Final answer:
The action leading to a positively charged thylakoid membrane involves the pumping of H+ ions into the thylakoid lumen by the electron transport chain, coupled with the splitting of water to release additional H+ and electrons.
Step-by-step explanation:
The action that contributes to the inside of the thylakoid membrane becoming positively charged during the light-dependent reactions is the active transport of hydrogen ions (H+). When a photon of light is absorbed by chlorophyll in the Photosystem II (PSII), it excites electrons that are subsequently transferred through the electron transport chain (ETC).
As these electrons move from PSII to Photosystem I (PSI), their energy is used by protein complexes within the ETC to pump hydrogen ions from the stroma into the thylakoid lumen, against their concentration gradient. Additionally, the splitting of water in PSII releases more H+ into the thylakoid lumen while generating electrons that re-enter the ETC and oxygen as a by-product. This accumulation of H+ creates a high concentration of positively charged hydrogen ions inside the thylakoid lumen, thereby leading to a decrease in pH (increased acidity) and establishing an electrochemical gradient across the membrane.
The potential energy stored in this gradient is then harnessed by ATP synthase to convert ADP to ATP as H+ ions flow back into the stroma through the enzyme, a process known as chemiosmosis. The result of these actions is the generation of ATP and NADPH, which are essential for the subsequent light-independent reactions (Calvin cycle) in photosynthesis.