1. Water and the Fitness of the Environment
Biochemistry
Water and the Fitness of the Environment
Water is essential to life on earth. It is important that each property of water be linked to biological processes through illustrative examples.

2. Water: The molecule that supports life on this planet!
Water is the biological medium for all life on Earth
All living organisms require water more than any other substance
Emphasize that the polar covalent bonds are actual chemical bonds where a single pair of electrons are shared between two atomic nuclei. BUT a H-bond is not an actual chemical bond, but rather an intermolecular force (IMF) that is not to be ignored. H-bonds can form only between the H attached to a highly electronegative element such as F, O or N (think “phone”) on one molecule and an unshared pair of electrons on a highly electronegative element on an adjacent molecule (F, O or N again).

3. Water: The molecule that supports life on this planet!
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
We’re still trying to confirm the existence of water on other heavenly bodies!

4. FOUR Emergent Properties of water contribute to Earth’s suitability for life
Water’s cohesive & adhesive behavior
Water’s ability to moderate temperature
Water’s expansion upon freezing
Water’s versatility as a solvent.
This in most definitely the main emphasis of this lesson! Refer to these properties over and over throughout the course. Sorry about the pun on “polar”, just couldn’t resist it!

5. What causes water molecules to both cohere and adhere?
Yep! Our good buddy the intermolecular force (IMF) named hydrogen bonding
Cohesion is when water molecules stick to each other.
Adhesion is when water molecules stick to some other type of substance like plant cell walls.
Also a good time to discuss capillary action with glass tubes. The thinner the tube, the more surface area vs. volume, the higher the water will “climb” since water’s adhesive force for glass is greater than the cohesive forces it has for other water molecules.
This animation should be embedded into your ppt if you are using PowerPoint Scroll over the bottom of the picture and you should see a bar that allows you to press “play”—the blue “PLAY” button is pretty, but useless, the control is hidden until you scroll over the picture.
Animation by scrolling over image

6. What causes water molecules to both cohere and adhere?
Surface tension is a measure of how hard it is to break the surface of a liquid.
Surface tension is a consequence of cohesion. What causes it? Go ahead, guess!
Hydrogen bonding, yet again!
This may well be “prior learning” from middle school. You can always do the “how many drops fit on a penny” demo/activity to reinforce water’s adhesive vs. cohesive forces. Students predict how many drops will fit on a penny and are surprised to see how high the “bubble” of water will get before the balance of cohesive forces are overcome and water spills off the penny. If you have an evil streak, you can use your own pennies, some of which you’ve covertly coated with a bit of bar soap to act as a surfactant! Great inquiry moment even if you’ve stacked the deck so to speak.

7. How does water moderate temperature?
Water has a high heat capacity.
Heat capacity is the amount of heat required to raise 1 gram of water by 1o Celsius (1 calorie = J)
What does that mean? It means water can absorb large quantities of heat without much change in its own temperature, thus it’s a good thermo regulator.
But, why? Go, ahead…guess!
Again, probably prior learning from physical science or chemistry I.

8. How does water moderate temperature?
Yep! Hydrogen bonding again! The fact that each water molecule can make TWO H-bonds each means they are really, really attracted to each other, thus more ENERGY must be added to the water sample to increase their molecular motion.
Remember, temperature is a measure of the average kinetic energy of a sample of molecules!
Emphasize that in the vapor* phase thermal E has overcome the attractive IMFs of H-bonding. In the liquid phase, the H-bonds are not static but constantly forming, “breaking” and reforming. In the solid phase, they are static, but the atoms are vibrating as a function of temperature. As a result, water has a high vapor pressure (good news regarding our oceans—they don’t evaporate easily) and a high boiling point (good news for our fish—enormous amounts of energy are needed to vaporize a pond, lake, stream, etc. )
*Technically a gas is a gas at room temperature, like nitrogen or oxygen or carbon dioxide, but a vapor is what we call the “gas” of a substance that is normally a liquid at room temperature, like water, alcohol, gasoline, etc.)

9. How does water moderate temperature?
We should also discuss this effect as it relates to “air” with regard to the amount of water vapor in the air or humidity. The more water vapor in the air, the more heat the air can absorb.
Large bodies of water absorb heat from warmer air and release stored heat to cooler air.
Try to explain this with regard to your local atmospheric conditions and geography so student can have some concrete examples.

10. It’s a 2-way street!
Water’s high specific heat can be traced to hydrogen bonding (IMFs)
Heat is absorbed when hydrogen bonds break
Heat is released when hydrogen bonds form
The high specific heat of water minimizes temperature fluctuations to within limits that permit life
Recite over and over to students: “It takes energy to break bonds, but energy is released when bonds form.” You’ll be glad you did by the time we reach cell respiration and photosynthesis!

