托福阅读真题+题目+答案：Breathing Inside an Egg

托福阅读真题+题目+答案：Breathing Inside an Egg

Tucked away inside its shell, a bird embryo has to breathe. But, rather than using lungs to draw air in and push carbon dioxide and water vapor out, a bird embryo relies on“diffusion"- -the natural movement of gases- -much as do insects (which also lack lungs). In fact, both insects and eggs use tiny pores, or holes, and pore canals that connect the outside with their interior. For birds there are hundreds or thousands of tiny pores distributed all over the shell surface. The pores connect, via a narrow tube, the embryo's blood supply to the outside world. The number of pores per egg varies markedly between species, partly but not entirely related to the size of the egg. Since the pores are fairly straight and run vertically from the inner to the outer surface, their length is usually similar to the thickness of the shell. Generally, the number and size of pores determine how much and how fast oxygen diffuses into the egg. As well as taking away unwanted carbon dioxide, the pores allow water vapor to escape from the developing embryo. As the embryo grows it generates water, referred to as metabolic water, produced as a result of the metabolism of food. Different foods generate different amounts of metabolic water. Fat, for example, yields comparatively high amounts of metabolic water.

1.According to paragraph 1, which of the following is NOT true of the pores found in bird eggs?

A They are most highly concentrated at the ends of the egg.

B They differ significantly in number from one bird species to the next.

C Their number is partly based on the size of the egg.

D They are about as long as the thickness of the egg shell.

2.According to paragraph 1, a relatively large source of water vapor inside an egg is

A the breakdown of fat

B the breakdown of carbon dioxide before it leaves the egg

C the pores of the egg

D the water that enters the egg when oxygen diffuses into it

Inside an egg the developing chick generates plenty of metabolic water from the fat-rich yolk (yellow part of the egg) as it grows. This water has to be removed; otherwise the embryo would drown in its own juices, so to speak, and it does this by allowing it to diffuse as water vapor through the pores in the shell. As a result, eggs lose weight during the course of incubation. What is remarkable is that, despite the huge variation across bird species in the size of eggs, the duration of incubation, and the relative size of the yolk, the loss of water between laying and hatching is always about 15 percent of the egg's initial weight. The water vapor lost during incubation ensures that the relative amount of water in the egg is the same in the chick at hatching as it was when the egg was laid. In other words, the composition of the newly laid egg has evolved through natural selection to ensure that the newly hatched chick has the right composition- -in terms of the amount of water in its tissues, too. This is achieved by adjusting-via natural selection- -the effective pore area such that all the metabolic water produced during development is eliminated before hatching. One consequence of this loss of water vapor is a space in the egg, roughly 15 percent of its volume, that becomes the air cell at the blunt, or flat, end of the egg and provides the amount of air needed by the chick just before it hatches.

3.According to paragraph 2, each of the following varies greatly among different bird species EXCEPT

A how big the eggs are

B how long the eggs spend incubating

C how much yolk the eggs contain

D the proportion of water lost between laying and hatching

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