1 chapter 2 and 3 the chemical context of life and properties of water
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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