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READING PASSAGE 3

The geography of taste buds

Scientific researchers mapping the geography of the human tongue have finally succeeded in locating and identifying the sensors for sour tastes

Scientists have long known that the surface of the tongue is covered with taste buds: onion- shaped bundles of cells that stand guard for the digestive system. Long before our modern world of supermarkets and restaurants, taste buds served as a crucial tool to maintain the daily existence of our ancestors. The bitter flavour of a wild almond told a prehistoric human foraging in African forests that the nut was poisonous, and they should spit it out. A sweet fruit, though, was safe to consume, and full of energy from carbohydrates. The presence of lions and other man-eating predators also added urgency to meal choices. 'If the thing didn't taste good, you might decide it's not worth it to be out in the open and at risk,' says Steve Miller, a taste researcher at the University of Maryland School of Medicine.>> Form đăng kí giải đề thi thật IELTS 4 kĩ năng kèm bài giải bộ đề 100 đề PART 2 IELTS SPEAKING quý đang thi (update hàng tuần) từ IELTS TUTOR

Much of what we 'taste' in everyday life is actually the work of our nose. The tongue can distinguish only five basic flavour sensations - sweet, sour, bitter, salt and umami, the 'savoury' taste of meat and cheese which indicates protein. Sour taste comes from acidic compounds, and, like bitter taste, it can be a sign to leave decomposing meat or vegetables. Unpleasant tartness in unripe fruits comes from too much citric acid. This is how a tree or shrub keeps hungry diners at bay until its seeds are mature enough to travel - via the diner's digestive system - to a new home.

Unlike animals, humans have acquired a taste for tartness. In the case of the Japanese pickle called 'umeboshi', the plums are harvested when still green and sour, before the tree fills them with sugar and turns them into a tantalising snack for birds and animals. Sour cream, yoghurt, lemonade, gorgonzola cheese, the German fermented cabbage 'sauerkraut' and other tart treats are eaten every day by millions of people.

Only a decade ago, popular myth held that separate areas of the tongue were responsible for each taste. Referred to as the 'mouth map', the concept probably originated with a German text mistranslated into English at the beginning of the 20th century. As recently as 1996, 'mouth map' diagrams still appeared in university neuroscience textbooks.

By the late 1990s, however, scientists had learned enough about molecular biology to turn their attention seriously to taste. They had discovered that thousands of taste buds cover the tongue, each containing 50 to 100 taste cells. Each cell has two poles: one end covered with taste receptors projecting from the tongue's surface, and one end inside the tongue that connects to the brain via nerves. When a person bites into a lemon, acidic molecules from the food bump into acid-sensitive taste receptors, activating a taste cell, which sends a 'sour' message to the brain.

The details of this process were long a matter of debate. In line with the 'mouth map' concept, some scientists believed that the taste buds in different areas of the tongue were filled with cells of only one type: the tip with sweet-sensing cells, the back with bitter-sensing cells, and so on. Others were not convinced and further suggested the location of cells was not an issue because each cell could sense all five tastes. However, no one had identified any of the taste receptors, so which cells responded to which tastes was unknown. The recent discovery of the sour-taste receptor is a major step forward. Researchers found that the receptor is a specialized protein channel in the membrane of the taste cell. When acid from a food, such as the citric acid in a lemon, enters the pore of this channel, it triggers a change in the cell's electrical charge, ultimately leading to a signal being sent to the brain.

Questions 27-40

Questions 27-31
Do the following statements agree with the information given in Reading Passage 3?

*In boxes 27-31 on your answer sheet, write*

TRUE - if the statement agrees with the information
FALSE - if the statement contradicts the information
NOT GIVEN - if there is no information on this

1. Taste buds were more critical for survival in the past than they are today.

2. The primary role of a sour taste was to identify foods rich in protein.

3. Plants produce sour-tasting acid in unripe fruit to protect their seeds.

4. Animals, unlike humans, generally enjoy the taste of sour cream and yoghurt.

5. The 'mouth map' theory was based on accurate scientific research.

Questions 32-36
Complete the summary below.

Choose NO MORE THAN TWO WORDS from the passage for each answer.

*Write your answers in boxes 32-36 on your answer sheet.*

The process of tasting begins when molecules from food interact with 32 .................... on the tongue. Each taste cell has a specific structure: one end has these receptors, while the other end connects to the 33 .................... . For a sour taste, 34 .................... molecules from food activate the relevant receptors. This activation causes the taste cell to send a message to the brain. For a long time, there were two main theories about how taste buds worked. One theory supported the 'mouth map' idea, suggesting that each 35 .................... of the tongue had cells for only one taste. The opposing theory proposed that the 36 .................... of the cells was unimportant because every single cell could detect all five basic tastes.

Questions 37-40
*Reading Passage 3 has eight paragraphs, A-H.*

Which paragraph contains the following information?

*Write the correct letter, A-H, in boxes 37-40 on your answer sheet.*

1. A list of everyday sour foods consumed by people.

2. The reason why the 'mouth map' continued to be taught for a long time.

3. A reference to the constant danger our ancestors faced while eating.

4. The recent scientific breakthrough that identified the specific mechanism for sensing sourness.

Answers:

1. TRUE

2. FALSE

3. TRUE

4. FALSE

5. FALSE

6. taste receptors

7. brain

8. acidic

9. area

10. location

11. D

12. E

13. B

14. H

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