第九篇 An Essential Scientific Process
All life on the earth depends upon green plants. Using sunlight, the plants produce their own food. Then animals feed upon the plants. They take in the nutrients the plants have made and stored. But that’s not all. Sunlight also helps a plant produce oxygen. Some of the oxygen is used by the plant, but a plant usually produces more oxygen than it uses. The excess oxygen is necessary for animals and other organisms to live.
The process of changing light into food and oxygen is called photosynthesis. Besides light energy from the sun, plants also use water and carbon dioxide. The water gets to the plant through its roots. The carbon dioxide enters the leaves through tiny openings called stomata. The carbon dioxide travels to chloroplasts, special cells in the bodies of green plants. This is where photosynthesis takes place. Chloroplasts contain the chlorophylls that give plants their green color. The chlorophylls are the molecules that trap light energy. The trapped light energy changes water and carbon dioxide to produce oxygen and a simple sugar called glucose.
Carbon dioxide and oxygen move into and out of the stomata. Water vapor also moves out of the stomata. More than 90 percent of water a plant takes in through its roots escapes through the stomata. During the daytime, the stomata of most plants are open. This allows carbon dioxide to enter the leaves for photosynthesis. As night falls, carbon dioxide is not needed. The stomata of most plants close. Water loss stops.
If photosynthesis ceased, there would be little food or other organic matter on the earth. Most organisms would disappear. The earth’s atmosphere would no longer contain oxygen. Photosynthesis is essential for life on our planet.
詞匯：
nutrient n.營養(yǎng)物
organism n.生物體，有機體
carbon dioxide n.二氧化碳
chloroplast n.葉綠體
molecule n.分子
vapor n.水蒸氣
oxygen n.氧氣
photosynthesis n.光合作用
chlorophyll n.葉綠素
glucose n.葡萄糖
cease v.停止
注釋：
1.Then animals feed upon the plants.動物以植物為食。
練習：
1.In the first paragraph，the word “excess” means
A heavy.
B extra.
C green.
D liquid.
2.Which of the following does not move through a plant’s stomata?
A Carbon dioxide.
B Water vapor.
C Oxygen.
D Food.
3.In the title, the term Essential Scientific Process refers to
A photosynthesis.
B the formation of glucose.
C global warming.
D water getting to the roots of plants.
4.This passage is primarily developed by
A explaining a process.
B telling a story.
C comparing and contrasting.
D convincing the reader of plants’ importance.
5.Another good title for this passage would be
A Oxygen and Carbon Dioxide.
B Plants and Their Roots.
C How Photosynthesis Works.
D Why Our Earth Needs Water.
答案與題解：
1.B 前文講到，植物產(chǎn)生的氧氣一部分被植物自身消耗了，但植物消耗的氧氣量遠小于它們產(chǎn)生的氧氣，因此可以推測這句話的意思應該是剩余的氧氣對于動物以及其他生物體的生存是至關重要的。excess在句中的意思是“超額的”，與extra“額外的”意思相近。
2.D 從第三段的第一、二句得知，二氧化碳、氧氣和水蒸氣都能從氣孔中通過，唯一一個沒有提到的是food“養(yǎng)分、食物”，因此該題選D項。
3.A 文章通篇都在講Photosynthesis，即光合作頌鈉作用和重要性，文章結尾又重申了Photosynthesis is essential for life on our planet，因此選A項。B項是光合作用的一個部分，C、D項則毫不相干。
4.A 文章先是介紹了進行光合作用所需的原料和組織，又介紹了光合作用的過程，因此整個邏輯應該是解釋過程，而不是講故事或比較對比。D項是“向讀者說明植物的重要性”，這確實是文章的一個目的，但不是文章的組織方式。
5.C 文章的主題是光合作用的基本原理，因此選項C。A、B項在文中有提及，但不是主旨，D項與本文無關。
