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BCH210 > Lecture 13: Membrane Structure > Flashcards

Lecture 13: Membrane Structure Flashcards

1
Q

Membrane functions

A
  • Act as barrier to regulate import/export of molecules
  • Compartmentalization
  • Cell-cell recognition
  • Signaling is mediated by proteins and lipids
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2
Q

Membrane properties

A
  • Composed of lipid bilayers that are impermeable to polar or charged molecules
  • Hydrophobic effect drives membrane formation of amphipathic molecules due to non covalent interactions
  • Membranes may/may not have carbohydrates covalently bound
  • Play key role in transport of molecules and transduction of information across membrane
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3
Q

Membrane fluidity

A
  • Proteins carry the movement of molecules across the membrane
  • Lipid and cholesterol affect membrane fluidity
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4
Q

Fluid-Mosaic Model

A
  • “Fluid” - membrane components can move rapidly in plane of membrane
  • “Mosaic” - diverse mixture of lipids on peripheral proteins and carbohydrates on the surface
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5
Q

Fluorescence Recovery After Photobleaching(FRAP)

A
  • Label cell surface molecule
  • Use laser to bleach fluorophore in small part of membrane
  • Measure mobilty of other molecules in region
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6
Q

What effects do detergents play on FRAP

A
  • detergents lyse the cell and unable to observe FRAP
  • crosslinker decrease recovery rate
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7
Q

Asymmetry of biological membranes

A
  • Two leaflets have different lipid/protein compositions
  • Addition of sugars to lipids is form of PTM and is important for membrane insertion and cell recognition
  • Flip-flop movement across hydrophobic membrane unfavorable
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8
Q

Enzymes mediate membrane assembly

A
  • Flippase: move PE and PS from outer membrane to cytosolic leaflet
  • Floppase: Moves phospholipids from cytosolic to outer leaflet
  • Scramblase: Moves lipids in either direction
  • Enzymes are ATP dependent
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9
Q

Membrane assembly

A
  • Enzymes assist with distribution of lipids and proteins on either side of bilayer
  • ATP hydrolysis of flippases and floppases drive movement of lipids from one membrane to another
  • Scramblase moves all lipids down concentration gradient producing a symmetrical membrane
  • Enzymes catalyze addition of oligosaccharides to proteins and lipids
  • Membrane synthesis in ER to ensure proper orientation
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10
Q

Lipids

A
  • Class of molecules providing structural support for cells and organelles, and can play a role in information transduction and signaling
  • Defined by physical properties
    • low solubility in water
    • high solubility in non polar solvents
  • Properties arise from presence of functional groups(amphipathic)
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11
Q

Lipid Bilayer Formation

A
  • Lipids aggregate in water to bury hydrophobic groups while polar groups interact with water
  • Non covalent forces drive assembly
  • Structure forms is based off lipid structure and chemical interactions
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12
Q

Biological Lipids

A
  • Storage: Triacylglycerides
  • Structural: Phospholipids, Sphingolipids, Glycolipids, Sterols
  • Signalling: Sterols, Omegas, Eicosanoids
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13
Q

Fatty Acids

A
  • Fatty acid chains are either saturated(no double bonds) or unsaturated(double bonds)
  • Doubled bonds are usually numbers relative to carboxylic acid but can also be numbered relative to terminal methyl group for omega fatty acidss
  • Can be free or bound to head group/backbone via ester bonds
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14
Q

Fatty Acid Nomenclature

A
  • # C : #double bonds
  • Monounsaturated: 1 C double bond
  • Poly: more than 1 double bond
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15
Q

Omega Fatty Acids

A
  • Omega 3 and omega 6 fatty acids are polyunsaturated
  • Humans cannot synthesize these omega fatty acids
  • Must be obtained by diet
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16
Q

Chain Saturation and Membrane Fluidity

A
  • Saturated: low fluidity, and high melting temperature
  • Unsaturated: high fluidity, low melting temperature
17
Q

Phase Transitions

A
  • Melting of membrane lipids is a phase transition from a solid phase to a liquid crystalline phase
  • Melting temperature an index of membrane fluidity
  • Measures how fluid the membrane is
  • Pure lipid samples have sharp, well-defined temperatures, while native membranes have broad peaks
18
Q

Membrane lipid diversity

A
  • Amphipathic molecules made by attaching fatty acyl chains to polar head groups
    • Glycerol-phospholipids
    • Sphingolipids
  • Variability exists in structure of fatty acid tails and polar head groups
  • Cholesterol most common steroid
19
Q

Cholesterol

A
  • Can be metabolized to other hormones needed for dietary lipid absorption
  • Can form complexes with sphingolipids, glycolipids, and some lipid-anchored proteins to form lipid rafts
  • Lipid rafts help moderate membrane fluidity and signal transduction
  • Modulate membrane fluidity
20
Q

Peripheral Membrane Proteins

A
  • Adhere to surface of lipid membranes or integral membrane proteins through non-covalent interactions
  • Can be removed by increasing salt/changing pH
21
Q

Integral Membrane Proteins

A
  • Completely span the membrane(transmembrane segments)
  • Require harsh conditions for purification(detergents or organic solvents)
22
Q

Lipid Anchored Proteins

A
  • Also require harsh detergents for membrane removal
  • Lipid chains are covalently attached to amino functional groups and side chains
  • GPI anchored proteins covalently linked through sugar chain and lipid anchor
23
Q

Membrane Protein Purification

A
  • Some membrane proteins can be removed using mild conditions that disrupt non-covalent interactions, while others need detergents to disrupt hydrophobic interactions
  • Detergents create micelles around hydrophobic regions to solubilize membrane proteins
  • Proteins can then be purified and analyzed
24
Q

Studying Membrane Proteins

A
  • Critical micelle concentration is concentration at which detergent spontaneously forms stable micelle structures
  • Some detergents are more mild and preserve structure(TX-100, Tween 20, and digitoxin), while others are more harsh and denature proteins(SDS)
25
Q

Predicting Membrane Spanning Segments

A
  • DNA sequencing usd to deduce primary sequence
  • Transmembrane segments composed on primarily hydrophobic amino acids
  • In alpha helix, 20 amino acids needed to span the membrane
26
Q

Determining Protein Topology

A
  • Hydropathy index for a stretch of amino acids can be determined by averaging the Kyte-Doolittle hydrophobicity values of all amino acids found in the segment
  • Move window one residue to the right and recalculate hydropathy index
  • Plot hydropathy index vs central amino acid in the sequence
27
Q

Identifying TM Segments

A
  • Glycophorin A: Contains a single transmembrane segment
  • Bacteriorhodopsin: Contains 7 TMs

BCH210 (23 decks)

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