Linear photosynthetic electron flow makes both NADPH and a proton gradient, but cellular demand for ATP and reducing power does not always match the ratio produced by linear flow.

Cyclic electron flow around photosystem I provides a way to increase proton-gradient formation and ATP synthesis without net production of NADPH or oxygen.

Electrons return to the transport chain instead of reducing NADP+

In linear flow, excited PSI electrons pass through ferredoxin and ultimately reduce NADP$^+$ to NADPH.

In cyclic flow, electrons from the PSI acceptor side are redirected back toward the plastoquinone/cytochrome b6f region and eventually return to PSI:

             light
              ↓
PSI → ferredoxin
 ↑       ↓
 └── electron-return pathway
        ↓
plastoquinone → cytochrome b6f → plastocyanin
        ↓
       PSI

Electron transfer through the cytochrome b6f region contributes to proton-motive-force generation. ATP synthase can therefore make additional ATP.

What cyclic flow does and does not produce

Because the electrons return to PSI rather than ending on NADP$^+$:

• no net NADPH is formed by the cycle;
• PSII and water oxidation are not required for the cyclic route itself;
• no net $O_2$ is generated by the cyclic route;
• additional proton-motive force can support additional ATP synthesis.

The functional comparison is

linear flow:     ATP + NADPH + O2
cyclic PSI flow: additional ATP, no net NADPH, no net O2

Cyclic electron flow is therefore a balancing mechanism. It changes how light-derived free energy is partitioned between ATP production and reducing-power production without itself fixing carbon.