1.1 Early observations Some of the earliest observations of macromolecules within living cells were of nucleic acids in the form of chromosomes. These long dark-staining objects, which became visible in the nucleus of cells at specific stages of cell division, were large enough to be detected using primitive light microscopes. Giant polytene chromosomes, found in certain cells such as the salivary gland cells of Drosophila (see Figure 1a), contain many thousands of copies of each chromosomal DNA align
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2.8.1 Adipose tissue In this unit, there have been several references to adipose tissue. You probably have a sense of a rather inert (and, perhaps, hard to shift!) tissue acting as a long-term energy store, but this is only a small part of the story. Adipose tissue consists of individual fat cells (adipocytes (Figure 12)) togeth
2.6 Obesity and ageing So, it seems that a part of the explanation for Ron's obesity, and the health problems that have led him to seek medical advice, may relate to the environment in which he is living, and more specifically the diet that he has chosen. It may also be a diet that is especially effective in activating the reward circuits in Ron's brain. However, Ron is also in his later middle age. A longitudinal study of people of this age in the USA suggests that average body weight increases by 1–2 kg per dec
1.5 Obesity At the time of writing (2004) 20% of the adult population of the UK is classified as obese. The number of obese children has doubled since 1982, 10% of six year olds and 17% of fifteen year olds are now classified as obese. As shown in Table 4, obesity is recognized when the BMI exceeds 30 and occurs quite simply wh
2.4 Summary of Section 2 The water cycle involves the movement of water, in all its forms, over, on and through the rocks near the surface of the Earth in a cycle. This cycle is driven by the Sun's energy and the Earth's gravity. The total volume of water in the cycle is virtually co
1 Energy use Until about 8000 years ago humans relied on hunting and gathering for food, and burning wood to keep warm. Their exact energy demands can at best only be estimated but to survive they probably needed about as much Author(s):
7.5 Outstanding issues Evidence from rotation studies shows that some AGNs do indeed contain compact, supermassive objects within them, though there is no direct evidence that these are black holes. Quasars were most abundant at redshifts of 2–3 and have been declining in number for the last 10 billion years. It seems probable that AGNs fade with time as the supply of accreting material is used up. There is speculation that AGNs may be rejuvenat
7.3 The central engine An object that fluctuates in brightness on a timescale Δt can have a radius no greater than R ∼ cΔt. The point-like nature of AGNs and their rapid variability imply that the emitting region is smaller than the size of the Solar System. The central engine of a typical AGN is believed to contain a supermassive black hole of mass ∼108M ⊙ and Schwarzschild radius
7.2 Types of active galaxy All active galaxies have a compact, energetic nucleus – an AGN. Seyfert galaxies are spiral galaxies with bright, point-like nuclei which vary in brightness. They show excesses at far infrared and other wavelengths, and have strong, broad emission lines. Quasars resemble very distant Seyfert galaxies with very luminous nuclei. They are variable. About 10% are strong radio sources thought to be powered by jets of material
Radio-quiet AGNs There has been a great deal of debate about whether there really are two different types of Seyfert or whether they can both be accounted for by the same model. For example, suppose that you look at the model AGN in Figure 36a from a direction not too far from the rotation axis of the torus. You will see
Narrow-line region The model places the narrow-line region much further out from the central engine where orbital speeds are lower; 200–900 km s−1 is typical for the NLR. An important consequence of the NLR being outside the dust torus is that it is always in view, so narrow lines will be seen even if the broad-line emitting gas is obscured. 5.3 The broad- and narrow-line regions In our model, the engine is surrounded by gas clouds (Figure 32). You have already seen how common these are in our own and other galaxies, so it is reasonable to expect them to be present in at least the spiral galaxies that contain AGNs. If these gas clouds are illuminated by ultraviolet or X-rays from the e 4.7 Jets You have seen that two kinds of active galaxies – quasars and radio galaxies – are often seen to possess narrow features called jets projecting up to several hundred kiloparsecs from their nuclei. If these are indeed streams of energetic particles flowing from the central engine, how do they fit with the accretion disc model? How could the jets be produced? The answers to these questions are not fully resolved, but there are some aspects of the model of the central engine which prob 4.2 The size of AGNs AGNs appear point-like on optical images. It is instructive to work out how small a region these imaging observations indicate. Optical observations from the Earth suffer from ‘seeing’, the blurring of the image by atmospheric turbulence. The result is that star-like images are generally smeared by about 0.5 arcsec or more. One can do much better with the Hubble Space Telescope where, thanks to the lack of atmosphere, resolved images can be as small as 0.05 arcsec. What does this mean in 3.5 Blazars Blazars appear star-like, as do quasars, but were only recognised as a distinct class of object in the 1970s. They are variable on timescales of days or less. All are strong and variable radio sources. There are two subclasses. BL Lac objects are characterised by spectra in which emission lines are either absent or extremely weak. They lie at relatively low redshifts. At first, they were mistaken for variable stars until their spectra were studied. (Their name derives from BL Lac Active galaxies
Figure 10 shows the spectral energy distribution of an active galaxy. 6.1 Introduction Reading 5 ends with a call for a move towards a more ‘deliberative democracy’ in which public engagement takes place in parallel with the development of new technologies, so that opportunities are provided for ongoing dialogue and influence between the two. To help to achieve this, the authors argue, ‘… now is the right time to start experi 4 The GM Nation? debate The UK's GM Nation? debate took place in the summer of 2003 – a time when both the political mood and scientific innovation had moved on a good deal in the eight years since the first UKNCC. Despite the relatively non-controversial introduction of the first GM products (notably a GM tomato paste and GM cheese) in the shops in the mid-1990s, public concerns grew in subsequent years. One particularly influential event was the arrival in the late 1990s of US and Canadian soya beans in Europe, 3.1 Introduction On an autumnal morning in November 1994, a group of people gathered at Regent's College, London, conscious that they were making history. The purpose of the meeting was to discuss an important issue: plant biotechnology and how it should be regulated in the UK. At the time, the genetic engineering of plants was emerging as a technology of great potential for the development of new pest-resistant, higher yield crops, although the technique hadn't attracted the degree of media attention and pub
