Biology 11.1 THE CELL SURFACEThe Structure and Function of an Extracellular Layer-Cells usually aren’t bounded simply by a plasma membrane-Bacteria, algae, fungi, and plants have cell walls-Animal cells have extracellular matrix-Consist of cross-linked network of long filaments embedded in a stiff surrounding material-Fiber composites are particularly rugged due to a combination of tension and compression resisting elements.The Cell Wall in Plants-Cell Wall is dynamic, can send signaling molecules that trigger reinforcement of walls in nearby cellsPrimary Cell Walls-Fibrous component of primary cell walls consist of long strands of cellulose, which are cross-linked by other polysaccharide filaments and bundled into stout, cable-likestructures termed microfibrils. -The microfibrils are synthesized by a complex of enzymes in the plasma membrane, forming a crisscrossed network.-Space between microfibrils is filled with gelatinous polysaccharides like pectins-Pectins-Molecules that are used to thicken jams and jellies-Polysaccharides in pectin are hydrophilic-keep cell wall moist-gelatinous components of the cell wall are synthesized in the rough endoplasmic reticulum and Golgi apparatus and secreted to the extracellular space.-Primary cell wall defines shape of a plant cell-Normal conditions: nucleus and cytoplasm fill entire volume of the cell and push plasma membrane against cell wall-Water enters cell through osmosis inflating plasma membrane and exerts a force against cell wall called turgor pressure.-Young plant cells secrete enzymes named expansins into cell wall matrix-Expansins catalyze reactions that allow the microfibrils in the matrix to slidepast one another-Turgor pressure then forces the wall to elongate and expand causing cell growthSecondary Cell Walls-As plant cells mature and stop growing they secrete a layer of material-secondary cell wall- inside the primary cell wall-Cells that form wood, secondary cell wall includes lignin-Lignin is a tough substance that forms an exceptionally rigid network.The Extracellular Matrix in Animals-Most animal cells secrete a fiber composite called extracellular matrix (ECM)-ECM’s most important function is structural support.-ECM contains more protein than cell wall.-Fibrous component of animal ECM dominated by cable-like protein collagen-Collagen proteins consist of three polypeptide chains that wind around each other.-ECM contain many collagen fibrils, each made up of many collagen proteins-Matrix that surrounds collagen and other fibrous components consist of gel-forming polysaccharides- most of which are attached to a protein core.-Collagen and common ECM proteins are elastic and bendable-Most ECM components are synthesized in rough ER, processed in Golgi apparatus, and secreted from cell through exocytosis.-Some protein-bound polysaccharides that form the composite material are synthesized by membrane proteins-Amount of ECM varies among types of cells and composition of ECM varies11.2 How Do Adjacent Cells Connect and Communicate?Cell-Cell Attachments in EukaryotesTight JunctionsDesmosomesSelective AdhesionThe Discovery of CadherinsCells Communicate via Cell-Cell GapsPlasmodesmataGap