Chapter 11: Cell-Cell Interactions

11.1 The Cell Surface 

• ==Virtually all plant cells are surrounded by a cell wall-a fiber composite that is the basis of major industries.==
• When plant cells first form, they secrete an initial fiber composite called a primary cell wall.
• The fibrous component of the primary cell wall consists of long strands of the polysaccharide cellulose. These strands are bundled into stout structures termed microfibrils.
• The spaces between microfibrils are filled with gelatinous polysaccharides such as pectins-the molecules that are used to thicken jams and jellies.
• The force exerted by the cell against the wall is known as turgor pressure.
• Expansins disrupt the hydrogen bonds that crosslink microfibrils to other polymers in the wall, loosening the structure and allowing the microfibrils to slide past one another. 
• As plant cells mature and stop growing, they may secrete an additional layer of material-a secondary cell wall-between the plasma membrane and the primary cell wall.
• In cells that form wood, the secondary cell wall also contains lignin, a complex polymer that forms an exceptionally rigid network.
• Most animal cells secrete a fiber composite called the extracellular matrix, or simply ECM.
• The fibrous component of animal ECM is dominated by glycoproteins named collagen
• The ground substance that surrounds collagen and other fibrous components of the ECM contains highly glycosylated, gel forming proteins called proteoglycans
• A tissue consists of similar cells that function as a unit.
• Membrane proteins called integrins bind to extracellular cross-linking proteins, including laminins, which in turn bind to other components of the ECM. 

11.2 How Do Adjacent Cells Connect and Communicate?

• Direct physical connections between cells are the basis of multicellularity. 
• Materials and structures that bind cells together are particularly important in epithelial-tissues that form external and internal surfaces. 
• A tight junction is a cell-cell attachment composed of specialized proteins in the plasma membranes of adjacent animal cells
• Desmosome is a strong cell-cell attachment particularly common in animal epithelial cells and certain muscle cells. 
• As the experiment continued, cells began to aggregate based on their origin-adhering to other cells of the same tissue type. This phenomenon is now called selective adhesion. 
• An antibody is a protein produced by an immune response that binds specifically to a unique type of molecule, often another protein 
• Cadherins are the linking proteins in desmosomes 
• In many animal tissues, structures called gap junctions connect adjacent cells.
• In plants, gaps through cell walls allow direct connections between the cytoplasm of adjacent cells which are called plasmodesmata
• Symplast is a continuous network of cytoplasm connected by plasmodesmata.
• Apoplast is the region outside the plasma membrane

11.3 How Do Distant Cells Communicate?

• Hormones are information-carrying molecules that are secreted by plant and animal cells into bodily fluids and act on distant target cells. 
• A signal receptor is a protein that changes its shape and activity after binding to a signaling molecule. 
• ==Important information regarding signal receptors include:== 
  • ==Receptors are dynamic== 
  • ==Receptors can be blocked== 
• Signal transduction is the conversion of a signal from one form to another.
• Many signal receptors span the plasma membrane and are closely associated with peripheral membrane proteins inside the cell called G proteins.
• Second messenger is a small, nonprotein signaling molecule or ion that elicits an intracellular response to the first messenger
• Protein kinases are enzymes that activate or inactivate other proteins by adding a phosphate group to them.
• This sequence of protein modifications that culminates in a cell response is termed a phosphorylation cascade.

11.4 Signaling between Unicellular Organisms

• The use of signaling pathways to respond to population density in prokaryotic and eukaryotic microbes is referred to as quorum sensing.
• ==Cell signaling helps organisms ranging from bacteria to blue whales receive information about their environment and respond appropriately to changing conditions.==