Ch.11 - Membrane Structure Flashcards
11A - The Lipid Bilayer 11B - Membrane Proteins
Describe the structure of phospholipids.
plipids have a P-containing, hphilic head linked to glycerol and two HC (hphobic) tails on one side and a small, hphilic molecule on the other; typ choline (phosphatidylcholine)
- Amphipathic - has both hphilic/phobic parts; property shared by other types of mem lipids, incl cholesterol (animal mems) and glycolipids (sugar as hphilic head).
- Crucial to spontaneous arrangement.
T/F: Diff types of mem lipids are all amphipathic.
True
Diff types of mem lipids are all amphipathic.
- Phospholipids, e.g. phosphatidylserine, -choline, etc.
- Sterols, e.g. cholesterol
- Glycolipids, e.g. galactocerebroside
Summarize the diffs b/w hphilic/phobic molecules.
- Hphilic - readily dissolve; either charged groups or uncharged polar groups; form either e-static attractions or H bonds w water.
-
Hphobic - insoluble; all—or almost all—of atoms are uncharged and nonpolar → cannot form favorable interactions w water.
- Instead, force adj water to reorganize into cage-like struc around them.
- Hydrophobic forces - cage-like struc is more highly ordered → reqs free energy → energy cost minimized when hphobic molecules cluster t/g, limiting contacts w surrounding water.
Why does a cage-like structure of water molecules form around hydrophobic molecules?
Hydrophobic forces - cage-like struc is more highly ordered → requires free energy → energy cost minimized when hphobic molecules cluster t/g, limiting contacts w surrounding water.
- cage is more highly ordered bc ea water molecule has fewer partners w wh to H-bond.
Any tear in the lipid bilayer will create a free edge. How are small/large tears fixed?
Bilayer self-healing - Any tear will create free edge that is exposed to water → energ unfav → bilayer spont rearranges to eliminate free edge.
- Small tears: spont. rearrange excludes water → repair of bilayer → restores single continuous sheet.
- Large tears: sheet may begin to fold in on itself → break into sep closed vesicles.
- Either case, free edges are quickly eliminated.
Only way a finite amphipathic sheet can avoid having free edges is to bend/seal → form boundary around closed space → spont rearrange.
In vitro, pure plipids in aq soln will form closed spherical vesicles called ________, wh vary in size fr ~25 nm to 1 mm in diam.
In vitro, pure plipids in aq soln will form closed spherical vesicles called liposomes, wh vary in size fr ~25 nm to 1 mm in diam.
In vitro, pure plipids in aq soln will form closed spherical vesicles called liposomes. In what ways are phospholipids able to move w/i such a structure?
Plipid movement w/i liposomes:
- “Flip-flop” - plipids v rarely move b/w monolayers; no proteins to facilitate process.
- Lateral movement - continuous exchange b/w neighbors in same monolayer; occurs as result of random thermal motions.
- Individual lipids can flex their HC tails and rotate rapidly about their long axis.
Same movements possible in cellular bilayers.
Fluidity of a lipid bilayers deps on its composition. How does the length and # of carbon double bonds of HC chains/tails affect fluidity?
Fluidity vs HC length & # C=C:
closer/more regular packing of tails → more viscous/less fluid.
- As HC length ↑ → tendency of HC tails to interact ↑ → fluidity ↓.
-
As # double bonds ↓ → # vdw interactions ↑ → fluidity ↓.
- Ea C=C in unsat tail creates small kink → more difficult for tails to pack tightly → fewer vdw interactions → fluidity ↑.
Fluidity of a lipid bilayers deps on its composition. How does the presence of cholesterol affect fluidity?
Fluidity vs cholesterol:
closer/more regular packing of tails → more viscous/less fluid.
- In animals, mem fluidity is modulated by inclusion of cholesterol.
- Cholesterols are short/rigid → fill spaces b/w adj plipids left by kinks in unsat HC tails → tend to stiffen bilayer → less flexible, less permeable, less fluidity.
- As temp ↑ → extra noncovalent bonds formed w cholesterol prevents pmem fr becoming excessively fluid, i.e. ↓ fluidity.
- At temp ↓ → rigidity of cholesterol prevents pmem fr excessively tight-packing, i.e. ↑ fluidity.
Mem plipids vary b/w __ to __ carbons; most typ __ to __ carbons.
Most plipids contain ___ unsaturated and ___ saturated HC tail.
Mem plipids vary b/w 14 to 24 carbons; most typ 18 to 20 carbons.
Most plipids contain one unsaturated and one saturated HC tail.
In animal cells, mem fluidity is modulated by inclusion of cholesterol. Is mem fluidity also modulated in bac/yeast cells? If so, how?
