UNIT 3: Part 1
Pathways that Harvest and
Store Chemical Energy
Hillis Textbook Chapter 6
Energy is stored in chemical bonds and can be
released and transformed by metabolic pathways.
 Chemical energy available to do work is termed
free energy (G).
 Five principles governing metabolic pathways:

1.
2.
3.
4.
5.
Chemical transformations occur in a series of intermediate
reactions that form a metabolic pathway.
Each reaction is catalyzed by a specific enzyme.
Most metabolic pathways are similar in all organisms.
In eukaryotes, many metabolic pathways occur inside
specific organelles.
Each metabolic pathway is controlled by enzymes that can
be inhibited or activated.
In cells, energy-transforming reactions are often coupled:
An energy-releasing (exergonic) reaction is coupled to an
energy-requiring (endergonic) reaction.



Adenosine triphosphate (ATP)
is a kind of “energy currency”
in cells.
Energy released by exergonic
reactions is stored in the
bonds of ATP.
When ATP is hydrolyzed, free
energy is released to drive
endergonic reactions.
An exergonic
reaction will
release energy,
allowing it to be
stored in the
ATP molecule!
An endergonic
reaction will
need energy,
which comes
from the
breaking of those
bonds in ATP!

The process of hydrolysis of an ATP molecule is
exergonic: ATP  H2O  ADP  Pi  freeenergy
ΔG is about –7.3 kcal


Free energy of the bond between phosphate groups is much
higher than the energy of the O—H bond that forms after
hydrolysis.
Phosphate groups are negatively charged, so energy is required to
get them near enough to each other to make the covalent bonds
in the ATP molecule.
Lots of
energy
Not as
much
energy
Energy can also be transferred by the transfer of
electrons in oxidation–reduction, or redox
reactions.
 Oxidation is the loss of one or more electrons.
 Reduction is the gain of one or more electrons.
Oxidation and reduction always occur together.

Transfers of hydrogen atoms involve transfers of electrons
(H = H+ + e–).
When a molecule loses a hydrogen atom, it becomes
oxidized.
The more reduced a molecule is, the more energy is stored
in its bonds.
Coenzyme NAD+ is a key
electron carrier in redox
reactions.
NAD+ (oxidized form)
NADH (reduced form)

Reduction of NAD+ is highly endergonic:



NAD  H  2e  NADH

Oxidation of NADH is highly exergonic:

NADH  H 
1

2
O2  NAD  H 2O
In cells, energy is released in catabolism (breaking
bonds) by oxidation…
Energy is then trapped by reduction of coenzymes
such as NADH…
BUT, energy for anabolic (building bonds)
processes is supplied by ATP, not NADH!
So, oxidative phosphorylation transfers energy
from NADH to ATP.

Oxidative phosphorylation
couples:
oxidation of NADH: NADH  NAD   H   2e  energy

with production of ATP:

The coupling is called
chemiosmosis—diffusion of
protons across a membrane,
which drives the synthesis of
ATP.
Chemiosmosis converts
potential energy of a proton
gradient across a membrane
into the chemical energy in
ATP.


energy  ADP  Pi  ATP
ATP synthase
synthesizes ATP!
 It is a membrane
protein with two
subunits:

F0 is the H+
channel; potential
energy of the proton
gradient drives the H+
through.

F1 has active sites
for ATP synthesis.
