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Work Sheet 14
Vitamin A and Vision
1. Identify provitamin A / -carotene, vitamin A / retinol, vitamin A ester / retinol
palmitate, retinaldehyde / retinal and retinoic acid (all-trans and 9-cis) in terms of
their positions on the pathway of vitamin A metabolism. Delineate which
reactions are reversible and which are not.
Describe the biological function of each of the forms of vitamin A.
Describe the role of vitamin A in the visual cycle, in particular:
What happens when a Photon enters?
What does the “door” being OPEN imply?
What does the “door” being CLOSED imply?
Define progressive vitamin A deficiency clinically and biochemically.
Discuss why excess vitamin A is toxic to skin, bones, liver and gut.
Classify vitamin A and its RAR with the ligands for PPAR, the thyroid hormone
receptor (TR) and the vitamin D receptor (VDR), i.e. review Group I receptors
and understand that Vitamin A fits into this group.