T.D. Saussure demonstrated in the early 19th century that water is essential for photosynthesis, contributing to plant mass.

Ruben and Kamen used isotopic oxygen (¹⁸O) in the 1940s to show that photosynthetic oxygen comes from water, not CO₂.

Autoradiography with ¹⁴C allowed Calvin to trace carbon’s path in the Calvin cycle, identifying intermediates.

¹⁴C traces carbon fixation in the Calvin cycle, while ¹⁸O confirms water as the oxygen source in photosynthesis.

Photosynthesis begins when a photon excites chlorophyll electrons, initiating the light-dependent reactions.

The oxygen atoms in glucose (C₆H₁₂O₆) come from CO₂ during the Calvin cycle, as confirmed by isotopic studies.

The Calvin cycle, a light-independent reaction, occurs in the stroma, where CO₂ is fixed into glucose.

Moll’s experiment covered half a leaf, showing no starch formation without CO₂, proving CO₂’s role.

The primary photochemical reaction, involving light absorption by chlorophyll, occurs in the thylakoid membrane.

Chlorophyll a absorbs blue light (around 430 nm) most efficiently, followed by red.

Thylakoids lack stromal enzymes like RuBisCO, needed for the Calvin cycle to produce sugar.

Carotenoids absorb light in wavelengths not captured by chlorophyll and transfer energy to chlorophyll.

Slow translocation causes photosynthate buildup, inhibiting photosynthesis via feedback.

PS II oxidizes water, releasing electrons, protons, and oxygen during photolysis.

Blackman’s 1905 law states that photosynthesis is limited by the slowest factor, like light or CO₂.

PEP carboxylase fixes CO₂ into oxaloacetic acid in C₄ plants’ mesophyll cells.

In C₄ plants, PEP carboxylase fixes CO₂ into oxaloacetic acid in mesophyll cells.

C₄ plants concentrate CO₂ in bundle sheath cells, minimizing photorespiration in hot conditions.

The Calvin cycle in C₄ plants occurs in the stroma of bundle sheath chloroplasts, using concentrated CO₂.

Carotenoids absorb light energy and transfer it to chlorophyll, aiding photosynthesis.

The Emerson effect revealed two photosystems (PS I and PS II) by showing enhanced photosynthesis with mixed wavelengths.

In C₃ plants, 3-phosphoglyceric acid (PGA) is the first stable product of the Calvin cycle.

The Z-scheme describes electron transport from PS II (water splitting) to PS I (NADPH formation), producing ATP and NADPH.

Photolysis in PS II splits water, releasing electrons, protons (H⁺), and O₂.

Electron transfer from cytochrome b₆ to cytochrome f in the cytochrome complex drives proton pumping, forming ATP.

Cyclic photophosphorylation, driven by PS I, uses light >680 nm (red light) to produce ATP.

Both cyclic and non-cyclic photophosphorylation produce ATP via electron transport.

Six turns of the Calvin cycle fix six CO₂ molecules to produce one glucose (C₆H₁₂O₆).

Phytoplankton release oxygen during photosynthesis, attracting aerobic bacteria.

Oxygen bubbles produced during daytime photosynthesis increase algae buoyancy, causing sinking at night.