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BiochemistryWater 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. 1Water: 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 substance2

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). 2Water: The molecule that supports life on this planet!Most cells are surrounded by water, and cells themselves are about 7095% waterThe abundance of water is the main reason the Earth is habitable3

Were still trying to confirm the existence of water on other heavenly bodies!3FOUR Emergent Properties of water contribute to Earths suitability for lifeWaters cohesive & adhesive behaviorWaters ability to moderate temperatureWaters expansion upon freezingWaters versatility as a solvent.

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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 couldnt resist it!4What 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.5

Animation by scrolling over imageAlso 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 waters 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 2010. Scroll over the bottom of the picture and you should see a bar that allows you to press playthe blue PLAY button is pretty, but useless, the control is hidden until you scroll over the picture. 5What 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!

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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 waters 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 youve covertly coated with a bit of bar soap to act as a surfactant! Great inquiry moment even if youve stacked the deck so to speak.6How 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 = 4.184 J)What does that mean? It means water can absorb large quantities of heat without much change in its own temperature, thus its a good thermo regulator.

But, why? Go, aheadguess!7Again, probably prior learning from physical science or chemistry I. 7How 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!

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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 oceansthey dont evaporate easily) and a high boiling point (good news for our fishenormous 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.)8How 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.

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Try to explain this with regard to your local atmospheric conditions and geography so student can have some concrete examples. 9Its a 2-way street! Waters high specific heat can be traced to hydrogen bonding (IMFs)Heat is absorbed when hydrogen bonds breakHeat is released when hydrogen bonds formThe high specific heat of water minimizes temperature fluctuations to within limits that permit life10

Recite over and over to students: It takes energy to break bonds, but energy is released when bonds form. Youll be glad you did by the time we reach cell respiration and photosynthesis!10Care 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.11

Have students identify trends, and then EXPLAIN the trends in terms of the concepts weve explained to them thus far. Require that they use proper vocabularydont let them get sloppy here! If theyre stuck ask leading questions such as: What do you notice about the temperature trends as we leave the coastline? 11Ever sweat? Ever perspire? Evaporation is transformation of a substance from liquid to vaporHeat of vaporization is the heat a liquid must absorb for 1 g to be converted to vaporAs 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

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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. Theyll 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 bodys thermo receptors perceive an absence of heat as cold.

12Water is a freak! It expands upon freezing!13

Ice: stable hydrogen bonds

Liquid water: transienthydrogen bondsVery few substances on the planet expand upon freezing. Among them are antimony, bismuth, gallium, germanium and siliconjust 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. 13Water is a freak! It expands upon freezing!14Ice 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 4C, which is excellent news if youre a fish!If ice sank, all bodies of water would eventually freeze solid, making life impossible on EarthYou 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 dont freeze unless the entire body of water freezes solid!14Water is a freak! It expands upon freezing!15

Hydrogen bondIce:Hydrogen bondsare stable Liquid water:Hydrogen bonds break and re-formIce floats!15Waters Versatility as a SolventA 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

16Most likely prior knowledge from middle school, physical science or chemistry I.16Waters Versatility as a SolventWater 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

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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.17Waters Versatility as a SolventWater can also dissolve compounds made of nonionic polar moleculesEven large polar molecules such as proteins can dissolve in water if they have ionic and polar regions

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Its all about electrostatic attractions! (Opposite charges attract.)18Hydrophilic vs. HydrophobicA hydrophilic substance is one that has an affinity for waterA hydrophobic substance is one that does not have an affinity for water; fears waterOil molecules are hydrophobic because they have relatively nonpolar bondsA colloid is a stable suspension of fine particles in a liquid (like milkfat suspended in waterYummy!)

19Students 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. 19Acids, 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).

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Whos got the proton (H+ )? Whichever species has the proton is the acid.20Acids, 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)

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Whoever is missing the proton is the base.21Acids, 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

22We teach early on that pure water doesnt conduct an electric current, but thats 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. 22Acids, Bases and Buffers. Oh, my!23Concentrations 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 waterbig 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!23Acids, 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

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BleachIf 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!) 24Acids, Bases and Buffers. Oh, my!Buffers

The internal pH of most living cells must remain close to pH 7Buffers are substances that RESIST changes in concentrations of H+ and OH in a solution, therefore they RESIST a change in pHMost buffers consist of an acid-base pair that reversibly combines with H+

25HOW does a buffer work? A buffer consists of a weak acid and its 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. 25Buffers in Action26Human activities such as burning fossil fuels threaten water qualityCO2 is the main product of fossil fuel combustionAbout 25% of human-generated CO2 is absorbed by the oceansCO2 dissolved in sea water forms carbonic acid; this process is called ocean acidificationAcidification: A Threat to Water QualityNonmetal oxides in water make acids. 26Buffers in Action27As 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.27Acid RainThe 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 snowAcid precipitation is rain, fog, or snow with a pH lower than 5.2Acid precipitation damages life in lakes and streams and changes soil chemistry on land 28The 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.

28Acid Rain: Before & After29

Sad. Just sad.29The pH Scale30

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 Brnsted-Lowry definitions of acids and bases. 30The pH Scale31In any aqueous solution at 25C 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 107, so[H+][OH] = 1014pH = log [H+]pH = (7) = 7 Dont be frightened! Its just that students should have done these calculations in Chemistry I class. 31The pH Scale32Acidic 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!32

Created by:

Ren McCormickNational Math & ScienceDallas, TX

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