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ENERGY AND METABOLISM
Chapter 6
BASIC CONCEPTS
Matter is anything that has mass and takes up space.
Weight is a measurement of the pull of the Earth's gravity on an object.
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Weight changes with distance.
Mass of an object is constant regardless of distance.
Energy is the capacity to do work.
Work is any change in the state or motion of an object.
Units of energy are expressed in kilojoules (kJ), a unit of work, or kilocalories (kcal), a unit of
heat energy.
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1 kcal = 4.184 kJ
Technically mass is a form of energy. E = mc2.
Energy can change form.
Kinetic energy is the energy of motion.
Potential energy is stored energy.
Heat energy cannot do cellular work.
THERMODYNAMICS
Thermodynamics is the study of energy and its transformations.
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System is the object being studied in thermodynamics.
Surrounding is the rest of the universe other than the system.
Closed systems do not exchange matter or energy with its surroundings.
Open systems exchange matter and/or energy with the surroundings.
1. First law of thermodynamics.
Energy of the universe is constant.
Energy-mass cannot be created nor destroyed.
2. Second law of thermodynamics.
The entropy of the universe is increasing.
Entropy is a measure of the disorder or randomness of a system.
When energy is converted from one form to another, some of the usable energy is converted to
heat and is dispersed in the surroundings.
At every step of energy transformation there is a loss of energy capable to do work.
No one process that requires energy conversion is 100% efficient.
Entropy or disorder in a closed system tends to increase spontaneously over time.
Heat energy is less organized and has high entropy (S).
METABOLISM
Metabolic reactions require energy transformations.
Metabolism allows the organism to move, grow, heal, reproduce, etc.
Anabolism refers to metabolic reactions that synthesize complex organic molecules from
simpler ones.
Catabolism is the metabolic reactions in which complex molecules are broken down into
simpler ones.
Every type of chemical bond has a specific amount of bond energy.
Bond energy is the energy required to break the chemical bond.
The total bond energy is total potential energy of the system. It is known as enthalpy (H).
Enthalpy can be measured as heat and it is also known as the heat content of the system.
Free energy (G) is the energy available to do work under the conditions of the system.
Free energy and entropy are inversely related in a system. As entropy increases, free energy
decreases.
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As randomness increases there is less energy available to do work.
Enthalpy (H), entropy (S) and free energy (G) are related by the equation G = H - TS.
T stands for temperature, which is the measurement of the kinetic energy of the molecules in
the system and it is held constant in a reaction.
The total amount of free energy cannot be measured but changes (, delta) in free energy can
be measured.
The following equation is used to express what happens in a chemical reaction G = H - TS.
No reaction can take place without a decrease in enthalpy, an increase in entropy or both.
Exergonic reactions release energy and are spontaneous, -G. Free energy decreases.
Spontaneous reactions may be fast or slow.
Activation energy is needed to start the reaction.
There is an increase in free energy in endergonic reactions.
The energy absorbed during the reaction must be supplied by the surroundings.
The free energy required to drive an endergonic reaction may be supplied by coupling it to an
exergonic reaction.
ADENOSINE TRIPHOSPHATE, ATP
In living cell, energy is temporarily stored in ATP.
ATP is a nucleotide made of adenosine, a nitrogenous base, ribose, a 5-C sugar, and three
phosphate groups.
ATP donates energy through the transfer of a phosphate group.
ATP is formed by the phosphorylation of ADP. This is an endergonic reaction that requires
energy input.
Phosphorylation occurs when a phosphate group is transferred to some other compound.
ATP is the link between exergonic catabolic reactions and endergonic anabolic reactions.
The cell maintains a high ration of ATP to ADP as glucose is oxidized in cellular respiration or
as radiant energy is trapped in photosynthesis.
Large quantities of ATP cannot be stored in the cell since ATP is used almost as soon as it is
synthesized.
ROLL OF REDOX REACTIONS
Energy can be transferred through the transfer of a phosphate group.
Energy can also be transferred through the transfer of electrons.
Electrons released through oxidation cannot exist in the free state in the cell.
Every oxidation reaction must be accompanied by a reduction reaction in which electrons are
accepted by other molecule, ion or atom.
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Redox reactions occur simultaneously.
Often occur in a series of reactions in which electrons are transferred from one
molecule to another.
Electron transfers are equivalent to energy transfers.
Most redox reactions involve the transfer of H atom.
When an electron singly or as part of H atom is transferred it carries with it some of the energy
stored in the chemical bond.
