biochem exam 3: chapter 12

what are the three significant biological consequences of the hydrophobic interactions of a lipid bilayer?

(1) lipid bilayers have an inherent tendency to be extensive (2) lipid bilayers will tend to close on themselves so that there are no edges with exposed hydrocarbon chains, and so they form compartments (3) lipid bilayers are self-sealing because a hole in a bilayer is energetically unfavorable

what are the 8 common features that all biological membranes have?

1. membranes are sheetlike structures, only 2 molecules thick, that form closed boundaries between different compartments 2. membranes consist mainly of lipids and proteins, mass ratio lipids to proteins ranging from 1:4 to 4:1. membranes also contain carbohydrates linked to lipids and proteins. 3. membrane lipids are small molecules that have both hydrophilic and hydrophobic moieties. lipids spontaneously form bilayers that are barriers to the flow of polar molecules. 4. specific proteins mediate distinctive functions of membranes (serve as pumps, channels, receptors, energy transducers, and enzymes) 5. membranes are noncovalent assemblies. the protein and lipids molecules are held together by many noncovalent interactions. 6. membranes are asymmetric. 7. membranes are fluid structures. lipid molecules and protein molecules diffuse rapidly in the plane of the membrane unless anchored by an interaction, however they do not readily rotate across the membrane. 8. most cell membranes are electrically polarized, the inside is negative (usually -60mV). the membrane potential plays a key role in transport, energy conversion, and excitability

what is a phospholipid molecule constructed from?

1. one or more fatty acids 2. a platform to which the fatty acids are attached 3. a phosphate 4. an alcohol attached to the phosphate

how do the membranes of archaea differ in composition from those of eukaryotes or bacteria?

1. the nonpolar chains are joined to a glycerol backbone by ether rather than ester linkages. the ether linkage is more resistant to hydrolysis. 2. the alkyl chains are branches rather than linear. they are built from repeats of a fully saturated 5-carbon fragment. the branched, saturated hydrocarbons are more resistant to oxidation. 3. the stereochemistry of the central glycerol is inverted

what chain length is most common for fatty acids in biological systems

16- and 18-carbon fatty acids are most common

what is the favored structure for most phospholipids and glycolipids in aqueous media

a bimolecular sheet rather than a micelle

what is a demyelinating disease (give an example)

a disease where myelin assembly is impaired or existing myelin is damaged an example is multiple sclerosis this proves the importance of myelination

what do phosphoglycerides consist of?

a glycerol backbone to which are attached two fatty acid chains and a phosphorylated alcohol

what is sphingomyelin

a phospholipid found in membranes that is not derived from glycerol

how can polar interactions be disrupted?

adding salts or by changing the pH

what is SDS-polyacrylamide gel electrophoresis?

allows us to visualize the protein components of a membrane

alternatively, how can the position of a double bond be denoted?

alternatively, the position of a double bond can be denoted by counting from the distal end, with the ω-carbon atom (the methyl carbon) as number 1

what type of moleules are membrane lipids

amphipathic molecules

what type of membrane protein is bacteriorhopsin

an archael protein that uses light energy to transport protons from inside to outside the cell, generating a proton gradient used to form ATP this protein is built almost entirely of α helices; seven closely packed α helices, nearly perpendicular to the plane of the cell membrane, span its 45-Å width

how many carbon atoms do fatty acids in biological systems usually contain?

an even number of carbon atoms, typically between 14 and 24

what membranes do eukaryotic cells have?

an external cell membrane (the plasma membrane) internal membranes that form the boundaries of organelles such as mitochondria, chloroplasts, peroxisomes, and lysosomes

what is a planar bilayer membrane?

another well-defined synthetic membrane, has electrical conduction properties that are readily studied by inserting electrodes into each aqueous compartment

what kind of structures are lipid bilayers?

because lipid bilayers are held together by many reinforcing, noncovalent interactions (predominantly hydrophobic), they are cooperative structure

what are biological membranes?

biological membranes define the boundaries of all cells, they are dynamic structures in which proteins float in a sea of lipids

which carbon atoms are often referred to as α and β, respectively

carbon atoms 2 and 3

what is the simplest glycolipid and what is it composed of

cerebroside, contains a single sugar residue either glucose or galactose

where is cholesterol absent from and where is it found?

