Created by Wyken Seagrave.
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We saw on the atoms page that some atoms are soft, and can stick to other atoms to create clumps called molecules. The forces which hold atoms together are called "covalent bonds". For a more detailed discussion of how and why molecules form, see the relevant page on the history of the universe website
One of the simplest, but perhaps the most important molecules in the universe is water. On Soccearth it would be about 25 millimetres long. In the diagram we also show some other small molecules which can teach us some basic facts about the way molecules build themselves.
Notice that water is made by combining oxygen with 2 hydrogen atoms. Therefore it's forumla is H2O. It turns out that hydrogen can only form one covalent bond with other atoms and oxygen can only form 2 such bonds.
Nitrogen, on the other hand, can form 3 bonds, hence it can form a molecule with 3 hydrogen atoms to form ammonia NH3. And finally carbon can form 4 covalent bonds, so methane has the formula CH4. Atoms cannot form more than 4 covalent bonds. Soo these 4 atoms - H, O, N and C - provide every possible number of bonds.
In the diagram above we also show the notorious molecule carbon dioxide. Here 2 oxygen atoms have each formed 2 covalent bonds with a single atom of carbon.
Carbon dioxide is often blamed for causing global warming, but in fact methane is a far more powerful greenhouse gas than carbon dioxide, and water itself is also a greenhouse gas!
The molecules of life are made principally by combining these 4 types of atoms in various different ways to build bigger and bigger structures. We show some of the larger molecules below. On the left we show a sample of DNA and on the right a chain of amino acids which forms part of a protein.
One of the main types of molecules contained in living things is DNA, which contains the information needed to build the cell and keep it functioning.
On Soccearth, a molecule of DNA would be about 12cm thick, about as wide as a street lamp column, but much, much longer.
DNA animation by Spiffistan; Street light image from www.stainton-metal.co.uk
Inside living things, the DNA is curled up very small. If all the DNA in a single human cell were stretched out it would wrap twelve times around Soccearth!
Another of the main types of cell used by life is protein, which is made from small molecules called amino acids.
The amino acids range in size, but from the smallest (such as alanine) to the largest (tryptophan) they are all about as large as tomatoes on Soccearth.
Cherry tomato image by Luc Viatour (www.Lucnix.be), plum tomato by Ulla
Amino acids are joined together into long chains. In fact, the DNA in a cell is simply a set of instructions specifying which amino acids should be joined together and in which sequence.
When a chain of amino acids is in contact with water, some of the amino acids stick to the water and are pulled to the outside, while others are oily and get squeezed into the middle of the group. The result is that the chain folds itself up into a unique shape, called a protein. The proteins are the worker molecules in a cell. They do all the jobs needed to keep the cell alive. For example, mammals have a protein called myoglobin in their muscles, which carries the oxygen the muscle requires in order to move.
Sugar bag image by Hanácká potravinárská spolecnost (www.hps.cz)
Living cells contain a host of fantastic tiny machines called "ribosomes" which are responsible for joining the amino acids together into chains.
A complete ribosome is about the same size and shape as a large beanbag seat.
50S ribosomal subunit animation by David S. Goodsell, RCSB Protein Data Bank. Beanbag image from www.greatbeanbags.com
Here we show one part of the two parts of a ribosomes. It consists of a mixture of proteins (shown in blue) and RNA chains (shown in orange and yellow). RNA is similar to DNA. The small patch of green in the center of the subunit is the site where the amino acids are joined together.
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