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Class 9 Science

Chapter 6: Tissues

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Tissues

Introduction

In unicellular organisms, a single cell performs all the basic functions of life. However, in multicellular organisms, different groups of cells perform different specialized functions. This is called division of labor.

Tissue: A tissue is a group of cells that are similar in structure and work together to perform a specific function.

Key Points:

  • Cells in a tissue have common origin (from same parent cell)
  • Cells in a tissue have similar structure
  • Cells work together for a common function
  • Study of tissues is called Histology

Types of Tissues

Based on the complexity of organisms, tissues are classified into two main groups:

1. Plant Tissues
2. Animal Tissues

PLANT TISSUES

Plant tissues are of two main types based on their ability to divide:

A. Meristematic Tissues (Growing Tissues)

Meristematic Tissue: Tissues whose cells have the ability to divide continuously are called meristematic tissues.

Characteristics:

  • Cells are small and immature
  • Cells are spherical or polygonal in shape
  • Have thin cell walls
  • Have large nuclei
  • Dense cytoplasm with few vacuoles
  • No intercellular spaces
  • Cells divide actively and rapidly
  • Cells are alive and metabolically active

Function: Growth of plants (increase in length and girth)

Types of Meristematic Tissues (Based on Position):

1. Apical Meristem:

  • Located at the tips of roots and shoots (apex)
  • Responsible for growth in length (primary growth)
  • Helps plant grow taller and roots grow deeper
  • Present at root tip and shoot tip

2. Lateral Meristem (Cambium):

  • Located along the sides of stem and roots
  • Responsible for growth in thickness/girth (secondary growth)
  • Makes stem and roots thicker
  • Examples: Vascular cambium, Cork cambium

3. Intercalary Meristem:

  • Located at the base of leaves or between mature tissues
  • Found at nodes (in grasses and bamboo)
  • Helps in regrowth of grass after cutting
  • Allows leaves to elongate

B. Permanent Tissues (Non-Growing Tissues)

Permanent Tissue: Tissues formed from meristematic tissues that have lost the ability to divide are called permanent tissues.

Formation: When meristematic cells divide, some daughter cells lose the ability to divide and become permanent tissues. This process is called differentiation.

Characteristics:

  • Cells have lost the ability to divide
  • Cells are mature and fully developed
  • Cells have specific shapes based on function
  • Have thick cell walls
  • Have smaller nuclei
  • Have large vacuoles
  • May have intercellular spaces

Types of Permanent Tissues:

I. Simple Permanent Tissues

Made up of only one type of cells that are similar in structure and function.

1. Parenchyma:

Structure:
  • Most common and simple tissue
  • Cells are living, thin-walled
  • Cells are loosely packed with intercellular spaces
  • Cells are spherical, oval, or polygonal
  • Large vacuoles present
  • Nucleus is present
Location: Found in stems, roots, leaves, flowers, and fruits Functions:
  • Storage of food (starch, proteins, oils)
  • Provides support to plant (turgidity)
  • Fills spaces between other tissues
  • Helps in gaseous exchange
Special Types:
  • Chlorenchyma: Parenchyma with chloroplasts for photosynthesis
  • Aerenchyma: Parenchyma with large air spaces for buoyancy in aquatic plants

2. Collenchyma:

Structure:
  • Cells are living
  • Cell walls are thickened at corners due to cellulose and pectin
  • Cells are elongated
  • Few intercellular spaces
  • Provides flexibility
Location: Below epidermis in leaf stalks (petioles) and stems Functions:
  • Provides mechanical support with flexibility
  • Allows bending without breaking
  • Provides strength to growing parts

3. Sclerenchyma:

Structure:
  • Cells are dead (no protoplasm)
  • Cell walls are very thick and hard
  • Thickening due to lignin deposition
  • Cells are long and narrow
  • No intercellular spaces
  • Very strong tissue
Location: Stems, around vascular bundles, in hard coverings of seeds and nuts Functions:
  • Provides mechanical strength and rigidity
  • Makes plant hard and stiff
  • Provides support to plant parts
  • Protects seeds
Example: Fibers in jute, coconut husk; hard shell of walnut

II. Complex Permanent Tissues

Made up of more than one type of cells that work together as a unit.

