Brock Biology Chapter 14: Microbial Metabolic Diversity

What do all microbes follow in terms of metabolic principles?

All microbes follow the same metabolic principles: energy conservation, obtaining reducing power during catabolism, and coupling energy to anabolism/biosynthesis.

What drives the tremendous diversity in microbial metabolism?

The tremendous diversity is driven by the modularity of metabolic reactions, allowing for the formation of new pathways through horizontal transfer and modification of enzymes.

What is the role of energy conservation in microbial metabolism?

All cells conserve energy by coupling electron flow to ATP synthesis.

What defines the tendency of donating or accepting electrons in redox reactions?

The tendency is defined by reduction potential (E'0).

How is ATP generated in fermentation?

ATP is generated primarily by substrate-level phosphorylation and does not require an external electron acceptor.

What is the difference between respiration and fermentation?

Respiration requires an external electron acceptor and generates ATP by oxidative phosphorylation, while fermentation does not.

What is the function of ATP synthase in respiration?

ATP synthase harnesses the proton motive force (pmf) to synthesize ATP.

What are anaerobic respirations?

Anaerobic respirations use electron acceptors other than O2 and yield less energy than aerobic respiration.

What is the most important assimilative process in microorganisms?

The most important assimilative process is CO2 fixation.

What is the Calvin Cycle?

The Calvin Cycle is the most widespread pathway for CO2 fixation, used by all oxygenic phototrophs.

What key enzyme is involved in the Calvin Cycle?

The key enzyme is Rubisco, which reduces CO2 to glyceraldehyde-3-phosphate.

What are carboxysomes?

Carboxysomes are proteinaceous microcompartments that contain Rubisco and protect it from O2.

What is the role of light reactions in photosynthesis?

Light reactions convert light energy into chemical energy, producing a proton motive force and ATP.

What is the difference between oxygenic and anoxygenic photosynthesis?

Oxygenic photosynthesis uses water as an electron donor and produces O2, while anoxygenic photosynthesis uses other electron donors and does not produce O2.

What are antenna pigments?

Antenna pigments are larger numbers of chlorophylls/bacteriochlorophylls that funnel light energy to reaction centers.

What is the function of chlorosomes in photosynthetic bacteria?

Chlorosomes capture low light intensities and function as giant antenna systems, allowing phototrophs to grow in low light conditions.

What types of organisms are considered photoautotrophs?

Photoautotrophs are organisms that use light energy to reduce CO2 to organic compounds.

What is the role of bacteriochlorophyll in anoxygenic phototrophs?

Bacteriochlorophyll absorbs light energy to drive photosynthesis in anoxygenic phototrophs.

What is the significance of the Great Oxidation Event in relation to the Calvin Cycle?

The Calvin Cycle originated before the Great Oxidation Event, indicating its long evolutionary history.

What are chemolithotrophs?

Chemolithotrophs are organisms that use inorganic electron donors for respiration.

What is the primary method of ATP generation in respiration?

ATP is generated by oxidative phosphorylation from electron transport.

What is the relationship between assimilative and dissimilative processes?

Assimilative processes consume energy to incorporate inorganic nutrients into cells, while dissimilative processes conserve energy by reducing electron acceptors and excreting them.

What is the role of reducing power in microbial metabolism?

Reducing power provides electronegative electrons for redox reactions and is essential for biosynthesis.

How do anaerobic organisms compete with aerobic organisms?

Aerobic organisms conserve more energy than anaerobic organisms, allowing them to outcompete in environments with available O2.

What is the significance of the proton motive force (pmf) in ATP synthesis?

The proton motive force drives ATP synthesis by providing the energy needed for ATP synthase to function.

What types of electron donors can be used in microbial respiration?

A wide variety of organic or inorganic electron donors can be coupled to a wide variety of electron acceptors.

What are carotenoids?

Widespread accessory pigments that are hydrophobic and embedded in photosynthetic membranes.

What is an example of a carotenoid?

β-carotene.

What colors do carotenoids typically exhibit?

Yellow, red, brown, or green.

What type of light do carotenoids absorb?

Blue light.

What is the primary function of carotenoids?

To act as photoprotective agents, quenching toxic oxygen species and preventing photooxidation.

What are phycobiliproteins?

Main light-harvesting systems of cyanobacteria and red algae chloroplasts.

What are the characteristic colors of organisms with phycobiliproteins?

Red or blue-green.

What do red phycoerythrin, blue phycocyanin, and allophycocyanin absorb?

Red phycoerythrin absorbs ~550 nm, blue phycocyanin absorbs ~620 nm, and allophycocyanin absorbs ~650 nm.

What are phycobilisomes?

Aggregated phycobiliproteins attached to cyanobacterial thylakoids.

What is the role of phycobilisomes in photosynthesis?

They allow cells to grow at lower light intensities.

What are oxygenic phototrophs?

Organisms that use both PS I (photosystem I) and PS II (photosystem II) in photosynthesis.

What is the 'Z scheme' in photosynthesis?

The interaction between PSI and PSII where energy is transferred from PSII to PSI.