Bac/yeast cells synth longer/shorter and unsat/sat fatty acid chains to accommodate environ.
- As temp ↑ → synth more longer/saturated FA chains → ↓ fluidity; & v-v.
Provide several reasons why mem fluidity is important.
Importance of mem fluidity:
- Enables many mem proteins to diffuse rapidly and interact w one/an, e.g. imp in cell signaling.
- Permits mem lipids/proteins to diffuse fr sites of synth to other regions of cell, e.g. transport.
- Ensures mem molecules are distributed evenly b/w daughter cells during reprod.
- Allows mems to fuse w one/an and mix.
Where in the cell does membrane assembly begin?
Mem assembly begins in the ER
- Euks: new plipids synth’d by enzymes bound to cytosolic surface of ER.
Summarize the process of synthesizing new phospholipids in euks.
Plipid synth in euks:
- Using free FAs as substrates, enzymes deposit new plipids exclusively in cytosolic monolayer of ER.
-
Scramblases randomly flip plipids fr cytosolic to noncytosolic monolayer (ER lumen side).
- new plipids now evenly distributed thru/o ER bilayer.
- Recall: spont flip-flip occurs v rarely.
- new mem on ER continually pinches off as vesicles → fuses w Golgi (cis).
- Flippases in Golgi mem seletively flip plipids fr noncytosolic to cytosolic monolayer.
- new mem on Golgi (trans) pinches off as vesicles → transported to other IC mems or pmem.
T/F: certain plipids are confined to one side of mem.
True
Certain plipids are confined to one side of mem.
Most mems are asymmetrical, and asymmetry preserved b/w organelles.
- Mems have distinct in/outside faces: cytosolic monolayer always faces cytosol; noncytosolic monolayer is exposed to either cell exterior (as w pmem) or to interior space (lumen) of an organelle.
- Conservation of orientation also applies to any embedded proteins.
- Proper orientation is critical for proper function.
Describe the location and orientation of glycolipids in cell mems.
Glycolipids show signif asymm distribution in cell mems: located primarily in pmem, and only in noncytosolic monolayer.
- Orientation: Sugar groups face cell exterior (noncytosolic) → form part of continuous coat of carbs that surrounds/protects animal cells.
- Enzymes in Golgi are oriented such that sugars added only to lipids in noncytosolic monolayer.
- There are no flippases that transfer glycolipids to cytosolic side → glycolipids remain trapped in noncytosolic monolayer.
Describe several functions that membrane proteins serve.
Mem proteins serve many functions:
- Transport partic nutrients, metabolites, and ions across bilayer.
- Anchor mem to macromolecules on either side.
- Receptors that detect chemical signals in environ → relay into cell interior
- Enzymes.
Proteins assoc w lipid bilayer in many ways. What are the two principle classes of mem proteins?
Integral - strongly assoc w mem; removed only by disrupting bilayer w detergents.
Peripheral - transiently assoc w mem.
Integral mem proteins are strongly assoc w mem and are removed only by disrupting bilayer w detergents. Describe several types of integral mem proteins.
-
Transmembrane - extend thru bilayer and into either side as single α helix, multiple α helices, or a rolled-up β sheet (β barrel).
- Amphipathic - hphobic regions near HC tails of plipids; hphilic regions exposed to aq environ on either side.
- Monolayer-associated α helix - attach to cytosolic side via an amphipathic α helix on protein’s surface; extends partially into bilayer.
- Lipid-linked - lie entirely on either side of bilayer, attached to mem only by 1+ covalently attached lipid groups.
Lipid-linked proteins are _______ (integral/peripheral) mem proteins that lie entirely on _______ (cytosolic, non, either) side of bilayer and attach to mem only by 1+ _______ (covalently, non) attached lipid groups.
Lipid-linked proteins are integral mem proteins that lie entirely on either side of bilayer and attach to mem only by 1+ covalently attached lipid groups.
Transmem proteins are integral proteins that extend thru bilayer and into either side. Describe the three common structures of xmem proteins.
Transmembrane - extend thru bilayer and into either side as single α helix, multiple α helices, or a rolled-up β sheet (β barrel).
- Amphipathic - hphobic regions near HC tails of plipids; hphilic regions exposed to aq environ on either side.
- Recall: integral mem proteins can only be removed by disrupting bilayer w detergents.
Monolayer-associated α helices attach to ________ (cytosolic, non, either) side via an amphipathic _______ (α helix, β barrel) on protein’s surface and extends partially into bilayer.
Monolayer-associated α helices attach to cytosolic side via an amphipathic α helix on protein’s surface and extends partially into bilayer.