The electron progressively loses free energy as it is transferred from molecule to molecule.
Nicotinamide adenine dinucleotide or NAD+ is a common electron acceptor.
XH2 + NAD+  X + NADH + H+
oxidized
a proton and two electrons were transferred
reduced
The energy stored in NADH is usually transferred to ATP.
Nicotine adenine dinucleotide phosphate or NADP+ is another acceptor molecule similar to
NAD+ and form the reduced NADPH.
NADPH provides energy to other reactions including some reactions of photosynthesis.
Flavin adenine dinucleotide or FAD, accepts hydrogens and becomes FADH2
The cytochromes are proteins that contain iron. The iron atom accepts electrons from H atoms
and transfers these electrons to other compounds.
ENZYMES
Enzymes are biological catalysts.
Enzymes consist only or mostly of proteins.
A catalyst increases the speed of a reaction without being consumed.
All chemical reactions require energy of activation.
Activation energy is the energy required to break existing bonds and begin the reaction.
Enzymes lower the activation energy required to initiate the reaction.
Enzymes combine with the substrate to form the enzyme-substrate complex.
The regions to which the substrates bind to the enzyme are called the active sites of the
enzyme.
The substrates come into close contact and react more readily.
When a substrate binds to an enzyme, the shape of the enzyme changes slightly producing
what is called an induced fit.
The substrate usually becomes distorted also and the bonds become strained and more easily
broken.
Enzyme names usually end in -ase.
The majority of enzymes is highly specific and catalyzes one single reaction or a few very
closely related reactions.
Some enzymes consist of two components, a protein called the apoenzyme, and another
chemical component called a cofactor.
A cofactor may be an inorganic ion or an organic molecule. Many trace elements act as
cofactors.
An organic, non-polypeptide cofactor that binds to an enzyme is called a coenzyme.
NADH, NADPH and FADH2 are coenzymes.
Most vitamins are coenzymes or form part of a coenzyme.
CONDITIONS FOR ENZYMATIC ACTIVITY
Optimal temperature for enzyme activity in humans is between 35 and 40oC.
High temperatures denature most enzymes causing the death of the organism.
Some bacteria living in hot springs can stand temperatures near 100oC.
Optimal pH for most human enzymes is between 6 and 8.
Changes in pH changes the charges in the enzyme and affect the H bonds that contribute to the
tertiary and quaternary structure of the enzyme, thus changing the shape of the protein and its
activity.
Pepsin, a protein-digesting enzyme works at a pH of 2 .
REGULATION OF ENZYMATIC ACTIVITY
Cells regulate enzymatic activity by regulating the amount of enzyme produced and by
regulating the conditions that influence the shape of the enzyme.
Enzymes are organized to perform a sequence of chemical reactions in which the product of
one reaction becomes the substrate of the next reaction.
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This is called a metabolic pathway.
Enzymatic activity may be regulated by...
1. A gene may be switched on by a signal from a hormone or some other cellular product. The
amount of enzyme present influences the rate of the reaction.
2. The rate of the reaction may be affected by either the substrate or enzyme concentration.
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If there is an excess of substrate, the rate of the reaction depends on the enzyme
concentration.
If the concentration of the enzyme is constant, increasing the substrate will increase the
rate of the reaction until all the enzyme molecules become saturated.
3. Feedback inhibition occurs when the final product inhibits one of the intermediate steps of
the pathway thus stopping further synthesis of the product.
Some enzymes have an allosteric site to which a substance can bind, changing the enzyme's
activity.
Substances that affect enzymatic activity by binding to allosteric sites are called allosteric
regulators. Some are inhibitors others are activators.
ENZYMATIC INHIBITION
Most enzymes can be inhibited or destroyed by some chemical agents.
Reversible inhibition occurs when the inhibitor forms a weak chemical bond with the enzyme.
There are to forms of reversible inhibition.
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Competitive inhibition happens when the substrate and the inhibitor compete for the
active site of the enzyme. If the concentration of substrate increases the rate increases.
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Noncompetitive inhibition occurs when the inhibitor binds to an allosteric site and
modifies the shape of the enzyme molecule rendering inactive.
Irreversible competition occurs when an inhibitor combines with an enzyme and permanently
inactivates it.
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Many poisons are permanent inhibitors, e.g. mercury, lead, cyanide, pesticides.
Many drugs are enzyme inhibitors that affect bacterial growth, e.g. sulfa drugs, penicillin.