Answer to Question 1
As mentioned, vitamin A also supports reproduction and regulates growth. In men, retinol participates in sperm development, and in women, vitamin A supports normal fetal development during pregnancy. Children lacking vitamin A fail to grow; given vitamin A supplements, these children gain weight and grow taller.
The growth of bones illustrates that growth is a complex phenomenon of remodeling. To convert a small bone into a large bone, some bone cells must undo parts of the bone before other cells can build new bone, and vitamin A participates in the dismantling. The cells that break down bone contain acid and enzymes that dissolve the minerals and digest the matrix. With the help of vitamin A, these bone-dismantling cells destroy selected sites in the bone, removing the parts that are not needed. After completing their work, the bone-dismantling cells die, leaving their excavation site to be rebuilt by the bone-building cells.
Answer to Question 2
phylloquinone (K1); menaquinone (K2)
Tip growth by a pollen tube.
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