Question 20N.3.hl.TZ0.8c
| Date | November 2020 | Marks available | [Maximum mark: 3] | Reference code | 20N.3.hl.TZ0.8c |
| Level | hl | Paper | 3 | Time zone | TZ0 |
| Command term | Explain | Question number | c | Adapted from | N/A |
The diverse functions of biological molecules depend on their structure and shape.
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.
[3]
Any three of:
cis-retinal binds to «the protein» opsin
OR
cis-retinal «binds to opsin and» forms rhodopsin ✔
opsin extends conjugation in retinal
OR
conjugation in rhodopsin is larger/more extended/involves more atoms than that 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 ✔
This question which centred on retinal as the key molecule in vision was well answered and many scored all three marks. The most common incomplete answer was "cis-retinal changing to trans-retinal" without stating that this occurs when visible light is absorbed. Some also were unclear of the difference between opsin and rhodopsin.
