6.1 Energy
A. Energy
1. Energy is capacity to do work; cells must continually use
energy to do biological work.
2. Kinetic energy is
energy of motion; all moving objects have kinetic energy.
3. Potential energy is
stored energy.
a. Water
behind a dam has potential energy that can be converted to kinetic energy.
b. Energy
within an atom lies in arrangement of its atoms in molecule; glucose has more
energy than its breakdown components, carbon dioxide and water.
B. Two Laws of Thermodynamics
1. First law of thermodynamics (also called the law of
conservation of energy)
a. Energy
cannot be created or destroyed; it can be changed from one form to another.
b. In an
engine, chemical energy of coal converts to heat; heat energy converts to
kinetic energy.
c. In human
body, chemical energy in food is converted to chemical energy in ATP and then
converted to mechanical energy of muscle contraction.
2. Second law of
thermodynamics
a. Energy
cannot be changed from one form into another without a loss of usable energy.
b. 25% of
chemical energy of gasoline is converted to move a car; rest is lost as heat.
c. When
muscles convert chemical energy in ATP to mechanical energy, some is lost as
heat.
d. Heat is
form of energy but quickly dissipates into the environment; because heat
dissipates,
it can never be converted back to the form of potential energy.
C. Entropy
1. Entropy is measure of randomness or disorder.
2. Organized usable forms of energy
have low entropy; unorganized/less stable forms have high entropy.
3. Energy conversions result in heat
and therefore the entropy of the universe is always increasing.
4. It takes a constant input of
usable energy from the food you eat to keep you organized.
6.2
Metabolic Reactions and Energy Transformations
A. Metabolism
1. Sum of all the biochemical reactions in a cell.
2. In a reaction A + B 
C + D, A and B
are reactants and C and D are products.
3. Free energy (
) is the amount
of energy that is free to do work after a chemical reaction.
4. Change in free energy is noted
as ; a
negative means that
products have less free energy
than reactants; the reaction occurs spontaneously.
5. Exergonic
reactions have a negative and energy
is released.
6. Endergonic
reactions have a positive ; products have
more energy than reactants; such reactions
can only occur
with an input of energy.
7. Reversible reactions
have a free energy difference near zero; such a reaction is at equilibrium.
8. Cells use product of a first
reaction as reactant in second reaction; such a process pulls first reaction
in one
direction.
B. Coupled Reactions
1. Occur when energy released by an exergonic
reaction is used to drive an endergonic reaction.
2. Energy released from ATP ADP + 
is used to
fuel many biological reactions.
3. ATP breakdown is coupled to a
reaction that requires energy; both reactions take place at same time in same
place.
4. When ATP breaks down to drive
reactions, some energy is lost as heat; overall reaction becomes exergonic.
C. ATP: Energy for Cells
1. ATP (adenosine triphosphate)
is energy currency of cells; when cells require energy, they
"spend" ATP.
2. Great demand for ATP requires
body to constantly produce ATP.
3. Small amount of ATP is constantly
recycled from ADP and — it is
continually made, broken down,
and remade
in cells.
4. The energy released from
ATP ADP + is just
about enough for most biological reactions.
D. Function of ATP
1. Chemical work: ATP supplies energy to synthesize
macromolecules that make up the cell.
2. Transport work: ATP
supplies energy needed to pump substances across the plasma membrane.
3. Mechanical work:
ATP supplies energy to move muscles, cilia and flagella, chromosomes, etc.
E. Structure of ATP
1. ATP is a nucleotide made of base adenine, sugar ribose, and three
phosphate groups.
2. ATP is called a
"high-energy" compound because a phosphate group is easily removed.
3. In cells, about 7.3 kcal per mole
is released when ATP is hydrolyzed to ADP + .
6.3
Metabolic Pathways and Enzymes
A. Reactions in Cells are Orderly
1. Metabolic pathways are orderly sequence of chemical reactions;
each step is catalyzed by a specific enzyme.
2.Metabolic pathways begin with
particular reactant, end with end product, and have many in intermediate steps.
3. One pathway leads to next; since
pathways use same molecules, a pathway can lead to several others.
4. Metabolic energy is captured more
easily if it is released in small increments.
5. A reactant is
substance that participates in reaction; a product is substance
formed by reaction.
6. Each step in a series of chemical
reactions is assisted by an enzyme.
7. Enzymes are
catalysts that speed chemical reactions without the enzyme being changed.
8. Every enzyme is specific in its
action and catalyzes only one reaction or one type of reaction.
9. A substrate is a
reactant in an enzymatic reaction.
B. Energy of Activation
1. For metabolic reactions to occur in a cell, an enzyme must usually be
present.
2. Without enzymes, activation is
achieved by heating reaction flask to increase molecular collisions.
3. Energy of activation
(Ea) is energy that must be added to cause molecules to react.
C. Enzyme-Substrate Complexes
1. Enzymes speed chemical reactions by lowering the energy of activation
(Ea) by forming a
complex with
their substrate(s) at the active site.
a. Active
site is small region on surface of enzyme where the substrate(s) bind.
b. When
substrate binds to enzyme, active site undergoes a slight change in shape that
facilitates
the reaction — this is called the induced-fit model.
2. Only a small amount of enzyme is
needed in a cell because enzymes are not used up.
3. Some enzymes actually
participated in the reaction (e.g. trypsin).
