Download ap carbohydrates and lipids

2
The Chemistry and Energy
of Life
Chapter 2 The Chemistry and Energy of Life
Key Concepts
2.1 Atomic Structure Is the Basis for Life’s
Chemistry
2.2 Atoms Interact and Form Molecules
2.3 Carbohydrates Consist of Sugar
Molecules
2.4 Lipids Are Hydrophobic Molecules
2.5 Biochemical Changes Involve Energy
Chapter 2 Opening Question
Why is the search for water important in
the search for life?
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
Living and nonliving matter is composed of
atoms.
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
Like charges repel; different charges attract.
Most atoms are neutral because the number
of electrons equals the number of protons.
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
Dalton—mass of one proton or neutron
(1.7 × 10–24 grams)
Mass of electrons is so tiny, it is usually
ignored.
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
Element—pure substance that contains only
one kind of atom
Living things are mostly composed of six
elements:
Carbon (C)
Hydrogen (H)
Nitrogen (N)
Oxygen (O)
Phosphorus (P)
Sulfur (S)
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
• The number of protons identifies an
element.
• Number of protons = atomic number
• For electrical neutrality:
protons = electrons
• Mass number is the number of protons
plus neutrons
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
Bohr model for atomic structure: atom is
largely empty space; the electrons occur in
orbits, or electron shells.
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
Bohr models are simplified, but useful in
understanding how atoms behave.
Behavior of electrons determines whether a
chemical bond will form between atoms
and what shape the bond will have.
Figure 2.1 Electron Shells
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
Octet rule: for elements 6–20, an atom will
lose, gain, or share electrons in order to
achieve a stable configuration of 8
electrons in its outermost shell.
When atoms share electrons, they form
stable associations called molecules.
Concept 2.2 Atoms Interact and Form Molecules
A chemical bond is an attractive force that
links atoms together in molecules.
There are several kinds of chemical bonds.
Table 2.1
Concept 2.2 Atoms Interact and Form Molecules
Covalent bonds form when two atoms
share pairs of electrons.
The atoms attain stability by having full
outer shells.
Each atom contributes one member of the
electron pair.
Figure 2.2 Electrons Are Shared in Covalent Bonds
Concept 2.2 Atoms Interact and Form Molecules
Carbon atoms have 6 electrons; 4 in the
outer shell.
They can form covalent bonds with four
other atoms.
Figure 2.3 Covalent Bonding
Table 2.2
Concept 2.2 Atoms Interact and Form Molecules
Properties of molecules are influenced by
characteristics of the covalent bonds:
• Orientation—length, angle, and
direction of bonds between any two
elements are always the same.
Example: Methane always
forms a tetrahedron.
Concept 2.2 Atoms Interact and Form Molecules
• Strength and stability—covalent bonds
are very strong; it takes a lot of energy
to break them.
• Multiple bonds
Single—sharing 1 pair of electrons
C H
Double—sharing 2 pairs of electrons
C C
Triple—sharing 3 pairs of electrons
N N
Concept 2.2 Atoms Interact and Form Molecules
Two atoms of different elements do not
always share electrons equally.
The nucleus of one element may have
greater electronegativity—the attractive
force that an atomic nucleus exerts on
electrons.
Depends on the number of protons and
the distance between the nucleus and
electrons.
Table 2.3
Concept 2.2 Atoms Interact and Form Molecules
If atoms have similar electronegativities,
they share electrons equally (nonpolar
covalent bond).
If atoms have different electronegativities,
electrons tend to be near the most
attractive atom, forming a polar covalent
bond.
Concept 2.2 Atoms Interact and Form Molecules
The partial charges that result from polar
covalent bonds produce polar molecules
or polar regions of large molecules.
Polar bonds influence interactions with other
molecules.
Polarity of water molecules determines
many of water’s unique properties.
Concept 2.2 Atoms Interact and Form Molecules
Hydrogen bonds:
Attraction between the δ– end of one
molecule and the δ+ hydrogen end of
another molecule.
They form between water molecules and
within larger molecules.
Although much weaker than covalent bonds,
they are important in the structure of DNA
and proteins.
