What Do You Think You Are The Science of What Makes You You - Brian Clegg Page 0,28

state. And in our food, just as is the case when we burn something, the energy stored in the bonds between atoms is released as the bonds are broken. Typically, the digestive system breaks down your food into smaller molecules which then undergo a process known as respiration.

THE SLOW BURN

Respiration is a slow form of combustion. We usually associate combustion with flames, which thankfully aren’t involved in the digestive process. But combustion is simply a matter of a fuel and an oxidising material, often the oxygen in the air, undergoing a chemical reaction whereby bonds are broken and energy released. These ‘bonds’ are not physical ties as we would expect in the usual use of the word, but something closer to the way that two magnets stick to each other.

As we have seen, the atoms that make up all matter – including your food – have a positive electrically charged nucleus surrounded by a cloud of negative electrically charged electrons. When two or more atoms bind together, they either share electrons between them, or one atom loses one or more electrons while another gains one or more. In the latter case, the result is that one ‘ion’ (atoms that have gained or lost electrons) is positively charged and the other negatively charged. These particles with opposite electrical charges attract each other.

Such chemical bonds can store energy away that is released when the bonds are broken. In your body this is happening all the time. Molecules derived from your food are combined with oxygen and the energy produced is stored in a special molecule called adenosine triphosphate (ATP), which is used to transfer the energy around the body to wherever it is needed to power everything from your brain to your muscles. This is where mitochondria come in.

As we have seen, mitochondria, sometimes called the power sources of the body, are thought to once have been independent bacteria that were incorporated into many of the cells of eukaryotes. Mitochondria react fuel molecules with oxygen and produce vast quantities of the ATP molecule, which are repeatedly recycled. This is such a major process in your body that you get through around your own bodyweight in ATP every day. The chemical energy of respiration is used to push protons – hydrogen nuclei – through a membrane, setting up an electrical gradient. As a result, the molecular machines in the mitochondria can make use of there being more electrical charge on one side of the membrane than the other to power their manufacture of ATP.

MEASURING OUR ENERGY

As we’re thinking of food here as our primary source of energy, it’s worth exploring the units we use to measure energy in, because they are particularly confusing when applied to food. The standard scientific unit of energy is the joule. An electrical device rated at one watt uses one joule each second. The watt is a unit of power – the rate at which energy is transferred. This results in the rather bizarre unit used by energy companies of kilowatt hours. One kilowatt hour is a thousand watts for one hour – which is 3,600 seconds – so is 3,600,000 joules. Measuring energy in kilowatt hours makes as much sense as measuring distance in ‘miles per hour hours’.

The good news is that we don’t measure food energy in kilowatt hours. The bad news is that it is rarely talked about in joules either. Food energy tends to be measured in calories, the scientific unit prior to the joule being introduced. Each calorie is the equivalent of 4.184 joules. As the joule is a small unit of energy, when food energy is displayed on packaging in joules, it’s usually as kilojoules, represented as kJ – thousands of joules. Similarly, when displayed as calories, kilocalories are used. However, to make things really confusing, this usage came in before we all became familiar with ‘kilo’ style prefixes, so kilocalories are habitually mislabelled as Calories – calories with a large C.‡

Getting an idea of the energy involved in food is important because, just as is the case with water, there is plenty of energy around on the Earth, but most of it is either not in the right place, or not in a form in which it can be easily used. Plants, in many respects, are better off than we are in this regard. They can get their energy directly from the Sun; with the exception of tidal, nuclear and geothermal energy, pretty well all the usable energy on our planet