听力总结

Algae Biofuel If you’re like many people, when you think of algae you picture the unsightly green film that forms on neglected swimming pools and fish tanks.^Scientists, however, have made some discoveries that might change the way you think about this common plant.^Researchers are working on ways to use algae to reduce our dependence on fossil fuels to power cars, trucks, and even airplanes. Algae has attracted attention because it is simple and economical to grow and carries fewer financial and environmental costs when compared to other alternatives.^Algae needs only water to grow, and can use fresh water, salt water, marshlands, and even wastewater for a habitat. The plant’s highly-adapted photosynthesis process allows it to thrive anywhere that abundant sunshine can be found. And even sunshine is optional for this versatile plant because scientists have discovered ways to grow algae in the dark using sugar as a food source.^To use algae as a replacement for fossil fuels, oil is extracted from the plant and developed into a biofuel that can be burned in diesel engines. The yield produced from the algae has proven far greater than other biofuels currently being developed. For example, soy produces an average yield of fifty gallons of biofuel per acre per year. Canola, another common alternative, produces roughly 130 gallons of biofuel per acre per year. In contrast, scientists have found that algae can produce up to 4,000 gallons of biofuel per acre per year.^In addition, because algae can be grown on water rather than land and without fertilizers, it may prove to be a more efficient and (1)environmentally sound alternative to the biofuels in use today. In the future, then, when you see algae you might think about more than just pond scum!

All This Carbon Dioxide is Making Me Hungry Carbon dioxide is a potent greenhouse gas.^Levels of carbon dioxide, or CO2, in the atmosphere are increasing at an unprecedented rate. But what impact do higher CO2 levels have on plants?^Plants use CO2 from the air along with sunlight and water to produce carbohydrates they use for energy. This process is called photosynthesis.^Higher atmospheric CO2 increases the rate of photosynthesis, and thus the amount of carbohydrates in plant leaves. Sounds beneficial, right? However, scientists have found that as CO2 levels go up, plant defenses might go down.^Researchers at the University of Illinois used a special open-air research facility that allowed them to grow soybean plants in different concentrations of CO2—without changing other factors such as sunlight, insects or rainfall.^They discovered that soybeans grown in fields with higher CO2 levels had much more insect damage and attracted more adult insect pests than those in plots with less CO2 in the air.^The insects might have been attracted by the higher carbohydrate levels in the leaves of plants grown in high CO2. But a higher carb diet wasn’t the only thing that encouraged infestation.^Normally, soybean plants under attack by insects will produce jasmonic acid. This inhibits the i nsects’ ability to digest leaves, which protects the plant from further attack. In high CO2 atmospheres, the plants were unable to produce this defensive chemical.^So, the dangers of rapidly increasing CO2 in our atmosphere might extend beyond climate change. Since soybeans grown at higher CO2 would lose a crucial defense pathway and attract more hungry beetles, other food crops might also be more vulnerable if CO2 levels continue to rise.

Clouds in the Kitchen Fill a kettle with water, then turn on the burner.^In a while, your kettle will

start belching white billowy stuff into the air. What is this stuff? Steam? Actually, no.^Steam is water that’s heated to two hundred twelve degrees Fahrenheit. Believe it or not, steam is invisible–you can see right through it! If you look closely at the end of your kettle’s spout, you’ll notice that the white stuff doesn’t start right away. It

begins billowing about half an inch away from the nozzle, with clear gas in between. This clear gas is the

actual steam. The billowy white stuff is what the steam turns into when it hits the drier, cooler air of your kitchen.^Those white billows are, in fact, clouds, not steam. In many ways, they are identical to the clouds you can see in the sky. The white color comes from tiny liquid water droplets that have condensed from

