2012届上海高考模拟试题

the extra oxygen to break down carbohydrates, which they use for fuel, and turn them into energy, so their metabolism is said to be aerobic (with air). You are now jogging along at a comfortable speed, still supplying sufficient oxygen to your muscle cells, so your exercise is still aerobic. As you increase your speed, your oxygen consumption will continue to rise, but at some point, it will stop rising. That is because your heart and lungs have reached their maximum capacity for supplying oxygen to the muscles via the bloodstream. But you can still run a lot faster. Your speed keeps increasing and eventually you are running as fast as you can, but your oxygen consumption has not changed. Your muscle cells can keep on firing without getting the extra oxygen they need because they are able to break down carbohydrates without using oxygen. This is called anaerobic (without air) metabolism. Sprinters (短跑运动员), who run at top speed, perform most of their hundred-meter race anaerobically; in contrast, long-distance runners, who have time to vary their speed, perform most of their event aerobically.

Anaerobic metabolism is far less efficient than aerobic metabolism but is capable of generating very high quantities of energy for short periods of time. It also generates lactic acid (乳酸) as a by-product. This is stored in the muscles and associated with muscle fatigue and general exhaustion. That is why sprinting cannot be kept up for very long. After sprinting, the lactic acid that has been stored in the muscles has to be broken down, and this requires oxygen, which explains why we breathe quickly after a hard run.

72. The body’s energy is usually obtained through the breakdown of _____.

A. carbohydrates B. lactic acid C. the bloodstream D. oxygen

73. When you run, your body begins anaerobic metabolism _____.

A. as you begin to pick up speed

B. after sprinting is completed

C. when your heart and lungs cannot supply enough oxygen

D. when the lactic acid in your muscles must be broken down

74. Which of the following statements is true?

A. Aerobic metabolism is more efficient than anaerobic metabolism.

B. Aerobic metabolism breaks down carbohydrates without oxygen.

C. Aerobic metabolism produces lactic acid.

D. Aerobic metabolism happens for only brief periods.

75. The passage implies that during sprinting, _____.

A. there is a dangerous pressure on the heart B. the body’s reserve of carbohydrates increases

C. the body breaks down carbohydrates both with and without oxygen

D. the body needs less energy than usual

Section C

Directions: Read the following text and choose the most suitable heading from A-F for each paragraph. There is one extra heading which you do not need.