how does the biological clock tick

A - Limitations of life span

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Our life span is restricted. Everyone accepts this as ‘biologically’ obvious. ‘Nothing lives for ever!’ However, in this statement we think of artificially produced, technical objects, products which are subjected to lớn natural wear and tear during use. This leads to lớn the result that at some time or other the object stops working and is unusable (‘death’ in the biological sense). But are the wear and tear and loss of function of technical objects and the death of living organisms really similar or comparable?

B - Fundamental differences in ageing of objects and organisms

Our ‘dead’ products are ‘static’, closed systems. It is always the basic material which constitutes the object and which, in the natural course of things, is worn down and becomes ‘older’. Ageing in this case must occur according to lớn the laws of physical chemistry and of thermodynamics. Although the same law holds for a living organism, the result of this law is not inexorable in the same way. At least as long as a biological system has the ability to lớn renew itself it could actually become older without ageing; an organism is an open, dynamic system through which new material continuously flows. Destruction of old material and formation of new material are thus in permanent dynamic equilibrium. The material of which the organism is formed changes continuously. Thus our bodies continuously exchange old substance for new, just lượt thích a spring which more or less maintains its sườn and movement, but in which the water molecules are always different.

C - Why dying is beneficial

Thus ageing and death should not be seen as inevitable, particularly as the organism possesses many mechanisms for repair. It is not, in principle, necessary for a biological system to lớn age and die. Nevertheless, a restricted life span, ageing, and then death are basic characteristics of life. The reason for this is easy to lớn recognise: in nature, the existent organisms either adapt or are regularly replaced by new types. Because of changes in the genetic material (mutations) these have new characteristics and in the course of their individual lives they are tested for optimal or better adaptation to lớn the environmental conditions. Immortality would disturb this system - it needs room for new and better life. This is the basic problem of evolution.

D - A stable life span despite improvements

Every organism has a life span which is highly characteristic. There are striking differences in life span between different species, but within one species the parameter is relatively constant. For example, the average duration of human life has hardly changed in thousands of years. Although more and more people attain an advanced age as a result of developments in medical care and better nutrition, the characteristic upper limit for most remains 80 years. A further argument against the simple wear and tear theory is the observation that the time within which organisms age lies between a few days (even a few hours for unicellular organisms) and several thousand years, as with mammoth trees.

E - The biological clock

If a life span is a genetically determined biological characteristic, it is logically necessary to lớn propose the existence of an internal clock, which in some way measures and controls the ageing process and which finally determines death as the last step in a fixed programme. Like the life span, the metabolic rate has for different organisms a fixed mathematical relationship to lớn the toàn thân mass. In comparison to lớn the life span this relationship is ‘inverted’: the larger the organism the lower its metabolic rate. Again this relationship is valid not only for birds, but also, similarly on average within the systematic unit, for all other organisms (plants, animals, unicellular organisms).

F - Energy consumption

Animals which behave ‘frugally’ with energy become particularly old, for example, crocodiles and tortoises. Parrots and birds of prey are often held chained up. Thus they are not able to lớn ‘experience life’ and so sánh they attain a high life span in captivity. Animals which save energy by hibernation or lethargy (e.g. bats or hedgehogs) live much longer than thở those which are always active. The metabolic rate of mice can be reduced by a very low consumption of food (hunger diet). They then may live twice as long as their well fed comrades. Women become distinctly (about 10 per cent) older than thở men. If you examine the metabolic rates of the two sexes you establish that the higher male metabolic rate roughly accounts for the lower male life span. That means that they live life ‘energetically’ - more intensively, but not for as long.

G - Prolonging your life

It follows from the above that sparing use of energy reserves should tend to lớn extend life. Extreme high performance sports may lead to lớn optimal cardiovascular performance, but they quite certainly bởi not prolong life. Relaxation lowers metabolic rate, as does adequate sleep and in general an equable and balanced personality. Each of us can develop his or her own ‘energy saving programme’ with a little self-observation, critical self-control and, above all, logical consistency. Experience will show that to lớn live in this way not only increases the life span but is also very healthy. This final aspect should not be forgotten.

Questions 1-6

Reading Passage 3 has seven paragraphs, A-G.

Choose the correct heading for paragraphs B-G from the list of headings below.

Write the correct number, i-x, in boxes 1-6 on your answer sheet.

List of Headings

i    The biological clock

ii    Why dying is beneficial

iii    The ageing process of men and women

iv    Prolonging your life

v    Limitations of life span

vi    Modes of development of different species

vii    A stable life span despite improvements

viii    Energy consumption

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ix    Fundamental differences in ageing of objects and organisms

x    Repair of genetic material



Paragraph A


1    Paragraph B
Answer: ix

2   Paragraph C
Answer: ii

3   Paragraph D
Answer: vii

4    Paragraph E
Answer: i

5    Paragraph F
Answer: viii

6    Paragraph G
Answer: iv

Questions 7-10

Complete the notes below.

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

Write your answers in boxes 7-10 on your answer sheet.

•    Objects age in accordance with principles of 7 and of 8
Answer: physical chemistry, thermodynamics IN EITHER ORDER
Answer: physical chemistry, thermodynamics IN EITHER ORDER

•    Through mutations, organisms can 9 better to lớn the environment
Answer: adapt

•    10 would pose a serious problem for the theory of evolution
Answer: immortality

Questions 11-14

Do the following statements agree with the views of the writer in Reading Passage 3?

In boxes 11-14 on your answer sheet, write

YES    if the statement agrees with the views of the writer

NO    if the statement contradicts the views of the writer

NOT GIVEN if it is impossible to lớn say what the writer thinks about this

11    The wear and tear theory applies to lớn both artificial objects and biological systems.
Answer: NO

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12    In principle, it is possible for a biological system to lớn become older without ageing.
Answer: YES

13    Within seven years, about 90 per cent of a human toàn thân is replaced as new.

14    Conserving energy may help to lớn extend a human’s life.
Answer: YES