3.1 Introduction Just how readily available are uranium resources, and do their distribution and cost impose restrictions on nuclear power generation? Compared to a coal-fired power station a nuclear power station requires far less fuel in terms of mass. You have seen that a 1 GW burner reactor requires 5000 t of natural uranium over 30 years, whereas a comparable modern coal-fired power station needs 10 000 t of coal every day. However, uranium does not occur naturally in metallic form, nor in the concentrat
4.5 Global distribution of coal Figure 35 shows the global distribution of coal deposits. The major areas are principally in the Northern Hemisphere; with the exception of Australia, the southern continents are relatively deficient in coal deposits. 4.3 The UK's coal reserves Production of large quantities of coal in the UK during the 19th and 20th centuries led to the progressive depletion of reserves. In 2005 underground mining was limited to the Carboniferous coalfields of Yorkshire and the East Midlands, with only one underground mine operating in South Wales. However, surface mining sites still work coal in most of the coalfields (Author(s): 1.7 How old is coal? Not surprisingly, the distribution of coal deposits through time corresponds closely to the origin and distribution of land plants. (This is discussed further in Section 4.) Coals are commonly found in rocks from Carboniferous times onwards, Devonian coals are rare, and pre-Silurian true coals are never found. This coincides with evidence for the evolution of land plants, which first appeared in Silurian times about 400 Ma (million years) ago, colonized the land surface rapidly through the De Introduction There are many environmental reasons why coal is a rather undesirable source of energy. Burning it introduces large amounts of gases into the atmosphere that harm the enviironment in a variety of ways, as well as other, sollid waste products. Coal extraction leads to spoil heaps and mines that scar the landscape, land subsidence that affects roads and buildings, and in some cases water pollution. With apparently so little going for it, why do we rely so much on coal to meet our energy n 7.5 Summary of Section 7 Site-directed mutagenesis is an important technique for studying protein function. Using recombinant DNA technology, selected amino acids can be substituted with different residues to alter the structure and function of a protein. One widely used method employs primer extension. SDM studies can help identify residues that are critical for interactions or catalytic activity. Protein–protein interactions can be studied in a number of different 7.3.1 Library-based methods for demonstrating an interaction between proteins As well as the biochemical approaches to studying protein–protein interactions, there is a variety of qualitative methods for screening ‘libraries’ of cloned genes or gene fragments whose protein products might interact with a protein of interest. Such an approach has the advantage that the genes that encode those proteins that bind are available immediately for expression, facilitating subsequent analysis of the protein. The two-hybrid system uses transcriptional activity 6.3 Lysozyme Lysozyme was the first enzyme for which the X-ray structure was determined at high resolution. This was achieved in 1965 by David Phillips, working at the Royal Institution in London. Phillips went on to propose a mechanism for lysozyme action that was based principally on structural data. The Phillips mechanism has since been borne out by experimental evidence, as we shall see later. Lysozyme is found widely in the cells and secretions (including tears and saliva) of vertebrates, and h 1.4.4 Covalent cross-linkages stabilise protein structure Proteins that are secreted by the cell, or are attached to the extracellular surface of the plasma membrane, can be subject to more extreme conditions than those experienced by intracellular proteins. Often, covalent cross-linkages stabilise these proteins by connecting specific amino acids within a polypeptide or between polypeptide chains in multisubunit proteins (see below). Typically such a linkage will be a covalent sulfur–sulfur bond which forms between the –SH groups of two cystein 1.2 The peptide bond and primary structure of proteins The primary structure of a protein is defined as the sequence of amino acids of which it is composed. This sequence ultimately determines the shape that the protein adopts, according to the spatial limitations on the arrangement of the atoms in the protein, the chemical properties of the component amino acid residues, and the protein's environment. The peptide bonds that link amino acid residues in a polypeptide are formed in a condensation reaction between the acidic carboxyl gr 3.6.2 The JAK–STAT pathway Another important protein kinase pathway is the JAK–STAT pathway. Cytokines (Section 2.2), are frequently used for signalling between cells of the immune system. Cytokine-induced signal transduction cascades are often direct pathways to the nucleus for switching on sets of genes. Janus kinases (JAKs, named after the two-faced Roman god) are a particular 3.6 Protein kinases Protein kinases phosphorylate proteins either at tyrosine residues (tyrosine kinases), or at serine and threonine residues (serine–threonine kinases), or on any of these three amino acids (dual-specificity kinases). All these activities are employed in signal transduction pathways (histidine kinases also operate in certain plant and bacterial pathways, but not in animals). You should now be familiar with receptor tyrosine kinases, and have seen in some detail how phosph 3.3.2 Phospholipase C (PLC) Members of this family of enzymes contain two catalytic domains and several protein binding domains (Figure 13). The PH domain can temporarily tether phospholipase C to the membrane by attachment mainly to PI(3,4)P2. We shall discuss two main isoforms of PLC: PLC-β, which is activated by a subset of trimeric G proteins (Gαq an 2.6 Summary Receptors comprise a limited number of structural motifs, which determine binding affinity and specificity of receptor–ligand complexes. Some ligands bind to several receptors and some receptors bind to several ligands. Acetylcholine is a good example of a ligand with two structurally different kinds of receptor. Nicotinic receptors are ion channels, which are found predominantly in skeletal muscle, and are stimulated by nicotine. Nicotinic 2.1 Introduction Every receptor has to be able to recognize its particular ligand in a specific manner, and become activated by it in such a way that it transmits the signal to the cell. We shall deal with receptor specificity and activation mechanisms. Then we shall see how the same principles of specificity and activation also apply to intracellular receptors. 1.6 Signalling proteins can act as molecular switches How does a signalling molecule actually convey a signal? With second messengers, it is easy to understand: they are produced or released in large quantities, diffuse to their target, to which they usually bind, bringing about a functional change, after which they are degraded or stored within a subcellular compartment (such as endoplasmic reticulum). With signalling proteins it is less obvious. Protein concentrations cannot fluctuate rapidly, and protein molecules cannot easily move within th Exploring iSpot As well as posting your own observations, it's good to see what other people are observing, and to help them with identifications too. The latest observations are always displayed on the home page, or you can see them listed with more details, or you can see them as a Author(s): 8.2 Extended radio sources In section 7.5 we studied the spectrum of the synchrotron emission, i.e. how the flux density of radiation depends on the frequency or wavelength of the radio emission. Using radio telescopes such as the VLA (Author(s): 7.4 Faraday depolarization Radiation of wavelength λ which starts off linearly polarized in a particular direction and travels through a plasma has its direction of polarization rotated by an angle where ne is the electron density, B| | is 7.2 Free-free radiation The blackbody spectrum is emitted when thermally emitting matter is optically thick. Optically thin matter can also emit thermal radiation. Whenever a charged particle is accelerated it emits electromagnetic radiation. When the acceleration is due to the electric field of another charged particle the emitted radiation is called free-free emission or bremsstrahlung. (Bremsstrahlung is a German word meaning ‘braking radiation’.) The radiation emitted by an optically thin, ther
Author(s):
