Chapter 2 WATER, THE SOLVENT OF LIFE
Water is the most abundant substance in living systems, making up 70% or more of the weight of most organisms. The first living organisms on Earth doubtless arose in an aqueous environment, and the course of evolution has been shaped by the properties of the aqueous medium in which life began.
This chapter begins with descriptions of the physical and chemical properties of water, to which all aspects of cell structure and function are adapted. The attractive forces between water molecules and the slight tendency of water to ionize are of crucial importance to the structure and function of biomolecules. We review the topic of ionization in terms of equilibrium constants, pH, and titration curves, and we consider how aqueous solutions of weak acids or bases and their salts act as buffers against pH changes in biological systems. The water molecule and its ionization products, and , profoundly influence the structure, self-assembly, and properties of all cellular components, including proteins, nucleic acids, and lipids. The noncovalent interactions responsible for the strength and specificity of “recognition” among biomolecules are decisively influenced by water’s properties as a solvent, including its ability to form hydrogen bonds with itself and with solutes.
This chapter emphasizes the following principles:
The solvent properties of water shaped the evolution of living things. Most small intermediates of metabolism, as well as nucleic acids and proteins, are soluble in water. Lipid bilayers, the likely forerunners of biological membranes, form spontaneously in water and are stabilized by their interaction with it. Although hydrogen bonds, ionic interactions, and the hydrophobic effect are individually weak, their combined effects powerfully influence the three-dimensional shape and stability of biological molecules and structures.
The ionization behavior of water and of weak acids and bases dissolved in water can be represented by one or more equilibrium constants. Most biomolecules are ionizable; their structure and function depend on their ionization state, which is characterized by equilibrium constants.
An aqueous solution of a weak acid and its salt makes a buffer that resists changes in pH in response to added acid or base. Biological systems are buffered to maintain a narrow pH range, in which their macromolecules retain their functional structure, which depends on their ionization state. Conditions that produce blood pH outside the range of 7.3 to 7.5 are life-threatening in humans.
Enzymes, which catalyze all of the processes inside a cell, have evolved to function optimally at near-neutral (physiological) pH. However, enzymes that function in intracellular compartments of low or high pH show their greatest activity and stability at those pH values.
The solvent properties of water shaped the evolution of living things. Most small intermediates of metabolism, as well as nucleic acids and proteins, are soluble in water. Lipid bilayers, the likely forerunners of biological membranes, form spontaneously in water and are stabilized by their interaction with it. Although hydrogen bonds, ionic interactions, and the hydrophobic effect are individually weak, their combined effects powerfully influence the three-dimensional shape and stability of biological molecules and structures.
The ionization behavior of water and of weak acids and bases dissolved in water can be represented by one or more equilibrium constants. Most biomolecules are ionizable; their structure and function depend on their ionization state, which is characterized by equilibrium constants.
An aqueous solution of a weak acid and its salt makes a buffer that resists changes in pH in response to added acid or base. Biological systems are buffered to maintain a narrow pH range, in which their macromolecules retain their functional structure, which depends on their ionization state. Conditions that produce blood pH outside the range of 7.3 to 7.5 are life-threatening in humans.
Enzymes, which catalyze all of the processes inside a cell, have evolved to function optimally at near-neutral (physiological) pH. However, enzymes that function in intracellular compartments of low or high pH show their greatest activity and stability at those pH values.