UNIT 2: Metabolic Processes
Chapter 4: Cellular Respiration
pg. 166 - 209
4.4: Anaerobic Pathways: Life without Oxygen pg. 190 – 194
There are organisms that can survive in environments with a poor
supply of oxygen or without oxygen and still perform cellular
respiration, known as anaerobic respiration. The problem is the energy
yield from anaerobic respiration is much less then aerobic respiration.
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Fermentation
Fermentation uses an organic molecule as a final electron acceptor, but does
not use an electron transport chain; therefore it is not considered a form of
cellular respiration.
In cells glycolysis is a universal that generates ATP, without the presence of
oxygen. NAD+ is used to remove electrons and hydrogen protons, as it is
reduced to NADH. The NADH must be oxidized to regenerate NAD+ to
continue the glycolysis. Fermentation is a process that can use NAD+ to
produce NADH, and then use NADH to regenerate NAD+.
There are two forms of fermentation that occurs in eukaryotic cells; alcohol
(ethanol) fermentation and lactate (lactic acid) fermentation.
Alcohol Fermentation
Plants, Bacteria and Yeasts
Alcohol fermentation – is a process in which pyruvate is decarboxylated,
producing a molecule each CO2 and of ethanol, and NAD+.
In alcohol fermentation, the glucose molecule undergoes glycolysis,
producing two molecules of pyruvate. During this process two NAD+ are
reduced to two molecules of NADH. The two molecules of pyruvate
undergo a decarboxylation reaction, removing two carbon dioxide molecules,
and producing two molecules of acetaldehyde.
The NADH produced earlier is used to reduce acetaldehyde to ethanol and
oxidize NADH to NAD+.
During the glycolysis reaction 2 molecules of ATP are produced through
substrate-level phosphorylation.
Glycolysis:
glucose + 2 ADP + Pi + 2 NAH+ → 2 ATP + 2 NADH + 2 pyruvate
Alcohol Fermentation:
2 pyruvate + 2 NADH + 2 H+ → 2 NAD+ + 2 CO2 + 2 ethanol
**Alcohol fermentation occurs naturally in the environment, as fruit ripens
or decomposes it starts to ferment.
Figure 2: Alcohol Fermentation reactions produces ethanol, pg. 191
Lactate Fermentation
Lactate fermentation – is a process in which pyruvate reacts with NADH
and is converted directly into lactate and regenerates NAD+.
This pathway is the primary pathway for bacteria and can supplement some
eukaryote systems. In humans lactate acid fermentation occurs in the muscle
cells when ATP demand is greater then the O2 supply for the electron
transport chain.
Glycolysis helps to maximize ATP production by continuing to proceed to
produce 2 ATP and 2 molecules of pyruvate. The excess pyruvate builds up
in the cell and is converted to lactate when there is a low supply of O2.
2 NAD+ are reduced to 2 molecules of NADH, the NADH is then used to
regenerate NAD+, as lactate is produced.
As oxygen levels return to normal, the process is reversed and lactate is
converted back to pyruvate (in the Liver) which then enters pyruvate
oxidation of the aerobic respiratory pathway.
Glycolysis:
glucose + 2 ADP + Pi + 2 NAH+ → 2 ATP + 2 NADH + 2 pyruvate
Lactate Fermentation:
2 pyruvate + 2 NADH + 2 H+ → 2 NAD+ + 2 CO2 + 2 lactate
** The rate at which oxygen is supplied to the mitochondrion is not always
sufficient to meet al of the energy demands of the cell. Therefore, lactate
production is ongoing throughout the duration of highly strenuous physical
activity.
Figure 5: Lactate Fermentation reactions that produce lactate, pg. 192
Alcohol and Lactate Fermentation summary
Anaerobic Respiration
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4.5: Biology Journal
pg. 195
4.6: Explore and Issue in Metabolism
pg. 197
Chapter 4: Summary
pg. 202
Chapter 4: Self-Quiz
pg. 203
Chapter 4: Review
pg. 204 - 209