Class 9 Science Notes Chapter 6 (Tissues) – Science Book

Alright class, let's get straight into Chapter 6 - Tissues. This is a fundamental chapter, building upon our understanding of cells. For government exams, questions often test your understanding of the types, structure, location, and function of various tissues. Pay close attention.

Chapter 6: Tissues - Detailed Notes

What are Tissues?

• A tissue is a group of cells that are similar in structure and/or work together to achieve a particular function.
• In multicellular organisms, cells group to form tissues, tissues organize into organs, organs into organ systems, and finally, the organism. This division of labour increases efficiency.

Are Plants and Animals Made of the Same Tissues?

No. There are distinct differences due to their different modes of life:

• Plants: Mostly stationary. Need supportive tissues for structural strength. Many tissues are dead, providing mechanical strength with less maintenance. Growth is often limited to specific regions.
• Animals: Mobile. Need more energy and living cells for movement, feeding, etc. Growth is generally uniform throughout the body. Tissues are more complex and specialized for locomotion and responsiveness.

I. Plant Tissues

Plant tissues are broadly classified into two main types:

1. Meristematic Tissues: Responsible for growth.
2. Permanent Tissues: Derived from meristematic tissues; lose the ability to divide and take up specific roles.

A. Meristematic Tissue (Meristems)

• Characteristics:
  • Cells are actively dividing.
  • Cells are small, generally isodiametric (roughly spherical or polygonal).
  • Thin cellulose cell walls.
  • Dense cytoplasm.
  • Prominent nuclei.
  • Vacuoles are absent or very small (as they don't store food/waste and need dense cytoplasm for division).
• Types based on Location:
  • Apical Meristem: Found at the growing tips (apices) of stems and roots. Responsible for increasing the length of the plant (primary growth).
  • Lateral Meristem (Cambium): Found along the sides of stems and roots (e.g., vascular cambium, cork cambium). Responsible for increasing the girth or diameter of the plant (secondary growth).
  • Intercalary Meristem: Located at the base of leaves or internodes (e.g., in grasses, bamboo). Helps in the longitudinal growth of organs like leaves and internodes, especially useful for grasses regenerating after being grazed.

B. Permanent Tissue

• Origin: Formed when cells derived from meristematic tissue differentiate (take up a specific shape, size, and function) and lose their ability to divide. This process is called differentiation.
• Types:
  • Simple Permanent Tissue: Composed of only one type of cell, performing a common function.
  • Complex Permanent Tissue: Composed of more than one type of cell, working together as a unit.

1. Simple Permanent Tissues

• Parenchyma:
  • Structure: Living cells, relatively unspecialized, thin cell walls (cellulose), usually loosely packed with large intercellular spaces. Cells are typically isodiametric.
  • Location: Found throughout the plant body (cortex, pith, mesophyll of leaves, packing tissue in xylem and phloem).
  • Functions: Storage (food, water, waste products), photosynthesis (if containing chloroplasts - called Chlorenchyma), buoyancy (if large air cavities are present, especially in aquatic plants - called Aerenchyma), packing tissue.
• Collenchyma:
  • Structure: Living cells, elongated, irregularly thickened cell walls, especially at the corners (due to pectin and cellulose deposition). Very little intercellular space.
  • Location: Found below the epidermis in dicot stems and leaf stalks (petioles). Absent in monocots, roots, and mature woody stems.
  • Functions: Provides mechanical support and flexibility to growing parts like young stems and petioles, allowing them to bend without breaking.
• Sclerenchyma:
  • Structure: Dead cells at maturity. Long, narrow cells with thick, lignified secondary walls (lignin makes them hard and impermeable). No intercellular spaces. Lumen (internal cavity) is very narrow.
  • Types:
    • Fibres: Long, narrow, pointed ends. Occur in bundles. (e.g., jute, flax fibres).
    • Sclereids (Stone cells): Short, irregular shapes. Found in fruit walls (nuts), pulp of fruits (guava, pear - gives the gritty texture), seed coats (legumes), leaves (tea).
  • Location: Found in stems (around vascular bundles), veins of leaves, hard coverings of seeds and nuts.
  • Functions: Provides mechanical strength, rigidity, and hardness to the plant parts. Makes the plant stiff.
• Epidermis & Cork (Protective Tissues):
  • Epidermis: The outermost single layer of cells covering the entire plant body (leaves, flowers, stems, roots). Cells are flattened, often without intercellular spaces.
    • Cuticle: A waxy, water-resistant layer secreted by epidermal cells (especially in aerial parts) to prevent water loss and mechanical injury/infection. Thicker in desert plants.
    • Stomata: Tiny pores present on the epidermis of leaves (sometimes stems). Each stoma is guarded by two kidney-shaped guard cells (bean-shaped in dicots, dumbbell-shaped in monocots). Functions: Gaseous exchange (CO2 intake, O2 release during photosynthesis) and transpiration (loss of water vapour).
    • Root Hairs: Epidermal cells of roots bear long, hair-like extensions that increase the surface area for water absorption.
  • Cork (Phellem): As stems grow older, the outer epidermis is replaced by a secondary meristem (cork cambium or phellogen). This forms layers of cork cells towards the outside.
    • Structure: Dead cells, compactly arranged, walls thickened with suberin (makes them impermeable to gases and water).
    • Function: Protection from water loss, mechanical injury, and microbial infection in older stems/roots. Commercial cork (used as stoppers, insulators) is derived from the Cork Oak tree.

