- 相关推荐
经典英语作文：秋天叶子为什么会变颜色
在平凡的学习、工作、生活中，大家或多或少都会接触过作文吧，根据写作命题的特点，作文可以分为命题作文和非命题作文。那要怎么写好作文呢？下面是小编为大家收集的经典英语作文：秋天叶子为什么会变颜色，仅供参考，大家一起来看看吧。
Why do leaves change color in the fall? asks Patricia Brown, of New York City.
Autumn's cool days are trimmed with deep blue skies and golden light, and brilliant leaves of yellow, orange and red. Leaves changing color in the fall are a tree's way of preparing for long winter, rather like we put up storm windows and pull warm clothes and blankets out of storage.
In summer, the leaves on trees like pin oaks and sugar maples are green because they are chock-full of the green pigment chlorophyll.
Trees need sunlight to produce chlorophyll. In turn, chlorophyll uses sunlight's energy to split water (H2O) into hydrogen and oxygen. Meanwhile, leaves also absorb carbon dioxide gas from the air. The end products of leaf chemistry: carbohydrates (homemade plant food for the tree), and oxygen, released into the air (the gas we need to breathe). The whole process is called photosynthesis.
Along with green chlorophyll, most leaves also contain yellow, orange and red-orange pigments celled carotenoids. Trees don't need light to make carotenoids. Botanists call them "helper pigments," because carotenoids absorb some sunlight and (nicely) pass the energy along to chlorophyll. We don't see much of these deputy pigments (carotene, lycopene, and xanthophyll) in summer, because they are masked by abundant green chlorophyll.
But the ever-shortening days of fall mean less daylight and colder weather. The average tree is rushing to save all the nutrients it can for its winter hibernation. Nitrogen and phosphorus are pulled from leaves for storage in branches. A layer of corky cells grows between the leaves' stems and their branches, reducing the leaves' supply of nutrients and water.
With diminished sunlight, water, and nutrients, chlorophyll synthesis slows. Old, worn-out chlorophyll breaks down at the usual rate--ironically, sunlight destroys it--so each leaf's stock gradually dwindles. And as the green fades, yellow and orange emerge from hiding.
Unlike the green and yellow pigments, red and purple pigments (anthocyanins, part of the flavonoid class) actually form in leaves in the autumn, tinting leaves scarlet and burgundy.
Botanists have long wondered why some trees are genetically programmed to manufacture anthocyanins in the fall. New research indicates that anthocyanins may be a tree's own sunscreen.
Anthocyanins are made in a leaf's sugary sap, with the help of lots of sun and cool temperatures. Botanists think that anthocyanins shield the leaves' fading photosynthesis factories from too much sunlight, rather like the pigment melanin protects our skin from the sun. While the red pigments act as a shield, the tree feverishly breaks down and pulls nutrients out of leaves and into its limbs and trunk before leaves drop or die.
Anthocyanins may also act like Vitamin C or E, scavenging so-called "free radicals" before they can do oxidizing damage to a fall leaf's fragile structure.
Upper and outer leaves tend to be reddest, since they are most exposed to sunlight and cold. In some trees, like sugar maples, the reds of the anthocyanins bined with the yellows of the carotenoids make
【经典英语作文：秋天叶子为什么会变颜色】相关文章：
会变的颜色教案01-16
《会变的颜色》说课稿04-29
会变颜色的房子教案05-18
会变颜色的房子 教案04-25
假如太阳会变颜色作文04-30
会变的秋天作文04-30
会变的颜色中班美术教案02-05
小班教案：会变颜色的水01-04
大班语言《会变颜色的房子》的教案02-07
中班美术活动《会变的颜色》教案01-12
