1. Water and Fitness in the Environment
Chapter 3:
Water and Fitness in the Environment
AP Biology
Mrs. Madalon

2. Overview: The Molecule That Supports All of Life
Water = biological medium on Earth
All living organisms require water more than any other substance
Cells:
Most cells surrounded by water
Cells are about 70–95% water
Abundance of water = main reason Earth is habitable
Why?
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3. Concept 3.1: The polarity of water molecules results in hydrogen bonding
Water is a polar molecule:
The opposite ends have opposite charges
Polarity  water molecules form hydrogen bonds with each other
Four of water’s properties that facilitate an environment for life are:
Cohesive behavior
Strong adhesion
Ability to moderate temperature
Expansion upon freezing
Versatility as a solvent
Concept 3.2: Four emergent properties of water contribute to Earth’s fitness for life
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4. Fig. 3-UN7

5. Concept 2.2 Atoms Interact and Form Molecules
Hydrogen bonds:
Attraction between the δ– end of one molecule and the δ+ hydrogen end of another molecule.
They form between water molecules and within larger molecules.
Although much weaker than covalent bonds, they are important in the structure of DNA and proteins.

6. Figure 2.4 Hydrogen Bonds Can Form between or within Molecules
Figure 2.4 Hydrogen Bonds Can Form between or within Molecules (A) A hydrogen bond forms between two molecules because of the attraction between an atom with a partial negative charge on one molecule and a hydrogen with a partial positive charge on a second molecule. (B) Hydrogen bonds can form between different parts of the same large molecule.

7. Water has strong cohesion & high surface tension
Cohesion: Hydrogen bonds hold water molecules together
helps the transport of water against gravity in plants
Surface tension: measure of how hard it is to break the surface of a liquid; related to cohesion
cohesion—water molecules resist coming apart when placed under tension.
Hydrogen bonding between liquid water molecules and solid surfaces allows for adhesion between the water and the solid surface.
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9. Water has strong adhesion
Adhesion: attraction between unlike substances
water and plant cell walls
Capillary Action: this adhesive property allows H2O to rise against the force of gravity.
moves H2O in plants

10. Moderation of Temperature
Water absorbs heat from warmer air; releases stored heat to cooler air
Kinetic energy: the energy of motion
Heat: measure of the total amount of kinetic energy due to molecular motion
Temperature: measures the intensity of heat due to the average kinetic energy of molecules
Celsius scale: measure of temperature using Celsius degrees (°C)
Calorie (cal): amount of heat required to raise the temperature of 1 g of water by 1°C
“Calories” on food packages are actually kilocalories (kcal)
1 kcal = 1,000 cal
Joule (J): another unit of energy
1 J = cal, or 1 cal = J
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11. Moderation of Temperature
A lot of heat energy is required to raise the temperature of water—the heat energy breaks the hydrogen bonds.
In organisms, presence of water shields them from fluctuations in environmental temperature.

12. Water’s High Specific Heat
Specific heat: the degree to which a substance changes temperature in response to a gain or loss of heat.
Water’s high specific heat  resists changing temperature  better for LIFE
Hydrogen bonding  water’s high specific heat
Heat absorbed when hydrogen bonds break
Heat is released when hydrogen bonds form
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13. Evaporative Cooling Evaporation: liquid  gas
Water has a high heat of vaporization: a lot of heat energy is required to change water from the liquid to gaseous state (to break the hydrogen bonds).
Thus, evaporation has a cooling effect on the environment. [Evaporative Cooling]
Sweating cools the body—as sweat evaporates from the skin, a large amount of heat is taken with it and you are cooled.
helps stabilize temperatures in organisms and bodies of water
Evaporative cooling: as a liquid evaporates, its remaining surface cools

14. Evaporative Cooling
Heat of Fusion: energy required to change water from a solid to liquid
Heat of vaporization: energy required to change water from a liquid to a gas
The horizontal lines between the different physical states of water indicate the absorption of energy without a rise in temperature. The energy associated with each of these transition has a special name…
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15. Insulation of Bodies of Water by Floating Ice
Hydrogen bonds in ice are more “ordered” = ice less dense  Ice floats
Water reaches its greatest density at 4°C
If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth

16. The Solvent of Life!!
Solution: liquid that is a homogeneous mixture of substances
Solvent: dissolving agent of a solution
Solute: substance that is dissolved
Aqueous solution: one in which water is the solvent
Water = versatile solvent due to its polarity
Hydration shell: When an ionic compounds dissolve in water, each ion is surrounded by a sphere of water molecules
Even large polar molecules like proteins can dissolve in water if they have ionic and polar regions
UNIVERSAL SOLVENT

17. Hydrophilic and Hydrophobic Substances
Solute Concentration in Aqueous Solutions
Most biochemical reactions occur in water
Chemical reactions depend on collisions of molecules thus on the concentration of solutes in an aqueous solution
Molarity (M): number of moles of solute per liter of solution
Hydrophilic: has an affinity for water; “water-loving”
Hydrophobic: does not have an affinity for water; “water-fearing”
How can you tell if a molecule is hydrophobic?
Colloid: stable suspension of fine particles in a liquid
Hydrophilic and Hydrophobic Substances
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18. Hydrophilic and Hydrophobic Substances
POLAR MOLECULES LIKE EACH OTHER!
Any polar molecule can interact with any other polar molecule through hydrogen bonds.
Hydrophilic (“water-loving”): in aqueous solutions, polar molecules become separated and surrounded by water molecules.
Nonpolar molecules are called hydrophobic (“water-hating”); the interactions between them are hydrophobic interactions.

