Ch.3 Life on Earth is Aqueous!

Predominance of Water

-3/4 of earth covered with water (liquid & solid)

-cells are 70-95% water

-all organisms require water for survival

~ 1 week survival time for human without water!

LE 3-2

Hydrogenbonds

Key properties of water defines behavior

-Polarity: partial positive and negative charges

-Hydrophilic nature: attracted to other water moleculesand charged particles

• Four of water’s properties - Cohesive behavior

- Ability to moderate temperature

- Expansion upon freezing

- Versatility as a solvent

LE 3-3

Water-conducting cells

100 µm

Cohesion & AdhesionDuring Transpiration

H2O

>100 ft

CohesionWater molecules hold together through H-bonds to other water molecules

ExampleCohesion helps transport water against gravity in plants from roots to stems during transpiration

Adhesion Water’s attraction to other charged surfaces

ExampleWater’s attraction to cell walls helps upward transport againstgravity

Surface Tension:Strong ordered film-like structure at interface of water and atmosphere

Held together through H-bonds

Strength creates surface for small organisms to move across

• Consequence– Lessens temperature fluctuations to within limits

that permit life

– Heat is absorbed to hydrogen bonds break– Heat is released when hydrogen bonds form

High specific heat

1 cal/g/oCAmount of heat gained or lost to change the temperature of 1g of water by 1ºC

Compare to alcohol: specific heat of 0.6 cal/g/oC

Evaporative Cooling– transformation of a substance from liquid to gas

– Heat of vaporization • The amount of heat 1 g of liquid must absorb to be converted to

gas (water: ~580 cal/g at 25oC)• remaining surface cools during evaporation, a process called

evaporative cooling

• Consequence• Evaporative cooling of water helps stabilize temperatures in

organisms and bodies of water• Perspiration: sensation?

Solid water (ice):

less dense than liquid because H-bonds more stable and ordered; expansion occurs

Consequence:

Ice floats on liquid water

Insulates; prevents temperature fluctuations

Example: ponds and lakes in wintertime

aquatic organisms survive in the liquid water beneath ice

Polar solvent

Dissolves other polar or charged solutesExamples: salts, polar proteins, nucleic acids

Creates an aqueous solution-through

Hydration shellsH-bonds

LE 3-6

Na+

Na+

Cl–Cl–

+

+

+

+

+

+

++

–

–

–

–

–

–

–

–

–

–

–

Hydration shells form aroundcations and anions

Causes salt crystals to dissolveIn H2O

LE 3-7a

Lysozyme molecule (protein)in a nonaqueous environment.

LE 3-7b

Lysozyme molecule in a aqueous environment.

Can you deduce what regions on lysozyme are positive and negative?

• Hydrophilic substance– Attracted to water due to charged or polar nature

e.g. salts (ionic)

• Hydrophobic substance-Repelled by water due to nonpolar nature

e.g. oils, fats (nonpolar)

Important when considering the plasma membrane.

Concepts of Hydrophilic and Hydrophobic

• Most biochemical reactions occur in water

• Most reactions are highly sensitive to pH

Aqueous chemistry in biological systems

What is pH and how does it relate to water?

Reactant-1 + Reactant-2 Product

Enzyme

LE 3-UN53

Hydroniumion (H3O+)

Hydroxideion (OH–)

Simplified to H+

Water occasionally produces protons (H+) and hydroxide ions (OH-)

Results:– molecule with the extra proton is now a

hydronium ion (H3O+)

– The molecule that lost the proton is now a hydroxide ion (OH-)

Water Dissociation-Hydrogen involved in H-bonds in H2O can lose electron

-H+ (proton) can bond with another H2O molecule

• Dissociation of water molecules• Rare in pure water (25oC)

» [H+]=10^-7 M» [OH-]=10^-7 M

• Changes in concentrations of H+ and OH- drastically affect the chemistry of a cell

Such changes alter the pH

pH-reflects the molar concentration of H+ in a solution

pH= -log[H+]

-increases in [H+] increase aciditye.g. HCl (hydrochloric acid) readily dissociates into H+ and Cl-

-increases in [OH-] raises alkalinity, decreases in acidity

e.g. the base NaOH (sodium hydroxide) readily dissociates into Na+ and OH-

The pH Scale• pH 7 occurs when [H+] =[OH-]

• Acidic solutions pH < 7, [H+] > [OH-]

• Basic solutions pH > 7, [H+] < [OH-]

• Most biological fluids: pH 6-8

LE 3-8pH Scale

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14Oven cleaner

Household bleach

Household ammonia

Milk of magnesia

Seawater

Pure waterHuman blood

UrineRainwater

Black coffee

Tomato juice

Vinegar, beer, wine,cola

Digestive (stomach)juice, lemon juice

Battery acid

Neutral[H+] = [OH–]

Inc

rea

sin

gly

Ac

idic

[H+]

> [

OH

–]

Inc

rea

sin

gly

Ba

sic

[H+]

< [

OH

– ]

Given:pH= -log[H+]

Constant: Water ion product 10^-14 M^2= [H+][OH-]

Calculating pH

What is the pH of a solution containing 10^-7 M H+? 10^-4 M

For the same solutions, what is the concentration of OH-?

Determine the concentration of H+ and OH- at pH 3.

Apparent small changes in pH value are really LARGE Exponential!

Calculate the difference between pH 7 and pH 4

[H+] is10^3 x larger

Buffers• pH of most living cells must remain close to pH 7

• Buffers minimize changes in [H+] and [OH- ]in a solution

• Most buffers consist of an acid-base pair that reversibly combines with H+

• H2CO

3 HCO

3- + H+

• carbonic acid bicarbonate

The Damage of Acid Precipitation

• Acid precipitation refers to rain, snow, or fog with a pH lower than 5.6

• Caused by the mixing of different pollutants with water in the air e.g. sulfur and nitrogen oxides

• Main source: combusted fossil fuels

• Acid precipitation can damage life in lakes and streams– Leaches geological buffers from soils– Solubilizes toxic heavy metals e.g. aluminum

LE 3-9

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Morebasic

Normalrain

Moreacidic

Acidrain