Peripheral mem proteins assoc only transiently w mem. Describe one key type of peripheral mem protein.
Protein-attached - peripheral mem protein bound indirectly to either side of mem via noncovalent interactions w other mem proteins.
T/F: proteins typ cross lipid bilayer as an α helix.
True
Proteins typ cross lipid bilayer as an α helix.
- Recall: all mem proteins have unique orientation in bilayer, specified during mem protein synth.
Xmem proteins contain specialized mem-spanning segments. What type(s) of side chains comprise the majority of these segments?
Specialized mem-spanning segments of xmem polypeptide chain run thru the hphobic interior of bilayer → comprised largest of AAs w hphobic side chains.
Peptide bonds that join successive AAs are typ _____ (polar/non), and as a result the polypeptide backbone is typ _____ (hphilic/phobic).
Peptide bonds that join successive AAs are typ polar, and as a result the polypeptide backbone is typ hphilic.
Due to the absence of water in the lipid bilayer interior, atoms of the polyp backbone are driven to form H-bonds w one/an. What does this indicate wrt how proteins typ traverse the bilayer?
No water in bilayer interior → atoms of polyp backbone driven to form H-bonds w one/an.
H-bonding is maximized if polyp chain forms a regular α helix → great majority of mem-spanning segments of polyp chains traverse the bilayer as α helices.
- In mem-spanning α helices, hphobic side chains are exposed on outside of helix.
In mem-spanning α helices, hphobic side chains are exposed on _______ (in/outside) of helix and atoms of polyp backbone form H-bonds w one/an on _______ (in/outside) of helix
In mem-spanning α helices, hphobic side chains are exposed on outside of helix → interact w HC tails; atoms of polyp backbone form H-bonds w one/an on inside of helix
What role do single-pass xmem proteins typ serve?
Single-pass xmem proteins typ function as receptors for EC signals.
What struc/func do multi-pass xmem proteins typ form/serve?
- Multi-pass xmem proteins typ form aqueous pores/channels wh allow xprt of small, water-soluble molecules.
- Often incl 1+ amphipathic regions—formed fr α helices containing both hphobic/philic side chains → α helices pack side-by-side in a ring, w hphobic side chains exposed to lipid bilayer and hphilic side chains forming the lining of a hphilic pore
T/F: channels can also be formed by proteins w single xmem α helix.
False
Channels cannot be formed by proteins w single xmem α helix; typ consist of a series of α helices.
Porin proteins are the most striking example of _______, wh form large, water-filled pores in mito/bac outer mems.
Porin proteins are the most striking example of β barrels, wh form large, water-filled pores in mito/bac outer mems.
- β barrels - β sheets rolled into cylinder, forming keglike structure.
- Much less common form of crossing mem comp to α helix.
- Porins allow passage of small nutrients, metabolites, and inorganic ions across outer mems (recall: double-mem struc), while preventing unwanted larger molecules fr crossing.
Detergents are the most widely used disruptive agents for destroying the lipid bilayer. Describe the structure of detergents.
Detergents:
- Only one hphobic HC tail (comp to 2 for plipids).
- Shaped like cones → aggregate into small clusters called micelles (monolayer) in aq soln.
- Recall: plipids - two HC tails → more cylindrical → liposomes (bilayer) in aq soln.
Detergents are the most widely used disruptive agents for destroying the lipid bilayer. Describe the function of detergents.
Detergents:
- When mixed in great excess w mems, hphobic ends of detergents interact w mem-spanning hphobic regions of xmem proteins and HC tails of plipids → disrupt lipid bilayer → sep proteins fr most plipids.
- Other end of detergent is hphilic → bring mem proteins into solution as protein–detergent complexes as well as solubilizes plipids.
- Protein–detergent complexes then sep fr one/an and fr lipid–detergent complexes.
Bacteriorhodopsin functions as a mem xprt protein wh pumps H+ out of cell. Describe this mechanism.
B.rhodopsin: Retinal absorbs photon → changes shape → causes protein—embedded in bilayer—to undergo series conform changes → transfers one H+ fr retinal to EC space.
- Ea b.rhod contains single light-absorbing nonprotein molecule: retinal.
- Retinal is hphobic and covalently attached to one of b.rhod’s 7 xmem α helices.
- Strategically placed polar side chains guide H+ thru protein as it undergoes conform changes; prevents interaction w bilayer.
- Retinal regenerated by taking up H+ fr cytosol → restores protein’s original shape → cycle repeats.
- Net: one H+ transported fr IC to EC.
