1.3 Membrane structure

Question 1

Structure of phospholipids

Answer 1

Consists of a polar head (hydrophilic) - composed of a glycerol and a phosphate molecule
Consists of two non-polar tails (hydrophobic) - composed of fatty acid (hydrocarbon) chains
Amphipathic - contain both hydrophilic and lipophilic regions

Question 2

Arrangement in membranes

Answer 2

Phospholipids arrange into a bilayer

Hydrophobic tail regions face inwards and shielded from surrounding polar fluids

Question 3

Properties of the phospholipid bilayer

Answer 3

The bilayer is held by weak hydrophobic interactions
Restrict the passage of many substances
Individual phospholipids can move within the bilayer, allowing fluidity and flexibility

Question 4

Integral proteins

Answer 4

permanently attached to the membrane and are typically transmembrane (span across the bilayer).

Question 5

Peripheral proteins

Answer 5

temporarily attached by non-covalent interactions and associate with one surface of the membrane.

Question 6

Structure of membrane proteins

Answer 6

Non-polar (hydrophobic) amino acids associate directly with the lipid bilayer.
Polar (hydrophilic) amino acids are located internally and face aqueous solutions.

Question 7

Transmembrane proteins structures

Answer 7

Single helices/helical bundles

Beta barrels

Question 8

Functions of membrane proteins

Answer 8

Junctions
Enzymes
Transport
Recognition
Anchorage
Transduction

Question 9

Junctions

Answer 9

serve of connect and join two cells together

Question 10

Enzymes

Answer 10

fixing to membranes localises metabolic pathways

Question 11

Transport

Answer 11

responsible for facilitated diffusion and active transport

Question 12

Recognition

Answer 12

may function as markers for cellular identification

Question 13

Anchorage

Answer 13

attachment points for cytoskeleton and extracellular matrix

Question 14

Transduction

Answer 14

function as receptors for peptide hormones

Question 15

Cholesterol

Answer 15

a component of animal cell membranes, where it functions to maintain integrity and mechanical stability.

Question 16

Functions of cholesterol

Answer 16

Immobilise the outer surface of the membrane, reducing fluidity.
Makes the membrane less permeable to very small water-molecule molecules that would otherwise freely cross
Separate phospholipid tails so prevent crystallisation.
Secure peripheral proteins by high density lipid rafts capable of anchoring the protein.

Question 17

Cholesterol at high temperatures

Answer 17

it stabilises the membrane and raises the melting point.

Question 18

Cholesterol at low temperatures

Answer 18

it intercalates between the phospholipids and prevents clustering.

Question 19

Fluid

Answer 19

the phospholipid bilayer is viscous and individual phospholipids can move position

Question 20

Mosaic

Answer 20

the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components

Question 21

First model of fluid-mosaic model

Answer 21

The first model was proposed in 1935 by Hugh Davson and James Danielli.
They proposed a model whereby two layers of protein flanked a central phospholipid bilayer
The model was described as a “lipo-protein sandwich” - the lipid layer was sandwiched between two protein layers

Question 22

Problems of the lipo-protein sandwich

Answer 22

Assumed all membranes were of a uniform thickness
All membranes would aave symmetrical internal and external surfaces
Did not account for the permeability of certain substances
Temperatures at which membranes solidified did not correlate with those expected under the proposed model

Question 23

Evidence of falsification

Answer 23

Membranes were discovered to be insoluble in water and varied in size.
They are mobile and not fixed in place.
Freeze fracturing was used to split open the membrane and releaved irregular rough surfaces within the membrane.