Obviously, the process described on the previous page is not a simple one. A ribosome is an extremely complex structure of enzymes and ribosomal RNA (rRNA) bonded together into a large molecular machine. A ribosome is helped by ATP, which powers it as it walks along the messenger RNA and as it stitches the amino acids together. It is also helped by transfer RNA (tRNA), a collection of 20 special molecules that act as carriers for the 20 different individual amino acids. As the ribosome moves down to the next codon, the correct tRNA molecule, complete with the correct amino acid, moves into place. The ribosome breaks the amino acid off the tRNA and stitches it to the growing chain of the enzyme. The ribosome then ejects the "empty" tRNA molecule so it can go get another amino acid of the correct type.
As you can see, inside every cell there are a variety of processes keeping the cell alive:
- There is an extremely long and very precise DNA molecule that defines all of the enzymes the cell needs.
- There are RNA polymerase enzymes attaching to the DNA strand at the starting points of different genes and copying the DNA for the gene into an mRNA molecule.
- The mRNA molecule floats over to a ribosome, which reads the molecule and stitches together the string of amino acids that it encodes.
- The string of amino acids floats away from the ribosome and folds into its characteristic shape so it can start catalyzing its specific reaction.
The cytoplasm of any cell is swimming with ribosomes, RNA polymerases, tRNA and mRNA molecules and enzymes, all carrying out their reactions independently of each other.
As long as the enzymes in a cell are active and all of the necessary enzymes are available, the cell is alive. An interesting side note: If you take a bunch of yeast cells and mistreat them (for example, place them in a blender) to release the enzymes, the resulting soup will still do the sorts of things that living yeast cells do (for example, produce carbon dioxide and alcohol from sugar) for some period of time. However, since the cells are no longer intact and therefore are not alive, no new enzymes are produced. Eventually, as the existing enzymes wear out, the soup stops reacting. At this point, the cells and the soup have "died."