11. Care to examine some data?
Formulate an explanation for the temperature data presented below. Your explanation should address the properties of water as they relate to thermoregulation.
Have students identify trends, and then EXPLAIN the trends in terms of the concepts we’ve explained to them thus far. Require that they use proper vocabulary—don’t let them get sloppy here! If they’re stuck ask leading questions such as: “What do you notice about the temperature trends as we leave the coastline?”

12. Ever sweat? Ever perspire?
Evaporation is transformation of a substance from liquid to vapor
Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to vapor
As a liquid evaporates, its remaining surface cools, a process called evaporative cooling
Evaporative cooling of water helps stabilize temperatures in organisms (sweating, panting, etc.) and bodies of water
If you can get a container of rubbing alcohol , 2 thermometers and a pipet, you can do a quick demo. Have a thermometer in the container that is clearly labeled alcohol. Your second thermometer is for establishing room temperature. Fill a pipet with alcohol, then have each student make a fist, and put a drop of alcohol on the “top side” of their fist. The alcohol will feel “cold” to them. They’ll be quite surprised that it is at room temperature. Ask them: “Where did the energy for the evaporation of the alcohol come from?” Ans: the heat in the environment and their own body temp. “Which has more thermal energy for transfer, you or the room?” Ans: Them! “Ask a student that answers correctly, to explain how they know “them”. Ans: Body temp = 98.6 F while room is about 75 F. “Why did you believe the alcohol was colder than the room temp?” Ans: The heat from our “body” was absorbed by the alcohol during its evaporation and our body’s thermo receptors perceive an absence of heat as “cold”.

13. Water is a freak! It expands upon freezing!
Very few substances on the planet expand upon freezing. Among them are antimony, bismuth, gallium, germanium and silicon…just in case you are asked! There are a few others, but expanding upon freezing is definitely not the norm! A good analogy to use with student involves filling a room with people. If all the people have their arms out, not as many people can fit in the room, so fewer “molecules” fit in the same defined space, thus some of the space is “wasted” or empty.
Ice: stable hydrogen bonds
Liquid water: transient
hydrogen bonds

14. Water is a freak! It expands upon freezing!
Ice floats in liquid water because hydrogen bonds in ice are more “ordered” forming a hexagonal shape with a hole in the middle, making ice less dense.
Water reaches its greatest density at 4°C, which is excellent news if you’re a fish!
If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth
You can also explain the pond or lake “turning over” in the late fall and early spring. As the surface water is cooled (fall) or warmed (spring) to 4o C, that layer of water sinks, displacing the water that was on bottom. That will continue in the fall until all the water is at 4o C. Now, the top layer cools to 3 (and floats on top of the 4o water layer) , 2, 1 and 0oC and freezes solid. Fish don’t freeze unless the entire body of water freezes solid!

15. Water is a freak! It expands upon freezing!
Hydrogen bond
Liquid water:
Hydrogen bonds
break and re-form
Ice floats!
Ice:
Hydrogen bonds
are stable

16. Water’s 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 or “dissolver”
The solute is the substance that is dissolved or “disolvee”
An aqueous solution is one in which water is the solvent
Most likely prior knowledge from middle school, physical science or chemistry I.

17. Water’s Versatility as a Solvent
Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily.
When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell
Avoid the use of the phrase “universal solvent” which would imply the container would also be dissolved! Emphasize that ionic and polar molecules are more attracted to water than to each other if dissolving occurs.

18. Water’s Versatility as a Solvent
Water can also dissolve compounds made of nonionic polar molecules
Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions
It’s all about electrostatic attractions! (Opposite charges attract.)

19. Hydrophilic vs. Hydrophobic
A hydrophilic substance is one that has an affinity for water
A hydrophobic substance is one that does not have an affinity for water; “fears water”
Oil molecules are hydrophobic because they have relatively nonpolar bonds
A colloid is a stable suspension of fine particles in a liquid (like milk—fat suspended in water—Yummy!)
Students should understand they need to be able to write the “big people” words in their responses. Train them to use the term, then define it quickly in their writing.

20. Acids, Bases and Buffers. Oh, my!
ACIDIC AND BASIC CONDITIONS AFFECT LIVING ORGANISMS
A bonded hydrogen atom within a water molecule can shift between two water molecules (from one molecule to the other).
“Who’s got the proton (H+ )?” Whichever species has the proton is the acid.

21. Acids, Bases and Buffers. Oh, my!
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–)
Whoever is missing the proton is the base.

