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Electric Backpack
Backpacks are convenient. They can hold your books， your lunch， and a change of clothes leaving your hands free to do other things. Someday， if you don"t mind carrying a heavy load， your backpacks might also power your MP3 player， keep your cell phone running， and maybe even light your way home.
Lawrence C. Rome and his colleagues from the University of Pennsylvania in Philadelphia and the Marine Biological Laboratory in Woods Hole， Mass. have invented a backpack thatmakes electricity from energy produced while its wearer walks. In military actions search-and-rescue operations and scientific field studies， people rely increasingly on cellphones global positioning system （GPS） receivers，night-vision goggles， and other battery powered devices to get around and do their work. The backpack"s electricity-generating feature could dramatically reduce the amount of a wearer"s load now devoted to spare batteries， report Rome and his colleagues in the Sept.9 science.
The backpack"s electricity-creating powers depend on springs used to hang a cloth pack from its metal frame. The frame. sits against the wearer"s back， and the whole pack moves up and down as the person walks. A gear mechanism converts vertical movements of the pack to rotary motions of an electrical generator， producing up to 7.4 watts.
Unexpectedly， tests showed that wearers of the new backpack alter their gaits in response to the pack"s oscillations， so that they carry loads more comfortably and with less effort than they do ordinary backpacks. Because of that surprising advantage， Rome plans to commercialize both electric and non-electric versions of the backpack.
The backpack could be especially useful for soldiers， scientists， mountaineers， and emergency workers who typically carry heavy backpacks. For the rest of us， power-generating backpacks could make it possible to walk， play video games， watch TV， and listen to music， all at the same time. Electricity-generating packs aren"t on the market yet， but if you do get one eventually just make sure to look both ways before crossing the street!
Backpacks are convenient because __________. 查看材料
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In the title， the term Essential Scientific Process refers to___________ .
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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 （叶绿题） ， special cells in the bodies of green plants. This is where photosynthesis （光合作用 ） takes place. Chloroplasts contain the chlorophylls （叶绿素） 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.
In the first paragraph， the word "excess" means___________ . 查看材料
This passage is primarily developed by___________ . 查看材料
Which of the following does not move through a plant‘s stomata？
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