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Read the text and decide if the following statements are true (T) or false (F).
1) All ozone gas is contained in the stratospheric ozone and is spread thinly and unevenly.
2) Ultraviolet radiation from the sun causes oxygen molecules to combine with other oxygen molecules to form ozone.
3) UV rays are dangerous to human beings, animals and plants because they burn so we should avoid the sun at all.
4) Eye cataracts, the leading cause of blindness in the world, is the most dangerous disease because of the sun.
As the sun’s radiation approaches the planet’s surface it can be scattered, reflected, or absorbed, intercepted and re-emitted. This is where the ozone layer comes into its own by scattering and reflecting harmful high energy ultraviolet radiation. Variations in temperature and pressure divide the Earth’s atmosphere into layers and the mixing of gases between the layers happens very slowly. That is why this 90% of the ozone stays in the upper atmosphere. This stratospheric ozone contains 90% of all ozone gas on the Earth but it is spread thinly and unevenly.
Life on earth has been safeguarded because of a protective layer in the atmosphere. This layer, composed of ozone, acts as a shield to protect the earth against the harmful ultraviolet radiation from the sun. Ozone is a form of oxygen with three atoms (O3) instead of two (O2). Through natural atmospheric processes, ozone molecules are created and destroyed continuously. Ultraviolet radiation from the sun breaks up oxygen molecules into atoms which then combine with other oxygen molecules to form ozone. Ozone is not a stable gas and is particularly vulnerable to destruction by natural compounds containing hydrogen, nitrogen and chlorine. Near the earth’s surface (the troposphere) ozone is an increasingly troublesome pollutant, a constituent of photochemical smog and acid rain. But safely up in the stratosphere, 11 to 48 km above the earth’s surface, the blue, pungent-smelling gas is as important to life as oxygen itself. Ozone forms a fragile shield, curiously insubstantial but remarkably effective.
This ozone filter efficiently screens out almost all the harmful ultraviolet rays of the sun; the ozone layer absorbs most of the dangerous UV-B radiation (radiation between UV-A which is allowed through and UV-C which is mainly captured by oxygen, as indicated in Figure 1.2). Any damage that is done to the ozone layer will lead to increased UV-B radiation. Increases of UV-B radiation have been clearly observed in areas experiencing periods of intense ozone depletion. Any increased UV-B that reaches the earth’s surface has a potential to cause considerable harm to the environment and to life on earth. A small decrease in the ozone layer could significantly increase the incidence of skin cancer, and could lead to an acceleration of the rarer but more virulent form of cancer known as coetaneous malignant melanoma. Increased UV-B could lead to increased incidents of eye damage, including cataracts, deformation of the eye lenses, and presbyopia. Eye cataracts, the leading cause of blindness in the world, are expected to increase considerably.
UV rays are dangerous to human beings, animals and plants because they burn. They can penetrate our skin and eyes and weaken our bodies’ immune system. That is why we should avoid long periods in the sun. To get the minimum daily dose of vitamin D only 15 minutes in the sun per day is enough. If we stay more than that, we might get sunburnt if no protection is used. Repeated sunburns and frequent tanning can cause premature ageing of the skin and, at worst, skin cancer such as melanoma (because of UV-A and UV-B). For the eyes the UV-B rays can cause a cataract (clouding of the eye lens). Most of the serious health problems appear only many years later.
ANTI-SOCIAL AMINO ACIDS GANG UP
Read the text and write an abstract of the text in 3-5 sentences.
Previously thought to be purely the preserve of proteins and peptides, scientists have discovered that the amino acid phenylalanine can form the toxic amyloid fibrils that are a hallmark of diseases such as Alzheimer’s and Parkinson’s.
‘This is the first time ever that it has been demonstrated that an amino acid, rather than a peptide or a protein, can form such a structure,’ lead researcher Ehud Gazit at Tel Aviv University in Israel tells Chemistry World. As well as throwing new light on the behaviour of amino acids, this work could lead to new treatments for the genetic disorder phenylketonuria (PKU), which is caused by high levels of phenylalanine.
PKU sufferers lack the enzyme phenylalanine hydroxylase, which breaks down phenylalanine. As a result, high levels of phenylalanine quickly build up in the blood, cerebrospinal fluid and brain, leading to seizures, organ damage and unusual posture. The disorder is particularly dangerous for children, because it retards brain development and can cause serious learning difficulties.
Fortunately, as we derive most of our phenylalanine from food and drink, PKU sufferers can control phenylalanine levels through their diet. This is why certain food products have the phrase ‘Contains a source of phenylalanine’ on their packaging; this is the case with many diet soft drinks, because phenylalanine is a component of the artificial sweetener aspartame.
Still, this is a fairly crude, and easily misjudged, way to control PKU, such that many teenage and adult sufferers have phenylalanine levels that are higher than would be ideal. A more effective method to control or even cure PKU is desperately needed. Unfortunately, the development of new treatments is hampered by the fact that scientists have struggled to elucidate the precise mechanism by which high phenylalanine levels result in organ and tissue damage.
Gazit suspected that the damage may be caused by phenylalanine forming amyloid fibrils. Most famously associated with Alzheimer’s disease, these ordered agglomerations of inappropriately folded proteins and peptides have been found to play a part in a wide variety of different diseases, including Parkinson’s disease, diabetes, atherosclerosis and rheumatoid arthritis.
Gazit’s suspicions were first raised by the fact that phenylalanine is an aromatic amino acid. He had previously shown that short aromatic peptide fragments can clump together to form amyloid-like fibrils and that a peptide consisting of two phenylalanine molecules can form nano-assemblies with amyloid-like structural properties.
So, together with colleagues, Gazit decided to investigate whether individual phenylalanine molecules could form amyloid-like fibrils, even though no amino acid was known to do this. His group used transmission electron microscopy and scanning electron microscopy to study high concentrations of phenylalanine in solution and found that phenylalanine did indeed spontaneously form amyloid-like fibrils. Exactly the same thing happened when he studied high concentrations in human serum.
Next, the team showed that these phenylalanine fibrils were toxic to cell lines, causing them to change shape and die off. When they then injected the fibrils into rabbits, he found that they naturally generated antibodies against the fibrils. By attaching fluorescent compounds to these antibodies, the group were able to image the phenylalanine fibrils.
Finally, using these fluorescent antibody probes, together with a dye known as Congo red that is regularly used to stain amyloid fibrils, the team searched for phenylalanine fibrils in biological samples. These samples came from brain tissue from a mouse genetically engineered to have a PKU-like condition and from human PKU sufferers. In both cases, Gazit's team unambiguously detected phenylalanine fibrils. This provides convincing evidence that phenylalanine can form amyloid-like fibrils and that these fibrils are responsible for the tissue damage seen in PKU patients.
Последнее изменение этой страницы: 2016-04-18; Нарушение авторского права страницы
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