Beggiatoa Bacteria and Sulfur Granules: Chemolithotrophy Explained

The electron transport chain in aerobic bacteria and mitochondria consists of protein complexes (I, II, III, IV) embedded in membranes. Electrons flow from NADH/succinate through these complexes to oxygen, creating a current that powers proton extrusion: four protons at Complex I, four at Complex III, and two at Complex IV. The resulting proton gradient drives protons back through ATP synthase, a rotary motor enzyme that synthesizes ATP from ADP and inorganic phosphate. This ATP serves as the universal energy currency of life, driving protein function, conformational changes, and intermediary biochemistry. The stoichiometry of proton pumping and ATP synthesis has been refined over decades, with the current consensus being approximately 10 protons required to synthesize one ATP molecule.

The electron transport chain consists of four protein complexes (I, II, III, IV) embedded in the inner mitochondrial membrane. These complexes transfer electrons from NADH and FADH2 to oxygen, releasing energy that pumps protons from the matrix to the intermembrane space. Complex I pumps 4 protons, Complex III pumps 4 protons, and Complex IV pumps 2 protons per electron pair. This creates an electrochemical gradient with both chemical potential (higher proton concentration in intermembrane space) and electrical potential (charge separation). The total electrochemical potential is approximately double the chemical component alone, storing potential energy for ATP synthesis. ATP synthase consists of two main functional components: F0 (embedded in the inner mitochondrial membrane) and F1 (protruding into the matrix). The F0 component contains a proton channel through which protons flow, while the F1 component contains catalytic sites for ATP synthesis. The F1 component includes three alpha subunits, three beta subunits, and gamma, delta, and epsilon subunits. The beta subunits are the catalytic sites where ATP synthesis occurs, while the alpha subunits play a regulatory role. The gamma subunit connects the F0 and F1 components and transmits rotational energy.

ATP synthase is the enzyme that synthesizes ATP during oxidative phosphorylation, utilizing the electrochemical gradient generated by the electron transport chain. The enzyme consists of three main components: the F1 component (extending into the mitochondrial matrix), the shaft (connecting F1 to F0), and the F0 component (embedded in the inner mitochondrial membrane). The F0 component acts as a proton pumper that translocates protons from the intermembrane space to the matrix, causing rotational movement. This rotation drives the shaft, which mechanically couples to the F1 component. The F1 component is a hexamer composed of three dimers, each containing an alpha and beta subunit, where all substrate binding and catalytic reactions occur.

The electron transport chain consists of four protein complexes (I, II, III, IV) and ATP synthase (complex V). Complex I (NADH dehydrogenase) accepts electrons from NADH, Complex II (succinate dehydrogenase) accepts electrons from FADH2, Complex III (cytochrome bc1 complex) transfers electrons to cytochrome c, and Complex IV (cytochrome c oxidase) transfers electrons to oxygen. Complexes I, III, and IV function as proton pumps, moving protons from the matrix to the intermembrane space. This creates a proton gradient that drives ATP synthesis through ATP synthase.

The electron transport chain (ETC) is a series of protein complexes in the mitochondrial inner membrane that transfers electrons from NADH and succinate to oxygen, generating a proton gradient used for ATP synthesis. The ETC comprises five complexes: Complex I (NADH ubiquinone reductase) receives electrons from NADH via FMN and Fe-S proteins; Complex II (succinate dehydrogenase) transfers electrons from succinate to ubiquinone; Coenzyme Q (ubiquinone) is a mobile hydrophobic carrier; Complex III (ubiquinol cytochrome c reductase) transfers electrons to cytochrome c while pumping four protons; Complex IV (cytochrome c oxidase) reduces oxygen to water while pumping two protons. The accumulated protons create a proton motive force that drives ATP synthase (Complex V), which uses mechanical rotation to catalyze ATP formation from ADP and inorganic phosphate. Complete glucose oxidation yields approximately 38 ATP.

Organisms are classified by energy source (phototrophs from sunlight, chemotrophs from chemicals) and carbon source (autotrophs from CO2, heterotrophs from organic compounds). This yields six types: photoautotrophs (plants), photoheterotrophs, chemoautotrophs, chemoheterotrophs (animals), photochemoheterotrophs, and chemolithoheterotrophs. Autotrophs synthesize their own organic matter; heterotrophs consume pre-existing organic matter. This classification explains how different organisms obtain energy and carbon for survival.

