Listen to part of a lecture in a biology class. Professor: Today we're going to discuss a fascinating adaptation in desert plants: the process of crassulacean acid metabolism, or CAM. Unlike most plants that open their stomata during the day to take in carbon dioxide for photosynthesis, CAM plants, such as cacti and succulents, open their stomata at night. Why? Because in the desert, daytime temperatures are extremely high, and opening stomata during the day would lead to rapid water loss through transpiration. So, at night, when it's cooler and more humid, CAM plants open their stomata and absorb carbon dioxide. They convert this carbon dioxide into an organic acid and store it in vacuoles. Then, during the day, with the stomata closed to conserve water, they break down this acid to release carbon dioxide for the Calvin cycle, which produces sugars. This allows them to photosynthesize without losing precious water. However, CAM photosynthesis is less efficient in terms of energy use, but the trade-off is survival in arid conditions.
❓ Câu hỏi According to the professor, why do CAM plants open their stomata at night?
A. To absorb more sunlight for photosynthesis
B. To minimize water loss during hot daytime hours
C. To release oxygen into the atmosphere
D. To store carbon dioxide in their roots
✅ Đáp án đúng
To minimize water loss during hot daytime hours
TOEFL Listening Tips
Skim the questions first to predict the content
Watch for signal words in lectures and conversations (however, actually, in fact)
For main idea questions, focus on the beginning; for detail questions, focus on examples and data
For attitude questions, pay attention to the speaker's tone and word choice
Take notes on keywords; don't try to write down every word
