Characteristics of Life

All living organisms share 8 characteristics that distinguish them from non-living things. The mnemonic MRS NERG can help you remember them:

Movement - response to stimuli by changing position
Respiration - releasing energy from food molecules
Sensitivity - detecting and responding to changes in the environment
Nutrition - obtaining food for energy, growth, and repair
Excretion - removal of metabolic waste products
Reproduction - producing new individuals of the same species
Growth - permanent increase in size and complexity

Plus Organisation: all living things are made of cells arranged in levels of increasing complexity.

Viruses challenge the definition of life: they have DNA or RNA and can replicate, but they cannot reproduce independently, carry out metabolism, or respond to stimuli. They need a host cell to reproduce.

Classification is the organisation of living things into groups based on shared characteristics. Modern classification uses three domains based on genetic (DNA) evidence.

The Three Domains (proposed by Carl Woese in 1990):

1. Bacteria - prokaryotic, most diverse and abundant. E.g. E. coli, Streptococcus
2. Archaea - prokaryotic, often found in extreme environments (hot springs, salt lakes)
3. Eukaryota - all organisms with membrane-bound nuclei: Protista, Fungi, Plantae, Animalia

Binomial Nomenclature (devised by Linnaeus): every species has a two-part Latin name - Genus species. E.g. Homo sapiens, Escherichia coli. The genus is capitalised, both italicised.

Taxonomic hierarchy: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species

Phylogenetic classification groups organisms according to their evolutionary relationships rather than their appearance. Relationships are worked out by comparing gene and protein sequences, especially ribosomal RNA genes, which change very slowly and are found in every organism.

The three domains

Carl Woese's rRNA comparisons showed that the prokaryotes are not one group. Life is now divided into three domains:

Archaea are genetically closer to Eukarya than to Bacteria, which is why the tree branches as it does. The full hierarchy runs domain, kingdom, phylum, class, order, family, genus, species, and each organism is given a two-part binomial name such as Homo sapiens.

Reading a phylogenetic tree

Each branch point is a common ancestor, and the shorter the distance back to a shared branch point, the more closely related two organisms are. Because sequence data is objective and can be reanalysed by anyone, trees are revised as more genomes are sequenced. Similar appearance can be misleading: unrelated species can look alike through convergent evolution, but their DNA still tells them apart.

Biology studies life at many levels of organisation. Each level is built from the one below it, and each new level shows properties the level below does not have. A chloroplast cannot survive alone, a leaf cell can photosynthesise, and only a whole ecosystem cycles nutrients.

The life processes

An organism is alive if it carries out all of these:

• Nutrition - obtaining food or making it. Autotrophic organisms make their own food (plants, algae, some bacteria); heterotrophic organisms take it in ready made (animals, fungi, most bacteria).
• Respiration - releasing energy from food as ATP in every living cell.
• Excretion - removing the waste products of metabolism, such as CO₂ and urea.
• Response - detecting and reacting to stimuli, which keeps the organism alive in a changing environment.
• Growth - a permanent increase in size and dry mass, from cell division and cell enlargement.
• Reproduction - producing new individuals, asexually or sexually, so the species continues.
• Organisation - all living things are made of one or more cells, the basic unit of life.

Metabolism and homeostasis

Metabolism is the sum of all chemical reactions in an organism. Anabolic reactions build large molecules and use energy (photosynthesis, protein synthesis); catabolic reactions break molecules down and release energy (respiration, digestion). Every one of these reactions is controlled by an enzyme.

Homeostasis is the maintenance of a stable internal environment despite changes outside. Body temperature, blood glucose, water balance and blood pH are all held within narrow limits, usually by negative feedback: a change is detected, a response is triggered that reverses the change, and conditions return to the set point.

Common mistake

Cars use fuel and move, and crystals grow, but neither carries out all the life processes. To decide whether something is alive, check the full list, not one feature.