Photosynthesis and Energy Sources

Heterotrophic Organisms and Carbohydrates

• Heterotrophic organisms, including humans and E. coli, derive energy from carbohydrates.
• Carbohydrates are primarily produced through the process of photosynthesis.

Photosynthesis Overview

• Photosynthesis transforms sunlight into chemical energy, performed mainly by phototrophic organisms like plants and some microbes.
• Microbial photosynthesis is crucial for various ecosystems.

Stages of Photosynthesis

• Light-dependent reactions absorb sunlight through pigments, generating ATP and NADPH/NADH.
• Light-independent reactions utilize ATP and NADPH from the first stage to synthesize sugar molecules from CO2.

Main Outputs of Photosynthesis

• The primary products include ATP (energy carrier), NADPH/NADH (energy carriers), and sugars (organic energy forms).

Eukaryotic Photosynthesis

• Occurs in chloroplasts, which contain double membranes, thylakoids (organized in granum), and stroma (the surrounding fluid).

Prokaryotic Photosynthesis

• Takes place in thylakoids formed from infolded plasma membranes, with pigments in membranes converting light energy to chemical energy.

Photosystems

• Comprised of a light-harvesting complex and a reaction center that releases electrons upon absorbing energy.

Photosynthetic Pigments

• Different pigments absorb various light wavelengths, including:
  • Bacteriochlorophylls (green, purple, red)
  • Carotenoids (orange, red, yellow)
  • Chlorophylls (green)
  • Phycocyanins (blue)
  • Phycoerythrins (red)
• Diverse pigments enhance light energy absorption.

Electron Transport System (ETS)

• High-energy electrons enter the ETS, resulting in the production of NADH or NADPH while ATP is generated through chemiosmosis.

Importance of Electrons in Photosynthesis

• Electrons released from the reaction center must be replenished for continuous photosynthesis.

Types of Photosynthesis

• Oxygenic Photosynthesis:
  • Found in plants and cyanobacteria.
  • Uses water (H2O) as an electron source and produces oxygen (O2) as a byproduct.
• Anoxygenic Photosynthesis:
  • Conducted by some bacteria using compounds like hydrogen sulfide (H2S) or thiosulfate (S2O3²⁻).
  • Does not produce oxygen but may generate elemental sulfur or sulfate ions.

Discovery of Photosystems

• Joan Mary Anderson identified two photosystems: Photosystem I (PSI) and Photosystem II (PSII).
• Different parts of chloroplasts yield distinct products; both photosystems are present in cyanobacteria and plant chloroplasts, while anoxygenic bacteria utilize only one.

Energy Production Mechanisms

• Noncyclic photophosphorylation generates both ATP and NADPH when required:
  • Electrons flow from PSII to PSI via ETS, with water splitting replenishing electrons at PSII.
  • PSI reduces NADP+ to form NADPH in the Z-scheme.
• Cyclic photophosphorylation is employed when ATP needs exceed NADPH needs, utilizing only PSI with electrons returning to PSI.

Light-Independent Reactions (Calvin Cycle)

• Converts CO2 into carbohydrates for energy storage, relying on ATP and NADPH from photosynthesis.
• Calvin-Benson Cycle Stages:
  • Fixation: RuBisCO enzyme adds CO2 to RuBP, forming 3-PGA.
  • Reduction: 6 ATP and 6 NADPH convert 3-PGA into G3P, used to create glucose.
  • Regeneration: Remaining G3P regenerates RuBP using 3 ATP, continuing the cycle.

Additional Insights

• RuBisCO is the most abundant enzyme on Earth, making up 30%-50% of soluble proteins in plant chloroplasts.
• The Calvin cycle is utilized by plants, photoautotrophic bacteria, and some non-photosynthetic chemoautotrophs.
• Other microorganisms may employ different systems for CO2 fixation.