题型：阅读选择 题类：常考题 难易度：普通
人教新目标（Go for it）八年级英语上学期期末试题
B
Have you ever wondered how and why a fall leaf changes color? We first have to understand what leaves are and what they do.
Plants are the world's food factories. Plants take water from the ground and take CO2 from the air. Plants use sunlight to turn water and CO2 into glucose(葡萄糖). Glucose is a kind of sugar. Using it as food, plants get energy and grow. Plants using sunlight to turn water and CO2 into sugar is called photosynthesis(光合作用). ChlorophyII helps photosynthesis. It gives plants their green color.
As summer ends and autumn comes, the days get shorter and shorter, and trees "know" to begin getting ready for winter. During winter, there is not enough light or water for photosynthesis. Trees rest during this time and live on the food they store during the summer. They begin to shut down their food — making factories. As the green chlorophyII disappears from the leaves, we begin to see yellow and orange leaves. These colors have been in the leaves all the time. We just can't see them in the summer, because they are covered up by the green chlorophyII.
The bright reds and purples we see in leaves are made mostly in autumn. In some trees, glucose is caught in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn turn this glucose into a red color. The brown color of trees is made from waste left in the leaves. It is mixture(混合物) of all these things. It makes the beautiful leaves during autumn.
Problems in 2050 Three Internet Posts |
Lily: I was staying at the Hotel Whale with my friends last week. After going shopping and having tea at the cafe, I went to back to my room to rest .But when I open the door, I saw that water was coming through the window! I knew I should not just stand there, but I did not know what to do ... |
Mike: At 11:30 a.m. the day before yesterday, I was having lunch in our Forever Green House while our fridge was ordering food. Suddenly a light went on and a lot of food started to arrive from the supermarket: meat, vegetable, fruit, milk, and so on. "Stop" I shouted at the fridge, but the food continue to come in... |
Benny: Daddy was driving us to the airport in his CJ3, and suddenly the car stopped. The hydrogen in the car failed to mix with oxygen, so the car did not work," Oh, no!" said Mum. "We are going to miss our plane. What shall we do?" |
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