Cell Membrane, Part 1

• define external boundaries of cell • regulate molecular traffic across boundary • eukaryotic cells: compartmentalization • central to energy conservation and cell-to-cell communication • flexible, self-sealing and selectively permeable to polar solutes and to allow growth and movement • can break and re-seal for fusion (exocytosis) or fission (endocytosis, cell division) • attachment surface

• separation of antiparallel processes • ex: anabolic and catabolic processes can be held in separate compartments – avoiding competition for substrate or interference in reactions FA synthesis in cytosol, FA oxidation in mitochondria) • separation of similar reactions serving different purposes • ex: similar reactions for different purposes, thus must be held in different compartments • fatty acid oxidation: • in mitochondria: energy production • in peroxisomes: heat production • coordination of reactions which are involved in the same pathway (energy efficiency) • ex: TCA and ETC are central point of energy metabolism in cells and are located in mitochondria

functional separation within the cell: • plasma membrane • nuclear membrane (inner and outer) 2 • ER and Golgi apparatus membrane • mitochondrial membrane (inner and outer)2 • peroxisomes and lysosomes

• regulates movement of material into and out of cell • facilitates electrical signaling btwn cells • defines boundaries of organelles and separates complex chemical reactions • -> multiple differing functions

• very thin film of lipid and protein molecules held together mainly by non covalent interactions • fluid, dynamic structures with most of their molecules able to move about in plane of membrane (fluid mosaic model) • arranged as continuous double layer (lipid bilayer) • relatively impermeable barrier to most water-soluble molecules (polar) • 3 main lipids: phospholipids, glycolipids and cholesterol • most membrane proteins span both parts of lipid bilayer some will only be in one layer

• separates cellular interior from exterior • exchanges information with environment in controlled manner • membranes are dynamic • membranes get information from cell about metabolic status • selective barriers, allow selective transport of molecules in and out of cell • contain enzymes • contain receptors

• fusion of 2 membranes central to many processes involving organelles and plasma membrane • vesicles bud from ER to transport synthesized lipids and proteins to other organelles and plasma membrane • processes involve membrane reorganization, fusion of 2 membrane segments without loss of continuity

• plasma membrane appears as 3-layer structure (osmium bound to the inner and outer surfaces of the membrane) separated by a less dense central region

• lipids (phospholipids, glycolipids, sphingolipids) • proteins (integral, peripheral) • carbohydrates • water • divalent cations • cholesterol (not present in most prokaryotes)

• proteins that are part of or interact with membranes • span lipid bilayer and mediate many functions of membranes: transport and catalysis of reactions • some transmembrane proteins serve as structural links that connect cytoskeleton through lipid bilayer to either ECM or to adjacent cell • others serve as receptors to detect and transduce signals • membrane proteins are targets of over 50% of all modern medicinal drugs

• often composed of one main type of phospholipid and contain NO CHOLESTEROL • mechanical stability is steadied by overlying cell wall

• contain large amounts of cholesterol and mixture of different phospholipids • different sterols in different organisms

• universal basis for membrane structure • bilayer structure is result of special properties of lipid molecules that cause spontaneous assemblage • lipid molecules make up about 50% of mass of most animal cell membranes, remainder is proteins • all lipid molecules in cell membranes are amphipathic: one hydrophilic (polar) head and hydrophobic (non polar) tail • most abundant membrane lipids -> phospholipids

• polar head group and 2 hydrophobic hydrocarbon tails (FAs) • tails can differ in length • one tail usually contains 1 or more cis-double bonds (unsaturated), while other does not (saturated) • double bond creates small kink in tail (important in formation of bilayer) • differences in length and saturation of fatty acid tails affect ability of phospholipid molecules to pack together and guarantee membrane fluidity

• spherical micelles (energy favorable shape) 2. bimolecular sheets - bilayers

• cone-shaped amphiphilic FATTY ACID molecules form micelles

• cylinder-shaped molecules (PHOSPHOLIPIDS) form bilayers

• energetically most favorable • stable • provides bilayer’s self-healing property: all free edges are avoided by closing on itself

• phosphatidylcholine • phosphatidylethanolamine • phosphatidylserine • sphingomyelin • these 4 phospholipids make up more than half lipid mass in most membranes

• only phospholipid that carries a NET NEGATIVE CHARGE at physiologic pH • others are neutral • PREVALENT IN INNER MEMBRANE LAYER

• present only in small quantities - important functionally (cell-signaling)

• prevalent in outer membrane layer

• prevalent in inner membrane layer

• prevalent in outer membrane layer