DQ PHYSIOLOGY Ankyrin and spectrin fix in place the 3Na*/2K+ pump that is found in all cell membranes Intermediate filaments are thought to give structural strength to cells and help them withstand mechanical stress. Actin filaments. Actin is an abundant cytosolic protein. It exists in F- actin, the polymerized, fibrous form, as a helical arrangement of monomeric G-actin chains. They are present throughout the cell and are concentrated in a narrow band just under the plasma membrane. A variety of proteins form anchoring links between this band and the elements of the plasma membrane. Actin has many additional functions in cells, including(1) aggregation into bundles so as to form microfilaments and (2)participation in movements of the cell surface, including phagocytosis Plasma membrane The plasma membrane defines the perimeter of the cell. Its special compo- Q b port/import functions of substances that were synthesized or are to metabolized within the cell control of intracellular composition, 3. recognition of other cells, and 4. interaction with neighboring cell Membrane structure The two major components are lipids and proteins in prop that among different tissues. The lipids can both rotate and move laterally within their membrane leaf, the proteins are relatively fixed in position because of cytoskeletal (Table 1-1) Lipids. More than half the lipid mass in plasma membranes is phospholipids and their physicochemical behavior imparts many of the characteristics that are associated with cell membranes. The plasma membrane also contains a high proportion of cholesterol. There are two classes of phospholipids: glycero-phospholipids and sphingolipids. Both contain a phosphorylated, charged head group and a pair of different noncharged hydrocarbon tails(Figure 1-5) In an aqueous medium, phospholipids arrange themselves in a double layer with the fatty acid tails facing one another so that the charged heads• Ankyrin and spectrin fix in place the 3Na+/2K+ pump that is found in all cell membranes. Intermediate filaments are thought to give structural strength to cells and help them withstand mechanical stress. Actin filaments. Actin is an abundant cytosolic protein. It exists in F￾actin, the polymerized, fibrous form, as a helical arrangement of monomeric G-actin chains. They are present throughout the cell and are concentrated in a narrow band just under the plasma membrane. A variety of proteins form anchoring links between this band and the elements of the plasma membrane. Actin has many additional functions in cells, including (1) aggregation into bundles so as to form microfilaments and (2) participation in movements of the cell surface, including phagocytosis. Plasma Membrane The plasma membrane defines the perimeter of the cell. Its special compo￾sition allows 1. export/import functions of substances that were synthesized or are to be metabolized within the cell, 2. control of intracellular composition, 3. recognition of other cells, and 4. interaction with neighboring cells. Membrane Structure The two major components are lipids and proteins in proportions that vary among different tissues. The lipids can both rotate and move laterally within their membrane leaf; the proteins are relatively fixed in position because of cytoskeletal anchoring (Table 1–1). Lipids. More than half the lipid mass in plasma membranes is phospholipids and their physicochemical behavior imparts many of the characteristics that are associated with cell membranes. The plasma membrane also contains a high proportion of cholesterol. There are two classes of phospholipids: glycero-phospholipids and sphingolipids. Both contain a phosphorylated, charged head group and a pair of different, noncharged hydrocarbon tails (Figure 1–5). In an aqueous medium, phospholipids arrange themselves in a double layer with the fatty acid tails facing one another so that the charged heads 8 PDQ PHYSIOLOGY