Water and the Fitness of the Environment Chapter 3 Water and the Fitness of the Environment

Water is a Polar molecule

It can Hydrogen bond with other water molecules Hydrogen bonding imparts several important properties Cohesion- sticks to other water Adhesion- sticks to other polar substances Surface tension- High specific heat- does not change temperature easily

Water is a good solvent (for ionic compounds and other polar substances) Solute: the substance getting dissolved. Solvent: the substance doing the dissolving

Water dissociates into hydroxide and hydronium This is often simplified into H+ and OH- The concentration of H+ and OH- ions in pure water is 10-7 M pH scale is a convenient way to express the number of hydrogen ions and hydroxide ions in solution pH= -log [H+] This means that the pH of Pure water is –log (10-7)= 7 thus pure water is neutral

Acids and Bases Acid: any molecule that will donate H+ to solution Base: any molecule that can accept an H+ from solution Examples in chemistry CH3COOH + H2O  CH3COO- + H3O + CH3COOH + NH3  CH3COO- + NH4 +

pH and living systems Living systems can be damaged by H+ and OH- ions These ions can denature proteins (cause them to become folded incorrectly) and cause damage to cellular parts. Living systems use buffers to maintain constant pH levels

Buffers in living systems A Buffer is a substance that minimizes changes in the concentration of H+ and OH- ions How? Buffers donate H+ ions if they become low or accept H+ ions if they become too high

Illustrative example: The blood Bicarbonate buffer Human blood is kept at a pH of 7.4 If pH drops below 6.8 or above 7.8 death may occur One of the main buffers responsible for keeping the blood at constant pH is the Carbonic acid/ bicarbonate buffer system H2O + CO2  H2CO3 HCO3- + H+ (Produced by metabolism)

Blood buffer demonstration Bromothymol blue is a pH indicator No Buffer: Water+ bromothymol blue With buffer: Water + sodium bicarbonate Make an observation about which solution can absorb more protons before changing pH 6.0 Neutal 7.6

NaHCO3 sodium bicarbonate dissolves in water NaHCO3  Na + HCO3- Bicarbonate becomes protonated by water forming a mildly basic solution HCO3- + H2O  H2CO3- + OH- This makes sense… Bromothymol is blue in basic solutions

Demo setup In one test tube 100 mL water and 4 drops Bromothymol blue In another test tube 1 g sodium bicarbonate , fill to 100 mL and 4 drops Bromothymol blue. Solutions should be blue. If not add a drop of ammonia so it turns blue Add one drop of vinegar at a time. Count the number of drops it takes to turn the solution yellow.

How do plants take up water? The TACT mechanism Transpiration Adhesion Cohesion Tension

Transpiration-Adhesion-Cohesion-Tension Plants need to exchange gasses for photosynthesis (CO2 in and O2 out) Gasses are exchanged through openings in the leaf’s waxy cuticle called stomata. Water also escapes through stomata

Transpiration-Adhesion-Cohesion-Tension Evaporation of water leaves empty space in the leaf mesophyll. Water molecules from further inside the leaf are pulled into this space because of tension or negative pressure. This is called Transpiration Pull. Think straw or syringe sucking up water

Transpiration pull

Transpiration-Adhesion-Cohesion-Tension Because of cohesion , Transpiration pull is transmitted all the way through the xylem from leaf to root. Adhesion of water to the hydrophilic walls of the xylem helps resist the downward pull of gravity.

Your task: Measure rate of transpiration and stomata density Consider the following for designing your experiment: What environmental conditions affect the rate of transpiration (temperature, humidity, light) Do all plants transpire at the same rate? Do all parts of a plant transpire? How does stomata density affect the rate of transpiration? How does the type of habitat the plant evolved in affect the rate of transpiration?

Measuring plant transpiration rate Plant’s soil is presoaked with water. Wrap soil portion in plastic bag and close tightly so that no evaporation can occur from the base. Take initial mass of plant including soil and bag. Place plant under experimental conditions. Follow your groups outlined data collection procedures

Other method you may see on AP test question

Leaf Stomata Casts Take stomata cast of the top and bottom of the leaf Count the number of stomata per field of view Measure the field of view and calculate stomata per unit area Estimate total leaf area of the plant