified into two main types: Meristematic and Permanent.

A. Meristematic Tissues (Tissues with Dividing Cells)

• Characteristics: Composed of actively dividing cells, responsible for growth in plants. Cells are small, isodiametric, with dense cytoplasm, thin cellulose walls, prominent nuclei, and lack vacuoles (or have very small ones).
• Types based on Location:
  • Apical Meristem: Located at the tips of roots and shoots (stems/branches). Responsible for increase in length (primary growth).
  • Lateral Meristem (Cambium): Found beneath the bark (in dicots) and in vascular bundles. Responsible for increase in the girth or diameter of the stem and root (secondary growth). Examples: Cork cambium, Vascular cambium.
  • Intercalary Meristem: Located at the base of leaves or internodes (e.g., in grasses, bamboo). Helps in longitudinal growth, especially after parts are removed by grazing animals.

B. Permanent Tissues (Tissues with Differentiated Cells)

• Characteristics: Derived from meristematic tissues; cells lose the ability to divide and take up specific roles (differentiation). They have a definite shape, size, and function. Can be simple or complex.

• I. Simple Permanent Tissues: Composed of only one type of cell, performing a common function.

  • Parenchyma:
    • Structure: Living cells, relatively unspecialized, thin-walled (cellulose), usually loosely packed with large intercellular spaces.
    • Location: Found throughout the plant body (cortex, pith, mesophyll of leaves, packing tissue in xylem and phloem).
    • Function: Storage (food, water), photosynthesis (if containing chloroplasts - called Chlorenchyma), provides buoyancy in aquatic plants (if large air cavities are present - called Aerenchyma), support.
  • Collenchyma:
    • Structure: Living cells, elongated, irregularly thickened at the corners (pectin and cellulose deposition), very little intercellular space.
    • Location: Found in leaf stalks, below the epidermis in stems (especially young dicot stems). Absent in monocots, roots, and older stems.
    • Function: Provides mechanical support and flexibility to growing parts like stems and leaves, allowing bending without breaking.
  • Sclerenchyma:
    • Structure: Dead cells, long and narrow or irregular shape, walls are heavily thickened with lignin (a chemical substance acting like cement), no intercellular spaces. Lumen (internal cavity) is very narrow.
    • Location: Found in stems (around vascular bundles), veins of leaves, hard coverings of seeds and nuts (e.g., coconut husk, grit in pear fruit).
    • Function: Provides mechanical strength, rigidity, and hardness to the plant parts.
    • Types: Fibres (long, pointed ends) and Sclereids (short, irregular shape, e.g., stone cells).

• II. Complex Permanent Tissues: Composed of more than one type of cell, working together as a unit. They transport water, minerals and food. Also called vascular or conducting tissues. Form vascular bundles (usually along with cambium in dicots).

  • Xylem:
    • Function: Conducts water and dissolved minerals upwards (unidirectional) from roots to leaves; also provides mechanical support.
    • Components:
      • Tracheids: Elongated, dead cells with lignified walls and tapering ends; conduct water through pits.
      • Vessels (or Tracheae): Tube-like structures formed by end-to-end fusion of dead cells (vessel members) with perforated or dissolved end walls; main water-conducting elements in angiosperms.
      • Xylem Parenchyma: Living cells; store food and help in lateral conduction of water.
      • Xylem Fibres (Sclerenchyma): Dead cells; provide mechanical support.
    • Note: Except for xylem parenchyma, all other xylem components are dead.
  • Phloem:
    • Function: Conducts prepared food (sugars produced during photosynthesis) from leaves to other parts (storage organs, growing regions) – transport can be bidirectional.
    • Components:
      • Sieve Tubes: Living, elongated, tube-like cells with perforated end walls called sieve plates. Lack nucleus at maturity. Transport sugars.
      • Companion Cells: Living parenchyma cells closely associated with sieve tube elements; have a prominent nucleus and dense cytoplasm; regulate the activity of sieve tubes.
      • Phloem Parenchyma: Living cells; store food and help in lateral transport.
      • Phloem Fibres (Bast Fibres): Dead sclerenchyma cells; provide mechanical strength (e.g., Jute, Hemp fibres).
    • Note: Except for phloem fibres, all other phloem components are living.

• III. Protective Tissue: Provides protection to the plant body.

