CVEN9886 Chap.3 Microbial Metabolism and Energy
Microbial Metabolism and Energy
This chapter classifies microbial energy processes by carbon source, energy source, electron donor, electron acceptor and products. It distinguishes autotrophy from heterotrophy and respiration from fermentation, then explains why aerobic and anaerobic zones can support different communities in the same system. ATP is treated as an energy-transfer currency linking catabolic release to anabolic work.
What this chapter covers
- 01
ATP production and hydrolysis as short-term coupling between energy release and cellular work
- 02
Catabolic energy release separated from anabolic synthesis of new cellular material
- 03
Photosynthesis and chemoautotrophy compared by carbon, energy and electron sources
- 04
Aerobic respiration with oxygen as the terminal electron acceptor
- 05
Anaerobic respiration using nitrate, sulphate, carbon dioxide or other inorganic acceptors
- 06
Fermentation, internal organic electron acceptors and reduced end products
Worked example · free
Diagnose nitrate removal without oxygen
- +1Identify glucose as the organic donor and carbon source.
- +1Identify nitrate as an inorganic electron acceptor.
- +1Classify the route as anaerobic respiration rather than fermentation.
- +1Link nitrate reduction, energy release and biomass growth without merging their carbon fates.
Key terms
- Autotrophy
- A mode of growth that builds biomass from inorganic carbon such as carbon dioxide.
- Respiration
- An energy process that transfers electrons to an inorganic terminal acceptor.
- Fermentation
- An energy process in which an organic compound accepts electrons without an external inorganic acceptor.
Microbial Metabolism and Energy FAQ
What is the difference between autotrophy and photosynthesis?
Autotrophy describes the carbon source, while photosynthesis describes light as the energy source. Chemoautotrophs also fix inorganic carbon but obtain energy from chemical oxidation rather than light.
Why is anaerobic respiration not fermentation?
Anaerobic respiration uses an inorganic acceptor such as nitrate, sulphate or carbon dioxide. Fermentation uses an organic acceptor and lacks that external inorganic respiratory endpoint.
How should I compare energy yields?
Use the direction supported by the course: aerobic respiration generally yields more ATP than anaerobic respiration. Avoid universal ATP totals unless the exact pathway and organism are specified.
How do I identify an unfamiliar pathway from reactor data?
Track the evidence in a fixed order: name the carbon source, identify the electron donor, identify the acceptor and verify the products. Oxygen absence only establishes an anaerobic condition; it does not distinguish nitrate respiration, sulphate reduction, methanogenesis or fermentation. Check whether an external inorganic acceptor is consumed before using the word respiration.
Then separate substrate converted for energy from carbon incorporated into biomass and close the material balance. If several pathways remain possible, state the ambiguity and request the product or electron-acceptor measurement that would resolve it.
Exam move
Build one process card for every metabolic route using five fixed fields: carbon source, energy source, electron donor, electron acceptor and products. Do not merge the fields. Autotrophy and heterotrophy classify carbon; phototrophy and chemotrophy classify energy; aerobic, anaerobic and fermentative pathways depend on electron transfer. Read each card aloud as a causal sentence so the labels remain attached to evidence.
Next, draw an electron-acceptor ladder for an environmental system. Begin with oxygen, then place the alternative acceptors supported by the teaching material and annotate the products expected as conditions become more reduced. Use the ladder to predict spatial succession through a sediment or biofilm and temporal succession in a sealed reactor.
For each prediction, state which substrate must still be available; acceptor presence alone cannot drive a reaction without an electron donor. Rework the nitrate-reactor example without looking at the solution. Mark four elements: organic donor, inorganic nitrate acceptor, nitrogen-gas product and biomass growth. Explain why no oxygen means anaerobic but does not mean fermentation.
Then change one input at a time—remove nitrate, introduce oxygen or exhaust glucose—and predict the new limitation before choosing a pathway. Create a separate ATP card. Define ATP as a continuously regenerated transfer currency that couples catabolic release to transport, movement, synthesis and regulation. Avoid treating it as a permanent storage reservoir.
Compare energy yields qualitatively unless a pathway-specific calculation is provided: explain why a stronger terminal acceptor can support a larger favourable transfer, while fermentation lacks the same external respiratory chain. Practise balancing the methanogenesis relationship by counting carbon, hydrogen and oxygen on both sides before interpreting the process.
For any unfamiliar equation, balance atoms and charge first, name oxidised and reduced products second, and only then connect it to an organism or habitat.
During timed revision, use a three-sentence answer frame: the observed donor and acceptor identify the pathway; the products and growth evidence support the classification; the named environmental change would shift the active process by altering availability or energetic opportunity. Finish with an engineering consequence such as aeration, carbon addition or zoning, plus the measurement that verifies response.
This method turns vocabulary into a process diagnosis and prevents oxygen status, growth and reaction rate from being treated as interchangeable observations. Check products as well as disappearing reactants before naming the pathway.