Biology I Chapter 2 The Chemistry of Life

The Water Molecule Like other molecules, water (H2O) is neutral. The positive charges on its 10 protons balance out the negative charges on its 10 electrons. Because of the angles of its chemical bonds, the oxygen atom is on one end of the molecule and the hydrogen atoms are on the other.

The Water Molecule With 8 protons in its nucleus, an oxygen atom has a much stronger attraction for electrons than does a hydrogen atom with its single proton so there is a greater probability of finding the shared electrons in water close to its oxygen atom than near its hydrogen atoms. As a result, the oxygen end of the molecule has a slight negative charge and the hydrogen end of the molecule has a slight positive charge.

Hydrogen Bonding Because of their partial positive and negative charges, polar molecules such as water can attract each other. The attraction between a hydrogen atom on one water molecule and the oxygen atom on another is known as a hydrogen bond.

Adhesion Adhesion between water and glass also causes water to rise in a narrow tube against the force of gravity. This effect is called capillary action. Capillary action is one of the forces that draws water out of the roots of a plant and up into its stems and leaves. Cohesion holds the column of water together as it rises.

Cohesion Since water molecules are attracted to each other to each other by hydrogen bonding, they tend to stick together: this property is called cohesion. Because of this cohesion, water has high surface tension. If you have ever seen an insect walking on the surface of a pond, and wondered what was going on, it is because of surface tension. Hydrogen bonds are holding the molecules of water together, forming a surface "skin" that an insect can walk on (if it spreads its weight out over the surface). Surface tension is responsible for water sticking together in small drops. In a slowly dripping faucet, watch the drop get bigger and bigger, until it finally falls into the sink. This happens because the attraction of the water molecules to each other is strong, holding them together until the drop gets to be a certain size, when the pull of gravity overcomes the cohesion of the water molecules.

Heat Capacity Because of the multiple hydrogen bonds between water molecules, it takes a large amount of heat energy to cause those molecules to move faster and raise the temperature of the water. Water’s heat capacity, the amount of heat energy required to increase its temperature, is relatively high. Large bodies of water, such as oceans and lakes, can absorb large amounts of heat with only small changes in temperature. This protects organisms living within from drastic changes in temperature. At the cellular level, water absorbs the heat produced by cell processes, regulating the temperature of the cell.

Solutions and Suspensions Water is not always pure; it is often found as part of a mixture. A mixture is a material composed of two or more elements or compounds that are physically mixed together but not chemically combined. Living things are in part composed of mixtures involving water. Two types of mixtures that can be made with water are solutions and suspensions.

Solutions If a crystal of table salt is placed in water, sodium and chloride ions on the surface of the crystal are attracted to the polar water molecules. Ions break away from the crystal and are surrounded by water molecules. The ions gradually become dispersed in the water, forming a type of mixture called a solution. In a saltwater solution, table salt is the solute—the substance that is dissolved. Water is the solvent—the substance in which the solute dissolves.

Suspensions Some materials do not dissolve when placed in water, but separate into pieces so small that they do not settle out. Such mixtures of water and nondissolved material are known as suspensions. Some of the most important biological fluids are both solutions and suspensions. Blood is mostly water. It contains many dissolved compounds, but also cells and other undissolved particles that remain in suspension as the blood moves through the body.

Acids, Bases, and pH Water molecules sometimes split apart to form hydrogen ions and hydroxide ions. This reaction can be summarized by a chemical equation in which double arrows are used to show that the reaction can occur in either direction.

The pH Scale Chemists devised a measurement system called the pH scale to indicate the concentration of H+ ions in solution. The pH scale ranges from 0 to 14. At a pH of 7, the concentration of H+ ions and OH– ions is equal. Pure water has a pH of 7.

The pH Scale Each step on the pH scale represents a factor of 10. For example, a liter of a solution with a pH of 4 has 10 times as many H+ ions as a liter of a solution with a pH of 5. A solution with a pH of 3 has 100 times as many H+ ions as a liter of a solution with a pH of 5.

Buffers The pH of the fluids within most cells in the human body must generally be kept between 6.5 and 7.5 in order to maintain homeostasis. If the pH is lower or higher, it will affect the chemical reactions that take place within the cells. One of the ways that organisms control pH is through dissolved compounds called buffers, which are weak acids or bases that can react with strong acids or bases to prevent sharp, sudden changes in pH.