Z

PLANT CELL WALLS

Overview of Multicellular Organisms

  • Multicellular organisms consist of various cell types organized into tissues.
  • Four major tissue types in animals:
    • Nervous system tissue
    • Muscle tissue
    • Epithelial tissue
    • Connective tissue
  • Tissues comprise cells and an extracellular matrix (ECM), which provides structural support and facilitates cell communication through cell junctions.

Differences in Tissues: Plants vs. Animals

  • Animals:
    • Mobile, cells need to be strong and agile.
    • Tissues must be able to change shape rapidly.
  • Plants:
    • Stationary, tissues require rigidity but can be fragile without a cell wall.

Structure and Function of Plant Cell Walls

  • Cell walls protect, limit shape, and provide structure to plant cells.
  • Act as extracellular matrix: synthesized and secreted by plant cells.
  • Cell wall characteristics depend on cell activity, affecting thickness and flexibility.

Types of Plant Cell Walls

  • Primary Cell Wall:
    • Formed when the cell is new and can expand as the cell grows.
  • Secondary Cell Wall:
    • Formed when cell growth stops; may involve thickening the primary wall.

Components of Cell Walls

  • Pectin in primary walls (stained green in cross-sections).
  • Cellulose in secondary walls (stained blue), providing structural integrity.
  • Primary cell walls demonstrate intact forms around cells, particularly in root structures.

Mechanical Strength and Growth of Cell Walls

  • Tensile strength is obtained through cellulose microfibrils aligned along stress lines.
  • Cellulose:
    • Long unbranched chains of glucose, resisting compression and tension.
  • Orientation of cellulose microfibrils influences cell growth direction.
  • Cell growth influenced by turgor pressure from internal water pressure.

Directional Growth Mechanism

  • If microfibrils grow around the cell, it grows upwards; if across, it grows outward.
  • The cell dictates the direction of microfibril growth, guiding its own growth pattern.

Cellulose Synthesis

  • Cellulosynthesizing proteins create cellulose externally from intracellular sugar monomers through enzymes on the extracellular side of the membrane.
  • Enzymes move along microtubules beneath the plasma membrane.
  • This movement leaves a trail of microfibrils oriented according to the microtubule direction, influencing the ECM's construction.

Summary of Mechanisms

  • Microtubules inside the cell regulate the orientation of external microfibrils by guiding the movement of cellulose-synthesizing enzymes.
  • Relationship between intracellular processes and extracellular structure is essential for appropriate cell wall functionality.