Madical brain Notes 10

  Atomic structure All atoms have a central, positively charged  nucleus , which is very dense, and makes up most of the mass of the atom. The nucleus is made up of two types of particle,  protons  and  neutrons . Protons carry a positive charge, and neutrons are uncharged,hence the nucleus overall is positively charged. It is surrounded by much lighter, and rapidly orbiting,  electrons  (Figure 2.1). These are negatively charged, the charge being equal (but of course opposite) to that of the protons, but they have only 1 / 1840 of the mass of either protons or neutrons. The attractive force between the positively charged protons and the negatively charged electrons holds the atom together. The number of protons in the nucleus is called the  atomic number , and ranges from 1 to over 100. The combined total of protons and neutrons is known as the  mass number . All atoms have an equal number of protons and electrons, so regardless of the atomi...

  Isotopes Although the number of protons in the nucleus of a given element is always the same, the number of neutrons can vary, to give different forms or  isotopes  of that element. Carbon-14 ( 14 C) is a naturally occurring but rare isotope of carbon that has eight neu-trons instead of six, hence the atomic mass of 14. Carbon-13 ( 13 C) is a rather more com-mon isotope, making up around 1 per cent of naturally occurring carbon; it has seven neutrons per atomic nucleus. The  atomic mass  (or atomic weight) of an element is the average of the mass numbers of an element’s different isotopes, taking into account the proportions in which they occur. Carbon-12 is by far the predominant form of the element in nature, but the existence of small amounts of the other forms means that the atomic mass is 12.011. Some isotopes are stable, while others decay spon-taneously, with the release of subatomic particles. The latter are called  radioisotopes ;  14 C is a...

  Chemical bonds The force that causes two or more atoms to join together is known as a  chemical bond , and several types are found in biological systems. The interaction between sodium and chloride ions shown in Figure 2.4 is an example of  ionic  bonding, where the transfer of an electron from one party to another means that both achieve a complete outer electron shell. There is an attractive force between positively and negatively charged ions, called an  ionic bond . Certain elements form ions with more than a single charge, by gaining or losing two or more electrons in order to achieve a full outer electron shell; thus calcium ions (Ca 2 +  ) are formed by the loss of two electrons from a calcium atom. The goal of stability through a full complement of outer shell electrons may also be achieved by means of sharing one or more pairs of electrons. Consider the formation of water (Figure 2.2); an oxygen atom, which has two spaces in its outer shell, can ...

  Acids, bases, and pH Only a minute proportion of water molecules, something like one in every 5  ×  10 8 , is present in its dissociated form, but as we have already seen, the H +  and OH −  ions play an important part in cellular reactions. A solution becomes acid or alkaline if there is an imbalance in the amount of these ions present. If there is an excess of H +  , the solution becomes  acid , whilst if OH −  predominates, it becomes  alkaline . The  pH  of a solution is an expression of the molar concentration of hydrogen ions: pH = − log 10 [H +  ] In pure water, hydrogen ions are present at a concentration of 10 − 7 M, thus the pH is 7.0. This is called neutrality, where the solution is neither acid or alkaline. At higher concentrations of H +  , such as 10 − 3 M (1 millimolar), the pH value is lower, in this case 3.0, so acid solutions have a value below 7. Conversely, alkaline solutions have a pH above 7. You wi...

  Biomacromolecules Many of the most important molecules in biological systems are  polymers , that is, large molecules made up of smaller subunits joined together by covalent bonds, and in some cases in a specific order.   Carbohydrates Carbohydrates are made up of just three different ele-ments, carbon, hydrogen and oxygen. The simplest carbohydrates are  monosaccharides , or simple sugars; these have the general formula (CH 2 O) n . They are classed as either aldoses or ketoses, ac-cording to whether they contain an aldehyde group or a ketone group (Figure 2.10). Monosaccharides can further be classified on the basis of the number of carbon atoms they contain. The simplest are trioses (three carbons) and the most important biologi-cally are hexoses (six carbons). Monosaccharides are generally crystalline solids which are soluble in water and have a sweet taste. They all reducing sugars, so called because they are able to reduce alkaline solutions of cupric ions (C...