1. Xylem (Wood):

Definition: Vascular tissue that transports water and minerals from roots to other parts of the plant.

Components:

  • Tracheids: Dead, tube-like cells with thick walls; transport water
  • Vessels: Dead, long tube-like structures; main water transport
  • Xylem Parenchyma: Living cells; store food and help in lateral conduction
  • Xylem Fibers: Dead cells; provide mechanical support

Characteristics:

  • Most cells are dead (except parenchyma)
  • Have thick, lignified walls
  • Form continuous tubes
  • No end walls between cells

Functions:

  • Transport water and dissolved minerals upward (unidirectional)
  • Provide mechanical support
  • Store food (xylem parenchyma)

Direction: Roots → Stems → Leaves (upward movement)

2. Phloem (Bast):

Definition: Vascular tissue that transports food (prepared by leaves) to all parts of the plant.

Components:

  • Sieve Tubes: Living cells without nucleus; transport food; have perforated end walls (sieve plates)
  • Companion Cells: Living cells with nucleus; help sieve tubes in transport
  • Phloem Parenchyma: Living cells; store food
  • Phloem Fibers: Dead cells; provide mechanical support

Characteristics:

  • Most cells are living
  • Sieve tubes have perforated end walls
  • Companion cells are closely associated with sieve tubes
  • Cells have thin walls

Functions:

  • Transport food (sugars) from leaves to all parts (bidirectional)
  • This process is called translocation
  • Provide mechanical support (phloem fibers)
  • Store food (phloem parenchyma)

Direction: Can move upward or downward (bidirectional)

III. Protective Tissues

1. Epidermis:

Definition: The outermost protective layer of plant body.

Structure:

  • Single layer of cells
  • Cells are flattened and closely packed
  • No intercellular spaces
  • Cells may have waxy coating (cuticle) on outer surface
  • Contains stomata (small pores)
  • Living cells

Location: Outer surface of leaves, stems, roots, flowers, fruits

Functions:

  • Protection from injury, water loss, infection
  • Cuticle prevents water loss (transpiration)
  • Stomata allow gaseous exchange and transpiration
  • Root hairs (epidermal cells) absorb water and minerals

Special Features:

  • Stomata: Tiny pores for gas exchange; surrounded by guard cells
  • Guard Cells: Kidney-shaped cells that control opening and closing of stomata
  • Cuticle: Waxy layer that prevents water loss

2. Cork (Secondary Cortex):

Definition: Protective tissue that replaces epidermis in older stems and roots.

Structure:

  • Made up of dead cells
  • Cells have thick walls
  • Walls are coated with suberin (waxy substance)
  • No intercellular spaces
  • Arranged in multiple layers
  • Contains lenticels (small pores)

Formation: Formed by cork cambium (secondary meristem) in older stems and roots

Functions:

  • Protection from mechanical injury
  • Prevents water loss
  • Protects from extreme temperatures
  • Lenticels allow gaseous exchange

Uses: Cork is obtained from cork oak tree; used for bottle stoppers, insulation

ANIMAL TISSUES

Animal tissues are more complex than plant tissues because animals need to perform more varied functions like movement, nervous coordination, etc.

Types of Animal Tissues:

1. Epithelial Tissue (Covering Tissue)

Definition: Tissue that covers body surfaces and lines internal organs and cavities.

Characteristics:

  • Cells are tightly packed with little intercellular material
  • Cells form continuous sheets
  • Cells rest on a basement membrane
  • Free surface may have cilia or microvilli
  • No blood vessels (avascular)
  • High regeneration capacity

Functions:

  • Protection from injury, chemicals, microbes
  • Absorption of substances
  • Secretion (glands)
  • Sensation
  • Movement of substances (ciliated epithelium)

Types of Epithelial Tissue (Based on Shape and Layers):

A. Simple Epithelium (Single Layer):

1. Squamous Epithelium:

  • Flat, thin cells like floor tiles
  • Nucleus is flat and central
  • Location: Lining of blood vessels, alveoli of lungs, walls of capillaries
  • Function: Allows easy diffusion of gases and nutrients; filtration

2. Cuboidal Epithelium:

  • Cube-shaped cells
  • Nucleus is round and central
  • Location: Kidney tubules, salivary glands, thyroid gland
  • Function: Secretion and absorption

