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Water on Mars ! Water on Mars! Evidences of water has been noticed on Mars. Mars is now a desert world, but erosion features seen on Mars today suggest that water once flowed there. The water was lost due to the thin Martian atmosphere and weak gravity. The abundance of water is a major reason Earth is habitable. Your objective in this topic is to develop a conceptual understanding of how water contributes to the fitness of Earth for life.

Learning Objectives

After completing the topic, the student will be able to:

  • Explore why water is considered as a fundamental factor for the existence of life on Earth.
  • Analyze how water's solid, liquid, and vapor state allow life to exist on Earth.
  • Deduce how the polarity of water molecules results in hydrogen bonding.
  • List four emergent properties of water that contribute to Earth's fitness for life.
  • Examine how dissociation of water molecules leads to acidic and basic conditions that affect living organisms.
  • Analyze how buffers prevent large pH changes in solutions.
  • Examine the cause, threat and prevention from acid precipitation.
View of Earth A view of Earth from space, showing our planet's abundance of water Water covers around 70 per cent of the Earth’s surface. This is why Earth looks blue from space, and why it is often called the Blue Planet.
Water - Elixir of life

Water (H2O) is fundamental to the existence of life as we know it. Water is the molecule that supports all of our life. As astronomers study newly discovered planets orbiting distant stars, they hope to find evidence of water on these far‐off celestial bodies, for water is the substance that makes possible life as we know it, here on Earth.

All organisms familiar to us are made mostly of water and live in an environment dominated by water. Water is the biological medium here on Earth, and possibly on other planets as well.

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co‐exists on Earth with its solid state, ice, and gaseous state (water vapor or steam). Water also exists in a liquid crystal state near hydrophilic surfaces.

Water in three states Water in three states Water in three states: liquid, solid (ice), and (invisible) water vapor in the air. Clouds are accumulations of water droplets, condensed from vapor–saturated air.
Abundance of Water

Life on Earth began in water and evolved there for 3 billion years before spreading onto land. Modern life, even terrestrial (land‐dwelling) life, remains tied to water. All living organisms require water more than any other substance. Human beings, for example, can survive for quite a few weeks without food, but only a week or so without water. Molecules of water participate in many chemical reactions necessary to sustain life. Most cells are surrounded by water, and cells themselves are about 70–95% water. Three‐quarters of Earth's surface is submerged in water. Although most of this water is in liquid form, water is also present on Earth as ice and vapor. Water is the only common substance to exist in the natural environment in all three physical states of matter: solid, liquid, and gas.

The abundance of water is a major reason Earth is habitable. In this chapter, we will learn how the structure of a water molecule allows it to form weak chemical bonds with other molecules, including other water molecules. This ability leads to unique properties that support and maintain living on our planet.

Our objective in this chapter is to develop a conceptual understanding of how water contributes to the fitness of Earth for life. As mentioned earlier, water is the most abundant substance in living systems, making up 70% or more of the weight of most organisms. The first living organisms doubtless arose in an aqueous environment, and the course of evolution has been shaped by the properties of the aqueous medium in which life began. This chapter begins with descriptions of the physical and chemical properties of water, to which all aspects of cell structure and function are adapted. The attractive forces between water molecules and the slight tendency of water to ionize are of crucial importance to the structure and function of biomolecules.

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