Question 18M.2.HL.TZ2.a
| Date | May 2018 | Marks available | [Maximum mark: 8] | Reference code | 18M.2.HL.TZ2.a |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Discuss | Question number | a | Adapted from | N/A |
Every cell is surrounded by a cell surface membrane which regulates the movement of materials into and out of the cell.
Discuss alternative models of membrane structure including evidence for or against each model.
[8]
a. early evidence showed membranes are partially permeable AND organic solvents penetrate faster than water
b. suggests they have non-polar regions
c. chemical analysis showed membranes consist mainly of proteins and lipids
d. layer of phospholipids spread over water, orientate themselves into monolayer with nonpolar/hydrophobic tails out of water and polar/hydrophilic heads in water surface
e. when shaken with water form micelles/particles with tails inwards away from water
f. Davson–Danielli model proposed phospholipid bilayer coated with protein molecules on both surfaces
g. evidence from electron microscopy «supported Davson–Danielli model»
h. three-layered structure/ sandwich/railway tracks/two dark bands with a light band between
i. model could not account for hydrophobic proteins / artifacts due to low resolution
j. fluorescent labelling / freeze fracturing later used to investigate membrane structure
k. led to Singer-Nicholson / fluid mosaic model of protein molecules floating in fluid lipid bilayer
l. shows particles/proteins project partially and sometimes right through lipid bilayer
m. indicates peripheral and integral proteins present
Accept any of the points clearly explained in an annotated diagram.
