BSCI330 EXAM 1 STUDY GUIDEChapter 10- Membrane Structure-Plasma membrane: encloses the cell, defines its boundaries, and maintains the essential differences between the cytosol and the extracellular environment.- Membrane-bound organelles for specialization- Ion gradients across the membrane used to synthesize ATP- Proteins that act as sensors of external signals; allow cells to change behavior in responseto environmental cues and transfer signals (and molecules) across membraneTHE LIPID BILAYER- (~5nm thick)- provides fluidity and impermeable barrier to passage of most water-soluble- Easily seen by electron microscopy- Major Lipids in Cell Membrane: Phosphoglycerides, sphingolipids, and sterolso ~5x106 lipid molecules in a 1umx1um area, or ~109 lipid molecules in membraneo All lipid molecules in membrane are amphiphilic: have a hydrophilic (polar) and hydrophobic (nonpolar) endo Phospholipids- one polar head, two hydrophobic hydrocarbon tails (fatty acids) Manufactured in the cytosolic monolayer of ER One tail typically has one more cis-double bond (unsatured), which createsa kink in the tail and makes the membrane more fluid Four major phospholipids in mammalian plasma membrane- phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, sphingomyelin, sphingosine Phosphoglycerides: have three-carbon glycerol backbone, two carbons attached to two fatty acid chains through an ester bond, and the third carbon is attached to a phosphate groupo Also contains: Cholesterol- sterol; up to one molecule for every phospholipid; contains arigid ring structure, attached to a single polar hydroxyl group, and a short non-polar hydrocarbon chain; orient themselves in the bilayer with their hydroxyl group close to the polar head groups of adjacent phospholipids Glycolipids- Transmembrane proteins- serve as structural links that connect cytoskeleton through the lipid bilayer to either the extracellular matrix or an adjacent cell, while others serve as receptors to detect and transduce chemical signals in the environment.-Phopholipids spontaneously form bilayers- Shape and amphiphilic nature of phospholipids cause them to form bilayers in aqueous environments.- lipid molecules aggregate to bury their hydrophobic hydrocarbon tails (energetically favorable) in the interior and expose hydrophilic heads to water in two ways: o spherical micelles (tails inward)o double-layered sheets, or bilayers (hydrophobic tails sandwiched between heads-The lipid bilayer is a two-dimensional fluid- contains 500-1000 different lipid species (phospholipid, cholesterol, glycolipid)- bilayers made in the form of spherical vesicle- liposomes (~25nm-1um in diam)- planar bilayers- black membranes (partitioned between two aqueous compartments)- Techniques to measure the motion of individual lipid molecules: fluorescent dye or gold particle attached to polar head group and follow diffusion of individual molecules in membraneo Lipid molecules readily exchange places with neighbors within a monolayer (~107times per second)- rapid lateral diffusion; lipid molecules diffuse length of a largebacterial cell (~2um) in about 1 secondo These studies show that individual lipid molecules rotate very rapidly about their long axis and have flexible hydrocarbon chains-The fluidity of lipid bilayer depends on its composition- Fluidity of cell membrane must be regulated precisely:o Membrane transport process and enzyme activities cease when bilayer viscosity isincreased beyond threshold level- Bilayer changes from a liquid state to a two-dimensional rigid crystalline (gel) state at a freezing point: phase transitiono If hydrocarbon chains are short or have double bonds, the temperature is lower forthis transition phase (more difficult to freeze)o Shorter chain lengths reduce tendency of the hydrocarbon tails to interact; cis-bonds create kinks in chains that make it difficult to pack together so membrane remains fluid at lower temperatureso Bacteria, yeast, and other organisms, whose temperatures fluctuate, adjust fatty acid composition in membranes to maintain a constant fluidity As temperature falls, cells synthesize fatty acids with more cis-bonds, avoiding the decrease in bilayer fluidity- Cholesterol modulates lipid bilayer propertieso Mixed with phospholipids, it enhances the permeability-barrier properties of lipid bilayero Inserts into bilayer with its hydroxyl group close to polar head of phospholipid; the platelike steroid rings interact (partially immobilize) regions of hydrocarbon chains closest to polar head group, makes lipid bilayer less deformable and decrease the permeability of the bilayer to small water-soluble molecules.o Prevents hydrocarbon chains from coming together and crystallizing-The asymmetry of the lipid bilayer is functionally important- RBCs: phospholipid molecules have choline in head group (outer monolayer) and terminal primary amino group in the inner layer (negative charge)- Converting extracellular signals into intracellular ones;o enzyme protein kinase C (PKC) is activated in response to various extracellular signals, it binds to the cytosolic face of plasma membrane (polar heads- phosphatidylserine is concentrated) and requires this negatively charged phospholipid for activityo Phospholipases activated by extracellular signals to cleave specifc phospholipid molecules, generating fragments that act as short-lived mediators-Glycolipids are found on the surface of all plasma membranes- Glycolipids: sugar-containing lipid molecules found exclusively in the noncytosolic monolayer of bilayer; extreme asymmetry; 5% of lipid molecules in outer monolayero Made from sphingosineo Self-associate, partly through hydrogen bonds between their sugars and partly through van der Waals forces between their long and straight hydrocarbon chainso Asymmetric distribution due to addition of sugar groups to the lipid molecules in the lumen of Golgi apparatuso Most complex: ganglioslides (net negative charge); alters electrical field across membrane and concentrations of ions at membrane surface (Ca2+)o Confined to the exposed surface of cell, so they help protect membrane against harsh conditions (low pH and high concentrations of degradative enzymes)o Function in cell recognition processesMEBRANE PROTEINS-Membrane proteins can be associated with the lipid bilayer in various ways- Transmembrane (Integral) proteins: amphiphilic, where their hydrophilic region is exposed to water on either side
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