The Chemistry of Life
Chapter 3
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Electrons
Each electron shell has a specific # of orbitals
Each orbital holds up to two electrons
Fig. 3.3
Atoms with incomplete
electron orbitals are
more reactive
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3.3 Molecules
A molecule is a group of atoms held together
by energy
The holding force is called a chemical bond
There are three kinds of chemical bonds
1. Ionic bonds
2. Covalent bonds
3. Hydrogen bonds
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Ionic Bonds - Formed by the attraction of oppositely charged ions
Everyday
table salt
NaCl
Crystal
Fig. 3.8 The formation of the ionic bond in table salt
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Covalent Bonds - Formed when two atoms share electrons
Fig. 3.9 Water molecules contain two covalent bonds
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Hydrogen Bonds
Formed by the attraction of opposite partial
electric charges between two polar molecules
Fig. 3.10 Hydrogen bonding in water molecules
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3.4 Hydrogen Bonds Give Water
Unique Properties
Heat Storage
A large input of thermal energy is required to
disrupt the organization of liquid water
This minimizes temperature changes
Ice Formation
At low temperatures, hydrogen bonds don’t break
Water forms a regular crystal structure that floats
High Heat of Vaporization
At high temperatures, hydrogen bonds do break
Water is changed into vapor
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3.4 Hydrogen Bonds Give Water
Unique Properties
High Polarity
Polar molecules are termed hydrophilic
Water-loving
All polar molecules that dissolve in water are
termed soluble
Nonpolar molecules are termed hydrophobic
Water-fearing
These do not form hydrogen bonds and are
therefore not water soluble
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3.4 Hydrogen Bonds Give Water
Unique Properties
Cohesion
Fig. 3.12
Attraction of water molecules
to other water molecules
Adhesion
Water strider
Attraction of water molecules
to other polar molecules
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3.5 Water Ionizes / pH
Covalent bonds within a water molecule sometimes break
spontaneously
H2 O
OH–
hydroxide
ion
+
H+
hydrogen
ion
A convenient way to express the hydrogen ion concentration of a
solution
_ log [H+]
pH
=
Acid - Dissociates in water to increase H+ concentration
Base - Combines with H+ when dissolved in water
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Acidic solutions
Neutral solutions
Balance between
H+ and OH–
Basic solutions
Fig. 3.14 The pH Scale
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3.6 Forming Macromolecules
Organisms are primarily made of four kinds
of molecules
Proteins
Nucleic acids
Carbohydrates
Lipids
These are termed macromolecules
They constitute the building materials and
machinery of the cell
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Protein Structure
Primary structure
The specific amino acid sequence of a protein
Secondary structure
The initial folding of the amino acid chain by
hydrogen bonding
Tertiary structure
The final three-dimensional shape of the protein
Quaternary structure
The spatial arrangement of polypeptides in a
multi-component protein
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Fig. 3.21 Levels of
protein structure
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Protein Structure
Proteins can be divided into two classes
1. Structural
2. Globular
Long cables
Provide shape/strength
Fibrin
Grooves and depressions
Enzymes
Keratin
Silk
Fig. 3.23
Fig. 3.24
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3.8 Nucleic Acids
Serve as information storage molecules
A nucleotide is composed of three parts
Five-carbon sugar
Nitrogen-containing base
Phosphate
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Fig. 3.26 The structure of a nucleotide
Nitrogenous bases
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3.8 Nucleic Acids
Two varieties
Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)
RNA
DNA
Sugar = Ribose
Sugar = Deoxyribose
Bases = A, G, C, U
Bases = A, G, C, T
Single-stranded
Double-stranded
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Space-filling
model
Fig. 3.28
The DNA
double helix
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3.9 Carbohydrates
Also referred to as sugars
Provide building materials and energy storage
Are molecules that contain carbon, hydrogen
and oxygen in a 1:2:1 ratio
Are of two main types
Simple carbohydrates
Complex carbohydrates
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Simple Carbohydrates
1. Monosaccharides
2. Disaccharides
Consist of one subunit
Fig. 3.29
Consist of two subunits
Fig. 3.30
Glucose
Chemical formula: C6H12O6
Sucrose
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3.10 Lipids
Large nonpolar molecules that are insoluble
in water
Three major types
Fats
Phospholipids
Steroids
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Fats
Used for long-term
energy storage
Also termed
triglycerides or
triacylglycerol
Composed of
three fatty acid
chains linked to
glycerol
Fig. 3.33
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Fats
Fatty acids can be saturated or unsaturated
Most plant
fats
Most animal
fats
Fig. 3.33
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Phospholipids
A modified fat
One of the three fatty acids is replaced by a
phosphate and a small polar functional group
Hydrophilic
head
Hyrophobic
tail
Fig. 3.34a
In water, phospholipids aggregate to form a
lipid bilayer
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Steroids
Composed of four carbon rings
Examples:
Cholesterol
Found in most
animal cell
membranes
Fig. 3.34b
Male and female sex hormones
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