You should spend about 20 minutes on Questions 1-14, which are based on Reading Passage 1 below.
Mutual harm
A In forests and fields all over the world, plants are engaged in a deadly chemical war to suppress other plants and create conditions for their own success. But what if we could learn the secrets of these plants and use them for our own purposes? Would it be possible to use their strategies and weapons to help us improve agriculture by preventing weeds from germinating and encouraging growth in crops? This possibility is leading agricultural researchers to explore the effects plants have on other plants with the aim of applying their findings to farming.
B The phenomenon by which an organism produces one or more chemicals that influence the growth, survival and reproduction of other organisms is called allelopathy. These chemicals are a subset of chemicals produced by organisms called secondary metabolites. A plant's primary metabolites are associated with growth and development. Allelochemicals, however, are part of a plant's defence system and have a secondary function in the life of the organism. The term allelopathy comes from the Greek: allelo and pathy meaning 'mutual harm'. The term was first used by the Austrian scientist Hans Molisch in 1937, but people have been noting the negative effects that one plant can have on another for a long time. In 300 BC, the Greek philosopher Theophrastus noticed that pigweed had a negative effect on alfalfa plants. In China, around the first century AD, the author of Shennong Ben Cao Jing described 267 plants that have the ability to kill pests.
C Allelopathy can be observed in many aspects of plant ecology. It can affect where certain species of plants grow, the fertility of competitor plants, the natural change of plant communities over time, which plant species are able to dominate a particular area, and the diversity of plants in an area. Plants can release allelopathic chemicals in several ways: their roots can release chemicals directly into the soil, and their bark and leaves can release chemicals into the soil as they rot. Initially, scientists were interested in the negative effects of allelopathic chemicals. Observations of the phenomenon included poor growth of some forest trees, damage to crops, changes in vegetation patterns and, interestingly, the occurrence of weed-free areas. It was also realised that some species could have beneficial effects on agricultural crop plants and the possible application of allelopathy became the subject of research.
D Today research is focused on the effects of weeds on crops, the effects of crops on weeds, and how certain crops affect other crops. Agricultural scientists are exploring the use of allelochemicals to regulate growth and to act as natural herbicides, thereby promoting sustainable agriculture by using these natural chemicals as an alternative to man-made chemicals. For example, a small fast-growing tree found in Central America, sometimes called the 'miracle tree', contains a poison that slows the growth of other trees but does not affect its own seeds. Chemicals produced by this tree have been shown to improve the production of rice. Similarly, box elder – another tree – stimulates the growth of bluestem grass, which is a tall prairie grass found in the mid-western United States. Many weeds may use allelopathy to become ecologically successful; a study in China found that 25 out of 33 highly poisonous weeds had significant allelopathic properties.
E There may be at least three applications of allelopathy to agriculture. Firstly, the allelopathic properties of wild or cultivated plants may be bred into crop plants through genetic modification or traditional breeding methods to improve the release of desired allelochemicals and thus improve crop yield. Secondly, a plant with strong allelopathic properties could be used to control weeds by planting it in rotation with an agricultural crop and then leaving it to rot and become part of the soil in order to inhibit the growth of weeds. Finally, naturally occurring allelopathic chemicals could be used in combination with man-made chemicals. Boosting the efficiency of man-made herbicides could lead to a reduction in the amount of herbicides used in agriculture, which is better for the environment.
F Despite the promising uses of allelopathic chemicals, agricultural scientists are still cautious. Firstly, allelopathic chemicals may break down and disappear in the soil more easily than artificial chemicals. Secondly, allelopathic chemicals may be harmful to plants other than weeds. Thirdly, allelopathic chemicals could persist in the soil for a long time and may affect crops grown in the same field as the allelopathic plants at a later date. Because the effects of allelopathic chemicals are not yet fully known, agricultural scientists will need to continue to study the biological war between plants.
You should spend about 20 minutes on Questions 15-26, which are based on Reading Passage 2 below.
Ordinary treasures
When Andy Warhol, one of the twentieth century's most influential artists, died his four-floor house was so full of items that the only rooms you could walk through were the kitchen and the bedroom. It turned out that Warhol had compulsive hoarding disorder, which is defined as the excessive accumulation of objects and a refusal to throw them away. But Warhol's case is not uncommon; around five per cent of Americans – nearly 15 million people – suffer from compulsive hoarding disorder. This disorder interferes with daily activities such as sleeping and cooking, and in an extreme form it can harm one's health, be a fire risk and even lead to death. Although researchers suspect that the disorder is more widespread in the West, cases of hoarding have been recorded in almost every country.
Twenty years ago, compulsive hoarding disorder was a relatively unexplored psychological phenomenon, often treated as an aspect of obsessive compulsive disorder – the compulsion to repeat a certain action over and over. However, it is now recognised as a separate disorder. Scientists from many disciplines, including psychologists, neurologists and behavioural researchers are looking at gene sequences within hoarders' DNA and scanning their brains to try to understand their behaviour in the hope that they can be helped.
There are several theories for the behaviour. First of all, hoarding appears to run in families and may have genetic causes, with family members often having similar issues. In a study of 219 families, researchers at Johns Hopkins University found that families with two or more hoarding members showed a linkage between hoarding behaviour and chromosome 14 – one of the 23 pairs of chromosomes that make up human DNA. A second theory states that the instinct to hoard may be an evolutionary survival strategy: there are plenty of examples of hoarding in the animal kingdom. The Arctic gray jay hoards around 100,000 berries and insects so that it has enough food for the long winter months. Humans, however, are the only species that take the strategy to extremes, sometimes filling their homes with so many objects that they eventually become uninhabitable.