2. Complex Permanent Tissues (Vascular/Conducting Tissues)

• These tissues transport water, minerals, and food. They form the vascular bundles.
• Xylem:
  • Function: Conducts water and dissolved minerals upwards from roots to leaves (unidirectional flow). Also provides mechanical support.
  • Components (4 types of cells):
    • Tracheids: Elongated, dead cells with lignified walls and tapering ends. Water moves through pits. The main water-conducting element in gymnosperms and pteridophytes.
    • Vessels (or Tracheae): Long, cylindrical tube-like structures made of vessel members joined end-to-end, with perforated end walls (or dissolved end walls). More efficient water conductors than tracheids. Found mainly in angiosperms. Dead cells with lignified walls.
    • Xylem Parenchyma: Living cells. Store food and help in the lateral conduction of water.
    • Xylem Fibres (or Xylem Sclerenchyma): Dead, sclerenchymatous cells. Provide mechanical strength.
• Phloem:
  • Function: Conducts prepared food (sugars, mainly sucrose) from leaves to storage organs and growing parts (bidirectional or multidirectional flow possible).
  • Components (4 types of cells in Angiosperms):
    • Sieve Tubes: Living, elongated, tube-like cells arranged end-to-end. End walls are perforated like a sieve, forming sieve plates. Lack nucleus at maturity, but cytoplasm is present. Transport sugars.
    • Companion Cells: Specialized living parenchyma cells closely associated with sieve tube elements (connected by plasmodesmata). Have a prominent nucleus and dense cytoplasm. Control the functions of the sieve tube element. Found only in Angiosperms.
    • Phloem Parenchyma: Living cells. Store food (starch, fat) and resins, latex, etc. Help in lateral transport. Absent in most monocots.
    • Phloem Fibres (Bast Fibres): Dead, sclerenchymatous cells (like xylem fibres). Provide mechanical strength. (e.g., Jute, Hemp, Flax fibres are phloem fibres).

II. Animal Tissues

Animal tissues are classified into four main types based on their function:

1. Epithelial Tissue: Covering and protective tissue.
2. Connective Tissue: Connects and supports other tissues.
3. Muscular Tissue: Responsible for movement.
4. Nervous Tissue: Responsible for transmitting stimuli and coordinating responses.

A. Epithelial Tissue (Epithelium)

• Characteristics:
  • Forms continuous sheets covering the external surface of the body and lining internal organs and cavities.
  • Cells are tightly packed with very little intercellular matrix/space.
  • Cells rest on a non-cellular basement membrane separating the epithelium from underlying connective tissue.
  • Avascular (lack blood vessels); nutrients diffuse from underlying connective tissue.
  • Functions: Protection, absorption, filtration, excretion, secretion, sensory reception.
• Types based on Cell Shape and Layers:
  • Simple Squamous Epithelium: Single layer of thin, flat, irregular-shaped cells. Like 'tiles on a floor'.
    • Location: Lining of blood vessels (endothelium), lymph vessels, air sacs (alveoli) of lungs, lining of mouth, oesophagus.
    • Function: Filtration, diffusion, protection.
  • Stratified Squamous Epithelium: Multiple layers of cells, with the deepest layers being cuboidal/columnar and outer layers flattened (squamous). Outer layers can be dead and keratinized.
    • Location: Skin (epidermis - keratinized), lining of mouth cavity (buccal cavity), pharynx, oesophagus (non-keratinized).
    • Function: Protection against mechanical stress, chemical damage, water loss, and pathogens.
  • Simple Cuboidal Epithelium: Single layer of cube-shaped cells with a central, spherical nucleus.
    • Location: Lining of kidney tubules, ducts of salivary glands, thyroid follicles.
    • Function: Secretion, absorption, mechanical support.
  • Simple Columnar Epithelium: Single layer of tall, pillar-like cells, usually with the nucleus located near the base.
    • Location: Lining of stomach, small intestine.
    • Function: Secretion (mucus, enzymes), absorption.
    • Ciliated Columnar Epithelium: Columnar cells possessing cilia (hair-like projections) on their free surface.
      • Location: Lining of the respiratory tract (trachea, bronchi), oviducts (fallopian tubes).
      • Function: Move mucus, particles, or eggs in a specific direction.
  • Glandular Epithelium: Epithelial cells modified to synthesize and secrete substances. Can form multicellular glands (e.g., salivary glands) or be unicellular (e.g., goblet cells secreting mucus in the intestine/respiratory tract).

