This chapter describes the structure of the water molecule and explores the many ways that polar covalent bonds and hydrogen bonds among water molecules affect organisms and their interactions with their environments. In addition, this chapter discusses topics including concentrations of solutions, hydrogen ion concentration (pH), and buffer solutions. | Chapter 3 Water and the Fitness of the Environment Overview: The Molecule That Supports All of Life Water is the biological medium on Earth. All living organisms require water more than any other substance Most cells are surrounded by water, and cells themselves are about 70–95% water. The abundance of water is the main reason the Earth is habitable. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Why does the abundance of water allow life to exist on the planet Earth? Figure Why does the abundance of water allow life to exist on the planet Earth? The polarity of water molecules results in hydrogen bonding The water molecule is a polar molecule: The opposite ends have opposite charges. Polarity allows water molecules to form hydrogen bonds with each other. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hydrogen bonds between water molecules Hydrogen bond – H + H O —— —— + + + – – – Figure Hydrogen bonds between water molecules Four emergent properties of water contribute to Earth’s fitness for life Four of water’s properties that facilitate an environment for life are: Cohesive behavior Ability to moderate temperature: Stable Expansion upon freezing: Less dense Ice floats Versatility as a solvent. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Cohesion Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion. Cohesion helps the transport of water against gravity in plants. Adhesion is an attraction between different substances, for example, between water and plant cell walls. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Water transport in plants Water-conducting cells Adhesion Cohesion 150 µm Direction of water movement Figure Water transport in plants Surface tension is a measure of how hard it is to break the surface of a liquid. . | Chapter 3 Water and the Fitness of the Environment Overview: The Molecule That Supports All of Life Water is the biological medium on Earth. All living organisms require water more than any other substance Most cells are surrounded by water, and cells themselves are about 70–95% water. The abundance of water is the main reason the Earth is habitable. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Why does the abundance of water allow life to exist on the planet Earth? Figure Why does the abundance of water allow life to exist on the planet Earth? The polarity of water molecules results in hydrogen bonding The water molecule is a polar molecule: The opposite ends have opposite charges. Polarity allows water molecules to form hydrogen bonds with each other. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Hydrogen bonds between water molecules Hydrogen bond – H + H O —— —— + + + – –
