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a. State of the Membrane
i. physical state of lipid of membrane described by its __
1. if you have unsaturated fatty acids kept relatively warm= lipid is __
a. at this temperature, the lipid bilayer is like a __
- fluidity (viscosity)
- fluid
- 2D liquid crystal
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i. molecules still retain a __, in this case, the long axes of the molecules tend toward a __, yet individual phospholipids can __ or __ within the __ of the bilayer
- specified orientation
- parallel arrangement
- rotate around their axis
- move laterally
- plane
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1. if temp is lowered= reaches a temp that changes state of bilayer
a. liquidà frozen gel (movement is __)
called __
- greatly restricted
- transition temperature
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a. Transition Temperature
i. depends on the ability of the lipid molecules to__, which depends on the __ that make it
1. __ have the shape of a straight, flexible rod
a. phospholipids with saturated chains do what?
- be packed together
- particular lipids
- saturated fatty acids
- pack together more tightly than those containing unsaturated chains
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1. __ have crooks in the chain at the sites of a double bond
a. the higher the degree of unsaturation of fatty acids, the __the transition temperature
ii. another factor is the __1. shorter= lower melting temperature
- Cis-unsaturated fatty acids
- lower
- length of the fatty acid chain
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i. cholesterol
1. because of their orientation within the bilayer, cholesterol molecules do what?
2. its presence tends to __ and __
3. increases __while decreasing ___of membrane
disrupt the close packing of fatty acyl chains and interfere with their mobility
abolish sharp transition temperatures
- creates a condition of intermediate fluidity
- durability
- permeability
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h. Importance of Membrane Fluidity (1)
i. it provides a perfect compromise between a rigid, ordered structure in which mobility would be absent and a completely fluid, nonviscous liquid in which the components of the membrane could not be oriented and structural organization and mechanical support would be lacking
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h. Importance of Membrane Fluidity (2)
- i. fluidity allows for interactions to take place within the membrane
- Clusters of membrane proteins to assemble and form specialized structures, like junctions, etc
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h. Importance of Membrane Fluidity (3)
i. because of fluidity, molecules that interact can come together, carry out the necessary reaction, and move apart
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h. Importance of Membrane Fluidity (4)
i. plays key role in membrane assembly
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Explanation of the Key role in membrane assembly aspect of membrane fluidity
1. membranes arise from __ only and their growth is accomplished by the __ of __ and__ into the __ of the membranous sheet
a. many of the most basic cellular process, including cell movement, growth, division, secretion, etc, depend on the movement of membrane components and would probably not be possible if membranes were rigid
- preexisting membranes
- insertion of lipids and proteins
- fluid matrix
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a. Maintaining Membrane Fluidity
i. internal temperature of most organisms (except birds and mammals) fluctuates with temp of __
ii. cells respond to changing conditions by __the types of __of which they are made
- external environment
- altering
- phospholipids
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i. maintenance of fluidity is an example of __at the cellular level and can be demonstrated in a variety of ways. Explain this.
- homeostasis
- if temp of a culture of cells is lowered, the cells respond metabolicallya. the initial “emergency” response is mediated by enzymes that remodel membranes, making the cell more cold resistant
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i. remodeling accomplished by:
- 1. desaturating single bonds in fatty acyl chains to form double bonds
- 2. reshuffling the chains between different phospholipid molecules to produce ones that contain two unsaturated fatty acids, which greatly lowers the melting temperature of the bilayer
- 3. cell also changes the types of phospholipids being synthesized in favor of one’s containing more unsaturated fatty acids
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Explain desaturation.
a. desaturation of single bonds to form double bonds is catalyzed by enzymes called desaturases
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Explain reshuffling.
a. accomplished by phospholipases, which split the fatty acid from the glycerol backbone, and acyltransferases, which transfer fatty acids between phospholipids
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a. as a result of these various enzymes, the physical properties of a cell’s membranes are matched to the prevailing __
environmental conditions
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i. maintenance of fluid membranes by adjustments in __ has been demonstrated in numerous organisms whose body temp changes from day to night, cold-resistant plants, and bacteria living in hot springs
ii. prokaryotic cells living at very high temperatures have plasma membranes that contain highly __
- fatty acyl composition
- unusual lipids
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a. Lipid Rafts
i. when membrane lipids are extracted from cells and used to prepare artificial lipid bilayers, __ and __ tend to __ into __that are more __and highly ordered than surrounding regions consisting primarily of __
- cholesterol and sphingolipids
- self-assemble
- microdomains
- gelated
- phosphoglycerides
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1. because of these distinctive physical properties, such microdomains tend to __
float within the more fluid and disordered environment of the artificial bilayerà called lipid rafts
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i. when added to these artificial bilayers, certain proteins tend to become concentrated in the __, whereas others tend to remain outside their boundaries
ii. __ are fond of the ordered regions of the bilayer
- lipid rafts
- GPI-anchored proteins
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i. lipid rafts are postulated to serve as __ that concentrate particular proteins, thereby organizing the membrane into __
1. thought to provide a favorable local environment for __to interact with other membrane proteins that __ from the __ to the __
- floating platforms
- functional compartments
- cell-surface receptors
- transmit signals
- extracellular space to the cell interior
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a. Asymmetry of Membrane Lipids
explain the lipid bilayer
i. lipid bilayer: two distinct leaflets have distinctly different lipid composition
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1. __ cannot penetrate the plasma membrane and are only able to digest __that reside in the __ of the bilayer
a. compared to the inner leaflet, the outer leaflet has a high concentration of __and a low concentration of __
- lipid-digesting enzymes
- lipids
- outer leaflet
- PC (and sphingomyelin)
- PE and PS
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i. it follows that the lipid bilayer can be thought of as composed of __
two more or less stable, independent monolayers having different physical and chemical properties
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Different classes of lipids have different properties: one and two
- 1. all glycolipids of plasma membrane are in outer leaflet and serve as receptors for extracellular ligands
- 2. phosphatidylethanolamine, which is in the inner leaflet, tends to promote the curvature of the membrane, which is important in membrane budding and fusion
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Different classes of lipids have different properties
three and four?
- 1. phosphatidylserine in the inner leaflet has a net negative charge at physiologic pH, which makes it a candidate for binding positively charged lysine and arginine residues
- 2. appearance of PS on the outer surface of aging lymphocytes marks the cells for destruction by macrophages, whereas its appearance on the outer surface of platelets leads to blood coagulation
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Different classes of lipids have different properties
five?
- 1. PI in the inner leaflet can be phosphorylated at different sites on the inositol ring, which converts the lipid into a phosphoinositide
- a. play a key role in transfer of stimuli from plasma membrane to cytoplasm and recruitment of proteins to the cytosolic face of the plasma membrane
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a. Membrane Carbs
i. depending on species and cell type, carbs in membrane range between __ by weight
1. more than __ of membrane carbs are linked to proteins to form __; remaining are covalently linked to lipids to form __
2-10%
- 90%
- glycoproteins
- glycolipids
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i. all carbs of plasma membrane face outward into __; carbs of internal cell membrane face away from __
ii. modification of proteins
1. addition of carb, __, is the most complex
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1. carbohydrate of __are short, branched, hydrophilic oligosacchardies
a. these oligosaccharides attached to membrane proteins and lipids can display __
b. even the same protein can display __ in different cells and tissues
- glycoproteins
- extensive variability in composition and structure
- different chains of sugars
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a. oligosaccharides may be attached to several different amino acids by __
i. important role in __ of a cell with its environment and __ to different cellular compartments
1. ex: blood type determined by this
- two major types of linkages
- mediating interactions
sorting of membrane proteins
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