In higher plants, photosystems II and I are found in grana stacks and unstacked stroma lamellae, respectively. To connect them, electron carriers negotiate tortuous multi-media paths and are subject to macromolecular blocking. Why does evolution select an apparently unnecessary, inefficient bipartition? Here we systematically explain this perplexing phenomenon. We propose that grana stacks, acting like bellows in accordions, increase the degree of ultrastructural control on photosynthesis through thylakoid swelling/shrinking induced by osmotic water fluxes. This control coordinates with variations in stomatal conductance and the turgor of guard cells, which act like an accordion's air button. Thylakoid ultrastructural dynamics regulate macromolecular blocking/collision probability, direct diffusional pathlengths, division of function of Cytochrome b₆f complex between linear and cyclic electron transport, luminal pH via osmotic water fluxes, and the separation of pH dynamics between granal and lamellar lumens in response to environmental variations. With the two functionally asymmetrical photosystems located distantly from each other, the ultrastructural control, nonphotochemical quenching, and carbon-reaction feedbacks maximally cooperate to balance electron transport with gas exchange, provide homeostasis in fluctuating light environments, and protect photosystems in drought. Grana stacks represent a dry/high irradiance adaptation of photosynthetic machinery to improve fitness in challenging land environments. Our theory unifies many well-known but seemingly unconnected phenomena of thylakoid structure and function in higher plants.

中文翻译：

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颗粒类囊体的结构和功能：解释高等植物的一个持久之谜


在高等植物中，光系统 II 和 I 分别存在于基粒堆叠和未堆叠的基质片层中。为了连接它们，电子载体通过曲折的多媒体路径并受到大分子阻挡。为什么进化会选择一种明显不必要的、低效的二分法？下面我们系统地解释一下这个令人费解的现象。我们认为，基粒堆叠就像手风琴中的风箱一样，通过渗透水通量引起的类囊体膨胀/收缩来增加对光合作用的超微结构控制程度。这种控制与气孔导度的变化和保卫细胞的膨胀相协调，其作用就像手风琴的空气按钮。类囊体超微结构动力学调节大分子阻塞/碰撞概率、直接扩散路径长度、细胞色素b ₆ f复合物在线性和循环电子传递之间的功能划分、通过渗透水通量的管腔 pH 值以及响应中颗粒和层状管腔之间 pH 动力学的分离到环境变化。由于两个功能不对称的光系统彼此距离较远，超微结构控制、非光化学猝灭和碳反应反馈最大限度地配合，以平衡电子传输与气体交换，在波动的光环境中提供稳态，并在干旱中保护光系统。格拉纳堆栈代表了光合作用机械的干燥/高辐照度适应性，以提高在具有挑战性的陆地环境中的适应性。我们的理论统一了高等植物中许多众所周知但看似无关的类囊体结构和功能现象。