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.
