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.