Final answer:
Linear electron transport in photosynthesis involves the production of NADPH and ATP through the flow of electrons from water to NADP+ using photosystem II and I, while cyclic electron transport recycles electrons within PSI to produce additional ATP without producing NADPH.
Step-by-step explanation:
During photosynthesis in the light-dependent reactions, there are two pathways for electron transport: linear electron transport (LET) and cyclic electron transport (CET). LET involves the flow of electrons from water to NADP+, forming NADPH and releasing oxygen as a byproduct. It begins when light energy is absorbed by photosystem II (PSII), exciting electrons that are then passed down the electron transport chain to a cytochrome complex that transfers protons across the thylakoid membrane. The energy released during these redox reactions helps generate a proton gradient that is used by ATP synthase to produce ATP. Electrons are then passed to photosystem I (PSI), which reduces NADP+ to NADPH. CET, on the other hand, involves electrons that are recycled within PSI and the electron transport chain connected to it. This process contributes to generating a chemiosmotic gradient without the production of NADPH and without the oxidation of water. This gradient is also harnessed by ATP synthase to produce ATP. CET is thought to protect the plants from over-reduction and provide additional ATP that may be needed during periods of high energetic demand.
In summary, the light-dependent reactions harness solar energy to produce ATP and NADPH—the energy carriers which then power the Calvin cycle to produce glucose. The LET pathway yields both ATP and NADPH, while CET primarily enhances ATP production.