hce_nthu
115年
生物與生化
第 47 題
The evolutionary adaptation of $C_4$ photosynthesis minimizes the photorespiratory deficit by ensuring a high local concentration of $CO_2$ near the enzyme rubisco. This mechanism relies on the spatial segregation of tasks, primarily in the bundle-sheath cells, whose chloroplasts are specialized to contain Photosystem I (PS I) but often lack functional Photosystem II (PS II). How does the absence of Photosystem II (PS II) in the bundle-sheath chloroplasts contribute to minimizing photorespiration, even though the cell is operating under conditions of intense light?
- A The resulting reliance on cyclic electron flow generates the low ratio of ATP to NADPH required to drive the reduction of $CO_2$ and outcompete the photorespiration pathway.
- B The four-carbon compound (e.g., malate) delivered from the mesophyll cells is designed to directly activate PS I, allowing it to generate the necessary high-energy electrons without relying on water.
- C The inhibition of $H_2O$ splitting, which normally occurs at PS II, prevents the release of $O_2$ gas directly into the local environment where the Calvin cycle and rubisco are confined.
- D The PS I-only system utilizes an alternate terminal electron acceptor, which binds to $O_2$ and converts it into water, thus chemically removing the substrate for photorespiration.
- E The bundle sheath cells primarily serve to regenerate the enzyme PEP carboxylase, a process that is independent of $O_2$ concentration and requires only the minimal ATP produced by cyclic flow.
思路引導 VIP
既然維管束鞘細胞的首要任務是讓 Rubisco 專心與二氧化碳結合，請試著回想：在光反應的過程中，哪一種產物會與二氧化碳產生競爭關係？而這個會造成干擾的物質，又是透過光反應中哪一個特定構造的運作所產生的呢？
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AI 詳解
AI 專屬家教
恭喜你準確掌握了 $C_4$ 植物特殊的生理構造細節！這道題目的核心在於理解 Rubisco 這位主角的「雙重身分」：它既能固定 $CO_2$ 進行光合作用，也會誤與 $O_2$ 結合導致耗能的光呼吸 (Photorespiration)。你的判斷非常正確，能連結光系統功能與細胞生化環境是解題的關鍵。
排除氧氣干擾的關鍵適應
在 $C_4$ 植物的維管束鞘細胞中，植物為了極大化 $CO_2$ 的利用效率，必須盡可能降低當地的氧氣濃度。由於 光系統 II (PS II) 是進行水分子光解（$2H_2O \to 4H^+ + 4e^- + O_2$）並釋放氧氣的場所，因此這些細胞選擇減少或不具備功能性的 PS II。如此一來，即使在強光下，維管束鞘細胞內部也不會因為光反應而產生額外的 $O_2$，確保 Rubisco 被包裹在一個高 $CO_2$、低 $O_2$ 的微環境中，徹底解決光呼吸帶來的產能虧損。
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