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An Introduction to
Metabolism
Metabolism/Bioenergetics
 Metabolism:
The totality of an organism’s
chemical processes; managing the material
and energy resources of the cell
1. Catabolic pathways: degradative process
such as cellular respiration; releases energy

2.
Exergonic
Anabolic pathways: building process such as
protein synthesis; photosynthesis; consumes
energy

Endergonic
Thermodynamics

Energy (E): capacity to do work;





Kinetic energy: energy of motion
Potential energy: stored energy
Thermodynamics: study of E transformations
1st Law: conservation of energy; E
transferred/transformed, not created/destroyed
2nd Law: transformations increase entropy (disorder,
randomness)
 Combo:
quantity of E is constant, quality is not
Free energy (G)
 Free
energy: portion of system’s E that can perform
work (at a constant T)
 Exergonic reaction: net release of free E to
surroundings (ΔG<0) - spontaneous
 Endergonic reaction: absorbs free E from
surroundings (ΔG>0) – NOT spontaneous
Gibb’s Free Energy Formula
 ΔG
= ΔH - T ΔS
 ΔG = Gibb’s Free Energy, the amount of
free energy available to a system
 Unit = kJ (kiloJoules)
 ΔH = heat of reaction (or Enthalpy), the
amount of heat energy in a system
 Unit = kJ (kiloJoules)
 T = temperature
 Unit (Kelvin  ̊C + 273)
 ΔS = entropy, the amount of
order/disorder in a system
 Unit = J/K (Joules per Kelvin)
What it means



ΔG – positive vs. negative
 + ΔG = reaction is endergonic (energy
must be supplied to make the reaction
occur)
 - ΔG = reaction is exergonic (generally, this
reaction is spontaneous and releases
energy)
ΔH – positive vs. negative
 + ΔH = reaction is endothermic (heat
energy is absorbed from the surrounding
area)
- ΔH = reaction is exothermic (heat energy is
released into the surrounding area)
What it means
 ΔS


– positive vs. negative
+ ΔS = the entropy of the system in
increasing (you have more free moving
molecules, things are breaking apart,
catabolism)
- ΔS = the entropy of the system is
decreasing (you have more structured
arrangement of molecules, tends to be
fewer, more complex, molecules,
anabolism)
 EXAMPLE:
HC2H3O2 has less entropy than 2 H+
(more number of molecules as opposed to
just more atoms in molecule)
EXAMPLE
A
biological reaction has an
exothermic reaction (ΔH) that
produces -55.8kJ of energy. If the
reaction is decreasing entropy (ΔS)
by -0.35kJ/K at a temperature of
25°C, what would be the free
energy (ΔG)? Is this reaction
exergonic (releasing energy) or
endergonic (absorbing energy)?
SOLUTION
ΔG
=?
ΔH = -55.8kJ
ΔS = -0.35kJ/K
T = 25°C
 = 298K
 ΔG
= ΔH – TΔS
 ΔG = -55.8kJ –
(298K)(-0.35kJ/K)
 ΔG = -55.8kJ +
104.3kJ
 ΔG = +48.5kJ
 NOTE: The reaction
is endergonic
QUESTIONS
 How
do you think the following
affects Free Energy? Does it make it
more likely to happen or less likely?
1. Increasing the temperature of a
system?
2. Decreasing the entropy?
3. Decreasing the enthalpy (heat of
reaction)?
ENERGETICS AND
REACTIONS
 All
reactions need energy to start (Energy
of activation)
 Many times this energy cost is
insurmountable
 Strategies to deal with energy cost


Couple reaction with another exergonic
reaction
Enzymes to reduce energy of activation
Energy Coupling & ATP





E coupling: use of
exergonic process to drive
an endergonic one (ATP)
Adenosine triphosphate
ATP tail: high negative
charge
ATP hydrolysis: release of
free E
Phosphorylation
(phosphorylated
intermediate): enzymes

KINASES
Enzymes: LOCK AND KEY
MODEL





Catalytic proteins:
change the rate of
reactions w/o being
consumed
Free E of activation
(activation E): the E
required to break bonds
Substrate: enzyme
reactant
Active site: pocket or
groove on enzyme that
binds to substrate
Induced fit model
Effects on Enzyme Activity
 Temperature
 pH
 Cofactors:

inorganic, nonprotein
helpers; ex.: zinc, iron,
copper
 Coenzymes:

organic helpers; ex.:
vitamins
Rate of enzyme activity
 There
are many actions that can speed
up the rate of enzyme activity (already
discussed)

Optimal pH, increased temp, increased
amount of enzyme, increased amount of
substrate
 Regardless
of optimal concentrations,
thing always limits the enzyme rates


Amount of enzyme
Once you have “maxed” out the use of
enzymes, the rate of reaction is constant
Enzyme Inhibitors
 Irreversible
(covalent);
reversible (weak bonds)
 Competitive: competes for
active site (reversible);
mimics the substrate
 Noncompetitive: bind to
another part of enzyme
(allosteric site) altering its
conformation (shape);
poisons, antibiotics