Organisms are classified by their energy and carbon sources into four main types: (1) Photoautotrophs use light energy and carbon dioxide to produce food through photosynthesis, (2) Chemotrophs use inorganic chemical substances as energy sources while using carbon dioxide as carbon source, (3) Chemoheterotrophs use organic compounds as both energy and carbon sources, and (4) Photoheterotrophs use light energy but obtain carbon from organic sources. The key distinction is whether organisms synthesize their own food or obtain it externally, and whether they use light or chemical substances as energy sources.

Organisms are classified into four main metabolic types based on two criteria: carbon source and energy source. Autotrophs obtain carbon from inorganic sources (like CO2), while heterotrophs obtain carbon from organic sources (like food). Photoautotrophs use light energy to build organic compounds from inorganic carbon (e.g., plants performing photosynthesis). Photoheterotrophs use light energy but obtain carbon from organic sources (e.g., some purple bacteria). Chemoautotrophs obtain energy from chemical reactions and use inorganic carbon sources. Chemoheterotrophs obtain both energy and carbon from organic compounds (e.g., humans, fungi).

Microorganisms are classified by carbon source (auto/hetero) and energy source (photo/chemo). Autotrophs obtain carbon from inorganic sources like CO2, while heterotrophs obtain carbon from organic compounds of other organisms. Phototrophs obtain energy from light, while chemotrophs obtain energy from chemical bonds. Combining these classifications creates four main categories: chemoautotrophs (energy from chemical bonds, carbon from inorganic sources), chemoheterotrophs (energy from chemical bonds, carbon from organic compounds), photoautotrophs (energy from light, carbon from inorganic sources), and photoheterotrophs (energy from light, carbon from organic compounds).

Organisms are classified by their energy and carbon sources. Chemotrophs obtain energy from chemical compounds: chemolithotrophs use inorganic compounds, while chemoorganotrophs use organic compounds (including humans, animals, and heterotrophic bacteria). Photoautotrophs (plants, algae) use light to excite electrons from water and fix CO2. Photoheterotrophs use light for energy but obtain carbon from organic compounds. This classification explains how diverse organisms meet their energy and carbon requirements through different metabolic strategies.

Hydrogen Sulphide (H₂S) is a dibasic acid that can donate two hydrogen ions per molecule, forming normal salts (like Na₂S) or acid salts (like NaHS) when reacting with bases. It burns in air with a blue flame, producing sulfur and water in limited oxygen or sulfur dioxide and water in excess oxygen. H₂S acts as a strong reducing agent, reducing oxidizing agents like potassium dichromate (K₂Cr₂O₇) from orange to green color. When H₂S gas is passed through aqueous solutions of metallic salts, insoluble black metallic sulfides precipitate (e.g., PbS, CuS, Ag₂S). H₂S gas can be identified by passing it through lead acetate paper, which turns black due to lead sulfide formation, or through alkaline sodium nitroprusside, which produces a violet color. H₂S is highly toxic and can rapidly affect the central nervous system, with no known antidote.

H2S exhibits several important chemical properties: (1) Combustion: H2S burns in air with a blue flame to form sulfur dioxide and water. (2) Reaction with metals: H2S reacts with active metals to form metal sulfides and hydrogen gas. (3) Reaction with metal oxides: H2S reduces metal oxides to metals. (4) Reducing nature: H2S acts as a reducing agent and can be oxidized to sulfur or sulfur dioxide. (5) Acidic nature: H2S is a weak acid that dissociates to form H+ and HS- ions. These properties make H2S useful in various chemical reactions and qualitative analysis.

Hydrogen sulfide (H2S) has a bent molecular structure with sulfur having 6 valence electrons and 2 unpaired electrons. It is a colorless gas heavier than air, highly toxic, and forms sulfhydric acid in water. Chemical properties include combustion: 2H2S + 3O2 → 2SO2 + 2H2O (sufficient oxygen) or 2H2S + O2 → 2S + 2H2O (limited oxygen). Sulfur has oxidation state -2, so H2S acts only as a reducing agent. Applications include production of sulfuric acid, sulfur, sulfides, and as an analytical reagent for detecting metal ions.

Some organisms can perform photosynthesis using hydrogen sulfide (H2S) instead of water as the electron donor, producing sulfur as a byproduct rather than oxygen. Sulfur compounds can precipitate to form minerals through chemical reactions, such as hydrogen sulfide reacting with metal ions to form metal sulfides. Iron sulfide minerals form when hydrogen sulfide reacts with iron in the environment. Quantitative analysis of the sulfur cycle involves measuring the amounts of sulfur in different forms and locations, revealing the scale and distribution of sulfur cycling processes.

Hydrogen sulfide (H2S) is discussed as a compound containing hydrogen and sulfur. The instructor explains its chemical properties and its role in acid-base chemistry, noting that it can act as an acid.