Question 19M.2.SL.TZ1.5c
| Date | May 2019 | Marks available | [Maximum mark: 7] | Reference code | 19M.2.SL.TZ1.5c |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Explain | Question number | c | Adapted from | N/A |
Explain how hydrophobic and hydrophilic properties contribute to the arrangement of molecules in a membrane.
[7]
a. hydrophilic is attracted to/soluble in water and hydrophobic not attracted/insoluble ✔
b. hydrophilic phosphate/head and hydrophobic hydrocarbon/tail in phospholipids ✔
c. phospholipid bilayer in water/in membranes ✔
d. hydrophilic heads «of phospholipids» face outwards/are on surface ✔
e. hydrophobic tails «of phospholipids» face inwards/are inside/are in core ✔
f. cholesterol is «mainly» hydrophobic/amphipathic so is located among phospholipids/in hydrophobic region of membrane ✔
g. some amino acids are hydrophilic and some are hydrophobic ✔
h. hydrophobic «amino acids/regions of» proteins in phospholipid bilayer «core» ✔
i. hydrophilic «amino acids/regions of» proteins are on the membrane surface ✔
j. integral proteins are embedded in membranes due to hydrophobic properties/region
OR
transmembrane proteins have a hydrophobic middle region and hydrophilic ends ✔
k. peripheral proteins on are on the membrane surface/among phosphate heads due to being «entirely» hydrophilic
OR
«carbohydrate» part of glycoproteins is hydrophilic so is outside the membrane ✔
l. pore of channel proteins is hydrophilic ✔
Allow mark points shown in clearly annotated diagram.
In any part of the answer, accept polar instead of hydrophilic and non-polar or apolar instead of hydrophobic.
Candidates struggled to explain the meaning of the term hydrophobic. Many stated the literal meaning – water-fearing, hence repelled by water. This was not accepted as hydrophobic molecules aren’t repelled by water. They appear to be, because water is more attracted to polar or ionic substances than to apolar/hydrophobic substances, but there is no chemical mechanism for repulsion and of course molecules do not fear each other. Many focused only on phospholipids, but stronger answers also included information on proteins and how the positions they occupy within the membrane depend on their hydrophobic and hydrophilic properties.