cholesterol is absent from prokaryotes but is found to varying degrees in virtually all animal membranes it constitutes almost 25% of the membrane lipids in certain nerve cells, but is essentially absent from some intracellular membranes

why are liposomes promising for several therapeutic applications?

drug delivery with liposomes often lessens its toxicity the selective fusion of lipid vesicles with particular kinds of cells is a promising means of controlling the delivery of drugs to target cells

describe the arrangement of β strands in porin

each strand is hydrogen bonded to its neighbor in an antiparallel arrangement, forming a single β sheet. the β sheet curls up to form a hollow cylinder that, as its name suggests, forms a pore, or channel, in the membrane

what additional functions do biological membranes do?

energy storage and information transduction the proteins associated with the membrane define these functions for any given cell

fatty acids are ionized at what pH?

fatty acids are ionized at physiological pH so its appropriate to refer to them according to their carboxylate form (i.e. palmitate or hexadecanoate)

how can phospholipid molecules arrange themselves in aqueous solutions

form a globular structure called a micelle, which is surrounded by water, and the hydrocarbon tails are sequestered inside, interacting with one another

alternatively, how else can phospholipid molecules be arranged

forming a lipid bilayer, composed of two lipid sheets also called a bimolecular sheet has a hydrophobic interior that acts as a permeability barrier the hydrophilic head groups interact with the aqueous medium on each side of the bilayer

what are more complex glycolipids?

gangliosides, they contain a branched chain of as many as seven sugar residues

what can the platform on which phospholipids are built be?

glycerol, a three carbon alcohol, or sphingosine, a more complex alcohol

how are glycolipids often oriented?

glycolipids are oriented in a completely asymmetric fashion with the sugar residues always on the extracellular side of the membrane

what is the second major class of membrane lipids

glycolipids, sugar-containing lipids

what is the major driving force for the formation of lipid bilayers?

hydrophobic interactions

in membranes, what is the orientation of cholesterol molecules?

in membranes, the orientation of the molecule is parallel to the fatty acid chains of phospholipids, and the hydroxyl group interacts with the nearby phospholipid head groups

what can integral membrane proteins also be composed of and what does this form?

integral membrane proteins may also be composed of β strands that form a pore in the mebrane

what do lipid bilayer membranes have a low permeability for?

ions and most polar molecules

what is the structure of cholesterol?

it is a steroid, build from four linked hydrocarbon rings a hydrocarbon tail is liked to the steroid at one end, and a hydroxyl group is attached at the other

why is water an exception permeability wise with lipid bilayers?

it traverses such membranes relatively easily because of its low molecular weight, high concentration, and lack of a complete charge

what are the two opposing sheats of a bilayer membrane called

leaflets

what are the glycolipids in animal cells derived from>

like sphingomyelin, the glycolipids in animal cells are derived from sphingosine

what is an exception to what lipid bilayer membranes have a low permeability for?

lipid bilayer membranes have a low permeability for ions and most polar molecules. the exception is water, water traverses such membranes relatively easily

what are liposomes?

lipid vesicles, aqueous compartments enclosed by a lipid bilayer

lipids found in ___ and ___ are similar, and lipids found in ___ are distinct

lipids found in eukaryotes and bacteria are similar, and lipids found in archaea are distinct, although they have many features related to membrane formation in common with lipids of other organisms

how are lipsomes formed?

liposomes are formed by suspending a suitable lipid, such as phosphatidylcholine, in an aqueous medium, and then sonicating (i.e., agitating by high-frequency sound waves) to give a dispersion of closed vesicles that are quite uniform in size

how are glycine-containing liposomes prepared?

liposomes containing glycine are formed by the sonication of phospholipids in the presence of glycine. free glycine is removed by gel filtration (dialysis or gel-filtration chromatography)

what are fatty acids

long hydrocarbon chains of various lengths and degrees of unsaturation that terminate with carboxylic acid groups

where are peripheral membranes bound?

many peripheral membrane proteins are bound to the surfaces of integral proteins, on either the cytoplasmic or the extracellular side of the membrane. others are anchored to the lipid bilayer by a covalently attached hydrophobic moiety/chain, such as a fatty acid

what properties enable phospholipids to form membranes

membrane formation is a consequence of the amphipathic nature of the molecules polar head groups favor contact with water, whereas hydrocarbon tails interact with one another in preference to water

what are membrane proteins responsible for?