22. Acids, Bases and Buffers. Oh, my!
Though statistically rare, the dissociation of water molecules has a great effect on organisms
About 2 in every billion water molecules exist as H+ and OH–
Changes in concentrations of H+ and OH– can drastically affect the chemistry of a cell
We teach early on that pure water doesn’t conduct an electric current, but that’s a lie. Since water “autoionizes” into H+ and OH-, water does conduct an ever so slight electric current. It is that small change in current that the electrode of a pH meters detects from the presence of additional positive or negative charge.

23. Acids, Bases and Buffers. Oh, my!
Concentrations of H+ and OH– are equal in pure water
Adding certain solutes, called acids and bases, modifies the concentrations of H+ and OH–
Biologists use something called the pH scale to describe whether a solution is acidic or basic (the opposite of acidic; also known as alkaline)
The pH scale was designed to compare weak acids and bases. Strong acids and bases dissociate completely in water. Weak acids and bases dissociate less than 10% in water—big difference! Further confusion abounds since strong acids have weak bonds which is why water can surround the molecule and “tug” the H away from the molecule, releasing more H+ ion, thus decreasing the pH. Weak acids have strong bonds and water alone cannot “tug” the H+ away from a weak acid molecule. I see why students get confused! Sometimes the language is not our friend!

24. Acids, Bases and Buffers. Oh, my!
Simply put,
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
If OH- is added to a solution that has excess H+, then water is formed as a product which effectively reduces the amount of H+ in solution and increases the pH. Ask students the pH of a neutral solution. They should answer “7”. Starting at 7, in the center of the board construct the following: More Acidic  7  More Basic (Alkaline) so students understand that the pH “numbers” decrease as amount of H+ in solution increases BUT the pH numbers increase as the amount of H+ decreases. (Kind of backwards!)
Bleach

25. Acids, Bases and Buffers. Oh, my!
The internal pH of most living cells must remain close to pH 7
Buffers are substances that RESIST changes in concentrations of H+ and OH– in a solution, therefore they RESIST a change in pH
Most buffers consist of an acid-base pair that reversibly combines with H+
HOW does a buffer work? A buffer consists of a weak acid and it’s conjugate base or vice versa. Since it has both an acid and a base within, it can resist a change in pH by neutralizing and “invader”. If a bit of strong acid is placed into a buffer system, then the base component of the buffer neutralizes it with only a small decrease in pH.

26. Buffers in Action Acidification: A Threat to Water Quality
Human activities such as burning fossil fuels threaten water quality
CO2 is the main product of fossil fuel combustion
About 25% of human-generated CO2 is absorbed by the oceans
CO2 dissolved in sea water forms carbonic acid; this process is called ocean acidification
Nonmetal oxides in water make acids.

27. Buffers in Action
As seawater acidifies, H+ ions combine with carbonate ions to produce bicarbonate
Carbonate is required for calcification (production of calcium carbonate) by many marine organisms, including reef-building corals
Students may be given specific data in situations like this and be called upon to interpret the data in the context of changes in pH.

28. Acid Rain
The burning of fossil fuels is also a major source of sulfur oxides (SOx) and nitrogen oxides (NOx)
These “socks and knocks” compounds react with water in the air to form strong acids that fall within rain or snow
Acid precipitation is rain, fog, or snow with a pH lower than 5.2
Acid precipitation damages life in lakes and streams and changes soil chemistry on land
The more nonmetal oxides we spew into the atmosphere, the more acids we create. Oxides of sulfur and nitrogen along with carbon, COx, SOx, and NOx all react with water (rain) to make an assortment of weak acids HCO2, HNO2, HNO3, HSO4, etc. Teach students that “socks” and “knocks” (oxides of sulfur and nitrogen) are responsible for acid rain.

29. Acid Rain: Before & After
Sad. Just sad.

30. The pH Scale
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
The scale was designed to compare WEAK acids and bases.
If asked, the graphic is using the Arrhenius and Brönsted-Lowry definitions of acids and bases.

31. The pH Scale
In any aqueous solution at 25°C the product of H+ and OH– is constant and can be written as the autoionization constant of water
The pH of a solution is defined by the negative logarithm of H+ concentration, written as
For a neutral aqueous solution, [H+] is 10–7, so
[H+][OH–] = 10–14
Don’t be frightened! It’s just that students should have done these calculations in Chemistry I class.
pH = –log [H+]
pH = –(–7) = 7

32. The pH Scale 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
This is the stuff they should know well!

33. Created by:
René McCormick
National Math & Science
Dallas, TX