Figure 2.4 Hydrogen Bonds Can Form between or within Molecules
Concept 2.2 Atoms Interact and Form Molecules
Hydrogen bonding contributes to properties
of water that are significant for life:
• Water is a solvent in living systems—a
liquid in which other molecules dissolve.
• Water molecules form multiple
hydrogen bonds with each other—this
contributes to high heat capacity.
In-Text Art, Chapter 2, p. 23
Concept 2.2 Atoms Interact and Form Molecules
A lot of heat energy is required to raise the
temperature of water—the heat energy
breaks the hydrogen bonds.
In organisms, presence of water shields
them from fluctuations in environmental
temperature.
Concept 2.2 Atoms Interact and Form Molecules
• Water has a high heat of vaporization:
a lot of heat energy is required to
change water from the liquid to gaseous
state (to break the hydrogen bonds).

Thus, evaporation has a cooling
effect on the environment.

Sweating cools the body—as sweat
evaporates from the skin, it absorbs
some of the adjacent body heat.
Concept 2.2 Atoms Interact and Form Molecules
• Hydrogen bonds give water cohesive
strength, or cohesion—water
molecules resist coming apart when
placed under tension.
• Hydrogen bonding between liquid water
molecules and solid surfaces allows for
adhesion between the water and the
solid surface.
In-Text Art, Chapter 2, p. 24
Concept 2.2 Atoms Interact and Form Molecules

Cohesion and adhesion allow narrow
columns of water to move from roots
to the leaves of plants.
• Surface tension: water molecules at
the surface are hydrogen-bonded to
other molecules below them, making
the surface difficult to puncture. This
allows spiders to walk on the surface of
a pond.
Concept 2.2 Atoms Interact and Form Molecules
Any polar molecule can interact with any
other polar molecule through hydrogen
bonds.
• Hydrophilic (“water-loving”): in
aqueous solutions, polar molecules
become separated and surrounded by
water molecules.
Nonpolar molecules are called
hydrophobic (“water-hating”); the
interactions between them are
hydrophobic interactions.
Figure 2.5 Hydrophilic and Hydrophobic
Concept 2.2 Atoms Interact and Form Molecules
When one atom is much more
electronegative than the other, a complete
transfer of electrons may occur.
This makes both atoms more stable
because their outer shells are full.
The result is two ions—electrically charged
particles that form when atoms gain or
lose one or more electrons.
Figure 2.6 Ionic Attraction between Sodium and Chlorine
Concept 2.2 Atoms Interact and Form Molecules
Cations—positively charged ions
Anions—negatively charged ions
Ionic attractions result from the electrical
attraction between ions with opposite
charges.
The resulting molecules are called salts or
ionic compounds.
Concept 2.2 Atoms Interact and Form Molecules
Ionic attractions are weak, so salts dissolve
easily in water.
place text art pg 25 here
Concept 2.2 Atoms Interact and Form Molecules
Functional groups—small groups of atoms
with specific chemical properties
Functional groups confer these properties to
larger molecules (e.g., polarity).
One biological molecule may contain many
functional groups that determine molecular
shape and reactivity.
Figure 2.7 Functional Groups Important to Living Systems (Part 1)
Figure 2.7 Functional Groups Important to Living Systems (Part 2)
Figure 2.7 Functional Groups Important to Living Systems (Part 3)
In-Text Art, Chapter 2, p. 26
Concept 2.2 Atoms Interact and Form Molecules
• Proteins—formed from different
combinations of 20 amino acids
• Carbohydrates—formed by linking
sugar monomers (monosaccharides) to
form polysaccharides
• Nucleic acids—formed from four kinds
of nucleotide monomers
• Lipids—noncovalent forces maintain the
interactions between the lipid
monomers
Concept 2.2 Atoms Interact and Form Molecules
Polymers are formed and broken apart in
reactions involving water.
