Chapter 3 Notes: Tissues in Action

What is a Tissue?

A tissue is a group of cells similar in structure that work together to perform a specific function. In multicellular organisms, cells group together to form tissues, tissues form organs, organs form organ systems, and organ systems form an organism. This organization leads to division of labour, increasing efficiency and enabling complex life processes. For example, muscle tissue enables movement and nervous tissue carries messages in animals, while xylem transports water and phloem transports food in plants.

Why are Plant and Animal Tissues Different?

• Plants are fixed and need rigid support → plant cells have a cell wall
• Animals move, so flexible cells are needed → no rigid cell wall
• Plants make their own food (photosynthesis); animals digest food from external sources
• Growth patterns and transport mechanisms differ between the two kingdoms

What is Meristematic Tissue?

Meristematic tissue consists of actively dividing cells responsible for plant growth. Their cells are small, thin-walled, have dense cytoplasm, large prominent nucleus, and lack vacuoles — all features enabling rapid and continuous cell division.

Three Types of Meristematic Tissue

Type	Location	Function
Apical meristem	Tips of roots and shoots	Increases length (height and root depth)
Lateral meristem	Along circumference of stem	Increases girth/thickness; produces annual rings
Intercalary meristem	Base of internode/just above node	Regrowth after cutting (e.g., grass after mowing)

Key Process: Differentiation

Some cells produced by meristematic tissue lose the ability to divide and undergo changes in structure and function to become permanent tissues specialized for support, transport, or storage. This process is called differentiation.

Simple Permanent Tissues (one type of cell)

1. Epidermis (Protective Tissue)

• Single layer of flat, tightly packed cells forming the outermost layer
• Covered by a waxy layer called cuticle (made of cutin) — reduces water loss and prevents parasite invasion
• Root hairs increase surface area for water/mineral absorption
• Stomata in leaves allow gaseous exchange and transpiration (transpiration pull helps water rise in xylem)

2. Parenchyma (Supporting Tissue)

• Living cells with thin walls and intercellular spaces
• Functions: food storage, photosynthesis (in green parts), buoyancy in aquatic plants (air spaces)

3. Collenchyma (Supporting Tissue)

• Living cells with unevenly thickened corners due to pectin deposition
• Provides flexibility — allows stems and tendrils to bend without breaking

4. Sclerenchyma (Supporting Tissue)

• Mostly dead cells with thick lignified walls — hard and strong
• Found in stems, leaf veins, seed coats (coconut husk, walnut shell)
• Provides structural rigidity

Complex Permanent Tissues (more than one type of cell)

Xylem

• Components: Tracheids, Vessels (dead, tubular, thick-walled — transport water), Xylem parenchyma (only living component — storage), Xylem fibres (provide strength)

Phloem

• Components: Sieve tubes (transport food from leaves), Companion cells (regulate sieve tubes; monitor sugar loading/unloading), Phloem parenchyma (store food, resin, tannins, latex), Phloem fibres (provide strength)

Three Tissue Systems in Plants

1. Dermal tissue system — outer covering (epidermis); protection and reduces water loss
2. Ground tissue system — main body; includes parenchyma, collenchyma, sclerenchyma
3. Vascular tissue system — xylem and phloem for conduction

Epithelial Tissue

Epithelial tissue forms the outer covering (skin) and lines internal organs (mouth, lungs, blood vessels, intestine). Cells are closely packed with very little intercellular space, preventing germ entry and water loss.

Type	Structure	Function	Location
Simple squamous	Single layer of thin, flat cells	Gas/material exchange	Blood vessels, lungs
Stratified	Many layers of cells	Protection	Skin, mouth, oesophagus
Cuboidal/Columnar	Cuboidal or tall pillar-like cells	Secretion/absorption	Glands, small intestine
Ciliated	Cells with hair-like cilia	Sensory functions, move substances	Nostrils, inner ear

Connective Tissue

Connective tissue connects and supports other tissues and organs. The key difference between types lies in their matrix (non-cellular material).

