Download Guangyi Wang Chemosynthesis (Chemolithotrophy)

OCN621: Biological OceanographyBioenergetics-II
Guangyi Wang
POST 103B
[email protected]
http://www.soest.hawaii.edu/oceanography/zij/education/ocn621/
Chemosynthesis (Chemolithotrophy)
Use of small inorganic molecules as an external energy source to power
CO2 reduction.
Examples:
2 NH4+ + 3 O2 Æ 2 NO2- + 4 H+ + 2 H2O
2 NO2- + O2 Æ 2 NO34F
Fe2+ + O2 + 4H+ Æ 4 F
Fe3+ + 2 H2O
HS- + 2 O2 Æ SO42- + H+
Shared characteristics:
1. Use energy from inorganic chemicals to generate ATP by electron
transport phosphorylations with O2 as terminal electron acceptor.
2. Use Calvin Cycle to fix CO2 into glucose.
3. Reducing agent NADP has to be generated by utilization of some ATP
not directly produced by the chemosynthetic process
process.
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Anaerobic Chemosynthesis
Some bacteria can live chemosynthetically without reducing O2 and
without assimilating CO2 through the Calvin Cycle.
Example: utilization of H2 + CO2 to produce methane
4 H2 + CO2 Æ CH4 + 2 H2O
The energy derived from this process is used to reduce CO2 to organics.
Bacterial metabolism is exceptionally diverse. Many chemical substrates
can serve as a source of energy for bacterial growth and production.
Chemosynthesis (by bacteria) is generally not as important as
photosynthesis in producing organic matter, but is clearly important in
understanding elemental cycling in the oceans.
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Definitions of nutritional modes
AUTOTROPHIC - self-nourishing, organisms with the ability to synthesize
organic molecules from CO2. All photolithotrophic and
chemolithotrophic organisms may be autotrophic, but many require
small amounts of organic molecules - vitamins or essential amino acids
which they cannot synthesize. These organisms are auxotrophic requiring
i i supplemental
l
t l nourishment.
i h
t
HETEROTROPHIC - depend entirely on organic molecules synthesized by
other organisms. Osmotrophic heterotrophs which take up organic
compounds by absorption through cell membrane. Phagotrophic
heterotrophs which ingest particulate food.
MIXOTROPHIC - organisms with mixed mode of nutrition. Some bacteria
(chemoorganotrophs) use energy from small organic molecules to
reduce CO2 to sugars. Other mixotrophic organisms (e.g., protozoans)
consume particulate food, but also contain functional chloroplasts or
endosymbionts.
Note: Both autotrophic and heterotrophic organisms manage to get small organic molecules into their cells, but
they require additional energy to do anything with them. All of energy from sunlight is used up in
photosynthesis; assimilation by heterotrophs requires (does not yield) energy. In order to live, all organisms
have to convert small organic molecules into chemical energy which can then be used to do work .
Extracting Energy from Organic Molecules
GLYCOLYSIS - a "fermentation" reaction – anaerobic decomposition of organics
into "waste" product, lactic acid. The process involves about 11 steps & does
not require a rigid organizational framework.
C6H12O6 Æ 2 C3H4O3 Æ 2 C3H6O3
glucose
2 ADP + 2 Pj Æ 2 ATP
2 NADox Æ 2 NADre
pyruvate
lactate
2 NADre Æ 2 NADox
If muscle tissue operates anaerobically (glycolysis) it builds up
lactic acid and an oxygen deficit
RESPIRATION - aerobic breakdown of food molecules yielding ATP. All basic
organic constituents (sugars, fatty acids, amino acids) can be broken down in
respiration, which begins where gylcolysis leaves off.
Complex reaction system >100+ steps organized in mitochondria (cell
"power plant")
KREBS CYCLE (aka: Citric Acid or Tricarboxylic Acid
-Acetyl CoA (2 carbon molecule) oxidized to CO2. Krebs Cycle
intermediates return to initial state & generate NADre from
electrons liberated in the process.
