Water, like carbon, has a special role in living things. It is needed by all known forms of life. Water is a simple molecule, containing just three atoms. Nonetheless, water’s structure gives it unique properties that help explain why it is vital to all living organisms.
Water, Water Everywhere
Water is a common chemical substance on planet Earth. In fact, Earth is sometimes called the "water planet" because almost 75% of its surface is covered with water. If you look at Figure below , you will see where Earth’s water is found. The term water generally refers to its liquid state, and water is a liquid over a wide range of temperatures on Earth. However, water also occurs on Earth as a solid (ice) and as a gas (water vapor).
Chemical Structure of Water
To understand some of water’s properties, you need to know more about its chemical structure. As you have seen, each molecule of water consists of one atom of oxygen and two atoms of hydrogen. The oxygen atom in a water molecule attracts negatively-charged electrons more strongly than the hydrogen atoms do. As a result, the oxygen atom has a slightly negative charge, and the hydrogen atoms have a slightly positive charge. A difference in electrical charge between different parts of the same molecule is called polarity , making water a polar molecule .
Opposites attract when it comes to charged molecules. In the case of water, the positive (hydrogen) end of one water molecule is attracted to the negative (oxygen) end of a nearby water molecule. Because of this attraction, weak bonds form between adjacent water molecules. The type of bond that forms between molecules is called a hydrogen bond .
Water and Life
The human body is about 70% water (not counting the water in body fat, which varies from person to person). The body needs all this water to function normally. Just why is so much water required by human beings and other organisms? Water can dissolve many substances that organisms need, and it is necessary for many biochemical reactions. The examples below are among the most important biochemical processes that occur in living things, but they are just two of many ways that water is involved in biochemical reactions.
Photosynthesis—In this process, cells use the energy in sunlight to change carbon dioxide and water to glucose and oxygen. Water is a reactant in this process. The reactions of photosynthesis can be represented by the chemical equation
6CO 2 + 6H 2 O + Energy → C 6 H 12 O 6 + 6O 2 .
Cellular respiration—In this process, cells break down glucose in the presence of oxygen and release carbon dioxide, water (a product), and energy. The reactions of cellular respiration can be represented by the chemical equation
C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + Energy.
Water is involved in many other biochemical reactions. As a result, just about all life processes depend on water. Clearly, life as we know it could not exist without water.
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