Three Stages in Detail 6.7-6.12
• Stage 1: GLYCOLYSIS = “splitting of sugar”
(6.7 pg 94)
– Takes place in the cytoplasm; *
– Anaerobic: does not require oxygen*
• Glucose is split into 2 molecules of pyruvic acid *
• Costs: 2 ATP molecules
• NAD+ (an electron carrier) is reduced to NADH when it accepts
two electrons from pyruvic acid.
• Produces: 4 ATP molecules via substrate-level phosphorylation.
• Glycolysis Yields: 2 ATP, 2 NADH, and 2 H+ ions.*
Diagram of Glycolysis
• Carbon atoms
– Glucose has 6 carbons
– Each pyruvate has 3 carbons
• There are 9 chemical steps
from glucose to pyruvate!
Substrate-level Phosphorylation• An enzyme transfers a phosphate group from a
substrate molecule directly to ADP, forming ATP
– This process produces a small amount of ATP in
both glycolysis and the citric acid cycle.
Glucose energy is now banked in a
combination of ATP and NADH:
– ATP is available for immediate use but,
– NADH must first pass electrons to the ETC
• Their stored energy is not available for use in the
absence of oxygen.
– Pyruvate still holds most of the energy of
glucose.
Details of Glycolysis
• Glycolysis is an example of a metabolic
pathway in which each chemical step leads to
the next one.
• See page 95
– Steps 1-4: energy investment phase (consumes energy).
– Steps 5-9: energy payoff phase (yields energy)
• Intermediates- compounds that form between
the initial reactant glucose, and the final
product, pyruvate.
•
Each pyruvic acid
1.
2.
3.
4.
5.
•
(6.8 pg 96)
loses a molecule of carbon dioxide,
The remaining two carbon compound is oxidized
.NAD+ is reduced to NADH
The two carbon compound combines with coenzyme A and
becomes Acetyl Coenzyme A (Acetyl-CoA).*
Total yield for both pyruvic acids:
2 CO2, 2 NADH, 1 H+
(6.9 pg 96-97)
Stage 2: CITRIC ACID CYCLE aka KREBS CYCLE
- Takes place in the mitochondria;*
– Aerobic: requires oxygen*
– Coenzyme A breaks off at this point
– The acetyl portion joins a 4-carbon molecule
•
–
•
This 6-carbon molecule (citrate) is processed through a
series of reactions
Cycle turns twice(once for each original Acetyl CoA) *
Each Acetyl CoA
1. produces 1 ATP and 2 carbon dioxide molecules
2. NAD+ and FAD (electron carriers) pick up energized electrons.
–
Yield per turn: 2 CO2, 1 ATP, 3 NADH, & 1 FADH2*
Stage 1 and 2: Total Energy Output
• Overall, how many energy-rich molecules has
the cell gained by processing one molecule of
glucose through glycolysis and the citric acid
cycle?
• 4 ATP
• 10 NADH
• 2 FADH2
– NADH and FADH2 must shuttle their high energy
electrons to the ETC before the cell can use this
energy.
•
Stage 3: Oxidative Phosphorylation
(6.10 page 98)
The folds (cristae) of the
mitochondrial membrane enlarges
its surface area and provides space
for:
1. Thousands of copies of the ETC
2. Many ATP synthase complexes
for chemiosmosis.
Diagram of Oxidative Phosphorylation
Why is this process called
oxidative phosphorylation?
• The energy derived from the oxidationreduction reactions of the ETC is used to
phosphorylate ADP.
– The exergonic reactions of the ETC produce
an H+ gradient.
– Through chemiosmosis, the energy stored in
this H+ gradient drives the endergonic
synthesis of ATP.
Stage 3: Oxidative Phosphorylation
(6.10 page 98)
• NADH & FADH give up electrons to the ETC
• Oxygen is the final electron acceptor in the chain*
– Reacts with 2 H+ ions and 2 electrons
– Produces one H20 molecule
• ETC Yields: 6 H20, 32-34 ATP*
Total Yields for Cellular Respiration Yields
(6.12 page 100)
• Cellular Respiration yields a total of 36-38 ATP:*
– Glycolysis 
– Krebs Cycle 
– ETC 
2
2 (one ATP per turn)
32-34
• Each NADH generates 3 ATPs by contributing its electrons to
the ETC
• Each FADH2 generates only 2 ATPs because it contributes its
electrons to the ETC at a later point and therefore, does not
provide as much energy towards establishing the H+ gradient.
An estimated tally of the ATP produced by
substrate-level and oxidative phosphorylation
Reasons for Variations in Total Yield of ATP
• Some energy of the H+ gradient may be used
for transport work instead of ATP production.
• NADH and FADH2 generate different
amounts of ATP (see previous slide)
– So oxidative phosphorylation may yield 32-34
ATP molecules depending on the type of electron
shuttle/taxi
– Cellular respiration may yield up to 38 ATPs per
glucose molecule, but that’s the max!