Recent psychological research, however, emphasises that hoarders do not just collect junk; nor are they lazy or disorganised, even if their homes are chaotic. Many hoarders have normal lives, with regular jobs and normal relationships with friends and family. Cognitively, hoarders tend to be emotional, attaching sentimental value to belongings that other people would discard. They also tend to be intelligent, well educated and more creative than average. However, they can be indecisive and may start several different projects at the same time.
Carol Mathews, a leading researcher into the condition, used magnetic resonance imaging (MRI) to show brain activity in the process of decision making. People with compulsive hoarding disorder display increased activity in an area of the brain related to decision making when asked to organise objects. This increased activity is due to their greater emotional attachment to possessions. In other tests, Mathews found that people with hoarding behaviour had difficulty grouping similar objects and remembering the sequence of things. In effect, people with compulsive hoarding disorder do not categorise objects in the same way as other people, and when they are asked to do so, show an increase in brain activity associated with the decision-making process. It seems that people with hoarding behaviour see and treat objects differently and might have a different appreciation of the physical world. For example, a pile of objects in the middle of a room may be seen as a work of art by a hoarder rather than just a heap of junk.
Treating hoarding effectively may depend upon whether we can identify specific character traits. Dr Monika Eckfield of the University of California, San Francisco, believes there are two different kinds of people with hoarding behaviour. She calls one kind impulsive-acquirers, who buy objects out of excitement and keep them because they are interested in them. The other type are the worried-keepers – the hoarders who acquire items passively and keep them in case they need them in future. Worried-keepers spend more time sorting and organising belongings. While both kinds of hoarders of either gender find it nearly impossible to throw anything away, more men than women belong to the former category whilst more women fall into the worried-keeper group.
You should spend about 20 minutes on Questions 27-40, which are based on Reading Passage 3 below.
Reading Passage 3
A A major cause of blindness in the industrialised world is age-related macular degeneration (AMD). It affects approximately three million people globally and accounts for around nine per cent of all blindness. These statistics are expected to double by the year 2020 as the world population increases. Scientists have been working on a new treatment for one type of the disease by using stem cells to repair damage to the retina, with positive results. Two women in America, both registered as blind, were given the new treatment and say their vision improved just weeks after they were injected with the stems cells. With such promising results for a condition which previously had no treatment, researchers are positive about the direction of the stem cell treatment.
B AMD usually affects the elderly although younger people can also develop a version of the condition. People with AMD typically have dark patches at the centre of their vision – the deterioration takes place over months and years. Although people with AMD do not lose their sight completely, they do lose central vision, which is vital for detailed work and activities like reading and driving, so that leading a normal life can become impossible over time. In the cases of the two American women, one was a graphic artist who began to lose her vision in her twenties. As she lost most of her central vision, she became unable to work, and then eventually it became impossible for her to recognise people or watch TV. The second woman became unable to recognise faces, had to stop driving and in the end could not leave her home.
C To understand AMD, we need to understand how our eyes work. Essentially a hollow ball, the eye has a number of layers. The outer layer consists of the white of the eye and the cornea. The cornea is the transparent area in front of the coloured iris and the black pupil at the centre of the iris. The middle layer of the eye includes blood vessels and the iris, which regulates the amount of light entering the eye. Just behind the iris is the lens, which focuses images on the retina, which covers the inside of the eyeball. The retina is the part of the eye that contains photoreceptors – cells that sense light. Nerve fibres from the photoreceptors in the retina join together to form the optic nerve, which then exits the eyeball and transmits visual information to the brain. The photoreceptors are of two types, rods and cones: the rods are sensitive to light intensity and the cones are sensitive to colour. They are mostly concentrated in the part of the retina called the macula. This is only the size of a grain of rice, but it is responsible for our central vision, most of our colour vision and our visual acuity, or sharpness of vision. The photoreceptor cells lie on a thin layer of cells that provide them with nutrients and carry away waste. When these underlying cells die or are damaged, the photoreceptor cells cannot function properly, and this leads to a loss of vision.
D Damage to the macula can result from a variety of factors. Age is the main risk factor but smoking also damages blood vessels and the structure of the eye. Smokers are three times more likely to develop AMD, as are people with poor diets. A diet lacking in fruit and vegetables cannot help the body defend itself against free radical molecules which damage cells. Fruit and vegetables contain antioxidants, which protect the body against these free radicals. Finally, people with high blood pressure are one and a half times more likely to contract AMD, as are those people with a family history of the disease.
E The new treatment for AMD, developed by Dr Robert Lanza at Advanced Cell Technology, involves changing embryonic stem cells into more specialised eye stem cells and injecting 50,000 of them into the layer of the eye that supports the photoreceptors. A stem cell is a primary cell that has the ability to divide and form specialised cells that perform various functions within the body. First, a single stem cell was taken from a human embryo and grown into a colony of millions of cells. The cells were checked to make sure they were healthy. Then another procedure encouraged the stem cells to develop into the type of cell that forms the layer under the photoreceptor cells. The eye stem cells were injected below the retina through a cut into the eyeball, where they filled in the gaps left by dead and damaged cells and began functioning again.
F For the two patients, the results have been stunning. Tests have indicated that healthy cells have grown where the stem cells were injected. The first woman has regained enough vision to be able to cycle. Meanwhile, the second woman can read, cook and go shopping by herself again. The hope is that the treatment could be a way forward for other currently incurable conditions so that other people can lead normal lives again.