  • Epidermis: Outermost single layer of cells. Covered by a waxy, water-resistant layer called the cuticle (thicker in desert plants) which prevents water loss and infection. Cells are flattened with no intercellular spaces. Contains small pores called stomata (singular: stoma), especially on leaves, guarded by two kidney-shaped guard cells. Stomata regulate gas exchange (CO2, O2) and transpiration (water loss). In roots, epidermal cells bear root hairs for water absorption.
  • Cork (Phellem): Formed in older stems and roots due to secondary growth (activity of cork cambium). Replaces the epidermis. Composed of dead cells, compactly arranged, with walls thickened with suberin (a waxy substance). Suberin makes cork impermeable to gases and water, protecting underlying tissues. Lenticels (pores in cork) allow gaseous exchange.

3. Animal Tissues

Animal tissues are classified into four main types based on their function and structure.

A. Epithelial Tissue (Covering/Protective Tissue)

• Characteristics: Covers body surfaces, lines body cavities and hollow organs. Cells are tightly packed with very little intercellular matrix, forming continuous sheets. Separated from underlying tissue by a fibrous basement membrane.
• Functions: Protection, absorption, secretion, filtration, sensation.
• Types:
  • Simple Squamous Epithelium: Single layer of thin, flat, irregular cells. Location: Lining of blood vessels (endothelium), lung alveoli, mouth, oesophagus. Function: Filtration, diffusion, protection.
  • Stratified Squamous Epithelium: Multiple layers of cells, with deeper layers cuboidal/columnar and outer layers flattened (squamous). Can be keratinized (skin) or non-keratinized (mouth lining, oesophagus). Location: Skin epidermis, lining of mouth, pharynx, oesophagus. Function: Protection against mechanical injury, chemical damage, entry of germs.
  • Simple Cuboidal Epithelium: Single layer of cube-shaped cells. Location: Kidney tubules, ducts of salivary glands, thyroid follicles. Function: Secretion, absorption.
  • Simple Columnar Epithelium: Single layer of tall, pillar-like cells. Nuclei usually near the base. Location: Lining of stomach, intestine. Function: Absorption (often have microvilli), secretion.
  • Ciliated Columnar Epithelium: Columnar cells bearing cilia (hair-like projections) on their free surface. Location: Lining of respiratory tract (trachea, bronchi), fallopian tubes (oviducts). Function: Move mucus, particles, or eggs in a specific direction.
  • Glandular Epithelium: Epithelial cells modified to secrete substances. Can form multicellular glands (e.g., salivary glands, sweat glands) or be unicellular (goblet cells in intestine). Function: Secretion (e.g., mucus, sweat, enzymes, hormones).

B. Connective Tissue

• Characteristics: Most abundant and widely distributed tissue. Connects and supports other tissues/organs. Cells are loosely spaced and embedded in an intercellular matrix. The nature of the matrix (fluid, jelly-like, dense, rigid) determines the tissue's function.
• Types:
  • Blood: Fluid matrix called plasma (contains proteins, salts, hormones). Contains free-floating cells: Red Blood Cells (RBCs/Erythrocytes - carry oxygen), White Blood Cells (WBCs/Leukocytes - defence), and Platelets (Thrombocytes - blood clotting). Function: Transport (gases, nutrients, hormones, waste), defence, clotting.
  • Bone: Hard, rigid matrix composed of calcium and phosphorus compounds (salts) and collagen fibres. Bone cells (osteocytes) are embedded in spaces called lacunae. Function: Forms skeletal framework, provides support and shape, protects vital organs, anchors muscles, produces blood cells (in bone marrow).
  • Cartilage: Solid but flexible matrix rich in proteins (chondrin) and sugars. Cartilage cells (chondrocytes) are found in lacunae, often in groups. Location: Tip of nose, external ear (pinna), trachea, larynx, ends of long bones (smoothens joint surfaces). Function: Provides support and flexibility.
  • Ligament: Dense, regular connective tissue. Very elastic due to elastin fibres. Connects bone to bone. Function: Strengthens joints, allows movement.
  • Tendon: Dense, regular connective tissue. Strong but less flexible, rich in collagen fibres. Connects skeletal muscle to bone. Function: Transmits the force of muscle contraction to move bones.
  • Areolar Tissue (Loose Connective Tissue): Jelly-like matrix containing various cells (fibroblasts, macrophages, mast cells) and fibres (collagen, elastin). Location: Found between skin and muscles, around blood vessels and nerves, in bone marrow. Function: Fills space inside organs, supports internal organs, helps in tissue repair.
  • Adipose Tissue: Aggregation of fat cells (adipocytes), each containing a large fat globule. Matrix is reduced. Location: Below the skin, around internal organs (kidneys, heart). Function: Stores fat (energy reserve), acts as an insulator (prevents heat loss), provides padding/shock absorption.

