光合作用Photosynthesis

光反应在叶绿体类囊体膜上进行：光系统 II 吸收光后由放氧复合体（Mn₄CaO₅ 簇）氧化水放出 O₂，电子经质体醌、细胞色素 b₆f 传给光系统 I，最终把 NADP⁺ 还原为 NADPH；同时形成的质子梯度驱动 ATP 合酶合成 ATP（“Z 形图”）。暗反应（卡尔文循环）在基质中进行：RuBisCO 把 CO₂ 固定到核酮糖-1,5-二磷酸上，再用 ATP 和 NADPH 还原为丙糖磷酸。每固定 1 个 CO₂ 需 3 ATP 和 2 NADPH。

The light reactions take place in the thylakoid membranes: photosystem II absorbs light and its oxygen-evolving Mn₄CaO₅ cluster oxidises water to O₂; electrons pass through plastoquinone and cytochrome b₆f to photosystem I, which finally reduces NADP⁺ to NADPH, while the proton gradient drives ATP synthase (the Z-scheme). The dark reactions, the Calvin cycle, take place in the stroma: RuBisCO fixes CO₂ onto ribulose-1,5-bisphosphate, and ATP and NADPH reduce the product to triose phosphate. Each CO₂ fixed needs 3 ATP and 2 NADPH.

1648 年海尔蒙特的柳树实验表明植物增重主要不来自土壤。1771 年普里斯特利发现植物能“恢复”被蜡烛燃烧污染的空气；1779 年英根豪斯证明这需要光照；1804 年德索绪尔证明植物吸收 CO₂ 和水。1941 年鲁本和卡门用 ¹⁸O 证明释放的氧来自水。1950 年代卡尔文用 ¹⁴C 阐明碳固定途径（1961 年诺贝尔化学奖）；1985 年米歇尔、胡贝尔和戴森霍费尔解析了细菌光合反应中心结构（1988 年诺贝尔化学奖）。

Van Helmont's willow experiment of 1648 showed that a plant's gain in mass does not come mainly from soil. In 1771 Priestley found that plants 'restore' air spoilt by a burning candle, in 1779 Ingenhousz showed this needs light, and in 1804 de Saussure showed that plants take up CO₂ and water. In 1941 Ruben and Kamen used ¹⁸O to show the oxygen comes from water. In the 1950s Calvin mapped carbon fixation with ¹⁴C (Nobel Prize in Chemistry 1961), and in 1985 Michel, Huber and Deisenhofer solved the structure of a bacterial reaction centre (Nobel Prize in Chemistry 1988).

C₄ 植物（玉米、甘蔗）和 CAM 植物（仙人掌、菠萝）通过先固定 CO₂ 为四碳酸来减少 RuBisCO 的光呼吸损失，适应高温干旱。光合作用的理论能量转化效率约 11%，实际作物通常只有 1–2%。人工光合作用研究用半导体光催化剂或分子催化剂分解水制氢、还原 CO₂ 制燃料。

C₄ plants such as maize and sugar cane and CAM plants such as cacti and pineapple first fix CO₂ into four-carbon acids, reducing losses to RuBisCO photorespiration and coping with heat and drought. The theoretical energy efficiency of photosynthesis is about 11%, while crops typically reach only 1–2%. Artificial photosynthesis research uses semiconductor photocatalysts or molecular catalysts to split water into hydrogen and to reduce CO₂ to fuels.