Importance of Water to Living Things

1. Chapter 3 Water

2. Why is Water Important to Living Things? • Water is the substance that makes possible life as we know it here on Earth • Covers ¾ of the Earth’s Surface • Makes up 70-95% of cells; most cells are surrounded by water • Only common substance to exist in all three physical states of matter – Solid, Liquid, and Gas – in the natural environment.

3. Polarity and Hydrogen Bonding • The O and H in a water molecules are held together by polar covalent bond • Polarity – Unequal charge distribution due to differences in electronegativity. O is more electronegative than H. • Hydrogen Bonding – Weak, intermolecular, transient attraction Each water molecule can make up to 4 bonds with other water molecules

4. Cohesion, Adhesion • Cohesion – Hydrogen bonds hold water molecules together. One water molecule can bond with up to 4 other water molecules at one time. • Adhesion – Allow water molecules to adhere to hydrophilic surfaces. Benefit to Life • Transport of water through the xylem of plants against gravity.

5. Water Cohesion/Adhesion • Allows water to move as continuous column upward through stems of plants

6. Surface Tension • Surface Tension – a measure of how difficult it is to stretch or break the surface of a liquid • Water has a greater surface tension than most other liquids • At the interface between water and air is an ordered arrangement of water molecules, hydrogen-bonded to one another and to the water below Benefit to Life • Some animals can stand, walk, or run on water without breaking the surface

7. Heat and Temperature • Anything that moves has kinetic energy, the energy of motion • The faster a molecule moves, the greater its kinetic energy. • Heat is a measure of the total quantity of kinetic energy due to molecular motion • Temperature measures the intensity of heat due to the average kinetic energy of the molecules – when the average speed of the molecules increases, a thermometer records this as a rise in temperature

8. High Specific Heat of Water • Amount of heat that must be absorbed or lost for 1g of substance to change its temp. by 1oC • For water, the specific heat is 1cal/g/oC • Water resists changes in its temp. • Heat is absorbed to break H-bonds; released when bonds form.

9. High Specific Heat Benefit to Life • Large bodies of water (i.e. oceans) can absorb and store a huge amount of heat. In winter, heat lost to the air can warm it and make coastal areas milder in climate than inland regions. • Stabilizes temperatures – favorable environment for life – keeps land/water temperatures within limits that permit life. • Living things are mostly water – resists change in temperature and maintains somewhat constant temp.

10. High Heat of Vaporization • Energy required to change 1g of substance from a liquid to a gas. • Water has a high heat of vap. – it takes a lot of energy to vaporize it. (580 cal at 25oC for 1 g of water) • An increase in heat breaks H-bonds releasing molecules to a gaseous state. Benefit to Life • The solar heat absorbed by tropical seas causes evaporation of water. As the moist air travels towards the poles it condenses to form rain

11. Evaporative Cooling • Water molecules with high kinetic energy (the “hottest” ones) evaporate (change from a liquid to a gas); remaining molecules are cooler. Benefit to Life • Stabilizes temperatures in lake and ponds. • Prevent terrestrial organisms from overheating. • High humidity prevents evaporation and sweat and makes us more uncomfortable.

12. Ice Floats • Water expands as it freezes; less dense as a solid (fewer molecules for same volume) • Hydrogen bonds in ice keep the molecules far enough apart to make ice less dense than liquid water at 4oC or above. Benefit to Life • Floating ice insulates bodies of water so they don’t freeze solid – keeps the water below the ice from the colder air

13. Ice Floats

14. Versatile Solvent • Polar water molecules cause ions and other polar solutes to dissolve by ionization.

15. – – + + + + – + – + Na+ – + + + + – Cl– + – – + – + + + – + + – + Spheres of Hydration Keeping Ions Separate = Dissolving Power

16. Versatile Solvent • Solution – a liquid that is a completely homogeneous mixture of two or more substances • Solvent – the dissolving agent of a solution • Solute – the substance that is dissolved • Aqueous solution- solution where water is the solvent Benefit to Life • Most chemical reactions in living things involve solutes dissolved in water

17. To Dissolve or Not to Dissolve? • Hydrophilic: A substance with an affinity/able to dissolve in water. • What kind of substances? • Hydrophobic: A substance that repels water/don’t dissolve. • What kind of substances?

18. Dissociation/Ionization • 2H2O H3O + OH- • H2O H+ + OH- • In pure water, one water molecule in every 554 million is dissociated. • The pH scale is based on the concentrations of H+ and OH- in solution

19. Acids and Bases • Acid – when dissolved in water, add H+ ions to the solution. HCl H+ + Cl- pH< 7 • Base – reduces the hydrogen ion concentration of a solution; work by either accepting hydrogen ions or dissociating to form hydroxide ions. NaOH Na+ + OH- pH>7

20. Response to a rise in pH Response to a drop in pH An Important Biological Buffer • Buffer – minimize changes in concentration of H+ and OH- ions in a solution • Buffers normally maintain the pH of human blood very close to 7.4 • Work by accepting H+ ions from the solution when they are in excess and donating H+ ions to a solution when they have been depleted H2CO3 HCO3- + H+ Carbonic Acid Bicarbonate ion Hydrogen ion H+ Donor (acid) H+ Acceptor (base)

21. Carbonic Acid-Bicarbonate Buffer System • When blood pH rises, carbonic acid dissociates to form bicarbonate and H+ H2C03 -----> HC03- + H+ • When blood pH drops, bicarbonate binds H+ to form carbonic acid HC03- + H+ -----> H2C03

22. Acid Precipitation • Mostly sulfur oxides and nitrogen oxides dissolved in rainwater. • Come from fossil fuels burned in factories and automobiles • Aquatic animal’s egg and young are vulnerable to low pH levels – can alter the structure of biochemical molecules and prevent them from carrying out essential chemical processes of life.

23. Acid Precipitation