3. Columnar Epithelium:

  • Tall, pillar-like cells
  • Nucleus is elongated at base
  • Location: Inner lining of intestine, stomach
  • Function: Absorption and secretion
  • May have microvilli to increase surface area

4. Ciliated Epithelium:

  • Columnar cells with hair-like projections (cilia)
  • Cilia can move
  • Location: Respiratory tract, fallopian tubes, spinal cord
  • Function: Movement of mucus, eggs, and other substances

B. Compound/Stratified Epithelium (Multiple Layers):

  • Made up of more than one layer of cells
  • Provides better protection
  • Limited role in secretion and absorption
  • Location: Skin, lining of mouth, esophagus
  • Function: Protection from wear and tear

Glandular Epithelium:

  • Specialized epithelial tissue for secretion
  • Forms glands
  • Types:
    • Exocrine Glands: Release secretions through ducts (sweat glands, salivary glands)
    • Endocrine Glands: Release hormones directly into blood (thyroid, pituitary)

2. Connective Tissue (Connecting and Supporting Tissue)

Definition: Tissue that connects, supports, and binds different tissues and organs.

Characteristics:

  • Cells are loosely arranged
  • Large amount of intercellular material (matrix)
  • Matrix can be solid, semi-solid, or liquid
  • Has blood vessels (except cartilage)
  • Most widely distributed tissue

Functions:

  • Connects and supports other tissues
  • Transports materials
  • Storage (fat)
  • Protection and defense (blood)
  • Repair damaged tissues

Types of Connective Tissue:

A. Areolar Connective Tissue:

  • Most common connective tissue
  • Cells and fibers are loosely arranged in semi-fluid matrix
  • Contains collagen fibers, elastic fibers, and cells
  • Location: Between skin and muscles, around blood vessels and nerves, in bone marrow
  • Functions: Fills spaces, supports organs, helps in repair

B. Adipose Tissue (Fat Tissue):

  • Cells are filled with fat droplets
  • Nucleus is pushed to periphery
  • Cells are spherical when isolated
  • Location: Below skin, around kidneys, between organs
  • Functions:
    • Storage of fats (energy reserve)
    • Insulation (prevents heat loss)
    • Acts as shock absorber
    • Fills spaces between organs

C. Dense Connective Tissue (Fibrous Tissue):

  • Fibers are densely packed
  • Less cells and matrix
  • Two types:
    • Tendons: Connect muscles to bones; mainly collagen fibers; strong but not flexible
    • Ligaments: Connect bones to bones; contain elastic fibers; strong and elastic

D. Skeletal Connective Tissues:

1. Cartilage:

  • Solid but flexible connective tissue
  • Matrix is semi-solid and contains protein and sugar
  • Cells are called chondrocytes
  • Found in chondrin matrix
  • No blood vessels (nutrients diffuse through matrix)
  • Location: Nose, ear, trachea, larynx, joints between bones
  • Functions:
    • Provides support and shape
    • Provides flexibility
    • Provides smooth surface for bone joints
    • Reduces friction

2. Bone:

  • Hard connective tissue
  • Matrix is solid and rigid
  • Matrix contains calcium and phosphorus salts
  • Cells are called osteocytes
  • Cells are present in spaces called lacunae
  • Hardest tissue in body
  • Has blood vessels
  • Location: Forms skeleton
  • Functions:
    • Provides shape and support to body (framework)
    • Protects vital organs (skull protects brain, ribs protect heart and lungs)
    • Helps in movement (attached to muscles)
    • Stores minerals (calcium, phosphorus)
    • Produces blood cells (bone marrow)

E. Fluid Connective Tissues:

1. Blood:

  • Fluid connective tissue
  • Matrix is liquid (plasma)
  • Contains three types of cells floating in plasma:
    • Red Blood Cells (RBCs/Erythrocytes): Contain hemoglobin; transport oxygen; no nucleus in mammals
    • White Blood Cells (WBCs/Leukocytes): Fight infections; have nucleus; can squeeze through blood vessel walls
    • Platelets (Thrombocytes): Help in blood clotting; cell fragments without nucleus
  • Functions:
    • Transports oxygen, CO₂, nutrients, hormones, waste
    • Fights infections
    • Helps in blood clotting
    • Maintains body temperature
    • Maintains pH balance

2. Lymph:

  • Similar to blood but colorless
  • Plasma without RBCs
  • Contains WBCs and plasma
  • Flows in lymphatic vessels
  • Returns tissue fluid to blood
  • Functions:
    • Carries digested fats from intestine
    • Drains excess fluid from tissues
    • Fights infections (lymphocytes)

3. Muscular Tissue (Contractile Tissue)

Definition: Tissue specialized for contraction and movement.

