Cell membrane Flashcards
what allows for archaeal membranes to withstand hostile living conditions
Nonpolar chains are joined to a glycerol backbone by ether rather than ester linkages
The ether linkage is more resistant to hydrolysis
alkyl chains of archaea are branched rather than linear
- branching is more resistant to oxidation than the unbranched chains of eukaryotic and bacterial lipids
Membrane lipid is an _____ molecule containing a hydrophilic and hydrophobic moiety
amphipathic
The two hydrophobic fatty acid chains are approximately ______ to each other
parallel
What are two forms that phospholipids and glycolipids take in aqueous media
Micelle and Lipid bilayer (bimolecular sheet)
what is a leaflet in terms of the lipid bilayer
the two opposing sheets
what is the more favored formation of a lipid in aqueous solution
Lipid Bilayer (bimolecular sheet)
why is the lipid bilayer prefered over a micelle
the two fatty acid chains of a phospholipid or a glycolipid are too bulky to fit into the interior of a micelle.
what is the size of a micelle
Typically less than 20nm
What forms a micelle
salts of fatty acids (such as sodium palmitate, a constituent of soap) readily form micelles because they contain only one chain.
Lipid bilayers form _______ by a sef-assembly process
spontaneously
What is the major driving force behind the formation of the lipid bilayers
*Hydrophobic interactions
furthermore van der Waals attractive forces between the hydrocarbon tails favor close packing of the tails.
lastly electrostatic and hydrogen-bonding attractions between the polar head groups and water molecules.
Why are lipid bilayers refered to as cooperative structures
because they are held together by many noncovalent interactions.
what are three significant biological consequences of hydrophobic interactions in lipid bilayers
lipid bilayers have an inherent tendency to be extensive
lipid bilayers are self-sealing because a hole in a bilayer is energetically unfavorable
lipid bilayers will tend to close on themselves so that there are no edges with exposed hydrocarbons chains, and so they form compartments.
A lipid bilayer (bimolecular sheet) can ave macroscopic dimensions, such as _____
10^6 nm (1mm)
what is a liposome
aqueous compartments enclosed by a lipid bilayer
what is sonicating
agitating by high-frequency sound waves
Liposomes can be formed with specific membrane proteins in them by
solubilizing the proteins in the presence of detergents and then adding them to the phospholipids from which liposomes will be formed.
The lipid bilayer has a ____ permeability for ions and most polar molecules.
Low
Why is water able to traverse the membrane relatively easily
Low molecular weight, high concentration, and lack of a complete charge.
Integral membrane proteins
interact extensively with the hydrocarbon chains of membrane lipids, and they can be released only by agents that compete for these nonpolar interactions.
Most integral proteins ___ the lipid bilayer
span
Peripheral membrane proteins
Are bound to membranes primarily by electrostatic and hydrogen-bond interactions with the head groups of lipids.
how can peripheral membrane bonds be disrupted
adding salts or changing the pH
what are two ways that peripheral proteins are bound to the membrane
on the surfaces of integral proteins, on either the cytoplasmic or the extracellular side of the membrane
anchored to the lipid bilayer by a covalently attached hydrocarbon chain, such as a fatty acid.
membrane proteins are more difficult to purify and crystalize than are ______ proteins
water-soluble.
Bacteriorhodopsin is composed of how many alpha helices
7
What is Bacteriorhodopsin (BR)
Arachaeal protein
is an integral protein
A light-driven proton pump, converts the energy of light into trans-membrane proton gradient that is used to synthesize ATP
What is Bacteriorhodopsin (BR) mostly composed of
almost entirely of alpha helices (with most of nonpolar amino acids) arranged almost perpendicularly to the bilayer plane
Membrane-spanning alpha helices are the most common structure motif in membrane proteins
Cyclooxygenase (COX)1 or Prostaglandin H2 synthase (PGHS) 1
is an integral protein but does not span the membrane
binds to the luminal leaflet of the ER
a homodimer that consists of primarily alpha helices
what are the two important features of membrane-protein structure
The parts of the protein that interact with the hydrophobic parts of the membrane are coated with nonpolar amino acid side chains, whereas those parts that interact with the aqueous environment are much more hydrophilic.
- the structures positioned within the membrane are quite regular and, in particular, all backbone hydrogen-bond donors and acceptors participate in hydrogen bonds.
