Biol 12:
1
Glucose Metabolism
1) The overall reaction for cellular metabolism is:
2) Prepare a chart which compares glycolysis, fermentation, the citric acid cycle, the
ETC and overall glucose metabolism:
Pathway
Location and
reactions
Starting
Materials
(include #s)
End Products
Net Energy
Harvest
Glycolysis
Fermentation
Transition
reactions
Kreb’s cycle
Electron
Transport
Chain
Overall
Glucose
metabolism
3) Glucose is a stable molecule. How are the reactions of its degradation all started?
4) In glycolysis, what happens to the glucose molecule when the 2 ATP’s are used?
5) How is NADH formed from NAD+?
6) In glycolysis, what happens to the two 3C molecules when ATP is formed?
7) When does fermentation occur?
8) Fermentation does not yield any E. Why is it necessary?
9) How are wine and beer made? Why does the [alcohol] not exceed 18%?
10) Is all of the energy from glucose converted to ATP? Explain.
Biol 12:
2
Glucose Metabolism
Complete Glucose Metabolism:
C6H12O6 + 6 O2 -----> 6 CO2 + 6 H2O + E
Occurs in two Stages:
Glycolysis
&
Cellular Respiration
-does not require O2
- requires O2
- occurs in the cytosol
- occurs in the
mitochondria
- see figure 8-1, page 142
1. Glycolysis
overall reaction:
glucose ----------> 2 pyruvic acid + 2 ATP
THIS process also occurs in two stages:
A. Glucose activation
- 2 ATP molecules each donate a
phosphate to make FDP (fructose
diphosphate)
- FDP is highly reactive
Biol 12:
3
Glucose Metabolism
2 ATP
2 ADP
glucose ------>------>------>--------> FDP
B. Energy harvest
- FDP splits into 2 PGAL
- PGAL go through a series of reactions
- the end product is 2 pyruvic acid
molecules AND 4 ATP, 2 NADH
4 ADP
4 ATP
2 PGAL ----->----->----->----->2 pyruvic acid
2 NAD+
2 NADH
NET GAIN: 2 ATP and 2 NADH (e carriers)
See Page 144 for details on glycolysis
FERMENTATION:
If O2 is not present, the two molecules of
pyruvic acid are used to regenerate the
NAD+ so that glycolysis can continue.
Biol 12:
4
Glucose Metabolism
- this process can occur in two ways:
NADH
NAD+
i) pyruvic acid ---------------> Lactic Acid
- toxic at high [ ]
In our bodies, lactic acid is reconverted to
pyruvic acid when O2 becomes available
- bacteria ferment yogurt, sour cream and
cheese this way
NADH NAD+
ii) pyruvic acid ---------------------> ethanol + CO2
- also toxic at high [ ]
- microorganisms, and most yeast regenerate
their NAD+ this way
Fermentation occurs when O2 is limited.
(anaerobic conditions)
However, under aerobic conditions, pyruvic
acid is transported across the mitochondrial
membrane and enters the second stage of
glucose metabolism:
2. Cellular Respiration
Biol 12:
5
Glucose Metabolism
- requires O2 to be the final electron
acceptor
Can be broken into 3 Stages:
a. Transition Reaction: after being shuttled
into the mitochondrial matrix, pyruvic acid is
broken down to acetyl CoA and 2 CO2 .
(pyruvic acid is oxidized during this process)
b. The Kreb’s cycle (citric acid cycle)
Acetyl CoA enters a cyclical series of
oxidation reactions producing 2 CO2 , 1 ATP,
3 NADH and I FADH2
c. The Electron Transport Chains
(respiratory chains)
The electron carriers, NADH and FADH2 are
shuttled to the inner membrane where they
donate their electrons and/or Hydrogens.
This ultimately drives the synthesis of ATP
via chemiosmosis.
Biol 12:
6
Glucose Metabolism
A. The Transition Reactions (formation
of Acetyl CoA)
- pyruvic acid is shuttled by facilitated
transport from the cytosol into the
mitochondrial matrix
- pyruvic Acid is combined with coenzyme A
to make Acetyl CoA and CO2 - NADH
captures 2 high E electrons in the process
Coenzyme A
pyruvic acid ------------------------------------>
CO2 + Acetyl CoA
(3 C)
NAD+ NADH
(2 C)
2. The Citric Acid Cycle (Kreb’s Cycle)
- a series of reactions that produce 2 CO2 ,
3 NADH, 1 ATP, and 1 FADH2
a) acetyl CoA combines with oxaloacetic
acid to form citric acid
- this process releases coenzyme A
b) citric acid goes through a series of
reactions, releasing high E electrons which
are picked up by NADH and FADH2 along
the way.
Biol 12:
7
Glucose Metabolism
c) on two occasions, a CO2 is lost, and one
of the reactions also losses enough E to
drive the synthesis of ATP
d)2 water molecules are also used for these
reactions
e) oxaloacetic acid is regenerated in the
final reaction
(see page 149 for the full cycle)
* Recall that for every glucose metabolized,
2 pyruvic acids are produced, and therefore
the Kreb’s cycle is completed twice per
glucose, producing 4 CO2 , 6 NADH, 2 ATP
and 2 FADH2 *
3. The Electron Transport Chain
- during the metabolism of glucose, 10
NADH and 2 FADH2 are produced.
- 2 NADH from glycolysis
- 2 NADH from formation of Acetyl CoA
- 6 NADH from the Kreb’s Cycle
- 2 FADH2 from the Kreb’s Cycle
Biol 12:
8
Glucose Metabolism
- The NADH and the FADH2 deposit their
electrons into an ETC that is embedded in
the mitochondrial inner membrane.
- ATP is formed via CHEMIOSMOSIS (the
same process as found in photosynthesis)
- the electrons donated by NADH have
enough E to pump 3 H+ from the matrix into
the intermembrane compartment
- the release of 3 H+ back out of the
intermembrane compartment drives the
formation of (approximately) 3 ATP
- the electrons donated by FADH2 enter at a
later stage in the ETC and therefore will only
generate 2 ATP.
** The Final Electron acceptor in the Respiratory
ETC is O2 !***
When the O2 accepts the E-depleted electrons,
it combines with Hydrogen to form H2O. (a
product of respiration!)
1/2 O2
+ 2 H+ + 2 e- -----------------> H2O
If there is no O2 around, the ETC backs up,
which will eventually shut down the Kreb’s cycle.
This is when fermentation will kick in!
Biol 12:
9
Glucose Metabolism
Using Fats or Proteins for Energy
Fats:(efficient E storage because of the long
Fatty Acid Chains)
fats --------------------------> glycerol
+
Fatty Acids
- converted to
- converted to
Acetyl CoA
PGAL
(one Acetyl CoA for
every 2C)
Proteins:
protein ----------------------> Amino acids
- enter Kreb’s cycle at different
places
depending on
the length of the R group
- must be “deaminated”
(removal of the amino
group)
before entering the cycle
- the amino group is
converted to NH3
and
excreted out of the cell