Madical brain Notes 10

  Why is energy needed? Like all other living things, microorganisms need to acquire energy in order to survive. Energy is required: ·                to maintain the structural integrity of the cell by repairing any damage to its constituents ·                to synthesise new cellular components such as nucleic acids, polysaccharides and enzymes ·                to transport certain substances into the cell from its surroundings ·                for the cell to grow and multiply ·               for cellular movement. Metabolism  is the term used to describe all the biochemi-cal reactions that take place inside a cell; it includes those r...

  Enzymes An enzyme is a cellular catalyst; it makes biochemical reactions proceed many times more rapidly than they would if uncatalysed. The participation of an enzyme can in-crease the rate of a reaction by a factor of millions, or even billions. Traditionally, all enzymes have been thought of as globular proteins, but around twenty years ago it was demonstrated (surprisingly) that certain RNA molecules also have catalytic properties. These  ribozymes  however, are very much in the minority, carrying out specific cut-and-splice reactions on RNA molecules, and in the present  context can be ignored. In this book we shall confine our discussion of enzymes to the protein type. Like any other catalyst, an enzyme remains unchanged at the end of a reaction. It must, however, at some point during the reaction bind to its  substrate  (the substance upon which it acts) to form an enzyme–substrate complex (Figure 6.2) by multiple weak forces such as electrostatic ...

  Principles of energy generation In this section, we shall consider how enzyme-catalysed reactions are involved in the cellular capture and utilisation of energy. Energy taken up by the cell, be it in the form of nutrients or sunlight, must be converted into a usable form. A simple analogy is selling goods for cash, which you can then use to buy exactly what you want. The ‘cash’ of cellular metabolism is a compound called  adenosine triphosphate (ATP) . ATP is by far the most important of a class of compounds known as high-energy transfer compounds, which store the energy *  from the breakdown of nutrients (or trapped by photosynthetic pigments) and release it when required by the cell. In  catabolic  reactions, in which molecules such as glucose are broken down, energy is released in the form of ATP, which can then be utilised in  anabolic  (synthetic) reactions. ATP has a structure very similar to the nucleotides found in RNA, except it has two addi...

  Oxidation -- reduction reactions Many metabolic reactions involve the transfer of electrons from one molecule to another; these are called  oxidation-reduction  or  redox  reactions. When a molecule (or atom or ion) loses an electron, it is said to be  oxidized . (Note, that despite the terminology, oxygen does not necessarily take part in the reaction.) Conversely, when an electron is gained, the recipient is  reduced  (Figure 6.14). Many metabolic reactions involve the loss of a hydrogen atom; since this contains one proton and one electron, the reaction is regarded as an oxidation, because an electron has been lost: The lactate in the example above, by losing two hydrogen atoms, has automatically lost two electrons and thus become oxidised to pyruvate. Oxidation reactions are always associated with the transfer of energy from the oxidised substance to the reduced substance Two important molecules that we shall encounter a number of times...

  Why glucose? By concentrating on glucose catabolism in this way, you may think we are ignoring the fate of other nutrient molecules. If you take another look at Figure 6.15, however, you will notice that the breakdown products of lipids, proteins and nucleic acids also find their way into our pathway sooner or later, having undergone transformations of their own. Glycolysis The initial sequence of reactions, in which a molecule of glucose is converted to two molecules of  pyruvate ∗  , is called  glycolysis (Figures 6.16 and 6.17). In the first phase of glycolysis, glucose is phosphorylated and its six-carbon ring structure rearranged, before being cleaved into two three-carbon molecules. In the second phase, each of these undergoes oxidation, resulting in pyruvate. Also known as the  Embden–Meyerhof pathway , glycolysis is used for the metabolism of simple sugars not just by microorganisms, but by most living cells. The pathway, which takes place in the cytop...

  Aerobic respiration We shall now examine the fate of the pyruvate produced as the end-product of glycol-ysis. As we have seen, this depends on whether the organism in question is aerobic or anaerobic. You will recall that during glycolysis, NAD +  was reduced to NADH. In order for glu-cose metabolism to continue, this supply of NAD +  must be replenished; this is achieved either by  respiration  or  fermentation . Respiration is the term used to describe those ATP-generating processes, aerobic or anaerobic, by which oxidation of a substrate occurs, with an inorganic substance acting as the final  electron acceptor . In  aerobic  respira-tion, that substance is oxygen; in  anaerobic  respiration, a substance such as nitrate or sulphate can fulfil the role. In most aerobic organisms, the pyruvate is completely oxidised to CO 2  and water by entering the  tricarboxylicacid (TCA) cycle , also known as the  Krebs cycle...