Explain how the structure of vitamin A is important to vision using section 35 of the data booklet.

Sketch the wedge and dash (3-D) representations of alanine enantiomers.

Identify the structural feature which enables rhodopsin to absorb visible light.

Vision is dependent on retinol (vitamin A) present in retina cells. Retinol is oxidized to the photosensitive chemical 11-cis-retinal and isomerizes to 11-trans-retinal on absorption of light.

Outline how the formation of 11-trans-retinal results in the generation of nerve signals to the brain.

Outline the change that occurs in the retinal residue during the absorption of visible light.

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.

Aspartic acid is obtained synthetically as a racemic mixture. Draw the three‑dimensional shape of each isomer showing their spatial relationship to each other. Use section 33 of the data booklet.

Classify, giving your reason, the hexose (six-membered) ring of sucrose as an α or β isomer.