Metabolism & Enzymes
AP Biology
2007-2008
From food webs to the life of a cell
energy
energy
energy
AP Biology
Flow of energy through life
 Life is built on chemical reactions

transforming energy from one form to
organic molecules  ATP
another
& organic molecules
sun
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solar energy 
ATP & organic molecules
organic molecules 
ATP & organic molecules
Metabolism
 Chemical reactions of life

forming bonds between molecules
 dehydration synthesis
 synthesis
 anabolic reactions

breaking bonds between molecules
 hydrolysis
 digestion
 catabolic reactions
AP Biology
That’s why
they’re called
anabolic steroids!
Examples
 dehydration synthesis (synthesis)
+
enzyme
H2O
 hydrolysis (digestion)
enzyme
H2O
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+
Examples
 dehydration synthesis (synthesis)
enzyme
 hydrolysis (digestion)
enzyme
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Chemical reactions & energy
 Some chemical reactions release energy
exergonic
 digesting polymers
 hydrolysis = catabolism

digesting molecules=
LESS organization=
lower energy state
 Some chemical reactions require
input of energy
building molecules=
MORE organization=
higher energy state
endergonic
 building polymers
 dehydration synthesis = anabolism

AP Biology
Endergonic vs. exergonic reactions
exergonic
endergonic
- energy released
- digestion
- energy invested
- synthesis
+G
-G
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G = change in free energy = ability to do work
Energy & life
 Organisms require energy to live

where does that energy come from?
 coupling exergonic reactions (releasing energy)
with endergonic reactions (needing energy)
+
digestion
synthesis
+
AP Biology
+
energy
+
energy
Adenine
• The bonds between the
phosphate groups of ATP
can be broken by hydrolysis.
– Energy is released from ATP
when the terminal phosphate
bond is broken.
• This release of energy
comes from the chemical
change to a state of lower
free energy, not from the
phosphate bonds
themselves.
AP Biology
Phosphate groups
Ribose
(a) The structure of ATP
Adenosine triphosphate (ATP)
Energy
Inorganic
phosphate
Adenosine diphosphate (ADP)
(b) The hydrolysis of ATP
How the Hydrolysis of ATP Performs Work
 The three types of cellular work (mechanical,
transport, and chemical) are powered by the
hydrolysis of ATP.
 In the cell, the energy from the exergonic reaction
of ATP hydrolysis can be used to drive an
endergonic reaction.
 Overall, the coupled reactions are exergonic.
AP Biology
GGlu  3.4 kcal/mol
Glutamic acid Ammonia
Glutamine
(a) Glutamic acid conversion to glutamine
Phosphorylated
intermediate
Glutamic acid
Glutamine
(b) Conversion reaction coupled with ATP hydrolysis
GGlu  3.4 kcal/mol
GGlu  3.4 kcal/mol
 GATP  −7.3 kcal/mol
Net G  −3.9 kcal/mol
AP Biology
GATP  −7.3 kcal/mol
(c) Free-energy change for coupled reaction
 ATP drives endergonic reactions
by phosphorylation, transferring
a phosphate group to some
other molecule, such as a
reactant.

The recipient molecule is now
called a phosphorylated
intermediate.
 ATP hydrolysis leads to a
change in a protein’s shape and
often its ability to bind to another
molecule.
AP Biology
The Regeneration of ATP
 ATP is a renewable resource that is regenerated by addition of a


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phosphate group to adenosine diphosphate (ADP).
The energy to phosphorylate ADP comes from catabolic reactions in
the cell.
The ATP cycle is a revolving door through which energy passes during
its transfer from catabolic to anabolic pathways.
What drives reactions?
 If reactions are “downhill”, why don’t they
just happen spontaneously?

because covalent bonds are stable bonds
starch
AP Biology
Why don’t
stable polymers
spontaneously
digest into their
monomers?
Activation energy
 Breaking down large molecules
requires an initial input of energy
activation energy
 large biomolecules are stable
 must absorb energy to break bonds

AP Biology
cellulose
energy
CO2 + H2O + heat
Too much activation energy for life
 Activation energy
amount of energy needed to destabilize
the bonds of a molecule
 moves the reaction over an “energy hill”

glucose
AP Biology
Not a match!
That’s too much
energy to expose
living cells to!
Reducing Activation energy
 Catalysts

reducing the amount of energy to
start a reaction
uncatalyzed reaction
Pheeew…
that takes a lot
less energy!
catalyzed reaction
NEW activation energy
reactant
product
AP Biology
Catalysts
 So what’s a cell got to do to reduce
activation energy?

get help! … chemical help… ENZYMES
Call in the
ENZYMES!
G
AP Biology