Chapter 12 Final Review: Essay
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1) What are the 8 characteristics of biological membranes?
- a. They are sheetlike structures, only two molecules thick, that form closed boundaries between different compartments
- b. Membranes consist mainly of lipids and proteins.
- c. Membrane lipids are small molecules that have both hydrophilic and hydrophobic moeieties
- d. Specific proteins mediate distinctive functions of membranes
- e. Membranes are noncovalent assemblies
- f. Membranes are asymmetric
- g. Membranes are fluid structures. Lipid molecules diffuse rapidly in the plane of the membrane
- h. Most cell membranes are electrically polarized
1) What properties of fatty acids and lipids affect their fluidity?
a. Fluidity is dependent on chain length and degree of saturation. Unsaturated fatty acids have lower melting points than do saturated fatty acids of the same length. Thus, short chain length and unsaturation enhance the fluidity of fatty acids and their derivatives.
1) What are the three types of membrane lipids?
a. Phospholipids, glycolipids, and cholesterol
1) What are phospholipids? Explain them.
- a. Phospholipids are constructed from four components: one or more fatty acids, a backbone, a phosphate, and an alcohol attached to the phosphate group. The different backbones can be glycerol, a three-carbon alcohol, or sphingosine.
- b. Phospholipids can be phosphoglycerides or sphingomyelin
a. They are derived from sphingosine, except that sphingomyelin’s primary hydroxyl is bound to something other than phosphorylcholine
a. It is the third major type of membrane lipid, has a structure that is different from phospholipids. It is a steroid built from four linked hydrocarbon rings. A hydrocarbon tail is linked to the steroid at one end, and a hydroxyl group is attached at the other end. The orientation is parallel to the fatty acid chains of the phospholipids, and the hydroxyl group interacts with the nearby phospholipid head groups.
1) Why are bimolecular sheets more favorable than micelles?
a. The reason is that the two fatty acid chains of a phospholipid or a glycolipid are too bulky to fit into the interior of a micelle. In contrast, salts of fatty acids readily form micelles because they contain only one chain. The formation of bilayers instead of micelles by phospholipids is important because it can extend to macroscopic dimensions.
1) How do lipid bilayers form?
a. They form spontaneously by a self-assembly process. It is rapid and spontaneous; and hydrophobic interactions are the major driving force for the formation of lipid bilayers. Water molecules are released from the hydrocarbon tails of membrane lipids as they become sequestered in the bilayer; and, van der Waals forces favor this close packing. Electrostatic and hydrogen-bonding attractions between the polar head groups and water molecules also facilitate its forming. Lipid bilayers are cooperative structures.
1) Why are hydrophobic interactions important?
- a. Lipid bilayers have an inherent tendency to be extensive
- b. Lipid bilayers close on themselves to make sure no edges are exposed
- c. Lipid bilayers are selcf-sealing because a hole is energetically unfavorable
1) How are liposomes formed?
a. They are formed by suspending a suitable lipid in an aqueous medium, and then sonicating to create closed vesicles.
1) How can one measure the permeability of a molecule?
a. Vesicles can be formed in a solution containing ion molecules by sonication. The ions will then be trapped in the vesicle. Afterwards, the solution undergoes gel filtration. After the solution is devoid of the ions, the permeability can then be determined by measuring the rate of efflux of the ion from the inner compartment of the vesicle to the solution. Protein-liposome complexes can also be created.
1) What is the permeability of membranes?
a. They have very low permeability for ions and most polar molecules except for water, which traverses easily due to its high concentration ad lack of a complete charge. The permeability of small molecules is correlated with their solubility in a nonpolar solvent relative to their solubility in water.
1) How might molecules traverse a lipid bilayer membrane?
a. First, it sheds its solvation shell of water; then, it is dissolved in the hydrocarbon core of the membrane; and, finally, it diffuses through this core to the other side of the membrane, where it becomes resolvated by water.
1) What are the types of proteins? And, how can we determine what they are?
a. There are integral and peripheral. We can determine what they are on the basis of how they are removed from the membrane. Integral proteins interact with the hydrocarbon chains and can be released only by detergents or organic solvents. Peripheral proteins are bound to membranes by electrostatic or hydrogen-bond interactions and can be removed by changing the salinity or pH.
1) What are the two ways that membranes can span the membrane?
- a. Alpha helices and Beta strands can span the membrane.
- b. For alpha helices, most of the amino acids in the membrane-spanning alpha helices are nonpolar and only a very few are charged. They either contact the hydrocarbon core or one another. Alpha helices are the most common structural motifs in membrane proteions.
- c. Beta strands can also be located in the membrane. Each strand is hydrogen bonded to its neighbor in an antiparallel arrangement, forming a single beta sheet, which curls up to form a hollow cylinder that forms a pore in the membrane. The outside surface is nonpolar and the inside of the channel is hydrophilic.
1) Aside from association to the membrane via hydrophobic groups, what are other methods for attaching to the membrane?
a. Thre are palmitoyl groups, which acttach to a cysteine residue by a thioester bond, a farnesyl group, which attaches to the cysteine residue at the carboxyl terminus, and a glycolipid structure called a GPI anchor, attached to the carboxyl terminus end.
1) How can one predict whether a transembrane helix exists?
a. We can ask whether a proposed helical segment is likely to be mre stable in a hydrocarbon environment or in water. WE do this by estimating the free-energy change when a helical segment is transferred from the interior of a membrane to water. The hydrocarbon core of a membrane allows an alpha helix with 20 residues. The way we determine this is by estimating the free-energy change that takes place when a hypothetica alpha helix formed of a window of proteins is transferred from the membrane interior to water.
1) What might a peak indicate in the hyropathy plot?
a. It can indicate an alpha helix. However, it can reach the criterion level, but be a beta strand, meaning no alpha helices are present.
1) What is the fluid mosaic model?
a. It states that membranes are 2D solutions of oriented lipids and globular protiens. The lipid bilayer is a solvent for integral proteins and a permeability barrier.
1) Why is membrane fluidity important? Explain fluidity.
a. Many processes, such as transport and signal transduction depend on it. The transition from rigid to fluid takes place as the temperature is raised above the melting temperature and depends on the length of the fatty acid chains and their degree of unsaturation. The presence of saturated fatty acids favors a rigid state. A cis double bond bends, interfering with tight packing and lowering the melting temperature. Shorter chains also interact less closely. Bacteria can regulate the fluidity fo their membrane by varying the number of double bonds and the length of their fatty acid chains. Animals control fluidity by regulating membrane fluidity
1) Explain receptor-mediated endocytosis.
a. Many cells take up molecuels through receptor-mediated endocytosis. a protein or larger complex binds to a receptor on the cell surface. After the receptor is bound, specialized proteins cause invagination; and, a protein called clathrin polymerizes into a lattice network around the growing membrane bud, called a clathrin coated pit. The invaginated membrane breaks off and fuses to form a vesicle.
1) Explain receptor mediated endocytosis in terms of iron.
a. Because iron is critica, and free iron is toxic, transport of iron ions must be controlled. Iron in the blood is bound to a protein transferrin. Cells requiring iron have transferrin receptors, which the transferrin bidns to initiate receptor-mediated endocytosis, internalizing the complexes within vesicles called endosomes. As endosomes mature, proton pumps within the vesicle membrane ower the pH, causing dissociation of iron from transferrin, where they pass intot eh cytoplasm. The iron-free transferrin complex is recycled and the cycle can begin again.
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