Bound water -

Water present in water-containing systems such as biological tissues, gels, colloids, or aqueous solutions that is bound to supporting tissues, dispersoids, or solutes. Bound water has different properties from normal water, such as not freezing at 0 °C or not acting as a solvent, so the amount of bound water can be measured by various methods. Known methods include the cobalt chloride method, freezing point depression method, calorimeter method, dielectric constant method, compressibility method (ultrasonic absorption method), nuclear magnetic resonance absorption method, vapor pressure method, sedimentation method, and diffusion method. However, since the definition of bound water is not necessarily strict, the amount of bound water often varies considerably depending on the measurement method and measurement conditions. The bond form of bound water is thought to be mainly hydrogen bonds with polar groups, but the bonds are not fixed, and it is generally thought that exchange between bound water and free water takes place. It has been pointed out that the cold resistance of insects and plants in low-temperature regions and the drought resistance of plants in high-temperature and low-temperature regions are due to the regulation of the amount of bound water. However, there are still many unknowns about the physiological role of bound water in living organisms.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

生物組織，ゲル，コロイド，あるいは水溶液などの含水系中に存在する水のうち，支持組織，分散質，あるいは溶質などに結合した水．結合水は0 ℃ で凍らない，あるいは溶媒作用を示さない，など通常の水と異なった性質を示すので，各種の方法で結合水量を測定できる．測定法として，塩化コバルト法，氷点降下法，熱量計法，誘電率法，圧縮率法(超音波吸収法)，核磁気共鳴吸収法，蒸気圧法，沈降法，あるいは拡散法などが知られている．しかし，結合水の定義が必ずしも厳密ではないので，結合水量は測定法や測定条件によって相当の相違が認められることが多い．結合水の結合形態は，おもに極性基との水素結合と考えられているが，結合は固定したものではなく，一般には，結合水と自由水との間の交換が行われていると考えられる．低温地域の昆虫や植物の耐寒性や，高温・低温地域の植物の耐乾性などは，結合水量の調節に起因していることが指摘されている．しかし，生体における結合水の生理的役割については，いまだ不明の点が多い．

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