The primary liquid serving as a solvent in biological systems is water. Water is essential for life, acting as a medium for biochemical reactions, transporting nutrients and waste, and regulating temperature. Its unique properties, such as high specific heat and polarity, make it an effective solvent for a wide range of polar and ionic compounds. This characteristic enables vital processes, including cellular respiration and photosynthesis, and facilitates the transportation of substances in organisms. Without water, life as we know it would be unsustainable.