key concept2.4Water Is Critical for Life

Your body is more than 70 percent water by weight, excluding the minerals contained in bones. Water is the dominant component of virtually all living organisms, and most biochemical reactions take place in this watery, or aqueous, environment. Water is an unusual substance with unusual properties. Under conditions on Earth, water exists in solid, liquid, and gas forms, all of which have relevance to living systems. Water allows chemical reactions to occur inside living organisms, and it is necessary for the formation of certain biological structures. In this section we will explore how the structure and interactions of water molecules make water essential to life.

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  • Biochemical reactions within cells occur in an aqueous environment.

  • Moles are used to quantify substances in biological fluids.

  • The acid–base properties of water and other compounds in cells allow reversible changes that affect biological functions.