5.4.1 Study Notes Modification of Cell Surfaces 

A. Cell Surfaces in Animals 

• extracellular matrix 

  • a meshwork of polysaccharides and proteins produced by animal cells

  • Collagen gives the matrix strength and elastin gives it resilience

  • Fibronectins and laminins bind to membrane receptors and permit communication between matrix and cytoplasm

    • these proteins also form “highways” that direct the migration of cells during development

  • Proteoglycans are glycoproteins that provide a packing gel that joins the various proteins in matrix and most likely regulate signaling proteins that bind to receptors in the plasma protein

• Junctions Between Cells 

  • points of contact between cells that allow them to behave in a coordinated manner

    • Adhesion junctions mechanically attach adjacent cells

  • desmosomes, internal cytoplasmic plaques, firmly attached to cytoskeleton within each cell, are joined by intercellular filaments; they hold cells together where tissues stretch (e.g., in heart, stomach, bladder)

• tight junctions, plasma membrane proteins attach in zipper-like fastenings

  • they hold cells together so tightly that the tissues are barriers (e.g., epithelial lining of stomach, kidney tubules, blood-brain barrier)

• gap junction allows cells to communicate; formed when two identical plasma membrane channels join

  • provide strength to the cells involved and allow the movement of small molecules and ions from the cytoplasm of one cell to the cytoplasm of the other cell

  • permit flow of ions for heart muscle and smooth muscle cells to contract

B. Plant Cell Walls

• plant cells are surrounded by a porous cell wall

  • it varies in thickness, depending on the function of the cell

• primary cell wall

  • composed of cellulose polymers united into threadlike microfibrils that form fibrils

  • cellulose fibrils form a framework whose spaces are filled by non-cellulose molecules

  • pectins allow the cell wall to stretch and are abundant in the middle lamella that holds cells together

  • Non-cellulose polysaccharides harden the wall of mature cells

  • Lignin adds strength and is a common ingredient of secondary cell walls in woody plants

  • Plasmodesmata are narrow membrane-lined channels that pass through cell walls of neighboring cells and connect their cytoplasms

    • allowing direct exchange of molecules and ions between neighboring plant cells