Photosynthesis: Light and Dark Reactions

Ever wondered how plants turn sunlight into food? Photosynthesis is basically nature's solar power system, happening inside tiny structures called chloroplasts. These contain stacks of membranes called thylakoids (grouped into grana) surrounded by a fluid called the stroma.

The light-dependent stage is where the magic starts. Chlorophyll pigments in the thylakoids absorb mainly red and blue light, reflecting green (which is why plants look green!). This energy powers two types of photophosphorylation - cyclic and non-cyclic - both creating ATP through chemiosmosis.

In non-cyclic photophosphorylation, water molecules get split through photolysis $H₂O → 2H⁺ + ½O₂ + 2e⁻$, releasing oxygen and providing electrons for the process. The energy also reduces NADP to NADPH, creating the chemical currency needed for the next stage.

The Calvin cycle $light-independent stage$ happens in the stroma, where RuBisCO enzyme fixes CO₂ to RuBP (ribulose bisphosphate). This creates unstable 6-carbon compounds that split into GP $glycerate-3-phosphate$, which then gets converted to glucose using the ATP and NADPH from the light reactions.

Quick Tip: Remember the overall equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ - six of everything going in, glucose and oxygen coming out!

Three main factors affect photosynthesis rates: light intensity $more light = more ATP and NADPH$, CO₂ concentration $more CO₂ = faster Calvin cycle$, and temperature (higher temps speed up enzymes until they denature). Understanding these helps explain why plants grow differently in various conditions.