Life, a term used to summarize the characteristic activities of all organisms—from cyanobacteria to plants and animals. The essence of life is a reproduction, the formation of identical or near identical copies of a complex structure from simple starting materials. The increase of complexity involved in the formation of living organisms from their precursors distinguishes the processes of biological growth and reproduction from physical processes such as condensation or crystallization. This local increase of complexity, or decrease of entropy, appears to contradict the Second Law of Thermodynamics which states that entropy must always increase in natural spontaneous processes. However, it can be shown that overall entropy always increases so long as the changes in the surroundings, as well as the changes in the organisms themselves, are included. There is thus no conflict between the basic laws of physics and chemistry and the existence of living organisms. But the understanding of the nature of living organisms has necessitated the creation of the new sciences of biochemistry and molecular biology, which have their own concepts, principles, and laws in addition to those of physics and chemistry.
All living organisms possess a genome, which is the set of instructions for making the body, and this is always composed of nucleic acid. It is usually DNA (deoxyribonucleic acid) or in the case of some viruses, RNA (ribonucleic acid). The genome consists of a number of genes each of which is a segment of nucleic acid coding for a particular type of protein molecule. Nucleic acids are linear polymers composed of four types of chemical unit (nucleotides, abbreviated as A, T, C, and G), that can occur in any order, and proteins are linear polymers composed of any number of twenty types of unit (amino acids). The relationship between the sequence of nucleotides in a gene and the sequence of amino acids in the corresponding protein is given by the genetic code. Each amino acid is encoded by 3 nucleotides, and since there are only 20 amino acids but 64 (=4×4×4) nucleotide triplets, most amino acids can be specified by more than 1 nucleotide triplet.
The molecular structure of DNA is a double helix. One strand contains the coding sequence of a gene and the other strand contains a complementary sequence determined by the pairing rules for the four nucleotides (A pairs with T and C pairs with G). When a genome is reproduced, the DNA double helix separates and a new complementary strand is synthesized alongside each of the old strands, the final result is the formation of two identical double helical DNA molecules.
There is a very wide diversity of proteins and they carry out most of the biochemical activities of living organisms, called metabolism. Many proteins are enzymes, bringing about chemical changes in other molecules under the conditions of temperature and concentration found in living organisms. These changes are required to build up the many macromolecules required to form the structure of the organism, including the proteins themselves, the carbohydrates (such as cellulose), and the lipids (fats). Other enzymes drive the metabolic processes needed to supply energy for this biosynthesis, in the case of animals this means the breakdown of food and in the case of plants the process of photosynthesis.
When a gene is active, it is copied into a strand of ribonucleic acid using the same complementarity rules as used for DNA replication. The RNA copy is called messenger RNA, or mRNA. This directs the synthesis of the protein by structures called ribosomes working along with a number of enzymes.
The simplest types of living organism are viruses, but with only a few genes they are unable to grow and reproduce without a supply of numerous enzymes and other macromolecules from more complex organisms. All free-living organisms are composed of cells, having an outer membrane composed of lipid and a genome composed of nucleic acid. The simplest free-living bacteria contain a genome of about 2,000 genes, which is sufficient to specify the entire organism. There are two fundamentally different types of bacteria. The Eubacteria comprise all the more familiar free-living and pathogenic bacteria, together with the Cyanophyta, or blue-green algae, while the Archaeabacteria comprises the methane producing, the halophilic, and the thermoacidophilic bacteria. The Archaeabacteria have a distinctive ribosome structure and their cell membranes contain ether- rather than ester-type lipids. The third major division of life on Earth consists of the Eukaryotes, or organisms composed of cells with nuclei. These include all animals and plants as well as numerous groups of single-celled algae and protozoa. In terms of complexity, Eukaryotes range from a few thousand essential genes in single-celled organisms up to about 30,000 for man and other vertebrates.
It is conceivable that life may exist in other solar systems which conform to the essential definition of self-reproducing complex structures, but which does not involve nucleic acid, or is not even composed of carbon compounds at all. Since the experimental methods currently used to search for life on other planets involve an assumption of carbon-based biochemistry, it might prove difficult to recognize such alien forms of life if they were encountered.