C. Muscular Tissue

• Characteristics: Composed of elongated cells called muscle fibres. Responsible for movement in the body. Contain contractile proteins (actin and myosin) that enable contraction and relaxation.
• Types:
  • Striated Muscle (Skeletal Muscle or Voluntary Muscle):
    • Structure: Long, cylindrical, unbranched fibres with distinct cross-striations (alternating light and dark bands). Multinucleate (nuclei are peripheral).
    • Location: Attached to bones (e.g., muscles of limbs, trunk, face).
    • Function: Responsible for voluntary body movements (walking, running, lifting). Fatigue easily.
  • Smooth Muscle (Unstriated Muscle or Involuntary Muscle):
    • Structure: Spindle-shaped (fusiform) cells, pointed at both ends. No striations. Uninucleate (single central nucleus).
    • Location: Walls of internal organs (stomach, intestine, bladder, blood vessels, iris of the eye).
    • Function: Responsible for involuntary movements (e.g., movement of food in alimentary canal, contraction/relaxation of blood vessels). Do not fatigue easily.
  • Cardiac Muscle (Involuntary Muscle):
    • Structure: Cylindrical, branched fibres. Faint striations. Uninucleate (single central nucleus). Cells are joined end-to-end by special junctions called intercalated discs which allow rapid spread of impulse.
    • Location: Found only in the walls of the heart.
    • Function: Responsible for rhythmic contraction and relaxation of the heart throughout life. Never fatigues.

D. Nervous Tissue

• Characteristics: Specialized tissue for receiving stimuli and transmitting signals (nerve impulses) rapidly from one part of the body to another. Controls and coordinates body activities.
• Components: Brain, spinal cord, and nerves are composed of nervous tissue.
• Neuron (Nerve Cell): The structural and functional unit of nervous tissue.
  • Structure:
    • Cell Body (Cyton or Soma): Contains the nucleus and cytoplasm (neuroplasm) with granular bodies called Nissl's granules.
    • Dendrites: Short, branched processes arising from the cyton; receive signals from other neurons.
    • Axon: A single, long process arising from the cyton; transmits nerve impulses away from the cell body. May be covered by a fatty myelin sheath (insulation, speeds up impulse transmission). The axon ends in branched nerve terminals.
• Nerve Impulse: The signal transmitted along the nerve fibre.
• Nerve: A bundle of axons (nerve fibres) bound together by connective tissue.

Multiple Choice Questions (MCQs)

1. Which of the following plant tissues has dead cells with lignified secondary walls?
   a) Parenchyma
   b) Collenchyma
   c) Sclerenchyma
   d) Chlorenchyma

2. Increase in the girth of a tree stem is primarily due to the activity of:
   a) Apical meristem
   b) Intercalary meristem
   c) Lateral meristem (Cambium)
   d) Epidermis

3. Which component of xylem is living?
   a) Tracheids
   b) Vessels
   c) Xylem parenchyma
   d) Xylem fibres

4. Flexibility in plants, allowing bending of parts like tendrils without breaking, is provided by:
   a) Parenchyma
   b) Collenchyma
   c) Sclerenchyma
   d) Xylem

5. The tissue responsible for the movement of food (sugars) in plants is:
   a) Xylem
   b) Phloem
   c) Parenchyma
   d) Cambium

6. Which type of animal tissue forms the lining of kidney tubules and ducts of salivary glands?
   a) Simple Squamous Epithelium
   b) Simple Cuboidal Epithelium
   c) Simple Columnar Epithelium
   d) Stratified Squamous Epithelium

7. Tendons and ligaments are examples of:
   a) Epithelial Tissue
   b) Muscular Tissue
   c) Nervous Tissue
   d) Connective Tissue

8. Which muscle tissue is voluntary, striated, and multinucleate?
   a) Smooth muscle
   b) Cardiac muscle
   c) Skeletal muscle
   d) Unstriated muscle

9. The matrix of which connective tissue is fluid?
   a) Bone
   b) Cartilage
   c) Blood
   d) Areolar tissue

10. The structural and functional unit of the nervous system is the:
    a) Nephron
    b) Neuron
    c) Osteocyte
    d) Chondrocyte

Answer Key for MCQs:

1. c) Sclerenchyma
2. c) Lateral meristem (Cambium)
3. c) Xylem parenchyma
4. b) Collenchyma
5. b) Phloem
6. b) Simple Cuboidal Epithelium
7. d) Connective Tissue
8. c) Skeletal muscle
9. c) Blood
10. b) Neuron

Make sure you revise these notes thoroughly. Pay special attention to the specific locations and functions of each tissue type, the components of complex tissues like xylem, phloem, and blood, and the differences between the three types of muscle tissues. These are common areas tested in competitive exams. Good luck with your preparation!