3.6

Cell Adhesion and Communication in Multicellular Organisms

Introduction to Cellular Cohesion and Communication

  • Multicellular organisms, including plants and animals, are composed of many cells that must work cooperatively.
  • Adhesion: Cells need to adhere to one another to prevent the organism's disintegration (e.g., a body or plant in heavy rain).
  • Communication: Cells in direct contact must also communicate to coordinate development and respond to environmental stimuli.
  • This section details the mechanisms by which plant and animal cells stick together and share signals.

Plant Cell Adhesion and Communication

Cell Walls
  • Presence: Cell walls surround the cell membranes of nearly all bacteria, archaea, fungi, algae, and plants.
  • Misleading Term: The term "cell wall" is often misleading; it's not merely an outlining barrier.
  • Functions:
    • Impart shape to the cell.
    • Regulate cell volume.
    • Prevent the cell from bursting when it takes in excessive water.
  • Composition: The wall itself consists largely of cellulose molecules.
    • Cellulose molecules are aligned into countless criss-crossing fibers, which provide significant strength.
Plasmodesmata
  • Definition: Plasmodesmata (singular: plasmodesma) are specialized channels that directly connect adjacent plant cells.
  • Function: They are essentially "tunnels" within the cell wall, facilitating communication and transport between cells.
  • Mechanism: Materials can move from one cell to another via a thin strand of cytoplasm that passes through each channel.
  • Significance: They allow plant cells to communicate with their neighbors through the cell wall, coordinating cellular activities.

Animal Cell Adhesion and Communication

Absence of Cell Walls
  • Animal cells fundamentally lack cell walls.
Extracellular Matrix (ECM)
  • Secretion: Many animal cells secrete a complex extracellular matrix.
  • Functions:
    • Holds animal cells together in tissues.
    • Coordinates many aspects of cellular life.
  • Context: The ECM is particularly important in tissues where cells are not in direct contact with one another.
Direct Cell-to-Cell Junctions
  • In other animal tissues, the outer membranes of adjacent cells directly connect to one another via several types of junctions.
1. Tight Junctions
  • Structure: Groups of tight junctions fuse animal cells together.
  • Function: They form an impermeable barrier between cells, effectively sealing off intercellular space.
  • Mechanism: Proteins anchored in the cell membranes connect to the actin cytoskeleton, joining cells into sheets.
  • Examples: Found in tissues lining the inside of the digestive tract.
  • Control of Movement: These connections control where biochemicals move by preventing fluids from leaking between the joined cells.
    • Practical Example: Tight junctions prevent potent substances like stomach acid from seeping into the surrounding tissues outside the stomach.
2. Anchoring (Adhering) Junctions
  • Structure: Connects an animal cell to its neighbors or to the extracellular matrix.
  • Mechanism: Proteins at each anchoring junction span the cell membrane and link directly to each cell's cytoskeleton.
  • Function: They provide strong mechanical attachments, holding cells in place.
  • Examples: These junctions are crucial for holding skin cells in place by anchoring them to one another and to the extracellular matrix, imparting strength and integrity to tissues under mechanical stress.
3. Gap Junctions
  • Structure: A gap junction is a protein channel that directly links the cytoplasm of adjacent animal cells.
  • Function: They allow for the rapid exchange of ions, nutrients, and other small molecules between cells.
  • Analogy: Gap junctions are analogous to plasmodesmata in plants, serving a similar function in intercellular communication and transport.
  • Examples: They link heart muscle cells to one another, which is critical for allowing groups of cells to contract together in a coordinated fashion, essential for the heart's pumping action.