4. A particular reactant(s) may
produce more than one type of product(s).
a. Presence
or absence of enzyme determines which reaction takes place.
b. If
reactants can form more than one product, enzymes present determine product
produced.
5. Every cell reaction requires its
specific enzyme; enzymes are named for substrates by adding "-ase."
D. Factors Affecting Enzymatic Speed
1. Enzymatic reactions are rapid (e.g., 2H2O2 H2O + O2 occurs
600,000 times/sec with catalase).
a. To
achieve maximum product per unit time, need enough substrate to fill active
sites.
b. Optimal
temperature and pH increase rates of enzymatic reaction.
2. Temperature and pH
a. As
temperature rises, enzyme activity increases because there are more molecular
collisions.
b. Enzyme
activity declines rapidly when enzyme is denatured at a certain
temperature; results
in change in shape of enzyme.
c. Each
enzyme has optimal pH that maintains its normal configuration.
d. A change
in pH alters ionization of side chains, eventually resulting in denaturation.
3. Enzyme Concentration
a. Enzyme
concentration is regulated by a cell.
b. Some
enzymes regulated by phosphorylation; molecules
received by membrane receptors turn
on kinases, which activate enzymes by phosphorylating them.
4. Enzyme Inhibition
a. Inhibition
is common means by which cells regulate enzyme activity.
b. In competitive
inhibition, another molecule is similar to enzymes substrate, competes
with
true substrate for enzyme’s active site, resulting in decreased product
formation.
c. In noncompetitive
inhibition, a molecule binds to allosteric
site, a site other than active site,
hereby changing the three-dimensional structure of enzyme and ability to bind
to its substrate.
d. Feedback
inhibition regulates activity of most enzymes; product produced by an enzyme
binds
to enzyme’s active site.
1) When product is abundant, active sites are full and enzyme activity drops.
2) When product is used up, inhibition is reduced and more product is produced.
3) Concentrations of products can be kept within narrow ranges.
4) Pathways can be regulated by feedback inhibition; end product of pathway
binds at an
allosteric site on the first enzyme of the pathway,
shutting down the pathway.
e. Cyanide
inhibits an essential enzyme (cytochrome oxidase) found in all cells.
5. Enzyme Cofactors
a. Many
enzymes require an inorganic ion or non-protein cofactor to
function.
b. Ions are
metals; the organic cofactors are coenzymes (e.g. vitamins)
that assist enzymes or
accept or contribute atoms to the reaction.
c. Vitamins
required in trace amounts for synthesis of coenzymes; become part of coenzyme’s
molecular structure; vitamin deficiency causes lack of coenzyme and lace of
enzyme action.
6.4
Metabolic Pathways and Oxidation-Reduction
A. Oxidation-Reduction
1. In oxidation-reduction (redox) reactions,
electrons pass from one molecule to another.
2. Oxidation is the
loss of electrons.
3. Reduction is the
gain of electrons.
4. Both reactions occur at the same
time because one molecule accepts electrons given up by another molecule.
B. Photosynthesis
1. Photosynthesis uses energy to combine carbon dioxide and water to produce
glucose in the formula:
6CO2
+ 6H2O + energy C6H12O6 + 6O2
2. Water has been oxidized and
carbon dioxide has been reduced.
3. Input of energy is needed to
produce high-energy glucose molecule.
4. Chloroplasts capture solar energy
and convert it by electron transport system to chemical energy of ATP.
5. ATP is used along with hydrogen
atoms to reduce glucose; when NADP+ (nicotinamide
adenine dinucleotide
phosphate)
donates hydrogen atoms (H+ + e) to a substrate during photosynthesis, substrate
has
accepted
electrons and is reduced.
6. The reaction that reduces NADP+
is:
NADP+
+ 2e- + H+
NADPH
C. Cellular Respiration
1. Overall equation for aerobic respiration is opposite that of photosynthesis:
C6H12O6
+ 6O2 6CO2 +
6H2O + energy
2. When NAD removed hydrogen atoms
(H+ + e) during cell respiration, the substrate has lost
electrons
and is oxidized.
NAD+
+ 2e- + H+
NADH
3. At the end of aerobic
respiration, glucose has been oxidized to carbon dioxide and water and
ATP have
been produced.
D. Electron Transport System
1. Both photosynthesis and respiration are metabolic pathways that use an
electron transport system
consisting
of membrane-bound carriers to pass electrons from one carrier to another.
2. High-energy electrons are
delivered to the system and low-energy electrons leave it.
3. Each time electrons transfer to a
new carrier, energy is released; ultimately used to produce ATP.
E. ATP Production
1. ATP synthesis was known to be coupled to the electron transport system.
2. Peter Mitchell received 1978
Nobel prize for chemiosmotic theory of ATP production.
3. In mitochondria and chloroplasts,
carriers of electron transport systems are located within a membrane.
4. H+ ions collect on one side of
membrane because they are pumped there by certain carriers.
5. The electrochemical gradient
across the membrane is used to provide energy for ATP production.
6. Particles called ATP synthase complexes span the membrane; each complex
contains a channel that
allows
H+ ions to flow down their electrochemical gradient.
7. Flow of H+ ions through the
channel provides the energy to drive ADP + ATP.
8. As solar energy is collected by
plants and converted to ATP, thylakoid membrane acts
as a dam to
maintain
energy gradient; formation of ATP resembles the turbines in a dam that couple
water flow
to formation
of electricity.