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Chapter 2 and 3The Chemical Context of Lifeand Properties of Water

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Essential Elements

• Essential elements in living things include carbon C, hydrogen H, oxygen O, and nitrogen N making up 96% of an organism

• A few other elements Make up the remaining 4% of living matter

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Deficiencies

(a) Nitrogen deficiency(b) Iodine deficiency (Goiter)

• If there is a deficiency of an essential element, disease results

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Trace Elements

• Trace elements Are required by an organism in only minute quantities

• Minerals such as Fe and Zn are trace elements

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Basics You Should Know

• Matter• Elements• Compounds• Atoms• Subatomic Particles• Atomic Number • Atomic Mass• Isotopes• Electron

Shells/Energy Levels• Electron Orbitals

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Periodic table– Shows the electron distribution for

all the elements

Secondshell

Helium2He

Firstshell

Thirdshell

Hydrogen1H

2He

4.00Atomic mass

Atomic number

Element symbol

Electron-shelldiagram

Lithium3Li

Beryllium4Be

Boron3B

Carbon6C

Nitrogen7N

Oxygen8O

Fluorine9F

Neon10Ne

Sodium11Na

Magnesium12Mg

Aluminum13Al

Silicon14Si

Phosphorus15P

Sulfur16S

Chlorine17Cl

Argon18Ar

Figure 2.8

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Why do some elements react?• Valence electrons

– Are those in the outermost, or valence shell

– Determine the chemical behavior of an atom

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Covalent Bonding

• A molecule– Consists of two or more atoms held

together by covalent bonds (sharing electrons)

• A single bond– Is the sharing of one pair of valence

electrons

• A double bond– Is the sharing of two pairs of valence

electrons

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ClClEach chlorine atom wants to gain one electron to achieve get 8

Chlorine forms a covalent bond with itself

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ClCl

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Cl Cl

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Cl Cl

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Cl Cl

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Cl Cl8

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Cl Cl

circle the electrons foreach atom that completestheir octets

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Cl Cl

circle the electrons foreach atom that completestheir octets

Each atom has 8 electrons because they share the electrons in the middle

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Cl Cl

circle the electrons foreach atom that completestheir octets

It is called a SINGLE BOND

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Cl Cl

circle the electrons foreach atom that completestheir octets

Single bonds are shownwith a dash

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O2

Oxygen

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How will two oxygen atoms bond?

OO

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OOEach atom has two unpaired electrons

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OO

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OO

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OO

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OO

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OO

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OO

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Both oxygen atoms want to gain two electrons.

OO

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OO

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OO

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OO

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OO

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OOBoth electron pairs are shared.

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8

OO

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8

OO

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two sets of shared electrons,

OOmaking a double bond

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OO=The double bond

is shown as two dashes.

OO

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Covalent Bonding

• Electronegativity– Is the attraction of a particular kind of

atom for the electrons in a covalent bond

• The more electronegative an atom– The more strongly it pulls shared

electrons toward itself

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Covalent Bonding

• In a nonpolar covalent bond– The atoms have

similar electronegativities

– Share the electron equally

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Figure 2.12

This results in a partial negative charge on theoxygen and apartial positivecharge onthe hydrogens.

H2O

–

O

H H+ +

Because oxygen (O) is more electronegative than hydrogen (H), shared electrons are pulled more toward oxygen.

• In a polar covalent bond– The atoms have differing

electronegativities– Share the electrons unequally

Covalent Bonding

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Ionic Bonds

• In some cases, atoms strip electrons away from their bonding partners

• Electron transfer between two atoms creates ions

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FK

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FK

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FK

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FK

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FK

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FK+ _

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FK+ _

The ionic bond is the attractionbetween the positive K+ ion

and the negative F- ion

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Ions

• An anion– Is negatively

charged ions

• A cation– Is positively

charged

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Ionic Substances

Na+

Cl–Figure 2.14

• Ionic compounds– Are often

called salts, which may form crystals

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Weak Chemical Bonds

• Several types of weak chemical bonds are important in living systems

• Van der Waals interactions– Intermolecular forces

• Weak chemical bonds– Reinforce the shapes of large molecules– Help molecules adhere to each other

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Molecular Shape and Function

• Structure determines Function!• The precise shape of a molecule

– Is usually very important to its function in the living cell

– Is determined by the positions of its atoms’ valence orbitals

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Shape and Function

• Molecular shape– Determines how biological molecules

recognize and respond to one another with specificity

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Morphine

Carbon

Hydrogen

Nitrogen

Sulfur

OxygenNaturalendorphin

(a) Structures of endorphin and morphine. The boxed portion of the endorphin molecule (left) binds toreceptor molecules on target cells in the brain. The boxed portion of the morphine molecule is a close match.

