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Water: The Vital Medium for Life on Earth

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  1. Chapter 3 Water & The Fitness of the Environment

  2. You Must Know • 1) The importance of hydrogen bonding to the properties of water • 2) Four unique properties of water, and how each contributes to life on Earth • 3) How to interpret the pH scale • 4) The importance of buffers in biological systems

  3. WATER!!! • 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

  4. 3.1 • The water molecule is a polar molecule: The opposite ends have opposite charges • Oxygen is slightly negative, Hydrogen is slightly positive • The Polarity of water molecules results in hydrogen bonding • Hydrogen bonds form between (-) Oxygen & (+) Hydrogen on ANOTHER water molecule • Can form a maximum of 4 hydrogen bonds at a time • The structure of water is the key to its properties • One Oxygen & Two Hydrogen’s bonded to form a molecule

  5. 3.2 • Four properties of water (the key are the hydrogen bonds) • Cohesive behavior • Ability to moderate temperature • Expansion upon freezing • Versatility as a solvent

  6. 1) Cohesion • Linking of like molecules • “water molecule joined to water molecule” • Allows for surface tension

  7. Adhesion • Clinging of one substance to another • “water molecule attached to some other molecule” • Water droplets on a mirror • Transpiration • Movement of water molecules up thin tubes (xylem) and evaporation through the openings in leaves • Uses Adhesion (xylem) & Cohesion (water to water)

  8. 2) Ability to moderate temperature • Kinetic energy is the energy of motion • Heat is a measure of the total amount of kinetic energy due to molecular motion • Temperature measures the intensity of heat due to the average kinetic energy of molecules

  9. Specific Heat: • Amount of heat required to raise or lower the temperature of a substance by 1 degree Celsius • Relative to most materials, the temp of water changes less when heat is lost or absorbed • This causes a high specific heat

  10. Water’s high specific heat can be traced to hydrogen bonding • Heat is absorbed when hydrogen bonds break • Heat is released when hydrogen bonds form

  11. This high specific heat results in: • Stable temperature of Earth’s oceans • Enables Earth to support vast quantities of both plant & animal life

  12. 3) Expansion upon freezing • Water is LESS dense as a solid than as a liquid (opposite in most other substances) • This is why ice floats • This keeps large bodies of water from freezing solid – this moderates temperatures & allows for LIFE

  13. 4) Versatility as a solvent • A solution is a liquid that is a homogeneous mixture of substances • A solvent is the dissolving agent of a solution • The solute is the substance that is dissolved • An aqueous solution is one in which water is the solvent

  14. A hydrophilic substance is one that has an affinity for water • A hydrophobic substance is one that does not have an affinity for water • Oil molecules are hydrophobic because they have relatively nonpolar bonds • A colloid is a stable suspension of fine particles in a liquid

  15. Most biochemical reactions occur in water • Chemical reactions depend on collisions of molecules and therefore on the concentration of solutes in an aqueous solution

  16. 3.3 • Acidic & Basic conditions affect living organisms • An acid is any substance that increases the H+ concentration of a solution • A base is any substance that reduces the H+ concentration of a solution

  17. A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other: • The hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H+) • The molecule with the extra proton is now a hydronium ion (H3O+), though it is often represented as H+ • The molecule that lost the proton is now a hydroxide ion (OH–)

  18. Changes in concentrations of H+ and OH– can drastically affect the chemistry of a cell • Concentrations of H+ and OH– are equal in pure water • Adding certain solutes, called acids and bases, modifies the concentrations of H+ and OH–

  19. pH Scale • In any aqueous solution at 25°C the product of H+ and OH– is constant and can be written as [H+][OH–] = 10–14 • The pH of a solution is defined by the negative logarithm of H+ concentration, written as pH = –log [H+] • For a neutral aqueous solution [H+] is 10–7 = –(–7) = 7

  20. Acidic solutions have pH values less than 7 • Basic solutions have pH values greater than 7 • Most biological fluids have pH values in the range of 6 to 8

  21. The internal pH of most living cells must remain close to pH 7 • Buffers are substances that minimize changes in concentrations of H+ and OH– in a solution • Most buffers consist of an acid-base pair that reversibly combines with H+

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