Chapter 3Water and the Fitness of the

EnvironmentAP minknow

•The importance of hydrogen bonding to the properties of water.•Four unique properties of water, and how each contributes to life on earth.•How to interpret the pH scale.•The importance of buffers in biological systems.

Checkup

1. Using bohr models draw the reaction of hydrogen and oxygen to form water.

2. How many neutrons does an oxygen atom have?

3. What is the difference between mass number and atomic mass?

4. What is an isotope? What is a radioactive isotope? What is one way a radioactive isotope can be used?

Water: The Molecule That Supports All of Life

• Water is the biological medium here on Earth– All living organisms require

water more than any other substance

– Three-quarters of the Earth’s surface is submerged in water

– The abundance of water is the main reason the Earth is habitable

3.1: The polarity of water molecules results in hydrogen bonding

• The polarity of water molecules– Allows them to form hydrogen bonds with each other– Contributes to the various properties water exhibits

Hydrogenbonds

+

+

H

H+

+

–

–

– –

3.2: Four emergent properties of water contribute to Earth’s fitness for life

1. Cohesion

2. Moderation of Temperature

3. Insulation of bodies of water by floating ice

4. The solvent of life (universal solvent)

1. Cohesion• Cohesion – the hydrogen

bonds holding a substance together. (water – water)

• Adhesion – the hydrogen bonds holding one substance to another. (water – glass)

• Capillary Action – water transport in plants. Uses Cohesion and Adhesion– Transpiration

• Surface tension – measure of how difficult it is to stretch or break the surface of a liquid. – Water has a greater surface

tensions than most liquids

2. Moderation of Temperature

• Kinetic Energy – energy of motion

• Thermal Energy (heat) – total energy within a substance– Calorie – amount of

heat energy to heat 1g water by 1°C

– Kcal – 1000c• Temperature –

average kinetic energy per molecule (Celsius Scale)

2. Moderation of Temperature

• Specific heat – the amount of heat absorbed or loss for 1g of a substance to change its temperature by 1°C– Water has high

specific heat capacity compared to other substances

– 1 cal/g/°C

2. Moderation of Temperature

• Evaporation• Heat of vaporization –

the amount of heat 1g of a liquid must absorb to be converted to a gas

• Evaporative cooling – as a liquid evaporates the surface of the remaining liquid cools– This occurs because the

“hottest” molecules leave

3. Insulation of bodies of water by floating ice

Liquid water

Hydrogen bonds constantly break and re-form

Ice

Hydrogen bonds are stable

Hydrogen bond

3. Insulation of bodies of water by floating ice

4. Solvent of Life• Water is claimed to be the

universal solvent.– Solution – homogeneous

mixture of two or more substances in the same phase

– Solute – substance which is dissolved (in case of liquids, substance with the least amount

– Solvent – substance which is dissolving another

– Aqueous solution – solution involving water

– Hydration shell – pocket formed by water molecules in order to dissolve a substance

4. Solvent of life• Hydrophilic –

attracted to water– Can be dissolved– Unless molecule

is too large– Colloid –

stable suspension of fine molecules in a liquid. (blood, milk)

• Hydrophobic – repel water– Non-ionic, non-

polar, can’t form H-bonds

4. Solvent of Life

• Solute concentrations in aqueous solutions– Concentration =

g solute / ml solvent

– Molarity – moles solute / Liter solution

Acidic and Basic conditions affect living organisms

• Water can dissociate– Into hydronium ions and hydroxide ions

• H+ (hydrogen ion) is used to represent the hydronium ion

• Changes in the concentration of these ions– Can have a great affect on living organisms

• Only 1 in 554 mil pure water molecules will diss.

H

Hydroniumion (H3O+)

H

Hydroxideion (OH–)

H

H

H

H

H

H

+ –

+

Figure on p. 53 of water dissociating

Acids and Bases• Acids [H+]>[OH-]

• Bases [H+]<[OH-]• When acids dissolve in water, they

release hydrogen ions—H+ (protons).– H+ ions can attach to other molecules and

change their properties.

• Bases reduce H+ concentration byaccepting H+ ions and/or release OH- ions

Strong Acid

HCl is a strong acid—the dissolution is complete.

ClHHCl

Weak AcidOrganic acids have a carboxyl

group:

Weak acids: not all the acid molecules dissociate into ions.

HCOOHCOOH

Strong BaseNaOH is a strong base.

OHNaNaOH

The OH– absorbs H+ to form water.

Weak BasesWeak bases:

• Bicarbonate ion

• Ammonia

• Compounds with amino groups

323 COHHHCO

43 NHHNH

32 NHHNH

Acids, Bases, pHpH = negative log of the molar

concentration of H+ ions.

H+ concentration of pure water is 10–7 M, its pH = 7.

Lower pH numbers mean higher H+ concentration, or greater acidity.

Acids, Bases, buffers• Living organisms

maintain constant internal conditions, including pH.– Buffers help maintain

constant pH by accepting or donating H+ ions.

– They are kept in excess in systems

• A buffer is a weak acid and its corresponding base.

323 COHHHCO

•If you add 0.001 mole

of a stong acid to:

•1L of pure water

the pH will go from

7 2.0

•1L of blood the pH

will only decrease

from 7.4 7.3

Figure 2.17 Buffers Minimize Changes in pH

2.4 What Properties of Water Make It So Important in

Biology?Buffers illustrate the law of mass action: addition of reactant on one side of a reversible equation drives the system in the direction that uses up that compound.

2.4 What Properties of Water Make It So Important in

Biology?Life’s chemistry began in water.

Water and other chemicals may have come to Earth on comets.

Water was an essential condition for life to evolve.