AP BIOLOGY

Detailed explanation-1: -There are two types of photosystems in the light-dependent reactions, photosystem II (PSII) and photosystem I (PSI). PSII comes first in the path of electron flow, but it is named as second because it was discovered after PSI.

Detailed explanation-2: -The series of light-dependent reactions begins when sunlight hits a molecule of chlorophyll, located in photosystem II. This excites an electron, which leaves the chlorophyll molecule and travels along the thylakoid membrane via a series of carrier proteins (known as the electron transport chain).

Detailed explanation-3: -These reactions occur within specialised membrane discs within the chloroplast called thylakoids and involve three steps: Excitation of photosystems by light energy. Production of ATP via an electron transport chain. Reduction of NADP+ and the photolysis of water.

Detailed explanation-4: -There are two kinds of photosystems: photosystem I (PSI) and photosystem II (PSII) (Fig. 3.3). PSII acts first during the light transformation process in photosynthesis, but it was named PSII because it was discovered second.

Detailed explanation-5: -Their names, however, are based on the time of their discovery. Photosystem I was discovered and named first, followed by photosystem II, which was discovered second. Another reason why photosystem II goes first is that photosystem I depend on the electron that is liberated by photosystem II.