What is life?

At my high school, we were taught that the defining life is easy. Living things obey the seven life processes – those being: movement, reproduction, sensitivity, nutrition, excretion, respiration and growth (the mnemonic being “Mrs Nerg”). Movement can include anything from physically ambling around to microscopic movements of cellular machinery. Reproduction is the production of offspring by parents. Sensitivity is the ability to detect one’s surroundings. Nutrition is the acquisition of materials for use by the body and respiration is the conversion of nutrients into energy; the products of this conversion are either excreted by the organism or used to help it grow.

This exclusive definition of life is simple: anything that does not adhere to every single process is not alive. It is a good working definition for GCSE biology, and one that belies the historical belief that living things and non-living things are entirely separate.  This is no longer the case, as scientists have realised the boundary between life and death is much more blurry…

All life on Earth is made from the same basic chemicals: proteins, sugar, fats, and a whole lot of water. None of these substances are alive by themselves, but mix them together in just the right way, and you get something that is alive as a whole. It is not proteins, sugar and fats that make living things alive, but the way that those chemicals interact – the interaction is metabolism. Life can be defined as a self-contained, self-sustaining continuous chemical reaction. The interactions result in the above life processes:  as if by magic, a chemical sludge is transformed into a living, breathing, moving beast.

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