The internal body clock
Read The internal body clock and check the IELTS Reading answers. From buffalo to bacteria, oaks to algae, all life follows the same relentless 24-hour cycle, which is driven by the rising and setting of the Sun. Or is it? Over 250 years ago, a French scientist performed a simple experiment that bl…
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From buffalo to bacteria, oaks to algae, all life follows the same relentless 24-hour cycle, which is driven by the rising and setting of the Sun. Or is it? Over 250 years ago, a French scientist performed a simple experiment that blew apart the idea that all life on Earth does the bidding of the Sun. Intrigued by the way some flowers open and close their leaves each day, Jean-Jacques de Mairan put a heliotrope in a darkened room and observed the effect. He was expecting the plant, robbed of sunlight, to cease its daily routine. To his astonishment, its leaves continued to open and close as if in response to some invisible timekeeper.
Vocabulary from this passage
10 words worth learning before you sit the test, each with the sentence it appears in.
- chronobiology noun
The branch of biology concerned with natural physiological rhythms and other phenomena related to the passage of time
Chronobiology is at the forefront of understanding how internal clocks achieve such astonishing regularity in organisms.
- circadian cycle noun
The natural internal process that regulates the sleep-wake cycle and repeats roughly every twenty-four hours
Research found that SCN cells kept up a circadian cycle even without any help from daylight.
- comprehensive adjective
Complete and including everything that is necessary or relevant; dealing with all or nearly all aspects
Dr. Hines published the most comprehensive study reviewing over 130 investigations into the claims of biorhythms.
- culminate verb
To reach a climax or point of highest development; to result in a final, significant outcome
Investigation into how the CLOCK gene works could culminate in better ways of coping with jet-lag.
- lockstep adjective
A standard procedure or set of steps that is followed rigidly or in close synchronization with others
Sunlight might be useful in keeping the myriad cellular clocks in lockstep with the environment.
- manifestation noun
An event, action, or object that clearly shows or embodies something, especially a theory or an abstract concept
Books emerged claiming that certain physical traits were manifestations of so-called internal biorhythms.
- pseudoscience noun
A collection of beliefs or practices mistakenly regarded as being based on scientific method but lacking factual evidence
Scientists have since dismissed biorhythms as pseudoscience, insisting the cycles have no basis in fact.
- sub-optimal adjective
Being below the highest quality or level of performance that is possible in a given situation
Critical days in the biorhythm theory were supposed to result in sub-optimal performance for the individual.
- synchronization noun
The operation or activity of two or more things at the same time or rate; keeping in step
Melatonin helps to keep body functions in synchronization with the actual time of day.
- therapeutics noun
The branch of medicine concerned with the treatment of disease and the action of remedial agents
We will need new strategies and therapeutics to reset the circadian clocks found throughout our bodies.
Answers with explanations
- 14. F
Paragraph F describes the 1971 finding with **fruit flies** (insects) having internal clocks. It then provides **support** for this by detailing **further evidence** from 1995 and 1997 involving mammals, confirming the cellular clock concept across species. Answer: F.
- 15. D
Paragraph D discusses Hines's review of biorhythm research. It provides specific **criticism** of how the **data was analysed**, citing **faulty mathematics** and **statistical errors** as reasons the studies were unreliable. Answer: D.
- 16. A
Paragraph A introduces de Mairan's **basic test** using a **heliotrope** (a type of plant). By putting it in a dark room, he proved its cycle was independent of the sun, which is the foundational experiment described. Answer: A.
- 17. E
The question describes a statement that the **length of natural cycles** varies **slightly** between living things. Scan for cycle lengths in numbers, which leads to the paragraph on 24-hour rhythms. It says organisms settle into **free-running cycles** that are close to, but **rarely exactly**, 24 hours, and gives humans a typical figure of around 24.5 hours. Close to but not exactly 24 hours matches varying slightly, so the answer is the paragraph on circadian cycles.
- 18. C
Paragraph C mentions **best-selling books** in the 1970s, which indicates a **theory** (biorhythms) that **became popular with the public**. The paragraph outlines the 23, 28, and 33-day cycles claimed by this theory. Answer: C.
- 19. daylight
The blank requires a **noun** representing what the SCN perceives via the eye. The text explicitly says the SCN **senses daylight** through **photosensitive cells** in the eye. Grammatically, 'daylight' functions as the object of perception. Answer: DAYLIGHT.
- 20. fruit flies
The blank requires identifying the **subject of research** in 1971. The passage identifies these subjects as **fruit flies**. The plural noun 'fruit flies' fits the syntactic requirement for the creatures involved in the gene discovery. Answer: FRUIT FLIES.
- 21. genes
The blank requires a **noun** for the specific biological components found in cells. The text states they found **genes** affecting rhythms. 'Genes' functions as the plural noun identifying the internal 'clocks' mentioned in the summary context. Answer: GENES.
- 22. circadian cycle
The blank requires a **noun phrase** for the cycle maintained by SCN cells. The text states the cells **kept up a circadian cycle**. This phrase describes the roughly 24-hour routine mentioned in the previous sentence of the summary. Answer: CIRCADIAN CYCLE.
- 23-24: BC
23: This is one of two answers in a pair, and either order is accepted: the correct pair is the failure of night workers and the road casualties. The option about **night-shifts** matches the passage's reference to people who **work at night** and are left **unable to think or act**, which paraphrases a failure to perform tasks. Windows and natural light are never discussed, and mood swings linked to the Sun are not mentioned, so those are unsupported distractors.
24: This is the second of two answers in a pair, in either order. The option about **casualties on the roads** at **specific times** paraphrases the statement that **traffic accident statistics** show **two deadly peaks**, at 4 a.m. and twelve hours later. These peaks occur when the internal clock makes people least alert. The option about declining efficiency on certain days relates to biorhythms, which the passage dismisses as pseudoscience, so it is not stated as linked to the clock.
- 25-26: CD
25: In Paragraph G, the author lists **unsolved issues**, including the **reason for... a link with sunlight**. The text explicitly states 'no-one knows' why organisms bothered to acquire this connection, matching the logic of option C. Answer: C.
26: This is part of a pair, and either order is accepted: the unsolved issues are why organisms link to sunlight and how every clock stays in step. The passage asks how all the **biochemical clocks** inside cells **stay in synchronization**, which matches the option about clocks staying in step with one another. It also asks why organisms acquired a **link with sunlight**. The signal **so far, no-one knows** confirms both remain unsolved. The cycle length and clock locations are already addressed in the text.