All living things react to stimuli, feed, grow, reproduce, repair themselves and die. Some of these attributes are found in non-living things – crystals ‘feed’ on salts dissolved in water, and thus grow; robots can respond to stimuli. So what sets living things apart?
The answer is the cell, the basic unit of life as we know it. An individual cell is tiny – the smallest fraction of a millimetre in diameter. But cells are among the most complex mechanisms known to science. Some are separate living organisms in their own right (see here ), while others play specialized roles in more complex multicellular organisms (see here ). There may be as many as 37 trillion cells in a human body.
Cells have the ability to absorb a wide range of raw materials from their environment and alter them chemically within themselves to create more complex compounds. It is this ability that enables them to repair damage, and to reproduce by dividing and redividing.
Four groups of chemicals are essential for the structure and functioning of a cell. Nucleic acids (DNA and RNA) encode genetic information and carry out the instructions embedded in that code (see here ). The second group consists of proteins, some of which are structural, while others are enzymes – catalysts that help drive chemical reactions. Proteins are made from simpler building blocks, the amino acids. The third group is the carbohydrates, some of which are building blocks while others store energy. The simplest carbohydrate is glucose, produced by plants during photosynthesis. Nearly all animals acquire the carbohydrates they need from plants. The final group is the lipids, key components of the membrane of the cell (see here ).
All these complex molecules are made up of a relatively small range of simpler molecules, most notably water and carbon. Water contributes oxygen and hydrogen to many other compounds. In addition, some two-thirds of a living cell consists of liquid water, in which more complex compounds are dissolved and transported. Carbon has the ability to combine with other elements to create a huge variety of organic compounds, many of which are soluble in water.
The origin of life?
How did simple molecules such as water and carbon come to form the more complex compounds necessary for life? The atmosphere of the early Earth consisted of gases emitted during volcanic eruptions, such as water vapour (H2 O), hydrogen (H2 ), nitrogen (N2 ), carbon dioxide (CO2 ) and carbon monoxide (CO). As these gases cooled, hydrogen combined with nitrogen to form ammonia (NH3 ) and with carbon dioxide and carbon monoxide to form methane (CH4 ). In the presence of ultraviolet light (as emitted by the Sun) and an electrical spark (e.g. a lightning flash), ammonia and methane can combine with water and carbon dioxide to form simple amino acids, which, if heated, can link up to form proteins. Similar reactions can result in the creation of the components of DNA. Alternatively, there is some evidence that not only the components of DNA but also the amino acids may have been brought to Earth by meteorites.
As the Earth cooled, water vapour in the atmosphere condensed to form the early oceans, in which many different minerals and gases were dissolved. It is possible that in this great chemical soup about 4 billion years ago the first self-replicating molecules – such as DNA – might have appeared.