BIO 208 Unit 2 Review of General Metabolism Concepts
Review - Important Concepts for Lectures over Metabolism
I assume that you have had an introduction to the basics of metabolism in an
introductory biology course. The metabolism you learned was probably entirely
focused on the types of metabolism that animal (maybe plant) cells carry out -aerobic respiration; perhaps you were exposed to lactic acid fermentation (When
muscles are working very hard, they may be temporarily depleted of oxygen,
muscle cells can perform lactic acid fermentation for a short period of time. The
lactic acid end products are secreted by the muscle cells into your tissues, and you
feel the lactic acid as muscle soreness). The microorganisms are tremendously
more diverse and complex in metabolic patterns than are Eucarya and I want to
spend our time emphasizing what microbes can do, not just covering what you
have already had in other courses.
So, if you do not remember the basics of metabolism you will need to review. The
following pages should serve as a reminder. If it doesn’t all come back to you then
read Chapter 5 in the text. If you have not had chemistry you will also need to read
Chapter 2.
Review of oxygen tolerance:
 Obligate anaerobe – does not require O2.
 Aerotolerant anaerobe – does not require O2 for .
 Microaerophile – needs a little O2 for metabolism, but less than amount present
in the atmosphere.
 Facultative anaerobe – can switch its metabolism based on whether or not O2 is
present.
 Aerobe (obligate aerobe) – requires O2 for metabolism.
Review of nutritional patterns:
Source of energy
Source of carbon
Chemicals
CO2 (used by autotrophs)
organic
Organic molecules (-C-C-C-) (used by heterotrophs)
inorganic
Light
Most common combinations of Energy gaining strategy plus Carbon gaining
strategy
Chemoorgano heterotrophs
Chemolitho autotrophs
Photo autotrophs
Photo heterotrophs
BIO 208 Unit 2 Review of General Metabolism Concepts
You should also know
Definitions of metabolism, anabolism, and catabolism
That ATP (Adenosine Tri Phosphate) is made to store energy and used to release
energy – it is the energy “currency” for the cell.
Pyruvate is a key intermediate molecule in many catabolic pathways.
Should understand basics of oxidations - reductions
Remember - A loss of an electron is called an oxidation; a gain of an electron is
called a reduction (remember as: LEO the lion says GER).
In biological molecules it is usually the entire H atom (electron and proton) that is
lost or gained, but not always. Sometimes the electrons are separated from the
proton and only the electrons are lost or gained; and sometimes it may be one H
atom + 1 electron (from a second H atom) that are lost or gained.
In any pair of molecules you can distinguish which is the oxidized and which is the
reduced:
Oxidized state
Contains more oxygen atoms OR
fewer hydrogen atoms AND
therefore has fewer electrons and is
less negative or more positive
Reduced state:
Contains fewer oxygen atoms OR
more hydrogen atoms AND
therefore has more electrons and is
more negative or less positive
Example pairs:
Glucose
C6H12O6
Pyruvate
C3H4O3
NAD+
NADH
Sulfate
SO4
Hydrogen sulfide
H2S
BIO 208 Unit 2 Review of General Metabolism Concepts
All cells need:
1. A source of carbon for making cellular molecules.
There are two strategies for obtaining carbon:
a. Recycle the C already present in some organic (-C-C-) molecule
b. Use CO2 from the atmosphere
2. A source of energy for performing all cellular work (building molecules,
transport across the plasma membrane, locomotion, etc.)
Energy is created by harvesting the electrons present in:
a. Organic
molecules.
(specifically the
electrons in the H atoms
in the molecules)
Hydrogen – showing the
proton and electron
like a sugar or
an amino acid
OR
b. Inorganic
molecules.
electrons in
molecules like
ammonia
hydrogen sulfide
The more electrons a molecule has, the more energy the molecule is capable of
yielding – so look at glucose compared to hydrogen sulfide – which molecule
should yield the most energy? (glucose – 12 H vs. 2 in H2S)
BIO 208 Unit 2 Review of General Metabolism Concepts
The electrons that are
released when bonds
are broken have to go
somewhere, so they get
passed from the donor
(the molecule that you
started with that had all
the electrons) to
intermediate electron
carriers.