membrane proteins are responsible for most of the dynamic processes carried out by membranes while membrane lipids establish a permeability barrier to polar molecules and ions, membrane proteins allow the transport of molecules and information across the membrane membrane lipids create the appropriate environment for the action of such proteins

what is the most common structural motif in membrane proteins?

membrane-spanning α helices

what is the protein content of membranes?

membranes vary in proteoin content, from less than 20% (myelin) to as much as 75% (energy-transduction membranes, such as the internal membranes of mitochondria and chloroplasts)

what are amphipathic molecules

molecules that contain both a hydrophilic and a hydrophobic moiety

describe the amino acids in the α helices of bacteriorhopsin

most of the amino acids in these membrane-spanning α helices are nonpolar/hydrophobic and only a very few are charged the polar and charged residues tend to be found in the cytoplasmic and extracellular regions this distribution of nonpolar amino acids is sensible because these residues are either in contact with the hydrocarbon core of the membrane or with one another

what important role does myelin play

myelin serves as an insulator and plays a critical role in enabling the rapid transmission of nerve impulses, or action potentials

which phosphoglyceride molecule is a key intermediate in the biosynthesis of other phosphoglycerides?

phosphatidate only small amounts are present in membranes, but it is a key intermediate in the biosynthesis of other phosphoglycerides

what are five common phosphoglycerides found in membranes?

phosphatidylserine phosphatidylcholine phosphatidylethanolamine phosphatidylinositol diphosphatidylglyceril

which class of membrane lipids are abundant in all biological membranes?

phospholipids

what are phosphoglycerides?

phospholipids derived from glycerol

what are the three common types of membrane lipids?

phospholipids, glycolipids, and cholesterol

describe the protein porin

porin is a protein from the outer membranes of bacteria such as E. coli and Rhodobacter capsulatus, it represents a class of membrane proteins with a completely different type of structure, a structure built from β strands and containing essentially no α helices

what are peripheral membrane proteins?

proteins that are bound to membranes primarily by electrostatic and hydrogen-bond interactions with the head groups of lipids

what are integral membrane proteins?

proteins that interact extensively with the hydrocarbon chains of membrane lipids, and they can be released only by agents that compete for these nonpolar interactions

what readily forms micelles?

salts of fatty acids (such as sodium palmitate, a constituent of soap) readily form micelles because they contain only one chain

what property is essential to membranes?

selective permability

what chain length and saturation enhances the fluidity of fatty acids and of their derivatives?

short chain length and unsaturation this causes lower MP -> enhances fluidity

how does chain length affect melting poitn

shorter chain length causes lower MP

when does myelination of neurons in the brain occur?

significant myelination of neurons in the brain occurs during infancy but persist throughout adolescence, meaning the brain is an actively developing organ throughout childhood

how do lipid bilayers form?

spontaneously by a self-assembly process in other words, the structure of a bimolecular sheet is inherent in the structure of the constituent lipid molecules the growth of lipid bilayers from phospholipids is rapid and spontaneous in water

where are fatty aid carbon atoms numbered from?

starting at the carboxyl terminus

what about archaeal membranes helps these organisms to withstand extreme conditions like high temp, low pH, or high salt concentration?

the ability of archaeal lipids to resist hydrolysis and oxidayion (ether linkage, branched alkyl chains)

what are the common alcohol moieties of phosphoglycerides?

the amino acid serine, ethanolamine, choline, glycerol, and inositol

what is sphingomyelin composed of?

the backbone is sphingosine, an amino alcohol that contains a long, unsaturated hydrocarbon chain the amino group of the backbone is linked to a fatty acid by an amide bond the primary hydroxyl group of sphingosine is esterified to phosphorylcholine

what is unique about the structure of myelin

the cells that generate myelin— oligodendrocytes in the brain, Schwann cells in the periphery—wrap their plasma membranes multiple times around the axon or the part of the neuron that conducts the electrical signal

what indicates how intimately a protein is associated with the membrane?

the ease with which a protein can be dissociated from the membrane

how does SDS-polyacrylamide gel electrophoresis allow us to visualize the protein components of a membrane?

the electrophoretic mobility of many proteins in SDS-containing gels depends on the mass rather than the net charge of the protein. the gel-electrophoresis patterns of membranes can reflect distinct protein composition. membranes performing different functions contain different repertoires of proteins

which components of a phospholipid are hydrophobic and which are hydrophilic?

