ch 14 part1

14.1 Foundational Principles of Metabolic Diversity: Energy and Redox

Overview

  • All microbes share metabolic principles, including energy conservation and coupling of catabolism to anabolism.

  • Tremendous diversity arises from the modularity of metabolic reactions.

Key Concepts

  • Energy Conservation: Microbes convert chemical/light energy into ATP.

  • Reducing Power: Acquired through catabolism using electronegative electrons for redox reactions.

  • Redox Balance: Necessary to regenerate oxidized electron carriers with external electron acceptors.

Electron Flow and ATP Synthesis

  • Energy is conserved by linking electron flow to ATP synthesis through redox reactions:

    • Electrons are transferred from donors to acceptors.

    • Various organic/inorganic electron donors and acceptors are available.

    • Types of ATP synthesis:

      • Substrate-level Phosphorylation: Direct generation of ATP during metabolic reactions.

      • Oxidative Phosphorylation: Involves electron transport chains and creates a proton motive force (pmf).

      • Photophosphorylation: Light-driven process to produce ATP.

  • Examples: Through specific metabolic pathways depend on microbial type.

Chemotrophic Metabolism

  • Fermentation:

    • Does not need external electron acceptors.

    • ATP generated chiefly through substrate-level phosphorylation.

    • Electron balance is maintained via metabolites excreted as fermentation products.

  • Respiration:

    • Requires external electron acceptors.

    • ATP is formed through oxidative phosphorylation utilizing electron transport and pmf.

    • Chemolithotrophs utilize inorganic donors such as hydrogen, sulfide, nitrite, ammonia, iron.

Anaerobic and Aerobic Respiration

  • Anaerobic Respiration: Uses non-O2 electron acceptors leading to less energy conservation compared to aerobic respiration due to the higher electron potential of O2.

  • In environments with limited O2, microorganisms adapt using anaerobic paths, affecting ecological competition.

Reducing Power and Biosynthesis

  • Cells depend on reducing power for biosynthesis and to maintain redox balance:

    • Utilizes low potential carriers (e.g., NAD(P)H).

    • In chemorganotrophs, power generated from organic substrates (like glucose).

    • Achieved by donating electrons to acceptors either in respiration or fermentation.

14.2 Autotrophic Pathways

Overview

  • Autotrophs: Can assimilate CO2 into cellular material, including most phototrophs and chemolithotrophs.

  • CO2 acts as a fundamental carbon source for biosynthesis.

The Calvin Cycle

  • Most prevalent pathway for fixing CO2.

  • Involves:

    • Utilization by all oxygenic phototrophs (plants, algae, cyanobacteria).

    • Key enzyme: RubisCO, which reduces O2 to glyceraldehyde-3-phosphate (G3P).

    • Requires significant resources: 12 NADPH and 18 ATP to produce one fructose-6-phosphate.

Other CO2 Fixation Pathways

  • Green Nonsulfur Bacteria: Fix CO2 into pyruvate.

  • Chemolithotrophic Archaea: Reduce bicarbonate/CO2 to acetyl-CoA.

  • Anaerobic bacteria and archaea: Use the most efficient CO2 fixation pathway, needing only 1 ATP for every 3 CO2 fixed.

14.3 Photosynthesis and Chlorophylls

Overview

  • Photosynthesis: The use of light energy for biosynthesis; characteristic of autotrophs.

  • Phototrophy: Not all phototrophs are strictly autotrophic; some use organic carbon sources (photoheterotrophs).

Light Reactions vs Light-Independent Reactions

  • Light Reactions: Convert light into chemical energy, creating proton motive force and ATP.

  • Light-Independent Reactions: Require ATP and reducing power to fix CO2.

    • In cyanobacteria: Water acts as an electron donor, producing O2 (oxygenic photosynthesis).

    • Anoxygenic photosynthesis employs alternative electron donors; does not release O2.

Photosynthetic Pigments

  • Chlorophylls: Main pigment involved in capturing light.

    • Absorb light and initiate energy conversion.

  • Bacteriochlorophylls: Present in anoxygenic phototrophs.

  • Pigment Diversity: Important ecologically, allowing different organisms to occupy similar environments by utilizing different wavelengths.

Reaction Centers and Antenna Pigments

  • Chlorophylls are integrated into photocomplexes within membranes.

  • Reaction Centers: Involved in energy conversion.

  • Antenna Pigments: Surround reaction centers, increasing the efficiency of light harvesting.

Chloroplasts and Chlorosomes

  • Eukaryotic Photosynthesis: Occurs in chloroplasts with internal thylakoids.

  • Prokaryotic Photosynthesis: Photosynthetic pigments integrated into cytoplasmic membranes (e.g., chromatophores, thylakoids).

  • Chlorosomes: Found in certain bacteria, allowing growth under low light conditions by acting as giant antenna systems.

14.4 Carotenoids and Phycobilins

Carotenoids

  • Widespread accessory pigments absorbing blue light.

  • Have protective roles against toxic photooxidation and are hydrophobic.

Phycobiliproteins and Phycobilisomes

  • Main light-harvesting systems found in cyanobacteria and red algae.

  • Crucial for utilizing lower light intensities, allowing survival in suboptimal environments.