Harvesting Chemical Energy

As open systems, cells require outside energy sources to perform
cellular work.

Only photosynthetic organisms have the ability to harness the energy
from the sun.

During Photosynthesis:
 CO2 and H2O are the raw materials used to make glucose
 Light energy is converted into chemical bond energy

Chemical bond energy can be released to drive metabolic reactions by
cellular respiration.

Note: the chemical elements are recycled, but the energy is not!
The storage and release of Free Energy

The free energy is stored and transferred by ATP (adenosine
triphosphate).

Recall, the phosphate bonds have stored energy; breaking these
bonds releases energy (to drive reactions); while forming new
phosphate bonds temporarily stores the chemical energy.

The compound receiving the phosphate group from ATP is said to be
phosphorylated and becomes more reactive in the process.

The phosphorylated compound loses its newly acquired phosphate
group as work is performed.

There are two types of phosphorylation:
1. Substrate level phosphorylation
 an enzyme cataslyzed reaction of Pi to ADP
2. oxidative phosphorylation
 forming of ATP directly through a series of enzyme
catalyzed redox reactions where oxygen is the final
electron acceptor (more about this in a bit).
REDOX Reactions Also Release Energy

Oxidation-reduction reactions = chemical reactions which involve a
partial or complete transfer of electrons from one reactant to another;
called «REDOX reactions» for short.
1. Oxidation = partial or complete loss of electrons
2. Reduction = partial or complete gain of electrons
“LEO says GER”
Loss of electrons = oxidation
Gain of electrons = reduction

The electron transfers require both a donor (becomes oxidized) and
an acceptor (becomes reduced).

Since electrons lose potential energy when they shift toward more
electronegative atoms, redox reaction that move electrons closer to
elements with a higher electronegativity (like oxygen) release energy.

Cellular Respiration is a redox process that (ultimately) transfers
hydrogen, including electrons with high potential energy, from sugar
(glucose) to oxygen.

Released energy is used to convert ADP to ATP.
How is the energy transferred to a molecule of ATP?

Essentially, glucose is broken down in a series of sequential steps so
that the free energy released from breaking bonds can be harnessed
(captured) bit by bit.

Eventually this harnessed free energy is transferred to molecules of
ATP as they are phosphorylated (i.e from ADP + Pi  ATP)
Why does the process need to occur in a sequence of steps?

Activation energy = requires extreme heating to start combustion of
glucose

Exergonic reation = large amount of energy is released; would
damage cells

Therefore, the combustion of glucose occurs by a series of enzyme
catalyzed steps, that result in:

At each step of the way, hydrogen (along with electrons) are
transferred to another substance (enzymes & coenzymes)

When this happens, a portion of the reaction’s total energy is
released and stored in ATP (ie. ATP is produced).
An Overview of Cellular Respiration
There are two types:

Anaerobic respiration (fermentation) = ATP-producing pathway in
which sugars are only partially degraded (low energy yield); proton
donors and acceptors are organic molecules.

Aerobic respiration = ATP-producing pathway in which sugar is fully
oxidized (high energy yield); ultimate electron acceptor is oxygen
(inorganic molecule)
Overall Equation:
(Aerobic)
Aerobic Cellular Respiration occurs in four stages:
1. glycolysis
2. pyruvate oxidation (occurs only in eukaryotic cells)
3. Citric Acid or Kreb’s Cycle
4. Electron Transport Chain (ETC)
Breaking down
glucose to get H+
(and electrons)
Using H+ (and
electrons) by releasing
them slowly 
trapping released
energy in ATP
Glucose + oxygen  carbon dioxide + water + energy
C6H12O6 + 6O2  6CO2 + 6H2O + 38 ATP (total)
Stepwise fall of Electrons

occurs due to special electron acceptors (NAD+ and FAD) and the
electron transport chain.

Hydrogen (and electrons) stripped from glucose is NOT transferred
directly to oxygen, but are first passed to a special electron
acceptor  NAD+
NAD+ (nicotinamide adenine dinulceotide) and FAD (flavin adenine
dinucleotide = are dinucleotides that function as a coenzyme in the redox
reaction of metabolism. NAD+ and FAD
 are found in all cells
 assist enzymes in electron transfer during redox reactions
What happens?
 During the oxidation of glucose, the electrons are transferred and
temporarily trapped in NAD+ and FAD.

This transferred is catalyzed by enzymes called dehydrogenases,
which:
o remove a pair of hydrogen atoms (2 electrons and 2 protons)
from glucose (substrate)
o deliver the two electrons (2e-) and one proton (H+) to NAD+ (or
2e- and 2H+ to FAD)
o release the remaining proton (H+) into the surrounding
solution.
Overall reactions:

The high energy electrons transferred to NAD+ and FAD are then
passed down the electron transport chain to oxygen, powering the
production of ATP by oxidative phosphorylation.
Electron Transport Chains





Consist of electron-carrier molecules built into cellular membranes
Accept energy rich electrons from reduced coenzymes (NADH and
FADH2)
Since electrons lose potential energy when they are transferred to a
more electronegative atom, this series of reactions releases energy.
Release energy from energy-rich electrons in a controlled stepwise
fashion  powering the production of ATP
Each successive carrier in the chain has a higher electronegative
then the carrier before, so the electrons are pulled downhill towards
oxygen (the molecule with the highest electronegativity), the final
electron acceptor.