Metabolism, Cell Respiration and Photosynthesis (AHL) - Photosynthesis (HL)

Photosynthesis occurs in two main stages: the Light-Dependent Reactions (in the thylakoid membrane) and the Light-Independent Reactions (in the stroma).

Light-Dependent Reactions involve the absorption of light by photosystems ($PSII$ and $PSI$). Photolysis of $H_2O$ occurs at $PSII$, releasing $O_2$, protons ($H^+$), and electrons ($e^-$).

Photophosphorylation: The flow of electrons through the Electron Transport Chain ($ETC$) generates a proton gradient. $H^+$ ions flow through $ATP$ synthase to produce $ATP$ from $ADP$ and $P_i$.

Reduction of $NADP$: $PSI$ absorbs light to excite electrons, which are then used to reduce $NADP^+$ to $NADPH$ via the enzyme $NADP$ reductase.

Light-Independent Reactions (Calvin Cycle): $CO_2$ is fixed to Ribulose bisphosphate ($RuBP$) by the enzyme Rubisco, forming two molecules of Glycerate-3-phosphate ($G3P$).

Reduction Phase: $G3P$ is reduced to Triose Phosphate ($TP$) using $ATP$ and $NADPH$ produced in the light-dependent stage.

Regeneration: For every six molecules of $TP$ produced, five are used to regenerate $RuBP$ (requiring $ATP$), while one exits the cycle to contribute to the formation of glucose ($C_6H_{12}O_6$).

Chloroplast Structure: The thylakoids provide a large surface area for light absorption; the small thylakoid lumen allows for rapid accumulation of a proton gradient; the stroma contains the enzymes for the Calvin cycle.