Cyclic Electron Flow

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• Utilizes components of photosystem I, as well as plastocyanin and the cytochrome
• Use of this pathway depends on levels of NADP
• One ATP is created per two electrons entering and completing the cycle -- Cyclic Phophorylation
• This process generates ATP when there are low levels of NADP

As we study the light reactions, one consideration to make is that intracellular NADP⁺ concentrations may not always favor reactions of photosystem I. In this case, an alternate pathway for ATP generation must be used. Cyclic Electron Flow is this alternate.

Cyclic electron flow produces ATP but not NADPH or O₂. It occurs when high levels of NADPH are already present. This process occurs by electrons leaving the photosystem I complex and, utilizing the cytochrome bf complex, are pumped through the membrane, creating a proton gradient.

As a web page I visited points out, the steps for cyclic electron flow haven't been elucidated. This same page points out that a benefit of cyclic electron flow is that the plant can use it to control pH of the thylakoid lumen.