the fatty acid components provide a hydrophobic barrier, whereas the remainder of the molecule has hydrophilic properties that enable interaction with the aqueous environment

which components of a membrane lipid are hydrophobic and which are hydrophilic

the fatty acid tail components provide the hydrophobic properties the alcohol and phosphate components, called the polar head group, provide the hydrophilic properties

how are glycolipids formed and how do they differ from sphingomyelin

the glycolipids in animal cells are derived from sphingosine. the amino group of the sphingosine backbone is acylated by a fatty acid, as in sphingomyelin. glycolipids differ from sphingomyelin in the identity of the unit that is linked to the primary hydroxyl group of the sphingosine backbone. in glycolipids, one or more sugars (rather than phosphorylcholine) are attached to this group

how are phosphoglycerides formed?

the hydroxyl groups at C-1 and C-2 of glycerol are esterified to the carboxyl groups of the two fatty acid chains. the C-3 hydroxyl group of the glycerol backbone is esterified to phosphoric acid. when no further additions are made, the resulting compound is phosphatidate (diacylglycerol 3-phosphate), the simplest phosphoglyceride

what does the lipid component of a biological membrane do

the lipid component prevents molecules generated inside the cell from leaking out and unwanted molecules from diffusing in

how are the major phosphoglycerides derived?

the major phosphoglycerides are derived from phosphatidate by the formation of an ester bond between the phosphate group of phosphatidate and the hydroxyl group of one of several alcohols

which parts of a pore is polar or nonpolar?

the outside surface of porin is appropriately nonpolar, given that it interacts with the hydrocarbon core of the membrane. in contrast, the inside of the channel is quite hydrophilic and is filled with water. this arrangement is accomplished by the alternation of hydrophobic and hydrophilic amino acids along each β strand

what is the permeability of small molecules correlated with?

the permeability of small molecules is correlated with their solubility in a nonpolar solvent relative to their solubility in water this suggests that a small molecule might traverse a lipid bilayer membrane in the following way: 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

describe the protein content of plasma membranes

the plasma membranes, or exterior membranes, of most other cells are much more metabolically active. they contain many pumps, channels, receptors, and enzymes. the protein content of these plasma membranes is typically 50%

what are the properties of fatty acids and of lipids derived from?

the properties of fatty acids and of lipids derived from them are markedly dependent on chain length and degree of saturation

what do the protein components of a biological membrane do

the protein components act as transport systems that allow the cell to take up specific molecules and remove unwanted ones

where is the systematic name for a fatty acid derived form

the systematic name for a fatty acid is derived from the name of its parent hydrocarbon by the substitution of oic for the final e Example C18 saturated fatty acid: octadecane one double bond: octadecenoic acid two double bonds: ocyadecadienoic acid three double bonds: octadectrienoic acid

what is cholesterol?

the third major type of membrane lipid, has a structure very different from that of phospholipids

why is a biomolecular sheet favored over a micelle structure?

the two fatty acid chains of a phospholipid or a glycolipid are too bulky to fit into the interior of a micelle

what do the types of membrane proteins in a cell reflect?

the types of membrane proteins in a cell are a reflection of the biochemistry occurring inside the cell

what forces are there with the polar head groups?

there are electrostatic and hydrogen-bonding attractions between the polar head groups and water molecules

what can liposomes be used for?

these structures can be used to study membrane permeability or to deliver chemicals to cells

what important role do the carbohydrate components of glycolipoids play?

they play an important role in cell-cell recognition, often on the extracellular surface of the cell membrane

what biological roles do lipids serve as?

they serve as fuel molecules, highly concentrated energy stores, signal molecules and messengers in signal-transduction pathways, and components of membranes **the first three roles we'll learn about in later chapters. we will now focus on lipids as membrane constituents.

do unsaturated or saturated fatty acids have lower melting points (at the same length)?

unsaturated fatty acids have lower MPs than do saturated fatty acids of the same length polyunsaturated fats are even lower

what forces pack the tails of the lipid bilayer

van der Waals attractive forces between the hydrocarbon tails favor close packing of the tails

what are lipids

water-insoluble biomolecules that are highly soluble in organic solvents such as chloroform

what are some methods researchers use to determine the 3D structure of proteins

x-ray crystallographic or electron microscopic methods

what is the position of a double bond represented by?

Δ followed by a superscript number Example cis-Δ9 means that there is a cis double bond between C atoms 9 and 10

what is the methyl carbon atom at the distal end of the chain called?

ω-carbon atom