B. Connective Tissue

• Characteristics:
  • Most abundant and widely distributed tissue type.
  • Connects, supports, binds, or separates other tissues or organs.
  • Cells are loosely spaced and embedded in an intercellular matrix.
  • The matrix can be jelly-like, fluid, dense, or rigid, determining the tissue's function. Matrix is secreted by the connective tissue cells themselves (except blood).
• Types:
  • Blood:
    • Matrix: Fluid matrix called plasma (contains proteins, salts, hormones, water).
    • Cells:
      • Red Blood Cells (RBCs or Erythrocytes): Biconcave, lack nucleus (in mammals), contain haemoglobin for O2 transport.
      • White Blood Cells (WBCs or Leukocytes): Irregular shape, nucleated, involved in defence (e.g., neutrophils, lymphocytes, monocytes).
      • Platelets (Thrombocytes): Cell fragments, involved in blood clotting.
    • Function: Transport of gases (O2, CO2), nutrients, hormones, waste products; defence against diseases; clotting.
  • Bone:
    • Matrix: Hard, rigid matrix made of calcium and phosphorus compounds (calcium phosphate) embedded in collagen fibres.
    • Cells: Bone cells (osteocytes) are embedded within spaces called lacunae.
    • Structure: Forms the skeleton. Hard and strong, non-flexible. Richly supplied with blood vessels.
    • Function: Provides structural framework, support, protection to vital organs (brain, lungs), anchorage for muscles, storage of calcium and phosphate, production of blood cells (in bone marrow).
  • Cartilage:
    • Matrix: Solid, pliable (flexible) matrix composed of proteins (chondrin) and sugars. Less hard than bone.
    • Cells: Cartilage cells (chondrocytes) are embedded in lacunae, often in groups.
    • Location: Tip of the nose, external ear (pinna), trachea, larynx (voice box), ends of long bones (smoothens joint surfaces), intervertebral discs.
    • Function: Provides support and flexibility, smoothens surfaces at joints. Avascular (usually).
  • Ligament:
    • Structure: Dense connective tissue. Very elastic and strong. Contains very little matrix. Connects bone to bone. Made mainly of elastin fibres.
    • Function: Holds bones together at joints, allows movement while providing stability.
  • Tendon:
    • Structure: Dense fibrous connective tissue. Great strength but limited flexibility (inelastic). Connects skeletal muscle to bone. Made mainly of collagen fibres.
    • Function: Transmits the force of muscle contraction to bones, causing movement.
  • Areolar Tissue (Loose Connective Tissue):
    • Structure: Cells (fibroblasts, macrophages, mast cells) and fibres (collagen, elastin) loosely arranged in a semi-fluid ground substance (matrix).
    • Location: Found between skin and muscles, around blood vessels and nerves, in the bone marrow. Fills space inside organs.
    • Function: Supports internal organs, helps in tissue repair, packing tissue.
  • Adipose Tissue (Fat Tissue):
    • Structure: Modified areolar tissue containing specialized cells called adipocytes that store fat. Fat globules push the nucleus and cytoplasm to the periphery.
    • Location: Found below the skin, between internal organs, around heart and kidneys.
    • Function: Fat storage, insulation (prevents heat loss), protective cushion for organs.

