Support and Movement in Organisms

Organisms require support and movement to maintain their shape, withstand gravity, protect internal organs, and actively seek food or mates. In plants, this is achieved through specialized tissues and growth responses, while in animals, various types of skeletons and joints facilitate these functions.

1. Plant Supporting Tissues

Plants rely on specific tissues to remain upright and withstand environmental stress:

• Collenchyma: Living cells with unevenly thickened primary walls (cellulose). They provide flexibility and support in young, growing stems and leaves.
• Sclerenchyma: Dead cells at maturity with thick, lignified secondary walls. They provide rigid mechanical support. Examples include fibers and sclereids.
• Xylem: While primarily for water transport, xylem vessels and tracheids are highly lignified, providing the main structural support for woody plants.
• Phloem Fibers: Provide additional tensile strength to the vascular bundles.

Distribution of Supporting Tissues

• Roots: Supporting tissues (xylem) are located at the center to resist pulling forces (tug-of-war effect).
• Stems: Supporting tissues are located towards the periphery (vascular bundles) to resist bending forces caused by wind.
• Leaves: Xylem and collenchyma are located in the midrib and veins to keep the leaf blade expanded.

2. Plant Movements and Responses

Plants respond to stimuli through three main types of movements:

• Tactic Movement (Taxism): Directional movement of a whole organism (e.g., swimming of moss sperm toward chemicals).
• Tropic Movement (Tropism): Directional growth response determined by the direction of the stimulus. Examples include Phototropism (light), Geotropism (gravity), Hydrotropism (water), and Thigmotropism (touch).
• Nastic Movement: Non-directional response where the direction of movement is independent of the stimulus direction (e.g., closing of Mimosa pudica leaves).

The Role of Auxins: Auxins are growth hormones produced at the shoot and root tips. In shoots, higher auxin concentration promotes cell elongation. In phototropism, auxins migrate to the shaded side, causing it to grow faster and bend the shoot toward the light.

3. Animal Skeletons

There are three main types of skeletons in animals:

• Hydrostatic Skeleton: Found in soft-bodied animals like earthworms; fluid-filled compartments provide support.
• Exoskeleton: Hard outer covering made of chitin (in arthropods) or calcium carbonate (in mollusks). It protects and prevents desiccation but limits growth, necessitating molting (ecdysis).
• Endoskeleton: Internal framework made of cartilage and bone found in vertebrates.

The Mammalian Skeleton

The mammalian skeleton is divided into the Axial Skeleton (Skull, vertebral column, ribs, sternum) and the Appendicular Skeleton (Pectoral and pelvic girdles, limb bones).

• Support: Provides a framework for the body.
• Protection: Skull protects the brain; ribs protect heart and lungs.
• Locomotion: Acts as levers for muscles.
• Respiration: The rib cage and sternum move to facilitate breathing.

4. Joints

Joints are points where two or more bones meet. They are classified by their degree of movement:

• Immovable/Fibrous: Sutures of the skull.
• Slightly Movable/Cartilaginous: Between vertebrae.
• Freely Movable/Synovial: Includes Ball and Socket (shoulder/hip), Hinge (elbow/knee), Pivot (neck), and Gliding (wrist).