Human Anatomy and Physiology

The water molecule

1.0 Introduction 1.0 Introduction 1.0 Introduction1.0 Introduction 1.0 Introduction 1.0 Introduction 1.0 Introduction

In this unit you will study the structure and properties of the water molecule. You will also study the biological implications of the properties of the water molecule on living things.

Can you imagine yourself going for a whole month without water? Well, the conclusion of such action is obvious! Water is extremely important for the existence of life on this planet. I am sure you know that water is a vital chemical constituent of living cells. And that it provides an environment for those organisms living in water. In fact about 70% of the earths’ surface is covered by water.

Some people say water is life. What is your comment concerning this statement?

During and upon completion of this unit you will be able to:

 Explain the partial dipolar nature of the water molecule and how it comes about.

 Explain how the tetrahedral structure of the water molecule arises

 Explain the relationship between the structure of the water molecule and the properties of water.

 Explain the biological implications of the different properties of water on the environment and living organism.

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In our study we will look at the structure of the water molecule and its biological importance.

Listed below are some of the properties of water that we will study:

i. Density and freezing properties

ii. High heat capacity

iii. High heat of vaporization

iv. High heat of fusion

v. Universal solvent properties

vi. High surface tension

When you read some A- level biology books, you will find some more properties of water. However, in this module we will limit ourselves only to the properties listed above.

1.2 Structure of the water molecule.

Before we study the properties of water and their biological implications, we need to know and understand the structure of the water molecule. This is because the properties of water arise from the structure of the water molecule.

The water molecule is formed from two hydrogen atoms and one oxygen atom joined together as shown in fig 1.1 below.

WATER MOLECULE

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Fig 1.1 shows the structure of a water molecule.

The bond formed between hydrogen and the oxygen atom is a covalent bond. The oxygen and the hydrogen atoms each contribute an electron to the shared pair of electrons. The oxygen atom, however, being more electronegative, pulls the shared electrons closer to its self and further away from the hydrogen atom. The result of this is that the oxygen atom assumes a partial negative charge while the hydrogen atom takes a partial positive charge. See fig 1.2

The atoms in the water molecule are arranged with the two H-O bonds at an angle of about 105° rather than forming a linear structure. The asymmetrical shape of the molecule arises from a tendency of the four electron pairs in the valency shell of oxygen to arrange themselves symmetrically at the vertices of a tetrahedron around the oxygen nucleus. The two pairs associated with covalent bonds holding the hydrogen atoms are drawn together slightly, resulting in the angle of 105° between these bonds. This arrangement results in a polar molecule, since there is a net negative charge toward the oxygen end (the apex) of the V-shaped molecule and a net positive charge at the hydrogen end. The electric dipole gives rise to attractions between neighbouring opposite ends of water molecules, with each oxygen being able to attract two nearby hydrogen atoms of

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two other water molecules. Such hydrogen bonding, as it is called, has also been observed in other hydrogen compounds. Although considerably weaker than the covalent bonds holding the water molecule together, hydrogen bonding is strong enough to hold the water molecules together and therefore keep water as a liquid at ordinary temperatures; its low molecular weight would normally tend to make it a gas at such temperatures.

Various other properties of water, such