- H+.cgrad across pmem grad used to synth ATP
________ proteins are a class of xmem proteins wh actively move small organic molecules and inorganic ions in/out of cells.
Pump proteins are a class of xmem proteins wh actively move small organic molecules and inorganic ions in/out of cells; e.g. bacteriorhodopsin.
The pmem is reinforced by the underlying _________.
The pmem is reinforced by the underlying cell cortex.
Most cell mems are reinforced by framework of proteins, attached via xmem proteins. Describe the diffs in these frameworks b/w animals, plants, yeasts, and bac.
Most cell mems are reinforced by framework of proteins, attached via xmem proteins.
- Plants, yeasts, and bacteria: rigid cell wall—mesh-work of protein/sugar/other macros that encases pmem.
-
Animals: pmem stabilized by meshwork of fibrous proteins, called cell cortex, attached to underside of mem.
- Abnormalities in cell cortex may lead to serious disease, e.g. abnormalities of spectrin—a dimeric protein of RBC cortex—leads to anemia.
Most animal cells cortices are partic rich in what type of proteins?
Most animal cells cortices are partic rich in motor proteins, e.g. actin and myosin.
Cells can confine partic proteins to localized areas w/i bilayer, creating specialized regions, or ____________, on cell/organelle surface.
Cells can confine partic proteins to localized areas w/i bilayer, creating specialized regions, or membrane domains, on cell/organelle surface.
Cells can confine partic proteins to localized areas w/i bilayer, creating specialized regions, or mem domains, on cell/organelle surface. Describe several examples.
- Pmem proteins can be tethered to EC/adj cell structures or to relatively immobile IC structures, partic the cell cortex.
- Cells can also create barriers that restrict partic mem components to one mem domain, e.g. tight junctions.
Describe how the epithelial cells that line the gut regulate the direction of intake/export.
Epithelial cells that line the gut:
- Xprt proteins involved in nutrient uptake confined to apical surface (faces gut contents).
- Xprt proteins involved in export of solutes fr epithelial cell into tissues/bloodstream confined to basal and lateral surfaces.
- Tight junctions maintain asymmetric distribution of mem proteins.
- Specialized junctional proteins form a continuous belt around cell where the cell contacts its neighbors → creates seal b/w adj pmems → mem proteins cannot diffuse past junction.
T/F: Most mem proteins are glycoproteins.
True
Most mem proteins are glycoproteins, w oligosacchs attached.
__________ are subclass of glycoproteins that contain 1+ polysacchs attached to protein.
Proteoglycans are subclass of glycoproteins that contain 1+ polysacchs attached to protein.
- Recall: glycoproteins typ contain mono-/di-/oligosacchs attached to protein, not polysacchs.
Sugar component of glycoproteins/lipids is located ______ (in/outside) of pmem → forms sugar coating called __________ or ________.
Sugar component of glycoproteins/lipids and proteoglycans is located outside of pmem (noncytosolic) → forms sugar coating called carbohydrate layer or glycocalyx.
What purpose does the carb layer, or glycocalyx, serve?
Glycocalyx helps protect cell surface fr mechanical damage.
- Oligo/polysacchs adsorb water → produce slimy surface → helps motile cells (e.g. white blood cells) squeeze thru narrow spaces and prevents sticking t/g or to walls of blood vessels.
T/F: carb layer/glycocalyx in multicell orgs is specific to ea cell type.
True
Carb layer/glycocalyx in multicell orgs is specific to ea cell type.
- Imp for recognition, e.g. immune sys (neutrophils).
Besides protecting cells fr mechanical damage, what other purpose do cell-surface carbs serve?
Cell-surface carbs also imp cell–cell recog/adhesion:
- Lectins (proteins) are specialized to bind to partic oligosacch side chains.
- Oligosacch side chains of glycopro/lipids are short (typ < 15 sugar units) but enormously diverse.
- Recall: sugars can be joined t/g in many diff arrangements → branched strucs.
By what methods can cell restrict proteins to specific regions of the pmem? Is a mem w many of its proteins restricted still fluid?
- Mobility of mem proteins is drastically ↓ if they are bound to other proteins, e.g. those of cell cortex/ECM.
- Some mem proteins are confined to mem domains by barriers, e.g. tight junctions.
- Bilayer fluidity is not signif affected by anchoring of mem proteins; the sea of lipids flows around anchored mem proteins like water around posts of a pier.
What would happen if plipids had only one HC tail instead of two?
You would have a detergent. The diameter of the lipid
head would be much larger than that of the hydrocarbon
tail, so that the shape of the molecule would be a
cone rather than a cylinder, and the molecules would
aggregate to form micelles rather than bilayers.