the steam.^More accurately, these billows are a type of cloud called a ―mixing cloud.‖ These can form when two separate air masses–with different temperatures and different amounts of water in them–mix together. In the case of your kettle, the hot, steamy gas cools rapidly in the kitchen air, and this sudden coolness is what makes some of the vapor condense.^Mixing clouds are pretty common, and they don’t need to start with steam. You see mixing clouds when you ―see your breath‖ on a cold winter day. You’ll find them rising from a bowl of warm soup. Wherever there’s a mixing cloud, you can bet some warm,moist air is mixing with air that’s cooler and drier. Dinosaur Oil Crisis^w:What killed the dinosaurs？^m:She is again？I mean have we done this one like a hundred times。A giant meteor smashed into the Earth and blocked out the sun，so the dinosaurs had nothing to eat and died。^W：But not all scientists agree with that theory. There’s some evidence that a giant oil fire might have had something to do with the dinosaurs’ demise.^ m:Oh，OK,sorry.W:No problem，because you are right。Scientists still agree that around sixty-five million years ago, a meteor or comet or something was the instigator. But it’s not clear that the impact alone would have been enough to end the reign of the dinosaurs. So one theory is that the explosion started global forest fires. All that fire shot so much carbon dioxide in the atmosphere that the planet heated up like an oven and baked th e dinos to death.m:I heard that theory but problem is that there isn’t as much charred plant debris as you’d expect if there really were such giant fires. ^W:Right，so what sparked a new theory was the discovery of tiny carbon spheres at dozens of dinosaur dig sites around the world. The spheres are created from burning fossil fuels like oil. So maybe the meteor exploded in the midst of a huge oil field in the Gulf of Mexico. First it would have vaporized the oil, then ignited it in the atmosphere, creating a huge, sky-bound fireball. The combination of fire, soot and global warming may have been enough to kill the dinosaurs.^m:Wow，and we think we've got an oil crisis。

Early-morning Shivers If you’ve ever stayed up the whole night through, you may have noticed that you begin to feel cold in the hours just before morning.^Supposing you normally go to sleep at eleven o’clock, on the night that you stay awake the shivers will hit you somewhere around four or five a.m. — just about the time when you might otherwise pull up an extra blanket and sleep for another few hours. Although people often assume that the shivers come at the coldest part of the night, the actual explanation is found within the body itself.^Sleep is one way of conserving energy. During the night, when we are inactive, it isn’t necessary to generate as much energy as we need for our daily business of moving around, finding food, and so forth. So, instead of maintaining the same temperature all day, which would waste a lot of our energy, our body temperature rises and falls in accordance to what is needed. This kind of pattern is called a circadian rhythm, because it takes one day to complete before starting over.^The circadian temperature rhythm continues to run, regardless of whether you yourself have decided to go to sleep or not. Your body, you might say, executes the same strategy for conserving energy during what were supposed to be your off-hours. Your internal temperature descends to its lowest point late at night when you would normally have been in deep sleep. This conservation-mode is experienced by you as a creeping sensation

that the room is getting colder and colder, when in fact it is your own body that is generating less and less heat.^Do this for several nights in a row, and your temperature rhythm will begin to adjust itself. But anytime you keep yourself awake when the body expects to be asleep, those early-morning shivers will return.

Honeybees in Winter^Many insects hibernate or migrate during the winter, but honeybees are more like us, they bundle up and wait for spring.^When winter comes, honeybees form a ball deep in their hive to regulate the hive temperature. This means they stay close together, vibrating their wing muscles to generate heat and keep the center of the hive around eighty-six to ninety-five degrees Fahrenheit. If the core temperature drops too low, the colony will die.^Bees also eat a lot of honey to stay warm, and the fastes t way to kill off a hive if you’re a beekeeper is to take out too much honey, too late in the season. Healthy beehives have at least twenty-thousand bees, and that’s a lot of mouths to feed.^Some bees on the outside of the ball will not stay warm enough and will die over the course of the winter, but its for the good of the colony. (Fortunately, we humans have central heat.)^With bees one always talks about the colony as a single entity. This is because each bee has a very specific role, but together they work as a unit. You could say each bee functions like a cell in your body. Honeybees start out cleaning the hive and feeding larvae, but will progress to collectors and foragers, and even hive guardians.^Honeybees try their best to keep the hive clean, so on a warm winter day, they clean house and fly a few hundred yards away to use the bathroom! This can be dangerous, though, because if it’s colder out than they thought, the bees might not make it back.

