9 Glossary Click on the link below to open the unit glossary.
8 Summary Energy is the basis of modern society. Other physical resources can only be effectively extracted, processed and transported if there is a ready supply of Author(s):
5 Nuclear energy Einstein's famous equation E = mc2 shows that mass (m) and energy (E) are proportional to one another. The constant c2 linking the two is the square of the speed of light c (3 × 108 m s−1). Implicit in the equat
4.3 Photosynthesis, respiration and decay Green plants absorb solar radiation and use its energy to fuel photosynthesis — a chem
4.2 The terrestrial carbon cycle Figure 1.10 shows the rates of natural carbon exchange between the terrestrial system and the atmosphere. Learning outcomes By the end of this unit you should be able to: explain the principles that underlie the ability of hydropower to deliver useable energy; outline the technologies that are used to harness hydropower; discuss the positive and negative aspects of hydropower in relation to natural and human aspects of the environment. 4.2 Iron transport It is obvious that iron must be transported around the human body. Firstly, it must be transported from the food in the gut to the places where it is required. Mostly, iron is required in the bone marrow, where red blood cells are formed. Red blood cells have a finite lifetime of about only four months, and old cells are destroyed, usually in the spleen. Iron from the destruction of these cells is then transported from the spleen back to the bone marrow to be recycled. Iron cannot be tr 4.1 Introduction As bacteria secrete such powerful chelators into the environment, iron in other organisms must be kept under very close control. Any free iron within an organism is likely to be chelated by a siderophore, which may lead to bacterial infection within the organism In this Section we shall examine the biochemical systems that handle iron within the human body. The two areas we shall study are iron transport and iron storage. 3.4.4 Sediment filling The lifetime of reservoirs can vary greatly. Many reservoirs have lasted for over a hundred years, but some may be useful for only a much shorter period—fifty years or so — not because of the general deterioration of the dam as it gets older, but because sediment accumulates in the reservoir. Rivers carry large amounts of mud, silt and sand in suspension, particularly during floods, and when a river enters a reservoir it slows down and the sediment carried in suspension is deposited on th 3.4.1 Calcium ions The Ca2+ concentration is normally low in the cytosol (~10–7 mol 1–1) compared with the extracellular space (~10–3 mol 1−1). There are several mechanisms for achieving this. The most widespread are ATP-dependent Ca2+ efflux pumps on the plasma membrane, which pump Ca2+ ions out of the cell. Muscle and nerve cells, where oscillations in intracellular Ca2+ concentration often occur, employ an additio 2.5 Intracellular receptors Signal receptors are usually located at the cell surface. However, it is important to remember that there are some groups of receptors that do not fit into the general signal transduction model set out in Figure 2, These are intracellular receptors, which bind small or lipophilic molecules such as steroid hormones, which can cross the cell membrane. The signalling pathways activated by these receptors seem quite simple compared with the other pathways we shall be dealing with, but the same pr 2.3.4 Recruiter receptors Enzyme-associated or recruiter receptors also form dimers (or oligomers) on activation by their ligand, in a similar way to receptors with intrinsic enzymatic activity. Dimerization facilitates an interaction between the cell surface receptor (which lacks a catalytic domain) and cytosolic proteins with enzymatic activity. In the case of receptors that associate with tyrosine kinases (called ‘tyrosine kinaseassociated receptors’, the most common in this group), it is the non-covalently lin 2.3.3 Receptors with intrinsic enzymatic activity Receptors with intrinsic enzymatic activity are the second biggest group of receptors after the GPCRs. They include four types according to the form of enzymatic activity of the intracellular domain (Figure 23a). Receptor tyrosine kinases (RTKs) On activation, the kinase domain phosphorylates tyrosine amino 2.3.2 Seven-helix transmembrane (7TM) receptors Although in unicellular organisms such as the yeast S. cerevisiae there are only two classes of 7TM receptors, the pheromone and glucose receptors, multicellular organisms have many more, accounting for up to 5% of all genes in C. elegans and 2% of genes in the human and Drosophila genomes. 7TM proteins have been classified into four classes, A, B, C (Table 1). Between them, they can bind a huge range of ligands including simple ions, nucleotides, lipids, steroids, modifi 2.3.1 Ion-channel receptors Nicotinic cholinergic receptors are probably the best studied of all receptors, firstly because they are present throughout skeletal muscle, and secondly because there are plenty of natural and synthetic toxins that bind specifically to this receptor. Furthermore, the technique of patch-clamp electrophysiology has made possible the detailed characterization of the properties of individual ion channels (Author(s): 2.3 Receptor activation Receptors may be activated by conformational change (for example, ion-channel receptors such as nicotinic receptors, and 7TM receptors such as muscarinic receptors and adrenergic receptors), by formation of dimers (such as receptors with intrinsic enzymatic activity and recruiter receptors) or by proteolysis. We shall now consider how each cell surface receptor class described in Author(s): 2.2 Receptor specificity Binding of an extracellular signal to its receptor involves the same type of interactions as those between an enzyme and its substrate. Receptor specificity depends on the binding affinity between the ligand and the binding site on the receptor. The dissociation constant (KD) describes the affinity between receptors and their ligands. Proteins can be thought of as consisting of various domains, and the different combinations of structural motifs in the extracellular re 1.9 Summary In a basic model of signal transduction, a signalling molecule binds to a specific receptor, and this activates a sequence (or web) of intracellular signalling molecules that spread the information to relevant parts of the cell, activating target molecules, which effect a cellular response. Signalling between cells can be contact dependent or via secreted signalling molecules. The latter comprise paracrine, autocrine, endocrine or electrical s 1.7 Localization of signalling proteins Since signalling proteins cannot diffuse as rapidly as small second messengers, they need be close to their downstream target in order to be able to function. Where they are located with respect to both their subcellular position and their immediate neighbours is therefore vitally important. The plasma membrane is usually the initial location, and proteins can be attached to the plasma membrane in various ways (Author(s): 1.2 Extracellular signals can act locally or at a distance First we shall consider the general types of intercellular signalling mechanism within multicellular organisms (Figure 3). Broadly speaking, cells may interact with each other directly, requiring cell–cell contact, or indirectly, via molecules secreted by one cell, which are then carried away to target cells.
