Part of a lecture about bioluminescence, the production of light by

Section 4. You will hear part of a lecture about bioluminescence, the production of light by living things in the ocean. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. MAN: Good afternoon. Today I'd like to introduce one of the most beautiful phenomena in the natural world, bioluminescence, the production of light by living organisms. Let me begin with what it actually is, and how it differs from the light we use every day. The light is created inside the organism's own body by a chemical reaction. Now, this is quite unlike an ordinary electric light bulb. A bulb wastes most of its energy as warmth, but a living light is astonishingly efficient. In fact, it is often described as 'cold' light, precisely because it gives off almost no heat at all; well over ninety per cent of the energy comes out as light rather than as warmth. MAN: Where do we find it? You might picture fireflies in a summer meadow, and indeed a few land creatures can glow. But these are very much the exception. Bioluminescence is overwhelmingly a feature of the ocean, and especially of its darkest depths. Scientists now estimate that the great majority of animals living in the deep sea are able to produce their own light, an extraordinary figure when you consider how little of the deep ocean we have actually explored. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. MAN: So how is the light made? People sometimes assume it must involve a gas such as nitrogen, or even phosphorus, but that is not the case. At the heart of the reaction is a light-producing molecule, known generally as luciferin. When luciferin is combined with oxygen, and helped along by a natural catalyst, energy is released, and that energy takes the form of light. In the sea this light is nearly always blue, and there is a good reason for that: blue light travels furthest through water, whereas red light is absorbed within a few metres. MAN: Now, why do animals go to all this trouble? There are three main functions, and it is worth being careful here, because the same trick is used for opposite purposes. The first is hunting. The deep-sea anglerfish, for example, dangles a glowing lure in front of its mouth; smaller creatures are drawn towards the light, and the anglerfish simply waits to catch its prey. The second function is the reverse, defence. When it is attacked, a certain squid does not flee into the dark; instead it ejects a bright cloud of light, rather like a smokescreen, to dazzle and confuse a predator while it escapes. And the third use is communication; in particular, many species flash distinctive patterns of light in order to attract a mate during the breeding season, each species with its own signal. MAN: Let me turn, finally, to how we humans are putting all this to use, because the applications are remarkable. The genes responsible for making light have become a powerful tool in medical research. Scientists attach them to particular cells as 'markers', so that the cells light up and their activity can be tracked, for instance in following the spread of a disease. There are environmental uses too. Some bioluminescent organisms dim or brighten in the presence of certain toxins, which means they can serve as living sensors, giving us early warning of water pollution before our own instruments would detect it. And looking further ahead, some researchers have a genuinely striking ambition: by transferring these same light-producing genes into plants, they hope one day to grow softly glowing trees that could be planted along our streets and gradually replace electric street lights altogether. Whether or not that comes to pass, it captures the imagination. We'll stop there for today. That is the end of Section 4. You now have some time to check your answers.