Chapter #8 - An Introduction to Metabolism (Integrated - With Chapter #9)
Overview The Energy of Life
1. The living cell is a chemical factory where thousands of reactions occur.
2. The cell apply the energy extracted in the process of cellular respiration to
perform various types of work.
I. An Organism’s Metabolism Transforms Matter and Energy, Subject to the
Laws of Thermodynamics
1. The totality of an organism’s chemical reactions is called metabolism.
A. Organization of the Chemistry of Life Into Metabolic Pathways
1. Metabolism as a whole manages the material & energy resources of the cell.
2. Metabolic pathways may be categorized as catabolic pathways or anabolic
Pathways.
3. Catabolic pathways is a degradative process. Catabolism in cellular respiration is a process whereas the sugar glucose and other organic fuels are
broken down in the presence of oxygen to carbon dioxide and water.
4. Anabolic pathways, in contrast consume energy to build complicated
molecules from simpler ones; they are sometimes called biosynthetic pathways.
B. Forms of Energy
1. Energy is the capacity to cause change.
2. Energy exists in various forms.
a) Kinetic energy – energy associated with the motion of objects.
b) A type of kinetic energy is heat or thermal energy.
c) Potential energy – energy that matter posses because of its location or
structure.
d) A type of potential energy is chemical energy.
C. The Laws of Energy Transformation
1. The study of energy transformations that occur in a collection of matter is
called thermodynamics.
D. The First Law of Thermodynamics
1. According to the first law of thermodynamics, the energy of the universe
is constant. Energy can be transferred and transformed, but it cannot be
created or destroyed.
2. The first law is also known as the principle of conservation of energy.
E. The Second Law of Thermodynamics
1. The second law of thermodynamics – Every energy transfer or transformation increases the entropy of the universe.
II. The Free Energy Change of a Reaction Tells Us Whether or Not the
Reaction Occurs Spontaneously
A. Free-Energy Change
1. Free energy is the portion of a system’s energy that can perform work when
temperature and pressure are uniform throughout the system, as in a living
cell.
2. Free energy is written as Delta G
B. Free Energy & Metabolism
1. Chemical energy can be classified as either exergonic or endergonic.
2. An exergonic reaction proceeds with a net release of free energy.
a) Exergonic reactions are those that occur spontaneous.
b) Endergonic reactions are those that absorb free energy from its
surroundings.
III. ATP Powers Cellular Work by Coupling Exergonic Reactions to
Endergonic Reactions
1. A cell does three main inds of work:
a) Chemical work, the pushing of endergonic reactions, which would not
occur spontaneously, such as the synthesis of polymers from monomers.
b) Transport work, the pumping of substances across membranes against
the direction of spontaneous movement.
c) Mechanical work, such as the beating of cilia, the contraction of muscle
cells, and the movement of chromosomes during cellular reproduction.
*The key feature in the way cells manage their energy resources to do this
work is energy coupling, the use of an exergonic process to drive an
endergonic one.
A. The Structure & Hydrolysis of ATP
1. ATP (adenosine triphosphate) is a nucleotide. ATP contains the sugar
ribose, with the nitrogenous base adenine and a chain of three phosphate
groups bonded to it.
2. The bonds between the phosphate groups of ATP can be broken by
hydrolysis.
a) When the terminal phosphate bond is broken a molecule of inorganic
phosphate leaves the ATP, which becomes adenosine diphosphate or
ADP.
b) This reaction is exergonic & releases 7.3kcal of energy per mole of
ATP hydrolyzed.
c) Because their hydrolysis releases energy, the phosphate bonds of ATP
are sometimes referred to as high-energy phosphate bonds, but the
term is misleading.
B. How ATP Performs Work
1. ATP hydrolysis drives endergonic reactions by phosphorylation, the transfer
of a phosphate group to specific reactants, making them more reactive.
2. The recipient of the phosphate group is said to be phosphorylated.
3. ATP hydrolysis also causes changes in the shape & binding affinities
of transport and motor proteins.
C. The Regeneration of ATP
1. ATP is a renewable resource that can be regenerated by the addition of
phosphate o ADP.
2. Catabolic pathways drive the regeneration of ATP from ADP and phosphate.
IV. Enzymes Speed Up Metabolic Reactions by Lowering Energy Barriers
1. An enzyme is a macromolecule that acts as a catalyst, a chemical agent
that speeds up a reaction without being consumed by the reaction.
A. The Activation Energy Barrier
1. In a chemical reaction, the energy necessary to break the bonds of the reactants
is the activation energy. EA.
B. Substrate Specificity of Enzymes
1. The reactant an enzyme acts on is referred to as the enzyme’s substrate.
2. The enzyme binds to its substrate forming an enzyme-substrate complex.
3. Only a restricted region of the enzyme molecule actually binds to the substrate.
This region, called the active site, is typically a pocket or groove on the
surface of the protein where catalysis occurs.
4. The specificity of an enzyme is attributed to a compatible fit between the
shape of its active site & the shape of the substrate.
C. Effects of Local Conditions on Enzyme Activity
1. Enzyme efficiently is affected by general environmental factors, such as
a) temperature
b) pH
2. Enzyme activity can also be affected by
a) chemicals that specifically influence that enzyme.
D. Effects of Temperature & pH
1. Each enzyme works better under some conditions than under others, because these optimal conditions favor the most active shape for the enzyme
molecule.
2. The rate of an enzymatic reaction increases with increasing temperature,
partly because substrates collide with active sites more frequently when
the molecules move rapidly. (This is up to a point)
a) Above that certain temperature, the speed of the enzymatic reaction drops
sharply.
b) Most human enzymes have optimal temperatures of about 35-40 degrees C
c) The thermophilic bacteria that live in hot springs contain enzymes with
optimal temperatures of 70 degrees C or higher.
3. The optimal pH values for most enzymes fall in the range of pH 6-8, both
there are exceptions. (Pepsin, a digestive enzyme in the human stomach,
works best at pH 2).
4. Trypsin, a digestive enzyme residing in the alkaline environment of the
human intestine has an optimal pH of 8 & would be denatured in the stomach.
E. Cofactors
1. Cofactors are enzymes that require nonprotein helpers for catalytic activity.
2. Cofactors that are organic molecules are more specifically called a coenzyme.
F. Enzyme Inhibitors
1. Certain chemicals selectively inhibit the action of specific enzymes.
2. Many enzyme inhibitors bind to the enzyme by weak interactions, in which
case inhibition is reversible.
a) Competitive inhibitors reduce the productivity of enzymes by blocking
substrates from entering active sites.
b) Noncompetitive inhibitors do not directly compete with the substrate to
bind to the enzyme at the active. Noncompetitive inhibitors impede
enzymatic reactions by binding another part of the enzyme.
V. Regulation of Enzyme Activity Helps Control Metabolism
A. Allosteric Regulation of Enzymes
1. Many enzymes are allosterically regulated:
a) Regulatory molecules, either activators or inhibitors, bind to specific
regulatory sites, affecting the shape & function of the enzyme.
b) In cooperativity, binding of one substrate molecule can stimulate
binding or activity at other active sites.
B. Feedback Inhibition
1. In feedback inhibition, a metabolic pathway is switched off by the inhibitory
binding of its end product to an enzyme that acts early in the pathway.
2. An example of feedback inhibition – as the end product accumulates, it
slows down its own synthesis by allosterically inhibiting the enzyme for
the first step of the pathway.
C. Specific Localization of Enzymes Within the Cell
1. Some enzymes are grouped into complexes, some are incorporated into
membranes, & some are contained inside organelles, increasing the efficiency
of metabolic processes.