Chapter 6: Photosynthesis

Early life forms were heterotrophs, dependent on external organic molecules. The evolution of cyanobacteria, which utilize water (H2O) as an electron source instead of less abundant compounds like hydrogen sulfide (H2S), resulted in the production of molecular oxygen (O2), fundamentally altering Earth’s atmosphere. The process occurs within the chloroplast, an organelle found predominantly in mesophyll cells, defined by a double envelope and an internal membrane system organized into flattened sacs called thylakoids, which stack to form grana. The internal space, the stroma, houses the enzymes (like Rubisco) necessary for carbohydrate synthesis. Photosynthesis consists of two stages: the light-dependent reactions, which use photon energy to generate ATP and NADPH, and the light-independent reactions (Calvin cycle), which use that stored energy to fix carbon dioxide (CO2) into sugars. Light absorption is initiated by primary pigments called chlorophylls and accessory pigments called carotenoids, which form light-harvesting antennae that rapidly transfer excitation energy to reaction centers (P680 or P700). Oxygen-releasing photosynthesis involves two sequential light-absorbing photosystems acting in series, known as the Z scheme. Photosystem II (PSII, P680) catalyzes the light-driven photolysis of water, supplying electrons and protons to the thylakoid lumen, and producing O2. These electrons move through the cytochrome b6f complex (which further translocates protons) and plastocyanin, eventually reaching Photosystem I (PSI, P700), which boosts them to a sufficiently high energy level to reduce NADP+ to NADPH. The resulting electrochemical proton gradient drives ATP synthesis via ATP synthase (CF1/CFo), a process called noncyclic photophosphorylation. An alternate path, cyclic photophosphorylation involving only PSI, produces extra ATP without generating NADPH or O2. Carbohydrate synthesis occurs in the Calvin cycle in the stroma, starting when the abundant enzyme Rubisco catalyzes the fixation of CO2 onto ribulose 1,5-bisphosphate (RuBP). Rubisco, however, can also catalyze a wasteful reaction with O2, initiating photorespiration, a process that involves chloroplasts, peroxisomes, and mitochondria. To overcome photorespiration, C4 plants evolved a spatial separation of initial CO2 fixation (in mesophyll cells using PEP carboxylase) and the Calvin cycle (in bundle-sheath cells), while CAM plants utilize a temporal separation, fixing CO2 at night and performing the Calvin cycle during the day. Finally, the chapter notes that the principle of light-excited pigments producing reactive oxygen species is harnessed in medicine through Photodynamic Therapy (PDT) to cause localized cell death.