Characteristics:

  • Cells are elongated and called muscle fibers
  • Contain contractile proteins (actin and myosin)
  • Can contract and relax
  • Specialized for movement

Function: Movement of body parts

Types of Muscular Tissue:

1. Striated Muscle (Skeletal/Voluntary Muscle):

  • Cylindrical, long, unbranched fibers
  • Multinucleated (many nuclei)
  • Shows alternate light and dark bands (striations)
  • Under conscious control (voluntary)
  • Location: Attached to bones
  • Functions:
    • Body movement and locomotion
    • Changes in body posture
    • Facial expressions
  • Contracts rapidly but tires quickly
  • Examples: Muscles of limbs, tongue, face

2. Smooth Muscle (Unstriated/Involuntary Muscle):

  • Spindle-shaped (tapering at ends)
  • Uninucleated (single nucleus)
  • No striations (no bands)
  • Not under conscious control (involuntary)
  • Location: Walls of internal organs
  • Functions:
    • Movement of food in alimentary canal
    • Contraction and relaxation of blood vessels
    • Movement in hollow organs
  • Contracts slowly and rhythmically
  • Does not tire easily
  • Examples: Muscles of stomach, intestine, blood vessels, iris

3. Cardiac Muscle (Heart Muscle):

  • Cylindrical, branched fibers
  • Uninucleated (single nucleus in center)
  • Shows striations (but less prominent)
  • Not under conscious control (involuntary)
  • Location: Only in heart
  • Function: Pumps blood throughout life
  • Contracts rhythmically and continuously
  • Never tires (works throughout life)
  • Can work independently (myogenic)

4. Nervous Tissue (Irritable Tissue)

Definition: Tissue specialized for receiving stimuli and conducting impulses.

Location: Brain, spinal cord, nerves

Components:

  • Neurons (Nerve Cells): Functional units that transmit impulses
  • Neuroglia: Supporting cells that protect and nourish neurons

Structure of Neuron:

  • Cell Body (Soma/Cyton): Contains nucleus and cytoplasm
  • Dendrites: Short, branched projections; receive impulses from other neurons
  • Axon: Long, single projection; transmits impulses away from cell body
  • Nerve Endings: Terminal branches at the end of axon
  • Myelin Sheath: Fatty covering on axon; insulates and speeds up transmission
  • Synapse: Junction between two neurons where impulse is transmitted

Functions:

  • Receives stimuli from environment
  • Converts stimuli into nerve impulses
  • Transmits impulses to different parts of body
  • Coordinates body activities
  • Controls and regulates body functions

Types of Neurons:

  • Sensory Neurons: Carry impulses from sense organs to brain/spinal cord
  • Motor Neurons: Carry impulses from brain/spinal cord to muscles/glands
  • Association Neurons: Connect sensory and motor neurons

Comparison Tables

Feature Meristematic Tissue Permanent Tissue
Cell Division Divides continuously Lost ability to divide
Cell Wall Thin Thick
Vacuoles Small or absent Large
Intercellular Spaces Absent Present
Function Growth Various functions
Feature Xylem Phloem
Function Transports water and minerals Transports food
Direction Unidirectional (upward) Bidirectional
Cells Mostly dead Mostly living
Cell Wall Thick, lignified Thin, cellulose
Components Tracheids, vessels, parenchyma, fibers Sieve tubes, companion cells, parenchyma, fibers
Feature Striated Muscle Smooth Muscle Cardiac Muscle
Striations Present Absent Present
Nuclei Many One One
Shape Cylindrical, unbranched Spindle-shaped Cylindrical, branched
Control Voluntary Involuntary Involuntary
Location Attached to bones Internal organs Heart only
Fatigue Tires quickly Does not tire Never tires

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