CHAPTER 2

Water and Lifehttp://www.youtube.com/watch?feature=player_embedded&v=ASLUY2U1M-8

Water and Life

– Life on Earth began in water and evolved there for 3 billion years.• Modern life still remains tied to water.• Your cells are composed of 70%–95% water.

– The abundance of water is a major reason Earth is habitable.

• Like no other common substance, water exists in nature in all three physical states:

Figure 2.10B

– as a solid– as a liquid– as a gas

Figure 19.29d

The Structure of a Water Molecule

• What atoms make up a water molecule?• How are these atoms joined together to

make a water molecule?• What type of bond joins the atoms

together?

– This makes the oxygen end of the molecule slightly negatively charged

– The hydrogen end of the molecule is slightly positively charged

– Water is therefore a polar molecule

• In a water molecule, oxygen exerts a stronger pull on the shared electrons than hydrogen

Figure 2.9

(–)

O

(–)

(+)(+)

H H

The Properties of Water

• Atoms in a molecule that form covalent bonds may share electrons equally, creating a nonpolar molecule

• If electrons are shared unequally, a polar molecule is created

• The charged regions on water molecules are attracted to the oppositely charged regions on nearby molecules– This attraction forms

weak bonds called hydrogen bonds

– Water model activity

Water’s polarity leads to hydrogen bonding and other unusual properties

Figure 2.10A

Hydrogen bond

Essential Question

• How does water rise from the roots of a redwood tree to the very top?

• www.campbellbiology.com

The Cohesion of Water

– Water molecules stick together as a result of hydrogen bonding.• This is called cohesion.• Due to cohesion water molecules form a continuous

column of water in the conducting tubes of a plant.• Cohesion is vital for water transport in plants.

The Adhesion of Water

– Water molecules stick to other polar (charged) molecules by hydrogen bonding• This is called adhesion.• Due to adhesion water molecules stick to the

charged walls of the conducting tubes keeping the column of water from falling due to gravity

• Cohesion is vital for water transport in plants.

Cohesion and Adhesion in Your Life

• Can you think of an example of cohesion and adhesion in your everyday life?

• Explain how these properties of water explain this example.

Water’s Life-Supporting Properties

– The polarity of water molecules and the hydrogen bonding that results explain most of water’s life-supporting properties:

• Water’s cohesive and adhesive nature • Water’s ability to moderate temperature• Floating ice• Versatility of water as a solvent

Essential Question

• How do insects walk on water?

• http://www.youtube.com/watch?v=V-cXzZt2iVk&feature=player_embedded

Surface Tension

Surface tension is the measure of how difficult it is to stretch or break the surface of a liquid.

• Hydrogen bonds give water an unusually high surface tension.

• It is as if a film is formed on the surface of the water and this allows insects to walk on the surface

Surface Tension in Your Life

• What is an example of surface tension in your life?

Essential Question

• Why do people sweat and some animals pant to reduce their body temperature

• http://www.youtube.com/watch?feature=player_embedded&v=lqwPS6wJN-c

How Water Moderates Temperature

• It takes a lot of energy to disrupt hydrogen bonds– Therefore water is able to absorb a great

deal of heat energy without a large increase in temperature

– As water cools, a slight drop in temperature releases a large amount of heat

– Because of hydrogen bonding, water has a strong resistance to temperature change.

– http://oceanservice.noaa.gov/education/pd/oceans_weather_climate/media/specific_heat.swf

– A water molecule takes a large amount of energy with it when it evaporates

– This leads to evaporative cooling– This is why people sweat and some animals

pant

Figure 2.12

Essential Question

• http://www.sumanasinc.com/webcontent/animations/content/propertiesofwater/water.html

• Why does ice float rather than sink?

• How would life in a lake be affected if ice sank and lakes froze from the bottom up?

The Biological Significance of Ice Floating

• Due to hydrogen bonds molecules in ice are farther apart than those in liquid water

– The density of ice is lower than liquid water.

• This is why ice floats.

– Since ice floats, ponds, lakes, and even the oceans do not freeze solid.

– How would life in a lake be affected if ice sank and lakes froze from the bottom up?

Water as the Solvent of Life

– A solution is a liquid consisting of two or more substances evenly mixed.

• The dissolving agent is called the solvent.

• The dissolved substance is called the solute.

Water as the Solvent of Life

• Solutes whose charges or polarity allow them to stick to water molecules dissolve in water– They form

aqueous solutions

Essential Question

• www.campbellbiology.com

• Why is your blood mostly water?

• Why is the inside and outside of your cells mostly water?

Acids, Bases, and pH

- Acid• A chemical compound that donates H+ ions to

solutions.

- Base • A compound that accepts H+ ions and

removes them from solution.

- To describe the acidity of a solution, we use the pH scale.

Figure 2.17

– Cells are kept close to pH 7 by buffers.http://www.youtube.com/watch?feature=player_embedded&v=NJyAme5GVF8

– Buffers are substances that resist pH change.• They accept H+ ions when they are in excess.• They donate H+ ions when they are depleted.

– Buffering is not foolproof.• Example: acid precipitation

• Some ecosystems are threatened by acid precipitation

• http://www.natgeoeducationvideo.com/film/1233/acid-rain

• Acid precipitation is formed when air pollutants from burning fossil fuels combine with water vapor in the air to form sulfuric and nitric acids

Acid precipitation threatens the environment

Figure 2.16A

– These acids can kill fish, damage buildings, and injure trees

– Regulations, new technology, and energy conservation may help us reduce acid precipitation

Figure 2.16B