Download Lecture Presentation to accompany Principles of Life

2
Life Chemistry and Energy
Chapter 2 Life Chemistry and Energy
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.
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 6
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—total number of protons and
neutrons
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
A Bohr model for atomic structure—the atom
is largely empty space, and the electrons
occur in orbits, or electron shells.
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
Behavior of electrons determines whether a
chemical bond will form and what shape the
bond will have.
Figure 2.1 Electron Shells
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
Atoms with unfilled outer shells tend to undergo
chemical reactions to fill their outer shells.
They can attain stability by sharing electrons
with other atoms or by losing or gaining
electrons.
The atoms are then bonded together into
molecules.
Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry
Octet rule—atoms with at least two electron
shells form stable molecules so they have
eight electrons in their outermost shells.
Concept 2.2 Atoms Interact and Form Molecules
Chemical bond is an attractive force that links
atoms together to form molecules.
There are several kinds of chemical bonds.
Table 2.1 Chemical Bonds and Interactions
Concept 2.2 Atoms Interact and Form Molecules
Ionic bonds
Ions are charged particle that form when an
atom gains or loses one or more electrons.
Cations—positively charged ions
Anions—negatively charged ions
Ionic bonds result from the electrical attraction
between ions with opposite charges.
The resulting molecules are called salts.
Figure 2.2 Ionic Bond between Sodium and Chlorine
Concept 2.2 Atoms Interact and Form Molecules
Ionic attractions are weak, so salts dissolve
easily in water.
Concept 2.2 Atoms Interact and Form Molecules
Covalent bonds
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 2.3 Electrons Are Shared in Covalent Bonds
Concept 2.2 Atoms Interact and Form Molecules
Carbon atoms have four electrons in the outer
shell—they can form covalent bonds with four
other atoms.
Figure 2.4 Covalent Bonding (Part 1)
Figure 2.4 Covalent Bonding (Part 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
• Degree of sharing electrons is not always
equal.
Electronegativity—the attractive force that an
atomic nucleus exerts on electrons
• It depends on the number of protons and the
distance between the nucleus and electrons.
Table 2.2 Some Electronegativities
Concept 2.2 Atoms Interact and Form Molecules
If two atoms have similar electronegativities,
they share electrons equally, in what is called
a nonpolar covalent bond.
If atoms have different electronegativities,
electrons tend to be near the most attractive
atom, in what is called a polar covalent bond
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
forms hydrogen bonds.
They form between water molecules.
They are important in the structure of DNA
and proteins.
Figure 2.5 Hydrogen Bonds Can Form between or within Molecules
Concept 2.2 Atoms Interact and Form Molecules
Water molecules form multiple hydrogen bonds
with each other—this contributes to high heat
capacity.
Concept 2.2 Atoms Interact and Form Molecules
A lot of heat 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 is required to change water from liquid
to gaseous state.
Thus, evaporation has a cooling effect on the
environment.
Sweating cools the body—as sweat evaporates
from the skin, it transforms some of the
adjacent body heat.
Concept 2.2 Atoms Interact and Form Molecules
Hydrogen bonds also give water cohesive
strength, or cohesion—water molecules
resist coming apart when placed under
tension.
• This permits narrow columns of water to
move from roots to leaves of plants.
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 2.6 Hydrophilic and Hydrophobic
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.
Figure 2.7 Functional Groups Important to Living Systems (Part 1)
Figure 2.7 Functional Groups Important to Living Systems (Part 2)
Concept 2.2 Atoms Interact and Form Molecules
Macromolecules
• Most biological molecules are polymers
(poly, “many”; mer, “unit”), made by covalent
bonding of smaller molecules called
monomers.
Concept 2.2 Atoms Interact and Form Molecules
• Proteins: Formed from different combinations
of 20 amino acids
• Carbohydrates—formed by linking similar
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
Cn ( H 2O) n
• Source of stored energy
• Transport stored energy within complex
organisms
• Structural molecules that give many
organisms their shapes
• Recognition or signaling molecules that 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 (Part 1)
Figure 2.9 Monosaccharides (Part 2)
Concept 2.3 Carbohydrates Consist of Sugar Molecules
Monosaccharides are covalently bonded by
condensation reactions that form glycosidic
linkages.
Sucrose is a disaccharide.
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.
Concept 2.3 Carbohydrates Consist of Sugar Molecules
Polysaccharides are large polymers of
monosaccharides; the chains can be
branching.
Starches—a family of polysaccharides of
glucose
Glycogen—highly branched polymer of
glucose; main energy storage molecule in
mammals
Cellulose—the most abundant carboncontaining (organic) biological compound on
Earth; stable; good structural material
Figure 2.10 Polysaccharides (Part 1)
Figure 2.10 Polysaccharides (Part 2)
Figure 2.10 Polysaccharides (Part 3)
Concept 2.4 Lipids Are Hydrophobic Molecules
Lipids are hydrocarbons (composed of C and
H atoms); they 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 role 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
They 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 3 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
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 contain one or more double bonds.
These acids cause kinks in the chain and
prevent molecules from packing together
tightly.
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 compound bound to glycerol
The phosphate group has a negative charge,
making that part of the molecule hydrophilic.
Figure 2.13 A Phospholipids
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 B Phospholipids
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
Metabolism—the sum total of all chemical
reactions occurring in a biological system at a
given time
Metabolic reactions involve energy changes.
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
Two basic types of metabolism:
Anabolic reactions link simple molecules to
form complex ones.
• They require energy inputs; energy is
captured in the chemical bonds that form.
Catabolic reactions break down complex
molecules into simpler ones.
• Energy stored in the chemical bonds is
released.
Figure 2.14 Energy Changes in Reactions (Part 1)
Figure 2.14 Energy Changes in Reactions (Part 2)
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: Disorder (entropy) tends to
increase.
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
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
As a result of energy transformations, disorder
tends to increase.
• Some energy is always lost to random
thermal motion (entropy).
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
One way to investigate the possibility of life on
other planets is to study how life may have
originated on Earth.
An experiment in the 1950s combined gases
thought to be present in Earth’s early
atmosphere, including water vapor. An
electric spark provided energy.
Complex molecules were formed, such as
amino acids. Water was essential in this
experiment.
Figure 2.16 Synthesis of Prebiotic Molecules in an Experimental Atmosphere (Part 1)
Figure 2.16 Synthesis of Prebiotic Molecules in an Experimental Atmosphere (Part 2)