ABSTRACT
Membranes Membranes form boundaries both around the cell (the plasma membrane) and around distinct subcellular compartments (e.g. nucleus, mitochondria, lysosomes, etc.) (Sections A1 and A2). They act as selectively permeable barriers allowing the inside environment of the cell or organelle to differ from that outside (Sections E3). Membranes are involved in signaling processes; they contain specific receptors for external stimuli and are involved in both chemical and electrical signal generation (Sections E5 and E6). All membranes contain two basic components: lipids and proteins. Some membranes also contain carbohydrate. The composition of lipid, protein and carbohydrate varies from one membrane to another. For example, the inner mitochondrial membrane has a larger amount of protein than lipid due to the presence of numerous protein complexes involved in oxidative phosphorylation and electron transfer (Section L2), whereas the myelin sheath membrane of nerve cells, which serves to insulate the cell electrically, has a larger proportion of lipid (Section E6).
