Video Transcript
What is the primary role of ATP
synthase in the light-dependent reactions? (A) To absorb wavelengths of light
to provide the energy needed for the light-dependent reactions. (B) To actively transport H+ ions
from the stroma and into the thylakoid lumen. (C) To actively transport electrons
along the electron transport chain. Or (D) to phosphorylate ADP to
create ATP.
The light-dependent reactions take
place in the chloroplasts, but specifically in the membranes of the thylakoids. The thylakoid membrane contains
many structures to help carry out the light-dependent reactions. These include photosystems,
electron carriers, and the enzyme ATP synthase. The question asks us specifically
about the role of ATP synthase. So let’s have a closer look at what
this enzyme does in the light-dependent reactions.
During the light-dependent
reactions, hydrogen ions are actively transported from the stroma and into the lumen
of the thylakoids. The concentration of hydrogen ions
in the thylakoid lumen increases, so it is higher than that of the stroma. The hydrogen ions now move from an
area where they are highly concentrated to an area where they are at a lower
concentration. They cannot just move through the
lipid bilayer but instead move down the channel of ATP synthase. This movement of hydrogen ions down
their concentration gradient is referred to as chemiosmosis.
In the light-dependent reactions,
the movement of hydrogen ions down their concentration gradient is coupled to the
production of ATP. In this reaction, ADP gains a
phosphate group. This is known as
phosphorylation. When ADP gains this phosphate
group, it forms ATP. If we look back at our question, we
can see a clue about the role of ATP synthase from the name alone. Synthase comes from the word
“synthesis,” which means to make. We now know that ATP synthase is
responsible for making ATP.
Now, we have enough information to
answer our question. The correct answer is (D). The primary role of ATP synthase in
the light-dependent reactions is to phosphorylate ADP to create ATP.
