Chapter 7 Microbial Metabolism: The Chemical Crossroads of Life
1. An Overview of Metabolism
Metabolism: The total of all chemical reactions occurring in the cell.
Two major parts
(1) Catabolism:
larger and more complex molecules are broken down into smaller, simpler molecules with the release of energy.
(2) Anabolism:
the synthesis of complex molecules from simpler ones with the input of energy.
Microorganisms usually use one of three sources of energy:
(1) Radiant energy from the sun (Phototroph)
(2) Oxidizing organic molecules to liberate energy
(3) Employing inorganic nutrients as energy sources
2. Fermentation: The Embden-Meyerhof Pathway
The breakdown of glucose
(1)The Glycolytic Pathway or Embden-Meyerhof-Parnas Pathway (EMP)
Substrate-level phosphorylation:
The high-energy phosphate on carbon one is subsequently donated to ADP to produce ATP.
(2)The Pentose Phosphate Pathway or Hexose Monophosphate Pathway (HMP)
(3)The Entner-Doudoroff Pathway(ED)
(4)The Tricarbocylic Acid Cycle (TCA)
3. Respiration and Electron Transport
Electron transport chain:
Being composed of a series of electron carriers that operate together to transfer electrons from donors, like NADH and FADH2, to acceptors, like O2.
Oxidative phosphorylation:
The process by which energy from electron transport is used to make ATP.
Three biological oxidizations:
Aerobic respiration:
A metabolic process in which molecules, often organic, are oxidized with oxygen as the final electron acceptor.
Anaerobic respiration:
An energy-yielding process in which the electron transport chain acceptor is an inorganic molecule other than oxygen.
Fermentation:
An energy-yielding process in which an energy substrate is oxidized without an exogenous electron acceptor. Usually organic molecules serve as both electron donors and acceptors.
A fermentation in which one amino acid is oxidized and a second amino acid acts as the electron acceptor.
4. An Overview of Alternate Modes of Energy Generation
Oxidation of inorganic molecules
Hydrogen-oxidizing bacteria
Nitrogen-oxidizing bacteria
Sulfur-oxidizing bacteria
Bacteria | Electron donor | Electron acceptor | Products |
Alcaligenes, Hydrogenophage, pseudomonas | H2 | O2 | H2O |
Nitrobacter | NO2- | O2 | NO3-,H2O |
Nitrosomonas | NH4+ | O2 | NO2-,H2O |
Thiobacillus denitrificans | S0,H2S | NO3- | SO42-,N2 |
Thiobacillus ferrooxidans | Fe2+,S0,H2S | O2 | Fe3+,H2O,H2SO4 |
Photosynthesis:
(1) The light reaction in Eukaryotes and Cyanobacteria
Noncyclic photophosphorylation
The process in which light energy is used to make APT when electrons are moved from water to NADP+ during photosynthesis; both photosystem I and photosystem II are involved.
（2）The light reaction in green and purple bacteria
NAD reduction in green and purple bacteria
Cyclic photophosphorylation：
The formation of ATP when light energy is used to move electrons cyclically through an electron transport chain during photosynthesis; only photosystem I participates.
Property | Eucaryotes and cyanobacteria | Green and purple bacteria |
Photosynthetic pigment | Chlorophyll a | Bacteriochlorophyll |
Photosystem II | Present | Absent |
Phtosysthetic electron donors | H2O | H2,H2S,S,organic matter |
O2 production | Oxygenic | Anoxygenic |
Primary products of energy conversion | ATP+NADPH | ATP |
Carbon source | CO2 | Organic and/or CO2 |
(3) Photosynthesis in Halobacterium
Relationship between catabolism and anabolism
Amphibolic pathway: metabolic pathways that function catabolically and anabolically
Most microorganisms can replace TCA cycles intermediates by CO2 fixation
1. Biosynthesis of Monomers
(1) The photosynthetic fixation of CO2
Reductive tricarboxylic acid pathway
Hydroxypropionate pathway
(2) Nitrogen fixation
The reduction of atmospheric gaseous nitrogen to ammonia by microorganisms
Nitrogen fixation occurs in:
Free-living bacteria;
Bacteria living in symbiotic association with plants;
Associative nitrogen-fixer;
(3) Peptidoglycan synthesis
