我喜欢吃桃子，

桃子的外形圆圆的

，像一个小小的皮球 ，最顶端还有一个小尖尖，有的桃子的尖还稍稍

歪一点，真有趣！我摸了摸桃子的外皮，感觉有点扎手仔细一看原来桃子长着许多细毛毛，沾的我满手都是

。看来桃子还穿了一身“毛衣服”呢，太好玩了。
我举起桃子，闻了闻，感觉有一种混杂着甜的而且可

以提神的香味。桃子的外表是浅黄色的，上面还附着一些大小不同的

红色。我把桃子的皮小心地扒下来，观察果肉的颜色。我发现，果肉的颜色是乳白色的，熟

透的地方是乳黄色的，还夹着几丝粉红色，和表皮的颜色不一样。我轻

轻地咬了一口，有一股清新的味道融入了口中，而且还有一种比蜂蜜都要甜的汁水流入满口。

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　　vitamin a is an essential human nutrient. its important part is the retinyl group, which can be found not in a single compound, but in several forms. in foods of animal origin, the major form of vitamin a is an alcohol (retinol), but can also exist as an aldehyde (retinal), or as an acid (retinoic acid). precursors to the vitamin (provitamins) are present in foods of plant origin as some of the members of the carotenoid family of compounds.[1]

all forms of vitamin a have a beta-ionone ring to which an isoprenoid chain is attached. this structure is essential for vitamin activity.[1] the orange pigment of carrot - beta-carotene - can be represented as two connected retinyl groups. retinyl group, being attached to a protein, is the only primary light absorber in visual perception, and the compound name is related to retina of eye.

vitamin a may be found in various forms:

retinol, the animal form of vitamin a, is a yellow, fat-soluble, vitamin with importance in vision and bone growth. since the alcohol form is unstable, the vitamin is usually produced and administered in a form of retinyl acetate or palmitate.
other retinoids, a class of chemical compounds that are related chemically to vitamin a, are used in medicine.