C. Muscular Tissue

• Characteristics:
  • Composed of elongated cells called muscle fibres.
  • Responsible for movement in the body.
  • Contain special contractile proteins (actin and myosin) that contract and relax to cause movement.
• Types:
  • Striated Muscle (Skeletal Muscle / Voluntary Muscle):
    • Structure: Long, cylindrical, unbranched fibres. Show alternate light and dark bands or striations. Multinucleated (syncytial - nuclei are peripheral).
    • Location: Attached to bones (hence 'skeletal'). Found in limbs, body wall, face, neck.
    • Function: Responsible for voluntary movements (walking, running, lifting). Fatigue easily.
  • Smooth Muscle (Unstriated Muscle / Involuntary Muscle):
    • Structure: Spindle-shaped (fusiform) cells with tapering ends. Uninucleated (nucleus is central). Lack striations.
    • Location: Walls of internal organs (hollow visceral organs) like the alimentary canal, blood vessels, iris of the eye, ureters, bronchi.
    • Function: Responsible for involuntary movements (peristalsis in the gut, constriction/dilation of blood vessels). Do not fatigue easily.
  • Cardiac Muscle (Involuntary Muscle):
    • Structure: Cylindrical, branched fibres. Uninucleated (usually central nucleus). Show faint striations. Unique feature: presence of intercalated discs (junctions between cells) that allow rapid spread of impulse.
    • Location: Found only in the wall of the heart.
    • Function: Rhythmic contraction and relaxation of the heart throughout life, pumping blood. Never fatigues. Involuntary control.

D. Nervous Tissue

• Characteristics:
  • Highly specialized tissue responsible for receiving stimuli and transmitting signals (nerve impulses) rapidly from one place to another within the body.
  • Controls and coordinates body activities.
• Cells:
  • Neurons (Nerve Cells): The structural and functional units of nervous tissue.
    • Structure:
      • Cell Body (Cyton or Soma): Contains the nucleus and cytoplasm (neuroplasm).
      • Dendrites: Short, branched extensions arising from the cyton. Receive signals from other neurons.
      • Axon: A single, long extension arising from the cyton. Transmits signals away from the cell body. May be covered by a myelin sheath (insulating layer). The end of the axon branches into axon terminals.
  • Neuroglia (Glial Cells): Supporting cells that surround and protect neurons, provide nutrients, and help in repair. They do not conduct nerve impulses.
• Location: Brain, spinal cord, and nerves.
• Function: Rapid communication, control, and coordination of body functions through the transmission of electrical and chemical signals (nerve impulses).

Multiple Choice Questions (MCQs)

1. Which type of plant tissue is responsible for the increase in the girth of a tree trunk?
   (a) Apical meristem
   (b) Intercalary meristem
   (c) Lateral meristem (Cambium)
   (d) Parenchyma

2. Aerenchyma is a modification of parenchyma found in:
   (a) Desert plants
   (b) Aquatic plants
   (c) Climbing plants
   (d) Trees growing on mountains

3. Which of the following plant tissues consists of dead cells with lignified walls?
   (a) Parenchyma
   (b) Collenchyma
   (c) Sclerenchyma
   (d) Phloem Parenchyma

4. The transport of food from leaves to other parts of the plant is carried out by:
   (a) Xylem
   (b) Phloem
   (c) Epidermis
   (d) Cambium

5. Which component of phloem is absent in most monocots?
   (a) Sieve tubes
   (b) Companion cells
   (c) Phloem parenchyma
   (d) Phloem fibres

6. The lining of kidney tubules and ducts of salivary glands is formed by which epithelium?
   (a) Simple Squamous
   (b) Simple Cuboidal
   (c) Simple Columnar
   (d) Stratified Squamous

7. Which connective tissue connects bones to bones?
   (a) Tendon
   (b) Cartilage
   (c) Ligament
   (d) Areolar tissue

8. Which type of muscle tissue shows striations and is involuntary?
   (a) Skeletal muscle
   (b) Smooth muscle
   (c) Cardiac muscle
   (d) None of the above

9. The structural and functional unit of the nervous system is the:
   (a) Axon
   (b) Dendrite
   (c) Neuron
   (d) Neuroglia

10. Fat is stored in the human body as:
    (a) Areolar tissue
    (b) Adipose tissue
    (c) Cartilage
    (d) Bone marrow

Answer Key for MCQs:

1. (c)
2. (b)
3. (c)
4. (b)
5. (c)
6. (b)
7. (c)
8. (c)
9. (c)
10. (b)

Make sure you revise these notes thoroughly. Understanding the specific locations and functions of each tissue type is crucial for exams. Good luck!