What occurs during the light reactions of oxygenic photosynthesis?

Water is split into oxygen and electrons, generating a proton motive force.

What is the outcome of noncyclic photophosphorylation?

Electrons do not cycle back, reducing NADP+ to NADPH.

What happens during cyclic photophosphorylation?

PSI electrons are returned to the electron transport chain to produce more ATP.

What is denitrification?

The reduction of nitrate to nitrogen gas, consuming fixed nitrogen.

What are common electron acceptors in anaerobic respiration?

Inorganic nitrogen compounds like nitrate.

What is the role of nitrate reductase?

Catalyzes the first step in dissimilative nitrate reduction.

What is the ecological impact of denitrifying microorganisms?

They can remove nitrate from fertilizers, leading to pollution and contributing to greenhouse gas emissions.

What is sulfate reduction?

The reduction of sulfate to hydrogen sulfide by sulfate-reducing bacteria.

What is the significance of sulfate-reducing bacteria?

They are obligate anaerobes that play a key role in the sulfur cycle.

What is assimilative sulfate metabolism?

The incorporation of sulfate for biosynthetic purposes to produce compounds like cysteine and methionine.

What is the oxidation state of nitrogen in nitrate?

+5.

What is the oxidation state of sulfur in sulfate?

+6.

What is the primary electron donor for sulfate-reducing bacteria?

Hydrogen (H2).

What is the first step in dissimilative nitrate reduction?

Catalyzed by nitrate reductase.

What is the consequence of denitrification in agriculture?

It can lead to the loss of nitrogen from the soil, affecting crop yields.

Which organisms are known to denitrify?

Many are Proteobacteria and facultative aerobes, such as Pseudomonas.

What is required to reduce sulfate (SO4^2-) to hydrogen sulfide (H2S)?

Eight electrons and many intermediates.

What enzyme activates sulfate (SO4^2-) to form adenosine phosphosulfate (APS)?

ATP sulfurylase.

In dissimilative reduction, what does APS reduce to?

Sulfite (SO3^2-) by APS reductase, releasing AMP.

What is the difference between dissimilative and assimilative reduction of sulfate?

Dissimilative reduction directly reduces APS to sulfite, while assimilative reduction adds phosphate to APS before reduction.

What enzyme reduces sulfite to hydrogen sulfide (H2S)?

Sulfite reductase.

How is proton motive force generated in dissimilative sulfate reduction?

Through electron transport.

What role does cytochrome c play in sulfate reduction?

It accepts electrons from hydrogenases and transfers them to protein complexes.

What do sulfate reducers typically oxidize to produce hydrogen (H2)?

Organic substrates.

What is the energy yield of ATP produced for each sulfate reduced to hydrogen sulfide?

Approximately 2 ATP.

What is the primary fermentation process for lactic acid bacteria?

Lactic acid fermentation.

What distinguishes homofermentative lactic acid bacteria from heterofermentative ones?

Homofermentative yields only lactic acid and two ATP/glucose, while heterofermentative yields lactate, ethanol, CO2, and one ATP/glucose.

What is the Entner-Doudoroff pathway?

A variant of the glycolytic pathway that converts glucose 6-phosphate to pyruvate and glyceraldehyde 3-phosphate.

What type of fermentation is characteristic of enteric bacteria like E. coli?

Mixed-acid fermentation.

What are the main products of mixed-acid fermentation?

Acetic acid, lactic acid, succinic acid, ethanol, CO2, and H2.

What are obligate anaerobes?

Organisms that cannot tolerate oxygen and often produce hydrogen from fermentation.

What is the primary ATP synthesis method for Clostridium species?

Substrate-level phosphorylation.

What major products do saccharolytic Clostridium species produce?

Butyric acid and hydrogen (H2).

What is the phosphoroclastic reaction?

The splitting of pyruvate to acetyl-CoA, CO2, and reduced ferredoxin (Fd).

How is redox balance achieved in Clostridium species?

By cytoplasmic hydrogenase that reduces H+ to H2.

What is the energy yield of alcoholic fermentation?

Negative 239 kJ/mol.

What is the energy yield of homolactic fermentation?

Negative 196 kJ/mol.

What is the energy yield of heterolactic fermentation?

Negative 216 kJ/mol.

What is the energy yield of propionic acid fermentation?

Negative 170 kJ/mol.

What is the energy yield of butyric acid fermentation?

Negative 264 kJ/mol.

What is the energy yield of butanol fermentation?

Negative 468 kJ/mol.

What distinguishes fermentation from respiration?

Fermentation lacks an external electron acceptor.

What is the role of substrate-level phosphorylation in fermentation?

It allows microbes to make ATP by transferring a phosphate bond to ADP.

What is a common characteristic of fermentation products?

They are metabolic intermediates excreted as fermentation products.

What is the main challenge of fermentative organisms compared to respiratory organisms?

They conserve much less energy.

What is the significance of aldolase in fermentation?

Its presence distinguishes homofermentative from heterofermentative bacteria.

What type of fermentation do Clostridium acetobutylicum perform?

Butanol/acetone fermentation.