• Condensation—removal of water links
monomers together
• Hydrolysis—addition of water breaks a
polymer into monomers
Figure 2.8 Condensation and Hydrolysis of Polymers (Part 1)
Figure 2.8 Condensation and Hydrolysis of Polymers (Part 2)
Concept 2.3 Carbohydrates Consist of Sugar Molecules
Carbohydrates
• Source of stored energy
• Transport stored energy within
organisms
• Structural molecules give many
organisms their shapes
• Recognition or signaling molecules can
trigger specific biological responses
Concept 2.3 Carbohydrates Consist of Sugar Molecules
Monosaccharides are simple sugars.
Pentoses are 5-carbon sugars.
• Ribose and deoxyribose are the
backbones of RNA and DNA.
Hexoses (C6H12O6) include glucose,
fructose, mannose, and galactose.
Figure 2.9 Monosaccharides
Concept 2.3 Carbohydrates Consist of Sugar Molecules
Monosaccharides are covalently bonded by
condensation reactions that form
glycosidic linkages to form
disaccharides.
place text art pg 27 here
Concept 2.3 Carbohydrates Consist of Sugar Molecules
Oligosaccharides contain several
monosaccharides.
• Many have additional functional groups.
• They are often bonded to proteins and
lipids on cell surfaces, where they serve
as recognition signals.
 The human blood groups (ABO) get
their specificity from oligosaccharide
chains.
Concept 2.3 Carbohydrates Consist of Sugar Molecules
Polysaccharides are large polymers; the
chains can be branching.
• Starches—polymers of glucose
• Glycogen—highly branched polymer of
glucose; main energy storage molecule
in mammals
Figure 2.10 Polysaccharides (Part 1)
Concept 2.3 Carbohydrates Consist of Sugar Molecules
Concept 2.3 Carbohydrates Consist of Sugar Molecules
Cellulose—the main component of plant cell
walls.
• It is the most abundant carboncontaining (organic) biological
compound on Earth.
• Very stable; good structural material
Concept 2.3 Carbohydrates Consist of Sugar Molecules
Concept 2.4 Lipids Are Hydrophobic Molecules
Lipids
Hydrocarbons (composed of C and H
atoms) that are insoluble in water because
of many nonpolar covalent bonds.
When close together, weak but additive van
der Waals interactions hold them
together.
Concept 2.4 Lipids Are Hydrophobic Molecules
Lipids:
• Store energy in C—C and C—H bonds
• Play structural roles in cell membranes
• Fat in animal bodies serves as thermal
insulation
Concept 2.4 Lipids Are Hydrophobic Molecules
Triglycerides (simple lipids)
• Fats—solid at room temperature
• Oils—liquid at room temperature
• Have very little polarity and are
extremely hydrophobic.
Concept 2.4 Lipids Are Hydrophobic Molecules
Triglycerides consist of:
• Three fatty acids—nonpolar
hydrocarbon chain attached to a polar
carboxyl group (—COOH) (carboxylic
acid)
• One glycerol—an alcohol with three
hydroxyl (—OH) groups
Synthesis of a triglyceride involves three
condensation reactions.
Figure 2.11 Synthesis of a Triglyceride
Concept 2.4 Lipids Are Hydrophobic Molecules
The fatty acid chains can vary in length and
structure.
In saturated fatty acids, all bonds between
carbon atoms are single; they are
saturated with hydrogens.
In unsaturated fatty acids, hydrocarbon
chains have one or more double bonds.
This causes kinks in the chain and
prevents molecules from packing together
tightly.
Concept 2.4 Lipids Are Hydrophobic Molecules
Because the unsaturated fatty acids do not
pack tightly, they have low melting points
and are usually liquid at room temperature.
place text art pg 30 here
Figure 2.12 Saturated and Unsaturated Fatty Acids (Part 1)
Figure 2.12 Saturated and Unsaturated Fatty Acids (Part 2)
Concept 2.4 Lipids Are Hydrophobic Molecules
Fatty acids are amphipathic; they have a
hydrophilic end and a hydrophobic tail.
Phospholipid—two fatty acids and a
phosphate group bound to glycerol;
The phosphate group has a negative
charge, making that part of the molecule
hydrophilic.
Figure 2.13 Phospholipids (Part 1)
Concept 2.4 Lipids Are Hydrophobic Molecules
In an aqueous environment, phospholipids
form a bilayer.