19. Hydrophilic and Hydrophobic
Hydrophilic and Hydrophobic (A) Molecules with polar covalent bonds are attracted to polar water (they are hydrophilic). (B) Molecules with nonpolar covalent bonds show greater attraction to one another than to water (they are hydrophobic). The color convention in the models shown here (gray, H; red, O; black, C; green, F) is often used.

20. Atoms Interact and Form Molecules
When one atom is much more electronegative than the other, a complete transfer of electrons may occur.
This makes both atoms more stable because their outer shells are full.
The result is two ions—electrically charged particles that form when atoms gain or lose one or more electrons. 20

21. Ionic Attraction between Sodium and Chlorine
Figure 2.6 Ionic Attraction between Sodium and Chlorine When a sodium atom reacts with a chlorine atom, the chlorine fills its outermost shell by “stealing” an electron from the sodium. In so doing, the chlorine atom becomes a negatively charged chloride ion (Cl–). With one less electron, the sodium atom becomes a positively charged sodium ion (Na+).

22. Atoms Interact and Form Molecules
Cations—positively charged ions
Anions—negatively charged ions
Ionic attractions result from the electrical attraction between ions with opposite charges. The resulting molecules are called salts or ionic compounds. 22

23. Atoms Interact and Form Molecules
Ionic attractions are weak, so salts dissolve easily in water.
place text art pg 25 here

24. Concept 3.3: Acidic and basic conditions affect living organisms
Hydrogen atom in hydrogen bond between two water molecules can shift from one to the other:
Hydrogen atom leaves electron behind transferred as a proton or hydrogen ion (H+)
Molecule with the extra proton is = hydronium ion (H3O+)
The molecule that lost the proton is now a hydroxide ion (OH–)
Water is in a state of dynamic equilibrium in which water molecules dissociate at the same rate at which they are being reformed
Water in state of dynamic equilibrium
Rate of water molecules dissociate = rate of water molecules reformed
RARE but changes in concentrations of H+ and OH– can drastically affect the chemistry of a cell
How?
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25. Acids and Bases
pH Scale: Scientists us to determine whether solution is acidic or basic
Most biological processes within pH range 6-8
Acid: any substance that increases the H+ concentration of a solution
pH less than 7
Base: any substance that reduces the H+ concentration of a solution
pH more than 7
[ H+ ] = [ OH– ] in pure water
Any aqueous solution at 25°C the product of H+ and OH– is constant
[H+][OH–] = 10–14
pH of a solution is defined by the negative logarithm of H+ concentration
pH = –log [H+]
Neutral aqueous solution
[H+] is 10–7 = –(–7) = 7

26. Acidic [H+] > [OH–] Neutral [H+] = [OH–] 7 Basic [H+] < [OH–] 14
Fig. 3-UN5
Acidic
[H+] > [OH–]
Acids donate H+ in
aqueous solutions
Neutral
[H+] = [OH–] 7
Bases donate OH–
or accept H+ in
aqueous solutions
Basic
[H+] < [OH–] 14

27. Buffers Internal pH of most living cells MUST remain close to pH 7
Buffer: substances that minimize changes in concentrations of H+ and OH– in a solution
Most buffers consist of acid-base pair
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28. Threats to Water Quality on Earth
Acid precipitation: rain, snow, or fog with a pH lower than 5.6
can fall far from pollutant source
can damage life in lakes and streams
Effects on soil chemistry are contribute to decline of some forests
Pollutants + water in air = Acid Rain
Burning fossil fuels  Poor water quality
CO2 is released by fossil fuel combustion:
“Greenhouse effect”
Acidification of the oceans
this decrease in the ability of corals to form calcified reefs
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29. WHAT DID WE COVER? Water is necessary for LIFE! Water is polar
Water has strong cohesion & high surface tension
Water has strong adhesion
Water has ability to moderate temperature
Water has a high specific heat capacity
Evaporative cooling
Ice floats- unique density properties
Water is the universal solvent
Hydrophilic vs. Hydrophobic pH

30. You should now be able to:
List and explain the four properties of water that emerge as a result of its ability to form hydrogen bonds
Distinguish between the following sets of terms: hydrophobic and hydrophilic substances; a solute, a solvent, and a solution
Define acid, base, and pH
Explain how buffers work
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