Chapter 3 Water and the Fitness of the Environment AP minknow The importance of hydrogen bonding to the properties of water. Four unique properties of water, and how each contributes to life on earth. How to interpret the pH scale. The importance of buffers in biological systems.

Checkup Using bohr models draw the reaction of hydrogen and oxygen to form water. How many neutrons does an oxygen atom have? What is the difference between mass number and atomic mass? What is an isotope? What is a radioactive isotope? What is one way a radioactive isotope can be used?

Water: The Molecule That Supports All of Life Water is the biological medium here on Earth All living organisms require water more than any other substance Three-quarters of the Earth’s surface is submerged in water The abundance of water is the main reason the Earth is habitable

3.1: The polarity of water molecules results in hydrogen bonding Allows them to form hydrogen bonds with each other Contributes to the various properties water exhibits Hydrogen bonds + H  –

3.2: Four emergent properties of water contribute to Earth’s fitness for life Cohesion Moderation of Temperature Insulation of bodies of water by floating ice The solvent of life (universal solvent)

1. Cohesion Cohesion – the hydrogen bonds holding a substance together. (water – water) Adhesion – the hydrogen bonds holding one substance to another. (water – glass) Capillary Action – water transport in plants. Uses Cohesion and Adhesion Transpiration Surface tension – measure of how difficult it is to stretch or break the surface of a liquid. Water has a greater surface tensions than most liquids

2. Moderation of Temperature Kinetic Energy – energy of motion Thermal Energy (heat) – total energy within a substance Calorie – amount of heat energy to heat 1g water by 1°C Kcal – 1000c Temperature – average kinetic energy per molecule (Celsius Scale)

2. Moderation of Temperature Specific heat – the amount of heat absorbed or loss for 1g of a substance to change its temperature by 1°C Water has high specific heat capacity compared to other substances 1 cal/g/°C

2. Moderation of Temperature Evaporation Heat of vaporization – the amount of heat 1g of a liquid must absorb to be converted to a gas Evaporative cooling – as a liquid evaporates the surface of the remaining liquid cools This occurs because the “hottest” molecules leave

3. Insulation of bodies of water by floating ice Liquid water Hydrogen bonds constantly break and re-form Ice Hydrogen bonds are stable Hydrogen bond

3. Insulation of bodies of water by floating ice

4. Solvent of Life Water is claimed to be the universal solvent. Solution – homogeneous mixture of two or more substances in the same phase Solute – substance which is dissolved (in case of liquids, substance with the least amount Solvent – substance which is dissolving another Aqueous solution – solution involving water Hydration shell – pocket formed by water molecules in order to dissolve a substance

4. Solvent of life Hydrophilic – attracted to water Can be dissolved Unless molecule is too large Colloid – stable suspension of fine molecules in a liquid. (blood, milk) Hydrophobic – repel water Non-ionic, non-polar, can’t form H-bonds

4. Solvent of Life Solute concentrations in aqueous solutions Concentration = g solute / ml solvent Molarity – moles solute / Liter solution

Acidic and Basic conditions affect living organisms Water can dissociate Into hydronium ions and hydroxide ions H+ (hydrogen ion) is used to represent the hydronium ion Changes in the concentration of these ions Can have a great affect on living organisms Only 1 in 554 mil pure water molecules will diss. H Hydronium ion (H3O+) Hydroxide ion (OH–) + – Figure on p. 53 of water dissociating

Acids and Bases Acids [H+]>[OH-] Bases [H+]<[OH-] When acids dissolve in water, they release hydrogen ions—H+ (protons). H+ ions can attach to other molecules and change their properties. Bases reduce H+ concentration by accepting H+ ions and/or release OH- ions

Strong Acid HCl is a strong acid—the dissolution is complete.

Weak Acid Organic acids have a carboxyl group: Weak acids: not all the acid molecules dissociate into ions.

Strong Base NaOH is a strong base. The OH– absorbs H+ to form water.

Weak Bases Weak bases: • Bicarbonate ion • Ammonia • Compounds with amino groups

Acids, Bases, pH pH = negative log of the molar concentration of H+ ions. H+ concentration of pure water is 10–7 M, its pH = 7. Lower pH numbers mean higher H+ concentration, or greater acidity.

Acids, Bases, buffers Living organisms maintain constant internal conditions, including pH. Buffers help maintain constant pH by accepting or donating H+ ions. They are kept in excess in systems A buffer is a weak acid and its corresponding base. If you add 0.001 mole of a stong acid to: 1L of pure water the pH will go from 7 2.0 1L of blood the pH will only decrease from 7.4  7.3

Figure 2.17 Buffers Minimize Changes in pH

2.4 What Properties of Water Make It So Important in Biology? Buffers illustrate the law of mass action: addition of reactant on one side of a reversible equation drives the system in the direction that uses up that compound.

2.4 What Properties of Water Make It So Important in Biology? Life’s chemistry began in water. Water and other chemicals may have come to Earth on comets. Water was an essential condition for life to evolve.