a solvent to many other molecules. This means that many chemicals that are important to life
are readily dissolved in water and can be distributed throughout an organism due to its
movement.
Water also has properties of cohesion and adhesion. Cohesion occurs when molecules of the
same kind tend to stick together. In water, this is due to hydrogen bond cohesion between water
molecules. Cohesion causes surface tension, which is the tendency of liquid surfaces to shrink to
minimize surface area. This is due to water molecules at the water-air surface interfacing and
forming stronger hydrogen bonds with water molecules below, causing a shrinking of the space
between them. Surface tension causes water droplets to form and allows solid matter to float at
the surface of water.
Adhesion, on the other hand, is the tendency of dissimilar molecules to be attracted to each
other. Adhesive forces can be strong between water and charged molecules and are responsible
for capillary action, which is the movement of liquid through spaces on its own, sometimes in
opposition to gravity. Capillary action is the result of adhesive forces between water and the
surface it is touching, which draws the liquid towards it. Due to cohesive forces, the water also
pulls more water molecules behind it. These properties of water are essential to all life on Earth.
For example, in plants, capillary action is responsible for moving water from the roots up through
the rest of the plant.
Carbon, hydrogen, nitrogen, and oxygen comprise 99% of all living matter. Organic molecules,
which include most molecules with carbon, are the basis of life on Earth.
Carbon has the unique chemical property of being able to form four bonds with other elements,
making it an ideal element to form the backbone of complicated biological molecules. Carbon-
based molecules are able to take on many configurations, as carbon can form single, double, or
triple bonds with other elements. These molecules can take on many shapes: rings, branches, or
long chains. Thus, carbon is the elemental basis of the major biological macromolecules:
carbohydrates, proteins, lipids, and nucleic acids. In addition to carbon, nucleic acidsand proteins
rely on nitrogen and phosphorus to build their structure, which we will discuss in more detail
below.
The Makeup and Properties of Macromolecules
Large biological molecules are the building blocks of life. For your AP exam, you should be familiar
with carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates, proteins, and nucleic acids
are usually types of molecules called polymers, which are structures made of repeating smaller
units called monomers. The monomers that make DNA are nucleotides, amino acids make
proteins, and sugars make carbohydrates. The monomer units in each of these cases are not
necessarily identical but are of the same kind of molecule. Large polymers are also called
macromolecules. Lipids, on the other hand, are not generally polymers, thus are not always