(b) Binding to endorphin receptors. Endorphin receptors on the surface of a brain cell recognize and can bind to both endorphin and morphine.

Naturalendorphin

Endorphinreceptors

Morphine

Brain cell

Figure 2.17

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Chemical Reactions

• Chemical reactions make and break chemical bonds

• Chemical reactions absorb or release ENERGY (chemical energy stored in bonds)

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Chemical Reactions• Photosynthesis

– Is an example of a chemical reaction

Figure 2.18

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Chemical Reactions

• Chemical equilibrium– Is reached when the forward and

reverse reaction rates are equal

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Importance of waterWater is 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

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Water FactsThree-quarters of the Earth’s surface is submerged in water

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

Figure 3.1

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Water and Life

Most animals and plants contain more than 60% water by volume. Without water life would probably never have developed on our planet.

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Water is PolarThe oxygen end (pole) of water has a negative charge The hydrogen end (pole) has a positive charge

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Hydrogen bonding

The hydrogens of one water molecule are attracted to the oxygen from other water molecules. This attractive force is called hydrogen bonding

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Properties of waterSix emergent properties of water contribute to Earth’s fitness for life

1. Cohesion/Adhesion

2. Surface tension

3. Temperature Moderation

4. High specific heat

5. Evaporative cooling

6. Universal solvent

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Cohesion and AdhesionHydrogen bonds give water its cohesive and adhesive properties.Cohesion-water is attracted to other waterAdhesion-water attracted to other materials

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Capillary ActionCohesion

Helps pull water up through the microscopic vessels of plants

Water conducting cells

100 µmFigure 3.3

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Surface TensionSurface tension

Is a measure of how hard it is to break the surface of a liquid

Is related to cohesion

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Moderation of TemperatureWater moderates air temperature by absorbing heat from air that is warmer and releasing the stored heat to air that is cooler

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Water’s High Specific HeatThe specific heat of a substance is the amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1ºC

1 cal/goC

4.186 J/goC

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Evaporative CoolingIs due to water’s high heat of vaporization

Allows water to cool a surface

Sweating cools the body as heat energy from the body changes sweat into a gas

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Insulation of Bodies of Water by Floating Ice

Solid water, or ice

Is less dense than liquid water and floats in liquid water

Insulates water & organisms below ice layer

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Insulation of Bodies of Water by Floating Ice

The hydrogen bonds in iceAre more “ordered” than in liquid water, making ice less dense (crystal lattice)

Liquid water

Hydrogen bonds constantly break and re-form

IceHydrogen bonds are stable

Hydrogen bond

Figure 3.5

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The Solvent of LifeWater is a versatile solvent due to its polarityIt can form aqueous solutionsCalled the universal solvent because so many substances dissolve in water

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Hydrophilic and Hydrophobic Substances

A hydrophobic substance does not have an affinity for waterNonpolarlipids

A hydrophilic substance has an affinity for waterPolar or ionicCarbohydrates, salts

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Water can dissociate Into hydronium ions (H+ or H3O+) and hydroxide (OH-) ionsChanges in the concentration of these ions Can have a great affect on pH in living organisms

H

Hydroniumion (H3O+)

H

Hydroxideion (OH–)

H

H

H

H

H

H

+ –

+

Figure on p. 53 of water dissociating

Effects of Changes in pH

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Acids and BasesAn acid

Is any substance that increases the hydrogen ion concentration of a solution

A base

Is any substance that reduces the hydrogen ion concentration of a solution (more OH- ions)

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BuffersThe internal pH of most living cells

Must remain close to pH 7

BuffersAre substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution