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Cellular Respiration: Harvesting Chemical Energy
Chemical elements essential to life are recycled but energy is not (remember the Laws of
Thermodynamics). Energy flows into the ecosystem as sunlight and back out as heat (Figure 9.2)
• Organic compounds store energy in their arrangements of atoms.
• Catabolic pathways systematically degrade complex organic molecules (rich in energy) to simpler
waste products, which store less energy. In the process, energy is transferred from the energy rich
nutrient molecules to energy rich molecules (e.g.,ATP) that can be used directly by the cell to
• These catabolic pathways include both anaerobic processes (e.g., fermentation and anaerobic
respiration) and aerobic respiration.
• Technically cellular respiration includes both aerobic and anaerobic respiration but historically the
term cellular respiration originated from the study of aerobic respiration. This course will focus on
• Organic compounds are degraded in the presence of oxygen to produce carbon dioxide, water and
usable forms of energy (i.e.,ATP)
Organic Compounds + O ▯ CO + H O + Energy (work + heat)
2 2 2
Organic compounds that can be used in aerobic respiration
• carbohydrates (e.g., glucose)
C 6 O12 66O ▯ ▯ ▯2▯ ▯ 6 CO + 62H O + En2rgy (ATP + Heat)
∆G = - 686 kcal/mol of glucose
Where does aerobic respiration occur?
II. What is Cellular Respiration?
1* Cellular respiration is how many cells transfer the energy stored in complex organic molecules
(“food”) to ATP.
2* Aerobic respiration happens to be the most prevalent and efficient catabolic pathway – it is used
by most eukaryotic and many prokaryotic species.
How do catabolic pathways that decompose glucose and other energy rich organic molecules
yield energy for the cell?
3* Cellular respiration is a controlled stepwise oxidation of organic molecules in a cell
• Enzymes catalyze oxidation via a series of small steps. Free energy is transferred via electrons to
carrier molecules (most often but not alwaysATP and NADH). The relocation of electrons releases
energy stored in organic molecules and this energy is ultimately used to synthesizeATP
What is Oxidation?
• During catabolism of organic molecules, electrons and the potential energy associated with the
electrons are relocated to different molecules. Ultimately, this free energy is used to drive the
synthesize ofATP (and generate heat).
• Transfer of one or more electrons (e ) from one reactant to another occurs in many chemical
• These e transfers are oxidation-reduction reactions or redox reactions.
Note: redox reactions always occur together (i.e., reduction can’t occur without oxidation also
oxidation - loss of electrons (e ) by a substance
reduction - gain of e by a substance
Xe + Y ▯ X + Ye -
reducing agent ▯ electron donor (X)
oxidizing agent ▯ electron acceptor (Y)
Note: Not all redox reactions involve the complete transfer of electrons from one substance to another.
Some redox reactions involve changing the degree of sharing of electrons in covalent bonds.
C 6 12 66 O ---->26 CO + 6 H O 2 Usable2form of Energy
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Electrons lose potential energy as they shift from a less electronegative atom to a more electronegative
atom. This potential energy is released and can be used to perform work.
In general – molecules rich in hydrogen are excellent fuels
III. Aerobic Respiration
• While glucose is typically used to teach anaerobic respiration, it is important to know that many
other energy rich organic molecules can be used to drive this catabolic process
• Aerobic respiration of glucose is the cumulative function of three metabolic stages:
2. CitricAcid Cycle (also known as Krebs cycle, Tricarboxylic acid cycle – TCA cycle)
3. Oxidative Phosphorylation: Electron Transport and chemiosmosis
1. Glycolysis (“splitting of sugar”)
• Technically not part of aerobic respiration but most cells deriving energy from glucose use
glycolysis to prepare starting material for the citric acid cycle
• occurs in the cytosol (for eukaryotic and prokaryotic cells)
• oxygen is not required and CO is 2ot released during this stage.All of the carbon atoms from one
glucose molecule are found in 2 molecule of pyruvate!!!
• 10 enzymatic steps (Figures 9.8 and 9.9 – You are not expected to memorize the steps in Fig 9.9 –
just have a look at the steps. We will talk about several key steps in lecture.)
For 1 molecule of glucose (C H 6 )12n6ering glycolysis, it goes through 10 chemical reactions divided
into two phases
A. Energy investment phase
• first 5 steps
• uses 2ATP
• produces 2 glyceraldehyde phosphate (this is a 3 carbon molecule)
B. Energy payoff phase
• last 5 steps
• produces 4 ATP
2 NADH (nicotinamide adenine dinucleotide)
2 pyruvate (all of the carbon atoms from glucose are found in pyruvate)
2 H 2
How are the 4ATP produced during glycolysis?
Substrate-level phosphorylation:. ATP produced in glycolysis is generated by substrate level
phosphorylation: when an enzyme (i.e., a kinase) transfers a phosphate group from a substrate molecule
toADP (Figure 9.7). Check out steps 7 and 10 in Figures 9.9.
How are the 2 NADH produced during glycolysis? See step 6 in Figure 9.9
+ - - +
Dehydrogenases remove 2 H and 2 e from the substrate (fuel molecule) and transfer 2 e and 1 H
to nicotinamide adenine dinucleotide (NAD ). NAD is a coenzyme.
Energy is transferred along with the electrons to NAD . The other proton is released as H into the
surrounding solution. (Figure 9.4)
H-C-OH + NAD C=O + NADH + H
If oxygen is present then energy stored in NADH can be used to generateATP by oxidative
phosphorylation (Electron transport and chemiosmosis) – see stage 3 of aerobic respiration
Approximately 25% of the energy stored in glucose is released during glycolysis.
How is this energy released?
Where is the remaining energy that was stored in glucose?
2. CitricAcid Cycle (also known as Krebs cycle or the tricarboxylic acid cycle)
• described by Hans Krebs - 1930s
If oxygen is present
then pyruvate is actively transported into the mitochondrion and oxidation is completed.
But what about prokaryotic organisms – where does this process occur??
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Prior to the citric acid cycle there is a junction step that links Glycolysis and the CitricAcid Cycle
Conversion of pyruvate ▯ acetyl CoA (catalyzed by a multi-enzyme pyruvate dehydrogenase complex).
This conversion involves the following three steps
a) Removal of pyruvate’s carboxyl group generating CO 2
b) The remaining two carbon molecule is oxidized forming an acetyl group (-COCH ). In the p3ocess, the
extracted electrons are transferred to NAD thereby storing energy in NADH and a H are produced
c) The acetyl group is attached to coenzymeA(a sulfur containing compound derived from a B
vitamin) forming a reactive product called acetyl CoA
CitricAcid Cycle (Figure 9.11, 9.12)
• 8 enzyme catalyzed steps (in eukaryotes, all enzymes are found in the mitochondrial matrix, except
for the enzyme that catalyzes step 6, it is found in the inner mitochondrial membrane)
• 2 carbon enter the cycle as an acetyl group and 2 different carbon leave as 2 CO 2
• The citric acid cycle is a cyclical process
• In the first step, acetyl (-COCH )3is added to oxaloacetate (4 carbon atoms) forming citrate (6
Every acetyl (-COCH ) e3tering the citric acid cycle results in