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Ch. 8
An Introduction to Metabolism
I. Introduction
A. The cell has thousands of chemical reactions
occurring within a microscopic space.
-Example: Cellular respiration - energy
from sugar is extracted.
II. Metabolism, Energy, and Life
A. Metabolic pathways
1. Metabolism is the sum of all chemical
reactions in an organism.
2. Enzymes accelerate chemical rxns.
3. Catabolic pathways release energy
by breaking down complex molecules to
simpler compounds.
Ex. Cellular respiration
4. Anabolic pathways consume energy to
build complicated molecules from simpler
compounds.
Ex. Synthesis of proteins from amino acids
B. Energy coupling: Energy from catabolic
pathways is used for anabolic pathways.
C. Bioenergetics is the study of how organisms
manage their energy resources.
D. How organism transform energy
1. Energy:
2. Kinetic energy:
3. Potential energy:
4. Chemical energy:
Organisms can convert one type of energy
to another (Ex. Slide)
E. The two laws of thermodynamics (the study
of energy transformations) in regards to an
open system:
1. The first law of thermodynamics: energy
can be transferred and transformed, but
it cannot be created or destroyed.
2. The second law of thermodynamics:
entropy increases with every energy
transformation or transfer. The universe
is becoming more random.
-much of the energy transformed in
the universe is transformed into heat
energy.
Combining the two laws, the quantity of
energy is constant, but the quality is not.
F. Organisms live at the expense of free
energy
1. Spontaneous processes are those that
can occur without outside help.
Spontaneous processes occur so that
a system may become more stable.
2. The free energy (G) in a system is
related to the total energy (H) and its
entropy (S) by this relationship:
G = H – TS,
where T is temperature in Kelvin units.
a. Free energy can also be a measure of
stability in a system. Systems high
in free energy are unstable. They will
lose free energy in spontaneous
processes.
•delta G = G final state - G starting state
Delta G is negative in spontaneous
processes.
3. Systems that are stable and are at
equilibrium have no change in free
energy.
Systems at equilibrium means that it
can do no work.
Systems at equilibrium must receive
energy from an outside source in
order to do work; it is
nonspontaneous.
G. Exergonic and Endergonic reactions in
metabolism
1. In an exergonic reaction, there is a
release of energy.
-delta G is negative. It is a spontaneous
reaction.
For the overall reaction of cellular
respiration:
C6H12O6 + 6O2 -> 6CO2 + 6H2O
delta G = -686 kcal/mol
2. In an endergonic reaction, energy is
absorbed.
-delta G is positive and free energy is
stored.
-it is nonspontaneous.
Photosynthesis is steeply endergonic,
powered by the absorption of light energy.
•Delta G = + 686 kcal / mol.
H. Metabolic Disequilibrium: reactions in a
closed system will eventually reach
equilibrium and do no work.
1. Cells maintain disequilibrium because
there is a constant flow of energy in
and out of a cell.
2. Sunlight is a source of free energy for
photosynthetic organisms.
Nonphotosynthetic organisms depend
on photosynthetic organisms for energy
in the form of organic molecules.
I. ATP powers cellular work
1. A cell does three main kinds of work:
•Mechanical work
•Transport work
•Chemical work
2. In most cases, ATP is the immediate
source of energy.
3. ATP (adenosine triphosphate) is a
type of nucleotide consisting of:
-nitrogenous base adenine
-ribose sugar and a chain of
-three phosphate groups
4. The bonds between phosphate groups
can be broken by hydrolysis.
Hydrolysis of the end phosphate group
forms adenosine diphosphate [ATP  ADP
+ Pi] and releases 7.3 kcal of energy per
mole of ATP under standard conditions.
In the cell delta G is about -13 kcal/mol.
5. The bonds between the phosphate
groups are referred to as high-energy.
However, these bonds are weak and
unstable. ADP + Pi is more stable.
ATP is more unstable than ADP because
each phosphate group has a negative
charge. These negative charges repel
one another.
6. When ATP is hydrolyzed, the Pi bonds
to another molecule. This molecule is
now phosphorylated, and energized.
7. ATP is a renewable resource that is
continually regenerated by adding a
phosphate group to ADP.
III.Enzymes
A. Enzymes are catalysts. Catalysts change
the rate of reaction without being destroyed.
B. Enzymes speed up metabolic reactions by
lowering energy barriers.
Activation energy is the amount of energy
necessary to push the reactants over an energy
barrier.
Enzymes work by lowering the activation
energy.
C. Enzymes are substrate specific
1. A substrate is a reactant that binds to the
enzyme. The enzyme catalyzes the
conversion of substrate to product.
2. The substrate binds to the active site
on the enzyme.
3. When the substrate binds to the active
site, the enzyme fits tighter around the
substrate. This is called induced fit.
4. The substrate is held to the active site by
weak bonds such as hydrogen and ionic
bonds.
5. R-groups of amino acids in the active
site catalyze of substrate to product.
6. Once the product is made, it leaves the
active site and the enzyme is free to take
another substrate.
7. Enzymes can catalyze reactions in both
forward and reverse directions.
D. A cell’s physical and chemical environment
can affect enzyme activity
1. Temperature
Each enzyme has an optimal
temperature.
2. pH: Most enzymes have an optimal pH
between 6-8.
However, digestive enzymes in the
stomach (pepsin) have an optimal pH
of 2, and intestinal enzymes (trypsin)
have an optimal pH of 8.
E. Cofactors: many enzymes require nonprotein helpers. These helpers are called
cofactors.
1. These cofactors may be attached to
the active site on enzymes, or they may
bind to the substrate.
2. Inorganic examples: zinc, iron, copper.
3. Organic example (called a coenzyme):
vitamins
F. Enzyme Inhibitors:
1. Competitive inhibitors:
2. Noncompetitive inhibitors:
3. Examples: DDT, penicillin
IV. The control of Metabolism
A. Molecules regulate enzyme activity:
Allosteric regulation
An activator will bind and change the shape
of the enzyme to its active form. An
inhibitor will bind and cause the enzyme to
maintain its inactive form.
B. Feedback inhibition: a metabolic pathway
is turned off by its end product, which acts
as an inhibitor of an enzyme within the
pathway.
C. Cooperativity: a substrate binds to the
enzyme, causing the enzyme to take the
active form. The enzyme’s multiple
subunits are primed to accept its
substrates.