NAD+ is a soluble
carrier present in the
cytoplasm. It is lacking
1 electron (1 H) and so
it can accept 1 electron
(1 H). As it accepts the
electron, it is reduced
to NADH.
Oxidized state
fewer H, fewer emore positive (NAD+)
Reduced state
more H, more e-
BIO 208 Unit 2 Review of General Metabolism Concepts
NAD+ is in limiting quantities in the cell and it must be regenerated if energy
production is to continue.
There are 2 ways to regenerate NAD+ from NADH :
1. NADH passes the electron to an
organic molecule like pyruvate –
this process is called fermentation as NADH loses the electron it
becomes oxidized to NAD+ again.
As pyruvate accepts the electron it
becomes reduced to acetic acid or to
ethanol, etc., which are excreted
from the cell, carrying waste
electrons with them. Acetic acid,
ethanol, etc. still have electrons, so
potential energy is lost in the
fermentation strategy.
2. NADH travels to the
cytoplasmic
membrane and passes
the electron off to the
electron transport
chain. This process is
called respiration.
(NADH then becomes
NAD+ )
electrochemical gradient - energy
Fig. 5.16
pH 8.5
BIO 208 Unit 2 Review of General Metabolism Concepts
The electrons are passed along the chain, generating two types of usable energy
along the way – electrochemical gradient and ATP - until they reach a final
electron acceptor, an inorganic molecule which can be:
a. oxygen (aerobic respiration)
As oxygen accepts
electrons it will become
reduced to H2O
OR
b. some other inorganic
molecule (anaerobic
respiration)
like nitrate
or sulfate
becomes reduced
to nitrite (NO2)
becomes reduced to
hydrogen sulfide (H2S)
Note – fermentation is NOT anaerobic respiration. By definition respiration
requires both an electron transport chain and an inorganic terminal electron
acceptor. Fermentation does not employ an electron transport chain and the
terminal electron acceptor is an organic molecule. Fermentation takes place in the
absence of oxygen, it can occur in anoxic and anaerobic environments, but it is not
respiration!
BIO 208 Unit 2 Review of General Metabolism Concepts
Comparison of Respiration vs Fermentation in Chemoorganotrophs
Initial electron donor:
examples:
Intermediary electron
carrier(s):
Final electron acceptor
examples:
final electron
acceptor reduced to:
example organisms
Potential net ATP
yield:
Respiration
organic molecule
carbohydrates, amino acids, lipids
NADH, FADH2, carriers in the
electron transport chain
inorganic molecule
O2
CO2, NO3, SO4
H2 O
Mitochondria,
E. coli,
Pseudomonas,
S. aureus
CH4, NO2, H2S
Methanogens,
E. coli,
Pseudomonas,
Sulfate-reducing
bacteria
as many as 38 if starting with 1
glucose by aerobic respiration
with an electron transport chain
containing all the cytochromes –
but often far fewer than 38 - but
still more than 2.
Fermentation
organic molecule
carbohydrates, amino
acids, lipids
NADH
organic molecule
pyruvate
lactic acid, acetic acid,
ethanol, etc.
Bifidobacterium,
Lactobacillus, E. coli,
Clostridium,
Bacteroides
2
BIO 208 Unit 2 Review of General Metabolism Concepts
Comparison of Respiration in Chemoorganotrophs vs Chemolithotrophs
Initial electron donor:
examples:
Electron donor oxidized
to:
Final electron acceptor
examples:
electron acceptor
reduced to:
example organisms
Chemoorganotroph
organic (-C-C-) molecule
carbohydrates, amino acids, lipids
CO2
inorganic molecule
CO2, NO3, SO4
O2 (aerobic
(anaerobic
respiration)
respiration)
Chemolithotroph
inorganic molecule
hydrogen gas, ammonia,
nitrate, hydrogen sulfide
water, nitrate, nitrite,
sulfuric acid
inorganic molecule
O2 (aerobic respiration)
H2 O
CH4, NO2, H2S
H2 O
Mitochondria,
E. coli,
Pseudomonas,
S. aureus
Methanogens,
E. coli,
Pseudomonas,
Sulfate-reducing
bacteria
Alcaligenes,
Nitrosomonas,
Nitrobacter,
Thiomargarita