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.
词汇：
nutrient n.营养物
organism n.生物体，有机体
carbon dioxide n.二氧化碳
chloroplast n.叶绿体
molecule n.分子
vapor n.水蒸气
oxygen n.氧气
photosynthesis n.光合作用
chlorophyll n.叶绿素
glucose n.葡萄糖
cease v.停止
注释：
1.Then animals feed upon the plants.动物以植物为食。
练习：
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.
答案与题解：
1.B 前文讲到，植物产生的氧气一部分被植物自身消耗了，但植物消耗的氧气量远小于它们产生的氧气，因此可以推测这句话的意思应该是剩余的氧气对于动物以及其他生物体的生存是至关重要的。excess在句中的意思是“超额的”，与extra“额外的”意思相近。
2.D 从第三段的第一、二句得知，二氧化碳、氧气和水蒸气都能从气孔中通过，唯一一个没有提到的是food“养分、食物”，因此该题选D项。
3.A 文章通篇都在讲Photosynthesis，即光合作頌钠作用和重要性，文章结尾又重申了Photosynthesis is essential for life on our planet，因此选A项。B项是光合作用的一个部分，C、D项则毫不相干。
4.A 文章先是介绍了进行光合作用所需的原料和组织，又介绍了光合作用的过程，因此整个逻辑应该是解释过程，而不是讲故事或比较对比。D项是“向读者说明植物的重要性”，这确实是文章的一个目的，但不是文章的组织方式。
5.C 文章的主题是光合作用的基本原理，因此选项C。A、B项在文中有提及，但不是主旨，D项与本文无关。
