Which substance makes up 50-70 percent of the body's weight and is essential for virtually every function of the cells and body?

• A. Water
• B. Proteins
• C. Lipids
• D. Carbohydrates

Answer: (A)

Water is the substance that makes up roughly half to two-thirds of the body's weight, and it is essential for almost every function of the cells and body. It serves as the universal solvent, dissolving ions and molecules so metabolic reactions can occur, and it provides the medium for transport of nutrients, hormones, and wastes in blood and across tissues. Water participates directly in many chemical reactions, including hydrolysis and condensation, and it helps maintain cell shape and turgor, lubricates joints, cushions organs, and stabilizes temperature through sweating and heat absorption. It also supports blood volume and tissue hydration, which are critical for circulation and metabolism. Although other macronutrients like proteins, lipids, and carbohydrates are important, they comprise a smaller portion of body weight, not the majority, making water the best answer.