1.3 Membrane Structure

Question 1

what is the structure of phospholipids? (3)

Answer 1

• 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

Question 2

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

Answer 2

• 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

Question 3

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

Answer 3

• 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)

Question 4

what are phospholipids embedded with? (2)

Answer 4

• embedded w/ proteins
  which can be permanent or temporary

Question 5

what two kinds of proteins are phospholipids embedded with?

Answer 5

• 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

Question 6

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

Answer 6

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

Question 7

what 2 tertiary structures do transmembrane proteins typically adopt?

Answer 7

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

Question 8

what 6 functions can membrane proteins serve?

Answer 8

• 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

Question 9

what is cholesterol’s function?

Answer 9

maintain integrity and mechanical stability

Question 10

where is cholesterol not found?

Answer 10

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

Question 11

what properties does cholesterol have? (3)

Answer 11

• 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

Question 12

what kind of properties do phospholipids have?

Answer 12

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

Question 13

what does cholesterol interact within the bilayer?

Answer 13

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

Question 14

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

Answer 14

• 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

Question 15

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

Answer 15

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

Question 16

what are 3 components of the plasma membrane?

Answer 16

• 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

Question 17

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

Answer 17

Davson and Danielli

Question 18

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

Answer 18

‘trilaminar’ appearance
means 3 layer

Question 19

what model did Davson and Danielli?

Answer 19

‘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

Question 20

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

Answer 20

• 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

Question 21

what is the evidence against the lipo-protein sandwich?

Answer 21

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

Question 22

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

Answer 22

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

Question 23

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

Answer 23

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

Question 24

who proposed the new model?

Answer 24

singer and nicolson

Question 25

what did singer and nicolson propose? (2)

Answer 25

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