Technoquest

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Questions for this column can be submitted by e-mail to sci.net@campus.bt.com

Q What are non-milk fats, and are they as fattening as milk fats?

Any fats not obtained from cows - oils from nuts, etc - are non- milk fats. All fats give about 9 kilocalories per gram, so are roughly equally fattening. However, different types of milk have different fat content.

Q How many cells are there in the human body?

About 50 million million (5x1013). But although some cells, such as nerve cells, do not divide in the normal human adult, many others - such as blood cells - can and do divide in response to different challenges. Also, people vary in size and so must have different numbers of cells in their organs.

Q How was the Turin shroud shown to date from the Middle Ages, not from the time of Jesus?

By carbon dating. Carbon-14 is a radioactive isotope of carbon, produced when cosmic rays hit nitrogen-14 nuclei in the atmosphere. It has a half- life of 5,700 years. The proportion in the atmosphere stays constant. However, animals and plants, which while they are alive contain the same ratio of carbon-14 atoms to stable carbon-12 atoms as the atmosphere,stop exchanging atoms with their surroundings when they die. So the proportion of carbon-14 falls as the atoms decay. When it has halved, for example, the specimen is 5,700 years old.

This dating method can be used for any organic material, such as bone, wood or cloth. One problem is knowing the concentration of carbon-14 in ancient times; fortunately, wood can also be dated by tree-ring measurements. Modern techniques can measure the ratio of carbon-14 to carbon-12 using very small samples - small enough for the Pope to give his consent for one to be taken from the shroud. It was found in 1988 to be no more than 750 years old.

Q What makes us sleepy?

A tiny, pea-like blob at the base of the brain - the pineal gland - seems to be important. It produces a chemical, melatonin, particularly in late evening, and this makes us feel sleepy. Scientists are trying to find out more about such substances.

Q What is the recommended daily intake of salt for an adult?

Our bodies need 1 gram of sodium chloride (salt) a day to survive. The UK average intake for adults is 8 to 10 grams a day. For health reasons, it is recommended that this be reduced to 5 to 6 grams - equivalent to a slightly heaped teaspoonfuln

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