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.