Sailing Over a Tidal Wave^Tsunamis are powerful waves caused by underwater earthquakes or eruptions that can crash over ten stories tall into a coastline.^If you were sailing far from shore, how would you feel about a tremendous tsunami heading your way?^Actually, you wouldn’t notice a tsunami at all. In deep water, a tsunami raises the ocean’s surface only about one or two feet—much less than ordinary ocean waves. What’s more, it takes up to an hour for a single tsunami to roll by. How can a wave be so deadly to a coastline, but so tame in the open ocean?^The energy of a wave has more to do with how deep the water churns than how tall the wave rises. Ordinary waves can be rather tall, but they don’t go very deep. What’s more there’s never much distance between one ordinary wave’s crest and the one following it.The distance between waves is called ―wavelength,‖ and it’s another secret of a tsunami’s strength.^While ordinary waves only tug at the surface water, tsunamis churn the water all the way to the deepest ocean floor! They also have very long wavelengths—a hundred miles or more. Although tsunamis raise the surface only a foot or so in deep water, they carry a lot of energy.^When a tsunami approaches a coastline, the water gets shallower. This causes friction at the front of the wave and slows it down. The faster moving back of the wave piles into the slower front, squeezing the whole hundred mile tsunami like an accordion. Just as a rug will crumple and rise if it’s scrunched by an opening door, the tsunami gets taller. By the time it hits the beach, it can be a hundred feet high!

The Color of Lint You go to the local laundromat to do your wash.In the spinning barrel full of sudsy water you load clothes of all colors: a bright red tee-shirt, blue jeans, a green bandanna. The original mix of clothing is as multi-colored as a rainbow. Why, then, does the lint that collects in the lint tray always come out the same color—a bland gray?^The answer lies in the way color perception works. Lint is a collection of tiny, loose fibers that come off your clothes due to normal wear and tear. As no cloth, and indeed no single strand of cloth, is

perfectly wound, these tiny threads will always be breaking away. Thus were you to load the washer with clothes of only one color—say a uniform purple—the lint would indeed be of that color.^In most cases, however, a

wash-load is a mixture of hues. The eye, upon examining a closely-knit combination of colors, merges them into a single, generalized sensation. This phenomenon is exploited by painters, who sometimes put tiny dots of different primary colors right next to each other on a canvas to create the illusion of a rich, single hue when viewed all together. If you pull apart a piece of lint carefully you will see that it is indeed composed of multiple colored threads; but as soon as you back away from it, the gray sensation returns.^In an idealized situation, where the threads contained all the primary colors in equal amounts, and were not obscured by dirt and other residues, the lint would appear to be white, not gray. But then, i f your lint were clean enough to see that effect clearly, you’d never need to wash your clothes.

The Physics of Water Towers Water towers take advantage of the force of gravity to provide pressure for the water they contain. Every vertical foot adds point-four-three pounds per square inch to the water pressure. Towns usually keep their water pressure between fifty and one-hundred pounds per square inch, so a simple equation tells them how high to build the tower. And since a typical tower contains a full d ay’s worth of water, the force of gravity can maintain the hydrostatic pressure of the water system even when the power goes out.^Actually they serve an important day-to-day purpose as well. A city’s water usage varies throughout the day, usually peaking in the early morning when many people are showering and washing. Water usage during this time can be four or five times higher than during other parts of the day. To maintain water pressure during peak hours of the day, a city could invest in a very powerful pump, but this would be expensive and wasteful, since its capacity would go unused for most of the day.^Let me guess，instead, the city draws on the water tower’s supply of water du ring these hours of high demand.^That's right, the water tower and pumps work together to keep the water pressure stable throughout the day.

