光合電子傳遞photosynthetic electron transport
光合作用中，受光激發(fā)推動的電子從H2O到輔酶Ⅱ（NADP+）的傳遞過程。光合色素吸收光能后，把能量聚集到反應(yīng)中心，一種特殊狀態(tài)的葉綠素a分子，引起電荷分離和光化學(xué)反應(yīng)。一方面將水氧化，放出氧氣；另一方面把電子傳遞給輔酶Ⅱ（NADP+），將它還原成NADPH，其間經(jīng)過一系列中間（電子）載體。綠色植物中，光合電子傳遞由兩個光反應(yīng)系統(tǒng)相互配合來完成。一個是吸收遠(yuǎn)紅光的特殊葉綠素a分子，最大吸收峰在700nm處，稱為P700。由P700和其他輔助復(fù)合物組成的光反應(yīng)系統(tǒng)，稱光系統(tǒng)I（PSI）。
另一個是吸收紅光的特殊葉綠素a分子，其吸收峰在680nm處，稱為P680。由P680和其他輔助復(fù)合物組成的光反應(yīng)系統(tǒng)，稱光系統(tǒng)Ⅱ（PSⅡ）。兩個光系統(tǒng)之間由細(xì)胞色素b6-f和鐵硫蛋白組成的復(fù)合物連接。在外加人工電子受體和供體的情況，光合鏈上的電子傳遞可分段進行。電子傳遞可偶聯(lián)氧的產(chǎn)生（水的光解）或消耗（O2作電子受體）。因此可用氧電極測定加入不同電子供體和受體之后氧氣的變化反映光合電子傳遞的分段反應(yīng)。
上圖是光合電子傳遞模式圖。紅色字體：DPC、DCBQ、DCP和MV代表人工電子受體和供體。DCMU是電子從PSII傳遞到PQ庫的抑制劑。1. 從水到甲基紫精（MV）的電子傳遞（耗O2反應(yīng)：每傳遞4個電子消耗1個O2，終產(chǎn)物為H2O2）2. 從二氯酚靛酚（DCPIP）到甲基紫精的電子傳遞（耗O2反應(yīng)：每傳遞一個電子消耗1個O2）。3. 從水到對二氯苯醌DCBQ的電子傳遞（放氧反應(yīng)：每傳遞4個電子釋放1個O2）。4. 從二苯卡巴肼（DPC）到甲基紫精的電子傳遞（耗氧反應(yīng)：傳遞1個電子消耗1個O2）。下表是測定光合電子傳遞分段反應(yīng)所需的體系和試劑：
基質(zhì)（低滲介質(zhì)） | 分段反應(yīng)過程 | 反應(yīng)體系 | 電子傳遞類型 | 備注 |
50mmol/L Tricine-KOH 50mmol/L KCl 5mmol/L MgCl2 PH 7.6 20-50μg chl/ml 葉綠體 | 完整的 電子傳遞過程 | 50μmol/L MV 5mmol/L NH4Cl 2mmol/L NaN3 | H2O→MV | 耗氧反應(yīng) |
PSI活性 | 50μmol/L MV 5mmol/L NH4Cl 2mmol/L NaN3 50μmol/L DCMU 2mmol/L DCPIP | DCPIP→MV | 耗氧反應(yīng) | |
PSII活性 | 5mmol/L NH4Cl 4mmol/L K3Fe(CN)6 1mmol/LDCBQ | H2O→DCBQ | 放氧反應(yīng) | |
PSI和PSII活性 | 5mmol/L NH4Cl 2mmol/L NaN3 0.5mmol/L DPC | DCP→MV | 耗氧反應(yīng) |
注意?。?！由于實驗樣品和條件的差異，測定光合電子傳遞分段反應(yīng)所使用的人工電子供體、受體以及反應(yīng)體系會有所不同，具體試劑和體系請根據(jù)實際情況選定。
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