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所在平台: Udemy |
课程主页: https://www.udemy.com/course/biology-photosynthesis/
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**Coursera 课程总结：植物光合作用** 本课程深入探讨了植物如何通过光合作用制造自身食物。 **核心内容：** * **光合作用的意义：** 绿色植物利用大气中的二氧化碳，通过叶片的气孔将其吸收，并转化为碳水化合物（主要是葡萄糖和淀粉）。 * **光合作用的场所：** 光合作用主要发生在植物的绿色部分，尤其是叶片。叶片中的叶肉细胞含有大量叶绿体，叶绿体是二氧化碳固定的关键场所。叶绿体内的类囊体膜是光反应的发生地，而基质则是化学合成途径（碳固定）的场所。 * **光合作用的两个阶段：** * **光反应：** 光能被色素分子吸收，并传递至反应中心叶绿素a（如PS I的P700和PS II的P680）。吸收光后，电子被激发并通过PS II 和 PS I 传递，最终形成NADH。此过程中，类囊体膜两侧会形成质子梯度，通过ATP合酶释放能量合成ATP。水在PS II处裂解，产生氧气、质子，并将电子传递至PS II。 * **碳固定反应（卡尔文循环）：** 二氧化碳通过RuBisCO酶与一种五碳化合物RuBP结合，生成两分子三碳化合物PGA。随后，PGA通过卡尔文循环转化为糖，并再生RuBP。该过程消耗光反应产生的ATP和NADPH。RuBisCO在C3植物中还会催化一种无益的氧化反应——光呼吸。 * **C4 植物的特殊光合作用：** 部分热带植物采用C4途径，其特点是二氧化碳固定的初始产物是四碳化合物，而卡尔文循环则在维管束鞘细胞中进行碳水化合物的合成。
SUMMARYGreen plants make their own food by photosynthesis. During this process carbon dioxide from the atmosphere is taken in by leaves through stomata and used for making carbohydrates, principally glucose and starch. Photosynthesis takes place only in the green parts of the plants, mainly the leaves. Within the leaves, the mesophyll cells have a large number of chloroplasts that are responsible for CO2 fixation. Within the chloroplasts, the membranes are sites for the light reaction, while the chemosynthetic pathway occurs in the stroma. Photosynthesis has two stages: the light reaction and the carbon fixing reactions. In the light reaction the light energy is absorbed by the pigments present in the antenna, and funnelled to special chlorophyll a molecules called reaction centre chlorophylls. There are two photosystems, PS I and PS II. PS I has a 700 nm absorbing chlorophyll a P700 molecule at its reaction centre, while PS II has a P680 reaction centre that absorbs red light at 680 nm. After absorbing light, electrons are excited and transferred through PS II and PS I and finally to NAD forming NADH. During this process a proton gradient is created across the membrane of the thylakoid. The breakdown of the protons gradient due to movement through the F0 part of the ATPase enzyme releases enough energy for synthesis of ATP. Splitting of water molecules is associated with PS II resulting in the release of O2 , protons and transfer of electrons to PS II.In the carbon fixation cycle, CO2 is added by the enzyme, RuBisCO, to a 5- carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA. This is then converted to sugar by the Calvin cycle, and the RuBP is regenerated. During this process ATP and NADPH synthesised in the light reaction are utilised. RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants: photorespiration.Some tropical plants show a special type of photosynthesis called C4 pathway. In these plants the first product of CO2 fixation that takes place in the mesophyll, is a 4-carbon compound. In the bundle sheath cells the Calvin pathway is carried out for the synthesis of carbohydrates.
