Photosynthesis — from sunlight to glucose

Photosynthesis is the process by which plants, algae, and certain bacteria convert light energy into chemical energy stored in glucose. The overall equation:

6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

The three phases

• Light Absorption: Chlorophyll in the leaf captures photons. The energy per mole of photons is calculated by E = hcNₐ/λ. At λ = 680 nm (P680 absorption peak), E ≈ 176 kJ/mol. At λ = 400 nm, E ≈ 299 kJ/mol. However, absorption efficiency varies with wavelength.
• Water Splitting: The absorbed energy splits H₂O molecules, releasing O₂ as a byproduct and generating electrons and protons. O₂ exits through the stoma. H⁺ protons flow FROM the Granum (thylakoid) DOWN INTO the Stroma — this proton gradient drives ATP synthesis.
• Carbon Fixation: CO₂ enters through the stoma and is fixed into organic molecules via the Calvin cycle, ultimately forming glucose (C₆H₁₂O₆, M = 180 g/mol) which travels through veins.

Measured quantities

• E = hcNₐ/λ — energy per mole of photons (Planck's law). At λ = 400 nm, E ≈ 299 kJ/mol. At λ = 680 nm, E ≈ 176 kJ/mol.
• λ = 400–700 nm — wavelength of light (visible spectrum). Chlorophyll absorption peaks at ~430 nm (blue) and ~680 nm (red).
• C = 180 g/mol — molar mass of glucose (C₆H₁₂O₆ = 6×12 + 12×1 + 6×16 = 180 g/mol)

Sources

• NCBI: Photosynthesis overview
• ScienceDirect: Chlorophyll absorption
• Chemistry Learner: Glucose