7.3 The cyclic pathway of photosynthesis  (Page 17/2)

The light-dependent reactions of photosynthesis use two photosystems to energize electrons that then enter an electron transport chain. As previously discussed, this electron transport chain results in the production of ATP and NADPH. However, ATP production can still happen even with just one photosystem.

ATP is required for most cellular activity, but NADPH is not. NADPH is generally only required when the photosynthesizer is actively making sugar from the light independent reactions (Calvin-Benson cycle). Since photosynthesizers don't need to make sugar all the time, they can save some energy by not producing NADPH all the time. The cyclic pathway of photosynthesis     produces ATP without producing NADPH.

The cyclic pathway of photosynthesis uses just one photosystem: photosystem I. This pathway is simpler than the non-cyclic reactions. It evolved first (in photosynthetic bacteria) and is present in nearly all photosynthetic organisms. Photosystem II and the non-cyclic pathway evolved later (still in photosynthetic bacteria), and spread with the evolution of photosynthetic protists and plants.

In the non-cyclic pathway, when photosystem I receives electrons from the electron transport chain, it re-energizes them. Those electrons then pass through another electron transport chain until they are accepted by NADP+. However, if there is not enough NADP+ to accept those electrons, the cycle does not immediately stop. Instead, electrons can be transported back to photosystem I, which can re-energize them again. This is called the cyclic pathway because no new electrons are ever needed ( [link] ). Electrons that leave photosystem I always return to it, so the same electrons are re-energized over and over again (as long as there is light hitting the photosystem). As electrons pass through the second electron transport chain, they are powering the transport of H+ ions across the membrane. As long as an H+ gradient exists, ATP can be made.

Figure 7.1

If no new NADPH is made (because there are no NADP+ molecules to accept electrons), no new sugar can be made. In certain situations, this is acceptable. During the winter, plants aren't actively making a lot of sugar, and therefore wouldn't need a lot of new NADPH. With the cyclic pathway, plants can save some time and energy.

Since photosystem I is accepting electrons that are returned to it, it is not accepting electrons from the previous electron transport chain. Therefore, the first electron transport chain will be backed up, which means that photolysis will not occur. Without photolysis, no water is broken down and therefore no oxygen is produced.

Section summary

The cyclic pathway of photosynthesis uses just photosystem I to generate ATP. This process occurs when sugar production is not immediately necessary and therefore no new NADPH is generated. Electrons are recycled from photosystem I to be continually re-energized until they can be accepted by NADP+. The cyclic pathway generates ATP, but neither oxygen nor sugar.