Metabolic Diversity in Microorganisms

Overview

Today's lecture focused on metabolic diversity, especially energy generation, electron transport, and related metabolic processes in bacteria and archaea, with review of key definitions and principles.

Metabolic Strategies and Classifications

Photoheterotrophs use light for ATP but cannot fix their own carbon.
Halobacteria are photoheterotrophic archaea using bacteriorhodopsin for light-driven proton gradients, not electron transport systems.
Halobacteria generate ATP via chemiosmosis but are not primary producers as they do not fix carbon.
Photoautotrophy (photosynthesis) uses light energy and often electrons from water, producing oxygen in oxygenic forms.

Electron Transport and Proton Gradients

Electron transport systems (ETS) run electrons through membranes to generate proton gradients.
Chemiosmosis uses this gradient to power ATP synthase for ATP production.
Voltage or membrane potential references charge separation across membranes, driving ATP synthesis.
ETS and electron transport chain (ETC) refer to the same concept.

Anaerobic vs Aerobic Respiration

Aerobic respiration uses oxygen as the terminal electron acceptor.
Anaerobic respiration uses non-oxygen compounds (e.g., sulfur, nitrogen) as terminal electron acceptors.
Exam strategy: if a terminal electron acceptor isn't oxygen, it's anaerobic.

Fermentation and Metabolic Processes

Fermentation recycles NAD+ by dumping electrons into waste products like lactic acid or alcohol.
Both bacteria and eukaryotes (like humans and yeast) perform fermentation.
The TCA cycle (Krebs cycle) fully oxidizes glucose to CO₂, extracting max electrons for ATP production.

Chemical Bonds and Polarity

Non-polar bonds: electrons shared equally (e.g., C-H).
Polar bonds: electrons shared unequally (e.g., C-O), leading to partial charges.
Understanding polarity is key for biochemical behavior.

Unique Biology of Halobacteria and Archaea

Halobacteria thrive in salty environments using bacteriorhodopsin, not ETS, to generate ATP.
Some archaea are methanogens; only archaea can do methanogenesis.
Both bacteria and archaea participate in nitrogen fixation; eukaryotes do not.

Applied Examples

Oxygen in mouthwash kills anaerobic bacteria causing bad breath.
Rotten egg smell in anoxic zones is from hydrogen sulfide, a sulfur compound produced by anaerobic respiration.

Key Terms & Definitions

Photoheterotroph — Organism using light for energy but relying on external carbon sources.
Chemiosmosis — ATP generation via proton gradients across membranes.
Electron Transport System (ETS) — Series of proteins moving electrons to set up proton gradients.
Fermentation — Process recycling NAD+ by transferring electrons to a waste product.
TCA Cycle (Krebs Cycle) — Pathway oxidizing sugars completely to CO₂ to extract electrons.
Aerobic Respiration — Uses oxygen as terminal electron acceptor.
Anaerobic Respiration — Uses compounds other than oxygen as terminal electron acceptor.
Membrane Potential — Voltage difference created by separation of charges across a membrane.
Polar Bond — Bond with unequal electron sharing, creating charge differences.
Non-polar Bond — Bond with equal electron sharing.

Action Items / Next Steps

Review textbook sections on chemiosmosis, ETS, and metabolic classifications.
Prepare for discussion on water properties and polarity next class.
Review notes on the TCA (Krebs) cycle, fermentation, and key metabolic pathways.