What are three types of lipid-linked modifications that allow for soluble proteins to link to the membrane
Lipids are covalently associated with proteins
- ) Palmitoylation of cysteine residues by a thiosester bond
- ) Farnesylation of cysteine residues at the C-terminus
- ) Glycosylphosphatidylinositol-link to the carboxyl terminus
what is the most common structural motif in membrane proteins
Membrane-spanning alpha helices
explain the arrangement of Beta strands in a porin
each strand is hydrogen bonded to its neighbor in an antiparallel arrangement, forming a single Beta sheet. The beta sheet curls up to form a hollow cylinder that, as its name suggests, forms a pore, or channel, in the membrane. The outside surface of the porin is appropriately nonpolar and interacts with the hydrocarbon core of the membrane.
Why is breaking a hydrogen bond within a membrane quite unfavorable
because little or no water is present to compete for the polar group
How wide is a typical hydrocarbon core
30 Angstroms wide
How many amino acids make up an alpha helix that can transverse a hydrocarbon cores width
20 amino acids
The pan of 20 residues chosen for calculation of free-engery change when transversing the width of the hydrocarbon core is called what
a window
A peak of _____ or more in a hydropathy plot based on a window of 20 residues indicates that a polypeptide segment could be a _________
+84 kJ mol-1, membrane-spanning alpha helix
A peak of +84 does _______ that a segment is a transmembrane helix. Why
Not prove, even soluble proteins may have highly non polar regions.
what is the lipid bilayers dual role
it is both a solvent for integral membrane proteins and a permeability barrier
membrane proteins are free to diffuse ______ in the lipid matrix unless restricted by special interactions
laterally
the transition of a molecule form one membrane surface to the other is called what
transverse diffusion or flip-flop
Why is the free-engergy barriers to flip-flopping of protein molecules larger than for lipids
proteins have more extensive polar regions
membrane fluidity is controlled by ______ and _____
fatty acid composition and cholesterol content
Why can membrane asymmetry be preserved for a long time
flip-flop of protein molecules has never been observed. An it is an extremely slow process for phospholipids to even flip-flop
The transition temperature depends on what
the length of the fatty acid chains and on their degree of unsaturation
The presence of saturated fatty acids favor a _____ state because what
rigid state, because their straight hydrocarbon chains interact very favorably with one another.
Why is the Tm lower in fatty acids with a cis double bond than in saturated fatty acids
The cis double bond produces a bend in the hydrocarbon chain. This bend interferes with a highly ordered packing of fatty acid chains.
how do bacteria regulate the fluidity of their membranes
They don’t possess cholesterol so the vary the number of double bonds and the length of their fatty acid chains
What are lipid rafts
highly dynamic complexes formed between cholesterol and specific lipids that contain the sphingosine backbone
and with GPI-anchored proteins.
fxn: moderation of membrane fluidity, making membranes less fluid but at the same time less subject to phase transitions.
concentrating proteins that participate in signal transduction pathways and may also serve to regulate membrane curvature and budding
Name 3 kinds of lipid-linked proteins
Palmitoylation of cysteine residues by a thioester bond
farnesylation of cysteine residues at the C-terminus
Glycosylphosphatidylinositol-link to the carboxyl terminus
Where does oxidative phosphorylation take place
in the inner-mitochondrial membrane
The matrix of the mitochondrion is the site of what
most of the TCA cycle and fatty acid oxidation
The outer mitochondrial membrane is what
quite permeable to most small molecules and ions
The inner mitochondrial membrane is what
impermeable to nearly all ions and polar molecules
How do ions and polar molecules cross the impermeable inner mitochondrial membrane
a large family of transporters shuttle metabolites such as ATP, Pyruvate, and citrate across the inner membrane
Where is the Mitochondrial porin located and what is its size
the outer mitochondrial membrane
30-35 kDa proe-forming proteins known as Voltage-dependent anion channel (VDAC)
What is the periplasm
The region between tow membranes containing the cell wall
What is one of the signs of mitochondria evolved from endosymbiosis with bacteria
the presence of two cell walls. An outer wall that is permeable and an inner that isn’t
why is free iron ions highly toxic to cells
ability to catalyze the formation of free radicals
What binds iron in the blood
Transferrin, can bind two Fe 3+ ions
