What a Wonderful World Read Online Free PDF Page A

complexes. 5 And the donated electrons, bursting with excess energy, hop from location to location down the wire.
    Focus on a single electron. As it hops down the wire, as inevitably as a ball rolling downhill, it drives hydrogen nuclei, orprotons, 6 through channels, or pores, in the cell membrane. 7 Since protons carry an electric charge – the opposite of electrons – this charges up one side of the membrane with respect to the other. Something like this happens in a battery; it creates an electric force field between the battery’s terminals. And, actually, this hints at what the super-energetic electron does as it thunders down the protein wire to an oxygen atom: it turns the cell membrane into a charged-up battery. The resulting electric force field across the membrane is stupendously powerful. It is comparable, in fact, to the electric field that, in a thunderstorm, breaks down the atoms in the air and unleashes a multimillion-volt bolt of lightning. 8
    You might imagine that the cells in your body should crackle with lightning. However, the tremendous electric force field extends over only the tiny thickness of a cell membrane – about 5 millionths of a millimetre – and other molecules intervene to stop this force field having its way. Interestingly, however, in programmed cell death, or apoptosis, this protective mechanism is turned off and cells are in effect killed off by their own internal lightning bolts.
    The powerful electric force field of the membrane battery drives a chemical reaction that creates adenosine triphosphate, or ATP. Such molecules are stores of energy; think of them as portable batteries. So, as the electron bounces down the protein wire, losing energy all the while, it leaves in its wake a large number of energy-packed ATP molecules. Released into the wild, these have the ability to power cellular processes wherever and whenever necessary.
    In the final analysis, you are battery powered. There are about a billion ATP molecules in your body, and all of these are usedand recycled every 1–2 minutes. Toys may require a handful of batteries that become flat in a few hours. Contrast this with your body, which uses up 10 million power packs
every second
. Thank goodness that, for human beings, batteries are included.
    Finally, the electron arrives at the end of the protein wire, exhausted of its energy. There it combines with the waiting oxygen atom. When a second electron from another hydrogen atom joins it, the oxygen atom achieves the highly desirable state of a filled outer shell of electrons. But this is not quite the end of the story. If the oxygen atom passes the electrons to a carbon atom – left behind when hydrogen was stripped from the food in the Krebs cycle – the result is a very stable molecule of carbon dioxide. And carbon dioxide, along with water vapour, is what oxygen-breathing animals exhale as waste.
Breathing
    So much for the chemistry of respiration; what about its physiology? Well, we breathe in air, of which about 20 per cent is oxygen. Only about a quarter of this actually gets used, so exhaled air still contains about 15 per cent oxygen. This is why it is possible to revive an unconscious person with exhaled breath via mouth-to-mouth resuscitation.
    Our breath is drawn deep down into our lungs, the inner surfaces of which have a structure on the smallest scale rather like a branching tree. The branches, known as alveoli, run alongside fine blood vessels, and oxygen molecules pass from them to red blood cells. The tree-like structure of the alveoli maximises the area over which this oxygen transfer can occur, maximising the amount of oxygen that can enter the blood stream. Remarkably,the surface area of a human lung is similar to that of a tennis court.
    When an oxygen molecule is transferred to a blood cell, it is picked up by a giant protein called haemoglobin. It is then ferried to a cell where the oxygen is combined with hydrogen stripped from food to liberate