What Is Energy? - The Renaissance of Renewable Energy - page 10

to survive. The oak and the rose, however - like all plants, all algae and some bacteria
- are autotrophs. This means they are able to synthesize their own food (from the Greek
autós , meaning self, and troph ē , meaning nourishment). They use solar energy to power a
reaction that converts water and carbon dioxide into organic biomass (sugar). We call this
chemical process photosynthesis (from the Greek photos , meaning light).
The synthesis of highly ordered sugar molecules from unordered and scattered
molecules of carbon dioxide and water could not happen spontaneously. That would
contravene the second principle of thermodynamics, according to which all things tend
towards decay and disorder. It is the input of solar energy that makes the transformation
possible.
Once an organism has obtained its food, whether through photosynthesis or by
consuming other organisms, it converts that fuel into energy. This process - essentially a
kind of cellular combustion - is known as respiration. Oxygen absorbed by the lungs is
distributed through red blood cells to every cell in the human body. In the cells, the oxygen
reacts with the organic fuel to produce energy. The by-products of this reaction - water and
carbon dioxide - are exhaled through the lungs and excreted through the kidneys. Plants
respire in a similar way, releasing carbon dioxide and water through their leaves.
Figure 1.2. All life depends on the ability of some organisms to transform inorganic
matter into organic matter using solar energy. The sun, therefore, is the first link in the
chain of life.
Many living organisms, including most animals, have a second way of converting
food to energy: fermentation. Unlike respiration, fermentation does not require oxygen.
In evolutionary terms, it is also far older than respiration, but around eighteen times
less efficient. Not surprisingly, then, all multicellular organisms, from plants to fungi
and animals, rely mainly on respiration. Organisms that depend entirely on fermentation
include yeasts and some bacteria. The pungent odours that emanate from rotting food are
caused by the by-products of fermentation (carbon dioxide, alcohol, lactic acid and acetic
acid). All methods of food storage and conservation, from smokehouses to fridges, aim
to prevent or slow down fermentation. Animals use fermentation as a kind of backup,
when more energy is needed than can be supplied using respiration, usually because of an
inability to breathe quickly enough. Humans also take advantage of fermentation to make
numerous food products - including alcoholic drinks, yogurt, bread and vinegar.