1.5 The physics and chemistry of coal formation Coal is a type of sediment made up mainly of lithified plant remains. But how does spongy, rotting plant debris become a hard seam of coal? As discussed earlier, plant material growing in mires dies, and then rots under anoxic conditions to form peat (by the process of humification). With time, the mire becomes covered with layers of sediment, the weight of which squeezes water and gas out of the pore spaces and compacts the vegetation. As subsidence allows deposition of further mireâ€
1.3.2 Peat formation in raised mires Mires can also form inland within low-lying depressions, provided the rate of precipitation exceeds the rate of evaporation (Figure 4a). Peat is impermeable and so its accumulation progressively impedes drainage. This attribute gives mires the ability to maintain a water table independent of the area surrounding them.
2 Odd one out The image below shows models of four mammals: Rhinoceros Whale Elephant Hippopotamus 7.3 Regulation of secretion Up to this point we have made a clear distinction between constitutive secretion and regulated secretion. In reality however the border is a bit more blurred. For example, many molecules are constitutively expressed on the surface of a cell, but their expression is increased in response to a particular stimulus. In other words, surface expression is determined by both constitutive and regulated secretion. Constitutive secretion is regulated primarily at the level of protein synthesis, whereas 6.2 Endocytosis Fluid-phase uptake by pinocytosis can be broadly categorised according to the size of the endocytic vesicle and this also relates to how the vesicle is coated (Figure 35). The rate of internalisation is directly proportional to (i) the concentration of extracellular molecules, (ii) the volume enclosed by the vesicle and (iii) the ra 4.5 Summary Targeting sequences at the N-terminus of proteins direct translation across the ER, and act as signals for import to the nucleus, mitochondrion and chloroplasts. Sequences at the C-terminus control traffic through the ER and the Golgi and to peroxisomes. Glycosylation is directed by signal sequences that act as targets for N-linked glycosylation in the ER and O-linked glycosylation in the Golgi apparatus. Glycosylation and remodelling of polys 3.7 Summary The formation of transport vesicles is initiated by small G proteins that insert into the donor membrane and assemble coat proteins. The coat proteins are COPI, COPII or clathrin, depending on the pathway, and the coat includes adaptor proteins that link the coat to the vesicle and its cargo. Epsins and dynamin are involved in the budding process. The vesicle cargo depends on the adaptor proteins, sorting proteins and receptors that are assemb 2.7 Summary Eukaryotic cells contain numerous distinct types of membrane-bound compartment. Transport vesicles move proteins and other molecules between the compartments. Proteins contain signalling sequences or patches that specify their destination compartment. Proteins destined for lysosomes, secretion or the plasma membrane are synthesised in the ER, transported to the cis Golgi, modified in the Golgi apparatus, and sorted and pa References 4.1 Glucose metabolism We are now in a position to draw together the major concepts and components of signalling, and show how they operate in one well-understood system, namely the regulation of the storage or release of glucose in the human body. From this, you will be able to recognize archetypal pathways represented in specific examples, you will be able to appreciate how the same basic pathways can be stimulated by different hormones in different tissues, and you will see how opposing hormones activate separat 3.5 Monomeric G proteins We shall discuss monomeric G proteins (also called small G proteins or small GTPases) separately from the trimeric G proteins for three reasons: their upstream activators are different, they tend to have different target proteins, and they commonly operate within different signalling pathways. What structural features and activities do monomeri Introduction Even the simplest organisms can detect and respond to events in their ever-changing environment. Similarly, within a multicellular organism, cells are surrounded by an extracellular environment from which signals are received and responded to. Extracellular events are decoded and transmitted to relevant parts of individual cells by way of a series of activation/deactivation steps involving many intracellular molecules. This relay of information along molecular pathways is called signal tra 7.1 Introduction You will already be aware of some of the many experimental techniques employed to study protein structure, including X-ray diffraction, CD, NMR and SDS–PAGE. There are also many techniques that have been developed to study protein function, of which several are described in this section. 5.3.2 Cooperative binding A feature of some proteins comprising more than one subunit is that binding of a ligand to its binding site on one subunit, can increase the affinity of a neighbouring subunit for the same ligand, and hence enhance binding. The ligand-binding event on the first subunit is communicated, via conformational change, to the neighbouring subunit. This type of allosteric regulation is called cooperative binding. Haemoglobin, as we have already discussed, is a tetramer consisting of two 5.2 All proteins bind other molecules All proteins bind to other molecules (generically termed ligands). Ligands that can bind to proteins include: ions, e.g. Ca2+; small molecules, e.g. H2O, O2 and CO2, glucose, ATP, GTP, NAD; macromolecules, i.e. proteins, lipids, polysaccharides, nucleic acids. These interactions are specific and key to the protein's function and, of course, are critically d 4.1 Introduction The availability of genomic sequence data from every major taxonomic group of organisms on Earth has allowed extensive comparisons to be made between their protein-coding regions, with over 800 000 protein sequences from these organisms being available for comparison in 2003. From these comparisons, it has become apparent that there is extensive homology between the amino acid sequences of many proteins, even between apparently distantly related organisms. In some proteins, this homology exte 3.1 Introduction That proteins contain functionally and physically discrete modules or domains is an important principle, one that will be reinforced as we examine the roles of specific proteins in a variety of different cellular processes. There are several advantages conferred by multidomain protein architecture: Creation of catalytic or substrate-binding sites These sites are often formed at the interface between two domains, typically a cleft. Movement 1.6 Fibrous proteins Most of the proteins described so far have been globular proteins. There are, however, some distinctive features that characterise fibrous proteins and we present here a general overview of these. Elongated fibrous proteins frequently play a structural role in the cell. They do not readily crystallise but tend to aggregate along their long axis to form fibres. X-ray diffraction studies of these fibres, in contrast to analysis of protein crystals, provides only very limited information on the 1.4.2 Protein fold Protein folds are often very extensive arrangements, combining elements of secondary and supersecondary structure. Some of the most common protein folds are described in Table 4: view document with examples of proteins that contain them. Notice that proteins can be conveniently divided into three classes, on the basis of the elements of secondar 8.7 Luminosity functions Samples of galaxies can be biased due to the flux limit of the sample that is observed. This is the so called Malmquist bias. Read Pe
Activity 9: Radio-quiet quasars
