lec 14 slide 19

How many photosystems do plants use in photosynthesis?

: Two — Photosystem II (PSII) and Photosystem I (PSI)

Which photosystem acts first in the light reactions?

Photosystem II (despite the numerical order)

Do both photosystems have antenna complexes?

Yes — each has its own light-harvesting antenna complexes, composed of different proteins

What is the path of electrons between the two photosystems called?

The Z-scheme, because the energy path resembles a “Z” shape

What happens in Photosystem II during the Z-scheme?

• Energy absorbed by antenna complexes

• Transferred via resonance to the special pair

• Charge separation occurs → electron leaves the special pair

• Special pair regains electron by splitting water → releases O₂

What happens to the electron after leaving Photosystem II?

It passes through the electron transport chain (ETC) to Photosystem I

What occurs in Photosystem I?

Energy from antenna complexes excites the electron → eventually used to reduce NADP⁺ to NADPH

Why is it called the Z-scheme?

Because the electron energy trajectory rises and falls like a Z as it moves from PSII → PSI → NADP⁺

What is the overall goal of the electrons in the Z-scheme?

To convert light energy into chemical energy by producing ATP and NADPH for the Calvin cycle