Water is essential for metabolism, serving multiple critical functions as a solvent, reactant, and transporter of nutrients.

As a fundamental component of life, water significantly contributes to the metabolic processes within the body. It functions primarily as a solvent, creating the environment in which most biochemical reactions take place. Since the majority of metabolic reactions occur in aqueous solutions, substances involved in these reactions are typically dissolved in water. For example, blood, which is composed of approximately $90\%$ water, transports oxygen from the lungs to the cells while also facilitating the removal of waste products such as carbon dioxide.

Additionally, water acts as a reactant in numerous metabolic reactions. During digestion, for instance, it participates in breaking down complex molecules into simpler forms through a process called hydrolysis. A notable example of this is the conversion of starch into glucose, where water molecules are utilized to cleave the bonds between individual glucose units. Likewise, in cellular respiration, water is generated as a by-product when glucose is oxidized to release energy.

Water also plays a vital role in the transportation of nutrients and waste products. It supports the absorption of nutrients from food within the digestive system and subsequently transports these nutrients to various parts of the body. Furthermore, water aids in the elimination of waste products through the excretory system. The kidneys filter waste from the blood and excrete it in urine, which is predominantly made up of water.

Moreover, water is crucial for maintaining body temperature through the process of sweating. When body temperature increases, sweat glands secrete sweat, which is primarily water. As sweat evaporates from the skin’s surface, it cools the body, thereby aiding in thermoregulation.

In summary, water is indispensable to metabolism. It serves as a solvent for biochemical reactions, a reactant in various metabolic processes, a transporter of nutrients and waste, and a regulator of body temperature. The absence of water would hinder these vital metabolic functions, underscoring the importance of staying hydrated for optimal health.