Cycle)
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Overview of Energy Extraction
Chemoorganotrophy (autotrophs and heterotrophs)
Glycolysis (anaerobic)
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Krebs Cycle (aerobic)
Respiratory Chain Phosphorylation
Electron transfer ultimately reduces O2 to H2O, but some of the energy liberated
in the process is conserved as ATP (3 ATP produced per molecule of NADre
processed).
Energy from the aerobic respiration of 1 glucose molecule:
2 ATP + 2 NADre
2 NADre
8 NADre
2 ATP + 12 NADre
x3 ATP/NADre respiratory chain
2 ATP + 36ATP = 38 ATP/molecule glucose
compare:
38 ATP/glucose - AEROBIC metabolism
2 ATP/glucose - ANAEROBIC metabolism
Anaerobic organisms must process 19X more organic substrate to produce
the same amount of ATP-energy as aerobic organisms. Aerobic organisms
will operate efficiently at relatively dilute substrate levels. Anaerobic
organisms have (by necessity) to be very simple and not very active to
minimize energy expenditure (bacteria, yeast – but also some protozoa, incl.
endosymbiotic chemotrophic bacteria). Also inhabit environments with high
substrate levels - e.g., decaying organic matter.
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Oxidation of Organic Matter with Different e- Acceptors
REACTION
CH20 + O2 Æ CO2 + H2O
5 CH2O + 4 NO3- Æ 4 HCO3- + CO2 + 3 H2O + 2 N2
CH2O + 3 CO2 + H2O + 2 MnO2 Æ 4 HCO3- + 2 Mn2+
CH2O + 7 CO2 + 4 Fe(OH)3 Æ 8 HC03- + 3 H2O + 4 Fe2+
2CH20 + SO4 Æ 2 HCO3- + H2S
CH20 Æ CO2 + CH4
ΔGo´ (kcal/mole)
-686
-570
570
-349
-114
-77
-58
ΔGo´ (kcal/mole) = free energy released per mole of glucose oxidized
CONCEPT: Some energetic transformations are more energetically favorable
than others. These will usually occur first under natural conditions - i.e., the
most energetically favorable terminal electron acceptor (O2) will be used until
it is no longer available, then the environment will favor organisms (bacteria)
capable of utilizing alternative electron acceptor to oxidize organic matter.
Biological Utilization of Chemical Energy
1. Energy “Currency” ATP - Economic analogy for the transformation of energy in the cell
- need for a "medium of exchange". Most biochemical reaction series requires elaborate
cell machinery and organization, and many specific enzymes. It is not efficient, and not
possible, for enzyme complexes to handle all possible combinations of substrates,
intermediates, and sources of energy. METABOLIC processes (e.g, respiration)
"oxidize" organic molecules, capturing some of their energy in a single molecule which
is recognized as the "energy donor" or "medium of exchange" in all subsequent
reactions. This molecule, ATP, is special because it carries a fixed amount of energy in
an easily released form – high energy phosphate bonds.
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2. Active Transport: work of moving molecules &ions against concentration
gradients
According to the 2nd Law of Thermodynamics - everything in universe tends toward
increased entropy (randomness). Therefore, energy must be expended to bring things
(e.g, molecules) into a more organized and concentrated state.
Functions of active transport:
1. Provides proper chemical environment for cellular processes (e.g., pH).
2. Brings needed substrates (glucose, amino acids) &essential minerals (nitrate,
phosphate, & important ions K+ and Ca++) where they are needed.
3. Gets rid of waste products (H+, Na+ , C02, lactic acid).
Characteristics of active transport:
1. A given systems is specific for a particular molecule or ion.
2. Transport occurs in a specific direction across membrane. Transport is accomplished
by enzymes at "active
active sites"
sites - i.e.,
i e substrate specific with specific orientation
(directionality) in cell membrane matrix.
3. Powered by ATP molecules which "fit" into an active sites and donates high energy
phosphate bonds to the process.
4. Works against continuous back diffusion (which occurs at a slower rate because it is
not enzyme aided). Active transport is generally a continuous process in living cells,
concentrations on either side of the membrane are maintained in "dynamic equilibrium"
2. Active Transport (cont.)
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