Week 2, Session 3: The Chemistry That Makes Life Possible

Estimated time: 75 minutes
NGSS connection: HS-LS1-6

Start small

The water in a glass looks simple, but every drop contains more molecules than you could count in a lifetime. Those molecules attract one another, dissolve substances, absorb heat, and participate in reactions. Much of biology becomes easier once you can explain why water behaves as it does.

By the end, you can

• connect atomic structure to chemical bonding;
• explain how polarity and hydrogen bonding give water life-supporting properties; and
• interpret the pH scale as a measure of hydrogen-ion concentration.

Learn the idea

Atoms contain positively charged protons and neutral neutrons in a nucleus, surrounded by negatively charged electrons. The electrons farthest from the nucleus, called valence electrons, strongly influence bonding. Atoms become more stable by sharing or transferring electrons.

In a covalent bond, atoms share electrons. In an ionic bond, one atom loses electrons and another gains them, producing oppositely charged ions that attract. Within a water molecule, oxygen pulls shared electrons more strongly than hydrogen. Oxygen becomes slightly negative and the hydrogens slightly positive. Water is therefore polar.

The weak attraction between the slightly positive hydrogen of one molecule and a slightly negative region of another is a hydrogen bond. Individually these bonds are weak. Together they give water remarkable properties. Cohesion helps water molecules cling to one another, supporting continuous water columns in plants. Adhesion helps water cling to other surfaces. Water's high specific heat allows it to absorb substantial energy before its temperature changes, which stabilizes oceans and body temperatures. Evaporation removes heat because the fastest molecules escape first.

Water is an excellent solvent for ions and many polar molecules. Its charged regions surround solute particles and separate them. Nonpolar substances such as oil do not mix well with water. This difference helps cell membranes form spontaneously.

Acids increase hydrogen-ion concentration, while bases reduce it or increase hydroxide ions. The pH scale is logarithmic. A solution at pH 4 has ten times the hydrogen-ion concentration of pH 5 and one hundred times that of pH 6. Cells use buffers to resist sudden pH changes because protein shapes and reaction rates depend on a narrow chemical range.

Carbon is another star of biological chemistry. With four valence electrons available for bonding, carbon can form chains, branches, rings, and double bonds. This versatility provides the backbone for life's large molecules.

Study the visual

Alt text: Water molecules form attractions around dissolved particles beside a protein, with familiar acidic, neutral, and basic samples nearby.

Look for the bent shape of water and the unequal charge distribution it creates. Explain why the water molecules would orient their oxygen side toward a positive ion and their hydrogen side toward a negative ion.

At-home model

Place one drop of water on wax paper and one on a clean glass plate. Without tasting anything, compare their shapes from the side. Water usually beads more strongly on wax because cohesion within the drop is stronger than adhesion to the nonpolar wax. Dry the surfaces afterward.

Check yourself

1. Why is water polar even though its total charge is zero?
2. Which property helps a pond resist rapid temperature change?
3. How many times more acidic is pH 3 than pH 5?
4. Why can a major pH change harm a cell?

Answer and feedback

1. Electrons are shared unequally and the molecule is bent, so partial charges do not cancel spatially.
2. High specific heat.
3. One hundred times in hydrogen-ion concentration.
4. Charge patterns and protein shapes change, interfering with enzymes, membranes, and reactions.

Key vocabulary: valence electron, covalent bond, ion, polarity, hydrogen bond, cohesion, solvent, pH, buffer

Exit reflection: Which property of water seems most important to your own cells, and why?