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Transcript
Chapter 9
Metabolism is the total of all chemical reactions occurring in the cell. The
flow of energy and the participation of enzymes make metabolism possible.
Note: The second law of thermodynamics describes the
randomness/disorder associated with a system as entropy. As physical and
chemical reactions proceed from reactant to product they do so in such a
way the entropy of the universe increases. So as energy moves through the
various trophic levels the randomness associated with that energy increases.
Metabolism can be divided in to two categories:
1. Catabolism – reaction resulting in large, complex higher energy
molecules being broken into small, simpler lower energy molecules.
a. Some of the energy associated with these molecules is used for
work (Free energy – G)
b. The remaining energy is released from the system as heat
(enthalpy – H)
2. Anabolism – reaction resulting in the formation of a large, complex
higher energy molecules from the binding of multiple smaller, less
complexed lower energy molecules.
a. These process increase the order of a system
Chemotrophic microorganisms vary in their energy sources as well as, their
electron acceptors. There are three metabolic processes that utilize organic
nutrients as electron acceptors (chemoorgnotrophs):
3. Fermentation – a process where the energy substrate is oxidized and
degraded without the participation of an exogenous or externally
derived electron acceptor ( aka respiration)
4. Aerobic respiration – the final electron acceptor is exogenous/
externally derived oxygen (O2)
5. Anaerobic respiration – the final electron acceptor is an exogenous
molecule other than oxygen
a. Nitrate
b. Sulfate
c. Carbon dioxide
d. Iron (III)
e. Fumerate
Electron obtained from inorganic nutrients is accomplished by
chemolithotrophs. The final electron acceptor can be any of the following;
a. O2
b. Nitrate
c. Sulfate
Chemoorganotrophic aerobic metabolism
1. Catabolism of an organic molecule
a. Protein
b. Carbohydrate
c. Lipid
2. Aerobic production of energy-transfer and intermediate molecules
a. ATP-energy transfer
b. NADH and FADH2 electron accepting/donating intermediates
c. This process can occur under anaerobic conditions also
3. Carbon nutrients are oxidized to produce more energy-transfer and
intermediate molecules
Process of Carbohydrate degradation in Microorganisms
Microorganisms have several pathways to catobolize glucose and other
sugars to pyruvate (stage 1);
1. Glycolysis
2. Pentose phosphate shunt/pathway
3. Entner-Doudoroff pathway
I. Glycolytic pathway – this pathway is also referred to as the EmbdenMeyerhof pathway
a. This is the most common of the 3 pathways
b. Occurs in the Cytoplasmic matrix of both prokaryotic and eukaryotic
cells
c. In a process called
phosphorylation,
phosphate groups
are added to the 6carbon molecule,
raising its free
energy to a state
that begins the
exergonic reaction.
d. In the second stage
of this process a
catabolic reactions
releases two 3carbon molecules
(Glyceraldeyde-3phosphate and
Dihydroxyacetone
phosphate) that
ultimately produces
ATP through an
anabolic process
call substrate-level
phosphorylation
e. Produces 2 ATP’s (net)
II. Pentose Phosphate Pathway – this pathway is also referred to as the
hexose monophosphate pathway
a. This pathway can occur simultaneously with either of the other two
pathways
b. Can occur under aerobic or anaerobic conditions
c. Is an amphibolic pathway (both catabolic and anabolic)
d. Has different pathways than the other pathways
1) The phosphorylated 6-carbon sugar is converted into a 5carbon sugar (pentose ribulose 5-phosphate
2) Carboxylation occurs (release of carbon dioxide from a
carboxly containing compound)
3) NADPH is produced (this compound is normally associated
with photosynthesis)
e. The 5-carbon compound then enters a variety of pathways resulting
in phosphorylated sugars containing between 3 to 7 carbons. There
are two unique enzymes responsible for these pathway;
1) Transketolase
2) Transaldolase
3) The result of these processes is the production of 3phosphoryated glucose molecules, a glyceraldehydes 3-
phosphate, and 3-carbon dioxide molecules
III.
f. Functions of this pathway
1) NADPH produced in this pathway serve as a electron source
for metabolism
2) The production of 4-carbon sugars (erythose 4-phosphate) is
used to synthesis pyridoxal (vitamin B6)
3) The production of the 5-carbon pentose ribose 5-phosphate
is need for carbon fixation in photosynthesis
4) Results in the production of ATP’s through the electron
transport system.
g. Commonly seen in E. coli and Bacillus subtilis
h. Produces 1ATP (net)
Entner-Doudoroff pathway
a. Begins much like
glycolysis, but produces
pyruvate in a shorter
pathway
b. Commonly seen in
Pseudomonas
aeruginosa
c. Produces 1 ATP (net),
1NADH, and 1NADPH
Fermentation:
In the absence of aerobic and anaerobic respiration, NADH is not oxidized
by electron transport chains because no external electron acceptor are
available, however NADH
must be oxidized back into
NAD+ to maintain glycolysis or
the reaction (glycolysis) will
stop.
1. many microbes simply
slow down/stop
pyruvate
dehydrogenase
2. pyruvate, or one of its
intermediate, then can
serve as an
endogenous
electron/hydrogen
acceptor for NADH
3. this process of NADH
reoxidation is call
fermentation.
Fermentation can lead to the production of ATP, but not as much as
aerobic/anaerobic respiration
1. in fermentation the substrate (pyruvate or a dirivative/carbohydrate) is
oxidized
2. ATP forms by substrate-level phosphorylation only
3. Oxygen is not needed
Types of fermentation:
There are 5 main types that we will discuss:
1. Alcohol fermentation
2. Lactic acid fermentation
a. homolactic
b. heterolactic
3. Mix acid fermentation
4. Butanediol fermentation
5. carbohydrate
1. Alcohol fermentation
a. Pyruvate is oxidized to
acetaldehyde
b. Acetaldehyde is
reduced to ethanol
c. Seen in fungi, bacteria,
algae, and protozoa
2. lactic acid
a. pyruvate is reduced to
lactate
b. occurs in bacteria,
algae, molds, protozoa,
and animal skeletal
muscles
c. homolactic fermenters
use lactate
dehydrogenase to
produce lactate only
directly from pyruvate(
enzyme lactate
dehydrogenase)
d. heterolactic produce
lactate, ethanol, and
carbon dioxide
(enzyme used is
phosphoketolase)
i. enterobacteriaceae
ii. important to production of wine, cheese and breads
3. Mixed acid
a. Pyruvate metabolism results in the production of ethanol, acetic
acid, succinic acid, lactic acid, and formic acid
b. If formic hydrogenlyase is present then formic acid is degrade to
hydrogen and carbon dioxide gases
i. Escherichia
ii. Salmonella
iii. Proteus
4. Butanediol fermentation
a. Converts pyruvate to acetone, and finally ethanol
b. More ethanol production than other pathways
c. Produced by Enterobacter, Serratia, Erwinia, and some Bacillus
species.