topic 1.3- membrane structure

Question 1

why do phospholipids form bilayers in water?

Answer 1

due to the amphipathic properties of phospholipid molecules

Question 2

what is an amphipathic molecule?

Answer 2

contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions

Question 3

describe the structure of a phospholipid

Answer 3

• a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
• and two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains

Question 4

describe how phospholipids spontaneously arrange into bilayers.

Answer 4

• hydrophobic tails face inwards and so are shielded from the surrounding polar fluids
• the two hydrophilic heads associate with the cytosolic (intracellular) and extracellular fluids

Question 5

what is are phospholipids held together by?

Answer 5

weak hydrophobic interactions between the tails

Question 6

give two properties of a phospholipid bilayer

Answer 6

• hydrophilic / hydrophobic layers restrict the passage of many substances
• fluidity and flexibility: individual phospholipids can move within the bilayer

Question 7

why is the fluidity of the phospholipid bilayer important?

Answer 7

allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)

Question 8

state the 6 functions of membrane proteins

Answer 8

Junctions
Enzymes
Transport
Recognition
And neurotransmitters
Transduction

Question 9

Define ‘junctions’ as a membrane protein function

Answer 9

cell adhesion to connect and join groups of cells together in tissues and organs.

Question 10

Define ‘enzymes’ as a membrane protein function

Answer 10

for immobilised enzymes with the active site on the outside, fixing to membranes localises metabolic pathways (eg small intestine)

Question 11

Define ‘transport’ as a membrane protein function

Answer 11

act as channels to allow hydrophilic particles across by facilitated diffusion, and as pumps for active transport which use ATP to move particles across the membrane.

Question 12

Define ‘recognition’ as a membrane protein function

Answer 12

May function as markers for cellular identification

Question 13

Define ‘and neurotransmitters’ as a membrane protein function

Answer 13

cell-to-cell communication, for example receptors for neurotransmitters at synapses

Question 14

Define ‘transduction’ as a membrane protein function

Answer 14

hormone binding sites (peptide hormone receptors), for example the insulin receptor.

Question 15

what two types of membrane proteins are there?

Answer 15

• integral proteins
• peripheral proteins

Question 16

describe integral proteins

Answer 16

• permanently attached to the membrane
• typically transmembrane (span across the bilayer)
• hydrophobic on at least part of their surface
• embedded in hydrocarbon chains at centre

Question 17

describe peripheral proteins

Answer 17

• temporarily attached by non-covalent interactions
• associate with one surface of the membrane, not embedded in the membrane
• hydrophilic on surface
• may be attached to surface of integral proteins/have a single hydrocarbon chain in the membrane

Question 18

the more active a membrane, the —– its protein content

Answer 18

higher

Question 19

what group of lipid substances does cholesterol belong to?

Answer 19

steroids

Question 20

why is cholesterol considered amphipathic?

Answer 20

Cholesterol’s hydroxyl (-OH) group is hydrophilic but the remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic

Question 21

what does cholesterol’s hydroxyl group align with?

Answer 21

towards the phosphate heads of phospholipids

Question 22

what do cholesterol’s steroid ring/hydrocarbon tail align with?

Answer 22

with the phospholipid tails

Question 23

state two functions of cholesterol in mammalian membranes

Answer 23

• controls membrane fluidity
• reduces permeability to some solutes

Question 24

how does cholesterol affect membrane fluidity in high/low temperatures?

Answer 24

at high temperatures, cholesterol reduces membrane fluidity through its interactions with the fatty acid tails, which stabilise the membrane . at low temperatures, cholesterol increases membrane fluidity by preventing the phospholipid tails from packing too close together.

Question 25

why does the fluidity of animal cell membranes need to be controlled?

Answer 25

too fluid-> unable to control what substances pass through
not fluid enough-> movement of cell and substances would be restricted

Question 26

describe the 2ary roles of cholesterol in mammalian membranes

Answer 26

3. reduces permeability to hydrophilic particles such as Na+ ions and H+ ions
4. helps membranes curve into a concave shape, which helps formations of vesicles during endocytosis

Question 27

draw the fluid mosaic model

Answer 27

refer elsewhere

Question 28

describe the Davson-Danielli model

Answer 28

• model whereby two layers of protein flanked a central phospholipid bilayer
• ‘lipo-protein sandwich’: the lipid layer was sandwiched between two protein layers

Question 29

what were the dark segments under the microscope identified as?

Answer 29

dark segments seen under electron microscope were identified (wrongly) as representing the two protein layers

Question 30

Give the three main pieces of evidence for the falsification of the Davson-Danielli model

Answer 30

• membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size
• Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place
• Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane

Question 31

How does the insolubility and variation in size of membrane proteins disprove the DD model?

Answer 31

Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane

Question 32

how did antibody tagging disprove the DD model?

Answer 32

showed membrane proteins could move and did not form a static layer

Question 33

how did freeze fracturing disprove the DD model?

Answer 33

rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure