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. Chemical transformations occur in a series of intermediate

reactions that form a metabolic pathway.

2. Each reaction is catalyzed by a specific enzyme.

3. Most metabolic pathways are similar in all organisms.

4. In eukaryotes, many metabolic pathways occur inside

specific organelles.

5. Each metabolic pathway is controlled by enzymes that can

be inhibited or activated.

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.

In cells, energy-transforming reactions are often coupled:

An energy-releasing (exergonic) reaction is coupled to an

energy-requiring (endergonic) reaction.

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: 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.

energyfreePADPOHATP i 2

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:

Oxidation of NADH is highly exergonic:

NADHeHNAD 2

OHNADOHNADH 2221

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:

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.

energyeHNADNADH 2

ATPPADPenergy i

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.