| Date | November 2017 | Marks available | 3 | Reference code | 17N.3.hl.TZ0.15 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Explain | Question number | 15 | Adapted from | N/A |
Question
Vitamins can be water-soluble or fat-soluble.
Retinal is the key molecule involved in vision. Explain the roles of cis and trans-retinal in vision and how the isomers are formed in the visual cycle.
Markscheme
Any three of:
cis-retinal binds to «the protein» opsin
OR
cis-retinal «binds to opsin and» forms rhodopsin
rhodopsin extends conjugation in retinal
OR
rhodopsin allows absorption of visible/blue/green light
when visible light is absorbed cis-retinal changes to trans-retinal
change «to trans-retinal» triggers an electrical/nerve signal
trans-retinal detaches from opsin AND is converted back to cis-retinal
OR
trans-retinal is converted back to cis-retinal through enzyme activity
Do not accept “cis-retinal to trans-retinal” alone without reference to absorption of visible light.
[Max 3 Marks]
