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Biology SL > 1.3 Membrane Structure > Flashcards

1.3 Membrane Structure Flashcards

1
Q

what is the structure of phospholipids? (3)

A
  • polar head (hydrophilic) composed of a glycerol and a phosphate molecule
  • consists of 2 non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
  • because they contain both hydrophilic and lipophilic (water-loving) regions they are AMPHIPATHIC
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2
Q

what is the arrangement of phospholipids in the membrane? (2)

A
  • phospholipids spontaneously arrange into a bilayer
  • hydrophobic tail regions face inwards and are shielded from the surrounding polar fluids, while two hydrophilic head regions associate w. the cytosolic and extracellular fluids
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3
Q

what are the properties of the phospholipid bilayer? (4)

A
  • bilayer is held together by weak hydrophobic interactions between the tails
  • hydrophilic/hydrophobic layers restrict the passage of many substances
  • individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility
  • the fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis/exocytosis)
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4
Q

what are phospholipids embedded with? (2)

A
  • embedded w/ proteins
    which can be permanent or temporary
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5
Q

what two kinds of proteins are phospholipids embedded with?

A
  • integral proteins - permanently attached to the membrane and are normally transmembrane (span across the bilayer)
  • peripheral proteins - temporarily attached by non-covalent interactions and associate w/ one surface of the membrane
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6
Q

how are amino acids of a membrane protein localised according to polarity? (2)

A
  • non-polar (hydrophobic) - amino acids associate directly w/ the lipid bilayer
  • polar (hydrophilic) - amino aids are located internally and face aqueous solutions
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7
Q

what 2 tertiary structures do transmembrane proteins typically adopt?

A
  • single helices/ helical bundles
  • beta barrels (common in channel proteins)
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8
Q

what 6 functions can membrane proteins serve?

A
  • Junctions - serve to connect and join 2 cells together
  • Enzyme - fixing to membranes localises metabolic pathways
  • Transport - responsible for facilitated diffusion and active transport
  • Recognition - may function as markers for cellular identification
  • Anchorage - attachment points for cytoskeleton and extracellular matrix
  • Transduction - function as receptors for peptide hormones
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9
Q

what is cholesterol’s function?

A

maintain integrity and mechanical stability

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10
Q

where is cholesterol not found?

A
  • absent in plant cells
    as these plasma membranes are surrounded and supported by a rigid cell wall made of cellulose
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11
Q

what properties does cholesterol have? (3)

A
  • amphipathic (both hydrophilic and hydrophobic regions)
  • cholesterol’s hydroxyl (-OH) group is hydrophilic and aligns towards the phosphate heads of phospholipids
  • remainder of molecule (steroid ring and hydrocarbon tail) is hydrophobic and associated w/ phospholipid tails
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12
Q

what kind of properties do phospholipids have?

A

phospholipid bilayers are fluid (the phospholipids are in constant movement relative to one another)

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13
Q

what does cholesterol interact within the bilayer?

A

interacts with fatty acid tails of phospholipids to moderate the properties of the membrane

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14
Q

what is cholesterol’s function in the membrane? (4)

A
  • immobilise the outer surface of the membrane, reducing fluidity
  • less permeable to very small water-soluble molecules that would otherwise freely cross
  • separate phospholipid tails and so prevent crystallisation of the membrane
  • secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein
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15
Q

why are cell membranes represented according to the fluid-mosaic model? (2)

A

fluid - the phospholipid bilayer is viscous and individual phospholipids can move position
mosaic - phospholipid bilayer is embedded w/ proteins

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16
Q

what are 3 components of the plasma membrane?

A
  • phospholipids - form bilayer w/ phosphate heads facing outwards and fatty acid tails facing inwards
  • cholesterol - found in animal cell membranes and functions to improve stability and reduce fluidity
  • proteins - may be integral (transmembrane) or peripheral and serve a variety or roles
17
Q

who proposed the first model that described the position of proteins within the bilayer?

A

Davson and Danielli

18
Q

what characteristic does the membrane exhibit under a transmission electron microscope?

A

‘trilaminar’ appearance
means 3 layer

19
Q

what model did Davson and Danielli?

A

‘lipo-protein sandwich’
2 layers of proteins with a central phospholipid bilayer
- dark segments seen under electron microscope were identified as representing the 2 protein layers

20
Q

what is wrong with the lipo-protein sandwich model? (4)

A
  • it assumed all membranes were of uniform thickness w/ a constant lipid-protein ratio
  • assumed all membranes would have symmetrical internal and external surfaces
  • didn’t account for the permeability of certain substances
  • temperatures at which membranes solidified did not correlate w/ those expected under proposed model
21
Q

what is the evidence against the lipo-protein sandwich?

A

membrane proteins were discovered to be insoluble in water and varied in size (unable to form uniform and continuous layer on outer surface)

22
Q

how did they show that the lipo-protein sandwich was wrong? (4)

A

fluoresent antibody tagging of membrane protein showed they were mobile and not fixed into place
- membrane proteins from 2 different cells were tagged w/ red and green fluorescent markers
- when 2 cells fuse the markers became mixed throughout the membrane of the fused cell
- demonstrated that the membrane proteins could move and did not form static layer

23
Q

what technique was used to expose the lipo-protein sandwich? (3)

A

freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane
- rough surfaces were interpreted as being transmembrane proteins
- demonstrating that proteins were not solely localised to the outside of the membrane structure

24
Q

who proposed the new model?

A

singer and nicolson

25
Q

what did singer and nicolson propose? (2)

A
  • proteins embedded within the lipid bilayer rather than existing as separate layers
  • fluid-mosaic model

Biology SL (38 decks)

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