Blood (Fluid connective tissue)

• Matrix: Liquid plasma (watery)
• Components: RBCs (contain haemoglobin — carry oxygen), WBCs (fight infection, cause pus/inflammation), Platelets (help in blood clotting)

Bone (Solid connective tissue)

• Matrix: Hard, rigid — contains calcium and phosphorus compounds
• Provides strength, support, and protection

Cartilage

• Matrix: Soft, jelly-like — flexible and cushioning
• Found at ends of bones; absorbs shock (e.g., between vertebrae)

Tendons and Ligaments

• Tendons: Connect muscles to bones — transmit muscle force to produce movement
• Ligaments: Connect bones to bones — provide stability, prevent dislocation

Types of Muscle Tissue

Feature	Skeletal Muscle	Smooth Muscle	Cardiac Muscle
Control	Voluntary	Involuntary	Involuntary
Shape	Long, cylindrical, unbranched	Spindle-shaped	Cylindrical, branched
Nuclei	Multinucleate	Single nucleus	Single nucleus
Striations	Prominent striations	No striations	Faint striations
Location	Attached to skeleton	Stomach, intestines	Heart only
Special feature	Conscious movement	Slow, continuous movement	Tireless rhythmic contractions

Nervous Tissue

• Forms the body's control and coordination network
• Cells called neurons (nerve cells) receive, process, and transmit messages
• Parts of a neuron:
  • Cell body — contains nucleus; controls cell activities
  • Dendrites — receive signals from other neurons
  • Axon — long fibre carrying messages from cell body to axon terminals
  • Axon terminals — transmit messages to other cells

The Musculoskeletal System

Composed of bones, muscles, joints, cartilage, tendons, and ligaments. Muscles pull on bones (via tendons) to produce movement at joints, under control of the nervous system.

Types of Joints

1. Ball and socket (shoulder, hip) — free movement in all directions (forward, backward, sideways, circular)
2. Hinge joint (elbow, knee) — movement in one direction only, like a door hinge; kneecap protects knee joint
3. Pivot joint (skull to backbone) — allows rotational/side-to-side movement
4. Fixed joints (skull bones) — no movement; protect the brain

Skeletal System

• Skull — protects brain, eyes, ears (joined by fixed joints)
• Vertebral column (spine) — 33 vertebrae with cartilage discs between them for cushioning and flexibility; allows bending and twisting
• Rib cage — 12 pairs of ribs joined to spine (back) and sternum/breast bone (front) by flexible cartilage; protects heart and lungs; expands/contracts during breathing

Totipotency — From One Cell to a Complete Plant

In 1958, F. C. Steward demonstrated that single phloem cells of carrot could regenerate into a complete plant when grown in a nutrient medium containing simple sugars and hormones. This showed that some mature plant cells can:

1. Dedifferentiate — regain the ability to divide (become unspecialized again)
2. Divide — form a mass of undifferentiated cells
3. Redifferentiate — form roots, shoot, and eventually a complete plant

This ability of mature plant cells to undifferentiate, divide, and redifferentiate to develop into a new complete plant is called totipotency. Such cells are called totipotent cells. This is similar to how a zygote divides and differentiates into an entire organism.

Crown Gall Disease

• Caused by the bacterium Agrobacterium tumefaciens
• Produces tumour-like swellings on stems due to rapid, uncontrolled cell division
• Scientists studied how this bacterium transfers genetic material into plant cells
• Application: Agrobacterium is now used as a tool in genetic engineering to introduce useful genes into plants for improved crops, disease-resistant varieties, and valuable phytochemicals

Applications of Totipotency

• Plant tissue culture: Growing complete plants from single cells in laboratory conditions
• Crop improvement: Rapid production of disease-free plants
• Conservation: Preserving endangered plant species
• Sipra Guha Mukherjee (with S. C. Maheshwari) made a breakthrough developing complete plants through anther culture, contributing to modern agriculture