The nonpolar, hydrophobic “tails” pack
together and the phosphate-containing
“heads” face outward, where they interact
with water.
Biological membranes have this kind of
phospholipid bilayer structure.
Figure 2.13 Phospholipids (Part 2)
Concept 2.5 Biochemical Changes Involve Energy
Chemical reactions occur when atoms
have enough energy to combine or change
bonding partners.
sucrose + H2O
glucose + fructose
(C12H22O11)
(C6H12O6)
reactants
(C6H12O6)
products
Concept 2.5 Biochemical Changes Involve Energy
Chemical reactions involve changes in
energy.
Energy can be defined as the capacity to do
work, or the capacity for change.
In biochemical reactions, energy changes
are usually associated with changes in the
chemical composition and properties of
molecules.
Concept 2.5 Biochemical Changes Involve Energy
All forms of energy can be considered as
either:
• Potential—the energy of state or
position, or stored energy
• Kinetic—the energy of movement; the
type of energy that does work; that
makes things change
Energy can be converted from one form to
another.
Concept 2.5 Biochemical Changes Involve Energy
Metabolism—sum total of all chemical
reactions occurring in a biological system
at a given time
Metabolic reactions involve energy changes.
Energy is either stored in, or released
from, chemical bonds.
A chemical reaction will occur
spontaneously if the total energy
consumed by breaking bonds in the
reactants is less than the total energy
released by forming bonds in the products.
Concept 2.5 Biochemical Changes Involve Energy
Two basic types of metabolism:
• Anabolic reactions link simple
molecules to form complex ones.
 They require energy inputs
(endergonic or endothermic; energy
is captured in the chemical bonds
that form.
Figure 2.14 Energy Changes in Reactions
Concept 2.5 Biochemical Changes Involve Energy
• Catabolic reactions: energy is
released (exergonic or exothermic)

Complex molecules are broken down
into simpler ones.

Energy stored in the chemical bonds
is released.
Concept 2.5 Biochemical Changes Involve Energy
Catabolic and anabolic reactions are often
linked.
The energy released in catabolic reactions
is often used to drive anabolic reactions—
to do biological work.
Concept 2.5 Biochemical Changes Involve Energy
The laws of thermodynamics apply to all
matter and energy transformations in the
universe.
• First law: Energy is neither created nor
destroyed.
• Second law: Useful energy tends to
decrease.
When energy is converted from one form to
another, some of that energy becomes
unavailable for doing work.
Figure 2.15 The Laws of Thermodynamics (Part 1)
Figure 2.15 The Laws of Thermodynamics (Part 2)
Figure 2.15 The Laws of Thermodynamics (Part 3)
Concept 2.5 Biochemical Changes Involve Energy
No physical process or chemical reaction is
100% efficient—some of the released
energy is lost in a form associated with
disorder.
This energy is so dispersed that it is
unusable.
Entropy is a measure of the disorder in a
system.
As a result of energy transformations,
disorder tends to increase.
Concept 2.5 Biochemical Changes Involve Energy
If a chemical reaction increases entropy, its
products are more disordered or random
than its reactants.
If there are fewer products than reactants,
the disorder is reduced; this requires
energy to achieve.
Concept 2.5 Biochemical Changes Involve Energy
Metabolism creates more disorder (more
energy is lost to entropy) than the amount
of order that is stored.
Example:
• The anabolic reactions needed to
construct 1 kg of animal body require
the catabolism of about 10 kg of food.
Life requires a constant input of energy to
maintain order.
Answer to Opening Question
Water is essential for life. One way to
investigate the possibility of life on other
planets is to study how life may have
originated on Earth.
Experiments in the 1950s combined gases
thought to be present in Earth’s early
atmosphere, including water vapor. An
electric spark provided energy.
Complex molecules formed, such as amino
acids. Water was essential in this
experiment.
Figure 2.16 Miller and Urey Synthesized Prebiotic Molecules in an Experimental Atmosphere
(Part 1)
Figure 2.16 Miller and Urey Synthesized Prebiotic Molecules in an Experimental Atmosphere
(Part 2)