Tricking Your Muscles Here’s a neat experiment: F ind an open doorway and stand inside it, facing into the room.^Now press your arms outward until the backs of your hands are pressing into the door frame. Go ahead and try it. Keep pressing outward, as hard as you feel comfortable with, for the next minute or so.^In the meantime, let’s talk about what your muscles are doing. In order to maintain the right amount of stiffness, your muscles need to know how much resistance they’re up against. To check this, your muscles have sensory receptors inside the muscle tissue itself which tell your nervous system just how stiff or relaxed these muscles are. For example, muscle spindles are thin, fluid filled sacks that measure your muscles’ stretch. Receptors in the tendons measure your muscles’ contraction. With this information feeding back from the muscle, your body can figure out how tight the muscle needs to be in order to keep it in the same place. Which, right now as you press the doorway, is pretty tight.^What will happen, though, if you suddenly take the resist ance of that door frame away? Let’s find out.^Step into the room again and completely relax your arms. If you’ve been pressing hard enough, you’ll be surprised to find your arms now drifting effortlessly upward, as though you had helium balloons tied to yo ur wrists.^What’s going on is this: While you were pressing, your muscles learned that they needed to be pretty stiff to stay in the same place. Stepping out of the doorway changed all that–and very suddenly. It takes your muscles a few moments

to learn about this new situation. Until that happens, your arms will continue to press outward against a door frame that’s no longer there.

Not So Silly Geese If you’ve ever seen a bicycle race, you might have noticed the riders lining up behind each other in straight columns.^Bicycle racers spend much of their energy overcoming wind resistance, so this lining up makes good sense. If you ride directly behind your opponent, she’ll cut through the air resistance for you, letting you save up energy for the finish line. What’s true for bicycle racers is true for other things too. Going in straight line formation is the best way to overcome wind resistance.^Which brings us to those silly geese. Like bicycle racers, geese fly in formation mainly to save energy. The formation they prefer however is shaped like the letter ―V.‖ What’s going on? Are geese just plain dumb?^Not at all! Remember, geese don’t only need to worry about air resistance—they need to stay aloft as well. To do this, goose wings are curved and tapered like those of an aircraft. The top surface bulges upward while the bottom stays flatter. Air rushes more quickly over the wing’s bulging top, and this creates a kind of upward suction. This phenomenon is called ―lift,‖ and it’s a goose’s secret for staying aloft.^Lift, however, is a peculiar thing. While most of it tugs upward on the goose, some of it spills away from the wing tips as a kind of updraft. This lingering updraft spreads out behind the goose in a V-shape like the wake of a boat. If another goose puts its wings inside this updraft wake, it gets an added upward boost itself. What’s the best formation to take advantage of this? It’s not a straight line, it’s a ―V‖!

Boiling Water Under Boiling Clouds You may be surprised to hear that it’s actually e asier to boil water on stormy days than it is when the weather is clear and sunny.^Can you guess why?^The reason is that the boiling point of water changes slightly in response to air pressure. High air pressure, found on sunny, clear days, means just what it sounds like: crowded air molecules are pressing down on the water in the pot, just like they press on the stove, the floor and on you. This causes the molecules that make up the water to be squeezed more tightly together, which means that the steam bubbles trying to form at the bottom of the pot have a harder job pressing outward. It’s somewhat like trying to stretch your arms in a crowded room: the more crowded it is, t he harder it will be for you to stretch without being bumped back again by someone else. ^On stormy days, the air pressure is relatively low; you have probably heard the weather person refer to a ―low pressure system‖ moving in just before it rains. When there is low air pressure the water in your pot is not compressed as much by the air around it. Because it’s easier to displace that water, the bubbles of steam form more easily at the bottom of the pot; and voila— boiling begins.^There is a catch, however. Although the boiling point of water — 212 degrees — is reduced on stormy days, and thus water boils faster, the temperature of that boiling water is also lower. That means it will actually take LONGER to cook things — an effect that more than counters the ten seconds or so you save in waiting for the boil.