Tissues in Action

Key Concepts

1. Introduction to Tissues

A tissue is a group of similar cells that work together to perform a specific function. In multicellular organisms, tissues allow for the division of labour, which increases the body's efficiency.

• Hierarchy: Cells → Tissues → Organs → Organ Systems → Organism.

2. Plant Tissues

Plant tissues are broadly classified based on their ability to divide:

A. Meristematic Tissues (Growth Tissues)

Consist of actively dividing cells with thin walls, dense cytoplasm, and large nuclei.

• Apical Meristem: Located at the tips of roots and shoots; increases length.
• Lateral Meristem: Located in a ring in the stem/root; increases girth (thickness).
• Intercalary Meristem: Located at the base of leaves or nodes; helps in regrowth after cutting/grazing.

B. Permanent Tissues

Formed when meristematic cells stop dividing and take on a permanent shape and function.

• Simple Permanent Tissues (One cell type):
  • Parenchyma: Living cells with thin walls; perform storage, photosynthesis, and support.
  • Collenchyma: Living cells with thickened corners; provide flexibility and mechanical strength to young stems.
  • Sclerenchyma: Dead cells with very thick, lignified walls; provide extreme strength (e.g., coconut husk).
  • Protective Tissue (Epidermis/Cork): Forms outer coverings; prevents water loss and entry of pathogens.
• Complex Permanent Tissues (Multiple cell types):
  • Xylem: Transports water and minerals upward from roots; consists of tracheids, vessels, xylem parenchyma, and xylem fibers.
  • Phloem: Transports food from leaves to other parts; consists of sieve tubes, companion cells, phloem parenchyma, and phloem fibers.

C. Plant Tissue Systems

1. Dermal: Outer protective covering.
2. Ground: Main body of the plant (Parenchyma, Collenchyma, Sclerenchyma).
3. Vascular: Conducting tissues (Xylem and Phloem).

3. Animal Tissues

Animal tissues are categorized into four main types:

A. Epithelial Tissue

Forms the outer covering and internal linings.

• Squamous: Flat cells for rapid diffusion (lungs, blood vessels).
• Stratified: Many layers for protection (skin).
• Cuboidal/Columnar: For secretion and absorption (glands, intestine).
• Ciliated: Have hair-like cilia to move substances (respiratory tract).

B. Connective Tissue

Connects and supports other tissues.

• Fluid: Blood (transports nutrients/gases).
• Hard/Rigid: Bone (supports the body; contains calcium/phosphorus).
• Flexible: Cartilage (cushions joints; found in nose/ears).
• Fibrous: Tendons (Muscle to Bone) and Ligaments (Bone to Bone).

C. Muscular Tissue

Responsible for all body movements.

• Skeletal Muscle: Voluntary, striated, multinucleate; attached to bones.
• Smooth Muscle: Involuntary, spindle-shaped; found in internal organs (stomach).
• Cardiac Muscle: Involuntary, branched, striated; found only in the heart.

D. Nervous Tissue

Specialized for communication.

• Neurons: Consist of a cell body, dendrites (receive signals), and an axon (transmits signals).

4. The Musculoskeletal System and Joints

The coordination of bones and muscles allows for movement and posture.

• Joints: Junctions between bones.
  • Ball and Socket: Free movement in all directions (Shoulder, Hip).
  • Hinge: Movement in one plane (Elbow, Knee).
  • Pivot: Rotation (Neck).
  • Fixed: No movement (Skull bones).

5. Specialized Concepts

• Totipotency: The ability of some mature plant cells to revert to a dividing state and develop into an entire new plant (demonstrated by F.C. Steward).
• Crown Gall Disease: Caused by Agrobacterium tumefaciens, which transfers genetic material to plant cells, causing tumors. This bacterium is now used as a tool in genetic engineering.