An Introduction to Metabolism
Chapter 8

Objectives
 Explain
how the nature of energy transformations is
guided by the two laws of thermodynamics
 Distinguish between endergonic and exergonic
reactions
 Describe how ATP functions as the universal energy
shuttle in cells
 Describe the structure of enzyme-substrate
interactions and how enzymes catalyze biological
reactions
Introduction
Characteristics of organisms are all the endproducts of the chemical reactions that
occur in their cells
 Chemical reactions carried out for the
purpose of energy transformation or making
necessary substances

Energy-The Capacity to do Work
Energy is described and measured by how it
affects matter
 Two types of energy:

 kinetic-energy
of motion
 potential-stored
energy because of structure or
location
 Example:
the energy stored in chemical bonds
G = Gibbs Free Energy
(Delta) G = Free energy available to do work in a
cell
 A - G means a rxn gives off energy; it provides
power

A + G means a rxn needs energy; it will not run
unless energy is first added
 Every rxn has a specific G


G Never changes!
Laws of Energy Conservation
Thermodynamics = study of energy transformations
Two laws govern energy transformation:

 First
law (energy conservation)
 total amount of energy in universe is constant
• can be transferred or transformed but cannot be
created or destroyed
 Second
law (entropy-disorder- increases)
 every energy transformation increases entropy
• energy available for doing useful work decreases
with every transformation
Energy Relationships in Living Things

Chemical reactions in cells either store or release
energy
 endergonic
reactions require input of energy
 energy input equals difference in potential energy
between reactants and products
exergonic reactions release energy
 energy released equals difference in potential energy
between reactants and products
 cellular
metabolism is sum total of all endergonic and
exergonic reactions in cells
Energy Relationships

ATP is cell’s energy shuttle
 most
cell reactions require small amounts of
energy
 food storage molecules contain large amounts
of energy
 energy in food molecules converted to energy
in ATP
 one
food molecule=many ATP (e.g. 1 x glucose=36
ATP)
Energy Relationships

Hydrolysis of ATP releases energy
 Most
energy is located in the covalent bond
between 2nd and 3rd phosphate groups

easily hydrolyzed
 forms
ADP and phosphate group
 ATP
ADP + Pi ( means PO4 = phosphate)
phosphate
group used to phosphorylate
cell chemicals-energizing reactions
So
ATP provides power in your cells
ATP synthesis

endergonic reactions of cellular respiration
phosphorylate ADP-reforms ATP

ADP + Pi (PO4 = phosphate) ATP

More about this in Chapter 9
Enzymes

Enzymes are large protein molecules that act as
biological catalysts

Energy of activation (EA) is “energy barrier”,
amount of energy needed to start a reaction

Enzymes can lower energy barriers = EA

Enzymes cannot lower G!
Enzyme Process

Specific enzymes catalyze each cell reaction
 reactant=substrate

reactant binds to enzyme active site
 substrate
 enzyme
converted to product
unchanged and releases product
Enzymes and Denaturation

Factors that affect enzyme activity
 temperature
 pH


salt concentration ( ions)
presence of co-factors
 These
factors may lead to denaturation
Denaturation
= disruption of the enzyme
structure due to adverse conditions
Example: PH to high or low
Question: How do you stop enzyme activity but
not destroy the enzyme? Answer: Inhibition
 Inhibitors
block enzyme action,
 competitive
inhibitors-bind to active site
 noncompetitive inhibitors-bind to second site on enzyme
 negative feedback-inhibition by product of reaction
 some
pesticides and antibiotics function by inhibiting
enzymes
 Inhibitors
most often work on a temporary basis
 BUT>>>>>>>
A+BC : G= - 8.6 kcal needs 8.6
kcal to run the reaction
A.
B.
True
False