Biology (Holt)

Water in Living Things

You may not realize it, but nearly 70 percent of your body is made
of water. About two-thirds of the molecules in your body are water
molecules. Your body’s cells are filled with water, and water is the
medium in which most cellular events take place. Your cells are also
surrounded by water, and water helps move nutrients and other sub-
stances into and out of your cells. What are some of the properties
of water that make it such an important substance for life?

Storage of Heat

Water heats more slowly and retains heat longer than many other
substances. For example, a pot of boiling water removed from a
stove takes a long time to cool down to room temperature. Many
organisms release excess heat through water evaporation. For exam-
ple, humans cool themselves by sweating. The water vapor lost
through the evaporation of sweat carries heat away from the body. In
organisms, this ability to control temperature enables cells to main-
tain a constant internal temperature when the external temperature
changes drastically. Water thus helps cells maintain homeostasis.

Cohesion and Adhesion

The hydrogen bonds between water molecules cause the cohesion of
liquid water. (koh HEE zhuhn)is an attraction between
substances of the same kind. Because of cohesion, water and other
liquids form thin films and drops, such as those shown in Figure 5.
Molecules at the surface of water are linked together by hydrogen
bonds like a crowd of people linked by holding hands. This attrac-
tion between water molecules causes a condition known as surface
tension. Surface tension prevents the surface of water from stretch-
ing or breaking easily.
Water molecules are also attracted to many
other similarly polar substances. (ad
HEE zhuhn)is an attraction between different
substances. Because of adhesion, some sub-
stances get wet. Adhesion powers a process,
called capillary action, in which water mol-
ecules move upward through a narrow tube,
such as the stem of a plant. The attraction of
water to the walls of the tube sucks the water
up more strongly than gravity pulls it down.
Water moves upward through a plant from
roots to leaves through a combination of cap-
illary action, cohesion, and other factors.

Adhesion

Cohesion

Water and Solutions Section 2

Objectives

Analyzethe properties of

water.

Describehow water

dissolves substances.

Distinguishbetween acids

and bases.

Key Terms

cohesion

adhesion

solution

acid

base

Figure 5 Cohesion.

Because of cohesion, water

forms drops like those on this

plant.

SECTION 2 Water and Solutions 31