Biology 11 1 THE CELL SURFACE The 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 cells Primary 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 like structures 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 slide past one another Turgor pressure then forces the wall to elongate and expand causing cell growth Secondary 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 gelforming 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 varies 11 2 How Do Adjacent Cells Connect and Communicate Cell Cell Attachments in Eukaryotes Tight Junctions Desmosomes Selective Adhesion The Discovery of Cadherins Cells Communicate via Cell Cell Gaps Plasmodesmata Gap Junctions