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

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. Fig. 3-UN6
Surface of Mars
Surface of Earth

4. Concept 3.1: The polarity of water molecules results in hydrogen bonding
Water molecule 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
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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5. Fig. 3-UN7

6. Cohesion Cohesion: Hydrogen bonds hold water molecules together
helps the transport of water against gravity in plants
Adhesion: attraction between different substances
water and plant cell walls
Surface tension: measure of how hard it is to break the surface of a liquid; related to cohesion
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7. 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

8. Water’s High Specific Heat
Specific heat: amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1ºC
Specific heat of water = 1 cal/g/ºC
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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9. Evaporative Cooling Insulation of Bodies of Water by Floating Ice
Evaporation: transformation of a substance from liquid to gas
Heat of vaporization: heat a liquid must absorb for 1 g to be converted to gas
Evaporative cooling: as a liquid evaporates, its remaining surface cools
helps stabilize temperatures in organisms and bodies of water
Hydrogen bonds in ice 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
Insulation of Bodies of Water by Floating Ice
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10. 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

11. Hydrophilic and Hydrophobic Substances
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
Most biochemical reactions occur in water
Chemical reactions depend on collisions of molecules thus on the concentration of solutes in an aqueous solution
Lots of chemical reactions would occur in…?
Molarity (M): number of moles of solute per liter of solution
Solute Concentration in Aqueous Solutions
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12. 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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13. 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

14. 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

15. 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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16. 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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17. Calcification rate (mmol CaCO3 per m2 per day)
Fig. 3-11
EXPERIMENT
RESULTS
Figure 3.11 What is the effect of carbonate ion concentration on coral reef calcification? 40
Calcification rate
(mmol CaCO3
per m2 per day) 20
150
200
250
300
[CO32–] (µmol/kg)

18. 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings