The uptake or assimilation by living organisms of a particular isotope in preference to another isotope of the same element. A well-known example in nature is the preferential fixation by photosynthetic organisms of the lighter (and vastly more abundant) isotope carbon-12 compared with the heavier carbon-13. The relative abundance of these two stable isotopes in biological material, whether living or dead, differs from that in the atmosphere and according to various factors, including the type of ecosystem, season, and atmospheric conditions. A record of these changes, based on preserved organic material, can provide insights into historical climatic conditions. In plants isotopic discrimination occurs chiefly because carbon dioxide (CO2) containing 13C diffuses more slowly than lighter CO2 containing 12C and also because the enzymes involved in photosynthetic carbon fixation discriminate between the two isotopes, particularly rubisco, the first enzyme to encounter CO2 in C3 plants (see C3 pathway). 12C/13C isotopic discrimination is less pronounced in C4 plants, in which CO2 is first incorporated by the C4 pathway. This gives C3 and C4 plants characteristically different isotopic signatures. Determination of 12C/13C ratios in plant material using mass spectrometry is informative about plant physiology and atmospheric conditions. For example, it indicates whether the stomata of a plant were mainly open or mainly closed during its lifetime, and hence whether conditions were likely to have been moist or dry.