Phenylurea is an herbicide used on corn fields to injure and kill dicot weeds after the seeds germinate. The herbicide functions by irreversibly binding to the D1 quinone-binding protein of photosystem II (PSII) and inhibiting the movement of electrons to plastoquinone, thereby shutting off the linear photosynthesis pathway. How will the pH of the stroma and thykaloid region change immediately after a leaf cell takes up phenylurea?

视频信息

答案文本 复制

视频字幕 复制

Phenylurea is a selective herbicide used in corn fields to control dicot weeds. It works by targeting photosystem two, specifically binding to the D1 quinone-binding protein. This binding blocks electron transport from photosystem two to plastoquinone, effectively shutting down the linear photosynthesis pathway. Under normal conditions, the photosynthetic electron transport chain maintains a proton gradient across the thylakoid membrane. Water splitting at photosystem two releases protons into the lumen. The cytochrome b6f complex pumps additional protons from the stroma into the lumen. Meanwhile, NADP plus reduction consumes protons in the stroma. This creates a pH gradient with an acidic lumen at pH 5 and an alkaline stroma at pH 8. When phenylurea binds to the D1 protein of photosystem two, it completely blocks electron transport. This stops the flow of electrons through the entire photosynthetic chain. Water splitting at photosystem two ceases, halting proton release into the lumen. The cytochrome b6f complex can no longer pump protons, and NADP plus reduction stops consuming protons in the stroma. All the processes that maintain the normal proton gradient come to a halt. The disruption of electron transport causes immediate pH changes in both compartments. In the thylakoid lumen, the pH increases from about 5 to 6, becoming less acidic. This happens because protons are no longer being released from water splitting or pumped in by the cytochrome complex. In the stroma, the pH decreases from about 8 to 7, becoming less alkaline. This occurs because protons are no longer being removed by pumping or consumed by NADP plus reduction. The normal proton gradient that drives ATP synthesis begins to collapse. In summary, phenylurea herbicide causes specific pH changes in leaf cells. The stroma pH decreases, becoming less alkaline, while the thylakoid lumen pH increases, becoming less acidic. This occurs because phenylurea blocks electron transport at photosystem two, stopping all the processes that maintain the normal proton gradient. Without water splitting, proton pumping, and NADP plus reduction, the gradient collapses, disrupting ATP synthesis and ultimately killing the plant through photosynthetic failure.